Applied Catalysis A: General 349 (2008) 116–124

Methane decomposition and catalyst regeneration in a cyclic mode oversupported Co and Ni catalysts

Jerry Li, Kevin J. Smith *

Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver V6T 1Z3, Canada

A R T I C L E I N F O

Article history:

Received 27 March 2008

Received in revised form 4 July 2008

Accepted 15 July 2008

Available online 23 July 2008

Keywords:

Methane decomposition

Partial oxidation

Carbon

Catalyst

Cobalt

Nickel

Cyclic reaction

A B S T R A C T

CH4 decomposition (CH4! C + 2H2) on supported Co and Ni catalysts has been studied using a fixed-bed,

oscillating microbalance reactor and a 5% CH4 in He feed reacted at 773 K and 101 kPa. After 45 min

reaction the catalysts were regenerated by reaction of the deposited carbon with O2 or CO2 at 773 K and

101 kPa. The effect of repeated CH4 decomposition–carbon oxidation cycles is reported. At the chosen

conditions, Ni was more active and more stable than Co during the CH4 decomposition step. Although

>90% of the carbon deposited on both Co and Ni was removed by reaction with O2, an oxidation of the

active metal also occurred. In subsequent CH4 decomposition steps, an induction period was necessary to

re-reduce the Co. Furthermore, the oxidized metal reacted with CH4 producing CO and CO2 as an impurity

in the H2. Removal of the carbon deposit by reaction with CO2 rather than O2 significantly decreased the

CO contamination of the produced H2 on the Ni catalyst. However, the carbon removal rate was 20�’s

slower with CO2 compared to O2. Several CH4 decomposition–carbon oxidation cycles were completed on

the Ni catalyst without a significant loss in activity, whereas the Co catalyst deactivated.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

Production of high purity H2 at low cost is a key requirement forthe development of the H2 economy [1]. Current thermochemicalprocesses for H2 production include steam reforming of CH4,partial oxidation of CH4 and CO2 reforming of CH4. Catalyticdecomposition of CH4 to C and H2 is a potential alternative route tohigh purity H2. Since only H2 and carbon are formed in thedecomposition process, purification of H2 is not required [2,3] andsince no CO is present, the H2 can be directly used for PEM fuelcells. However, large amounts of carbon produced during thisendothermic reaction lead to catalyst deactivation. Consequently,two-step processes have been proposed in which catalyticdecomposition of CH4 (CH4! C + 2H2) is followed by a secondstep that regenerates the catalyst by introducing O2, steam or CO2

to remove the deposited carbon by oxidation [2,4–7]. The metalcatalysts are also re-oxidized in the regeneration step and they arenormally re-reduced in H2 prior to repeating the CH4 decomposi-tion step [2,5]. In one study [8], no re-reduction was performed butthe H2 purity generated under these conditions was not reported.The ability to operate the two-step process without the need for anintermediate re-reduction of the catalyst in H2, could have

* Corresponding author. Tel.: +1 604 822 3601; fax: +1 604 822 6003.

E-mail address: kjs@interchange.ubc.ca (K.J. Smith).

0926-860X/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.07.011

important practical implications for a cyclic process. For example,continuous operation in a reactor-regenerator mode, similar tothat used in fluid catalytic cracking may then be possible.

In the present work, we report on the stability of supported Coand Ni catalysts during repeated cycles of CH4 decompositionfollowed by removal of the deposited carbon by reaction with O2 orCO2. The catalysts were not re-reduced following the carbonremoval step. Unlike most previous studies [2,4–8], the gaseousproducts were monitored continuously by mass spectrometry(MS) while simultaneously measuring the mass change of thecatalyst, using a tapered element, oscillating microbalance thatwas configured to operate as a fixed-bed micro-reactor. Co and Nicatalysts supported on Al2O3 [9–12], modified by the addition ofMgO and CeO2, have been investigated. MgO was used to enhancethe reducibility of the metal oxide precursor [13] and CeO2 wasintended to promote COx formation in the oxidation step [14–16].

2. Experimental

The supported Co and Ni catalysts were prepared by stepwiseincipient wetness impregnation. Pre-dried (373 K for 8 h) g-alumina (Sasol Germany GmbH, +95%) with a surface area of211 m2/g and pore volume of 0.814 mL/g was used as the support.The g-alumina was impregnated with a 1.1-M aqueous solution ofMg prepared from Mg(NO3)2�6H2O (BDH Chemical Ltd., 99%). Afterimpregnation, the sample was dried in air at 373 K for 2 h, and then

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124 117

calcined at 1073 K for 8 h. The modified alumina support (MgO/Al2O3), obtained after calcination, had a MgO content of 6.50 wt.%.A second impregnation of the MgO/Al2O3 support was carried outwith an ethanol (Aldrich, 95%) solution of 0.75 M Ce prepared fromCe(NO3)3�6H2O (Aldrich, 99%). After impregnation, the sample wasdried in air at 373 K for 8 h and then calcined at 748 K for 3 h. Themodified alumina support (CeO2/MgO/Al2O3) obtained aftercalcination had a CeO2 loading of 14.4 wt.%.

The Ni or Co was then added in a final impregnation. The Cocatalyst was prepared by impregnation of the CeO2/MgO/Al2O3

support with a 1.85-M aqueous solution of Co prepared fromCo(NO3)2�6H2O (Acros Organics, ACS Reagent). The metal loadingwas 12 wt.% Co on the CeO2/MgO/Al2O3 support. The Co catalystswere vacuum-dried at 378 K for 37 h and then calcined at 723 K for10 min. The Ni catalyst was prepared by impregnation of the CeO2/MgO/Al2O3 support with a 1.79-M aqueous solution of Ni preparedfrom Ni(NO3)2�6H2O (Aldrich, 99.999%). The metal loading was12 wt.% Ni on the CeO2/MgO/Al2O3 support. The Ni catalysts weredried in air at 383 K for 37 h and then calcined at 723 K for 10 min.

Temperature programmed reduction (TPR) of the calcinedcatalyst precursors was performed in a stainless steel micro-reactor (length = 50 mm and inside diameter = 7.7 mm). Approxi-mately 0.30 g of a catalyst precursor was loaded into theisothermal zone of the reactor and heated from room temperatureto 623 K at a rate of 10 K/min and held at 623 K for 60 min under60 mL/min Ar (Praxair, UHP, 99.999%) flow. The catalyst was thencooled to room temperature and the gas stream switched to 10%H2/90% Ar (Praxair, UHP, 99.999%). The catalyst was heated at 10 K/min to 1007 K in 60 mL/min of the reducing gas. The effluent gaspassed through a drying trap to a thermal conductivity detector(TCD) to quantify the H2 consumption. The degree of reduction wascalculated from the ratio of the actual H2 consumption to thetheoretical H2 consumption needed to completely convert themetal oxide to metal. The measured H2 consumption wascalibrated using completely reducible Cu2O (Rocky Mt. ResearchInc. 99.999%) as a standard.

X-ray diffraction (XRD) patterns of the prepared catalysts wereobtained with a Rigaku Multiflex diffractometer using Cu Karadiation (l = 1.5406 A), a scan range of 2u from 208 to 808 with astep size of 0.048. Particle size of the metal or metal oxide wasobtained from the XRD data using the Scherrer equation. Theformation of carbon deposits was examined by transmissionelectron microscopy (TEM) using a FEI TECNAI G2 electronmicroscope with an acceleration voltage of 200 kV. TEM specimenswere prepared by grinding the catalyst to a fine powder beforedispersing the catalysts in ethanol (Aldrich, 95%) and applying onedrop of the suspension onto a 200-mesh copper grid with aFormvar coating, stabilized with evaporated carbon film.

The single-point BET surface area of the catalysts was measuredusing a Flowsorb II 2300 Micromeritics analyzer and a 30-mol% N2/70 mol% He mixture at a flow of 15 mL (STP)/min. The sample wasdegassed at 398 K for approximately 3 h prior to measurement.

A tapered element oscillating microbalance (TEOM, Rupprecht& Patashnick Co., Inc. TEOM series 1500 Pulse Mass Analyzer) wasused for reaction. The TEOM is configured as a fixed-bed micro-reactor in which the real time mass change of the solid catalystplaced in the reactor can be determined. CH4 decomposition andcarbon oxidation were measured in the TEOM at atmosphericpressure, monitoring catalyst mass change while also using aquadrupole mass spectrometer (VG ProLab) for continuousproduct gas analysis. Approximately 0.10–0.15 g of calcinedsupported metal catalyst was loaded into the TEOM. The calcinedcatalyst was reduced in situ by flowing 40 vol.% H2 at 823 K and300 mL(STP)/min for approximately 2 h. After reduction, He wasintroduced to the TEOM for 15 min to flush any residual H2. The

temperature was adjusted to the designated temperature for CH4

decomposition during this He flush period. Subsequently, a 5% CH4

in He gas mix at 200 mL(STP)/min was flowed through the reactorbed for up to 45 min, such that the total carbon deposited wasabout 12 mg. On the Ni catalyst a time of 25 min was required,whereas on the Co catalyst a period of 45 mins was required. Oncethe CH4 was introduced, the mass change of the catalyst wasmeasured by the TEOM and the effluent gases were analyzed by theMS, recording the intensity of mass peaks corresponding to H2, He,CH4, H2O, CO, CO2 and O2. After the CH4 decomposition step wascompleted, He was introduced to flush residual gases from thesystem and the temperature was again adjusted to the desiredtemperature for the subsequent regeneration step (773 K). Duringthe regeneration step, O2 or CO2 was introduced to the TEOM andthe mass change of the catalyst and gas concentration were bothmeasured. In the present work, a single CH4 decomposition stepand a subsequent partial oxidation step is referred to as one cycle.Upon completion of the 1st cycle, a He flush was again required toremove residual gases and stabilize the reactor temperature.Importantly, the second CH4 decomposition step was then startedwithout an intermediate re-reduction of the catalyst in H2. Thesequence of CH4 decomposition–carbon oxidation was repeatedfor several cycles.

The catalysts examined by temperature programmed oxidation(TPO) were reduced in the TEOM in a flow of 40% H2 at 823 K for 2 hfollowed by cooling to 323 K under He flow and holding at thistemperature for 15 min. The reactant gas was then switched fromHe to 1% O2/99% He (Praxair, UHP, 99.999%) and the reduced Nicatalyst was held at 323 K for 8 min followed by heating from323 K to 823 K at a ramp rate of 5 K/min, holding at 823 K for15 min. The mass profile obtained by TEOM during the TPOrepresents the mass change due to the oxygen uptake of thecatalyst. A blank run was also performed in order to obtain themass change due to the rise in reactor temperature. The adjustedmass change of the catalyst during TPO was obtained bysubtracting the data from the blank run.

3. Results and discussion

3.1. Catalyst properties

The TPR profiles of the Co and Ni catalysts are compared inFig. 1. The degree of reduction of the Co3O4 increased from 45%(Fig. 1A) to 72% when MgO was added to the Al2O3 support (Fig. 1B)and was 73% for the CeO2/MgO/Al2O3 supported Co3O4 catalystprecursor (Fig. 1C and Table 1), calculated after correction for theCeO2 reduction shown in Fig. 1E. Data reported for the TPR of12 wt.% CeO2/Al2O3 [17] showed reduction peaks at 818–975 K and1147–1190 K, due to a strong interaction between CeO2 and Al2O3.In the present study, the interaction between CeO2 and Al2O3 wasreduced by the presence of MgO and consequently, reductionoccurred at approximately 600 K and 900 K (Fig. 1E).

TPR of supported Co3O4 occurs in two steps [18]: (i)Co3O4 + H2! 3CoO + H2O and (ii) 3CoO + 3H2! 3Co + 3H2O. Thefirst step is faster and results in a sharp, low temperature peak,whereas the second step involves reduction of CoO that tends tohave a stronger interaction with the support, resulting in a broader,high temperature peak. The theoretical H2 consumption ratio in (i)and (ii) is 1:3 whereas the measured ratio (Fig. 1C) was 1:3.4. Fig. 2shows the XRD patterns of the calcined Co catalyst precursor, thereduced Co catalyst and the same catalyst after reaction. Thepresence of Co3O4 in the calcined precursor is confirmed andreduction of Co3O4 to Co is also evident (Fig. 2B), although asignificant portion of Co3O4 remained after reduction, in agree-ment with the incomplete reduction measured by TPR.

Fig. 1. Temperature-programmed reduction of Co and Ni catalysts. The catalyst

samples were heated at a rate of 10 K/min in a 10% H2/90%Ar gas mixture at

60 mL(STP)/min. (A) Calcined Co–Al2O3 catalyst precursor, (B) calcined Co–MgO/

Al2O3 catalyst precursor, (C) calcined Co–CeO2/MgO/Al2O3 catalyst precursor, (D)

calcined Ni–CeO2/MgO/Al2O3 catalyst, and (E) calcined CeO2/MgO/Al2O3 catalyst

support.

Fig. 2. X-ray diffractograms of Co catalysts; *, CeO2 and�, Al2MgO4. (A) Calcined Co–

MgO/Al2O3 catalyst precursor, (B) reduced Co–MgO/Al2O3 catalyst, (C) calcined Co–

CeO2/MgO/Al2O3 catalyst precursor, and (D) Co–CeO2/MgO/Al2O3 catalyst after CH4

decomposition at 773K and carbon removal with O2 at 773 K.

Fig. 3. X-ray diffractograms of Ni catalysts before and after CH4 decomposition and

carbon oxidation in cyclic mode; *, CeO2 and �, Al2MgO4. (A) Calcined Ni–CeO2/

MgO/Al2O3 catalyst precursor, (B) reduced Ni–CeO2/MgO/Al2O3 catalyst, (C) Ni–

CeO2/MgO/Al2O3 catalyst after CH4 decomposition at 773 K, (D) Ni–CeO2/MgO/

Al2O3 catalyst after carbon removal with O2 at 773 K, and (E) Ni–CeO2/MgO/Al2O3

catalyst after carbon removal with CO2 at 773 K.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124118

Accounting for the H2 consumption by CeO2 reduction, thedegree of reduction of the NiO for the 12 wt.% Ni–CeO2/MgO/Al2O3

catalyst shown in Fig. 1D was 85%. The first peak of the reductionprofile of the calcined Ni–CeO2/MgO/Al2O3 precursor is assigned tothe reduction of bulk NiO, whereas the second and third peaks areattributed to a stronger interaction between smaller NiO particlesand the CeO2 surface of the support. The data also show that theNiO is relatively easy to reduce compared to CeO2, since thetemperature associated with the reduction peak of NiO (500 K) waslower than that of CeO2 (600 K).

The XRD data of the calcined catalyst precursor (Fig. 3A) and thereduced Ni–CeO2/MgO/Al2O3 (Fig. 3B) confirmed the high degreeof reduction measured by TPR, with no NiO peaks present in thereduced sample. Furthermore, the peak intensity for CeO2

decreased for the reduced Ni–CeO2/MgO/Al2O3 compared to thecalcined Ni–CeO2/MgO/Al2O3, indicative of some CeO2 reductionobserved during TPR as well.

Table 1 summarises the properties of the Co and Ni catalysts. TheCo crystallite size, as estimated by X-ray line broadening, was similarto that of the Ni and the degree of reduction of the Co catalyst wasless than that of the Ni, indicating that overall the Ni catalyst had asignificantly higher number of active sites than the Co.

Table 1Properties of the supported Co and Ni catalysts used in the present study

Co–CeO2/MgO/Al2O3 Ni–CeO2/MgO/Al2O3

BET area of reduced catalyst (m2/g) 111

Degree of reduction (mol%) 73 85

Crystallite size (nm) Co3O4 Co NiO Ni

Calcined catalyst 14.6 11.0a 14.2 12.0a

Reduced catalyst – – – 15.9

Used catalyst after CH4 decompositionb – – – 26.1

Used catalyst after oxidationb 12.1 9.1a 18.3 15.4a

a Calculated from the oxide assuming a Ni/NiO molar volume ratio of 0.84 and a Co/Co3O4 ratio of 0.75.b After 4 CH4 decomposition and carbon oxidation cycles.

Fig. 4. CH4 decomposition activity on Co–CeO2/MgO/Al2O3 catalyst: (A) and Ni–CeO2/MgO/Al2O3 catalyst (B) measured at 773 K using 5%CH4/He at 200 mL(STP)/min with

0.15 g catalyst. Data show catalyst mass and exit gas molar flows rates of CH4, H2, CO and CO2.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124 119

3.2. Comparison of Co and Ni catalysts

Fig. 4 compares the Co and Ni catalyst activities by presentingthe measured catalyst mass and the molar flow rates of the gasesleaving the reactor versus time-on-stream during CH4 decom-position in the 1st cycle. The reaction was performed immediatelyafter the in situ H2 reduction of the calcined catalyst precursor. Theinitial H2 molar flow rates observed for Co and Ni catalysts weredifferent, and the decline in H2 molar flow rate with time indicatedthat a much faster deactivation occurred on the Co catalyst thanthe Ni catalyst. In previous work, catalyst deactivation on the Cowas ascribed to the formation of encapsulating carbon [19],whereas on Ni catalysts, encapsulating carbon forms at highertemperatures [5]. At 773 K, the initial CH4 decomposition rate onthe Ni catalyst was 12.3 mmol/(g cat. s), faster than that on the Cocatalyst (9.4 mmol/(g cat. s)), and comparable to values of16.7 mmol/(g cat. s) reported for Ni/HZSM-5 [5] and 7.0 mmol/(g cat. s) reported for Ni/CeO2 [20] at the same reaction tempera-ture. At the end of the 45 min CH4 decomposition step, the amountof carbon deposited on the Co was 0.72 gC/gCo whereas after the25-min CH4 decomposition step on Ni, the carbon deposited was0.97 gC/gNi. The cumulative H2 yield, expressed as the H2/M moleratio (M = Co or Ni), was 7.1 and 9.5 on the Co and Ni, respectively.Also note that Fig. 4 shows that COx was not detected in the productgenerated over the Ni catalyst, whereas a small amount of CO wasproduced in the first 10 min of reaction over the Co catalyst, with apeak concentration of about 0.17 vol.% CO in the product gas.Contamination of the produced H2 by CO was reported previouslyby Choudhary et al. [5] on Ni catalysts. In that study, CO formationwas favored at high temperatures and was shown to be dependentupon the support [5]. The source of CO was ascribed to –OH groupspresent on the H-ZSM5 and SiO2 supports [5]. In the present work,no CO contamination was observed on the Ni catalyst that hadbeen reduced in H2, likely because of the lower reaction

temperature used for CH4 decomposition, and the differentthermal treatments of the catalyst support.

In several previous studies on Ni catalysts, much higher yieldsof H2 and carbon were reported because the CH4 decompositionstep was carried out in pure CH4 and/or was continued until thecatalyst was completely deactivated by the carbon deposit [2,6–8].For example, Takenaka et al. [7] reported a H2 yield ofapproximately 1792 mol H2/mol Ni on Ni/SiO2 catalyst operatedat 823 K for a period of 140 min. At 973 K, Ogihara et al. [21]reported a much lower yield (0.1 mol H2/mol Ni) on Ni/Al2O3

catalysts as a consequence of rapid catalyst deactivation at thehigher temperature. At the same temperature, Venugopal et al.[22] reported a maximum yield of 606 mol H2/mol Ni for a 30-wt.%Ni/SiO2 catalyst. The wide range of yields is partly a consequence ofvariations in metal crystallite size, metal loading, support andreaction conditions. High carbon loading makes thermal sinteringduring the regeneration step more likely [6,7] because of the highheat release associated with the oxidation of carbon. Furthermore,in a practical cyclic operation, the CH4 decomposition reactionwould not necessarily be conducted to the point of completedeactivation of the catalyst. Indeed, recycling the catalyst with lowcarbon yield for regeneration prior to complete deactivation maybe beneficial and it is for these conditions that the results of thepresent study are most pertinent.

The focus of the present work was to determine the stability ofthe Co and Ni catalysts for stepwise CH4 decomposition and carbonoxidation. Accordingly, following CH4 decomposition, the catalystswere exposed to O2 to remove the deposited carbon. Fig. 5compares the change in catalyst mass and the COx productsgenerated over the Co and Ni catalysts during the 1st oxidationstep at 773 K. In both cases, CO2 was the major product with ahigher CO selectivity on the Ni catalyst than the Co catalyst. Theoxidation occurred at a higher rate on the Ni catalyst compared tothe Co catalyst (as deduced from the slope of the catalyst mass

Fig. 5. Oxidation of carbon deposit from Co–CeO2/MgO/Al2O3 catalyst (A) and Ni–CeO2/MgO/Al2O3 catalyst (B) measured at 773 K using 5%O2/He at 200 mL(STP)/min with

0.15 g catalyst. Data show catalyst mass and exit gas molar flows rates. These data were measured after the 1st CH4 decomposition step reported in Fig. 4.

Fig. 6. CH4 decomposition activity during the 2nd cycle measured on Co–CeO2/MgO/Al2O3 catalyst (A) and Ni–CeO2/MgO/Al2O3 catalyst (B) at 773 K using 5%CH4/He at

200 mL(STP)/min with 0.15 g catalyst. Data show catalyst mass and exit gas molar flows rates of CH4, H2, CO and CO2.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124120

Fig. 7. Temperature-programmed oxidation of (A) 12 wt.% Ni–CeO2/MgO/Al2O3, (B)

12 wt.% Ni–MgO/Al2O3 and (C) the difference between A and B. The catalyst sample

was heated at a rate of 10 K/min in a 1% O2/90%He gas mixture at 60 mL(STP)/min.

Fig. 8. Measured versus calculated catalyst mass increase during TPO of (i) 12 wt.%

Ni–CeO2/MgO/Al2O3, (ii) 12 wt.% Ni–MgO/Al2O3 and (iii) 5 wt.% Ni–CeO2/MgO/

Al2O3. The calculated mass was based on the oxidation of the Ni present in the

catalyst.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124 121

versus time plots of Fig. 5), and the reaction was completed within2.5 min on the Ni catalyst (Fig. 5B) compared to 4.2 min on the Cocatalyst (Fig. 5A). The overall reduction in catalyst mass shown inFig. 5, when corrected for the mass gain by the oxidation of thereduced Co or Ni, corresponded to>90% carbon removal from bothcatalysts by oxidation.

Following removal of the carbon, the second reaction cycle wasinitiated without a re-reduction of the catalyst. Fig. 6 shows themeasured activity profiles during CH4 decomposition and clearlythe catalyst activity in the 2nd cycle was different to that observedin the 1st cycle (Fig. 4), especially for the Co catalyst (Fig. 6A).Fig. 6A shows that the Co catalyst had no significant activity whenthe 2nd cycle of CH4 flow was first introduced into the reactor.However, following an induction period of about 30 min, duringwhich there was no measurable change in the catalyst mass nor theeffluent gas composition, the CH4 decomposition reaction started.Simultaneously, significant quantities of CO and CO2 wereproduced and the catalyst mass decreased momentarily, beforeincreasing as more carbon was deposited. These data suggest aslow reduction of the Co3O4 by CH4, possibly due to a slowdiffusion of CH4 through a carbon overlayer to the Co3O4, followedby CH4 decomposition to yield C and H2. The produced H2 thenrapidly reduces the remaining Co3O4. The maximum CO concen-tration in the effluent of 0.24 vol.% occurred almost immediatelywith the consumption of CH4, and the COx content of the effluentthen decreased slowly as the reaction proceeded. Note, however,that on the Ni catalyst no induction period occurred—immediatelyupon introduction of the CH4 the CO, CO2 and H2 products wereobserved. The catalyst mass also showed an initial decreaseconsistent with the reduction of NiO and the maximum COconcentration in the effluent was 0.57 vol.%. On Ni, both CO andCO2 present in the effluent decayed to zero much more rapidlythan what was observed on the Co catalyst. In addition, the Nicatalyst produced much more CO2 than the Co catalyst. Theseresults demonstrate a more complete and faster reduction by CH4

of NiO than Co3O4. Furthermore, the absence of an induction periodon the Ni catalyst indicated that after oxidation, the supported Nicatalyst was readily reduced by CH4, as reported in the literaturepreviously [6–8], whereas the Co catalyst was not. Also note thatthe Co catalyst had more encapsulating carbon deposited duringCH4 decomposition, compared to the Ni catalyst, as evidenced bythe rapid deactivation of Co shown in Fig. 4A. The induction periodobserved on the Co catalyst, which notably increased as thenumber of decomposition–oxidation cycles increased, suggeststhat the encapsulating carbon was not readily removed duringoxidation, resulting in the rate of the subsequent reduction ofCo3O4 being determined by a slow diffusion rate of CH4 through acarbon overlayer. As the number of cycles increased the overlayerof encapsulating carbon would grow, leading to a longer diffusionpath of the reactant to the Co3O4 particles.

Prior to the 1st cycle, the Ni catalyst precursor had been reducedin H2 and the extent of reduction was 85%. The absence of COx

during the 1st CH4 decomposition step therefore implies that anyun-reduced NiO, the support (Al2O3) and the promoters (MgO andCeO2) were not further reduced by CH4 at the reaction conditions.During the subsequent oxidation, the uptake of O2 by the catalyst istherefore most likely related to the oxidation of the Ni and willdepend on the Ni content of the catalyst, rather than the Al2O3 orthe MgO. The TPR data of Fig. 1 also suggests that some O2 uptakemay be due to the re-oxidation of Ce species. TPO was used toexamine the oxygen uptake of the Ni catalysts. TPO was carried outon 12 wt.% Ni–CeO2/MgO/Al2O3, 12 wt.% Ni–MgO/Al2O3 (i.e. noCeO2) and 5 wt.% Ni–CeO2/MgO/Al2O3 catalysts. Fig. 7 shows themass of the Ni catalyst with (Fig. 7A) and without CeO2 (Fig. 7B)during TPO. The difference between the two mass profiles (Fig. 7C)

shows a peak at 450 K, ascribed to oxygen uptake by the reducedCeO2. Calculation showed that the mass increase due to thereduced CeO2 was<15% of the overall mass increase of the reducedNi–CeO2/MgO/Al2O3, suggesting that the oxygen uptake by thereduced CeO2 was low compared to the oxygen uptake by the Ni (asshown in Fig. 7A and B). Furthermore, Fig. 8 shows the correlationbetween the overall increase in mass of each catalyst measured bythe TEOM during TPO and the corresponding theoretical increasein mass due to the oxidation of Ni, calculated according to the Nicontent of the catalyst. The high correlation coefficient (R2 = 0.99)associated with the linear fit indicates that the overall increase inmass during TPO is strongly related to the Ni content of the catalystand not the support. Thus, the oxygen uptake of the reducedcatalyst was dominated by the content of Ni and was due to theoxidation of Ni to NiO. Also, in the 2nd CH4 decomposition step(Fig. 6B) the total amount of O present in the produced COx

(210 mmoles) was almost equivalent to the Ni content of thecatalyst (204 mmoles). Hence we conclude that the NiO reductionby CH4 and the associated release of oxygen as COx will also dependon the metal content of the catalyst. The reduction and oxidation of

Fig. 9. Initial CH4 decomposition activity and total H2 produced during the CH4

decomposition step of each of several cycles of CH4 decomposition and carbon

oxidation. (&) Co–CeO2/MgO/Al2O3 catalyst and 5%CH4/He at 200 mL(STP)/min

reacted at 773 K for 45 min followed by 5%O2/He at 200 mL(STP)/min at 773 K for

45 min. (&) Ni–CeO2/MgO/Al2O3 catalyst and 5%CH4/He at 200 mL(STP)/min

reacted at 773 K for 25 min followed by 10%O2/He at 200 mL(STP)/min at 773 K for

5 min.

Fig. 11. Cumulative CO/H2 molar ratio calculated from the total H2 and CO produced

at 773 K during each CH4 decomposition step of several repeated CH4

decomposition/carbon oxidation cycles over (&) Ni–CeO2/MgO/Al2O3 and (&)

Co–CeO2/MgO/Al2O3 catalysts. Reactions conditions as stated in Fig. 9.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124122

the Ni catalyst during each cycle were confirmed by the XRDanalysis shown in Fig. 3.

The initial CH4 decomposition rate and the total H2 producedduring the CH4 decomposition period are compared for a series ofrepeated decomposition/oxidation cycles over the Co and Nicatalysts in Fig. 9. The data show that deactivation occurred on theCo catalyst as the number of cycles increased, whereas nosignificant deactivation had occurred on the Ni catalyst after 6cycles. However, after the 1st cycle, some COx was generatedduring the subsequent CH4 decomposition steps of each cycle andFig. 10 shows the extent of the contamination of the produced H2

by reporting the cumulative CO/H2 molar ratio calculated from thetotal H2 and CO produced during each CH4 decomposition step.Clearly, the amount of COx increased with the number of cycles onCo, whereas on Ni there was a slight decrease after the 2nd cycle.

Fig. 11 shows similar differences between the Co and Nicatalysts in the oxidation step over repeated cycles. The CO

Fig. 10. Initial C oxidation rate, CO selectivity and total carbon removal during the

carbon oxidation step of each of several cycles of CH4 decomposition and carbon

oxidation over (&) Ni–CeO2/MgO/Al2O3 and (&) Co–CeO2/MgO/Al2O3 catalysts.

Reactions conditions as stated in Fig. 9.

selectivity and the carbon oxidation rate remained relativelyconstant on the Ni catalyst, whereas both decreased on the Cocatalyst as the number of cycles increased. The carbon removalduring each oxidation cycle was >90% for both catalysts, andremained relatively constant from cycle to cycle. As indicatedpreviously, in the case of Co catalyst, the presence of encapsulatingcarbon that is not completely removed by the oxidation, mayexplain the more rapid deactivation of the Co catalyst compared tothe Ni catalyst.

X-ray diffractograms of the Ni catalyst, following CH4 decom-position and oxidation, are shown in Fig. 3. After CH4 decomposi-tion the presence of graphitic carbon was identified by a peak atapproximately 2u = 268, as shown in Fig. 3C. During partialoxidation, the carbon generated by CH4 decomposition reactedwith O2, and therefore, Fig. 3D shows no graphite peak, whereasthe oxidation of Ni to NiO is clear. Table 1 compares the Ni particlesize estimated from XRD after reaction. The data show that the Nicrystallite size increased after CH4 decomposition compared to thereduced catalyst, and the TEM images of Fig. 12 show qualitativelythe same trend of Ni particle size change among the samples.However, after oxidation the equivalent Ni particle size (estimatedfrom the NiO line-broadening) was very similar to that of thereduced catalyst. In previous work, the loss in activity of Nicatalysts during repeated CH4 decomposition-carbon oxidationcycles [6] was ascribed to increases in the Ni particle size as aconsequence of sintering that occurred during the carbon removalstep. The data of the present study show that with low carbonloadings, removal of the carbon and catalyst regeneration can beachieved without a significant growth in the Ni particle size, andhence the Ni catalyst remains active during several repeated CH4

decomposition–carbon oxidation cycles.In summary, the Ni catalyst showed a better performance in the

cyclic reactions examined herein compared to the Co catalystbecause of a higher CH4 decomposition activity, facile reduction ofthe NiO by CH4, and better stability as indicated by a higher initialCH4 decomposition rate and H2 production from cycle to cycle.

3.3. Comparison of catalyst regeneration by O2 and CO2

The reaction of the deposited carbon with O2 converted the Ni(or Co) to NiO (or Co3O4) and the metal oxide provided the oxygenthat resulted in the formation of COx during the subsequent CH4

Fig. 12. TEM micrographs of Ni–CeO2/MgO/Al2O3 catalyst (A) after reduction; (B) after CH4 decomposition and (C) after carbon oxidation with O2/He.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124 123

decomposition step. To reduce the formation of COx during CH4

decomposition, the formation of the metal oxide during the partialoxidation step must be minimized. An alternative approach forcarbon removal that offers less likelihood of metal oxide formationis to react the deposited carbon with CO2 instead of O2 [6,7]. In thepresent study, the calcined Ni catalyst precursor was reduced in

situ as before prior to cyclic CH4 decomposition followed by carbonremoval with CO2. A high CO2 concentration (40 vol.%) and longreaction duration (100 min) were applied at 773 K to ensure thatthe reaction between the deposited carbon and CO2 was complete.

Fig. 13. Comparison of CH4 decomposition and carbon removal cycles over Ni–CeO2/MgO

removal with 10% O2/He at 773 K for 5 min, (~) carbon removal with 40% CO2/He at 7

Fig. 3D and E compares the XRD of the Ni catalyst after the finalcycle of carbon removal by reaction with O2 or CO2, respectively.NiO was identified on the catalyst for which the deposited carbonwas removed by partial oxidation with O2 but no Ni peak waspresent, whereas, the XRD of the catalyst reacted with CO2 onlyshowed the presence of Ni. The data indicate that removing thedeposited carbon with CO2 limited the formation of NiO, to thepoint that it was not identified by XRD, and with a limitedformation of NiO after carbon removal by CO2, the formation of COduring the subsequent CH4 decomposition was reduced.

/Al2O3 catalyst using different oxidation conditions for carbon removal: (~) carbon

73 K for 40 min.

J. Li, K.J. Smith / Applied Catalysis A: General 349 (2008) 116–124124

Fig. 13B shows a lower CO/H2 ratio in the product duringsubsequent CH4 decomposition cycles after reaction in CO2

compared to O2. Similar, high carbon removal percentages wereachieved with CO2 and O2 (Fig. 13D) and hence the number ofregenerated active Ni sites should be the same as what wereavailable initially. Consequently a similar initial CH4 decomposi-tion rate was observed for both experiments as shown in Fig. 13A.However, the carbon removal rate by CO2 was much slower (by afactor of approximately 20) compared to that obtained with O2, asshown in Fig. 13C.

Although the amount of CO in the produced H2 was reduced byreacting deposited carbon with CO2 instead of O2, therebyminimizing the formation of NiO, the carbon removal rate by CO2

was very low. Furthermore, unlike the exothermic O2 reaction,which can provide heat for the CH4 decomposition reaction, the CO2

reaction is highly endothermic. Both factors limit the use of CO2 asthe oxidant for cyclic methane decomposition and carbon oxidation.

4. Conclusions

The Ni catalyst reported herein was more active for CH4

decomposition than Co. The subsequent removal of the depositedcarbon with O2 resulted in oxidation of both Co and Ni. Aninduction period was necessary to re-reduce the Co in subsequentCH4 decomposition steps. The Ni catalyst was also more stable thanthe Co catalyst during cyclic CH4 decomposition–carbon oxidationcycles. The solid oxygen associated with the oxidized metal reactedwith carbon during subsequent CH4 decomposition steps, produ-cing CO and CO2 as an impurity in the H2. Removal of the carbondeposit by reaction with CO2 yielded CO-free H2. However, thecarbon removal rate was 20�’s lower with CO2 compared to O2.

Acknowledgement

Funding for the present study from the Natural Sciences andEngineering Research Council of Canada is gratefully acknowl-edged.

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