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Applied Catalysis A: General 468 (2013) 359– 369

Contents lists available at ScienceDirect

Applied Catalysis A: General

j ourna l h om epage: www.elsev ier .com/ locate /apcata

eO2–SiO2 supported nickel catalysts for dry reforming of methaneoward syngas production

un Hin Taufiq-Yapa,b,∗∗, Sudarnoa,b, Umer Rashidc,∗, Zulkarnain Zainala,b

Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, MalaysiaDepartment of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, MalaysiaInstitute of Advanced Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

r t i c l e i n f o

rticle history:eceived 7 June 2013eceived in revised form 6 September 2013ccepted 11 September 2013vailable online xxx

a b s t r a c t

Supported nickel catalysts (5 wt.% Ni) on binary CeO2–SiO2 (CS) with different ceria loading were pre-pared by wet impregnation and evaluated under catalytic dry reforming of methane (DRM) reaction toproduce syngas. Analytical methods of characterization i.e. EDX, BET surface area, XRD, H2-TPR, CO2-TPD, TEM, SEM and TGA were conducted to analyze the physico-chemical properties of the preparedsamples as well as to identify the carbon formation of the spent catalysts. The results showed that the

eywords:ry reformingyngasickel catalysteriaarbon formation

properties of CeO2–SiO2 (Ni/xCS) catalysts were superior to the Ni/SiO2 and Ni/CeO2 catalysts, in terms ofparticle sizes, Ni dispersion, reducibility and basicity. The catalyst evaluation showed that ceria additionon the Ni-supported catalysts influenced the catalytic performances and hindered the carbon formationsignificantly. In this study, Ni/CS catalyst with 9 wt.% ceria exhibited good properties, high catalytic per-formance, elevated stability and low carbon deposition, thus considered to be the best catalyst with theoptimal amount of ceria.

. Introduction

Nowadays, global warming and diminishing supplies of fossiluels have attracted much attention. Thus, scientists, engineersnd governments are demanded to participate in solving theseroblems by any means of environmentally friendly technologies.arbon dioxide (CO2) and methane (CH4) are two harmful greenouse gases that considerably contribute to the global warming1]. Dry reforming of methane (DRM) has received great attentionue to its environmental benefits from utilizing the two greenhouseases and producing highly valuable synthesis gas (syngas, H2 andO) as a feedstock [2,3]. Moreover, the syngas produced throughRM process (Eq. (1)) has a H2/CO ratio of ∼1, which is more com-atible for many downstream processes such as hydroformylation,arbonylation and Fischer–Tropsch synthesis [2,4–7].

H4 + CO2 � 2CO + 2H2 �H◦298 K = −247 kJ/mol (1)

2 + CO2 � H2O + CO �H◦298 K = −41 kJ/mol (2)

∗ Corresponding author. Institute of Advanced Technology, Universiti Putraalaysia, 43400 UPM Serdang, Selangor, Malaysia. Tel.: +60 389467393.

∗∗ Corresponding author at: Catalysis Science and Technology Research Cen-re, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor,

alaysia. Tel.: +60 3 89466809; fax: +60 3 89466758.E-mail addresses: yap@science.upm.edu.my (Y.H. Taufiq-Yap),

mer.rashid@yahoo.com (U. Rashid).

926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apcata.2013.09.020

© 2013 Elsevier B.V. All rights reserved.

CH4 → C + 2H2 �H◦298 K = −75 kJ/mol (3)

2CO → C + CO2 �H◦298 K = 172 kJ/mol (4)

Based on Eq. (1), syngas can be produced from CH4 and CO2 in areaction that is strongly endothermic and requires high tempera-tures (usually above 700 ◦C). However, carbon formation (coking)most likely occurs from CH4 cracking (Eq. (3)) and CO dispro-portionation (Eq. (4)), causing deactivation or destruction of thecatalysts and a blocked catalyst bed. In addition, H2O and excess ofCO can be formed from reversed-water gas shift (RWGS) reactionas shown in Eq. (2), making the catalyst more sensitive to metalsintering [2]. Thus, the DRM reaction has not been installed in largescale due to those reaction problems [8–10].

Reforming catalysts are divided into two major groups which ofnickel based, catalysts and noble metal based catalysts [11]. Noblemetals catalysts, such as Pt, Ru and Rh, are more resistant to cokingbut are very expensive and have limited availability, so they are notpreferable for use in industry. Ni-based catalysts are more suitableand widely used for CO2 reforming of methane because of theirproper activity, availability and cheaper but are low coking resis-tance [12]. Therefore, the development of Ni-based catalysts hasbecome an interesting research-object, directed to find effective

catalysts with excellent activity and stability. These enhancementsmight be achieved by improving (i) the natural properties of thesupport, (ii) the preparation method of the support and catalysts,and (iii) promoter addition [13].

3 talysis

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60 Y.H. Taufiq-Yap et al. / Applied Ca

Silica (SiO2) has been extensively used as a carrier for supportedetal catalysts due to high surface area and excellent metal surface

ispersion [14–19]. Ceria (CeO2) is also widely used as a support andromoter additive in various catalysts for many oxidation reactions,

ncluding the dry reforming [20], auto thermal reforming [21] andartial oxidation of methane [22]. Ceria is a very attractive pro-oter for the methane reforming process due to its high oxygen

torage capacity, redox activity as well as its extraordinary abil-ty to enhance metal dispersion [20,22,23]. The high surface area ofeO2 can also be used as a promoter, providing higher reactivity andxcellent coking resistance [15,24]. For example, ceria has been suc-essfully used for Ni/Al2O3 catalysts in DRM reaction for reducingarbon formation and increasing activity and stability [25].

The CeO2–SiO2 mixed-oxide was successfully synthesized viaeposition precipitation, which gave better properties in termsf higher interaction of Se and Si and superior metal dispersionompared to the similar support prepared by impregnation [26].ccording to our knowledge, the use of binary oxide CeO2–SiO2s catalyst support has not been reported widely in DRM reaction.herefore, this study is focused on the influence of ceria addition toilica on CeO2–SiO2 system for supported nickel catalysts. Further-ore, a series of ceria addition to CeO2–SiO2 has been prepared and

sed as Ni catalyst support for dry reforming of methane reaction.

. Experimental

.1. Support and catalyst preparation

The binary CeO2–SiO2 oxides with various ceria loading (3,, 18, and 30 wt.%) were prepared by deposition precipitationDP) and used as catalyst support. First, a stoichiometric amountf Ce(NO3)3·6H2O (Aldrich) solution was added into a two-neckask containing silica (Kieselgel 60). The pH of the mixture wasdjusted to ∼9–10 by adding NH4OH solution. Then the mixtureas stirred at 60 ◦C for 24 h. Finally, the solvent was removed

y rotary evaporation and drying in an oven at 80 ◦C overnight.he solid was crushed, sieved and calcined at 500 ◦C for 4 h. Therepared CeO2–SiO2 supports were denoted as xCS, x = wt.% ceriao SiO2. Besides, CeO2 oxide was also prepared by decomposinge(NO3)3·6H2O at 550 ◦C for 4 h in air. The obtained solid wasrushed and sieved.

The catalysts, Ni/SiO2, Ni/CeO2 and Ni/xCS (5 wt.% Ni), were pre-ared by a wet impregnation method. Ni(NO3)2·6H2O (Aldrich)olution was added into the supports (SiO2, CeO2 and xCS) in backerlass. The mixture was stirred for 5 h at room temperature, and thenhe water was removed by rotary evaporation and drying at 80 ◦Cvernight. The solid catalysts were crushed, sieved and calcined at00 ◦C for 4 h. All the catalysts were designated as Ni/SiO2, Ni/3CS,i/9CS, Ni/18CS, Ni/30CS and Ni/CeO2.

.2. Catalysts characterization

The chemical composition of the catalysts was determined bynergy dispersive X-ray fluorescent (EDX) analysis using a Shi-adzu EDX-720 spectrometer. The surface area, pore volume and

ore size diameter of the samples were determined using N2dsorption–desorption technique with a Thermo Finnigan Sorp-omatic Instrument, model 1900. Prior to analysis, the samplesere degassed at 150 ◦C overnight. Crystalline structures of the

amples were investigated using X-ray diffraction (XRD) with a Shi-adzu XRD-6000 diffractometer with Cu K� radiation (� = 1.541 A)

perated at an ambient temperature (at 30 kV, 30 mA) with a scan-ing rate of 2◦/min.

The reducibility and basicity of the catalysts were examinedy hydrogen temperature programmed reduction (H2-TPR) and

A: General 468 (2013) 359– 369

carbon dioxide temperature programmed desorption (CO2-TPD)on a Thermo Finnigan TPD/R/O 1100 Instrument. Prior to analyses,20 mg of samples was placed in a quartz tube reactor (i.d. Ø = 6 mm)and then pretreated. For H2-TPR, the sample was pretreated underN2 gas flow (20 ml/min) at 150 ◦C for 10 min and cooled to ambienttemperature. The analysis was performed using 5% H2/Ar flow(25 ml/min) from 60 to 900 ◦C with a ramp of 10 ◦C/min. ForCO2-TPD, the catalyst was pretreated under N2 flow (20 ml/min) at300 ◦C for 30 min and cooled to ambient temperature. Then, it wasexposed to CO2 gas (30 ml/min) for 60 min and purged with N2flow again for 30 min to remove excess CO2 on the catalyst surface.The analysis was performed using helium gas flow (at 25 ml/min)from 60 to 900 ◦C with a ramp of 10 ◦C/min. The amount of H2consumption and CO2 desorption was detected by a thermalconductivity detector (TCD).

The morphology of the samples was identified using SEM witha LEO 1455 Variable Pressure scanning electron microscope. Theanalysis was performed on gold-coated samples and operated atan accelerated voltage of 20 kV. The particle size and active sitedispersion of the catalysts were identified qualitatively using trans-mission electron microscopy (TEM) on a Hitachi (H-7110) scanningsystem. The catalysts were dispersed in 75% acetone and sonicatedfor 15 min. The dispersed catalysts were deposited on a copper gridfor analysis.

2.3. Catalytic evaluation

The catalytic evaluation for dry reforming of methane was per-formed in a continuous flow system using a fixed bed stainless steelmicro-reactor (i.d. Ø = 6 mm, h = 34 mm). The reactor was connectedto a mass flow gas controller and an online gas chromatograph (GC)(Agilent 6890N; G 1540N) equipped with Varian capillary columnsHP-PLOT/Q and HP-MOLSIV. Prior to reaction, approximately 0.20 gcatalyst was reduced by flowing 5% H2/Ar (25 ml/min) from 100 to700 ◦C and holding for 3 h. The reforming reaction was performedby flowing a gas mixture consisting of CH4/CO2 = 50/50% mol, at arate of 50 ml/min from 100 to 800 ◦C at 10 ◦C/min, then holding for10 h (1 atm, GHSV = 15,000 ml/gcat h).

2.4. Analysis of used catalysts

The carbon formation of the spent catalysts was analyzed usingXRD, TEM, SEM and TGA. Except for TGA, the procedures of thesetechniques followed the characterization descriptions detailedabove. The temperature program oxidation (TPO) was performedon a TGA/SDTA 851e METTLER TOLEDO instrument using ∼10 mgof the spent catalysts and heating from 25 to 1000 ◦C (10◦/min) in5% O2/He (50 ml/min) flow. Quantitatively, the amount of carbonformation was determined by means of mass loss of samples.

3. Results and discussion

3.1. Textural properties

The physicochemical properties of the supports and catalystsare given in Table 1. Surface elemental composition of the catalystspresented in the table was obtained from energy dispersive X-rayfluorescent (EDX) analysis. This analysis was conducted to confirmthe appropriate percentage shift for each metal component. Specif-ically, the percentage of Si and Ni gradually decreased on Ni/xCScatalysts with increasing amount of ceria. For instance, the metal-loid silicon component of Ni/SiO2 was 67.97%, which decreased to

63.21, 56.24, 46.26, and 38.32% after 3, 9, 18 and 30% CeO2 loadingon SiO2, respectively.

The specific surface area (SSA), pore volume and pore diameterof the support and catalysts are also presented in Table 1. Naturally,

Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369 361

Table 1Textural properties of supports and catalysts.

Support/catalysts Metal loading(wt.%)

Chemical compositiona Surface area(m2/g)b

Cumulativepore volume(cm3/g)c

Average poresize (nm)d

Bulk density(g/ml)

Cee Nif Si Ce Ni

SiO2 – – – – – 408 0.99 6.44 0.473CS 3 – – – – 233 1.51 8.66 0.359CS 9 – – – – 232 0.87 9.82 0.3618CS 18 – – – – 221 0.71 9.62 0.3830CS 30 – – – – 221 0.65 10.40 0.44CeO2 – – – – – 63 0.22 10.66 0.89Ni/SiO2 – 5 67.97 – 30.61 294 0.87 7.58 0.40Ni/3CS – 5 63.21 7.325 28.42 250 0.86 10.32 0.31Ni/9CS – 5 56.24 17.93 25.03 241 0.77 9.68 0.35Ni/18CS – 5 46.26 30.36 22.67 220 0.73 9.90 0.37Ni/30CS – 5 38.32 41.99 19.07 214 0.61 10.70 0.38Ni/CeO2 – 5 – – – 67 – –

a Investigated by EDX (surface analysis).b Calculated by BET equation.c BJH desorption total pore volume.

SafYa

d BJH desorption average pore diameter.e wt.% of CeO2 on silica.f wt.% of Ni on support.

iO2 as the main support had the highest surface area (408 m2/g)

nd CeO2 prepared by thermal decomposition had the lowest sur-ace area (63 m2/g). The results were similar to those reported byang et al. [23]. A variance of surface area values was obtainedmong the xCS supports with the range of 233–221 m2/g. Those SSA

Fig. 1. Scanning electron micrographs of selected supports a

values are smaller than that of SiO2 significantly, indicating that

ceria addition by DP method influenced the mixed oxide surfaceproperties.

The surface area of Ni/xCS catalysts had been slightly higher thanthat of xCS supports after the nickel impregnation. This might be

nd catalysts: (a) SiO2; (b) Ni/SiO2; (c) 9CS; (d) Ni/9CS.

362 Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369

20 30 40 50 60 70

CeO JCPDS -043-1002

= CeO= SiO

CeO

SiO

30CS

18CS

9CS

3CS

Degree (2θ)

Inte

nsity

(a.u

.)

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20 30 40 50 60 70

NiO JCPDS-065-2901CeO2 JC PDS-04 3-1002

= CeO2= NiO= SiO2

Ni/CeO2

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO2

Inte

nsity

(a.u

.)

reduced under a H2 flow prior to reaction. Fig. 4 shows the XRDpatterns of the reduced Ni catalysts. The characteristic nickel (Ni◦)peaks appeared at 2� = 44.5◦ and 51.8◦ corresponding to (1 1 1)and (2 0 0) reflections, respectively (JCPDS card no. 04-0850) [32].

Ni JCPDF 004-0850

Red Ni/CeO

= CeO2= Ni= SiO2

Red Ni/30CS

Red Ni/18CS

Red Ni/9CS

Red Ni/3CS

Red Ni/SiO

Inte

nsity

(a.u

.)

Fig. 2. XRD patterns of supports.

uggested that there was probably either an incomplete calcinationf xCS or the influence of impregnation during preparation of thei/xCS. In this work, Ni/SiO2 had the largest surface area (294 m2/g)nd Ni/CeO2 had the lowest surface area (67 m2/g) due to the nat-ral properties of the oxides. However, the addition of more CeO2ecreased the specific surface area of catalysts due to CeO2 particlesepositing and agglomerating on the SiO2 surface during catalystreparation [22]. In another study, Mishra and Rao [27] observedhat particle size of CeO2 was affected due to the addition of diffentonencentarion of ceria content in the CeO2–ZnO composite oxide.rnivec et al. [28] also reported the variation in the surface area bydding the different concentraion of Ni–Co active metals. On thether hand, the pore diameter did not decrease after impregnationf the nickel, indicating that the nickel particles were located onlyn the surface of the support.

To understand the influence of ceria addition by DP, the bulkensity of sample was also measured and shown in Table 1. Theable shows that the support modified by ceria addition has lowerensity than SiO2. The CeO2 had the highest density of 0.89 g/mlecause of its natural properties (molecular weight). On the otherand, the insertion of nickel to the supports made the nickel cata-

ysts had lower density than the supports. Moreover, four selectedamples were investigated using SEM (Fig. 1). The morphology witharge particle size of SiO2 is shown in Fig. 1a. For Ni/SiO2, there waso much difference of SiO2 particles after impregnation processnd Ni particles were only dispersed on SiO2 surface (Fig. 1b). InCS support, the SiO2 particles were reduced in size (Fig. 1c).

This indicates that DP has significantly changed the morphol-gy of those prepared supports mainly SiO2 due to chemical andhermal treatments during preparation. This indicates that CeO2articles are dispersed well on the SiO2 surface. The higher disper-ion of ceria would assist the high dispersion of Ni active sites onhe catalyst surface [22].

.2. Crystallographic and structural properties

The crystallographic and structural properties of the preparedupports and catalysts samples were studied by X-ray diffraction,s shown in Figs. 2 and 3, respectively. For 3CS, no CeO2 peaksere observed in the XRD spectra, but the amorphous silica peak

∼21.61◦) was decreased significantly. This suggested that the crys-alline structure of the support was strongly influenced by ceriaddition. Furthermore, with higher amounts of ceria (≥9 wt.%), four

◦ ◦ ◦ ◦

eO2 peaks were clearly seen at 28.5 , 33.2 , 47.4 and 56.4 , whichorresponded to a cubic ceria structure for (1 1 1), (2 0 0), (2 2 0)nd (3 1 1) reflections, respectively (JCPDS no. 43-1002) [29]. Thepectra intensity of four ceria peaks increased with increasing ceria

Degree (2 )

Fig. 3. XRD patterns of supported nickel catalysts.

loading. Between 30CS and CeO2, the four ceria peaks were simi-lar, which could be assumed that there is an aggregation processof ceria particles on the 30CS surface. These XRD patterns matchedthose previous studies [22,26,29,30]. Similar results was obtainedby Mishra and Rao [27], when ceria content was increased from20 to 80 mol% in the CeO2–ZnO mixed oxide, the intensity of ZnOdecreased quite rapidly while that of the CeO2 was increased.

The XRD patterns of the prepared catalysts are shown in Fig. 3.Three characteristic peaks of cubic NiO were observed in Ni/SiO2and Ni/xCS at 2� of 37.2◦, 43.3◦ and 62.9◦ representing the (1 0 1),(0 1 2) and (1 1 0) reflections, respectively (JCPDS no. 65-2901)[30,31]. However, these peaks decreased with increasing ceria load-ing, indicating that CeO2 addition influenced the catalyst structureand nickel dispersion. NiO peaks were not observed in the XRDpattern of Ni/CeO2 due to the physical properties of pure CeO2.Catalysts with higher ceria loading (≥9 wt.%) showed lower NiOpeaks, indicating that the NiO particles were well distributed onthe Ni/xCS surface.

A nickel metal (Ni) phase is needed in the oxidative DRM reac-tion as the active sites of the catalysts. Hence, the catalysts were

20 30 40 50 60 70

Degree (2 )

Fig. 4. XRD patterns of reduced catalysts.

Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369 363

2; (b) U

Crpdics

Fig. 5. TEM images of reduced catalysts: (a) Ni/SiO

ompared to Fig. 3, NiO peaks were not observed for any of theeduced catalysts, indicating that NiO (Ni2+) particles were com-letely reduced to Ni◦. On the other hand, CeO2 peaks were hardly

etected in Ni/3CS and a low CeO2 peak (28.52◦) was observed

n catalysts that have ceria loading higher than 9 wt.%. This indi-ates that the nickel particles were well distributed on the Ni/xCSurface.

sed Ni/9CS; (c) Ni/18CS; (d) Ni/30CS; (e) Ni/CeO2.

Crystallite sizes of nickel particles were calculated using Debye-Scherrer formula based on the XRD data (1 1 1 reflection of Ni◦ peak)and presented in Table 2. Ni metal particle was gradually changed

to be the smaller size with increasing ceria loading. This result couldbe attributed that the ceria addition to the support system reducedthe size of Ni particles, and consequently, the Ni dispersion wasenhanced. The dispersion and particle size was also identified by

364 Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369

Table 2Amount of H2 consumed, CO2 uptake during TPR and TPD analysis.

Catalysts H2-TPR CO2-TPD Ni size (nm)a

T peak (◦C) H2 consumed (�mol/g) Total T peak (◦C) CO2 uptake (�mol/g)

Ni/SiO2 370 1096.99 1096.99 344 655.27 15.46Ni/3CS 377 1809.91 1809.91 581 725.03 14.24Ni/9CS 384 2066.57 2066.57 597 725.72 12.40

Ni/18CS413 1340.14

1604.20 600 750.03 11.75749 264.06

Ni/30CS402 1483.19

2001.80 591 483.76 11.22778 518.61

Ni/CeO2

230 523.83

3528.69 602 577.88 N.D.306 933.21393 1307.32876 764.33

Nlysts (

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.D. = not defined.a Determined by Debye–Scherrer equation based on XRD pattern of reduced cata

EM technique. Fig. 5 shows the TEM images of selected reducedatalysts pretreated under 5% H2/Ar flow at 700 ◦C for 3 h. Fromhe figure, the reduced catalysts contained small spherical-blackarticles dispersed on CeO2–SiO2 supports that were assigned toi metal particles. The average Ni particle size of reduced Ni/SiO2as larger than that of reduced Ni/xCS. For reduced Ni/30CS, thei particle appeared only rarely. This rarity might have been due

o the amount of CeO2 being too high. These TEM images con-rmed the nickel-metal particle sizes calculated from XRD datasing Debye–Scherrer equation. It could be concluded that ceriaddition decreased the size and increased the dispersion of Ni par-icles on the catalysts.

.3. Reducibility and basicity

H2-TPR analysis was conducted for both selected supports andatalysts. Reducibility of SiO2, 9CS and CeO2 is presented as TPRrofiles in Fig. 6. As can be seen, there was no reduction peakbserved for SiO2 carrier. However, in 9CS, two reduction peaksere observed clearly at 534 and 753 ◦C, which indicated the

eduction behavior of CeO2. Specifically, the peaks correspond tohe reduction of capping oxygen (O2− or O− anions) on the sur-ace at lower temperature and the reduction of ceria, CeO2 to

e2O3 (Ce4+ → Ce3+) at a higher temperature, which were gradu-lly reduced by elimination of O2− anions of the lattice [15,33,34].or the CeO2 sample, three H2 consumption peaks also appearedt around 392, 510 and 806 ◦C indicating the reduction behavior

200 300 400 500 600 700 800 900

806oC

753oC534oC

510oC

392oC

CeO2

9CS

SiO2

Temperature (oC)

H2

Con

sum

ptio

n (a

.u.)

Fig. 6. H2-TPR profiles of selected supports: SiO2, 9CS and CeO2.

plane 1 1 1).

as seen for 9CS above, which were in agreement with a previousstudy [33]. As Gao et al. [15], the high-temperature regions forCeO2 reduction of 9CS indicated that there was a strong interactionbetween CeO2 and SiO2 formed during the preparation.

TPR profiles of supported Ni catalysts are depicted in Fig. 7. Thereduction behavior of the catalysts was quite different than thoseof the supports. The TPR profile showed a major peak among thecatalysts, occurring in a temperature maximum between 385 and418 ◦C, and indicated the reduction of NiO to Ni (Ni2+ → Nio) on thecatalyst surface. For Ni/SiO2, the peak was observed at 370 ◦C, shif-ting slightly to 377 ◦C for the Ni/3CS catalyst. Furthermore, the mainpeak at higher temperatures shifted with increasing ceria addition.This indicated that the addition of ceria changed the properties ofthe supports and influenced the interaction between NiO and thesupports. A similar observation was reported by Feio et al. [35] andMontoya et al. [36], where the H2 consumption and reduction peakof catalysts increased with an increase of ceria loading.

However, the amount of hydrogen consumption of Ni/xCS cata-lysts was higher than that of Ni/SiO2, as presented in Table 2. Amongthe catalysts, Ni/9CS gave the largest amount H2 consumption of2066.57 �mol/g. In detail, the TPR profiles of Ni/CeO2 were sim-ilar to those seen in Kambolis et al. [37], where three reductionpeaks were observed 232, 311 and 396 ◦C. This was contrary toother reports, such as those of in Ref. [20,36], where TPR profilesof Ni–ceria supported catalyst had only two peak ranges at higher

temperatures (410–417 ◦C and 800–880 ◦C). These two peaks wereattributed to the reduction of NiO and ceria, respectively. Thesedifferences might have been due to the preparation method used

200 300 400 500 600 700 800 900

876oC

778oC

749oC

393oC306oC

230oC

402oC

413oC

384oC

377oC

370oC

Ni/CeO2

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO2

Temperature (oC)

H2 C

onsu

mpt

ion

(a.u

.)

Fig. 7. H2-TPR profiles of catalysts.

Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369 365

Table 3Catalytic performance of catalysts, amount of carbon deposit and surface area decrement of used catalysts.

Catalysts CH4 conversion (%) CO2 conversion (%) CO selectivity (%) H2/CO BET surface areac (m2/g) Decrease of surface area (%)

Initiala Finalb Initial Final Initial Final Initial Final

Ni/SiO2 95.5 96.6 93.8 91.8 76.4 73.6 1.14 1.22 174 40.82Ni/3CS 97.1 94.3 96.5 93.2 76.1 78.3 1.20 1.09 133 46.80Ni/9CS 97.3 97.0 96.7 96.5 75.5 76.7 1.22 1.16 168 30.30Ni/18CS 98.1 97.4 96.9 97.3 72.6 75.5 1.21 1.14 112 49.00Ni/30CS 91.0 92.8 88.71 90.8 78.6 78.8 1.03 1.07 81 62.15Ni/CeO2 97.8 90.7 96.45 86.7 73.5 78.0 1.25 1.02 25 62.70

ebtc

nCCstptbasa

3

CFcaNvfimtsha

a

catalyst. The high methane conversion on Ni/SiO2 is might be dueto a preferable CH4 dissociation and the low CO2 conversion can beassociated with the low adsorption of CO2 by the catalyst. On the

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

CH

4C

onve

rsio

n (%

)

Ni/SiO2Ni/3CSNi/9CSNi/18CSNi/30CSNi/CeO2

a

a Initial time of reaction at 800 ◦C (after 5 min).b Final time of reaction at 800 ◦C (after 580 min).c Surface area of used catalysts.

ither for the support or catalysts. Based on these results, it coulde concluded that the CeO2 addition and that the preparation ofhe catalysts plays an important role in reducibility and hydrogenonsumption.

The basicity of the catalysts was evaluated by CO2-TPD tech-ique using the same apparatus as TPR analysis. Fig. 8 shows theO2-TPD profiles of the supported Ni catalysts, and the amount ofO2 desorbed is summarized in Table 2. From the figure, Ni/SiO2howed desorption peak for CO2 at low temperature (344 ◦C), whilehe promoted catalysts, Ni/xCS, showed the peak at a higher tem-erature (e.g. 597 ◦C for Ni/9CS). This phenomenon indicated thathe addition of ceria into the CeO2–SiO2 system by DP enhanced theasicity catalysts to be stronger and increased the amount of CO2dsorbed. According to previous studies [15,17], the enhanced basicites of the catalysts were favorable for accelerating the absorptionnd activation of CO2 as well as hydrocarbon.

.4. Catalytic evaluation

The catalytic behaviors of the catalysts, including CH4 andO2 conversions, CO selectivity and H2/CO ratio are presented inigs. 9–11. Fig. 9 shows the effect of reaction temperature on theonversion of CH4 and CO2 in the DRM reaction over the cat-lysts. Ni/18CS and Ni/9CS showed the highest activities whilei/SiO2 gave the lowest values. However, the value of those con-ersions increased with increasing reaction temperatures. Similarnding was obtained by Liu et al. [38] that during dry reforming ofethane, the concentration of CH4 formation was increased with

emperature by using the Ni/ZiO2–SiO2 catalyst. The CH4 conver-ion reached almost 100% at the maximum temperature of 800 ◦C;

ence the temperature was expected to be an appropriate temper-ture for DRM reaction.

The reaction was remained for 10 h at 800 ◦C to ascertain the cat-lytic stability of the catalysts. Fig. 10 demonstrates the CH4 and

300 400 500 600 700

602 C

591 C600 C

597 C

581 C

344 C

Ni/CeO

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO

Temperature ( oC)

CO 2 d

esor

bed

(a.u

)

Fig. 8. CO2-TPD profiles of catalysts.

CO2 conversions versus reaction time on stream and it revealedthat all catalysts showed high catalytic performance (conversion>90%). The conversions slightly increased with increasing cerialoading up to 18 wt.% and decreased drastically at the ceria loadingof 30 wt.% for Ni/xCS catalysts. This could be associated that therewas a difference of the accessible active-site number on the cata-lyst surface because of different ceria loading. However, betweenNi/18CS and Ni/9CS that gave excellent performance, the activitieswere relatively similar, and it could be assumed that 9 wt.% of ceriais the optimum ceria loading. Although high CH4 conversion wasobtained but instead low CO2 conversion was observed for Ni/SiO2

Temperature ( oC)

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

CO

2C

onve

rsio

n (%

)

Temperature (oC)

Ni/SiO2Ni/3CSNi/9CSNi/18CSNi/30CSNi/CeO2

b

Fig. 9. The effect of reaction temperatures on (a) CH4 and (b) CO2 conversions overthe catalysts.

366 Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369

50

55

60

65

70

75

80

85

90

95

100

0 50 600550500450400350300250200150100

CH

4C

onve

rsio

n (%

)

Time on stream (min)

Ni/SiO2 Ni/3CS Ni/ 9CS

Ni/18CS Ni/30CS Ni/CeO2

a

50

55

60

65

70

75

80

85

90

95

100

0 50 600550500450400350300250200150100

CO

2C

onve

rsio

n (%

)

Time on stream (min)

Ni/SiO2 Ni/3CS Ni/ 9CS

Ni/18C S Ni/ 30C S Ni/ CeO2

b

Ff

octcB(b

0

10

20

30

40

50

60

70

80

90

0 50 600550500450400350300250200150100

CO

Sel

ectiv

ity

(%)

Time on stream (min )

Ni/SiO2 Ni/3CS Ni/9CSNi/18CS Ni/30CS Ni/CeO2

a

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0 50 600550500450400350300250200150100

H2

/CO

rat

io

Time on stream (min )

Ni/SiO2 Ni/3CS Ni/ 9CS

Ni/18C S Ni/ 30C S Ni/ CeO2

b

ig. 10. Stability tests of (a) CH4 and (b) CO2 conversions over the catalysts as aunction of time (reaction temperature at 800 ◦C, GHSV 15,000 ml g−1 h−1).

ther hand, Ni/CeO2 exhibited a steady decrease in the reactantonversion because of the carbon formation on the active site ofhe catalysts. Generally, the conversion of CH4 was higher than the

onversion of CO2, which is likely due to side-reactions in such ofoudouard reaction (2CO → C + CO2) and methane decompositionCH4 → C + 2H2). These reactions not only produce coke (carbon)ut also enhance H2 production and reduce CO production. The

Fig. 12. XRD patterns of used catalysts: (a) reduced

Fig. 11. Catalytic performance of (a) CO selectivity and (b) syngas (H2/CO) ratio asa function of time.

coke formed by those side reactions is the main causes of cata-lyst deactivation [39]. This result has similarities to some previous

works [40,41]. They reported that Ni catalysts promoted by ceriaexhibited higher CH4 conversion than CO2 conversion at a temper-ature above 750 ◦C.

and used Ni/SiO2; (b) series of used catalysts.

Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369 367

2; (b)

pailwt

Fig. 13. TEM images of used catalyst: (a) U Ni/SiO

The plot of CO selectivity and H2/CO ratio of all the catalysts areresented in Fig. 11. As it can be seen, the Ni/xCS and Ni/CeO2 cat-lysts exhibited higher CO selectivity than Ni/SiO2 catalysts. This

s in agreement with the CO2-TPD results, which Ni/SiO2 gave theowest basicity. In addition, the H2/CO ratio for all the catalysts

as slightly higher than unity (H2/CO > 1) which corresponds tohe CH4 and CO2 conversions explained above. This result was

U Ni/9CS; (c) U Ni/18CS; (d) Ni/30CS; (e) Ni/CeO2.

contrasted with other works, where the most references reportedthat the ratio of H2/CO was lower than 1 because of the RWGS reac-tion. However, specifically in this works, the RWGS reaction was

probably not favorable in high temperature. There are some pre-vious studies [40–43] that obtained similar results where the H2to CO ratio was higher than 1. The higher ratio is probably due tothe un-favorable RWGS reaction that makes an excess of H2 [40].

368 Y.H. Taufiq-Yap et al. / Applied Catalysis A: General 468 (2013) 359– 369

ed used catalysts: (a) Ni/SiO2; (b) Ni/9CS.

Saepd

3

tStaFcctclcp

ucpmdamecccCcCwtar

pieabd

20

30

40

50

60

70

80

90

100

900.00800.00700.00600.00500.00400.00300.00200.00100.00

Mas

s lo

ss (

%)

o

Ni/SiO2

Ni/3CS

Ni/9CS

Ni/18C S

Ni/30C S

Ni/CeO2

Fig. 14. SEM micrographs of select

ome oscillating patterns of H2/CO ratio were observed for the cat-lysts which could be ascribed to the effect of carbon formation andlimination during reaction [44]. Table 3 summarizes the catalyticerformance of all the catalysts as well as the amount of carboneposit and surface area of the spent catalysts.

.5. Analysis of used catalysts

After reaction, the spent catalysts were collected to identifyhe carbon formation and other characteristics using XRD, TEM,EM–EDX and TGA. The bulk-phase of the spent catalysts was iden-ified using XRD (Fig. 12). The carbon (graphite) peak was observedt 2� 26.5◦ in all the spent catalysts (JCPDF no. 26-1080) [45].ig. 12a compares the XRD patterns of the reduced and used Ni/SiO2atalysts and it confirms that carbon phase was formed on the spentatalyst during DRM process. Among the catalysts, Ni/SiO2 showedhe most intense of graphitic peak than the Ni/xCS and Ni/CeO2atalysts. The peak was significantly decreased in the Ni/xCS cata-yst. This might be due to a high oxygen storage capacity given byeria promoter in CeO2–SiO2 system that can suppress the cookingrocess [41,46,47].

The deposit of carbon of the spent catalyst was also examinedsing TEM, which is shown in Fig. 13. Compared to the reducedatalysts (Fig. 5), the spent catalysts showed a significant change,articularly in structure, morphology, carbon formation and nickeletal sintering. As shown in the figure, filamentous carbon formed

uring DRMC process was observed in the used catalysts, whichlmost certainly formed during DRM. However, the carbon for-ation had clear differences between the spent catalysts. For

xample, the deposited filamentous carbon covered the Ni parti-les on Ni/SiO2 catalyst severely. Meanwhile, smaller amounts ofarbon formation were observed only in certain parts for Ni/xCSatalysts, which the Ni particles sintered and separated from theeO2–SiO2 system (see the small images of Fig. 13b–d) and thearbon was hardly deposited on the nickel particles attached ateO2–SiO2 system. This result demonstrated that CeO2 additionas successful in reducing and even preventing carbon forma-

ion. Therefore, a CeO2–SiO2 support prepared by DP is considered well-developed support for nickel-based catalysts for methaneeforming processes.

The sintering of Ni particles indicated that the strong metal sup-orts interaction (SMSI) between Ni active sites and xCS support

s not quite strong. Consequently, this may make the Ni particles

asily sintered and then the carbon formation covered the liber-ted nickel species. As it is already known, a strong interactionetween active site (Ni) and support is an important aspect forry reforming catalysts in order to prevent catalyst deactivation

Temperature ( C)

Fig. 15. TGA profiles of used catalysts.

caused by sintering and carbon formation. Thus, the improvementof ceria’s role as a promoter is necessary to inhibit the formationof carbon deposition by mobilizing the oxygen-exchange capacity[2,36]. Two spent catalysts of Ni/SiO2 and Ni/9CS were selected forscanning electron microscopy analysis, as shown in Fig. 14. As it canbe observed, carbonaceous species (coke) concealed some parts ofthe catalyst surface. The carbon formation appeared worse in usedNi/SiO2 (Fig. 14a) than in the used Ni/9CS catalyst (Fig. 14b).

For quantitative determination of carbon deposits, thermalgravimetry analysis (TGA) was conducted by flowing oxygen witha temperature elevation. Fig. 15 shows the TGA profile by meansof mass loss percentage. The mass loss was regarded as the carbondeposits burned by O2 and released as CO2. Used Ni/SiO2 showedthe highest amount of mass loss, which indicated the most car-bon deposition. Among the Ni/xCS catalysts, the amount of carbondeposits increased with increasing of CeO2 addition. It could berelated to the activities of each catalyst. The accumulation of carbondeposits is caused by the reactions of CH4 decomposition (Eq. (3))and CO disproportionation (Eq. (4)) that occur during DRM process.However, the carbon amount of Ni/xCS catalysts was lower thanthat of Ni/SiO2 and Ni/CeO2. These observed results were consistentwith the other spent catalyst analyses.

4. Conclusions

The influence of ceria addition to binary CeO2–SiO2 synthesizedvia deposition precipitation for nickel catalysts was investigated.

talysis

NepaottX9N

A

o

R

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Y.H. Taufiq-Yap et al. / Applied Ca

i catalysts supported on CeO2–SiO2 (Ni/xCS) showed better prop-rties than Ni/SiO2 and Ni/CeO2 in terms of reducibility, basicity, Niarticle size and metal dispersion. These characteristics eventuallyffected the catalytic performance, stability and carbon formationf the catalysts. The catalytic-performance results showed that allhe Ni/xCS catalysts gave high activity. In addition, Ni/SiO2 gavehe highest amount of carbon deposits, which was confirmed usingRD, TEM, SEM–EDX and TGA. Binary CeO2–SiO2 support with

wt.% ceria addition was suggested to be the best composition fori/CeO2–SiO2 catalysts.

cknowledgment

Financial support from Malaysian Ministry of Science, Technol-gy and Innovation is gratefully acknowledged.

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