Fuel Processing Technology 92 (2011) 1868–1875

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Fuel Processing Technology

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Methane reforming reaction with carbon dioxide over SBA-15 supported Ni–Mobimetallic catalysts

Tao Huang, Wei Huang ⁎, Jian Huang, Peng JiKey Laboratory of Coal Science and Technology, Ministry of Education of China and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, China

⁎ Corresponding author. Tel./fax: +86 3516018073.E-mail address: huangwei@tyut.edu.cn (W. Huang).

0378-3820/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.fuproc.2011.05.002

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 March 2011Received in revised form 27 April 2011Accepted 3 May 2011Available online 28 May 2011

Keywords:Methane reformingCarbon dioxideNi–Mo bimetallicMo2CCarbon depositionShell-like

A series of mesoporous molecular sieves SBA-15 supported Ni–Mo bimetallic catalysts (xMo1Ni, Ni=12 wt.%,Mo/Ni atomic ratio=x, x=0, 0.3, 0.5, 0.7) were prepared using co-impregnation method for carbon dioxidereforming of methane. The catalytic performance of these catalysts was investigated at 800 °C, atmosphericpressure, GHSV of 4000 ml·gcat

−1·h−1 and a V(CH4)/(CO2) ratio of 1 without dilute gas. The result indicatedthat the Ni–Mo bimetallic catalysts had a little lower initial activity compared with Ni monometallic catalyst,but it kept very stable performance under the reaction conditions. In addition, the Ni–Mo bimetallic catalystwith Mo/Ni atomic ratio of 0.5 showed high activity, superior stability and the lowest carbon deposition rate(0.00073gc·gcat−1·h−1) in 600-h time on stream. The catalysts were characterized by power X-ray diffraction,N2-physisorption, H2-TPR, CO2-TPD, TG and TEM. The results indicate that the Ni–Mo bimetallic catalysts havesmaller metal particle, higher metal dispersion, stronger basicity, metal–support interaction and Mo2Cspecies. It is concluded that Mo species in the Ni–Mo bimetallic catalysts play important roles in reducingeffectively the amount of carbon deposition, especially the amount of shell-like carbon deposition.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Hydrogen is not only a clean burning fuel but, it is also one of themain building molecules in the synthesis of hydrocarbons. Carbondioxide reforming of methane has been studied in detail, because of thepossibility of converting two of the cheapest carbon-containingmaterials into useful synthesis gas (syngas) [1–5]. The syngas has alowerH2/CO ratio than those available fromsteamreformingandpartialoxidation of methane, which is preferred for the synthesis of valuableoxygenated chemicals and long-chain hydrocarbons [6,7].

In the past decade, Nickel-based catalysts [8–10] and noble metal-supported catalysts (Rh, Ru, Pd, Pt, Ir) [11–13] were found to havepromising catalytic performance in terms of conversion and selectivityfor carbon dioxide reformingmethane. However, the high cost of noblemetals makes them a less than ideal choice. In spite of that rapiddeactivation owing to carbon deposition or sintering of active metals athigh temperatures is always observed, Ni-based catalysts have attractedconsiderable interest for their low costs and high activities. It has beenproved that the presence of modifiers [14] can inhabit the cokeformation. One of most important options for enhancing activity andstability in methane-reforming reactions is to use bimetallic catalysts.The transition-metal carbides, especially tungsten and molybdenumcarbide,were reported tohave excellent catalytic activity and selectivityin reformingprocess [15]. However, theWCandMo2C catalysts can only

exhibit stable activity at elevated pressure. At low pressure, thereactivity of these catalysts decreases dramatically after a short periodof time on stream because of oxidation by CO2 [15,16]. Borowiecki andco-workers [17,18] reported that the addition of Mo to Ni/Al2O3

catalysts could reduce the deactivation rate in reforming process.Borowiecki et al. [18] showed that introduction of small amounts

of molybdenum considerably reduces the detrimental effect of carbondeposit formation and increases the activity of methane steamreforming. Quincocess et al. [19] found that the addition of Moincreased the interaction between the Ni species and support, and alsoprevented the growth of Ni particles.

The SBA-15molecular sieve, whichpossesses high surface area (600–1000 m2/g), thermal stability, a hexagonal structure of mesopores withsize 4.6–30 nm and thicker walls (3.1–6.4 nm), may be used as apromising catalysts support, especially for reactions happening at hightemperature [20,21]. However, to date few investigations have beendone on the effect of variation of molybdenum content on theperformanceofNi-based SBA-15 catalysts in the reactionof dry-methanereforming. In this paper, the Ni–Mo/SBA-15 bimetallic catalystscombined the advantages of Ni and Mo species with the characteristicof SBA-15 have excellent catalytic performance.

2. Experimental

2.1. Catalyst preparation

The SBA-15 support was synthesized easily, according to theprocedure described elsewhere [20,21]. The triblock copolymer P123

1869T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

(EO20PO70EO20, Aldrich) was used as the structure-directing agentand tetraethyl orthosilicate (TEOS, Sinopharm Chemical Reagent Co.,Ltd) as the silica source. The P123 was dissolved in the solution ofdeionized water and 4 mol/l hydrochloric acid solution and stirred at40 °C for 0.5 h and then tetraethyl orthosilicate (Sinopharm ChemicalReagent Co., Ltd) was slowly added to the mixture with vigorousstirring at 40 °C for 24 h to get gel. The gel mixture was transferredinto a Teflon bottle and aged at 100 °C for 24 h. The obtained solid wasfiltered, washed with deionized water, and died for 24 h in desiccator.After that the sample was calcined at 550 °C for 5 h using a heatingrate of 2 °C·min−1 from room temperature to 550 °Cin air to removethe organic template.

The Ni–Mo/SBA-15 catalysts were prepared by the incipientwetness co-impregnation, in which Ni content (12 wt. %) was keptconstant, while the Mo/Ni atomic ratio was varied between 0 and 0.7.The SBA-15 support was added into the aqueous solution of nickelnitrate (Ni(NO3)2·6H2O, Aldrich) and ammonia heptamolybdate((NH4)6Mo7O24·4H2O, Aldrich), with stirring at 80 °C, till it producesa viscousmixture. After that, themixturewas dried at 100 °C overnightand then calcined at 550 °C for 6 h in air to obtain Ni–Mo/SBA-15catalysts. According to the Mo/Ni ratio, the Ni–Mo/SBA-15 catalystswere denoted as 0.7Mo1Ni, 0.5Mo1Ni, 0.3Mo1Ni, Ni, respectively.

2.2. Catalyst characterization

The crystalline phases of the samples were measured by X-raydiffraction (XRD) using a Shimadzu XRD-6000 X-ray powder diffrac-tometer with Cu Kα radiation (40 kV, 200 mA) in steps of 0.02° with ascanning rate at 8°/min from 20° to 80° under atmospheric pressure.

The reducibility of catalysts was investigated by temperatureprogrammed reduction (TPR) with TP-5000 (Xianquan china) instru-ment. The testswere performed using a quartz reactor containing 0.1 gof the catalyst in the flow of reducing gas (5%H2 balancedwith N2) at aflow rate of 20 cm3·min −1 and a heating rate of 10 °C·min−1 fromroom temperature to 850 °C.

The basicity of catalystswasmeasured by temperature programmeddesorption of CO2 (CO2-TPD). A sample of about 0.1 gwas held in quartztube and then pretreated at 200 °C for 30 min in a He atmosphere, aftercooled down to room temperature, the pretreated sample was exposedsample was exposed in CO2 for 30 min. Finally, the sample was purgedwithHe at 50 °C for 30 min. CO2-TPDwas carried outwith a ramp rate of10 °C·min−1 from room temperature up to 850 °C under ultra highpurity He stream.

Specific surface areas of the samples were determined withnitrogen adsorption using a Carlo Erba Sorp-tomatic 1990 porosi-meter. The samples were degassed at 250 °C and 0.01 Pa for 12 h priorto measurement.

The amount of carbon deposition on used catalysts was analyzedby thermogravimetry (TG, NETZSCH STA 409 C). A certain amount ofused catalysts was heated from 50 °C to 900 °C in the air with aheating rate of 10 °C·min−1.

Characterization of transmission electron microscopy (TEM) wascarried out on a FEITECNAI G2F-20 with a tension voltage of 200 kV.

2.3. Catalyst evaluation

The evaluation of catalyst activity and stability was carried out in afixed-bed quartz reactor with an inner diameter of 5 mm and a lengthof 800 mm. Prior to the reaction, the catalysts was reduced in flowingof mixture gas (20% H2 balanced with N2) at 800 °C for 3 h. Thecatalyst of 400 mg with the particle size of 40–60 mesh diluted with1.0 g quartz sand (60–80 Mesh) was held on a quartz wool. Thereactant feed composed of an equimolar mixture of CH4 and CO2 wasintroduced into the reactor at a rate of 40 ml/min (gas hourly spacevelocity (GHSV) of 4000 ml·gcat−1·h−1) at atmospheric pressure.Reaction product was analyzed by a gas chromatograph (Haixin

GC 950) equipped with TCD. Argon (99.99%) was used as carrier gas.Huang and Cheng [16] have reported that a blank DMR experimentwith only the quartz wool at 850 °C showed a CH4 conversion of lessthan 4%.

3. Results and discussion

3.1. Phase composition

The small-angle XRD patterns of samples were showed in Fig. 1a.Three well-resolved peaks of SBA-15 are indexable as (100), (200) and(210) reflections associated with p6mm hexagonal. The intensities ofscattering reflections ((100), (200) and (210)) decrease obviously withthe increasing of Mo/Ni atomic ratio, indicating that the ordering of themesoporous structures of SBA-15 is declined.

The phase structure of the unreduced Ni–Mo/SBA-15 catalysts wasanalyzed byXRD. The patterns of the samples are depicted in Fig. 1b. TheNi–Mo/SBA-15 patterns show rather sharp and intensive reflections at37.2°, 43.2°, 62.8°, 75.5° and 79.2° assigned to NiO phases. Theintensities decrease obviously with the increasing of Mo/Ni atomicratio, suggesting that the introduction ofMo can improve the dispersionof Ni, and make Ni particle size smaller (Table 1). When Mo/Ni ratioreached 0.7, a Ni–Mo oxide species was formed (β-NiMoO4), whosediffraction peaks present at 23°–30° (see Fig. 1b) [22], so the 0.7Ni1Mocatalyst showedbothβ-NiMoO4 andMoO3phases (theMoO3 reflectionsat 24.74°, 26.24°, 32.97°, 33.56°), but the diffraction peaks of β-NiMoO4

andMoO3 phases overlap at 23°–30°. TheNi―Obondof theNiMoO4 hashigher bond energy than that of NiO phase [23]. The stronger Ni―Obond increases the difficulty of reduction of Ni2+ to Ni0, resulting inproducing smaller nickel crystallites, which are relatively stable towardsintering and forming carbon. This result was proved further by H2-TPRtest in the following.

XRD patterns of catalysts reduced at 800 °C for 3 h shown in Fig. 1c.The characteristic peaks at 40.8° and 72.8° for the catalysts weredefined for Mo phases. The intensities of Mo and NiO diffraction peaksincreased obviously with the increasing of Mo content. However, thediffraction peaks of Ni phase presented at 44.4°, 52.1° and 76.3°decreased dramatically with the increasing of Mo/Ni atomic ration.There was a certain amount of Ni in strong interaction with supportprobably still included in the oxide matrix [24]. The addition of Mosignificantly reduced the reduction of NiO. These results indicated thatMo species can effectively improve the interaction between Ni andsupport.

XRD patterns of used catalysts were shown in Fig. 1d. The charac-teristic peaks of Ni–Mo bimetallic catalysts at 39.3°, 52.1°, 61.5°, 69.5°,74.6° and 75.5° were ascribed to Mo2C (PDF 35–0787). The intensitiesof Mo2C diffraction peaks of 0.5Mo1Ni and 0.7Mo1Ni catalysts werestronger than that of 0.3Mo1Ni catalyst. It was indicated that theMo2Cspecies was formed in Ni–Mo bimetallic catalysts after reaction.

3.2. Temperature programmed reduction (H2-TPR) and Temperatureprogrammed desorption (CO2-TPD)

Temperature programmed reduction (H2-TPR) were carried out toinvestigate the reducibility of the Ni–Mo/SBA-15 catalysts and theinteraction between the metal special and support. The H2-TPRprofiles of catalysts with different Mo/Ni atomic ratio are shown inFig. 2. The reduction peak of 1Ni catalyst at 384 °C is ascribed to Nispecies (less stable NiO and strong interactional NiO). The TPR curvesof catalysts (0.3Mo1Ni and 0.5Mo1Ni) exhibit a main reduction peaksat 462 and 477 °C that can be ascribed to the reduction of NiO, β-NiMoO4 andMoO3, to the first step of reduction (fromMo6+ toMo4+)of polymeric octahedral Mo species [25,26]. The shoulder peaks at 602and 707 °C are due to the reduction of Mo4+ derived from 0.3Mo1Niand 0.5Mo1Ni catalysts, respectively. The reduction peaks of0.7Mo1Ni catalyst are presented at 446, 533 and 806 °C, which can

a

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

(100)In

tens

ity (

a.u.

)

(110)(200)

2-Thete (degree)

SBA-15

1Ni

0.3Mo1Ni

0.5Mo1Ni0.7Mo1Ni

b

20 30 40 50 60 70 80

Inte

nsity

(a.

u.)

2-Thete (degree)

0.7Mo 1Ni

0.5Mo 1Ni

0.3Mo 1Ni

1Ni

c

30 40 50 60 70 80 90

Inte

nsity

(a.

u.)

2-Thete (degree)

1Ni

0.3Mo1Ni

0.5Mo1Ni

0.7Mo1Ni

d

40 50 60 70 80

0.5Mo1Ni

0.7Mo1Ni

Inte

nsity

(a.

u.)

2-Thete (degree)

0.3Mo1Ni

Fig. 1. a. Small-angle XRD patterns of fresh catalysts. Fig. 1b. XRD patterns of fresh catalysts. (▽) β-NiMoO4 ; (□) MoO3; (○) NiO. Fig. 1c. XRD patterns of catalysts reduced at 800 °Cfor 3 h. (■) NiO; (○) Mo; (▼) Ni. Fig. 1d. XRD patterns of catalysts after reaction. (○) Mo2C; (▽) Ni.

1870 T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

ascribe to NiO and NiMoO4, β-NiMoO4 and MoO3, MoO2, respectively.These results perfectly agree with the XRD result. Meanwhile, it isclearly showed that the reduction peaks of Ni species (▼) shift to hightemperature (Fig. 2), suggesting that the introduction of Mo canimprove the interaction between Ni species and catalyst environment.However, the reduction temperature of Ni species become lower,when the addition of too much amount of Mo in catalyst (0.7Mo1Ni).

The catalysts promoted with molybdenum exhibits less reducible[19,27] than the monometallic Ni catalysts ascribed to the increase ofthe metal–support interaction and a better nickel dispersion. Turlieret al. [28] reported that the catalysts with a larger degree of reduction

Table 1Physicochemical properties and carbon formation rate of the catalysts.

Sample Mo:Ni(atomic ratio)

BET surface a

area (m2/g)Ni crystal

Before c

1Ni – 427.2 20.60.3Mo1Ni 0.3:1 323.1 19.50.5Mo1Ni 0.5:1 288.8 10.70.7Mo1Ni 0.7:1 260.4 15.7

a Calcined at 550 °C for 5 h.b Calculated from the full width at half maximum (FWHM) of the reflection of Ni(111) pc After reduction at 800 °Cfor 3 h.d After reaction at 800 °C.e Calculated from Ni particles size of catalysts before and after reaction.f Calculated from TG. The reaction time for carbon dioxide reforming of methane over

respectively. The average carbon deposition rate of 0.5Mo1Ni catalyst after 250-h time on s

showed smaller nickel dispersion. The conclusion can be drawn thatMo species obviously reduce the Ni particles size and improve theinteraction between the Ni species and environment.

Gadalla andBower [29], Zhang andVerykios [30], Horiuchi et al. [31],and Yamazaki et al. [32] found that carbon deposition is suppressedwhen the metal is supported on a metal oxide with strong Lewisbasicity. CO2-TPD experiments were conducted to investigate the Lewisbasicity of the catalysts in this work. Fig. 3 shows the CO2-TPD analysisresults of promoted and un-promoted Ni/SBA-15 catalysts. It can beseen that main desorption peak of CO2 was detected on the surface ofcatalysts, with the increasing of Mo loading, the adsorption of CO2 was

size (nm) b Average Ni particlesgrowth rate (nm/h)e

Average carbondeposition rate(gc·gcat−1·h−1)f

After d

22.3 0.028 0.0101321.7 0.017 0.0042020.5 0.016 0.0007320.9 0.019 0.00204

lane in the XRD using the Scherrer equation.

1Ni, 0.3Mo1Ni, 0.5Mo1Ni and 0.7Mo1Ni catalysts was 60-h, 130-h, 600-h and 250-h,tream is 0.00198 gc·gcat−1·h−1.

50

100

0.5Mo 1Ni

50

100

0.7Mo 1Ni

50

100

0.3Mo 1Ni

0 50 100 150 200 250

50

100

1Ni

Con

vers

ion

of C

H4

(%)

Time on stream (h)

Fig. 4. The catalytic stability of Nimonometallic andNi–Mobimetallic catalysts at reactionconditions: T=800 °C, P=1 atm, GHSV=4000ml·g1·h−1, V(CH4)/V(CO2)=1:1.

0 200 400 600 800 1000

Inte

nsity

(a.

u.)

Temperature (oC)

0.3Mo 1Ni

1Ni

0.7Mo 1Ni

0.5Mo 1Ni

533

446

806

477

706

602

462384

Fig. 2. TPR profiles of Ni monometallic and Ni–Mo bimetallic catalysts with differentatomic ratio.

1871T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

increased and the desorption peak of CO2 shifted to higher tempera-tures. However, the basicity of 0.7Mo1Ni catalyst got weakest. It issuggested that the 0.5Mo1Ni catalyst possessed the strongest basicitywhich has optimalMo/Ni atomic ratio (0.5:1). It was confirmed that theability of the catalysts to chemisorb CO2 [33] was increased when theLewis basicity of catalysts increased. Therefore, the ability of CO2 ineliminating intermediate carbons formed from CH4 dehydrogenationwill be improved due to its absorption ability. These results indicatedthat Mo might improve CO2 chemisorption and dissociation, whichwould supply enoughoxygenspecies for thegasificationof intermediatecarbons. Ultimately, the lifetime of catalysts were promoted dramati-cally as result of its absorption ability.

3.3. Catalyst activity and stability

The activity and stability data of catalysts are shown in Figs. 4 and5. Itshows the CH4 conversion of catalysts within 250-h test and long-termstability test of 0.5Mo1Ni catalyst, respectively. The addition ofMo to theNi/SBA-15 catalyst led to different changes in the catalytic properties, inagreement with previous studies over steam reforming of CH4 [16,34].The 1Ni monometallic catalyst exhibited a higher initial conversion ofCH4 than Ni–Mo bimetallic catalysts, and the initial conversion followedthe order: 1Ni(97.2%)N0.3Mo1Ni(95.5%)N0.5Mo1Ni(94.3%)N0.7Mo1Ni(94.1%). The main reasons are that Ni monometallic catalyst with highreductiondegree ofNiO (Figs. 1c and2) andBET surface area (Table1). As

0 200 400 600 800

Inte

nsity

( a

.u. )

0.7Mo1Ni

0.5Mo1Ni

0.3Mo1Ni

Ni

646

671

671

589

Temperature (oC)

Fig. 3. CO2-TPD spectra of catalysts reduced at 800 °C for 0.5 h.

Mo loading was increased, more NiO cannot be reduced and covered byMo species. Figs. 1c and 2 showed that the reduction degree of NiOdecreases dramatically with the increasing of Mo/Ni atomic ration. Theinitial conversion of catalysts followed the same order as the NiOreduction degree, suggesting that the amount of Ni reduced in catalystsplays an important role in initial conversion. Swaan [10] reported that theinitial activity was found to depend essentially on the state of the nickelphase (reduction and dispersion) and little on its environment (support,additive). Liu and Au [35] reported that the degree of catalyst reductionwas an important factor affecting the initial activity. In addition, thecatalystswith high BET surface area canprovide large contact area for thereactants, and consequently, resulting in high reaction activity. The BETsurface area of catalysts were decreased with the increasing of Moloading as result of the insertion of the more Mo components into thepores of SBA-15 and the subsequent change in the SBA-15 channels.

Fig. 4 clearly shows that the activity of 1Ni, 0.3Mo1Ni, 0.5Mo1Niand 0.7Mo1Ni catalysts dropped during the 60-h, 130-h, 250-h and250-h stability test, respectively, but the activity of 0.5Mo1Ni catalyststayed the same as its initial value after 250-h time on stream. Thelong term stability test of 0.5Mo1Ni catalyst was carried out toinvestigate its reality performance (Fig. 5). Fig. 5 clearly shows thatthe ratio of H2 to CO stably keeps at about 0.96 and the conversion ofmethane keeps about 94% for 600-h, which may indicate that thiscatalyst had superiority stability and the occurrence of the reversewater–gas shift (RWSR) but of less significance. Bradford and Vannice[36,37] confirmed that the RWGS reaction is near equilibrium over a

0 100 200 300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

1.2

H2/

CO

mol

e ra

tio

Time on stream (h)

0

20

40

60

80

100

Con

vers

ion

of C

H4

(%)

Fig. 5.H2/CO and CH4 conversion as a function of time on stream of 0.5Mo1Ni bimetalliccatalysts at reaction conditions: T=800 °C, P=1 atm, GHSV=4000 ml·g−1·h−1, V(CH4)/V(CO2)=1:1.

a b

c1 d

c2

c2

Fig. 6. TEMmicrographs of reacted catalysts: (a)Morphology of the deposit distributions on 1Ni after 60-h; (b)Morphology of the deposit distributions on 0.5Mo1Ni after 600-h timeon stream; (c1) High- resolution micrograph of shell-like carbon deposition encapsulated by carbon layer obtained on both catalysts; (d) High-resolution micrograph of typicalstructure of a carbon tube obtained on both catalysts.

1872 T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

wide range of temperatures and therefore it is the major side reactionin the CO2 reforming of CH4 system. Its existence reduces the H2/COmoral ratio.

3.4. Carbon deposition

The catalyst testing and carbon analysis indicated that carbondepositionwas themain reason for catalyst deactivation indry-methanereforming. The higher ratio ofMo/Ni does not cause a further increase inthe stability. Carbon deposit on the surface of the catalysts wasinvestigated by thermo-gravimetric (TG) and transmission electronmicroscopy (TEM). The average rate of carbon deposit was measuredusing TG, whose results are shown in Table 1. It is clearly exhibited thatthe Ni monometallic catalyst had much higher coke deposition rate(0.01013 gc·gcat−1·h−1) in comparison with Ni–Mo bimetallic catalysts

Fig. 7. Influence of the metal–support interaction on the mode

(0.00420 gc·gcat−1·h−1 for 0.3Mo1Ni; 0.00037 gc·gcat−1·h−1 for 0.5Mo1Niand 0.00204 gc·gcat−1·h−1 for 0.7Mo1Ni). Meanwhile, the 250-h time onstream test also clearly showed that Ni–Mo bimetallic catalysts havebetter stability (Fig. 4). The deactivation rate follows the same order asthe carbon deposition rate: 1NiNN0.3Mo1NiN0.7Mo1NiN0.5Mo1Ni(Table 1). In addition, Table 1 showed that the Ni particles in catalystsexist thedifferentdegrees of sintering, suggesting that the sinteringofNiparticles is another factor that lead to deactivation of catalysts.

Fig. 6 showed representative TEM general views of the 1Ni(Fig. 6a) and 0.5Mo1Ni (Fig. 6b) catalysts at 800 °C after 60-h and600-h time on stream, respectively. From Fig. 6, it can be seen that twokinds of carbon deposits are formed on 1Ni (Fig. a) and 0.5Mo1Ni (Fig. b)catalysts, which consist of carbon tubes (whisker-like) and encapsulatingcarbon (shell-like)[24]. Carbon deposit on 1Ni (Fig. 6a) catalyst is mostlyin the form of shell-like carbon, whereas whisker carbon species, with a

of growth of carbon deposit. Metal is away from support.

Fig. 8. Influence of the metal–support interaction on the mode of growth of carbon deposit. Weak metal–support interaction.

1873T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

hollow internal channel, is mainly formed on the surface of 0.5Mo1Nicatalyst. As for 1Ni catalyst, metal particles completely encapsulated bycarbon layers, so that they probably do not play a catalytic role. There arethree kinds of carbon deposits [38]: true filaments, tubes and shell, butonly the shells had a deactivating character. Kroll and Swaan studied thedeactivation of Ni-based catalysts in the dry methane reforming byvarious techniques [10,39]. They found that whisker-like carbon possessessentially no or little toxicity, but shell-like carbon deposits have hightoxicity,which could progressively encapsulate theNiparticles, hinderingthe access of the reacting gasses to the active surface.

Fig. 6 (c1 and d) presented high-resolution micrograph of twokinds of deposits on the surface of catalysts. The detail of shell-likecarbon deposit (Fig. 6c1) is also presented at a higher resolution inFig. 6c2. From Fig. 6c2, the distance between the carbon layers showedis about 0.337 nm which is close to that of perfect graphite(0.335 nm), indicating that these carbon has a graphite structure. Inaddition, shell-like carbon deposit with carbon layers nearly parallelto the catalyst particle surface, so tight shells are created. Thethickness of these carbon layers are more than 10 nm; consequently,the deactivation occur as result of the surface covered by carbondeposit. The whisker carbon deposit is showed at higher resolution inFig. 6d. Fig. 6 (b and d) presented that the most of nickel particles arelocated at the tubes tip and have been carried away from the supportsurface during the growth process. The size of Ni particle roughlyequal to that of the internal diameter of the associated carbonnanotube, and indicated that inside diameter of the tube mainlydepends on the size of the metal particle. Furthermore, carbonnanotubes with carbon layers nearly parallel to the tube axis. Thedistance between the carbon layers is also about 0.337 nm, suggestedthat these carbon layers also with a graphite structure.

Bradford and Vannice [6] found that a common reaction interme-diate consisted of aNi-carbide (Ni3C)phase [40] asfirst step towhatevercarbon species generation. Audier et al. [41] revealed that the changes incatalyst composition, enabling modifications in the reactivity of theactive phase, could effectively influence the structures and morphologyof carbon deposits. In addition, Kroll et al. [39] pointed out that facetedand flat particles may facilitate carbon tubes formation and shell-likecarbon deposit may be easily produced on smoothed and sphericalparticles. From Fig. 6b, most encapsulating carbon formed on sphericaland large particles. This indicated that the decoration of Mo effectivelychanged the catalyst particle surface and metal–support interaction,which hindered the formation of shells. Meanwhile, the smaller metal

Fig. 9. Influence of the metal–support interaction on the mode of

particles presented on 0.5Mo1Ni catalyst was also one of the contribu-tions to the prevention of the shells formation.

3.5. Coking mechanism

According to the above analysis, the Ni–Mo bimetallic catalystsshowed better catalytic performance and lower carbon depositionrate, which can be ascribed to its high carbon resistance. The mainfactors contributing to the prevention of carbon deposition are strongmetal–support interaction, strong basicity, small metallic partials andthe formation of Mo2C species. Ruckenstein and Wang reported thatthe strong interaction between Ni particles and support was one ofthe contributions to good catalytic performance [42,43].We now use aseries of simple model to explain the coking mechanism of thecatalysts with different metal–support interaction. Fig. 7 clearlyshows that metal particle which is away from support is easilyencapsulated by carbon layers. When the metal particle is away fromsupport, carbon species derived from the dissociative adsorption ofCH4 are evenly distributed on the surface of Ni particles. After the Ni3Cformed on the surface of metal particle, the Ni particle is slowlyencapsulated by carbon layers [40]. Therefore, the shell-like carbondeposit make the catalyst deactivate rapidly. Fig. 8 shows a weakmetal–support interaction. In this mode, metal is easily lifted fromsupport as soon as the formation of carbon nucleation (Fig. 8).Therefore, the metallic particles disengaged from support are easilysintered and encapsulated with carbon layer, which result in catalystdeactivation. Fig. 9 shows the growth of carbon nanotubes on thesurface of metal with strong metal–support interaction by theextrusion mode. Therefore, the strong interaction between metalparticle and support will prevent metal particle being carried awayfrom the supported surface during the growth process of carbonnanotubes [44,45] (Fig. 9). Therefore, we believe that the metal–support interaction plays important role in the formation of differentkinds of carbon deposits. In this paper, the Ni–Mo bimetallic catalystshave stronger metal–support interaction as compared with 1Nimonometallic catalyst, which can be seen from the result of H2-TPR(Fig. 2). Furthermore, the kind of carbon deposit on 1Ni catalyst ismostly in the form of shell-like carbon, but the carbon nanotubes ismainly formed on 0.5Mo1Ni catalysts, which is clearly seen fromFig. 6. Therefore, the Ni–Mo bimetallic catalysts with strong metal–support interaction effectively prevent the formation of shell-likecarbon deposit with high toxicity.

growth of carbon deposit. Strong metal–support interaction.

Fig. 10. Conceptual model of CO2 adsorption and activation over the catalysts for CO2 reforming CH4. (a) Catalyst with weak basicity. (b) Catalyst with strong basicity. (c) Catalystwith strongest basicity. (○) Carbon species on the surface of metal. (□) Adsorbed and activated CO2 on the surface of support.

1874 T. Huang et al. / Fuel Processing Technology 92 (2011) 1868–1875

In addition, the ability of resistance to carbon deposition of theNi–Mobimetallic catalyst is related to the property of CO2 adsorption andactivation [46]. The carbon species formed on the support and on themetal were removed by reaction with adsorbed surface CO2 andadsorbed oxygen species. The CO2-TPD profiles (Fig. 3) showed thatNi–Mo bimetal catalysts have stronger basicity. Therefore, when thereforming reaction was carried out, the reactant consisted of a very highmolar ration CO2/CH4 was adsorbed on the surface of Ni–Mo bimetalliccatalysts at the same feed ratio of CO2/CH4,which result in decreasing therate of CH4 cracking on the surface. At the same time, there will be ahigher surface coverage of adsorbed CO2which lead to an increase in therate of deposit gasification at the expense of the competing reactions thatresult in carbon formation (Fig. 10). The rate of the deposit gasificationand the rate of coking nearly achieve equality, which is one of thecontributions resulting in stable catalytic performance. It indicated thatthe ability of coking resistivity is directly correlated with the basicity ofNi–Mo bimetal catalysts.

The obvious advantage of 0.5Mo1Ni bimetallic catalyst was the verysmall size of the metallic Ni particles, calculated from the Scherrerequation (Table 1). This result was in agreement with the conclusionrevealed by TEM micrograph (TEM result of reduction catalyst is notshowed in this paper). The metallic particles size of reduction catalystsfollowed the order: 1Ni (20.6 nm)N0.3Mo1Ni (19.5 nm)N0.7Mo1Ni(15.7 nm)NN0.5Mo1Ni (10.7 nm). In the previous research, manyinvestigators have reported the metallic particle size was a criticalfactor to inhibit carbon formation. Zhang and Tang pointed out thatthe carbon formation is closely related with the metallic particles sizeand below a critical particles size (10 nm), the formation rate of carbondecreased dramatically [3,47]. Segner pointed out that the activation ofcarbon dioxide is structure-sensibility, small particle size prompting itsactivation [48]. The superior stability of 0.5Mo1Ni catalyst indicated thatthe critical metallic particle for Ni–Mo bimetallic (in thin paper) toinhibit carbon formation is 10 nm. This result is in agreement with theprevious report [3,47].

Claridge et al. [15] found that no macroscopic carbon wasdeposited on the molybdenum and tungsten carbide catalystsduring the catalytic reactions, suggesting that carbide catalystshave a high resistance to carbon deposition. York et al. [49]reported that no bulk carbon deposition was observed onmolybdenum and tungsten carbides for the dry reforming, partialoxidation and steam reforming of methane to synthesis gas usingstoichiometric feedstock. Fig. 1d clearly showed that Mo2C specieswas formed in Ni–Mo bimetallic catalysts. Table 1 and Fig. 4demonstrated that Ni–Mo bimetallic catalysts have a lower carbondeposition rate and growth rate of Ni particles, and better catalyticperformance, respectively. These results indicated that the forma-tion of Mo2C in bimetallic catalysts improve the resistance tocarbon deposition and Ni sintering effectively. So Ni–Mo bimetalliccatalysts showed excellent performance.

4. Conclusion

Ni–Mo bimetallic catalysts supported on SBA-15 prepared usingco-impregnation method demonstrated excellent catalytic perfor-

mance for CO2 reforming of CH4 to synthesis gas in comparison with1Ni monometallic catalyst. Adjusting the atomic ratio Mo/Ni is able toimprove the interaction between metal particle and support, metallicparticle size and basicity. And the optimal Mo/Ni atomic ratio is 0.5:1in this paper. In addition, the formation of Mo2C in bimetallic catalystsprevents the growth of Ni particles effectively. The Ni–Mo bimetalliccatalysts not only have good coking and sinterization resistivity butalso change the kinds of carbon deposits; consequently, the bimetalliccatalysts have high activity and excellent stability. These outstandingperformances are directly correlated with strong metal–supportinteraction, strong basicity, small metal particles and Mo2C species.It is concluded that Mo species in the Ni–Mo bimetallic catalysts playimportant roles in inhibiting Ni sinterization and the formation ofcarbon deposition, especially the shell-like carbon deposition.

Acknowledgments

The authors gratefully acknowledge the financial support from theKey Project of Chinese National Programs for Fundamental Researchand Development (973 Program 2005CB221204) and the NaturalScience Fund of China (20676087).

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