Ž .Solid State Ionics 141–142 2001 191–195www.elsevier.comrlocaterssi

Growth of filamentous carbon from lithium and cesium bromidedoped NirSiO catalysts2

Colin Park a,), Mark A. Keane a,b

a Department of Chemical Engineering, UniÕersity of Leeds, Leeds LS2 9JT, UKb Department of Chemical and Materials Engineering, UniÕersity of Kentucky, Lexington, KY 40506-0046, USA

Abstract

The catalytic growth of filamentous carbon, from the decomposition of ethylene, has been studied over the temperaturerange 673–898 K using NirSiO doped with LiBr and CsBr. Promotionrinhibition effects due to the presence of these2

adatoms have been assessed; an optimum alakali metal loading is reported for Li, while the presence of Cs inhibited thereaction at all temperatures. Transmission electron microscopy has been used to characterise the ordered nature of thefilamentous carbon growth. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Filamentous carbon; NirSilica; Alkali metal; Bromine; Ethylene decomposition

1. Introduction

The synthesis of ordered carbonaceous materialshas gained considerable momentum since the discov-

w xery of buckminsterfullerene 1 , multi-rsingle-walledw x w xcarbon nanotubes 2 , graphitic nanofibres 3 and

w xrelated materials 4 . The commercial significance ofthese materials has reached new heights due to theirapplication as adsorbents, selective catalyst supportsw x5 and, more recently, as potential hydrogen storage

w xmaterials 6 . Ordered carbon nanofibres possessunique chemical and physical properties that serve toinfluence the catalytic behaviour of deposited metalsw x5,7 . Moreover, the renewed interest in nanosized

Ž .carbon materials diameters -10 nm for electron-ics applications has prompted a surge in the number

) Corresponding author. Tel.: q44-113-233-2415; fax: q44-113-233-2405.

Ž .E-mail address: checwp@leeds.ac.uk C. Park .

of articles devoted to the synthesis and characterisa-w xtion of these structured materials 8,9 .

Ž .The use of an electropositive adatom Cs toinduce ordering and reconstruction of amorphouscarbon at room temperature has been reported by

w xStevens et al. 10,11 . These authors also noted thatCsrnanoporous carbon exhibited a strong affinity forhydrogen, where Cs readily donated electrons to

`initiate the rupture of C H bonds in benzene, whilethe carbon served to stabilise the intermediate radical

w xanions. In a recent communication 12 , we reportedthe growth of ordered carbon filaments at 553 K,from a modified NirSiO catalyst during the gas2

phase hydrodechlorination of chlorobenzene. Themodification resulted in the incorporation of an ap-preciable Br and K component onto the catalystsurface. The temperature at which filamentous car-bon growth was achieved was at least 150 K lowerthan that quoted for conventional catalytic routes.We attributed this low temperature carbon growth to

0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0167-2738 01 00758-5

( )C. Park, M.A. KeanerSolid State Ionics 141–142 2001 191–195192

combined electron withdrawalrdonation by the halo-genralkali metal on the catalyst surface, which in-duces a restructuring of the active sites to favourdestructive chemisorption of chlorobenzene and or-dered precipitation of carbon. We have extended thiswork to consider the growth of filamentous carbonfrom an ethylene feedstock, promoted using NirSiO2

doped with LiBr and CsBr. The effects of varyingalkali metal loading are considered, and the yield andcharacteristics of the deposited carbon nanofibres arereported and discussed.

2. Experimental

2.1. Catalyst preparation and actiÕation

The NirSiO parent catalyst was prepared by a2

standard incipient wetness procedure where the silicaŽ 2 y1.support fumed silica, Surface Area )200 m g

was impregnated with a 2-butanolic solution ofŽ .Ni NO . The promoted catalysts were prepared by3 2

impregnating the parent NirSiO with methanolic2

solutions of LiBr and CsBr to realise the desiredalkali metal loading. The Ni, Li and Cs contents

Ž .were determined to within "2% by atomic absorp-Ž .tion spectrophotometry VarianSpectra AA-10 , em-

ploying calibration standards supplied by BDHChemicals. The Br content was measured by titration

Žusing a Metrohm 716 automatic titration unit AgNO3.titrant, combined Ag electrode . Carbon monoxide

chemisorption was employed to characterise the sup-ported Ni sites where the catalyst was cooled, fol-lowing the reduction step, to 298 K in dry He and a

Ž .fixed volume 10 ml of CO pulsed into the Hecarrier gas stream; the concentration of CO exitingthe reactor was measured using an on-line thermal

Ž .conductivity detector TCD . The injection of COwas repeated until the downstream peak area wasconstant, indicating surface saturation with CO. Thecatalyst was thoroughly flushed with dry He for 1 h

Žto remove any physisorbed CO, and then ramped 25y1 .K min to 1073 K with a continual monitoring of

the CO content in the exiting gas; data acquisitionŽand analysis were performed using the JCL 6000 for

.Windows software package. Upon completion of theCO TPD sequence, a series of calibration test peaks

were taken at ambient temperature to quantify theCO uptake; reproducibility was better than "3%.

2.2. Catalytic reactor system

ŽAll catalytic reactions were carried out in situ.directly after catalyst activation under atmospheric

pressure in a fixed bed continuous flow silica reac-tor, over the temperature range 673 KFTF898 Kusing a 4:1 vrv C H rH reactant mixture. The2 4 2

reactor temperature was monitored continuously by athermocouple located in the catalyst bed; catalysttemperature was constant to within "1 K. Thecatalytic measurements were made at a WrQ ratios2=10y5 g h cmy3, where W represents the weightof activated catalyst and Q is the inlet volumetricfeed rate of ethylene. The product stream was sam-pled at regular intervals by means of a heated gas-sampling valve, and analysed by on-line gas chro-

Ž .matography Cambridge GC94 chromatograph ,employing a DB-1 50 m=0.20 mm. i.d., 0.33-mm

Ž .capillary column J&W Scientific in conjunctionwith the JCL6000 data collectionrmanipulationpackage. The catalyst was typically left on-streamfor 60 min to allow a uniform growth of carbon. Thereactor was subsequently cooled to ambient tempera-ture where the sample was passivated in a 2% vrvO rHe mixture, before any weight changes to the2

catalyst, due to carbon deposition, were determined.The structural characteristics of the catalyst and de-posited carbon were probed by high-resolution trans-mission electron microscopy using a Phillips CM200

w Ž . Ž .FEGTEM. All gasses He 99.99% , C H 99.95%2 4Ž .xand H 99.99% were supplied by BOC and dried2

by passage through activated molecular sieves beforeuse.

3. Results and discussion

The chemical composition of the activated parentand doped NirSiO catalysts and pertinent CO2

chemisorption results are given in Table 1. Thesurface weighted average Ni particle sizes are alsoincluded in Table 1; these are based on the TEManalysis of over 500 individual particles. The COuptake was appreciably lower for the alkali bromide

( )C. Park, M.A. KeanerSolid State Ionics 141–142 2001 191–195 193

Table 1Ž .Chemical composition, surface weighted average particle size and the CO uptake and desorption temperatures T of the activated parentmax

and alkali bromide doped NirSiO2

Ž .Ni, Alkali metal, NirBr Particle CO uptake T Kmaxa y1Ž . Ž . Ž . Ž .wrw % wrw % mol ratio size nm mmol g Ni

NirSiO 11 – – 12.9 105 6632

Li–NirSiO -A 9 0.1 25.3 7.1 87 383, 513, 10732

Li–NirSiO -B 8 0.4 6.8 13.0 74 378, 508, 10732

Li–NirSiO -C 8 1.0 2.4 10.5 56 518, 10732

Li–NirSiO -D 8 6.0 7.0 20.3 11 525, 883, 10732

Cs–NirSiO -A 8 0.5 35.2 7.4 59 453, 791, 10732bCs–NirSiO -B 11 9.3 46.2 – 74 553, 888, 10432

a Particle size measured by TEM.bCs layer obstructed analysis by TEM.

doped samples and, in the case of the Li–NirSiO2

samples, declined further with increasing alkali metaladdition. The latter may be attributed, in the case ofthe Li–NirSiO -D sample, to a decrease in Ni dis-2

persion, due to the formation of surface metal halidethat is known to be particularly mobile and prone to

w xsintering 13 . The decrease in CO uptake may alsobe indicative of a spreading of Li or Cs over the Nisurface; a masking of supported Ni by alkali adlayerintroduced by impregnation has been noted else-

w xwhere 14 . Carbon monoxide desorbed from theŽ .parent NirSiO catalyst in one step T s663 K ,2 max

but doping with LiBr or CsBr introduced additionalŽ . Žlow 373 K-T -563 K and high T G1043max max

.K temperature peaks. The presence of the low tem-perature peaks are diagnostic of a weaker COrNiŽ . Ž .stronger Nirsupport interaction s due to electrondonation from the alkali metal, in agreement with

w xconclusions drawn by Arena et al. 15 . The desorp-

Fig. 1. Weight of carbon deposited as a function of reactionŽ .temperature for the decomposition of ethylene over NirSiO e ,2

Ž . Ž .Li–NirSiO -B B and Cs–NirSiO -A ` .2 2

tion of CO at a higher temperature can be ascribed tothe presence of residual electronegative Br that servesto strengthen the NirCO bond. It is evident thatdoping with LiBr or CsBr induced significant changesin the electronic structure of surface Ni sites.

The influence of reaction temperature on carbonproduction from both the parent and modified cata-lyst is illustrated in Fig. 1 and Table 2; the maximumyield of carbon was achieved from Li–NirSiO -B at2

w x723 K. It is well established 16 that the generationof solid carbon from a hydrocarbon feed, over sup-ported metal catalyst, involves a destructivechemisorption of the reactant on specific crystallo-graphic faces of the metal particle, followed bydiffusion into and through the particle with precipita-tion at the faces to yield an ordered carbon filament.

ŽThe presence of the alkali metal alone introduced as. w xa hydroxide had little effect on carbon yield 17 ,

and the optimum promotion requires the presence ofboth electropositive and electronegative adatoms. Thecarbon yield is, however, strongly dependent on both

Table 2Influence of alkali metal dopants on carbon yield at the statedtemperatures after 60 min on stream

y1Ž .Carbon yield g gC Ni

723 K 773 K 873 K

NirSiO 64 72 362

Li–NirSiO -A 75 80 202

Li–NirSiO -B 125 122 282

Li–NirSiO -C 93 95 422

Li–NirSiO -D 72 122 322

Cs–NirSiO -A 60 53 112

Cs–NirSiO -B 8 6 22

( )C. Park, M.A. KeanerSolid State Ionics 141–142 2001 191–195194

dopant loading and reaction temperature; the LiBrtreatment also limited carbon generation under cer-tain process conditions. The observed promotion ofcarbon growth over the LiBr doped sample must belinked to an enhanced chemisorption and decomposi-tion of ethylene, as a result of electronic perturba-tions in the active metal centres by the presence ofresidual Br on the catalyst surface. In marked con-trast, doping with CsBr only served to inhibit carbondecomposition, an effect that we ascribe jointly to

Ž .the low Br content high NirBr ratios in the acti-Ž .vated catalyst Table 1 and excessive masking of

surface Ni by Cs that, in effect, occluded the activesites from the incoming reactant. The TEM analysisof the Cs doped catalysts revealed a thin and highlymobile layer of Cs covering sections of the surface, afeature particularly pronounced at high Cs loadings.At T)823 K, all the doped samples delivered alower yield of carbon when compared with the par-ent NirSiO sample. At such elevated temperatures,2

blockage of the active sites by the increasingly mo-bile alkali metal adatoms must contribute to an inhi-bition of carbon growth.

The carbon generated in these studies was solelyfilamentous in nature, grown in a whisker-like mode,where the active metal particle could be found at thegrowing tiprcentre of the filament. Preliminary lowmagnification TEM studies have revealed that thedegree of filament curvature increased significantly

upon moving from Li to Cs, suggesting subtle pertur-bations in the electronic structure of the Ni particles.A restructuring of the Ni particles was also apparent,Ni crystallites adopt more regular geometrical shapesupon the addition of LiBr or CsBr, rather than therounded, spherical appearance of Ni crystallites onNirSiO , i.e. a stronger support interaction is in2

evidence. Upon closer examination at high resolutionŽ .Fig. 2a , the arrangement of the graphite platelets ofthe parent NirSiO is mainly parallel to the fibre2

axis with the remainder at an angle to the fibre axisin a ‘herringbone’ configuration. Carbon from Li andespecially Cs doped NirSiO catalysts were more2

helical in nature where the ordered carbon plateletsŽ .were parallel to the edge of the filament Fig. 2b,c .

The introduction of Cs, a bulkier and less volatileadatom than Li, to the parent catalyst can be ex-pected to have a pronounced effect on the diffusioncharacteristics of the carbon through the particleresulting in an anisotropic growth and the observedhelical structures. The addition of the smaller Liadatom has a lesser impact on the diffusion charac-teristics; therefore, the generated carbon is similar innature to that of the parent catalyst. One interestingfeature is the multi-directional growth of filamentouscarbon from pyramidal-shaped particles in the doped

Ž .Cs samples Fig. 2c , again indicative of pronouncedelectronic perturbations induced by the presence ofthe dopant.

Ž . Ž . Ž .Fig. 2. Representative HRTEM images of the filamentous carbon growth from a NirSiO , b Li–NirSiO -B and c Cs–NirSiO -A2 2 2

after 60 min on stream at 773 K.

( )C. Park, M.A. KeanerSolid State Ionics 141–142 2001 191–195 195

4. Conclusions

Passage of a C H rH mixture over NirSiO in2 4 2 2

the temperature range 673–873 K generated filamen-tous carbon where doping the catalyst with LiBrŽ .0.4% wrw Li elevated carbon deposition. Higherconcentrations of Li or treatment with CsBr onlyserved to mask or block the active Ni, inhibiting thedecomposition reaction. The temperature at whichthe maximum yield of filamentous carbon occurs isdependent on the alkali metal loading. The presenceof residual electronegative Br on the catalyst surfaceenhances ethylene decomposition to solid carbon bya charge transfer mechanism. Significant changes tothe structural features of the filamentous carbon wereimposed by the introduction of the alkali bromide,i.e. the occurrence of ordered helical structures inaddition to an increase in the overall degree ofnanofibre curvature. The latter was most pronouncedwith the addition of CsBr, the bulkier alkali metaladatom, serving to alter the diffusion characteristicsof the Ni crystallites and inducing an anistropiccarbon growth.

Acknowledgements

This work was funded by a grant from the RoyalSociety. The authors are grateful to Dr. R. Brydsonfor his assistance with the TEM analysis.

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