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Nonoxidative Activation of Methane

Tushar V. Choudhary,# Erhan Aksoylu, and D. Wayne Goodman*

Department of Chemistry, Texas A&M University, College Station, Texas, USA

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

II. FUNDAMENTAL SURFACE SCIENCE STUDIES . . . . . . . . . . . . . . . . . . . 154

A. Tungsten Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

B. Ni Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

C. Platinum and Palladium Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

D. Electron and Photo-Induced Dissociation of Methane on Pt(111) . . . 159

E. Rh and Ir Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

F. Ru Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

III. LOW TEMPERATURE COUPLING OF METHANE . . . . . . . . . . . . . . . . . 161

IV. PYROLYSIS AND HIGH-TEMPERATURE COUPLING OF

METHANE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

A. Methane Pyrolysis and Dehydrogenative Coupling of Methane

in Homogeneous Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167

B. Catalytic Pyrolysis and Nonoxidative Coupling of Methane at

High-Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

1. Carbonaceous, Oxide, and Metal Catalysts . . . . . . . . . . . . . . . . . . . 170

2. Zeolite-Supported Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

V. PRODUCTION OF HYDROGEN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175

151

DOI: 10.1081/CR-120017010 0161-4940 (Print); 1520-5703 (Online)

Copyright q 2003 by Marcel Dekker, Inc. www.dekker.com

#Current address: Phillips Petroleum Company, Bartlesville, OK 74004, USA.*Correspondence: D. Wayne Goodman, Department of Chemistry, Texas A&M University, College

Station, Texas, USA; Fax: (979) 845-6822; E-mail: [email protected].

CATALYSIS REVIEWSVol. 45, No. 1, pp. 151–203, 2003

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016

©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.

VI. FORMATION OF CARBON FILAMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . 179

VII. SYNTHESIS OF DIAMOND FILMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

VIII. CONCLUDING REMARKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

ABSTRACT

Effective utilization of methane remains one of the long-standing problems in catalysis.

Over the past several years, various routes, both direct and indirect, have been

considered for the conversion of methane to value-added products such as higher

hydrocarbons and oxygenates. This review will focus on the range of issues dealing with

thermal and catalytic decomposition of methane that have been addressed in the last few

years. Surface science studies (molecular beam methods and elevated-pressure reaction

studies) involving methane activation on model catalyst systems are extensively

reviewed. These studies have contributed significantly to our understanding of the

fundamental dynamics of methane decomposition. Various aspects of the nonoxidative

methane to higher hydrocarbon conversion processes such as high-temperature coupling

and two-step low-temperature methane homologation have been discussed.

Decomposition of methane results in the production of COx-free hydrogen (which is

of great interest to state-of-the-art low-temperature fuel cells) and various types of

carbon (filamentous carbon, carbon black, diamond films, etc.) depending on the

reaction conditions employed; these issues will be briefly addressed in this review.

Key Words: Surface science; Model catalysts; Carbon filaments; Methane coupling;

Aromatization; Diamond films; Natural gas; Hydrogen production.

I. INTRODUCTION

Over the past several years, considerable time and effort has been invested in the study

of methane activation.[1 – 16] The main goal of these efforts has been to find effective

processes to develop the technology for the efficient and environmentally benign utilization

of natural gas. Activation of methane represents an intensely challenging problem due to its

refractory nature. A plethora of investigations have addressed the issue of methane

conversion over the past three decades and a number of excellent reviews can be found in

the literature[17 – 24] Conversion of methane to hydrocarbons can be effected via an indirect

or a direct route as observed in Fig. 1. The indirect route involves the production of

hydrocarbons via intermediates formed from the reaction of methane with steam, oxygen,

HCl, etc.,[17] whereas the direct route involves coupling of methane in the presence of

oxygen (oxidative coupling of methane) or nonoxidative coupling (high-temperature

coupling and low-temperature two-step methane homologation). Methane can also be

converted to carbon and hydrogen via catalytic/homogeneous decomposition and the COx-

free hydrogen so produced is further utilized in current low-temperature fuel cells.

Choudhary, Aksoylu, and Goodman152



Steam reforming of methane, currently the major route to methane conversion, can be

represented as:

CH4 þ H2O ! CO þ 3H2

This highly endothermic process results in the formation of synthesis gas that can be

further processed into methanol and ammonia. The methanol to gasoline (MTG) process

can then be utilized for the production of gasoline.[25] Alternatively, synthesis gas can be

directly processed into hydrocarbons via the Fischer–Tropsch process.[26] Steam

reforming of methane has been extensively reviewed[27] and will not be a focus of this

review.

Hydrocarbon production via synthesis gas is expensive and rather circuitous,

therefore there was considerable excitement in the scientific world when Keller and

Bhasin[28] first reported the direct conversion of methane into ethylene in 1982. Following

this pioneering work, oxidative coupling of methane (OCM) became one of the most

pursued topics of research in methane activation. Detailed reviews of the process can be

found in the literature.[29 – 31] Usually OCM is carried out between 900 and 1200K under

atmospheric pressures. Basic oxides promoted with alkali metal salts and/or alkaline earth

metal salts are important OCM catalysts.[30,32] The reaction results in C2 hydrocarbon

yields of less than 25%–30% because the selectivity of the desired C2 products is

hampered by the undesired COx species formed due to the presence of oxygen in the feed.

Enhancement of the C2 yield can be obtained by separating the desired products from the

reactants by avoiding further oxidation of these products. Aris and co-workers utilized a

simulated counter-current moving-bed chromatographic reactor to obtain significantly

higher yields of C2 hydrocarbons,[33] whereas Vayenas and co-workers employed a gas

recycled electro-catalytic reactor-separator to obtain a 85% yield of ethylene.[34]

However, economic considerations prohibit the large-scale implementation of such

Figure 1. Simplistic schematic representation of different methods of activating methane.

Nonoxidative Activation of Methane 153



reactors. Choudhary et al.,[35] have suggested distribution of oxygen feed for increasing

both the conversion and selectivity and also reducing the hazardous nature of the OCM

process. The large input of work (catalyst synthesis and reactor design) into OCM has

significantly improved the process over the years[36 – 41] but inspite of the improvements, it

is currently far from being commercially viable. Catalytic CO2 reforming of methane to

syngas in the presence of steam and/or oxygen has also been extensively investigated in

the last few years.[42 – 44]

The thermal and catalytic decomposition of methane has been extensively studied for

fundamental and applied reasons. In this review we will focus on the range of issues that

have been addressed in the last few years related to methane activation. Numerous articles

have dealt with methane decomposition on single crystal surfaces, studies that have

provided important, fundamental insights into the dynamics of methane decomposition.

These studies will be comprehensively covered in this review. Issues related to low-

temperature and high-temperature nonoxidative coupling of methane will be addressed.

Decomposition of methane to produce hydrogen has been proposed as an economical

process to produce hydrogen.[45,46] Various studies related to the process will be reviewed.

We would also like to dwell on the various applications such as formation of pyrolytic

carbon, diamond films, and carbon-nanofibers/filaments arising from decomposition of

methane. This review will not address issues dealing with the reaction of methane with

oxygen, chlorine, steam, ammonia, etc.

II. FUNDAMENTAL SURFACE SCIENCE STUDIES

The last three decades have seen the development of an array of surface analytical

techniques that allows surfaces to be defined at the atomic level. Ultra-high vacuum

(UHV) surface science studies have played a key role in relating the atomic structure,

composition, and electronic properties of surfaces to catalytic activity and selectivity.[47 –

49] The limitation of the “pressure gap” has been overcome by coupling an apparatus for

measurement of reaction kinetics at elevated pressures with a contiguous UHV chamber

for surface analysis.[50 – 53] This approach has facilitated a direct comparison of reaction

rates measured on single crystal surfaces with those measured on supported high-surface

area catalysts. Moreover, it has allowed detailed studies addressing reaction mechanisms,

activity relationships, the effect of promoters and inhibitors on catalytic activity, and in

certain cases, the identification of reaction intermediates by post-reaction surface

analysis.[54,55]

Methane dissociation has attracted attention in recent years due to its industrial

relevance. It is considered to be the rate-determining step for the steam reforming of

natural gas and this has led to considerable interest in investigating the reactive sticking

process of methane on metal single crystals.[56] Methane chemisorption has been studied

on a variety of transition metal surfaces by employing both “molecular beams” as well as

“bulb techniques.” These studies have shown that dissociation of methane on transition

metals proceeds via either a direct dissociative mechanism (DDM) or a precursor/trapping

mediated mechanism (PMM). In DDM, dissociation takes place on impact with the

surface. Initial adsorption probabilities for DDM do not strongly depend upon surface

temperature and the initial reaction probability is expected to increase with the increase in




incident kinetic energy. For PMM, a molecular precursor/trapped state forms that is

accommodated to the surface temperature. This trapped precursor can then either desorb

from the surface or undergo dissociative chemisorption. This leads to a strong dependence

of the initial adsorption probabilities on the surface temperature. Contrary to DDM, for

PMM the initial reaction probability is expected to decrease with an increase in the

incident kinetic energy. There has been considerable debate in the literature over the

controversial issue of the mechanism of methane dissociation over transition metal

surfaces. The studies undertaken by various groups on different metal surfaces are briefly

summarized below.

A. Tungsten Surfaces

Winters investigated the dissociative chemisorption of CH4 and its various deuterated

forms (CD3H, CD4, CDH3, and CD2H2) on tungsten wires. Methane dissociation results

showed two temperature regimes of activated adsorption behavior exhibiting activation

energies of 11.3 kJ/mol ð600K , T , 1000KÞ and 42.6 kJ/mol

ð1200K , T , 2600KÞ:[57] Interestingly, he observed an absence of temperature

dependence on the ratio of sticking probability (S0) of one methane isotope to another.[58]

Based on experimental results and model calculations, Winters proposed a vibrationally

activated tunneling mechanism for the dissociation of CH4 on tungsten.

Rettner et al. were the first to employ molecular beam techniques for investigating the

dissociative chemisorption of methane.[59] Molecular beam studies permit considerable

control with respect to reactant parameters such as translational energy (Etrans) and

incidence angle. Their studies showed a dramatic increase (,105) in sticking probability

of CH4 on W(110) for a 100 kJ/mol increase in translational energy. A translationally

activated tunneling model was proposed to explain their results. In contrast to these

studies, investigations undertaken by Lo and Ehrlich[60] on the CH4(CD4)/W(211) surface

led them to believe that a tunneling mechanism was not consistent for the dissociative

chemisorption of methane, instead they advocated a mechanism involving vibrational

excitation without the influence of tunneling.

B. Ni Surfaces

Since steam reforming catalysts are Ni-based, considerable attention has been given

to the interaction of methane with Ni surfaces. Bootsma and coworkers[61] showed that

there was no methane dissociation on Ni(110) at room temperature without the aid of

electron sources. The authors observed a sticking coefficient in the order of 1028 at

temperatures between 473 and 579K. Two possible kinetic decomposition processes

involving activation energies of 88 and 130 kJ mol21 were proposed; however, the authors

did not distinguish between the two due to lack of sufficient accuracy in their kinetic

results. Subsequently, they also investigated the sticking probabilities of methane with the

other low-index Ni crystal phases at temperatures between 523 and 618K for methane

pressures of ca. 1022 Torr.[62] While a sticking probability of 5 £ 1029 was observed for




Ni(100), there was no reaction on the Ni(111) surface (within detection limits) indicating

an initial sticking coefficient of # 1 £ 10210:Ceyer and coworkers employed molecular beam experiments concomitant with high-

resolution electron-energy spectroscopy (HREELS) to investigate the interaction of

methane with Ni(111) surface.[63] The molecular beam of methane was produced by high-

pressure, adiabatic expansion of a CH4 diluted in He mixture. The dissociation probability

showed an exponential increase by two orders of magnitude on increasing the translational

energy (Etrans) from 50 to 71 kJ mol21 at a constant surface temperature of 475K. Studies

using HREELS indicated the presence of methyl (CH3) species on the surface after the

methane chemisorption experiments. Interestingly, the authors observed that the surface

temperature (Ts) had no influence on the dependence of the sticking probability with

translational energy. Experiments with CD4 showed a strong kinetic isotopic effect. Based

on their results the following was concluded:

(i) Methane dissociation proceeded via DDM.

(ii) Vibrational energy (Evib) and (Etrans) were both effective in promoting methane

dissociation.

(iii) Increase in Evib/Etrans promoted the deformation of the molecule such that the C

atom was effectively brought closer to the Ni surface. Moreover, it was not

essential to completely deform the molecule as the light H-atom could tunnel

through the barrier once it was sufficiently narrow.

Beebe, et al.[64] employed bulb experiments to thoroughly investigate methane

interaction kinetics with the low-index planes of Ni surfaces. These experiments were

carried out under high-incident methane flux (1 Torr) conditions as compared to molecular

beam studies. Time-dependent carbon buildup experiments at 450K showed that the

methane reactivity increased in the order Nið111Þ , Nið100Þ , Nið110Þ: Table 1

summarizes the activation energy and sticking probability results for the systems

investigated. The difference in sticking probabilities for the different low-index planes

clearly illustrates the structure sensitivity of the methane dissociation process on Ni. The

activation energies for CH4/Ni(111) and CH4/Ni(110) systems were quite similar, but the

activation energy for CH4/Ni(100) was smaller by a factor of 2. A large kinetic effect for

CH4 vs. CD4 was observed for Ni(100), whereas no such effect was observed for Ni(110)

surface. The rates measured in this work were about a magnitude higher than the adjusted

rates for catalytic methane steam reforming, rationalized by the fact that in these studies,

initial rates were measured on a clean surface. Based on this, the authors claimed that

Table 1. Summary of activation energies and initial

methane sticking co-efficients at 500K.[64]

Surface

Activation energy

CH4 (CD4), kJ mol21Initial sticking

co-efficient (CH4)

Ni(111) 52.7 (—) 1 £ 1028

Ni(100) 26.8 (62.3) 6 £ 1028

Ni(110) 55.6 (52.3) 1 £ 1027




the rates measured in their work effectively represented the upper limit for the unpromoted

steam reforming catalyst. As a part of this work the authors also provided a comparison of

this study with other studies performed on similar systems. Bootsma and coworkers

reported much lower sticking coefficients and an absence of dependence of initial sticking

coefficient on surface temperature.[61,62] This was in contrast to work by Beebe et al. who

reported a temperature dependence for both Ni(111) surface and Ni(100) surface and

higher sticking coefficients. This deviation was attributed to the presence of an

equilibrated gas above the Ni single crystals (at high flux such as 1 Torr) in the case of

Beebe et al. as opposed to the nonequilibrated behavior expected at pressures of 1023 to

1022 Torr used by Bootsma et al.[62] Though there was an excellent agreement for the

CH4/Ni(111) system studied by Beebe et al. (bulb method: high flux experiments) and

Ceyer and coworkers [molecular beam technique[63]], the results involving bulb

experiments for the CH4/Ni(100)[64] system differed appreciably from the molecular beam

experiments performed by Hamza and Madix on the same system.[65] It is also interesting

to note that Hamza and Madix reported a linear dependence of the sticking coefficient with

Etrans and much larger sticking coefficients (2 to 3 order magnitude) at a given incident

energy for the CH4/Ni(100) system as compared to the exponential dependence seen by

Ceyer and coworkers for the CH4/Ni(111) system.[63] Chorkendorff et al. employed x-ray

photo-electron spectroscopy (XPS) to study the CH4/Ni(100) system[66] and reported an

activation energy which was twice that reported by Beebe et al. on the same system.[64]

Bulb experiments performed by Yates and coworkers[67] showed that the dissociative

chemisorption of CH4 on Ni(111) proceeded via DDM at pressures relevant to catalytic

processes. At 1 Torr CH4 pressure and surface temperature of 600K, each methane

molecule collision was found to result in the deposition of 4 £ 1028 C atoms.

Beckerle et al.,[68,69] showed that methane dissociation could also be affected by

collision of inert gas atoms (Ne, Ar, and Kr) with CH4 physisorbed on Ni(111) surface

(surface temperature ðTsÞ ¼ 46KÞ: They proposed a hammer mechanism, which involved

impulsive transfer of some kinetic energy of the inert gas atom (Ar or Ne) to the

physisorbed CH4, followed by dissociative chemisorption of methane upon its collision

with the Ni surface to yield an adsorbed methyl group. Subsequently, the same group

reported the transformation of methane to benzene under UHV conditions.[70] The methyl

radicals formed from by the hammer method (as described previously) dissociated into CH

species at higher surface temperature and finally formed benzene via an ethylene

intermediate.

Jiang and Goodman[71] investigated the effect of sulfur poisoning on the

chemisorption of methane on Ni(100) surface. The influence of sulfur on the initial

methane decomposition activity is summarized in Table 2. The authors proposed a “simple

site-blocking mechanism” to explain their experimental results and the fit to a simple first-

order Langmurian form suggested that each sulfur atom was responsible for blocking three

“dissociation sites.” Campbell et al.,[72] reported that methane dissociation on Ni(100)

proceeded via PMM. The reactivity of methane was found to be lower on NiO thin film

[compared to Ni(100)] and exhibited an apparent activation energy of ca. 37 kJ/mol.

Employing both molecular beam experiments and bulb experiments, Chorkendorff and

coworkers[73,74] observed that the CH4 chemisorption on Ni(100) proceeded via DDM,

which was in contradiction to the results by Campbell et al.[72] In one of their previous

studies on the CH4/Ni(100) system, they had observed an apparent activation energy of




52 kJ/mol[66] which was less than the value (ca. 59 kJ/mol) they reported in their later

work.[74] They attributed this to the use of a thermal finger in their later work, which

ensured that the methane was completely equilibrated to the crystal.

Recently, Juurlink et al.,[75] combined the use of laser excitation and molecular beam

experiments to probe the CH4/Ni(100) system. Enhancement in dissociative chemisorp-

tion of methane by singular excitation of v3 (antisymmetric stretching vibration) was

observed for all the translational energies employed in the study. Translational energy was

found to be more effective in enhancing the sticking probability than the vibrational

excitation of v3.

C. Platinum and Palladium Surfaces

Employing molecular beam methods on the CH4/Pt(111) system, Luntz and Bethune

found that the sticking probability (S0) did not show a perfect exponential dependence on

the translational energy (Etrans).[76] This behavior was contrary to that observed on other

systems[59,63] where S0 was found to increase exponentially with Etrans. Moreover, a

modest dependence of S0 with surface temperature (Ts) was observed. To explain the

dependence of S0 on all three parameters viz. Etrans, Evib, and Ts a model invoking

thermally assisted tunneling was proposed.[77] The coupling of a tunneling barrier to the

lattice could successfully explain the Ts dependence of S0, which was important since a

simple DDM involving tunneling could not account for the Ts dependence of S0. A second

study related to the same system by Valden et al.,[78] was in good agreement with the work

by Luntz and Bethune.[76] The presence of pre-adsorbed oxygen was found to be

detrimental to CZH activation on Pt(111). It was believed that the mechanism for

dissociation of methane on the clean and oxygen-precovered Pt(111) surface was similar.

Chesters and coworkers studied methane dissociation on both Pt(111) and Pt0.25Rh0.75

alloy and found that the methane sticking behavior was similar for both substrates.[79]

Klier and coworkers investigated methane dissociation on Pd(679) single crystal

employing bulb techniques.[80] The apparent activation energy for the process was

estimated to be ca. 44.7 kJ/mole. No CH4 dissociation occurred below a threshold pressure

Table 2. Methane decomposition rate on Ni(100) shown at different

sulfur coverages and temperatures.[71]

Sulfur coverage

(ML)

Temperature

(K)

Initial methane decomposition rate

(molecules site21 sec21)

0.008 550 0.013

600 0.019

0.04 550 0.011

600 0.017

0.1 600 0.014

0.2 550 0.003

0.28 550 0.002

600 0.003




of 0.2 Torr at 500K, based on which the authors suggested DDM as the route for

dissociative chemisorption of methane. The dissociation of methane resulted in formation

of carbon in form of disordered fractional monolayers at 400–500K and disordered

multilayer clusters at temperatures greater than 500K.

Based on molecular beam experiments, Valden et al. concluded that dissociative

chemisorption of methane on Pd(110) proceeded via DDM.[81] The linear decrease in S0

with increasing preadsorbed oxygen coverage was similar to that observed in case of Pt

(111).[78] The first-order Langmuir kinetics provided a good fit for their experimental data

and indicated that each oxygen atom was responsible for poisoning ca. 2 dissociation sites

for methane.[82]

Recently Walker and King carried out a comprehensive molecular beam study on the

CH4/Pt(110) system.[83] A striking feature of this work was the decrease in S0 until

Etrans ¼ 100 meV: Thereafter, a sharp increase in S0 was observed with increasing Etrans.

The authors felt that the anomalous behavior of S0 at low Etrans did not involve a precursor

state (as suggested in [84]) but was in fact related to dynamic steering in a direct

dissociation process. Thus, supposedly there were two different potential energy pathways

operating at low and high Etrans. The surface temperature was found to promote the direct

dissociation processes in good agreement with previous work.[73,76,85]

D. Electron and Photo-Induced Dissociation of Methane on Pt(111)

Vacuum-ultraviolet (VUV) photons are required for dissociation of gas-phase

methane as it is transparent in the visible to ultraviolet region. But use of VUV photons for

this purpose is not pragmatic, as it is expensive and arduous to obtain from conventional

sources of light. Gruzdov et al. overcame this problem by photo-dissociating methane on

Pt(111).[86] Methane physisorbed at 40K on Pt(111) was shown to undergo dissociation

when irradiated with 193 nm photons. Adsorbate photo-dissociation on metal surfaces is

postulated to proceed[87] either via direct excitation of adsorbed molecules by photons or

the transfer of an electron to the low-lying negative state of the adsorbate initiated by

substrate excitation. The authors favored the latter mechanism to explain the photo-

induced dissociation of methane on Pt(111). To acquire further rinformation, Yoshinobu

and coworkers studied the same system using infrared reflection absorption spectroscopy

(IRAS).[88] Employing elegantly designed experiments, they showed that distortion of

methane symmetry from tetrahedral (Td) to C3v was essential for its photo-dissociation.

Only a part of the first layer [on Pt(111)] was deformed to C3v symmetry and, hence,

photo-dissociated. The reaction became self-limiting as some of the reaction products

rendered the other methane molecules inactive by modifying the surface.

White and coworkers investigated the electro-induced dissociative chemisorption of

CH4 on Pt(111).[89] Irradiation of physisorbed CH4 on Pt(111), a ðTs ¼ 55KÞ with

electrons resulted in the selective production of CH3 and H species. The authors felt that

the selective formation of CH3 species was due to the fact that dissociation of CH4 on

surfaces was restricted to fewer number of pathways as quenching of the excited CH4 state

by the substrate played an important role (unlike that for gas-phase dissociation). It was

believed that the dissociation chemistry was initiated by impact excitation of the

physisorbed methane.




E. Rh and Ir Surfaces

Brass and Ehrlich[90] proposed that activation energy measurements could help in

providing an understanding about the dissociation mechanism. Domination by

translational activation would result in single-activation energy for measurements on an

isothermal gas and surface as well as for a hot gas incident on a cold surface. But

involvement of internal degrees of freedom in the activation process would result in two

different activation energies for the two cases. For the CH4/Rh film system, they observed

an activation energy of 20.9 kJ/mol for isothermal gas and surface conditions.[91,92] The

activation energy was found to be much higher ca. 46 kJ/mol when the measurements were

carried out under nonisothermal conditions (i.e., hot gas striking the Rh film held at 273K).

The difference between the two activation energies was found to equal the estimated

desorption energy of molecular methane on the same surface. Based on these results the

authors claimed that the activation of methane on Rh films was dominated by vibrational

excitation of the gas.[93]

For the CH4/Ir(110) system, Seets et al.[84] observed an initial decrease (in the low-

energy region) in methane dissociation probability with increasing Etrans (up to a certain

value of Etrans). This was then followed by an increase in methane reactivity with

increasing Etrans. These results were interpreted in terms of PMM and DDM domination at

low and high Etrans, respectively. The authors found a good correspondence between their

molecular beam results and bulb experiments. Based on their results it was proposed that,

while DDM dominated at high gas temperatures (industrial relevant conditions), PMM

dominated at low gas temperatures. A similar set of studies were also carried out on the

Ir(111) surface.[94] The two surfaces were found to have a similar qualitative behavior.

Methane was found to be more reactive on the Ir(110) surface as compared to Ir(111)

surface. The experimental results indicated that PMM was more important for Ir(111)

surface than for the Ir(110) surface. In a separate study, Weinberg and coworkers

investigated the CH4(CD4)/Ir(111) system employing bulb methods.[95] PMM was

observed to dominate at low pressures (below 1023 Torr), whereas both pathways (PMM

and DDM) were believed to be active when the translational energy was increased by

diluting methane in argon at a total pressure of 1 Torr.

F. Ru Surfaces

Interference of the substrate signal precludes the effective use of Auger electron

spectroscopy to detect fractional monolayers of carbon on the Ru surface.[96] To

circumvent this, Wu and Goodman employed the d (CH) intensity obtained from HREELS

to estimate the sticking probability of methane on Ru(0001).[97] The estimated sticking

coefficients varied from 1026 to 1027 in the temperature range 500–650K. The apparent

activation energy for the methane dissociation on Ru(0001) was estimated to be

35.5 kJ/mol. Molecular beam studies on the same system provided excellent agreement for

the activation energy measurements (37 kJ/mol) and good agreement for the sticking

probabilities.[98]

An exhaustive study was undertaken in our laboratory to investigate the different

surface intermediates formed during methane chemisorption on Ru surfaces. Temperature




programmed desorption (TPD) and HREELS[99] were employed for identifying the

hydrocarbonaceous species formed on Ru(0001) and Ru(1120) surfaces. Methane

dissociation on Ru(1120) resulted in the formation of three different hydrocarbonaceous

species, namely methylidyne (CH), vinylidene (CCH2), and ethylidyne (CCH3) species. In

contrast, methane decomposition on Ru(0001) led to the formation of only CH and CCH2

species. For both substrates, only the graphitic phase was observed above 700K.

Additional information on the graphitic carbon (inactive form) was obtained by utilizing

scanning tunneling microscopy (STM).[100] On the Ru(0001) surface ðTs ¼ 800KÞ; the

carbon was present as small clusters (attached to the step edges of the surface) having a

diameter of 1.0–1.5 nm and apparent heights of 0.2–0.3 nm. The hexagonal super-

structure of graphite monolayers was observed at 1300K on the Ru(0001) surface, showing

a sharp corresponding LEED ð11 £ 11Þ pattern. In contrast, methane decomposition on

Ru(1120) surface produced three-dimensional particles of graphite. This difference in

growth patterns of carbon atoms was attributed to the structural differences between the

two substrates.

In order to confirm that similar surface intermediates are formed during methane

decomposition on supported real-world Ru catalysts, neutron vibrational spectroscopy

(NVS) studies were undertaken.[101] Neutron vibrational spectroscopy has several

advantages over conventional vibrational techniques in that it allows accurate qualitative,

as well as quantitative, analysis of surface hydrocarbonaceous species on supported metal

catalysts at ambient to high pressures. Furthermore, NVS is not limited by selection rules

and therefore in principle allows detection of all vibrational modes of the adsorbed

species. It is noteworthy that these studies showed the presence of similar surface species

(methylidyne, vinylidene, and ethylidyne) on the Ru/Al2O3 catalyst as those observed on

the model Ru catalysts after methane chemisorption. Moreover a similar trend was also

observed for the stability of the various surface intermediates on the Ru surfaces.

Since most of the surface science studies have focussed primarily on understanding

the fundamental methane chemisorption process, an extensive program was undertaken in

our laboratory to take these studies a step further, i.e., to investigate the transformation of

the surface species formed during methane chemisorption and relate them to the

transformation kinetics. These studies were undertaken to provide mechanistic

information about the low-temperature coupling of methane on Ru surfaces and are

described in the next section.

III. LOW TEMPERATURE COUPLING OF METHANE

The nonoxidative low-temperature homologation of methane has been proposed as an

alternative to OCM in recent years. Two research groups independently introduced the

process,[102,103] which involves methane decomposition in the first step followed by

hydrogenation of the surface carbonaceous species in the second step to obtain C2þ

hydrocarbons. Thermodynamics prohibits the direct formation of ethane from methane.

To circumvent this thermodynamic limitation, Van Santen and coworkers operated the

process at two different temperatures.[104] Dissociative adsorption of methane was carried

out on silica-supported transition metal catalysts (Ru, Rh, and Co) at 700K, followed by a

hydrogenation step at a lower temperature (ca. 373K). Temperature-programmed surface




reaction of the carbonaceous species (after the chemisorption step) revealed three different

carbonaceous species on the surface (similar to the work by McCarty and Wise[105]). The

highly active carbidic phase Ca was held primarily responsible for the formation of higher

hydrocarbons. Cg, which gas graphitic in nature, was highly unreactive and had a

proclivity for methane formation at higher temperatures, whereas the Cb (amorphous

carbon) had intermediate reactivity-producing predominantly methane with very small

amounts of C2þ hydrocarbons. The hydrogenation products obeyed the Flory–Schulz–

Anderson distribution and a maximum yield of 13% for C2þ hydrocarbons was obtained at

an optimum surface coverage of 0.18 on the Ru/SiO2 catalyst.

Amariglio and coworkers carried out the two-step process under isothermal

conditions.[103] In the first step, pure methane (flow-rate: 400 ml min21) was passed over

100 mg of the EUROPT-1 catalystat 523K. Ethane was produced along with hydrogen but

was approximately an order of magnitude less than the latter. Hydrogenation (hydrogen

flow: 50 ml min21) of the adsorbed species at 523K in the second step resulted in the

formation of saturated hydrocarbons ranging from C1–C7; 19.3% of the adsorbed methane

was converted to C2þ hydrocarbons. Though the C2þ yield increased to ca. 29% on

carrying out the process at 473K, the relative amount of higher hydrocarbons produced

was lower due to the decrease in the amount of methane adsorbed in the chemisorption

step. Comparison studies of Co, Ru, and Pt catalysts for the homologation of methane

indicated that Ru was the most active catalyst.[106] The temperature at which maximum

hydrocarbon production took place for Ru, Pt, and Co was 433, 523, and 548K with

corresponding C2þ yields of 36.9%, 19.3%, and 7.5%, respectively. For Co and Ru the

product distribution shifted towards higher hydrocarbons with decrease in temperature,

whereas the reverse trend was observed for Pt.[107,108] This temperature effect was

attributed to differences in the hydrogenolysis activities; Pt being far less effective than

Ru. The C2þ yield was significantly improved for the EUROPT-1 catalyst by operating the

process in a batch reactor and employing Pd catalyst as hydrogen trap.[109,110] A 16-min

exposure to methane at 523K followed by hydrogenation at the same temperature gave

C2þ yield of ,42.5% with respect to the consumed methane. The total amount of methane

homologated for a 16-min exposure was 24.6m-moles, yet 9.34m-moles for a 1-min

exposure, indicating a weak influence of the exposure time on the amount of homologated

methane. The thermodynamic limitation was overcome in the isothermal process by

removing hydrogen from the surface in the first step at low pressures (high flow-rates of

methane were used) and then supplying it in the hydrogenation step at 1 bar.[111]

Though the basic idea behind both the dual temperature (DT) and isothermal

temperature (IT) homologation is the same, striking differences are apparent between the

two. A greater variety of C2þ hydrocarbons was observed in the isothermal process.[111]

The maximum yield for hydrocarbons in a DT process on Ru-based catalyst passed

through a maximum with respect to carbon surface coverage.[104] In contrast, the IT

process showed an increase in the amount of homologated products with increasing

surface coverage (methane exposure time).[109] Higher temperatures employed in the first

step of the DT process resulted in higher yields of hydrocarbons compared to the IT

process. The Cg form of carbon, which Van Santen and coworkers referred to, was not

observed in the IT process.[107] Another major difference was that, while van Santen and

coworkers insinuated that CZC bond formation was primarily involved in the

hydrogenation step, Amariglio and coworkers proposed CZC bond formation in




the chemisorption step.[112,113] This controversy involving CZC bond formation was

further brought to light in form of two letters.[114,115] Experiments carried out in vacuum

prompted Hlavathy et al. to propose that CZC bond formation could take place in the

chemisorption step as well as the hydrogenation step. In response, Amariglio et al. referred

to their previous work, which involved flushing of the surface with CO after the

chemisorption step.[112] This had resulted in production of a number of hydrocarbons (up

to C8), including olefins, providing evidence for formation of CZC bond in the first step.

In good agreement surface science studies in our laboratory (as described in the previous

section) have shown direct spectroscopic evidence for the CZC bond formation during the

methane chemisorption step on both model[99] as well as supported real-world Ru

catalysts.[101]

Solymosi and coworkers investigated the two-step process on different metal-support

systems.[116 – 118] Ruthenium was found to be the most active catalyst for methane

decomposition (per metal site) followed by Rh, Ir, Pd, and Pt, whereas the efficacy of C2

hydrocarbon production followed the order Pt . Ru . Rh . Ir . Pd: On Pd supported

catalysts, other than methane, the only hydrocarbons observed were ethane and

propane.[117] Dissolution of hydrogen (formed in the decomposition step) in the Pd

crystallites represented an interesting feature of the supported Pd/CH4 system. The

hydrogen desorption temperature on Pd\SiO2 catalyst was in good agreement to that

observed on Pd (679).[80]

Goodman and coworkers observed an ethane yield of ,15% for an optimized carbon

coverage of 30–35%[119] on Ru/SiO2 for the DT methane homologation process. Studies

related to effect of multiple cycles indicated a large drop in catalyst activity after three

cycles, after which the catalyst had to be regenerated to regain its initial activity.

Experiments undertaken at higher pressures did not result in an appreciable decrease in the

ethane yield. Carstens and Bell investigated the different forms of carbon formed after

methane dissociation on Ru/silica catalyst.[120] The distribution of the carbonaceous

species (Ca, Cb, and Cg) was found to be dependent on the aging time as well as carbon

coverage. Additionally, their studies indicated a higher yield of C2þ hydrocarbons on a

carburized Ru catalyst than on a noncarburized catalyst.

Smith and coworkers investigated the DT methane homologation on supported Co

catalysts.[121 – 123] The amount of methane decomposed per Co site was found to be higher

on Co/Al2O3 than on Co/SiO2, whereas the latter showed higher activity in the

hydrogenation step and greater C2þ selectivity. This was attributed to the more facile

metal to support migration of the carbonaceous species in case of Co/Al2O3. It was

emphasized that high C2þ yields in the second step required the formation of CH species

on the surface in the first step. Interestingly, higher selectivity for C2þ was obtained in the

second step when argon was used instead of hydrogen.

Boskovic et al. investigated the effect of potassium as a promoter for the methane

homologation reaction on cobalt catalysts.[124] Their results indicated that the effect

caused by potassium was strongly dependent on the supports utilized (silica and alumina).

The methane-decomposition activity per site showed a larger increase on alumina-

supported catalysts (as compared to Co/SiO2). For the hydrogenation step, a greater

increase in selectivity for C2þ hydrocarbons was observed on the silica-supported Co

catalyst and only a marginal increase in C2þ selectivity was observed for Co/Al2O3.

However, the addition of potassium was found to decrease the hydrogenation activity,




resulting in a decrease in C2þ yield. Solymosi and Cserenyi observed an increase in ethane

selectivity in the methane chemisorption step on introduction of Cu in the Rh/SiO2

catalyst.[125] Additionally, the selectivity of C4–C6 hydrocarbons was also enhanced in the

hydrogenation step. As opposed to Cu addition, which altered the product distribution,

addition of molybdenum to Rh/Al2O3 induced the removal of coke by hydrogen at lower

temperatures.[126] The presence of Mo was believed to restrict the mobility of the residues,

and thereby the aging.

Methane chemisorption and the subsequent hydrogenation of the carbonaceous

species was investigated on bimetallic systems by Guczi and co-workers.[127 – 129] The

amount of methane chemisorbed was found to be much larger on the Co–Pt/NaY system

as compared to Co/NaY or Pt/NaY systems. This was attributed to the increase in

reducibility of Co ions in NaY due to the presence of the added Pt.[130] Moreover, the Co–

Pt/NaY system exhibited 100% conversion of adsorbed methane (present as adsorbed CHx

species) and a selectivity of 83.6% for C2þ hydrocarbons for the hydrogenation step. The

authors proposed that the enhancement was a result of a synergistic effect of Co–Pt on the

CZC bond formation in the hydrogenation step.

Garnier et al. utilized a Pd–Ag membrane reactor for the two-step methane

homologation on Ru catalyst.[131] As compared to a standard fixed bed reactor, the

temperature required for the same methane conversion (first step) was drastically reduced

for the Pd–Ag reactor. For a 60% methane conversion, the temperature required for the

Pd–Ag reactor was only 553K, whereas it was 673K for a fixed-bed reactor. The use of

low temperature in the first step had a positive effect on the C2þ selectivity in the

hydrogenation step. No deactivation was observed for the six cycles studied

(decomposition at 573K and hydrogenation at 373–393K). Effectively, the use of Pd–

Ag membrane resulted in the decrease of the “temperature gap” referred to by Van Santen

and coworkers for the DT process.

Marceau et al. investigated the hydrogenation step of the methane oligomerization

process on EUROPT-1 catalyst.[132] Based on their results, the authors proposed the

following:

(i) The first few minutes of the hydrogenation reaction resulted mainly in the

production of ethane and pentane.

(ii) Methane chemisorption in the first step led to the formation of cyclic precursors

of pentane.

(iii) CZC bond formation also took place in the hydrogenation step via a “statistical

and dynamical homologation,” which in turn led to the formation of C2–C5

alkanes.

(iv) Hydrogenolysis of the heavier carbonaceous compounds led to the formation

of higher hydrocarbons.

Recently, Bradford investigated the effect of pressure and metal-support interactions

on the methane homologation reaction on Ru- and Pt-based catalysts.[133] Increase in

pressure favored higher hydrocarbon formation in good agreement with previous studies

on Ni–Cu/SiO2 catalysts.[134] The results further indicated that employment of a support

exhibiting mild metal-support interactions could influence the product distribution.




As mentioned earlier, extensive efforts were undertaken in our laboratory to

understand the mechanism for the two-step low-temperature methane-coupling process on

Ru surfaces. To address this objective, the surface intermediates formed on the model

catalyst surfaces were identified by an array of surface science techniques.

Correspondingly, kinetic studies of the process were also undertaken on model and

the real-world catalyst surfaces. Since the studies involving the detection of surface

intermediates have been described in detail in the previous section, here emphasis will be

placed on describing the kinetic studies and relating the rates of the reactions with the

concentration of the surface reaction intermediates.

Kinetics studies were undertaken on Ru(0001) and Ru(1120) as a function of methane

chemisorption temperature and hydrogenation temperature.[135,136] Under the experimen-

tal conditions employed, the maximum yield in ethane/propane was obtained at 500K

(first-step temperature) on both Ru (0001) and Ru (1120) single-crystal catalysts. The

ethane yield obtained as a function of methane chemisorption temperature is shown in

Fig. 2; ethane formation rates passed through a maximum for both surfaces. The

hydrocarbon products, which were similar for the two surfaces, consisted of 16% ethane

and 2% propane under optimum conditions.

The mechanistic information was obtained by relating the stability of the surface

intermediates with the kinetic studies. The important points related to stability of the

surface species after methane chemisorption on the Ru model surfaces are summarized

below[99,100]:

1. Only graphitic species were observed on the surface after methane

chemisorption at $800K.

2. The ethylidyne species were stable only at low temperatures, and transformed to

vinylidene species upon heating.

3. The methylidyne and vinylidene species were stable from ,400–700K.

Since the ethane yield was significant only between ,400–700, the graphitic species

and the ethylidyne species were deemed unimportant for the methane coupling reaction.

Although the methylidyne and vinylidene species were both stable under the relevant

temperature region (corresponding to maximum ethane yield), the latter species were

considered to be the direct precursors for ethane, justified by the fact that the vinylidene

species can be directly hydrogenated to ethane, while methylidyne species require both

polymerization as well as hydrogenation for ethane formation.

Figure 2. Mechanism for the two-step methane homologation on Ru-based catalysts.[136]




The following mechanism can then be proposed (illustrated in Fig. 3). The methane-

chemisorption step involves the formation of surface methylidyne and vinylidene species

along with hydrogen. In the hydrogenation step, the vinylidene species are directly

hydrogenated to ethane while methylidyne intermediates are involved in two competitive

reactions: (i) hydrogenation to methane, and (ii) transformation to vinylidene species. It is

noteworthy that similar ethane yields were obtained for the model Ru catalysts and real-

world Ru/SiO2 catalysts.[119] This, coupled with the fact that same surface intermediates

(along with similar trend in stability of surface intermediates) were observed for both

surfaces, implies that the proposed mechanism for the two-step low-temperature methane-

coupling is applicable to all Ru surfaces.[137] This combined study exemplifies the

effectiveness of surface science studies to probe catalytic reaction mechanisms.

IV. PYROLYSIS AND HIGH-TEMPERATURE COUPLING OF METHANE

The methane molecule is different from other paraffins in that it cannot be further

differentiated into lighter hydrocarbons. Coupling and formation of higher hydrocarbons

requires splitting of a CZH bond[138 – 141] in methane, but its high-dissociation energy

(435 kJ/mol) makes this highly endothermic. For this reason, high temperatures are

required for the direct (one-step) nonoxidative coupling of methane. This field has been

Figure 3. Ethane yield over model (B) Ru(001) and (X) Ru(1120) catalysts as a function of

temperature for the low-temperature methane-coupling reaction.[119]




a focus of large number of studies in the past few years. Here, these studies have been

classified into homogeneous (noncatalytic) and catalytic high-temperature coupling of

methane. The investigations involving catalytic coupling of methane have been further

split into two subsections based on the catalyst system employed, (i) carbonaceous and

oxide catalysts and (ii) zeolite-based catalysts.

A. Methane Pyrolysis and Dehydrogenative Coupling ofMethane in Homogeneous Systems

Thermodynamically, methane is unstable in terms of its elements (carbon and

hydrogen) from 803K, but is more stable than the other hydrocarbons up to 1303K[142]

(Fig. 4). Gueret et al. carried out an exhaustive investigation of the thermodynamics of

methane pyrolysis.[142] Based on these thermodynamic calculations, the authors predicted

the following:

(i) Low methane conversions below 1473K, with benzene as the principal

product, followed by ethylene and traces of acetylene.

(ii) High propensity of the CZC and CZH bonds to rupture above 1473K

(iii) Factors leading to stabilization of the radicals formed during the coupling

reaction would be favorable for the endothermic dissociation reaction.

Figure 4. Gibbs free energy of formation for various hydrocarbons as a function of

temperature.[142]




(iv) The exothermic formation of the double bonds would enhance the dissociation

process.

(v) High pressures were detrimental to the splitting of the CZH bond and resulted

in low methane conversions.

Experimental studies by Holmen et al. revealed that high temperature and short

residence times were favorable for acetylene formation in the thermal coupling of

methane.[143] Acetylene yields greater than 85% could be obtained at temperatures higher

than 2000K and reaction times of ca. 1022 seconds. The maximum attainable yield of

acetylene declined sharply with decreasing temperatures. Auto-acceleration in the

methane decomposition process at low conversion was attributed to heat transfer from the

reactor wall to the gas. The rate of carbon formation escalated with increase in the partial

pressure of acetylene above a critical limit.[156] The inlet limiting pressure was found to

correspond to a maximum outlet partial pressure of acetylene of ca. 20 Torr. A recent study

by Sun et al. involving methane pyrolysis in a hot filament reactor yielded methane

conversion of 19.7% and a hydrocarbon selectivity of 68% at 1548K and residence time of

0.1 seconds.[144]

The overall reaction in the thermal coupling of methane is generally believed to

involve a stepwise dehydrogenation at high temperatures.[145 – 147]

2CH4 ! 2C2H6 þ H2 ! C2H4 þ H2 ! C2H2 þ H2 ! 2C þ H2

Arutyonov et al.[148] observed that at 1200K (static reactor), the concentration of all the

three C2 hydrocarbons (C2H6,C2H4, and C2H2) maximized simultaneously with time and,

hence, argued that although the above sequence accurately described the kinetics in the

low temperature region, it became less apparent at higher temperatures.

To date different experimental techniques such as tubular flow reactors, shock tube

reactors, and plasma reactors have been employed to investigate methane pyrolysis at the

temperatures high enough for the production of acetylene.[149 – 154] Studies undertaken in

shock-tube[155] reactors showed higher overall activation energies as compared to studies

in flow-reactors. This was attributed to the existence of a higher temperature difference

between the walls of the reactor and the bulk gas at high temperatures in case of flow-

reactors.[156] The overall methane decomposition was assigned a first-order reaction[157 –

159] by the majority of research groups in conflict with the observation by Eisenberg and

Bliss.[160]

Kinetic studies of the methane decomposition process (under static conditions) by

Arutyonov et al.[148] pointed to the possibility of obtaining C2 hydrocarbons in amounts

greater then the thermodynamic equilibrium quantities as the concentration of C2

hydrocarbons were found to pass through a maximum in pyrolysis. The concentration of

ethylene in the product mixture was ca. 10 times that of acetylene and ethane at 1100K,

whereas the concentration of acetylene and ethylene was comparable at 1300K. On

diluting methane with hydrogen ðCH4 : H2 ¼ 1 : 3Þ; there was a threefold decrease in the

pyrolysis rate constant.

Billaud et al.[161] investigated the influence of hydrogen dilution, temperature, and

residence time on the conversion and product yields during methane pyrolysis. Hydrogen

dilution had a detrimental effect on methane conversion, but decreased coke formation.




Ethylene and acetylene were obtained as the major products and ethane, propene,

propadiene, propane, butadiene, butenes, and cyclopentadiene as minor gaseous products.

Liquid products such as benzene, toluene, xylenes, and naphthalene were also observed.

The C2H4, C2H2, and C6H6 yields maximized between 7%–8% with residence time,

whereas the coke yield continued to increase. The studies suggested the existence of a

competition between C2 (ethylene and acetylene) formation and carbon deposition. The

formation of heavy products proceeded via acetylene as an intermediate rather than

ethylene at high conversions. The work concluded that reduction in coke formation and an

increase in C2 yields were possible by carrying out the pyrolysis of methane in presence of

hydrogen at high temperatures and low residence times.

The effects of hydrogen[161] (enhanced C2 yields, decreased benzene selectivity, and

suppressed coke formation) can be explained based upon a mechanism proposed by Olsvik

and Billaud[140]:

CH4 þ Hz , CH3zþ H2 ðIÞ

C2H4 þ CH3z , CH4 þ C2H3 ðIIÞ

C2H2 þ C2H3z , C4H5 ðIIIÞ

C2H2 þ C4H5z , C6H6 þ H ðIVÞ

In the elementary step (I), the addition of hydrogen leads to a smaller methane conversion

and to a lower CH3z concentration. As a result, the steps which included CH3z, specifically

(II), become less important, leading to a smaller C2H4 consumption and an increase in its

concentration. Since the concentration of C2H3 decreased, reaction (III) occurs at a slower

rate leading to a decrease in the production of C4H5. This effectively results in lower

production of C6H6 via (IV), thus, eventually leading to an increase in the concentration of

C2H2.

Coke (carbonaceous material containing hydrogen) is one of the undesired by-

products of the pyrolysis reaction.[162] According to one of the mechanisms, coke

formation proceeds via further reactions of aromatics (which are formed by the

trimerization of acetylene).[163] Further details regarding the mechanisms for carbon

formation during the methane decomposition reaction can be found in references.[163,164]

The stringent requirement of high temperatures for methane-coupling reactions places

very strong limitations on the reactor design. Heat recovery and transfer of heat pose a

severe challenge in the thermal-coupling process. Based on the method of heat supply and

removal from the reactor, the processes were grouped into four as follows[147]:

(i) The processes in which the energy source was an electric arc (Huels

process).[165]

(ii) The processes in which part of the methane feed was burnt in the reactor with

oxygen (BASF).[166]

(iii) Regenerative pyrolysis process, which employed a cyclic heating and pyrolysis

(Wulff process).[167]

(iv) Isothermal pyrolysis, which involved the introduction of heat through the

reactor wall by resistive heating.




Detailed information on the complex mechanisms involved in the noncatalytic high-

temperature coupling of methane can be obtained in.[147]

B. Catalytic Pyrolysis and Nonoxidative Coupling of Methane

at High-Temperatures

1. Carbonaceous, Oxide, and Metal Catalysts

Catalyst systems have been investigated to increase the kinetics of the methane

coupling process. Fang and Yeh[168] investigated the catalytic effect of ThO2/SiO2

surfaces on the methane-coupling process. Operating at low conversions ðT ¼

1073K; Pmethane ¼ 15 TorrÞ; the authors observed a ca. 300 times faster product generation

rate than that in homogeneous methane decomposition process. Ethane and ethylene were

the major products formed, whereas propylene and acetylene were obtained in smaller

concentrations. Van Santen and coworkers investigated methane pyrolysis on a range of

catalysts with varied specific surface areas.[169] Their results indicated that the conversion

and product selectivities in the methane pyrolysis process were not catalyst-specific, but

instead were controlled by the specific surface area. Low specific areas resulted in highest

yields for gaseous (ethylene and acetylene) and liquid products (light aromatics), whereas

high surface areas resulted in the production of graphitic coke and hydrogen.

Methane pyrolysis on pitch-based carbon fibers at 1273K by Mochida and

coworkers[170] resulted in steady-state conversions of 20% and high selectivity (upto 67%)

for C2 hydrocarbons with low levels of carbon deposition. Although they admitted that the

reaction mechanism and role of the fiber surface was not very clear, they speculated that

the pyrolysis was initiated by the radical fission of methane either on the surface or in the

gas phase. The resulting methyl radicals first supposedly coupled on the surface into C2

species, leading to formation of higher hydrocarbons and, finally, formed pyrolitic carbon.

Kharatyan et al.[171] investigated the high-temperature kinetics of methane (2–

100 mm Hg, 1673–2473K) on Mo wire. Their results indicated that, with a pressure less

than or equal to 3 mm Hg, pyrolysis of methane obeyed a linear law at all temperatures. At

T , 1973K and P . 3 mm Hg; the process obeyed a linear law in the initial stages,

whereas the process was described by parabolic law after a certain time period. At

pressures equal or greater than 25 mm Hg, the parabolic law applied from the very

beginning. The pyrolysis rate of methane was found to be much higher on molybdenum

carbides than on tungsten carbides.[172] The other notable difference on the tungsten

surface was that, in this case, the methane pyrolysis complied with a linear law over the

whole range of temperatures (1673–2923K) and pressures (2–100 mm Hg) were

investigated.

Potapova et al.[173] carried out the decomposition reaction on indium oxide ðT ¼

973–1073KÞ with methane–ethane mixtures having 50–95 vol% methane. Results

showed that the ethane decomposition rate decreased in the presence of methane whereas

increasing ethane concentration in the mixture enhanced methane conversions. This effect

was attributed to the participation of atomic hydrogen in competitive reactions. The

detrimental effect of methane on ethane decomposition was limited in the presence of the

indium oxide catalyst (as compared to the homogeneous thermal-cracking process).




The authors claimed that the concentration of atomic hydrogen was far greater (3–8 times)

in the catalytic decomposition of ethane–methane mixtures than during the thermal

process, resulting in enhancement of the methane conversion rate and decrease in the

methane inhibition effect upon ethane conversion.

Investigation of methane pyrolysis by Kushch et al.[174] on fullerene black showed

that the methane conversion decreased with time due to pyrolitic carbon formation. Use of

methane diluted in argon instead of pure methane did not result in the inhibition of

pyrolytic carbon formation. The influence of H2 as a diluent (at low hydrogen contents)

was found to be identical to that of argon in the initial part of the reaction, but upon

accumulation of pyrocarbon, a higher selectivity to ethylene and lower selectivity for

propylene was obtained. The dehydrodimerization of methane became the dominant

process with the increase of the H2 content in the initial mixture (above 45%). The

methane conversion was found to be lower than that with a smaller H2 content (or a similar

argon dilution) and the only reaction products were ethylene and propylene. Since there

was no pyrocarbon accumulation, the activity and selectivity remained constant with time.

Kurosaka et al.[175] observed that 3 wt% Pt-added sulfated zirconia gave a steady

conversion of 0.23% at 773K for a period of 5 hr in a flow reactor (0.5 g cat, 10 ml/min

methane). Ethane and ethylene were obtained as products in a ratio of 9:1. Platinum

supported on zirconia without the sulfate was found to sinter, resulting in deactivation. The

authors emphasized that deployment of sulfur acid (as opposed to using ammonium

sulfate) as a sulfating reagent led to the synthesis of superior catalysts.

2. Zeolite-Supported Catalysts

The studies involving zeolite-based catalysts are different from the investigations

discussed previously in that the objective in this case is to convert methane selectively to

aromatic compounds. Wang et al.[176] pioneered the nonoxidative dehydrogenative

methane aromatization reaction on ZSM-5-based catalysts. HZSM-5 catalysts showed a

1.4% conversion of methane with 100% selectivity to benzene at 973K. The conversion of

methane was greatly enhanced (without any detrimental effect to benzene selectivity) by

loading the zeolite with Mo or Zn cations. Mo/HZSM-5 exhibited the best activity: 7.2%

and 4.4% for the SiO2/Al2O3 ratios of 50 and 25, respectively. The catalyst was found to be

stable without any loss in activity and selectivity under the reaction conditions employed

for the study. The authors claimed that the activation of methane proceeded via a

carbenium-ion mechanism with proton sites Zn2þ and Mo6þ acting as the hybrid acceptors

in HZSM-5, Zn/HZSM-5, and Mo/HZSM-5, respectively. Subsequent studies[177]

revealed that the Bronstead acidity, channel structure, and the state and location of Mo

species in the zeolite played a vital role in the performance of the catalyst. Increase in

temperature increased methane conversion and aromatics selectivity but decreased C2

selectivity. Molybdenum-loading studies showed that the methane conversion passed

through a maximum at 2–3% Mo loading. High calcination temperatures and High Mo

loadings (.6%) resulted in the blockage of channels, leading to loss in catalytic activity.

A mechanism involving heterolytic splitting of methane in the presence of Bronsted acid

sites of HZSM-5 zeolite (leading to the formation of a molybdenum-carbene-like complex

CH2 ¼ MoO3 intermediate) was proposed.




Solymosi and coworkers investigated various aspects related to the methane

aromatization process.[178 – 183] Studies pertaining to MoO3 on various support oxides[178]

showed a common trend: initial induction period (varied with the support) followed by a

maximum in conversion, which finally decreased with time. The highest methane

conversion was observed on MoO3/SiO2 (but was less than that on MoO3/ZSM-5[176]) and

the least on MoO3/MgO. The dominant products were H2, CO, H2O, and CO2 in the initial

part of the reaction on MoO3/SiO2. Aromatic formation (benzene and trace amounts of

toluene) started late in the reaction (ca. 45 min) and its selectivity increased with reaction

time. Studies related to the induction period led the authors to believe that the partial

reduction of Mo6þ was required for the formation of hydrocarbons (as opposed to Ref.

[176] which assumed the active site to be Mo6þ species). Mo2C and Mo2C mixed with

HZSM-5 were found to be relatively inactive for the process, whereas Mo2C deposited in a

finely divided state on ZSM-5 (synthesized as in [184]) was found to be a highly active

catalyst for the dehydrogenation and aromatization of methane.[179,181] Molybdenum

carbide was proposed to be the active species responsible for the methane to ethylene (the

precursor for benzene formation) conversion.[181] Subsequently, the same group employed

surface science techniques (AES, XPS, TPD, HREELS) to study the generation and

reaction of CxHy species on the Mo2C/Mo(111) surface[182,183] in order to obtain

mechanistic information.

Lunsford and coworkers[185 – 189] carried out a series of investigations aimed at

determining the nature of the active catalytic sites. It is noteworthy to mention that in these

studies, nitrogen was used an internal standard so as to obtain accurate methane

conversions and to evaluate quantitatively coke formation. Catalyst results obtained at

973K ðGHSV ¼ 800 h21Þ showed an initial activation period, after which a sustained

selectivity of ca. 70% for benzene was obtained for several hours. Unlike benzene, the

selectivity for naphthalene passed though a maximum (ca. 20%) and then declined rapidly

with reaction time. The catalyst was stable (with methane conversions of 8–10%) and

showed a small decrease in catalytic activity over a period of 16 hr. The original activity of

the catalyst could be completely restored by oxidation treatment of the spent catalyst at

973K. Characterization of the catalyst with XPS indicated the gradual reduction of the

Mo6þ species with reaction time, finally leading to formation of Mo2C. Based on their

study of the induction period, the authors proposed that the coke-modified Mo2C surface

was the active species responsible for the formation of ethylene (reaction intermediate)

rather than clean Mo2C.[186] The XPS results indicated the presence of three different

types of surface carbon[188] on the active Mo/HZSM-5 catalyst:

(i) Graphitic carbon that was dominantly present in the zeolite channel system.

(ii) Carbidic carbon (from Mo2C) that was mainly located at the outer surface of

the catalyst.

(iii) Hydrogen-poor, sp-type carbon that was predominantly present on the outer

surface of the zeolite.

The last type of carbon was held responsible for the deactivation of the catalyst.

Subsequent studies on different transition metal ions (TMI)/H ZSM-5[187] indicated the

following trend in activities for methane conversion: Mo . W . Fe . V . Cr: The CO

pre-reduction treatments were found to enhance the catalytic activity. Catalysts obtained




by impregnation method showed a superior performance compared with those synthesized

by a solid-state ion exchange with a TMI salt. This was attributed to the presence of fewer

Bronsted acid sites in case of solid-state ion-exchanged catalysts. XPS studies revealed the

presence of transition metal suboxides[189] on all impregnated catalysts, with the exception

of Mo/HZSM-5; Mo2C was observed in this case. The authors concluded that the active

phase responsible for the reaction was different for different transition metals.

Characterization studes on the Mo/HZSM-5 catalyst related to Mo2C and the induction

period by Howe and coworkers[190] was in good correspondence with the work of

Lunsford and coworkers.[186]

Meriaudeau et al. carried out methane and ethylene aromatization over Mo/HZSM-5

and HZSM-5.[191] Surprisingly, the results indicated that ethylene aromatizion was more

facile on Mo/HZSM-5 (which has lower acid sites) than on HZSM-5. This led the authors

to express concern about ethylene being the key intermediate in methane aromatization

(which is the commonly accepted mechanism).[192] The authors suggested acetylene as

being the intermediate in methane aromatization. It was speculated that if ethylene was

indeed the key intermediate, then its transformation to benzene would not be a simple acid

(zeolitic protons) catalyzed reaction as proposed in the literature.[178,186]

Zhang et al.[193] compared the effect of different molecular sieve supports on the

catalytic performance of Mo-based catalysts and found that the zeolite structure exerted a

a significant influence on the catalytic activity. Zeolites possesing two-dimensional pore

diameters (such as ZSM-5, ZSM-8, and ZSM-11) in the vicinity of the dynamic diameter

of a benzene molecule (0.6 nm) were suggested to be excellent supports for methane

aromatization.

Jiang et al.[194] investigated the induction period of the methane aromatization

reaction on Mo/HZSM-5 employing pulse technique. In parallel to previous studies[178,186]

they observed that the induction period involved the gradual reduction of the Mo6þ

species. The carbonaceous residue resulting from further methane decomposition then

interacted with the partially reduced Mo species to form the active centers for the reaction.

In a second study involving the induction period, Ma et al.[195] concluded that the initial

methane activation was the most crucial step in the methane aromatization reaction.

Liu and Xu[196] utilized pyridine as a proble molecule to characterize the acid sites of

Mo/HZSM-5 and gain insights into the interaction between Mo species and HZSM-5.

Results indicated the presence of Mo species predominantly on OH-groups associated

with silanol groups and bridged hydroxyl group [Si(OH)Al]. Based on the catalytic

evaluation results and FT-IR results, the authors reiterated that Mo/HZSM-5 was a bi-

functional catalyst and claimed that ca. 60% of the original Bronsted acid sites in HZSM-5

were required for optimum performance in the methane dehydro-aromatization reaction.

Catalyst characterization studies by Xu and coworkers[197] indicated the presence of

two types of Mo species in the Mo/HZSM-5 catalyst: (i) polynuclear Mo species viz.

MoO3 species (with pressed octahedral coordination) and MoOx species (with square-

pyramidal coordination), present on the external surface; and (ii) mononuclear Mo species

located in the viscinity of framework aluminum. Elegantly designed NMR spectroscopic

experiments[198] were employed to follow the change in Bronstead protons during the

aromatization reaction. These results clearly illustrated the importance of the Bronsted

acid sites in the methane aromatization reaction. The same group further studied methane

dehydo-armatization on Mo supported on phosphoric and rare-earth containing




penta-sil-type high-silica catalyst (HZRP-1).[199] The Mo/HZRP-1 was found to have

greater activity for the reaction than Mo/HZSM-5 at high Mo-loadings. This was attributed

to the better stability of HZRP1 zeolite framework at high-Mo loadings. NMR studies

were subsequently employed to characterize the Mo/HZRP-1 catalysts.[200] The

phosphorous atoms had a greater propensity to replace framework silicon atoms at

high-Mo loadings and calcination temperatures, which consequently resulted in the

variation of the acidic property of the catalyst. Under similar experimental conditions,

Mo/MCM-22[201] was found to give a greater yield of benzene, but a lower yield of

napthalene as compared to Mo/HZSM-5. Interestingly, the authors found a similarity in

the behavior of the aromatization reaction on both the catalysts.

Ichikawa and coworkers used Ar as an internal standard in order to obtain accurate

quantitative results on conversions and selectivities of the various products obtained

during the dehydro-aromatization of methane.[202] Along with benzene and toluene, higher

aromatics such as naphthalene, methyl-napthalene, phenanthrene, and anthracene and

their derivatives were also detected in trace amounts. The Mo/HZSM-5 catalyst having a

SiO2/Al2O3 ratio of 40 was found to give maximum benzene formation and minimum coke

formation.[202]

Addition of Fe and Co to the Mo/HZSM-5 catalyst resulted in a significant

enhancement in the conversion of methane to benzene and naphthalene.[203] Similarly,

introduction of Cu (by ion-exchange method) into Mo/HZSM-5 improved both the activity

as well benzene selectivity for the methane dehydro-aromatization process.[204] In

contrast, addition of phosphorous and lithium to Mo/HZSM-5 reduced the catalytic acidity

and activity.[205]

Choudhary et al. achieved high methane aromatization activity on H-galloaluminate

ZSM-5 type zeolite by co-feeding methane with higher alkenes and alkanes.[206] A

mechanism involving hydrogen transfer was invoked for describing the methane

activation mechanism. Ichikawa and coworkers included CO and CO2 in the methane feed

and observed an enhanced stability of Mo/HZSM-5 due to decrease in coke

formation.[207,208] Re/HZSM-5 catalyst was found to give comparable methane

conversions and product selectivities (to Mo/HZSM-5) in the methane dehydrogenation

and aromatization reaction. As in case of Mo/HZSM-5, addition of a few percent of

CO/CO2 in the methane feed was found favorable for the stability of Re/H-ZSM-5.[208]

Yuan et al. observed that addition of optimum amounts of oxygen was beneficial for the

stability of the Mo/HZSM-5 for aromatic formation.[209] But the presence of oxygen in

greater than a certain critical value resulted in total oxidation and OCM processes being

dominant.

Recently, Dantsin and Suslick[210] have employed a sonochemical preparation

method for preparation of a Mo2C/HZSM-5 catalyst. The synthesis procedure consisted of

irradiation of molybdenum hexacarbonyl and HZSM-5 with ultrasound at 20 kHz for 3 hr

at 358K in inert atmosphere. The resulting catalysts consituted of ca. 2 nm sized Mo2C

particles, which embellished the external surface of the HZSM-5 support. Methane

aromatization studies on the catalyst gave comparable conversion and selectivity to

benzene as compared to conventionally synthesized catalysts.

It is noteworthy that the studies described in the high-temperature coupling of

methane section provide valuable information for hydrogen production via methane

decomposition (described in the next section). The objective of the coupling reations is to




increase C2þ yields and decrease carbon formation. In complete contrast, in case of the

hydrogen production process, it is essential to eliminate C2þ so as to obtain 100%

selectivity for hydrogen. Therefore, factors which are favorable for the direct methane

coupling process, such as extremely high temperatures, very short contact times, low

surface area catalysts, and catalysts that promote CZC coupling, should be avoided to

increase hydrogen yields. The validity of this is confirmed in the following section. One

hundred percent selectivity for hydrogen ðH2=CH4 ¼ 2Þ is obtained under the operating

conditions, which are chosen such that they are completely unfavorable for the methane-

coupling process.

V. PRODUCTION OF HYDROGEN

Hydrogen, being a clean source of energy, is predicted to be the “fuel of the

future.”[211,212] Hydrogen also plays an important role in many other processes as

illustrated in Fig. 5.[211] The figure clearly indicates that the demand for hydrogen is on the

rise. Among the fossil fuels, methane has the highest H/C ratio and thus is the most

obvious source for hydrogen. Steam reforming of methane represents the current trend for

hydrogen production. Other popular methods of hydrogen production include autothermal

reforming and partial oxidation. However, all these processes involve the formation of a

large amount of CO2 as a by-product. CO2, which is a major greenhouse gas, is of

environmental concern.[213] To circumvent this, CO2-free hydrogen production via

methane decomposition has been suggested.[45,46,214,215] Since only hydrogen and carbon

are formed in the decomposition process, separation of products is not an issue. The other

main advantage is the simplicity of the methane decomposition process as compared to

Figure 5. Applications of hydrogen.[211]




conventional methods. For example, the high- and low-temperature water-gas shift

reactions and CO2 removal step (involved in the conventional methods) are completely

eliminated.

Universal Oil Products developed the Hypro-process for hydrogen production based

on methane decomposition.[216] The process utilized a fluidized-bed reactor-regenerator

with 7% nickel on alumina as the catalyst. The thermal decomposition of methane was

carried out at ,1150K, whereas the regeneration took place at ,1475K. The reactor

effluent consisted of ca. 94% hydrogen ðH2=CH4 ¼ 2Þ; with the rest being mainly

unreacted methane. Noyes at the United Technologies Corporation proposed a process

which involved methane decomposition on nickel deposited on glass fibers.[217] The

process carried out at 1123K resulted in a production of high-density carbon and, thus,

required less frequent removal of carbon.

Steinberg proposed an interesting concept to reduce CO2 emission.[214] According to

the process, hydrogen produced via methane decomposition could be further used to

convert CO2 to methanol, which could then be used as fuel in automotive engines. The

process as he envisaged could decrease the CO2 emission by two-thirds from the

transportation and power generation sectors in use currently. Steinfeld et al. employed

solar power to crack methane for hydrogen production.[218,219] This approach further

decreases CO2 emissions by eliminating the fuel consumption required for the

endothermic methane-decomposition reaction. However, it should be noted that there

are severe technical difficulties involved in the large-scale use of solar reactors for

methane decomposition.

Muradov investigated methane decomposition on supported Ni and Fe catalysts over a

wide range of temperatures.[215] Results indicated that these catalysts required oxidative

regeneration to restore catalytic activity, resulting in the production of carbon-oxides in

the regeneration step. Carbon catalysts were used to circumvent the problem of carbon

removal. Among the different carbon materials studied, activated carbon produced from

coconut shells was found to have the highest initial activity, whereas graphite had the least.

This activity difference was attributed to the structure and size of the carbon crystallites.

The studies involving binary mixtures of methane with other hydrocarbons over inert

supports indicated that acetylene addition enhanced the steady-state methane

decomposition rate. No such effect was observed when propane was used. Carbon

resulting from acetylene decomposition was found to be more active towards methane

decomposition than that produced from methane.

Ishihara et al. achieved a significant increase in yield of hydrogen by using a

membrane reactor.[220] Ni supported on silica was the catalyst of choice and the hydrogen-

permeable membrane was made of a palladium–silver alloy. The permeated hydrogen was

swept by argon gas and increasing the argon flow-rate was found to have a positive effect

on methane conversion. A significant increase in methane decomposition was observed

above 723K, with the overall conversion exceeding equilibrium conversions. Results

indicated that contact time longer than 50 g cat h mol21 and sweep argon gas flow-rates

higher than 200 ml min21 were favorable for the process.

Fuel cells are gaining rapid importance, as they are environmentally benign and have

high efficiency. The stringent requirement of low levels of CO in the hydrogen stream

[ppm levels for the current proton-exchange membrane (PEM) fuel cells] adds substantial

cost to the operation of fuel cells using hydrogen from conventional sources such as steam




reforming, partial oxidation, and auto-thermal reforming of methane.[221] Recently, Zhang

and Amiridis investigated methane cracking over silica-supported catalysts[222] to produce

CO-free hydrogen. A 20% CH4 in He stream (T ¼ 823K and GHSV ¼ 30000 h21) gave

an initial methane conversion of 35%, which decreased with time until the catalyst was

completely deactivated after ca. 3 hr. Hydrogen (stoichiometric: H2=CH4 ¼ 2), along with

filamentous carbon were observed as the products. Regeneration with steam resulted in

complete restoration of the catalytic activity. A total of 3.4 moles of hydrogen were

produced per mole of methane consumed (decomposition þ regeneration). The TEM

results indicated that the external graphitic skin of the filamentous carbon was resistant

towards steam regeneration, whereas the inner less-graphitic carbon was easily removed in

the regeneration step. The external graphitic surface was roughly estimated to about 30%

of the total carbon deposited.

It has been recently noted that catalytic decomposition of methane may lead to CO

formation via reaction of the carbonaceous residue with the oxygen of the oxidic

support.[223] It is extremely important to monitor CO levels in the product stream as the

state-of-the-art PEM cells have very stringent CO-tolerance levels (ppm level). To address

this issue, the low levels of CO (in the hydrogen stream from methane decomposition)

were quantitatively estimated in our laboratory by methanation of CO and subsequent

analysis by flame ionization detection.[224 – 226] The step-wise methane steam-reforming

process for the production of CO-free hydrogen[224,227] involves the catalytic

decomposition of methane in step I to produce CO-free hydrogen and surface carbon

and/or hydrocarbonaceous species, followed by a separate step in which the surface

hydrocarbonaceous species is removed via reaction with water (step II), regenerating the

catalyst with respect to step II. Figure 6 shows the amount of methane reacted in step I and

the amount of carbon removed in step II as a function of cycle number (for a catalyst

loading of 200 mg at 648K). The amount of methane reacted is denoted by data points

below the abscissa, whereas the gas-phase carbon containing products obtained in step II

are shown above the abscissa. On an average, 93% of the carbon deposited in step I was

removed in steam gasification step and the catalytic activity was sustained throughout the

16 cycles studied. The average molar amount of hydrogen produced per mole of methane

consumed in step I was 1.1 (remaining hydrogen was present on the surface as

hydrocarbonaceous species) and contained less than 20 ppm CO. The total molar amount

of hydrogen produced per mole of methane consumed in Steps I and II was ca. 3.0. The

overall process was found to run optimally between 648–673K and a surface coverage of

0.1–0.2 monolayer equivalents.

While the above experiments were carried in a pulse mode at low temperatures,

Amiridis and coworkers investigated the same process at higher temperature in a

continuous mode.[228] Methane was cracked on 15% Ni/SiO2 catalyst at 973K for 3 hr, and

the catalyst subsequently regenerated with steam. This process was successfully repeated

for 10 cycles. X-ray diffraction analysis, after successive cracking–regeneration cycles

showed no change in the structure of nickel particles. Choudhary and coworkers[229]

carried out the methane decomposition step and steam gasification in two parallel reactors

by switching between the feed gas (methane containing stream) and steam at known time

intervals. Operating in a cyclic manner the authors observed no increase in pressure drop

(which may occur due to excess carbon deposition on the catalyst surface). The feed

switch-time was found to play an important role in determining the performance of a given




catalyst for the cyclic process. Both Ni/ZrO2 and Ni/Ce(72)NaY were found to be

potentially interesting catalysts for the step-wise steam reforming process.

Recent investigation by Choudhary et al. emphasized the dependence of the type of

carbon formed and amount of CO evolved on the nature of the support.[225] The XPS

studies revealed the presence of two forms of carbon (graphite and carbidic) at low

temperatures (723K), whereas only the graphite form was observed at higher temperatures

(873K). The rate of CO formation was observed to be lowest on Ni/SiO2 and the highest on

Ni/HZSM-5 (shown in Fig. 6). All the catalysts investigated showed a common trend for

the rate of CO formation, i.e., high initial rates followed by much lower stabilized rates.

The CO content in the hydrogen stream was ca. 50 ppm, 100 ppm, and 250 ppm for

Ni/SiO2,NiSiO2/Al2O3, and Ni/HY, respectively. For all the catalysts, stoichiometric

amounts of hydrogen ðH2=CH4 ¼ 2Þ were produced.

It is noteworthy that the production of hydrogen from methane decomposition

provides a dual advantage in the sense that, along with hydrogen, the side products formed

such as pyrolytic carbon, carbon black, and carbon filaments also have applications. The

special physical and mechanical properties of pyrolytic carbon make it suitable for

applications such as heat shields,[230] cladding material,[231] and bio-medical devices.[232]

Growth of pyrolytic carbon by methane decomposition has been studied using hot-wall

technique,[233,234] rotating furnace,[235] and tumbling bed.[236] Plasma techniques[237] and

thermal decomposition of methane in a regenerative gas heater[238,239] can be employed

Figure 6. Reaction cycles on 88% Ni/Zirconia ðcatalyst loading ¼ 200 mgÞ at 648K.[224]




for formation of carbon black and hydrogen. Carbon black finds applications in the rubber

industry. Amongst the various side-products formed during hydrogen generation from

methane cracking, carbon filaments are considered to be the most interesting. The issues

related to carbon filaments are addressed in the next section.

VI. FORMATION OF CARBON FILAMENTS

Carbon formed from the interaction of hydrocarbons with metals has been extensively

investigated due to its significance in various catalysis reactions.[240 – 244] Particularly,

interesting is the formation of carbon filaments on supported metal catalysts. Figure 7

shows a single filament of carbon. It is noteworthy that the formation of these carbon

filaments (CF) are studied for two entirely different reasons. In steam-reforming of

methane, formation of such filamentous carbon can eventually cause destruction of the

catalyst.[245] It is therefore of interest to study the mechanism and various other aspects of

CF formation to restrict the formation of this type of carbon. On the other hand, these

materials (CF) possess a variety of unique properties with prospective applications such as

catalyst supports, reinforcement material, selective adsorption agents, and in energy

storage devices.[246 – 252] Therefore, in the last few years a great deal of effort have been

directed towards optimization of the process condition for CF formation and to tune the

properties of the CF for desired applications.

Baker et al. proposed an influential model for the growth of CF.[253] According to this

basic model, the first step involves the adsorption and decomposition of hydrocarbons on

certain faces of the metal particle. The second step involves dissolution of some of the

carbon species into the bulk and diffusion through the metal particle from the hotter

Figure 7. TEM image of single filament of carbon.[225]




leading faces (as a result of exothermic hydrocarbon decomposition) to the cooler rear

faces, where carbon is precipitated from the solution to form CF. Further growth is

inhibited when the hydrocarbon decomposition is prevented due to encapsulation of the

leading face of the metal particle by a carbon layer. The above model and others as well

advocate the presence of a temperature gradient as the driving force for the bulk diffusion

of carbon through the metal particle.[253,254]

However, the temperature gradient-driven diffusion model encounters difficulty in

giving a completely satisfactory explanation for endothermic surface reactions, such as

methane decomposition, which have led to the postulate that a concentration gradient is

the driving force for the diffusion.[255 – 260] Rostrup-Nielsen and Trimm proposed the

presence of a concentration gradient[255] due to the higher solubility of carbon in metal in a

metal-gas system as compared to that of a metal carbon system. Studies by Sacco,

et al.,[256] and Kock et al.[257,261] suggested that metal carbides were necessary

intermediates for CF growth. Based on the magnetization measurements and temperature-

programmed reduction experiments, Kock et.al.[257] proposed that the driving force for

filament growth was a gradient in carbon content of “nonstoichiometric carbides.”

Drawing from the work by Schouten et al.[61,62] involving methane decomposition on

single crystals, Alstrup modified the “carbide intermediates” model, suggesting the

presence of a surface carbide (and not bulk carbides) during the steady-state growth of

carbon filaments.[258] Lund and coworkers, on the other hand, disputed the necessity for

the assumption of the surface carbide species.[262] Recently, Snoeck et al.[260] have

presented a thermodynamic basis for the difference in solubility at the metal–gas and

metal–carbon interface. The gas-phase carbon solubility was found to equal the solubility

of CF in nickel at the coking threshold.

As mentioned earlier, CF formation in processes such as steam reforming can lead to

catalyst destruction and clogging of the reactor. Based on controlled-atmosphere electron-

microscopy experiments, Baker and coworkers proposed a mechanism for coke formation

in steam-cracker tubes.[246] According to this mechanism, CF formed on Ni or Fe–Ni

particles, on account of having large surface area, provide excellent collection centers for

amorphous carbon. Inhibition of these CF would thus result in diminishing the problem of

accumulation of amorphous carbon and, hence, the coking problem. With this objective,

Baker and coworkers embarked on a series of investigations dealing with various aspects

of CF formation.[263] The effect of a number of oxide additives on the yield of CF formed

from the interaction of acetylene with Ni–Fe surfaces was investigated. The studies

indicated that while MoO3, WO3, and Ta2O5 hindered CF formation by only reducing the

carbon solubility in the catalyst particles, SiO2 reduced both carbon solubility as well as

diffusion through the catalyst particle.

Rostrup-Nielsen investigated the effect of added sulfur on carbon formation from

methane decomposition as well as methane steam-reforming. It was found that the added

sulfur had a larger retarding effect on the rate of carbon formation than on the steam-

reforming reaction.[264] The explanation provided for the above was that the nucleation of

carbon required a larger number of active catalyst sites compared to the steam-reforming

reaction. Above a sulfur surface coverage of 0.7, filamentous carbon was no longer

observed and instead amorphous carbon having a plate-like shape or octopus carbon

(having several amorphous-like carbon threads attached to a single metal particle) was

observed. Bernardo et al. extended this study to the silica-supported Ni–Cu alloy




systems.[265,266] The investigation indicated that low loadings of Cu resulted in the

formation of primarily filamentous carbon, whereas octopus carbon was predominantly

formed at 80 at% Cu content.

Carbon filaments have various features such as high surface area, unique adsorption

properties, and interesting mechanical/electrical properties, all of which manifest their

potential in various applications. The abundance of methane makes it a prime feedstock

for growth of filamentous carbon. Several investigations have been undertaken with

regard to CF growth and properties optimization. Tibbets and coworkers studied

methane decomposition on nanometer-sized Fe particles for CF formation.[259] It was

found that deposition of ca. 2.8 nm layer of carbon resulted in the inhibition of CF

growth. Bennissad et al.[267,268] investigated CF formation on Fe-based catalysts using

CH4–H2 mixtures at temperatures to 1423K. Under these condition, thicker fibers (ca

1m) were obtained, but when heating was stopped at 1323K, the normal structure of CF

was observed.

Baker and coworkers investigated CF formation from acetylene, ethylene, and

methane decomposition on Ni and Ni–Cu catalysts.[269,270] The investigation revealed

that the nature of the catalyst particles strongly affected the structural characteristics of

CF. While particles rich in Ni resulted in the formation of smooth filaments, Cu-rich alloy

particles gave rise to filaments having a spiral conformation. The filament size (25–

100 nm) was found to be strongly dependent on particle size of the catalyst. Rodriguez,[247]

in a review on CF, stressed that a “supported-metal particle arrangement” was favorable

for tuning the CF size within a desired range.

Fenelonov et al. investigated CF formation on various Ni-based catalysts.[271] The

texture of the filamentous carbon resulting from methane decomposition was found to

weakly depend on the texture and composition of the catalysts employed. The adsorption

properties of CF as measured by the authors were a clear indication of the potential use of

these materials as adsorbents. Shaikhutdinov and coworkers carried out an extensive study

on CF formation from methane on coprecipitated Ni–alumina and Ni–Cu–alumina

catalysts.[272 – 274] The XRD analysis indicated that, unlike the Ni–alumina catalyst, the

Ni–Cu–alumina catalyst did not exist in a spinel form.[272] Surface characterization using

AES indicated that introduction of Cu resulted in greater dispersion of alumina over the

surface. The amount of CF formed per gram of the catalyst (Gc) increased with increasing

Ni content in the Ni–alumina catalyst, but was dramatically small for pure Ni powder.[273]

The maximum Gc ¼ 240 gCF=gcat; was obtained for a 3% Cu–87% Ni catalyst. Addition

of Cu decreased the rate of CF formation but greatly increased the life-time of the catalyst.

Also, for samples with more than 9 wt% Cu, some octopus carbon was observed in good

agreement with studies by Rostrup-Nielsen.[266] Based on their study, the authors divided

the CF formation process into three stages. The first step, which was the induction period,

involved the dissolution of carbon into the Ni particles leading to the formation of pear-

shaped particles in the Ni–alumina catalysts and quasi-octahedral shaped ones in case of

the Ni–Cu–alumina catalysts. The second stage involved the lengthening of the

filamentous carbon. Finally, the last stage was catalyst deactivation, which was attributed

to fragmentation or encapsulation of the catalyst particle by carbon. The morphology and

surface structure of CF (F1), produced on Ni–alumina catalysts, and carbon (F2),

produced in case of Ni–Cu–alumina catalysts, were studied by STM and high-resolution

transmission electron microscopy (HRTEM).[274] The carbon surface of the filaments was




found to be rough and was proposed to be formed by misoriented edge planes of graphite

crystallites. In case of the F1, the basal graphite planes lay inclined to the fiber axis,

whereas the basal planes were perpendicular to the filament axis for F2. The HRTEM

micrographs indicated a closed-layer structure on the edges for F2, which was contrary to

the open structure observed for graphite crystallites.

Chen et al.[275] found that the decomposition of CH4/CO on Ni–MgO catalysts

resulted in the formation of small- and uniform-diameter carbon nano-tubes (outer

diameter: 15–20). Experimental conditions such as feed gas-employed, space velocity,

and reaction temperature were found to influence the rate-determining step of the process.

In a subsequent study, Holmen and coworkers[276] observed that the rate determining step

of the filamentous carbon growth on Ni/a-alumina was dependent on the carbon

deposition. Also, the induction and autoacceleration periods were influenced by the

presence of hydrogen and promoters such as Mg and Ca.

Kuvshinov and coworkers observed that the variation of the CH4:H2 feed ratio

could result in modification of the CF texture.[277] Their work also suggested that the

surface area of carbon growth centers was an important parameter for determining the

maximum CF yield on Ni catalysts using pure methane. Li et al employed a Ni–

alumina catalyst prepared from Feitknecht compound for producing CF from

methane.[278] The total amount of CF formed was found to increase with increasing Ni

content of the catalyst. Though the rate of CF formation was higher in this work

(compared to Shaikhutdinov and coworkers[274]) the total amount of CF formed in the

latter case was greater.

Contrary to previous studies,[255,257,270,274] no induction period was observed in this

work. The total amount of CF formed was dependent on the reduction temperature as well

as the reaction temperature. Although the rate of CF formation increased at higher

temperature, there was a decrease in the total yield of CF due to rapid deactivation of the

catalyst. The study was extended to study the doping effects of Cu on the CF

formation.[279] In this work, addition of small amounts of Cu not only increased the total

amount of CF formed, but also increased the growth rate at 873K. This was unlike the

work by Shaikhutdinov et al.,[274] where addition of small amount of Cu had decreased the

growth rate but increased the overall CF yield by greatly increasing the lifetime of the

catalyst. As previously observed,[274] addition of larger amounts of Cu had a detrimental

effect on the performance of the catalyst. Interestingly, it was found that the high-Cu-

content catalyst was the most effective catalyst for CF formation at higher temperature

when methane was co-fed with hydrogen.

Recent studies by Shaikhutdinov et al.[280] indicated that CF produced on the Cu–

Ni–alumina catalyst was an excellent support material for Ni in the methane

decomposition reaction. Comparison of this work with their previous studies[274] led

the authors to believe that the initial growth mechanism for CF formation was

independent of the support. However, the texture and composition of the support was

found to have a profound effect on the stability of the catalyst for accumulation of

carbon. X-ray diffraction studies[281] on high-loaded Ni–silica showed that the nickel

particles seemed to “self-organize” to the optimum size (30–40 nm) during the course

of the methane decomposition reaction, i.e., smaller particles underwent sintering to

form larger particles, whereas larger particles were found to undergo dispersion. The

authors proposed that deactivation occurred when the distance separating the metal




particles (present on filament ends) increased to an extent such that reversible merging

and dispersion of the Ni particles was prevented.

Carbon filaments formation on Co–alumina catalysts was found to be maximized at

60–75% content of Co.[282] No induction period was observed for the Co–

alumina catalysts, which was contrary to their previous experience involving Ni–alumina

catalysts.[273] Also in this case, a different variety of filaments (not observed on Ni-based

catalysts) with hallow-like core morphology were observed. Recently, Ermakova and

coworkers[283,284] achieved a very large yield of CF from methane decomposition at 823K.

The Ni-based catalyst employed for the process was synthesized by impregnation of nickel

oxide with a solution of the precursor of a textural promoter (silica, alumina, titanium

dioxide, zirconium oxide, and magnesia). The 90% Ni–10% silica catalyst was found to

be the most effective catalyst with a total CF yield of 375 gCF/gcat. The maximum yield of

CF was found to strongly depend on the Ni particle size. The optimum particle size

(10–40 nm) was obtained by varying the calcination temperature of NiO.

Recent studies by our group addressing methane decomposition on Ni-based catalysts

revealed a dependence of the type of carbon formed on the support employed.[225]

Filamentous carbon was observed on Ni/SiO2, Ni/HY, and Ni/SiO2/Al2O3 over the entire

temperature range studied (723–973K) whereas filamentous carbon was observed on

Ni/HZSM-5 at lower temperatures (723K), but not at high temperatures (.923K). At

higher temperatures an encapsulating type of carbon was observed which resulted in rapid

deactivation of the catalyst.

VII. SYNTHESIS OF DIAMOND FILMS

In this section, the formation of diamond-like (DCL) films from methane mixtures

will be briefly addressed. The interest in these films stem from their unique properties;

good thermal conductivity, extreme hardness, chemical intertness, low friction coefficient,

and optical transparency. These properties can be exploited in applications such as optical

coatings, wear-resistant coatings, heat sinks for high-power devices and microelectronic

devices.

Diamonds occurring naturally or those prepared at high pressures are in the form of

powder or grains, but the material in this form precludes the exploitation of its properties

in practical applications. Working at low pressures, Derjaguin et al.[285] Aisenberg and

Chabot,[286] Weissmantel et al.,[287] and Angus et al.[288] pioneered the synthesis of

diamond in the form of thin films. The DLC films are generally deposited by the chemical-

vapor deposition (CVD) methods, including hot-filament CVD[289 – 291] electron-assisted

CVD,[292,293] direct current,[294,295] radio frequency[296 – 299] microwave-plasma

CVD,[300 – 302] laser-induced CVD,[303] and combustion methods.[304]

Methane is one of the most commonly used carbon sources for growth of diamond-

like films. Work in this area has been primarily directed towards obtaining good-quality

DLC films at fast deposition rates and understanding the various mechanisms involved in

the film synthesis.[305 – 310] The general (simplistic form) mechanism involves the

dissociation of molecular hydrogen into H atoms, which then assists the reactions between

diluted gaseous hydrocarbons. The hydrocarbonaceous radicals (transient active species)

formed then subsequently transform to diamond. The concentration of methane in




the starting mixture is found to strongly affect the quality of the synthesized

film.[306,311,312] Since large initial-methane concentration results in increasing the rate

of DLC film synthesis, efforts have been directed towards obtaining good-quality DLC

films from high-concentration mixtures of methane.[306,313] Based on isotopic labeling and

high gas flow-rate studies, methane was found to be distinctly more efficient than

acetylene for DLC film growth.[314] A large number of techniques have been used to

evaluate the quality of the DLC films viz. Raman spectroscopy,[315 – 317] x-ray

diffraction,[316,318,319] scanning electron microscopy,[305,320,321] EELS,[322] XPS,[320] and

TEM.[322] Detailed information pertaining to diamond-like films may be obtained from a

number of excellent reviews found in the literature.[323 – 326]

VIII. CONCLUDING REMARKS

The scientific community has put enormous efforts into the study of methane

activation. Fundamental surface science studies related to methane on transition metal

surfaces have been effectively employed to address the problem. These studies have

provided invaluable information relevant to promoter/poison effects, reaction

mechanisms, structure sensitivity/insensitivity for reactions, etc., in heterogeneous

catalysis. High-temperature coupling of methane to produce C2þ hydrocarbons has

been studied extensively from the early part of the twentieth century and the process is

now well understood. Unfortunately, the stringent conditions of extremely high

temperatures and low yields has resulted in limiting the widespread use of this

technology. The low-temperature two-step methane coupling process represents an

interesting approach, but suffers from disadvantages such as low yields and catalyst

deactivation. The dehydroaromatization is scientifically very interesting; however, the

process is severely limited by thermodynamics. Further work is required before any of

the above techniques become commercially viable. Methane decomposition to produce

hydrogen for fuel-cell devices seems to be an attractive method for natural gas

utilization. The by-products of the decomposition process such as carbon nanotubes,

pyrolytic carbon, and carbon black also have applications. Currently considerable

efforts are being directed toward exploring this possibility in our laboratory and

elsewhere. Filamentous carbon formation has been extensively studied to meet two

different objectives: first to eliminate or drastically reduce the formation of carbon

filaments on the catalysts in the steam reforming of methane, and second, to enhance

the rate of filamentous carbon formation. The unique properties exhibited by these

nano-structures can be exploited in a wide variety of applications. Continued research

efforts have been also made towards producing better-quality diamond films at

acceptable deposition rates.

Along with the aforementioned processes, various other routes of methane conversion

(steam, oxygen, etc.) are being explored (these have not been reviewed in this work).

Though no technology currently represents a panacea for the methane conversion

problem, the relentless efforts of the scientific community should hopefully soon lead to

the successful utilization of methane more economically and energy efficiently, while

minimizing environmental pollution.




ACKNOWLEDGMENTS

We would like to thank all the researchers whose contribution has been used in this

review. We would like to acknowledge the support of Department of Energy, Office of

Basic Sciences, and the Robert Welch Foundation. TVC acknowledges V.R. Choudhary

for helpful discussions and the Link Foundation for the Link Energy Fellowship.

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