Methane Decomposition: Production ofHydrogen and Carbon Filaments

BY T.V. CHOUDHARYa AND D.W. GOODMANb

aConocoPhillips Company, Bartlesville Technology Centre, Bartlesville 74004,USA

bDepartment of Chemistry, Texas A&M University, College Station, TX-77843, USA

1 Introduction

Hydrogen, presently, finds application as a chemical rather than a fuel incommercial operations. However, being a non-polluting source of energy,hydrogen is predicted to be the ‘‘fuel of the future’’.1 One of the most potentialapplications for hydrogen is to power fuel cells. Major automobile manufac-turers are currently working towards developing fuel cell vehicles; such vehiclesare expected to significantly curtail the pollution from the transportationsector. Fuel cells, because of their modular nature, can be utilized to provideheat and electricity not only to single homes but also to provide a large amountof electricity to a large grid network.

Fuel cells can be broadly classified into two types; high temperature fuel cellssuch as molten carbonate fuel cells (MCFCs) and solid oxide polymer fuel cells(SOFCs), which operate at temperatures above 923 K and low temperature fuelcells such as proton exchange membrane fuel cells (PEMs), alkaline fuel cells(AFCs) and phosphoric acid fuel cells (PAFCs), which operate at temperatureslower than 523 K. Because of their higher operating temperatures, MCFCs andSOFCs have a high tolerance for commonly encountered impurities such as COand CO2 (COx). However, the high temperatures also impose problems in theirmaintenance and operation and thus, increase the difficulty in their effectiveutilization in vehicular and small-scale applications. Hence, a major part of theresearch has been directed towards low temperature fuel cells. The low tem-perature fuel cells unfortunately, have a very low tolerance for impurities suchas COx; PAFCs can tolerate up to 2% CO, PEMs only a few ppm, whereas theAFCs have a stringent (ppm level) CO2 tolerance.

Methane, due to its abundance and high H/C ratio (highest among allhydrocarbons) is an obvious source for hydrogen. Steam reforming of methanerepresents the current trend for hydrogen production. Other popular methods

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of hydrogen production include autothermal reforming and partial oxidation.However, all these processes involve the formation of large amount of COx asby-product.2–4 Hydrogen generated by these conventional methods can beutilized by the low temperature fuel cells only if COx (CO for PEM and CO2 forAFC) are completely eliminated (to ppm levels) from the stream prior to itsintroduction into the fuel cell. The process required to eliminate CO from thehydrogen produced in the steam reformer is briefly described below. The steamreformer products containing B10% CO (depending on the feedstock andconditions employed) are passed into water gas shift reactors (WGSs) whereCO is reacted with water to form CO2 and hydrogen.5 Generally two WGSreactors are used in series (high temperature and low temperature) to minimizethe amount of water. The WGS shift reactors are extremely bulky. Finally, theCO content is reduced to a few ppm in the preferential oxidation reactor(PROX). The hydrogen can be introduced in the fuel cell only after thiscircuitous procedure of removing CO. AFCs would additionally require re-moval of CO2 to ppm levels. Also, it is known that high levels of CO2 in thehydrogen stream can be detrimental for the performance of PEM fuel cells.6

Other conventional process of hydrogen production such as partial oxidationand auto-thermal reforming also entail similar procedures for COx removal.Removal of COx to ppm levels from the hydrogen stream makes the processextremely complex and bulky and thereby prohibits the use of the existinghydrogen production technology for use in vehicular and small-scale stationaryfuel cell applications.

Hydrogen production routes, which do not require complex COx removalprocedures, are therefore desired for fuelling low temperature fuel cells. Re-cently, there has been a great deal of interest in investigating the catalyticdecomposition of natural gas (whose major constituent is methane) for produc-tion of hydrogen. Since only hydrogen and carbon are formed in the decompo-sition process, separation of products is not an issue.7 The other main advantageis the simplicity of the methane decomposition process as compared to conven-tional methods. For example, the high- and low-temperature water-gas shiftreactions and CO2 removal step (involved in the conventional methods) arecompletely eliminated. This review will address the following topics related tothe methane decomposition process: (a) fundamentals of methane decomposi-tion, (b) effect of support and promoters on the methane decomposition process(c) alternate reactor design for improving the process yields and (d) catalystregeneration. Catalyst regeneration is extremely important for the practicalapplication of the clean hydrogen production process; issues related to catalystregeneration by steam, air and CO2 will be summarized separately. Sincehydrogen production via methane decomposition is a relatively new field thereare several unresolved issues. This review will attempt to bring forth these issues.

Under certain process conditions, high yields of carbon filaments can beobtained on the catalyst during the catalytic decomposition of methane. Cur-rently, there is a great interest in these carbon filaments, as the unique propertiesexhibited by these materials can be exploited in a number of applications suchas catalyst support, energy storage devices, selective adsorption agents and

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reinforcement materials. In this review we will focus on factors that influencecarbon filament formation rates/yields and properties.

This review will not address the (relatively low yield) synthesis of specialtysingle walled carbon nanotubes; a recent review on this topic can be foundelsewhere.8

2 Hydrogen Production

The theoretical hydrogen formation reaction via decomposition of methane canbe represented as:

CH4 - 2H2 þ C DH1073 ¼ 90.1 kJ/mol(CH4)

This moderately endothermic process results in the formation of 2 moles ofhydrogen per mole of methane consumed above a certain threshold reactiontemperature. A gradual catalyst deactivation is expected due to the accumula-tion of carbon on the catalyst. The catalyst can be regenerated by removing thecarbon on the catalyst in a separate step. Thus, hydrogen production by thisapproach involves two distinct steps: (a) catalytic decomposition of methaneand (b) regeneration of catalyst.

ðaÞ CH4 !2H2 þ C

ðbÞ CþH2O=O2=CO2 !COx þH2 and clean catalyst surface

At the outset, this section will address studies related to the catalytic methanedecomposition step and then subsequently describe the work undertaken on thecombined step-wise reforming (two step) process.

2.1 Catalytic Decomposition of Methane for Hydrogen Production. – Themethane decomposition reaction for hydrogen production has garneredconsiderable interest in the past 4–5 years. The recent interest in this approachfor producing hydrogen stems from the stringent requirement of CO-freehydrogen for the proton exchange membrane fuel cells. For vehicularand small scale stationary applications, it is necessary that the fuel reformerbe compact; this is difficult for the conventional processes since high COconversion efficiencies require large water gas shift reactors. Recent studieshave addressed different issues such as methane decomposition fundamentals,support/promoter effects and reactor design. The ensuing discussion will showthat while some interesting issues about catalytic methane decomposition (as amethod for generating pure hydrogen) have been uncovered, a significantamount of work still needs to be undertaken for better understanding thisprocess.

2.1.1 Methane Decomposition Fundamentals. The fundamentals of methanedecomposition have been extensively investigated on model-single crystal cat-alysts;9,10 an exhaustive review on this subject can be found elsewhere.11

Herein, only the studies undertaken on the fundamentals of methane

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decomposition reaction on high surface area catalysts will be described. Otsukaand co-workers used hydrogen-deuterium exchange studies to investigate themethane decomposition reaction mechanism over Ni/SiO2.

12 An isotopic effectwas observed for the CH4/CD4 decomposition on the catalyst. Furthermore,substituted H–D methanes were not observed when a CH4–CD4 mixture wasdecomposed over the Ni/SiO2 catalyst. Based on this the authors suggested thatthe first C–H bond cleavage was the rate determining step for the decompo-sition of methane to carbon and hydrogen. In line with this theory, the authorsalso observed a reverse isotopic effect between hydrogen and deuterium whenthe carbon deposited on the catalyst was hydrogenated back to methane. Themethane decomposition mechanism was further studied by performing thefollowing set of experiments sequentially:

(i) decomposition of 12CH4

(ii) decomposition of 13CH4

(iii) hydrogenation of deposited carbon.

The studies showed that the carbon that was deposited last was hydrogen-ated first; thus indicating that there was no significant scrambling between thecarbon atoms.

Using an array of catalyst characterization techniques, the same groupfurther investigated the structural changes of the Ni species in the Ni/SiO2

catalyst during the methane decomposition reaction.13 Prior to the reaction, theNi species on the catalyst were in the metallic state. The Ni metal particles werefound to aggregate [X-Ray Diffraction (XRD) studies] as soon the catalyst wascontacted with methane. Following this initial aggregation at the onset of themethane decomposition reaction, no significant change in the structure of theNi species was observed until towards the end of the reaction. During the rapidcatalyst deactivation stage, Ni K-edge X-Ray Absorption Near Edge Structure(XANES) studies indicated the formation of Ni carbide species.

As will be discussed later, along with Ni/SiO2 the Ni/TiO2 catalyst is also apromising catalyst for the methane decomposition reaction. Zein et al.14 havevery recently investigated the kinetics of methane decomposition on a Ni/TiO2

catalyst. Their studies suggested a first order rate law for the decompositionreaction and activation energy of 60 kJ/mol. Interestingly, their studies indi-cated that the methane adsorption step on the catalyst surface was the ratedetermining step. This is in contradiction to studies on the Ni/SiO2 catalystwherein,12 the scission of the first C–H bond was proposed as the rate deter-mining step for the methane decomposition reaction.

Carbon-based catalysts have also been considered for the methane decom-position reaction.15 Yoon and co-workers have recently investigated the kinet-ics of methane decomposition on activated carbons as well as on carbonblacks.16,17 In case of activated carbons the authors observed mass transporteffects in the catalyst particles and also significant pore mouth plugging. Thereaction order was found to be 0.5 and the activation energy was found to beB200 kJ/mol for the different activated carbon samples. On the other hand, for

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the carbon black catalysts a reaction order of near unity was observed and theactivation energy was lower than that observed for the activated carbons.

Recent studies have shown that unreduced Ni catalysts,18 depending on thesynthesis procedure, are also efficient for the hydrogen production reaction.Since most fundamental studies have been undertaken on reduced Ni catalysts(Ni0), it will be interesting to investigate methane decomposition fundamentalson unreduced Ni catalysts.

2.1.2 Effect of Support. Methane decomposition on various Ni-supportedcatalysts has been extensively investigated by the Goodman group.19,20 Thesestudies were directed towards understanding the role played by the support indetermining the nature of surface carbon and CO content in the hydrogenstream. Time on stream methane activity studies at a reaction temperature of823 K revealed comparable initial methane decomposition activities for the Ni/HY, Ni/HZSM-5, Ni/SiO2 and the Ni/SiO2/Al2O3 catalysts.

20 However, unlikethe Ni/SiO2, Ni/HY and Ni/SiO2/Al2O3 catalysts, which showed methaneconversion activity for several hours, a rapid deactivation (in ca. 1 h.) wasobserved in case of Ni/HZSM-5. Transmission Electron Microscopy (TEM)images of Ni/HZSM-5 catalyst after the reaction showed an encapsulating typeof carbon (Figure 1), which explained the rapid deactivation of the catalyst. Onthe other hand, carbon filaments (Figure 2) were observed in case of Ni/SiO2,Ni/HY and Ni/SiO2/Al2O3 catalysts.19 The presence of the Ni particle at theapex of the carbon filaments elongates the life-time of the catalyst. It is

Figure 1 TEM image of Ni/HZSM-5 after methane decomposition at 823 K19

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noteworthy that while there was rapid deactivation of the Ni/HZSM-5 catalystat 823 K, the catalyst had a much greater stability at 723 K. TEM imagesrevealed the presence of carbon filaments at lower temperatures (723 K), whichresulted in greater catalyst stability for methane conversion. X-Ray photoelec-tron Spectroscopy (XPS) of the spent samples showed the presence of carbidicand graphitic carbon at low reaction temperatures (r723 K), whereas only thegraphitic species were observed at higher temperatures. This is in excellentagreement with studies on model Ni catalysts (single crystal).21

Previously it has been noted that methane decomposition may lead to COformation via reaction of the carbonaceous residue with the oxygen of the oxidesupport.22 Since the CO content in the hydrogen stream is a critical parameter forthe PEM fuel cells, it is necessary to achieve an accurate quantification of CO(ppm levels). Although this aspect has been neglected in most studies, in ourstudies particular attention was devoted towards the CO quantification issue.19,20

Quantitative estimation of CO (to ppm levels) was achieved by utilizing theanalysis system showed in Figure 3. The effluents from the reactor were firstintroduced in the thermal conductivity detector (TCD) for detection of hydrogenand methane; Ar was employed as a carrier gas. Analysis of CO was carried outby converting it into methane in a methanizer prior to its introduction in a flame

Figure 2 TEM image of carbon filaments formed after methane decomposition at 823 K

on Ni/HY19

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ionization detector (FID). Essentially 100% conversion efficiency (CO to meth-ane) was achieved by operating the methanizer at 673 K with large amounts ofhydrogen. This was accomplished by routing all of the hydrogen flow to the FIDthrough the methanizer. An auxillary flow of carrier gas was employed to obtainthe optimum carrier/hydrogen ratio for maximum detector sensitivity.

The CO formation rates showed a common trend for all the catalysts; highinitial rates that rapidly decreased with time and finally stabilized.20 The rate ofCO formation was found to increase with increasing temperatures and decreasewith increasing gas space velocities (decrease in contact time). The spacevelocity effect was especially pronounced in the initial period of the methanedecomposition reaction. The CO content in the hydrogen stream at a reactiontemperature of 823 K was ca. 50, 100 and 250 ppm for Ni/SiO2, Ni/SiO2/Al2O3 and Ni/HY respectively after the CO-formation rates had stabilized.Diffuse Reflectance Infra-Red Spectroscopy (DRIFTS) studies showed thepresence of approximately 55 m-moles of hydroxyl species at 823 K on 0.1 g ofthe Ni/SiO2 catalyst. The hydroxyl groups on the support were held responsiblefor CO formation during the methane decomposition reaction. The authorsproposed that supports with a greater content of reactive hydroxyl groups ata given methane decomposition temperature would show higher CO formation.It should be noted that some of the support hydroxyl groups may also beinvolved in CO2 formation, further complicating the issue. Since CO2 isrelatively benign to the PEM fuel cell, the CO2 content was not measured inour study.19 However, the knowledge of CO2 content is important to estimatethe effect of support hydroxyl groups on the CO formation. Extensive studiesinvolving different catalyst supports (effect of temperature on hydroxyl groups)coupled with methane decomposition studies (quantitative detection of ppmlevels of CO and CO2) under different process conditions will be neededto obtain a satisfactory understanding about the influence of support on theCO formation.

Figure 3 Schematic of the experimental set-up employed to study the methane decom-

position reaction; methanizer coupled with the FID was used to quantify ppm levels of CO

in the hydrogen stream20

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Takenaka et al. have investigated the effect of several supports (Ni-based) onthe activity and stability for the methane decomposition reaction.23 While thecatalysts with SiO2, TiO2 and graphite supports showed high activities and longlife-times, the catalysts with Al2O3, MgO and SiO2–MgO2 supports were foundto be inactive. Characterization of the catalysts by XRD and XANES revealedthat in case of the active catalysts Ni was present in the metallic state, whereasin case of the inactive catalysts Ni formed an oxide compound with the support.Studies on catalysts with different silica supports showed that the catalystactivity/stability was also dependent on the pore structure of the support; thesilica support devoid of pore structure was found to enhance the catalystactivity/stability.

While a large number of studies have been undertaken related to the effect ofsupport on hydrogen production activity/stability, the influence of Ni particlesize has not been studied in detail. Otsuka and co-workers have recentlyreported that the Ni metal particles within a specific size range (60–100 nm)for a Ni/SiO2 showed longest catalytic life for the methane decompositionreaction.24 However, more information is desirable on this topic; for example itwill be interesting to systematically investigate the effect of particle size on thehydrogen production rate. Since the hydrogen production rate and the catalyststability are both important to the practical application of this process, it isimportant that future studies address both these aspects simultaneously.

2.1.3 Bimetallic Catalysts and Promoters. Shah and co-workers comparedthe methane decomposition activities and stabilities for monometallic (Pd, Moor Ni) and bimetallic M–Fe (M ¼ Pd, Mo or Ni) catalyst above 673 K.25 Theirstudies showed that the bimetallic M–Fe catalysts produced hydrogenat significantly higher rates than the monometallic (M) catalysts. The Pd–Fe catalyst was found to be the most active methane decomposition catalyst at973 K.

Chen et al. have investigated the effect of Cu content on the methanedecomposition activity and stability of bimetallic Ni–Cu/Al2O3 catalysts.26

The 2Ni–1Cu–Al catalyst was found to be superior to the 15Ni–3Cu–2Al, 3Ni–3Cu–2Al and 1Ni–1Cu–1Al catalysts; high activity required optimized levels(not too high and not too low) of Cu in the catalyst. The authors believed thatthe introduction of Cu (especially at high levels) transformed the catalyst into aquasi liquid state between 973 and 1013 K thus making them less stable. Similarto this study, Li and co-workers (who investigated a series of Ni–Cu–Nb2O5

catalysts) also observed that optimized levels of Cu were required to maximizehydrogen yields. The best catalyst (65Ni–25Cu–5Nb2O5) gave a yield of 7274mol H2/mol Ni.27,28

The methane decomposition reaction is severely constrained by equilibrium.A few studies have also been undertaken to circumvent the equilibrium con-straints.29,30 Otsuka and coworkers used the addition of CaNi5 to Ni/SiO2 forcheating equilibrium.29 The physical mixture of CaNi5 and Ni/SiO2 showedgreater than equilibrium methane (decomposition) conversion due to thehydrogen absorption property of CaNi5.

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2.1.4 Reactor Design. The studies discussed until this point were all under-taken in conventional fixed bed reactors; this segment focuses on methanedecomposition studies in membrane and fluidized bed reactors. Application ofmembrane reactors for hydrocarbon reforming is known to increase the yield ofhydrogen.31 In line with this, Ishihara et al. observed greater than equilibriummethane conversions using a 90%Pd10%Ag hydrogen permeable membranereactor during the decomposition of methane over Ni/SiO2 catalyst.30 Thepermeated hydrogen was swept by argon (Ar) gas and an increase in the Arflow-rate was found to enhance the hydrogen production. The positive effect inthe hydrogen yields was more pronounced at higher temperatures (4700 K)due to higher hydrogen permeability at these temperatures. A constant con-version of 70% (10% CH4 in N2) was observed at 773 K over a time period of60 h. Contact times larger than 50 g-cat � h �mol�1 and sweep Ar flow rateshigher than 200 ml �min�1 were found to be favorable for the process. Utili-zation of sweep gas on the permeate side results in dilution of the permeatedhydrogen; this can be a serious limitation for producing pure hydrogen. Also,membrane reactors have a tendency to get fouled; hence when considering amembrane reactor it is important to address the fouling issue.

Recently Weizhong and co-workers have used a two stage fluidizedbed reactor to study the methane decomposition reaction over a Ni–Cu/Al2O3 catalyst.32 The temperature in the lower stage of the reactor was heldconstant at 773 K, while the temperature in the upper stage was controlledbetween 773 K and 1123 K. Operation at higher temperatures is desired as itincreases the hydrogen production rates. Unlike the fixed bed reactor/singlestage fluidized bed reactor studies, wherein a rapid catalyst deactivationwas observed at 1123 K, the catalyst showed significantly lower deactivationrate in the two stage fluidized bed reactor (upper stage at 1123 K and lowerstage at 773 K). The authors believed that the two stage temperature operationdecreased the disparity in carbon production and diffusion rates (whichwas responsible for rapid catalyst deactivation in fixed bed/single stage fluid-ized bed reactors operating at high reaction temperatures). While this isan interesting concept, the suggested reactor design may be too complex forpractical operation.

2.2 Step-wise Methane Reforming: Regeneration Issues. – The catalyst isgradually expected to deactivate due to accumulation of carbon on the catalystsurface during the methane decomposition reaction. This means that after acertain reaction time period, the catalyst has to be regenerated or replacedwith a new catalyst (expensive approach). The latter approach could be usedfor synthesizing carbon filaments with high yields. However, frequent replace-ment of catalyst is not a practical approach for hydrogen generation. It istherefore essential to employ the regeneration strategy for hydrogen produc-tion. The hydrogen production process therefore consists of two steps (a)methane decomposition (Step I) and (b) catalyst regeneration (Step II). Thissegment, which will focus on the combined hydrogen production process (step-wise reforming), has been sub-divided by the type of regeneration gas used

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(steam/air/CO2). Issues related to regeneration efficiency and energy efficiencywill be addressed in this section.

2.2.1 Step-wise Reforming with Steam Regeneration. The step wise methanesteam reforming process, which involves the catalytic decomposition of meth-ane in step I and steam regeneration in step II, has been investigated by ourgroup at relatively low reaction temperatures.33,34 The studies were performedin a pulse mode so as to ensure accurate quantitative analysis of the carbonremoved in the regeneration step. No catalyst deactivation was observed in thisstudy; in contrast consequent pulsing of methane without intermittent regen-eration (i.e. without Step II) showed an exponential deactivation of the catalyst.The amount of surface carbon removed varied from 92% to 100% (of theamount deposited in Step I) in the various cycles; 95% of the carbon wasremoved on an average. The CO content in the hydrogen produced in step I wasless than 20 ppm. The average amount of hydrogen produced per mole ofmethane consumed in Step I was 1.1, thus indicating the presence of hydro-carbonaceous residue on the catalyst surface. Recent neutron vibrationalstudies have revealed the presence of methylidyne (CH), vinylidene (CCH2)and ethylidyne (CCH3) species on Ni–based surfaces after methane dissociationat low temperatures (o673 K).35 The ethylidyne species were found to be lessstable than the vinylidene and methylidyne species with increasing methanedecomposition temperatures.

Amiridis and co-workers employed a continuous flow reactor to study thestep-wise steam reforming process.36 In the first step, methane was decomposedover 15% Ni/SiO2 catalyst at 923 K and space velocity of 30000 h�1 for 3 h. Inthe second step, the catalyst was regenerated with steam until no hydrogen wasobserved in the product stream. Ten reaction cycles performed as describedabove showed no significant decrease in catalytic activity.36 XRD patternscollected after individual cycles suggested that a large fraction of the carbondeposited in Step I was removed in the regeneration step. It is noteworthy thatthere was no significant change in the crystallite size of Ni during the reaction-regeneration cycles. In agreement our recent studies on a pulse mass analyzerbalance have indicated that ca. 75% of the surface carbon (deposited in Step Ion Ni/Al2O3/SiO2 at 823) can be removed during the steam regeneration step at823 K in the continuous flow mode.20

Choudhary and co-workers have investigated the step-wise steam reform-ing process in two parallel reactors;37,38 methane decomposition and carbongasification were carried out simultaneously by switching a methane contain-ing feed and steam containing feed between the two reactors at pre-deter-mined time intervals. Amongst the various Ni supported catalysts (ZrO2,MgO, ThO2, CeO2, UO3, B2O3, MoO3, HZSM-5, Hb, NaY, Ce(72)NaY andSi–MCM–41) screened for this cyclic reaction, Ni/ZrO2 and Ni/Ce(72)NaYwere found to be the most suitable catalysts. The degree of carbon removal bysteam increased significantly on increasing the regeneration temperature from773 K to 873 K.39 Since issues related to pressure drop are extremelyimportant for practical operation, the pressure drop across the reactor was

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also monitored in this study. An exponential increase in pressure drop wasobserved across the reactor during the methane decomposition reaction aftera certain threshold of carbon deposition was exceeded. This study highlightsthe importance of optimizing the switch time between the methane decom-position reaction and regeneration reaction (optimizing run lengths for thetwo processes).

Both methane decomposition and regeneration by steam are endothermicprocesses, and hence the step-wise steam reforming (like conventional methanesteam reforming) is expected to be an energy intensive process. The hydrogengeneration process is expected to be more energy efficient for air/oxygen basedregeneration; studies related to step-wise air/oxygen methane reforming aresummarized in the next section.

2.2.2 Step-wise Reforming with Air/Oxygen Regeneration. The step-wise re-forming with air has been employed in the past by Universal oil products(Hypro Process).40 The process utilized a 7% Ni/Al2O3 catalyst in a fluidizedbed reactor-regenerator. Catalytic decomposition of methane occurred in thefluidized bed reactor atB1150 K followed by regeneration with air atB1475 Kin the fluidized bed regenerator. The product stream consisted of 93–95%hydrogen and unreacted methane.

We have also recently investigated the reaction/regeneration (by air) cycleson Ni/HZSM-5 at 723 K in a fixed bed reactor.20 In this case, the methanedecomposition step was performed for 1 h following which the catalyst wasregenerated using an oxidation–reduction cycle. There was no apparent de-crease in catalytic activity throughout the 12 cycles studied at 723 K. Similarly,Zein and Mohamed observed stable catalytic activity for six methane decom-position-regeneration cycles on a 15MnOx–20NiO/TiO2 catalyst.

41

Monnerat et al. have investigated the methane decomposition and airregeneration process over a Ni gauze catalyst (Ni-grid with Raney type outerlayer).42 Their studies revealed an optimal reaction performance when the cycleconsisted of 4 min of reaction period followed by 4 min of regeneration period.In a second study, Mirodatos and co-workers investigated the same process ona Pt/CeO2 catalyst at 673 K under forced unsteady-state conditions.43 No COwas detected in the products under these conditions in either the cracking or theoxidative regeneration steps.

Utilization of air can effectively increase the energy efficiency of the processas the exothermic regeneration step can be employed to drive the endothermichydrocarbon decomposition step. However on the flip side, air regenerationmay lead to sintering of the catalyst especially in fixed bed reactors. Villacampaet al. investigated several reaction-regeneration cycles on a co-precipitated Ni/Al2O3 catalyst.

44 Although, the initial activity for hydrogen was recovered aftereach regeneration step, the regenerated catalyst had a significantly higherdeactivation rate. This effect was most prominent after the first catalystregeneration. The increase in deactivation rate was attributed to the sinteringof Ni. Otsuka and co-workers, on the other hand, observed an excellentstability for Ni/Al2O3 and Ni/TiO2 and catalysts for the step wise reforming

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process.45 This may be related to the following differences in the catalystreactivation procedures between the two studies:

(i) The regeneration temperature was significantly lower in the latter study(smaller exotherm expected).

(ii) Also no reduction step was employed in between the regeneration and thereaction step in the latter study (smaller exotherm expected).

Based on this, it is apparent that the exothermic catalyst reactivation reactionsneed to be appropriately controlled to avoid Ni sintering/catalyst deactivation.

While steam regeneration appears to be more benign for the catalyst life(several reaction-regeneration cycles), regeneration by air is more energy effi-cient. It may therefore be interesting to perform the regeneration using acombination of steam and air (oxy-steam regeneration). This topic deservesattention in future investigations.

2.2.3 Step-wise Reforming with CO2 Regeneration. While most of the work todate has focused on regeneration by steam/air/oxygen, few studies involvingregeneration by CO2 have also been undertaken. Takenaka et al. investigatedthe step-wise reforming reaction with CO2 on Ni/SiO2, Ni/Al2O3 and Ni/TiO2

catalysts; the methane decomposition reaction was carried out at 823 K, whilethe carbon gasification by CO2 was performed at 923 K.46,47 The supportsplayed a crucial role in determining the hydrogen production stability for theprocess. A gradual decrease in the hydrogen yield (total hydrogen produced ineach cycle) was observed for the Ni/SiO2 catalysts during consecutive reaction-regeneration cycles. However, there was no decrease in the hydrogen yield forthe Ni/Al2O3 and Ni/TiO2 catalysts for several consecutive reaction regenera-tion cycles. The author claimed a495% conversion of the carbon to CO in theregeneration step. The structural changes of Ni species for the different cata-lysts occurring during the consecutive reaction-regeneration cycles were mon-itored by XANES, XRD and Scanning Electron Microscopy (SEM) to enhancethe understanding of the role played by the different supports.48 Based on thesestudies the authors arrived at the following conclusions:

(i) Ni particles in the 60–100 nm range are most effective for the methanedecomposition reaction.

(ii) While the fresh Ni/SiO2 catalyst had Ni particles in the 40–100 nm range,consecutive reaction-regeneration reactions led to sintering/agglomerationof Ni particles (4200 nm were observed). The Ni sintering was responsiblefor the inferior performance of the Ni/SiO2 catalyst.

(iii) The Ni/Al2O3 and Ni/TiO2 catalyst on the other hand maintained theoptimized size distribution of Ni particles throughout the consecutivereaction-regeneration cycles and hence were superior catalysts.

Due to its highly endothermic nature, regeneration by CO2 is unfortunately avery energy intensive process.

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3 Production of Carbon Filaments by Catalytic Methane Decomposition

Carbon filaments (CF), due to their unique properties, have the potential to beused in several applications such as selective adsorption agents, in energystorage devices, catalyst supports and reinforcement materials.49–56 In the lastfew years significant efforts have been directed towards optimization of theprocess condition for CF formation. Researchers have investigated CF forma-tion on different metal-based catalysts. In this section, the studies related toproduction of CF (mainly issues related to rates/yields and quality) have beencategorized based on the metals used for catalyzing the methane decompositionreaction. This section will not address issues related to the CF growth mech-anism; detailed information about the CF growth mechanism may be found inreference.53

3.1 Ni-based Catalysts. – Ni-based catalysts have been by far the mostinvestigated catalysts for the CF formation via methane decomposition. Thismay be attributed to the high CF yield obtained on Ni-based catalysts. CF yieldis defined as the total amount of CF formed per gram of catalyst at completedeactivation. Since the catalyst has to be essentially replaced for subsequent CFformation, from an economics point of view it is desirable to achieve extremelyhigh CF yields.

Shaikhutdinov and co-workers have exhaustively investigated CF formationyields on co-precipitated Ni-alumina and Ni-Cu-alumina catalysts.57,58 Theamount of CF formed per gram of the catalyst was found to increase withincreasing Ni content in the Ni-alumina catalyst. However, the CF yield wasfound to be radically small for pure Ni powder.57 In good agreement, studies byToebes et al. also showed negligible CF formation from methane decomposi-tion on unsupported Ni catalysts.59 Low CF yields from methane decomposi-tion on unsupported Ni catalyst have been attributed to the presence of largeNi particles (50–1000 nm) with low index planes, since low index planes areincapable of dissociating the unreactive methane molecules. Li et al. employeda Ni-alumina catalyst prepared from Feitknecht compound for maximizing CFyields from methane.60 Similar to the work by Shaikhutdinov and co-work-ers,57 the total amount of CF formed was found to increase with increasing Nicontent of the catalyst. The total amount of CF formed was dependent on thereduction temperature as well as the reaction temperature. Although the rate ofCF formation increased at higher temperature there was a decrease in the totalyield of CF due to rapid deactivation of the catalyst.

Ermakova and co-workers manipulated the Ni particle size to achieve largeCF yields from methane decomposition.61,62 The Ni-based catalysts employedfor the process were synthesized by impregnation of nickel oxide with a solutionof the precursor of a textural promoter (silica, alumina, titanium dioxide,zirconium oxide and magnesia). The optimum particle size (10–40 nm) wasobtained by varying the calcination temperature of NiO. The 90% Ni–10%silica catalyst was found to be the most effective catalyst with a total CF yieldof 375 gCF/gcat. XRD studies by the same group on high loaded Ni-silica

176 Catalysis, 2006, 19, 164–183

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 largerparticles were found to undergo dispersion.63 The authors proposed thatdeactivation occurred when the distance separating the metal particles (presenton filament ends) increased to an extent such that reversible merging anddispersion of the Ni particles was prevented.

Several studies have considered the addition of Cu to Ni-based catalysts forenhancing the methane decomposition CF yields. Studies by Shaikhutdinovand co-workers showed that the addition of Cu decreased the rate of CFformation but greatly increased the stability of the catalyst.57 The maximumCF yield (240 gCF/gcat) was obtained for a 3% Cu-87%Ni catalyst. Li et al. alsostudied the doping effects of Cu on the CF formation.64 In this work, additionof small amounts of Cu not only increased the total amount of CF formed, butalso increased the growth rate at 873 K. This was unlike the work byShaikhutdinov et al. where addition of small amount of Cu had decreasedthe growth rate but increased the overall CF yield by significantly increasingthe life time of the catalyst.57 Addition of large amounts of Cu had adetrimental effect on the performance of the catalyst at relatively low methanedecomposition temperatures, however the high Cu content catalyst was foundto be the most effective catalyst for CF formation at higher temperature whenmethane was co-fed with hydrogen. Reshetenko et al. also used the Feitknechtcompound as a precursor for preparing copper(8–45%) promoted Ni catalystsand carried out a detailed investigation of the methane decomposition reac-tion.65 The highest CF yield (525 gCF/gcat) was obtained on the 75Ni-15Cu/Al2O3 catalyst at 898 K.

In general, Ni-based catalysts in their reduced (Ni0) forms are used for CFgeneration from methane. However, some recent studies have shown that itmay not be necessary to pre-reduce the Ni catalysts. Qian and co-workersobserved methane conversions approaching equilibrium on an unreduced Ni–Cu/Al2O3 catalyst in a fluidized bed reactor.66 The corresponding methaneconversion for the reduced catalyst was significantly lower from the onset of thereaction. The CF yields were also considerably higher for the unreducedcatalysts. The authors believed that in situ reduction of the lattice oxygen (incase of the unreduced catalyst) provided energy for the endothermic methanedecomposition process. Also, since some the hydrogen produced in the reactionwas consumed in situ, this assisted in shifting the equilibrium in the direction ofCF formation. This is in contrast to recent fixed bed reactor studies by Suib andco-workers, wherein a significantly higher initial methane conversion for thereduced Ni catalyst (40% Ni/SiO2 catalyst prepared from nitrate salts) wasobserved as compared to the unreduced version.18 However, these studies dosuggest that unreduced catalysts depending on the synthesis procedure mayprovide large CF yields. Since the pre-reduction treatment is an importantprocess parameter for CF formation, it would be worthwhile to obtain adetailed understanding apropos the effect of catalyst reduction on methaneconversions/CF yields.

177Catalysis, 2006, 19, 164–183

The large surface area of CF makes it an attractive candidate for catalystsupport material. Avdeeva and co-workers used CF with different texturalproperties as supports for Ni to study the methane decomposition reaction.67,68

Ni supported on the CF, which was obtained from methane decompositionon a Ni–Cu/Al2O3 at 898 K, showed the highest yield for secondary carbon(224 gCF/gNi). Highly porous CF supports were found to be most effective forthe secondary generation of CF.

Compared to studies related to CF optimization fewer studies have beenundertaken related to the nature/quality of the CF formed from the methanedecomposition process. Studies by Baker and co-workers on Ni and Ni–Cucatalysts revealed that the structural characteristics of CF were stronglyinfluenced by the nature of the catalyst particles.69,70 While particles rich inNi resulted in the formation of smooth filaments, Cu-rich alloy particles gaverise to filaments having a spiral conformation. The filament size (25–100 nm)was found to be strongly dependent on particle size of the catalyst. Themorphology and surface structure of CF (F1) produced on Ni-alumina cata-lysts and carbon (F2) produced in case of Ni-Cu-alumina catalysts were studiedby scanning tunneling microscopy (STM) and High-Resolution TransmissionElectron Microscopy (HRTEM) by Shaikhutdinov et al.58 The carbon surfaceof the filaments was found to be rough and was formed by misoriented edgeplanes of graphite crystallites. In case of the F1 the basal graphite planeslay inclined to the fiber axis, whereas the basal planes were perpendicular tothe filament axis for F2. HRTEM micrographs indicated a closed layer struc-ture on the edges for F2, which was contrary to the open structure observedfor graphite crystallites. Kuvshinov and co-workers observed that the CFtexture could be modified by changing the CH4:H2 feed ratio.71 Theirwork also suggested that the surface area of carbon growth centers was animportant parameter for determining the maximum CF yield on Ni catalystsusing pure methane.

From a practical view point it is essential to have an excellent understandingof the CF yields in relation to desired CF properties (surface area, structural/mechanical properties etc). Unfortunately this aspect of CF productionhas been seriously neglected. In a couple of studies, the surface area of theCF formed during methane decomposition process has been related to the CFyields.65,72 These studies clearly show that the surface area of the CF andCF yields are both strongly dependent (however in a different way) on thecatalyst and process conditions. The BET surface area of the CF obtained onthe catalyst, which showed highest CF yield (catalyst: 75Ni–15Cu/Al2O3 andyield: 525 gCF/gcat), was 233 m2/g. On the other hand, the CF with highestsurface area (286 m2/g) was obtained with a 45Ni–45Cu/Al2O3 catalyst, whichhad a corresponding CF yield of only 118 gCF/gcat. While the CF BET surfacearea was found to decrease with increasing methane decomposition tempera-tures, the CF yield was found to pass through a maximum with increasingreaction temperature.72 The above study clearly demonstrates the importanceof optimizing the CF yields and CF quality simultaneously.

178 Catalysis, 2006, 19, 164–183

3.2 Fe and Co-based Catalysts. – Catalysts which are not Ni-based have lowCF formation rates/yields. However, it is interesting to consider metals such asFe and Co since the properties of the CF depend on the metal employed forcatalyzing the methane decomposition reaction.

Ermakova and co-workers have investigated the methane decompositionreaction on un-supported Fe powder as well as supported Fe catalysts.73 Withthe exception of SiO2 support, the other supports (Fe/Al2O3, Fe/Al2O3, Fe/TiO2) gave similar CF yields as unsupported Fe powder (17 gCF/gcat). Theinteresting behavior of Fe/SiO2 system motivated them to investigate it ingreater details.74 Their studies showed that the silicates depending on theirconcentration in the catalyst could have either a promoting effect or aninhibiting effect on CF formation. The Fe/SiO2 catalyst with optimal silicatecontent showed a yield of 45 gCF/gFe. It should be noted that although the yieldof CF on Fe-based catalysts is small, Fe-based catalysts produce predominantlythin walled CF (considered to be more valuable than other CF).

Bennissad et al. investigated CF formation on Fe-based catalysts using CH4–H2 mixtures at temperatures to 1423 K.75,76 Under these conditions thickerfibers (ca. 1 m) were obtained, but when heating was stopped at 1323 K, thenormal structure of CF was observed. Shah et al. investigated CF formation onbimetallic Fe–M (M ¼ Pd, Mo or Ni) catalysts.25 The bimetallic catalysts werefound to be more active for the CF formation than the corresponding mon-ometallic catalysts. While only CF formation was observed at the methanedecomposition reaction temperature range of 973–1073 K, amorphous carbonand carbon flakes were observed concomitant with CF at reaction temperaturesabove 1173 K.

Otsuka and co-workers have recently investigated the structural changes ofFe species and the nature of the CF formed during the methane decompositionreaction on Fe2O3/Al2O3 and Fe2O3/SiO2 catalysts.

77 XANES studies showedthat during the methane decomposition reaction the smaller sized Fe particleswere transformed into Fe3C while the larger particles were converted intocarbon atom saturated g-Fe species. The supports had a profound effect indetermining the nature of the CF formed in the process; multi-walled CF andchain like carbon fibers were formed on Fe/Al2O3, while CF composed ofspherical carbon units were formed along with chain like carbon fibers onFe2O3/SiO2 catalysts.

Avdeeva and co-workers studied the methane decomposition reaction on Co-alumina catalysts with varying concentration of Co.78 The CF formation wasfound to be maximized at 60–75% content of Co. No induction period wasobserved for the Co-alumina catalysts, which was contrary to their previousexperience involving Ni-alumina catalysts.57 Also in this case a different varietyof filaments (not observed on Ni-based catalysts) with hollow-like core mor-phology were observed. Takenaka et al. have recently investigated the effect ofsupports on CF formation for Co-based catalysts.79 The Co/Al2O3 and Co/MgO catalysts were found to be superior to the Co/SiO2 and the Co/TiO2

catalysts for CF formation. Based on catalyst characterization studies the

179Catalysis, 2006, 19, 164–183

authors claimed that the 10–30 nm range for Co particles was preferred for CFformation. The authors also found that the temperature had a significantinfluence in determining the nature of the CF. Multi-walled CF were formed inthe temperature range of 873–973 K, whereas helically coiled and bamboo-likeCF were preferentially formed at 1073 K.

Smith and co-workers investigated the effect of metal support interaction onthe CF formation on a series of Co-silica catalysts.80 The metal supportinteraction was manipulated by addition of either BaO, La2O3 or ZrO2 to silica.The rate of catalyst deactivation was found to increase with the increase in themetal support interaction. Competition between CF formation and encapsulat-ing carbon formation controlled the catalyst deactivation rate. In case of thecatalysts with high metal support interaction, the encapsulating carbon forma-tion was dominant and hence led to a rapid deactivation of the catalyst.

It is unfortunate that very few studies have been undertaken which relate themethane decomposition process conditions and CF yields to the CF quality(surface area, structure/texture, mechanical strength etc). From a processapplication view point it is extremely important to comprehensively investigatethis aspect of CF formation.

4 Concluding Remarks

Catalytic methane decomposition has received considerable attention in recentyears. The reaction has been investigated for two main applications (a)production of hydrogen and (b) synthesis of carbon filaments. The importantconditions necessary for clean hydrogen production are as follows:

(i) High conversion of methane (to COx-free hydrogen) to avoid costlyproduct separation.

(ii) Absence of pressure drop issues across reactor(iii) Effective regeneration of catalyst(iv) Stable life of catalyst over several cycles

While the methane decomposition step has been extensively investigated,unfortunately less attention has been devoted to other aspects. To avoid apressure drop it is important to optimize the run lengths for the methanedecomposition step and the regeneration step. In order to assess the commercialviability of the process, it is important to study the process over several cycles(4100). If the metal-based catalysts are not reduced in between the decompo-sition and deactivation stage, significant amount of CO may be formed due toreaction of the metal-oxide with methane during the initial stages of themethane decomposition reaction. Most studies have employed argon gas witha thermal conductivity detector to analyze product gases. Such an analysisprocedure is not astute for accurate quantification of ppm levels of CO/CO2.Special attention should be paid towards this analysis as the main advantageclaimed in this process is the production of clean hydrogen (without furtherneed for purification).

180 Catalysis, 2006, 19, 164–183

Following are the important conditions necessary for carbon nanotubesynthesis

(i) High yield of CF(ii) CF with desired properties

While several studies have been undertaken to optimize the CF yield, only a fewstudies have addressed the quality factor for CF. It is important to tune thesynthesis procedures such that CF yields/properties are consistent with thedesired applications.

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