Journal of Natural Gas Chemistry 13(2004)191–203
Advances in the Partial Oxidation of Methane to Synthesis Gas
Quanli Zhu1,2∗, Xutao Zhao1, Youquan Deng2
1. Petrochemical Research Institute of Lanzhou Petrochemical Company, China National Petroleum Corporation,
Lanzhou 730060, China; 2. State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
[Manuscript received November 10, 2004; revised November 26, 2004]
Abstract: The conversion and utilization of natural gas is of significant meaning to the national economy,even to the everyday life of people. However, it has not become a popular industrial process as expecteddue to the technical obstacles. In the past decades, much investigation into the conversion of methane,predominant component of natural gas, has been carried out. Among the possible routes of methaneconversion, the partial oxidation of methane to synthesis gas is considered as an effective and economicallyfeasible one. In this article, a brief review of recent studies on the mechanism of the partial oxidation of
methane to synthesis gas together with catalyst development is wherein presented.
Key words: methane partial oxidation, synthesis gas, catalyst, reaction mechanism
1. Introduction
Natural gas, which is mainly composed of
methane, is an abundant resource found over the
world and is predicated to outlast the oil reserves
by a significant margin [1]. Most of these reserves,
however, are situated in the areas far away from the
markets of highest energy consumption, and the ex-
pensive cost of compression, transportation and stor-
age, makes the utilization of natural gas as an unpre-
possessing proposition. Contrary to it, petroleum is
relatively cheap and it can be conveniently disposed.
In order to make the utilization of natural gas more
economically viable, a large amount of investigation
into the conversion of methane to liquids or higher hy-
drocarbons has been carried out in the past decades.
Unfortunately, the productive rate in these processes
is still lower than what is expected, because these
products resulted from methane partial oxidation are
usually more chemically active than methane, which
limits methane converting to the expected products.
For example, in the process of direct oxidative cou-
pling of methane to ethylene, the highest productive
rate was no more than 30% [2,3], and in the pro-
cess of direct oxidation of methane to methanol [4] or
formaldehyde [5], the highest productive rate was 8%
and 4%, respectively. It was recently reported that a
50% of methanol productive rate was achieved by a
pilot plant in homogeneous catalysis, but still lower
than what is expected [6,7]. On the other hand, the
mercury and concentrated sulfuric acid was used in
this process, and the resulted sulfuric dioxide should
to be re-oxidized for recycle. Although there are in-
dustrial processes of the direct conversion of methane,
such as the oxidation of methane and ammonium or
amine to cyanide [8] and the pyrolysis of methane to
acetylene [9], their marked disadvantage is that these
processes are needed to operate at very high tempera-
ture, usually above 1300 K. Because of these reasons,
it is very difficult for natural gas to compete with
petroleum at present.
In order to elevate the additional value of
methane, the utilization of methane can be theoret-
ically carried out via two pathways: one is the di-
rect conversion of methane, such as, as mentioned
above, the oxidative coupling of methane to ethy-
∗ Corresponding author. E-mail: [email protected]
192 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004
lene, the direct oxidation of methane to methanol
or formaldehyde, etc. It is impossible for these pro-
cesses to be applied to mass production unless great
breakthroughs of these technologies are achieved. The
other is the indirect conversion of methane, namely,
converting methane to any other products via syn-
thesis gas, which is the mainly practical route for the
methane conversion at the present time.
Nowadays, there are three methods for produc-
ing synthesis gas from methane: the steam reform-
ing, the dry reforming and the partial oxidation, of
methane. Compared with the former two, the partial
oxidation of methane (POM) possesses characteristics
as follows: (1) POM reaction is a mild exothermic
reaction, while the former two are endothermic reac-
tions. Thus, the industrial process based upon POM
is energy saving. In view of this, the utilization of
POM combined with steam reforming or dry reform-
ing is more effective. (2) The molar ratio of H2 to
CO in the resulted synthesis gas is close to 2 if POM
reaction is carried out according to stoichiometric ra-
tio. This kind of synthesis gas containing little CO2 is
an ideal feedstock for downstream processes, such as
methanol synthesis, etc. (3) POM can be carried out
under the condition of very high gas hourly space ve-
locity (GHSV), which makes the process require less
investment and less production scale to achieve the
same or larger capacity.
However, carbon depositing over catalyst bed was
unavoidable even if POM was carried out precisely
under the condition of 2:1 of the molar ratio of CH4
to O2, or less than 2:1, and the coke formation was
even worse at higher temperature. POM, thus, did
not get proper attention until the oil crisis. Since the
eighties’ of last century, Green and co-workers [10, 11]
have done much work for the renaissance of study on
POM. They used noble metals as POM catalysts, and
obtained the synthesis gas with compositions close
to thermodynamic equilibrium. POM is, henceforth,
paid much attention in the catalytic cycle around the
world.
2. Brief thermodynamic analysis of methane
partial oxidation
Possible pathways via which methane is converted
are shown in Figure 1. At high temperature, the main
reaction products between methane and oxygen are,
however, limited to CO, CO2, H2O and H2, [12,13],
apart from some intermediates. Product distribution
depends to a great extent upon the employed catalyst,
temperature, pressure and the ratio of methane to
oxygen in feedstock, as well as kinetic factors. Three
reactions possible to occur during POM are briefly
expressed in Figure 2, wherein some thermodynamic
information is included.
Figure 1. Possible route for methane conversion
Figure 2. Thermodynamic representation of POM
The calculated product gas distribution at the
thermodynamic equilibrium under the condition of
atmospheric pressure and input methane-to-oxygen
ratio of 2:1 versus temperature, based the reactions
in Figure 2, is shown in Figure 3 [14], regardless of
carbon deposition [14]. It can be observed in Figure
3 that the selectivity to CO and H2 increases with
increasing reaction temperature. In fact, very high
methane conversion (>90%) and selectivity (>90%)
to synthesis gas can be obtained above 1000 K. Like-
wise, the equilibrium gas compositions versus pressure
were done by Lunsford and co-workers [15]. Their
results indicated that the partial pressure of CH4,
CO2 or H2O in the equilibrium gas compositions in-
creased with elevated total pressure, which means
that high pressure is unfavorable to POM to synthesis
gas. However, elevated temperature can compensate
this pressure effect. In other words, from the point of
view of thermodynamics it is feasible for the reaction
Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 193
of methane and oxygen to synthesis gas at increased
pressure to be commercially used.
Figure 3. Equilibrium gas compositions for
methane partial oxidation, 1 bar, 2:1
of CH4:O2
(1) p(H2), (2) p(CH4), (3) p(CO), (4) p(CO2), (5) p(H2O)
It is generally accepted that POM could be cat-
alyzed to, or almost, to the thermodynamic equilib-
rium by group VIII metal catalysts [16,17]. How-
ever, Choudhary and his co-workers [18–20] obtained
the yields of synthesis gas much higher than that
calculated according to the thermodynamic equilib-
rium, over the metal oxide-supported nickel or cobalt,
within the temperature range from 723 to 773 K,
under very short residence time. They also investi-
gated the steam reforming and the dry reforming un-
der the same condition, and it was found that the
result was higher than that predicted by thermody-
namics. Based upon these data, they regarded that
POM reaction occurred under non-equilibrium con-
dition, and the mechanism was different from that
under equilibrium condition [10,11]. Green and co-
workers [14] studied the phenomenon described by
Choudhary et al, and it was found that the temper-
ature of catalyst bed increased with gas flow rate.
Therefore, this result was consistent with the pred-
ication by thermodynamics if the real reaction tem-
perature was taken into account.
3. Catalysts for POM to Synthesis Gas
POM reaction to synthesis gas can be carried out
without employment of a catalyst, but it occurs at
very high temperature, usually above 1400 K in the
flame [21]. The employment of catalyst can facilitate
the light-off of POM and promote it to thermody-
namic equilibrium. The catalysts for POM to synthe-
sis gas can be divided into three groups: Ni, Co and
Fe, noble metal and early transition metal carbide.
3.1. Ni, Co and Fe catalysts
The earliest work on the catalytic partial oxida-
tion of methane to synthesis gas was performed by
Liander [22], Padovani and Franchetti [23] and Pret-
tre et al. [24], who obtained high yields of synthe-
sis gas with ca. 2:1 of H2/CO molar ratio, within
the temperature range from 1000 to 1200 K, at at-
mospheric pressure. They proposed that a sequence
of reactions including total oxidation and reforming
reactions of methane was taking place over nickel cat-
alyst. The calculated equilibrium gas compositions
based upon those reactions shown in Figure 2 gave a
good agreement with the observed exit gas composi-
tions, which implied that the thermodynamic equilib-
rium was established in all cases, if carbon deposition
was ignored.
Vermeiren et al. [25] reported the results of ox-
idizing methane with air over nickel catalyst, and
drew similar conclusions to Prettre’s [24]. They com-
pared the POM activity with methane steam reform-
ing over the similar nickel catalyst and found that
POM was 13 times faster than the latter. Thus, they
presumed that there were extra reaction pathways,
which greatly accelerated the methane converting in
methane/oxygen mixture.
Lunsford and co-workers [15] studied in detail
the alumina-supported nickel catalyst bed exposed to
POM atmosphere using XRD and XPS techniques,
and found that three zones were formed in catalyst
bed. The outer zone was made up of NiAl2O4 phases,
which was moderately active for total oxidation of
methane. The mid zone included NiO and Al2O3 par-
ticles, which was thought to complete the total oxi-
dation of methane. The inner zone contained metal-
lic nickel particles, it was suggested that at this zone
the reforming reaction was catalyzed to thermody-
namic equilibrium. XPS studies indicated that only
products of CO2 and H2O were formed, without for-
mation of carbon deposition over the catalyst sur-
face below 973 K. At 1023 K, the surface carbon de-
position increased to monolayer companied with the
194 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004
higher methane conversion. Their results also indi-
cated that the amount of surface carbon deposition
was affected by the ratio of methane to oxygen in
feedstock, a higher ratio resulting in more carbon de-
position, and vice versa.Nickel is active component for POM, but the
nickel species with different oxidative state plays a
different role in surface reaction steps, as mentioned
above. In general, metallic nickel is beneficial to
the production of synthesis gas, while nickel species
with oxidative number ≥2 trends to catalyze the to-
tal combustion of methane. The distribution of sur-
face nickel species with different oxidative number
depends upon the support properties and synthesis
procedure. When referring to nickel-based catalysts,
its activity and stability are indispensable topics to
be touched. In order to enhance the POM activity
and stability of nickel-based catalyst, one approach is
to choose suitable supports from miscellaneous ma-
terials. Choudhary and co-workers [18–20] carefully
studied catalysts of nickel supported on Yb2O3, MgO,
CaO, TiO2, ZrO2, ThO2 and UO2, as well as alumina
doped with rare earth oxides. It was found that the
catalyst containing CaO, MgO, rare earth oxide and
alumina had higher activity under the condition of
short residence time. For nickel catalyst containing
ThO2, UO2 and ZrO2, the activity had the order as
follows, NiO/ThO2 > NiO/UO2> NiO/ZrO2. TiO2
or SiO2, as support, was not suitable for POM to syn-
thesis gas due to the easier sintering of nickel oxide
and the inertness of binary metal oxide at high tem-
perature.
Ruckenstein et al. [26–28] carefully studied
NiO/MgO catalyst system. It was found that the
high activity originated from the formation of solid
solution, nickel atoms evenly dissolved in the crystal
lattices of MgO. In addition, MgO, due to its weak
basicity, can prevent catalyst from carbon depositing
to some extent [29,30]. These functions of alkaline
earth oxide were also observed in other catalyst sys-
tems [31–35]. The alkaline earth oxide can improve
the dispersion of nickel due to the strong interaction
between nickel and alkaline earth oxide, and also it is
the strong interaction, the highly dispersed nickel par-
ticles, once formed can be prevented from agglomer-
ating, and can be stabilized. Although alkaline earth
oxide supported or modified nickel catalyst exhibited
high POM activity, the deactivation is unavoidable
due to the carbon deposition and the loss of nickel at
high reaction temperature [19].
Among the nickel-based catalysts, it was found
that nickel supported on the perovskite-sturctured
materials, e.g. Ni/Ca0.8Sr0.2TiO3, prepared by cit-
rate method, exhibited good ability to restrain itself
from carbon depositing. Negligible amount of carbon
deposition was found over its surface after 150 hours’
run [36–40]. It was claimed that this kind of sup-
port could control the size of dispersed active phases
below the threshold value needed to generate car-
bon deposition. Therefore, oxygen species over cat-
alyst surface can react with carbon deposition, which
leads to nickel particles prevent from being covered
[41]. Another kind of active support is hydrotalcite-
structured mixed oxide. It is also claimed that this
support can control the dispersed nickel particles size
[42,43]. However, the activity and selectivity for this
kind of catalyst depended to a great extent upon the
reducibility, concentration of nickel oxide and resi-
dence time of reactants. The reducibility of catalyst
relates to the properties of precursor, such as compo-
sition and pretreatment etc. High Mg/Al ratio leads
to less formation of spinel-type species, which is less
active for POM [43].Some other materials, for example, CaAl2O4,
AlPO4-5 and calcium phosphate/hydroxyapatite, etc.
[33,44,45], were tried to be used as the support of
POM catalysts. CaAl2O4 and calcium phosphate sup-
ported nickel catalysts showed excellent performance
of sintering and carbon depositing resistance, and
therefore they showed higher methane conversion and
selectivity to H2. AlPO4-5 also gave good catalytic
performance; however, the phase transformation to
tridymite-structured species caused the specific area
of catalyst and activity to be lost quickly.
Another approach to elevate activity and stability
is the modifying support. The employment of alka-
line earth oxide [35,40,44,46,47] usually leads to the
formation of solid solution. In this case, active com-
ponent is highly dispersed. There is also very strong
interaction existed between alkaline earth oxide and
active phase due to its chemical activity, which results
in anchoring of dispersed active particles, further pre-
vents these particles from agglomerating. The weak
basicity of alkaline earth oxide can restrain the car-
bon depositing to some extent. For example, carbon
deposition was hardly observed over the catalyst af-
ter 500 hour’s run [46]. Other effective modifier is
rare earth oxide [48–53]. These catalysts exhibited
long-term stability [51]. The promotion of rare earth
oxide may be resulted from its oxygen storage/release
capacity, which lands itself to oxidizing surface de-
posited carbon. It was also believed that it could
Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 195
restrain catalyst from sintering at high temperature
because of its strong interaction with active compo-
nent. It was also reported that the improved activity
was attributed to the enhanced reducibility of active
component after the addition of rare earth oxide [53],
because it usually accepted that metallic component
is highly active for POM to synthesis gas.Furthermore, other active component such as Fe,
Co, Pd, Rh, etc. were tried to modify the nickel-
based catalyst for the purpose of improving the sta-
bility and activity. Provendier and co-workers [54]
found that Fe could stabilize nickel catalyst. Us-
ing sol-gel method, They synthesized LaNixFe(1−x)O3
(0< x <1), perovskite-structured mixed oxide, a pre-
cursor of highly active catalyst for POM to synthesis
gas. For these catalysts, stability was improved as
the amount of added iron increased, owing to the re-
versible migration of nickel from the bulk to surface.
Choudhary et al. [20] pointed out that cobalt addition
to Ni/Yb2O3, NiO/ZrO2 or NiO/ThO2 catalyst can
reduce the formation rate of carbon deposition and ac-
tivation temperature of catalyst, which resulted from
improved reducibility of nickel species by the cobalt
addition. The addition of noble metal, although it is
very active for the POM, led to the change of nickel
chemical state or the distribution of nickel species
with different oxidative state. Thus, the tempera-
ture distribution along the longitudinal catalyst bed
was also changed, usually the hot spots disappeared.
The variation of nickel species with different oxidative
state resulted in reasonable distribution between the
total combustion and reforming of methane occurred
along the longitudinal catalyst bed during the process
of POM to synthesis gas [55].
The synthesis procedure of catalyst can affect its
activity and selectivity to a great extent. Xu et al.
[56] prepared alumina supported nickel catalyst us-
ing microemulsion method, and its stability was im-
proved due to an increased coking resistance. Li et al.
[57] prepared Ni/SiO2 catalyst using monodisperse
silica sol and rather high POM activity was achieved,
though SiO2 is an inferior support. Highly coking
resistant nickel catalyst is also synthesized using co-
precipitation method [58]. In general, an important
step in preparation is to improve the dispersion of ac-
tive phase and the interaction between active phases
and support. Both of these factors determine the
chemical state of active component, namely, the activ-
ity of catalyst. So the pretreatment of catalyst, like
the determining step of synthesis procedure, is very
crucial to the activity and selectivity.
Studies on Fe- or Co-based catalysts showed that
the activity of these catalysts was inferior to Ni-based
catalyst because they show higher activity for the to-
tal combustion of methane [49,59,60]. For these sup-
ported catalysts, the activity for POM to synthesis
gas has the order as follows: Ni>Co>Fe. It was re-
ported by Swaan et al. [61] that the cobalt based
catalyst was active only when it was promoted by the
substance that can facilitate its reduction, and this
was the reason that cobalt catalyst with higher load-
ing had higher activity [62]. The support plays an
important role in determining the activity and stabil-
ity of catalyst. Wang et al. [63] found that MgO is
an effective among the alkaline earth oxide supported
cobalt catalysts, and that the calcination temperature
threw a great impact upon the activity and stability.
The employment of cobalt together with noble metal
may be a good idea because cobalt is difficult to sin-
ter. Pt-Co catalyst system showed high activity [64].
It was usually thought that the active species of cobalt
catalyst was metallic cobalt, and that the stability
depended upon its preparation. Moreover, the deac-
tivation of cobalt containing catalyst resulted from
the sintering of active components and formation of
CoAl2O4 [63]. In fact, it was important to choose the
support, for instance, Co/ZrO2 showed high activity,
while Co/La2O3 deactivated rapidly [65].
Comprehensive investigation into Ni- or Co-based
catalyst has been performed, while less attention has
been paid to Fe-based catalysts. The deactivation of
Ni- based catalyst mainly attributed to carbon de-
position and nickel loss at high temperature and high
GHSV. The utilization of cobalt and iron, due to their
higher melting points, if substituting for nickel, may
give better performance. Other elicitation from a ref-
erence [17] on steam reforming is to control the size
of active phase and introduce some modifier into cat-
alysts for the purpose of improving the stability of
catalyst.
3.2. Noble metal catalysts
Green and co-workers obtained high yields of syn-
thesis gas over all noble metal catalysts, as well as
over the rare-earth ruthenium pyrochlores. These
catalysts catalyzed methane conversion to synthesis
gas with yields and selectivity closely approaching to
the thermodynamic calculations. For palladium cat-
alyst, like nickel, substantial carbon deposition was
observed, while for iridium and rhodium catalyst, no
macroscopic carbon deposition was observed [10,11].
196 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004
Poirier and co-workers [66] carried out POM to
synthesis gas experiment at extremely high GHSV
(0.893 molCH4/(kg·s), CH4/O2/He=8/4/3), namely
under the condition that products were dominated
by kinetics, and it was found that Rh was more ac-
tive than nickel, even though at very low loading
(0.015wt%Ru/Al2O3).
Hochmuth et al. [67–70] studied the monolith
supported noble metal catalysts for POM to syn-
thesis gas. The results of pilot plant test at high
GHSV showed that Pt or Pd and the like, was ex-
tremely effective for the production of synthesis gas
from methane. They drew the conclusion that the
complete oxidation of methane had carried out at the
foreside before reforming reaction occurred at the rear
part of catalyst bed. Schwiedernoch et al. [71] also
drew the similar conclusions.
The activity of noble metal for POM to synthe-
sis gas not only depends upon noble metal itself, but
also relates to the preparation procedure and support
properties. Basile and co-workers [72] used anion clay
as precursors of noble metal based catalysts for the
activation of methane and found that the synthesis
gas production activity increased according to the or-
der Rh>Ru∼Ir�Pt>Pd. The best catalytic perfor-
mance was observed for a 1% Rh content (atomic ra-
tio) and Rh content above 1% did not increase the
activity, unlike Ru based catalysts. The results by
Yan et al. [73] indicated that the conversion and se-
lectivity were relative stable over the Rh based cat-
alyst, while they were changeful over Ru based cat-
alyst. Furthermore, the pulse reaction showed that
only CO was formed as carbonaceous product over
Rh catalyst, while CO and CO2 were formed over
Ru catalyst. This implies that the reaction mech-
anisms over these two catalysts are different. As for
the Ir based catalysts during POM reaction to synthe-
sis gas, the activity order of supports was as follows:
TiO2 ≤ZrO2 ≤Y2O3 >La2O3 >MgO≤Al2O3 >SiO2
[74]. A series of rare earth supported noble metal
catalysts was studied, among them, Pt/Gd2O3 and
Pd/Sm2O3 gave preferable catalytic performance [75].
Nevertheless, the selectivity to CO was higher than
that to H2, and it was ascribed to the reverse reaction
of steam reforming. They thought that alkali earth or
rare earth metal oxide played the double roles: one is
to disperse noble metal and the other is to improve
the selectivity.Ruckenstein and coworkers [76–78] investigated
into the effect of different structured magnesia, rare
earth metal oxide, as well as other stable supports
on Rh based catalysts. It was found that the com-
pound formation between Rh and support depended
upon the calcination temperature. No Rh compound
was formed over γ-Al2O3 and SiO2, while LaRhO3,
MgRh2O4, YRhO3 and RhTaO4 were formed over
La2O3, MgO, Y2O3, Ta2O5, respectively, if calcined
at properly high temperature. Among them, La2O3
can provide better catalytic activity and selectivity af-
ter adequate calcinations. The catalyst stability and
the interaction between active metal and support can
be improved at higher reaction temperature. Among
the non-reductive metal oxide supported catalysts,
the activity decreased according to the order as fol-
lows: La2O3 < γ-Al2O3 ≤MgO, after 100 hours’ run.
Rh catalysts supported on variedly structured mag-
nesia hardly showed visible difference, though three
Rh species, Rh2O3, MgRh2O4 and a compound of Rh
and MgO, were observed over the used catalysts. The
higher stability of MgO supported catalyst was as-
cribed to the stronger interaction between the active
metal and support. Clausen et al. [79] investigated
into the local structure of dispersed Rh particles us-
ing in situ X-ray adsorption fine structure technique,
and it was found that metal particle size increased
significantly during the treatment in hydrogen, while
no structure change was observed during POM re-
action, and not influenced by the residence time of
reagents. In the other hand, methane conversion and
selectivity depend upon the residence time. Ruck-
enstein et al. [80] also investigated into the effect
of Rh content in alumina supported catalysts on the
catalytic performance, and it was found that almost
the same methane conversion and selectivity was pro-
vided when Rh content was within the range from 0.5
to 5.0wt%.Jones and co-workers [81] studied the performance
of Eu2Ir2O7 catalyst using in situ X-ray diffraction
and Mass spectrometry and found that the pyrochlore
structure of iridate catalyst was destroyed at the out-
set of catalysis, giving an active catalyst that was
shown to comprise particles of iridium metal of about
3 nm in diameter supported on europium oxide. The
sudden increase of synthesis gas, monitored by mass
spectrometry, corresponded to the onset of reduction
of pyrochlore. Ashcroft et al. [82] drew similar con-
clusions in a study of iridium pyrochlore catalysts us-
ing in situ energy dispersive X-ray diffraction by syn-
chrotron radiation.
Kunimori et al. [83] found excellent catalytic
properties of RhVO4/SiO2 and un-promoted Rh/SiO2
catalysts for the POM to synthesis gas, above 90% of
Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 197
methane conversion at 973 K. They also compared
the activity of the two catalysts over temperature
range of 573–973 K at ambient pressure, using a feed
of CH4/O2 with a molar ratio of 2 diluted with ni-
trogen. It was found that the onset of activity oc-
curred around 773 K for RhVO4/SiO2 catalyst, while
for Rh/SiO2 the catalyst exhibited activity at tem-
perature above 873 K. The examination of the used
catalysts indicated that the average Rh particle size
in RhVO4/SiO2 catalyst was smaller than that in
the un-promoted catalyst, Rh/SiO2. Therefore, the
difference of activity at low temperature was ascribed
to the active metal particle size, morphology and its
interaction with the support.The activity of the catalysts of 1% Pd supported
on oxides including IIIA-IVA metal oxide and rare
earth oxide, were investigated at 1023 K, using GHSV
of 5000 h−1 and CH4/O2 ratio of 8:1 [84]. The
methane conversion varied from 33.4% to 66.9%, but
surprisingly they all gave more than 99% of selectivity
to CO, with no data for hydrogen selectivity. How-
ever, the methane conversion in all cases exceeded the
theoretical maximum (25%) for synthesis gas produc-
tion under the fixed ratio of methane to oxygen. In
addition, the GHSV was set at 5000 h−1 in their work,
which is very small as compared with those used by
other researchers. It is thus probable that carbon de-
position over palladium catalysts is responsible for the
difference in methane conversion.
Platinum supported on alumina doped with zir-
conia gave very excellent performance, which was as-
cribed to the improved oxygen mobility brought by
the introduction of zirconia [85]. The properties of
support significantly affect the activity and selectivity
of Pt-based catalyst via adjusting interaction between
support and active component [86]. The studies on Pt
catalyst revealed that the deactivation of Pt catalyst
was mainly due to the agglomeration of dispersed Pt
particles and carbon deposition [87,88].
The modification of support can affect the oxida-
tive state of active component, which is the key factor
to determine the activity and selectivity. Elmasides
et al. [89] reported that for the Ru/TiO2 catalyst,
the introduction of W6+ into TiO2 led to stabiliza-
tion of oxidative ruthenium, which resulted in lower
conversion and selectivity, while the introduction of
Ca2+ cation led to the formation of metallic ruthe-
nium, which resulted in higher conversion and selec-
tivity.
Recently, noble metal based membrane reactor
has attracted a lot of attention. Using this kind of
membrane reactor, POM to synthesis gas can be car-
ried out at lower temperature, while higher CO and
H2 selectivity can be achieved. Armord et al. [90]
found that Pd based membrane reactor can elevate
H2 production during methane converting to synthe-
sis gas or liquid fuel. Kikuchi et al. [91] found that us-
ing noble metal based membrane catalysts POM can
be carried out at temperature below 773 K if the feed-
stock is poor in oxygen. The methane conversion and
CO selectivity can be elevated through removing H2
from the reactor. It was also found that carbon depo-
sition occurred after steam was depleted, while it can
be avoided by replenishing steam. The nickel-based
membrane reactor was also reported lately [92] and
high methane conversion and selectivity was reached.
Monolith reactor or monolith supported catalysts,
similar to the membrane reactor, was tried to be ap-
plied to POM to synthesis gas [93,94]. For rhodium
impregnated foam monoliths, very high methane con-
version (>90%), CO selectivity (>90%) and complete
conversion of oxygen was achieved during POM to
synthesis gas under the adiabatic condition, using ex-
tremely short residence times of between 10−4 and
10−2 seconds and the feedstock with stoichiometric
ratio. Under the same condition, H2 selectivity for
Pt based catalyst decreased to 70%, whilst the Pd
catalyst promoted the carbon deposition.
Another interesting noble metal based catalyst
system is a mixed oxide (Ba3NiRuTaO9) with per-
ovskite structure [95]. At 1173 K, it can provide 95%
of methane conversion and 98% of H2 selectivity. At
1070 K, it can catalyze the complete conversion of
ethane, obtaining 94% of synthesis gas, but there is no
transformation of perovskite structure post-catalysis
and no carbon deposition formed.
3.3. Early transition metal carbides and other
catalysts
Early transition metal carbides, particularly of
molybdenum and tungsten, exhibited excellent cat-
alytic performances in a large number of reactions,
which were usually catalyzed by noble metal based
catalysts. York et al. [96,97] obtained high yields of
synthesis gas using supported molybdenum or tung-
sten carbides at elevated pressure and temperature.
But it was deactivated rapidly at ambient pressure, re-
sulting in metal dioxide (MO2). In addition, if POM
was carried out under stoichiometric conditions, no
carbon deposition was observed on the used catalysts.
A study of the relative activity of molybdenum car-
198 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004
bide to the noble metals demonstrated that it had an
activity similar to iridium, both per active site and per
gram [96], while high space velocity is unfavorable to
the stability of carbide catalysts. The deactivation of
the catalyst may result from the oxidation of carbides
into oxides, followed by vaporization of oxides under
ambient pressure. However, under the elevated pres-
sure, the vaporization was choked up, since high pres-
sure prevented the carbide, possessing much higher
boiling point than its oxide counterpart, from being
transformed into oxides, especially in the reducing at-
mosphere. The atmosphere imposes influence greatly
upon the stability of carbide, particularly at high tem-
perature. The atmosphere of steam and CO2 goes
against the retention of oxides, but hydrogen and CO
favor retaining carbides [98]. Recent study shows that
addition of some transition metals can significantly
increase the catalyst activity and stability [99]. With
the addition of transition metal promoters, the cata-
lyst activity can be as high as the noble metal catalyst
even at very high space velocity and pressure condi-
tions, but there is much less carbon deposition over
the carbide catalysts. Other example is β-SiC with
medium surface area used as support [100]. β–SiC
supported nickel catalyst showed stable and high ac-
tivity for POM, and the hot spots usually occurred on
alumina supported catalysts were removed due to the
high thermal conductivity of β–SiC, and the carbon
nanofilament growth was scarcely observed.
Besides above POM catalysts, it was reported by
Otsuka et al. [101] that cerium dioxide could trans-
form methane into synthesis gas with a molar ratio of
2 for H2 to CO within temperature range from 873
to 1073 K. It was demonstrated that during the re-
dox cycle of ceria, methane is directly converted to
H2 and CO. Carbon dioxide resulting from the oxida-
tion of methane in gas phase is reduced by partially
reduced cerium cation and CO is given as the only
product. The addition of Pt to cerium dioxide cata-
lyst accelerated the formation of synthesis gas, while
the reduction of catalyst over 60 minutes led to the
synthesis gas with a molar ratio of H2 to CO higher
than 2. The latter case implies the formation of car-
bon deposition after a period of reduction. However,
cerium is used as a modifier at more time to improve
the oxygen mobility.
4. Methane partial oxidation mechanism
The mechanism of methane partial oxidation to
synthesis gas was dealt with and debated widely in
the literature. As far as it goes, it can be divided into
two categories: one is the indirect oxidation mecha-
nism involving methane total combustion, and steam
and dry reforming reactions, which is often referred to
as the “Combustion and Reforming Reactions Mecha-
nism” (CRR); the other is the direct oxidation mech-
anism in which surface carbon and oxygen species re-
act to form primary products, known as the “Direct
Partial Oxidation Mechanism” (DPO).
4.1. CRR mechanism
First mention of the CRR mechanism was made
by Prettre et al. [24]. Their experiments, later re-
peated by Vermeiren et al. [25], indicated that the
longitudinal temperature profile of the catalyst bed
was not uniform, namely, markedly higher tempera-
ture of the front part of catalyst bed than that of the
rear part and the furnace temperature, as shown in
Figure 4[25].
Figure 4. Schematic representation of the tempera-
ture in POM catalyst beds
Choudary et al. [59] carried out the POM reac-
tion using Ni/MgO catalyst under high GHSV con-
dition in order to get non-equilibrium product dis-
tribution. When York et al. [102] repeated the ex-
periment reported by Choudary et al, it was found
that the hot spots were formed. These results indicate
that exothermic reactions occurred at first and then
followed by endothermic reactions. As depicted in
Figure 2, the total combustion of methane, and then
followed by steam and dry reforming of methane. In
fact, nickel and noble metal are very effective catalyst
for the steam shift and steam reforming of methane.
According to CRR mechanism, synthesis gas is sec-
ondary product. Green et al. [11] in their latter ex-
periment investigated into the effect of reaction con-
ditions on the product distribution in the process of
POM to synthesis gas, and explained why there is
lower selectivity of synthesis gas and higher selectivity
Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 199
of CO2 and H2O under the condition of higher GHSV
or higher ratio of O2/CH4, using CRR mechanism. At
the same time, they pointed out that hydrogen and
carbon monoxide were formed as secondary products.
4.2. DPO mechanism
Hickman and Schmidt et al. [68–70] considered
H2 and CO as primary products during POM to syn-
thesis gas under adiabatic condition at very short
residence time. When they doubled the residence
time, the conversion and selectivity were improved.
When substituting Pt-10% Rh wire net for monolith
supported catalyst, the conversion and selectivity in-
creased with increasing the gas velocity of feedstock.
This phenomenon is conflicted with the case in Ref.
[11]. If the gas velocity is fixed and the layer number
of Pt-Rh wire net is increased (not less than 3), no
difference of conversion and selectivity was observed,
and product distribution was away from the equilib-
rium of steam shift or steam reforming reaction, com-
panied with a lower ratio of H2/CO than that calcu-
lated according to thermodynamic equilibrium. All
these facts cannot be explained by CRR mechanism.
In order to elucidate the phenomena mentioned above,
the DPO mechanism was put forward. In this mecha-
nism, synthesis gas is produced as a primary product.
CH4 = C(ads) + 4H(ads)
C(ads) + [O]s = CO(ads) = CO(g)
2H(ads) = H2(g)
They constructed a model incorporating the ele-
mentary adsorption, desorption and surface reaction
steps involved in a mechanism, of which some of the
most important steps are shown in the above equa-
tions. According to this model, the product selectiv-
ity over Pt or Rh catalyst can be forecasted.
Recently, the studies of POM specific mechanism
under specific conditions have become popular. When
Weng et al. [103–105] investigated into the reduc-
tion of Rh and Ru catalyst using FTIR technique, it
was found that CO was formed as a primary prod-
uct of POM reaction over reduced or really working
Rh/SiO2 catalyst, DPO pathway was the main route
of formation of synthesis gas over Rh/SiO2 catalyst.
In contrast to this, CO2 was a primary product of
POM reaction over Ru/Al2O3 or Ru/SiO2 catalyst
[73]. Synthesis gas was formed over Ru-based cata-
lyst by way of CRR mechanism. Of course, the oxygen
content in feedstock can alter the reaction direction.
Transient response and isotope exchange tech-
nique have also been used to investigate into the POM
mechanism. Nakagawa et al. [106,107] reported that
synthesis gas was formed over Ir/TiO2 and Rh/SiO2
catalysts via CRR mechanism. However, the en-
dothermic reaction, methane decomposed to hydro-
gen, carbon and dehydrogenated methane group, ini-
tially occurred, followed by a reaction: carbon and
the dehydrogenated methane group oxidized by oxy-
gen to COx species. As for POM performed over sup-
ported Rh catalyst, its reaction pathway depended
greatly upon the properties of the support. Ruck-
enstein et al. [108,109] reported that during POM
reaction over Ni/SiO2 catalyst, CH, CH2 and CH3
species were formed, which means that methane is ac-
tivated via dissociation, and the amount of methane
taking part in isotope exchange was larger than the
amount of methane converting to CO and CO2. It was
concluded that methane dissociating is not the rate-
limiting step. Over the un-reduced Ni/SiO2 catalyst,
methane directly reacted with oxygen without disso-
ciation of methane. Jin et al. [110] drew a similar
conclusion regarding alumina supported nickel cata-
lyst. Temperature can throw influence upon the POM
pathway. Within the temperature from 973 to 1023
K, POM proceeds mainly via the pathway of the dis-
sociation of methane, whilst at the temperature of
1123 K, CRR mechanism makes a rather contribu-
tion to POM [26]. Li et al. [111] made a point of
producing a high yield of synthesis gas requiring the
catalyst with reduced state. As reported in many
References. [45,55,73,112–114], metallic active com-
ponent, not only nickel but also noble metal, was very
effective to produce H2 and CO. Surface state deter-
mines reaction mechanism, and plays an important
role in determining conversion and selectivity.Judging by current evidences for the partial oxida-
tion of methane to synthesis gas, it is possible for the
two mechanisms to occur over nickel or noble metal
catalysts, but the real pathway depends upon the real
conditions. The pivotal factor is the chemical state of
active component element: metallic state prefers to
methane dissociation reaction, followed by surface re-
action with oxygen species to synthesis gas; while the
active component element with higher oxidative num-
ber facilitates deeper oxidation of H2 and CO, as ob-
served over Ru catalyst by Balint et al [115]. However,
the oxidative state of active component relates to the
properties of support, modifiers, pretreatments, re-
action temperature and oxygen partial pressure, etc.
Other important reason is the active surface oxygen
200 Quanli Zhu et al./ Journal of Natural Gas Chemistry Vol. 13 No. 4 2004
species and its mobility. Of course, kinetic factors
may exert an influence on it, even changes its direc-
tion. This is the reason why two reaction mechanisms
seem to be possible over all catalysts.
The two mechanisms are also applied to elucidate
the POM to synthesis gas over carbide catalysts [116].
(1) DPO type mechanism: this involves surface
species similar to those shown for the DPO mecha-
nism discussed earlier. However, it is likely that syn-
thesis gas is not a primary product over the carbide
catalysts, and that CO2 and H2O are important re-
action intermediates.
(2) Redox mechanism: O2, CO2 or H2O in the
reactor can react with surface carbide carbon species,
generating vacancies and oxide species. These vacan-
cies can then react with carbon from methane disso-
ciation, returning the site back to the carbidic. This
is shown below for the reaction of CH4 and CO2.
Mo2C + 5CO2 = 2MoO2 + 6CO
2MoO2 + 5CH4 = Mo2C + 4CO + 10H2
The most probable mechanism over carbide cata-
lysts is the redox mechanism according to the results
obtained by Xiao et al. [99] using in situ Raman and
pulse techniques. A possible model for the reaction is
given in Figure 5 [99].
Figure 5. Model of 2CH4+O2 reaction to synthesis
gas over molybdenum carbide catalyst
Oxygen first reacts with the carbide, producing
CO; the oxide or oxycarbide surface is then reduced
by methane to produce CO and H2. Because the re-
carburization of the oxide surface by methane is a
slow process and endothermic reaction, the carbide
catalyst is stable only under a condition of high pres-
sure and low space velocity [99].
5. Problems and future studies
Non-catalytic homogeneous partial oxidation of
methane to synthesis gas is well established. For ex-
ample, in Sarawak, Malaysia, Shell have been suc-
cessfully operated a highly selective process for pro-
ducing synthesis gas at high temperatures, typically
above 1400 K, and pressures of around 5–7 MPa,
as a part of the Middle Distillate Synthesis process
(SMDS) [117]. Out of question, employment of cat-
alysts would markedly lower the operating temper-
ature required for the production, which makes the
process more economically attractive [118]. However,
more work should be done to solve the following main
problems for the practical application of this process.
(1) Carbon deposition over the reactor and cata-
lyst bed. There are two possible routes for the forma-
tion of carbon, namely methane decomposition and
the Boudouard reaction.
CH4 = C(s) + 2H2(methane decomposition reaction)
2CO = C(s) + CO2(Boudouard reaction)
Studies by Claridge et al. [119] demonstrated that
both reactions are thermodynamically favorable un-
der reaction conditions typically for methane partial
oxidation, but that the source of carbon deposition is
mainly resulted from the methane decomposition. Ev-
idence for this was given by observing the amount of
carbon deposited on a nickel catalyst in pure methane
or carbon monoxide atmosphere; at high tempera-
tures the pure methane gave much more deposited
carbon than pure carbon monoxide, while support-
ing evidence arose from the fact that carbon built up
from the front of the catalyst bed, where methane
partial pressure was at its highest. Two types of car-
bon are formed on the partial oxidation catalysts as
shown in Figure 6[14]: (a) encapsulate carbon, which
envelops the nickel particles resulting in deactivation,
and (b) whisker carbon, which grows from the face of
the nickel particles and does not alter the rate of syn-
thesis gas formation, but is likely to eventually result
in reactor clogging. Detailed studies on the carbon
formation mechanism have been carried out for the
related steam reforming reaction [120–123]. To sup-
press the carbon deposition, more work needs to be
done on the catalyst preparation and reaction condi-
tion optimization. For example, some steam may be
Journal of Natural Gas Chemistry Vol. 13 No. 4 2004 201
added to the feedstock to eliminate the hot spots in
the catalyst bed, and also may be helpful to suppress
the carbon deposition.
Figure 6. Micrograph showing carbon deposited
over a nickel catalyst after POM
(2) Active component loss during the POM to
synthesis gas, particularly nickel catalyst. The over-
all POM reactions are mildly exothermic, while it may
occur by two steps, initial combustion and then dry
and wet reforming. In the first step, a large amount
of heat is given off, which can melt the active metal,
leading to peeling the active metal off the support.
Because nickel has a lower melting point, lower than
other active components, such as noble metal or Co
and Fe, thus, it may be easier to deactivate. However,
this can be improved by strengthening the interaction
between support and metal, and carrying out the re-
action at milder temperature.
(3) Noble metal catalysts have shown superior ad-
vantages to nickel metal catalyst in carbon deposition-
resistance, but the carbon deposition is still unavoid-
able at high temperature, because of the acidic sup-
port and the methane decomposition, etc. As re-
ported by Albertazzi et al. [87], carbon deposition,
together with sintering of noble metal particles, re-
sulted in the deactivation. Also a high loading of no-
ble metal is required to sustain the high activity, thus
the cost of the catalyst is very high. The combina-
tion of a small amount of noble metal with transition
metals such as Co, Ni or Fe may be a wise way to
decrease the catalyst cost and improve catalyst activ-
ity and stability. New alternative catalysts such as
transition metal carbide are expected to decrease the
catalyst cost and improve the catalyst stability.
(4) Because of the thermodynamic restriction, the
POM reaction under high pressure often gives rise to
more CO2 and H2O formation. To improve the prod-
uct distribution, the feedstock needs to be optimized.
New technology such as membrane catalyst and re-
actor are expected to lead the reaction to a dynamic
state, and thus to release from the thermodynamic
restriction. The combination of POM and steam re-
forming as an alternative, to increase CH4 conversion
and synthesis gas production, is also possible. In addi-
tion, a more stable catalyst system being able to resist
carbon depositing under excessive methane feedstock
is very important to increase the selectivity to CO
and H2. A high CH4/O2 ratio is also desired from the
view of safety, because lower CH4/O2 (≤1.5) ratio
increases the danger of explosions, and this is of par-
ticular importance when high pressure is employed.
6. Conclusions
As mentioned above, a great number of chemicals
and fuel can be obtained from methane by way of
synthesis gas, while the direct conversion of methane
is economically infeasible due to its intrinsic barrier.
Therefore, the partial oxidation of methane to syn-
thesis gas is likely to become more important in the
future, particularly when alternative sources of energy
are required. Early work showed that nickel was an
active catalyst for this reaction. Now it has been fol-
lowed up, particularly in the past 2–3 decades. At
present, a number of potential alternative catalysts
have been discovered for carbon free methane partial
oxidation, including the noble metals and molybde-
num and tungsten carbide. However, there are also
some problems such as carbon deposition for nickel
catalyst, the stability for transition metal carbide and
so on, to be resolved. In order to make POM to syn-
thesis gas become popular industrial process, further
study on this subject is required.
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