Accepted Manuscript

Title: CeO2-SiO2 supported nickel catalysts for dry reformingof methane towards syngas production

Authors: Yun Hin Taufiq-Yap Sudarno Umer RashidZulkarnain Zainal

PII: S0926-860X(13)00562-0DOI: http://dx.doi.org/doi:10.1016/j.apcata.2013.09.020Reference: APCATA 14458

To appear in: Applied Catalysis A: General

Received date: 7-6-2013Revised date: 6-9-2013Accepted date: 11-9-2013

Please cite this article as: Y.H. Taufiq-Yap, U. Rashid, Z. Zainal, CeO2-SiO2 supportednickel catalysts for dry reforming of methane towards syngas production, AppliedCatalysis A, General (2013), http://dx.doi.org/10.1016/j.apcata.2013.09.020

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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CeO2-SiO2 supported nickel catalysts for dry reforming of methane towards

syngas production

Yun Hin Taufiq-Yap a,b,*, Sudarno a,b, Umer Rashidc,*, Zulkarnain Zainal a,b

a Catalysis Science and Technology Research Centre, Faculty of Science, Universiti

Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysiab Department of Chemistry, Faculty of Science, Universiti Putra Malaysia,

43400 UPM Serdang, Selangor, MalaysiacInstitute of Advanced Technology, University Putra Malaysia, UPM 43400,

Serdang, Selangor, Malaysia

*Corresponding authors:

Dr. Umer Rashid, Institute of Advanced Technology, University Putra Malaysia, UPM 43400, Serdang, Selangor, Malaysia; Email: umer.rashid@yahoo.com, Off. No. +60389467393

Prof. Dr. Yun Hin Taufiq-Yap, Catalysis Science and Technology Research Centre, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. Email: yap@science.upm.edu.my; Tel: + 60-3-89466809; Fax: + 60-3-89466758

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ABSTRACT

Supported nickel catalysts (5 wt% Ni) on binary CeO2-SiO2 (CS) with different ceria

loading were prepared by wet impregnation and evaluated under catalytic dry

reforming of methane (DRM) reaction to produce 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 prepared

samples as well as to identify the carbon formation of the spent catalysts. The results

showed that the properties of CeO2-SiO2 (Ni/xCS) catalysts were superior to the

Ni/SiO2 and Ni/CeO2 catalysts, in terms of particle sizes, Ni dispersion, reducibility

and basicity. The catalyst evaluation showed that ceria addition on the Ni-supported

catalysts influenced the catalytic performances and hindered the carbon formation

significantly. In this study, Ni/CS catalyst with 9 wt% ceria exhibited good

properties, high catalytic performance, elevated stability and low carbon deposition,

thus considered to be the best catalyst with the optimal amount of ceria.

Key words: Dry reforming; Syngas; Nickel catalyst; Ceria; Carbon formation

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1. Introduction

Nowadays, global warming and diminishing supplies of fossil fuels have

attracted much attention. Thus, scientists, engineers and governments are demanded

to participate in solving these problems by any means of environmentally friendly

technologies. Carbon dioxide (CO2) and methane (CH4) are two harmful green house

gases that considerably contribute to the global warming [1]. Dry reforming of

methane (DRM) has received great attention due to its environmental benefits from

utilizing the two greenhouse gases and producing highly valuable synthesis gas

(syngas, H2 and CO) as a feedstock [2, 3]. Moreover, the syngas produced through

DRM process (Eq.1) has a H2/CO ratio of ~1, which is more compatible for many

downstream processes such as hydroformylation, carbonylation and Fischer–Tropsch

synthesis [2, 4-7].

CH4 + CO2 2CO + 2H2

H2 + CO2 H2O + CO

∆Ho298 K = -247 kJ/mol

∆Ho298 K = - 41 kJ/mol

(Eq. 1)

(Eq. 2)

CH4 C + 2H2

2CO C + CO2

∆Ho298 K = - 75 kJ/mol

∆Ho298 K = 172 kJ/mol

(Eq. 3)

(Eq. 4)

Based on (Eq. 1), syngas can be produced from CH4 and CO2 in a reaction that is

strongly endothermic and requires high temperatures (usually above 700 °C).

However, carbon formation (coking) most likely occurs from CH4 cracking (Eq. 3)

and CO disproportionation (Eq. 4), causing deactivation or destruction of the

catalysts and a blocked catalyst bed. In addition, H2O and excess of CO can be

formed from reversed-water gas shift (RWGS) reaction as shown in Eq. 2, making

the catalyst more sensitive to metal sintering [2]. Thus, the DRM reaction has not

been installed in large scale due to those reaction problems [8-10].

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Reforming catalysts are divided into two major groups which of nickel based,

catalysts and noble metal based catalysts [11]. Noble metals catalysts, such as Pt, Ru

and Rh are more resistant to coking but are very expensive and have limited

availability, so they are not preferable for use in industry. Ni-based catalysts are

more suitable and widely used for CO2 reforming of methane because of their proper

activity, availability and cheaper but are low coking resistance [12]. Therefore, the

development of Ni-based catalysts has become an interesting research-object,

directed to find effective catalysts with excellent activity and stability. These

enhancements might be achieved by improving (i) the natural properties of the

support, (ii) the preparation method of the support and catalysts, and (iii) promoter

addition [13].

Silica (SiO2) has been extensively used as a carrier for supported metal

catalysts due to high surface area and excellent metal surface dispersion [14-19].

Ceria (CeO2) is also widely used as a support and promoter additive in various

catalysts for many oxidation reactions, including the dry reforming [20], auto thermal

reforming [21] and partial oxidation of methane [22]. Ceria is a very attractive

promoter for the methane reforming process due to its high oxygen storage capacity,

redox activity as well as its extraordinary ability to enhance metal dispersion [20, 22,

23]. The high surface area of CeO2 can also be used as a promoter, providing higher

reactivity and excellent coking resistance [15, 24]. For example, ceria has been

successfully used for Ni/Al2O3 catalysts in DRM reaction for reducing carbon

formation and increasing activity and stability [25].

The CeO2-SiO2 mixed-oxide was successfully synthesized via deposition

precipitation, which gave better properties in terms of higher interaction of Se and Si

and superior metal dispersion compared to the similar support prepared by

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impregnation [26]. According to our knowledge, the use of binary oxide CeO2-SiO2

as catalyst support has not been reported widely in DRM reaction. Therefore, this

study is focused on the influence of ceria addition to silica on CeO2-SiO2 system for

supported nickel catalysts. Furthermore, a series of ceria addition to CeO2-SiO2 has

been prepared and used as Ni catalyst support for dry reforming of methane reaction.

2. Experimental

2.1. Support and catalyst preparation

The binary CeO2-SiO2 oxides with various ceria loading (3, 9, 18, 30 wt %)

were prepared by deposition precipitation (DP) and used as catalyst support. First, a

stoichiometric amount of Ce(NO3)3.6H2O (Aldrich) solution was added into a two-

neck flask containing silica Kieselgel 60). The pH of the mixture was adjusted to ~9-

10 by adding NH4OH solution. Then the mixture was stirred at 60 °C for 24 h.

Finally; the solvent was removed by rotary evaporation and drying in an oven at 80

°C overnight. The solid was crushed, sieved and calcined at 500 °C for 4 h. The

prepared CeO2-SiO2 supports were denoted as xCS, x= wt% ceria to SiO2. Besides,

CeO2 oxide was also prepared by decomposing Ce(NO3)3·6H2O at 550 °C for 4 h in

air. The obtained solid was crushed and sieved.

The catalysts, Ni/SiO2, Ni/CeO2 and Ni/xCS (5 wt% Ni), were prepared by a

wet impregnation method. Ni(NO3)2.6H2O (Aldrich) solution was added into the

supports (SiO2, CeO2 and xCS) in backer glass. The mixture was stirred for 5 h at

room temperature, and then the water was removed by rotary evaporation and drying

at 80 °C overnight. The solid catalysts were crushed, sieved and calcined at 500 °C

for 4 h. All the catalysts were designated as Ni/SiO2, Ni/3CS, Ni/9CS, Ni/18CS,

Ni/30CS and Ni/CeO2.

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2.2. Catalysts characterization

The chemical composition of the catalysts was determined by energy

dispersive X-ray fluorescent (EDX) analysis using a Shimadzu EDX-720

spectrometer. The surface area, pore volume and pore size diameter of the samples

were determined using N2 adsorption-desorption technique with a Thermo Finnigan

Sorptomatic Instrument, model 1900. Prior to analysis, the samples were degassed at

150 °C overnight. Crystalline structures of the samples were investigated using X-ray

diffraction (XRD) with a Shimadzu XRD-6000 diffractometer with Cu-Kα radiation

(λ=1.541 Å) operated at an ambient temperature (at 30 kV, 30 mA) with a scanning

rate of 2°/min.

The reducibility and basicity of the catalysts were examined by hydrogen

temperature programmed reduction (H2-TPR) and 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 under N2

gas flow (20 ml/min) at 150 °C for 10 min and cooled to ambient temperature. The

analysis was performed using 5% H2/Ar flow (25 ml/min) from 60 to 900 °C with a

ramp of 10 °C/min. For CO2-TPD, the catalyst was pretreated under N2 flow (20

ml/min) at 300 °C for 30 min and cooled to ambient temperature. Then, it was

exposed to CO2 gas (30 ml/min) for 60 min and purged with N2 flow 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 H2 consumption and CO2 desorption was detected by a thermal

conductivity detector (TCD).

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The morphology of the samples was identified using SEM with a LEO 1455

Variable Pressure scanning electron microscope. The analysis was performed on

gold-coated samples and operated at an accelerated voltage of 20 kV. The particle

size and active site dispersion of the catalysts were identified qualitatively using

transmission electron microscopy (TEM) on a Hitachi (H-7110) scanning system.

The catalysts were dispersed in 75% acetone and sonicated for 15 min. The dispersed

catalysts were deposited on a copper grid for analysis.

2.3. Catalytic evaluation

The catalytic evaluation for dry reforming of methane was performed in a

continuous flow system using a fixed bed stainless steel micro-reactor (i.d. Ø = 6

mm, h = 34 mm). The reactor was connected to a mass flow gas controller and an

online gas chromatograph (GC) (Agilent 6890N; G 1540N) equipped with Varian

capillary columns HP-PLOT/Q and HP-MOLSIV. Prior to reaction, approximately

0.20 g catalyst was reduced by flowing 5% H2/Ar (25 ml/min) from 100 to 700 °C

and holding for 3 h. The reforming reaction was performed by flowing a gas mixture

consisting of CH4/CO2 = 50/50 % mol, at a rate of 50 ml/min from 100 to 800 °C at

10 °C/min, then holding for 10 h (1 atm, GHSV = 15,000 ml/gcat h).

2.4. Analysis of used catalysts

The carbon formation of the spent catalysts was analyzed using XRD, TEM,

SEM and TGA. Except for TGA, the procedures of these techniques followed the

characterization descriptions detailed above. The temperature program oxidation

(TPO) was performed on a TGA/SDTA 851e METTLER TOLEDO instrument using

~10 mg of the spent catalysts and heating from 25 to 1000 °C (10°/min) in 5% O2/He

(50 ml/min) flow. Quantitatively, the amount of carbon formation was determined

by means of mass loss of samples

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3. Results and discussion

3.1. Textural properties

The physicochemical properties of the supports and catalysts are given in

Table 1. Surface elemental composition of the catalysts presented in the table was

obtained from energy dispersive X-ray fluorescent (EDX) analysis. This analysis was

conducted to confirm the appropriate percentage shift for each metal component.

Specifically, the percentage of Si and Ni gradually decreased on Ni/xCS catalysts

with increasing amount of ceria. For instance, the metalloid 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 loading on SiO2, respectively.

The specific surface area (SSA), pore volume and pore diameter of the

support and catalysts are also presented in Table 1. Naturally, SiO2 as the main

support had the highest surface area (408 m2/g) and CeO2 prepared by thermal

decomposition had the lowest surface area (63 m2/g). The results were similar to

those reported by Yang et al.[23]. A variance of surface area values was obtained

among the xCS supports with the range of 233 – 221 m2/g. Those SSA values are

smaller than that of SiO2 significantly, indicating that ceria addition by DP method

influenced the mixed oxide surface properties.

The surface area of Ni/xCS catalysts had been slightly higher than that of xCS

supports after the nickel impregnation. This might be suggested that there was

probably either an incomplete calcination of xCS or the influence of impregnation

during preparation of the Ni/xCS. In this work, Ni/SiO2 had the largest surface area

(294 m2/g) and Ni/CeO2 had the lowest surface area (67 m2/g) due to the natural

properties of the oxides. However, the addition of more CeO2 decreased the specific

surface area of catalysts due to CeO2 particles depositing and agglomerating on the

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SiO2 surface during catalyst preparation [22]. In another study, Mishra and Rao [27]

observed that particle size of CeO2 was affected due to the addition of diffent

conencentarion of ceria content in the CeO2-ZnO composite oxide. Crnivec et al.

[28] also reported the variation in the surface area by adding the different

concentraion of Ni-Co active metals. On the other hand, the pore diameter did not

decrease after impregnation of the nickel, indicating that the nickel particles were

located only on the surface of the support.

To understand the influence of ceria addition by DP, the bulk density of

sample was also measured and shown in Table 1. The table shows that the support

modified by ceria addition has lower density than SiO2. The CeO2 had the highest

density of 0.89 g/ml because of its natural properties (molecular weight). On the

other hand, the insertion of nickel to the supports made the nickel catalysts had lower

density than the supports. Moreover, four selected samples were investigated using

SEM (Fig. 1). The morphology with large particle size of SiO2 is shown in Fig. 1a.

For Ni/SiO2, there was no much difference of SiO2 particles after impregnation

process and Ni particles were only dispersed on SiO2 surface (Fig. 1b). In 9CS

support, the SiO2 particles were reduced in size (Fig. 1c).

This indicates that DP has significantly changed the morphology of those

prepared supports mainly SiO2 due to chemical and thermal treatments during

preparation. This indicates that CeO2 particles are dispersed well on the SiO2 surface.

The higher dispersion of ceria would assist the high dispersion of Ni active sites on

the catalyst surface [22].

3.2. Crystallographic and structural properties

The crystallographic and structural properties of the prepared supports and

catalysts samples were studied by X-ray diffraction, as shown in Fig. 2 and 3,

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respectively. For 3CS, no CeO2 peaks were observed in the XRD spectra, but the

amorphous silica peak (~21.61°) was decreased significantly. This suggested that the

crystalline structure of the support was strongly influenced by ceria addition.

Furthermore, with higher amounts of ceria (> 9 wt%), four CeO2 peaks were clearly

seen at 28.5, 33.2, 47.4 and 56.4°, which corresponded to a cubic ceria structure for

(111), (200), (220) and (311) reflections, respectively (JCPDS no. 43-1002) [29].

The spectra intensity of four ceria peaks increased with increasing ceria loading.

Between 30CS and CeO2, the four ceria peaks were similar, which could be assumed

that there is an aggregation process of ceria particles on the 30CS surface. These

XRD patterns matched those previous studies [22, 26, 29-30]. Similar results was

obtained by Mishra and Rao [27], when ceria content was increased from 20 to 80

mol% in the CeO2-ZnO mixed oxide, the intensity of ZnO decreased 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/SiO2 and Ni/xC-S at 2θ of

37.2°, 43.3° and 62.9° representing the (101), (012) and (110) reflections,

respectively (JCPDS no. 65–2901) [30, 31]. However, these peaks decreased with

increasing ceria loading, indicating that CeO2 addition influenced the catalyst

structure and nickel dispersion. NiO peaks were not observed in the XRD pattern of

Ni/CeO2 due to the physical properties of pure CeO2. Catalysts with higher ceria

loading (> 9 wt%) showed lower NiO peaks, indicating that the NiO particles were

well distributed on the Ni/xCS surface.

A nickel metal (Ni) phase is needed in the oxidative DRM reaction as the

active sites of the catalysts. Hence, the catalysts were reduced under a H2 flow prior

to reaction. Fig. 4 shows the XRD patterns of the reduced Ni catalysts. The

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characteristic nickel (Ni°) peaks appeared at 2θ = 44.5° and 51.8° corresponding to

(111) and (200) reflections, respectively (JCPDS card no. 04-0850) [32]. Compared

to Fig. 3, NiO peaks were not observed for any of the reduced catalysts, indicating

that NiO (Ni2+) particles were completely reduced to Ni°. On the other hand, CeO2

peaks were hardly detected in Ni/3CS and a low CeO2 peak (28.52°) was observed in

catalysts that have ceria loading higher than 9 wt%. This indicates that the nickel

particles were well distributed on the Ni/xC-S surface.

Crystallite sizes of nickel particles were calculated using Debye-Scherrer

formula based on the XRD data (111 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 could be attributed that the ceria addition to the support

system reduced the size of Ni particles, and consequently, the Ni dispersion was

enhanced. The dispersion and particle size was also identified by TEM technique.

Fig. 5 shows the TEM images of selected reduced catalysts pretreated under 5%

H2/Ar flow at 700 °C for 3 h. From the figure, the reduced catalysts contained small

spherical-black particles dispersed on CeO2-SiO2 supports that were assigned to Ni

metal particles. The average Ni particle size of reduced Ni Ni/SiO2 was larger than

that of reduced Ni/xCS. For reduced Ni/30CS, the Ni particle appeared only rarely.

This rarity might have been due to the amount of CeO2 being too high. These TEM

images confirmed the nickel-metal particle sizes calculated from XRD data using

Debye-Scherer equation. It could be concluded that ceria addition decreased the size

and increased the dispersion of Ni particles on the catalysts.

3.3. Reducibility and basicity

H2-TPR analysis was conducted for both selected supports and catalysts.

Reducibility of SiO2, 9CS and CeO2 are presented as TPR profiles in Fig. 6. As can

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be seen, there was no reduction peak observed for SiO2 carrier. However, in 9CS,

two reduction peaks were observed clearly at 534 and 753°C, which indicated the

reduction behavior of CeO2. Specifically, the peaks correspond to the reduction of

capping oxygen (O2- or O- anions) on the surface at lower temperature and the

reduction of ceria, CeO2 to Ce2O3 (Ce4+ Ce3+) at a higher temperature, which were

gradually reduced by elimination of O2- anions of the lattice [15, 33, 34]. For the

CeO2 sample, three H2 consumption peaks also appeared at around 392, 510 and 806

°C indicating the reduction behavior as seen for 9CS above, which were in

agreement with a previous study [33]. As Gao et al. [15], the high-temperature

regions for CeO2 reduction of 9CS indicated that there was a strong interaction

between CeO2 and SiO2 formed during the preparation.

TPR profiles of supported Ni catalysts are depicted in Fig. 7. The reduction

behavior of the catalysts was quite different than those of the supports. The TPR

profile showed a major peak among the catalysts, occurring in a temperature

maximum between 385 and 418 °C, and indicated the reduction of NiO to Ni

(Ni2+Nio) on the catalyst surface. For Ni/SiO2, the peak was observed at 370 °C,

shifting slightly to 377 °C for the Ni/3C-S catalyst. Furthermore, the main peak at

higher temperatures shifted with increasing ceria addition. This indicated that the

addition of ceria changed the properties of the supports and influenced the interaction

between NiO and the supports. A similar observation was reported by Feio et al. [35]

and Montoya et al. [36], where the H2 consumption and reduction peak of catalysts

increased with an increase of ceria loading.

However, the amount of hydrogen consumption of Ni/xCS catalysts was

higher than that of Ni/SiO2, as presented in Table 2. Among the catalysts, Ni/9CS

gave the largest amount H2 consumption of 2066.57 μmol/g. In detail; the TPR

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profiles of Ni/CeO2 were similar to those seen in Kambolis et al. [37], where three

reduction peaks were observed 232, 311 and 396 °C. This was contrary to other

reports, such as those of in ref. [20, 36], where TPR profiles of Ni-ceria supported

catalyst had only two peak ranges at higher temperatures (410-417 °C and 800-880

°C). These two peaks were attributed to the reduction of NiO and ceria, respectively.

These differences might have been due to the preparation method used either for the

support or catalysts. Based on these results, it could be concluded that the CeO2

addition and that the preparation of the catalysts plays an important role in

reducibility and hydrogen consumption.

The basicity of the catalysts was evaluated by CO2-TPD technique using the

same apparatus as TPR analysis. Fig. 8 shows the CO2-TPD profiles of the supported

Ni catalysts, and the amount of CO2 desorbed are summarized in Table 2. From the

figure, Ni/SiO2 showed desorption peak for CO2 at low temperature (344 °C), while

the promoted catalysts, Ni/xCS, showed the peak at a higher temperature (e.g. 597 °C

for Ni/9CS). This phenomenon indicated that the addition of ceria into the CeO2-

SiO2 system by DP enhanced the basicity catalysts to be stronger and increased the

amount of CO2 adsorbed. According to previous studies [15, 17], the enhanced basic

sites of the catalysts were favorable for accelerating the absorption and activation of

CO2 as well as hydrocarbon.

3.4. Catalytic Evaluation

The catalytic behaviors of the catalysts, including CH4 and CO2 conversions,

CO selectivity and H2/CO ratio are presented in Fig. 9 to 11. Fig. 9 shows the effect

of reaction temperature on the conversion of CH4 and CO2 in the DRM reaction over

the catalysts. Ni/18CS and Ni/9CS showed the highest activities while Ni/SiO2 gave

the lowest values. However, the value of those conversions increased with increasing

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reaction temperatures. Similar finding was obtained by Liu et al. [38] that during dry

reforming of methane, the concentration of CH4 formation was increased with

temperature by using the Ni/ZiO2-SiO2 catalyst. The CH4 conversion reached almost

100% at the maximum temperature of 800 °C; hence the temperature was expected to

be an appropriate temperature for DRM reaction.

The reaction was remained for 10 h at 800 °C to ascertain the catalytic

stability of the catalysts. Fig. 10 demonstrates the CH4 and CO2 conversions versus

reaction time on stream and it revealed that all catalysts showed high catalytic

performance (conversion > 90%). The conversions slightly increased with increasing

ceria loading up to 18 wt.% and decreased drastically at the ceria loading of 30 wt.%

for Ni/xCS catalysts. This could be associated that there was a difference of the

accessible active-site number on the catalyst surface because of different ceria

loading. However, between Ni/18CS and Ni/9CS that gave excellent performance,

the activities were relatively similar, and it could be assumed that 9 wt.% of ceria is

the optimum ceria loading. Although high CH4 conversion was obtained but instead

low CO2 conversion was observed for Ni/SiO2 catalyst. The high methane

conversion on Ni/SiO2 is might be due to a preferable CH4 dissociation and the low

CO2 conversion can be associated with the low adsorption of CO2 by the catalyst. On

the other hand, Ni/CeO2 exhibited a steady decrease in the reactant conversion

because of the carbon formation on the active site of the catalysts. Generally, the

conversion of CH4 was higher than the conversion of CO2; which is likely due to

side-reactions in such of Boudouard reaction (2CO C + CO2) and methane

decomposition (CH4 C + 2H2). These reactions not only produce coke (carbon)

but also enhance H2 production and reduce CO production. The coke formed by

those side reactions is the main causes of catalyst deactivation [39]. This result has

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similarities to some previous works [40, 41]. They reported that Ni catalysts

promoted by ceria exhibited higher CH4 conversion than CO2 conversion at a

temperature above 750 °C.

The plot of CO selectivity and H2/CO ratio of all the catalysts are presented

in Fig. 11. As it can be seen, the Ni/xCS and Ni/CeO2 catalysts exhibited higher CO

selectivity than Ni/SiO2 catalysts. This is in agreement with the CO2-TPD results,

which Ni/SiO2 gave the lowest basicity. In addition, the H2/CO ratio for all the

catalysts was slightly higher than unity (H2/CO > 1) which corresponds to the CH4

and CO2 conversions explained above. This result was contrasted with other works;

where the most references reported that the ratio of H2/CO was lower than 1 because

of the RWGS reaction. However, specifically in this works, the RWGS reaction was

probably not favourable in high temperature. There are some previous studies [40-

43] that obtained similar results where the H2 to CO ratio was higher than 1. The

higher ratio is probably due to the un-favourable RWGS reaction that makes an

excess of H2 [40]. Some oscillating patterns of H2/CO ratio were observed for the

catalysts which could be ascribed to the effect of carbon formation and elimination

during reaction [44]. Table 3 summarizes the catalytic performance of all the

catalysts as well as the amount of carbon deposit and surface area of the spent

catalysts.

3.5. Analysis of used catalysts

After reaction, the spent catalysts were collected to identify the carbon

formation and other characteristics using XRD, TEM, SEM-EDX and TGA. The

bulk-phase of the spent catalysts was identified using XRD (Fig. 12). The carbon

(graphite) peak was observed at 2θ 26.5 in all the spent catalysts (JCPDF no. 26-

1080) [45]. Fig. 12a compares the XRD patterns of the reduced and used Ni/SiO2

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catalysts and it confirms that carbon phase was formed on the spent catalyst during

DRM process. Among the catalysts, Ni/SiO2 showed the most intense of graphitic

peak than the Ni/xCS and Ni/CeO2 catalysts. The peak was significantly decreased in

the Ni/xCS catalyst. This might be due to a high oxygen storage capacity given by

ceria promoter in CeO2-SiO2 system that can suppress the cooking process [41, 46,

47].

The deposit of carbon of the spent catalyst was also examined using TEM,

which is shown in Fig. 13. Compared to the reduced catalysts (Fig. 5), the spent

catalysts showed a significant change, particularly in structure, morphology, carbon

formation and nickel metal sintering. As shown in the figure, filamentous carbon

formed during DRMC process was observed in the used catalysts, which almost

certainly formed during DRM. However, the carbon formation had clear differences

between the spent catalysts. For example, the deposited filamentous carbon covered

the Ni particles on Ni/SiO2 catalyst severely. Meanwhile, smaller amounts of carbon

formation were observed only in certain parts for Ni/xCS catalysts, which the Ni

particles sintered and separated from the CeO2-SiO2 system (see the small images of

Fig. 13b, c and d) and the carbon was hardly deposited on the nickel particles

attached at CeO2-SiO2 system. This result demonstrated that CeO2 addition was

successful in reducing and even preventing carbon formation. Therefore, a CeO2-

SiO2 support prepared by DP is considered a well-developed support for nickel-based

catalysts for methane reforming processes.

The sintering of Ni particles indicated that the strong metal supports

interaction (SMSI) between Ni active sites and xCS support is not quite strong.

Consequently, this may make the Ni particles easily sintered and then the carbon

formation covered the liberated nickel species. As it is already known, a strong

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interaction between active site (Ni) and support is an important aspect for dry

reforming catalysts in order to prevent catalyst deactivation caused by sintering and

carbon formation. Thus, the improvement of ceria’s role as a promoter is necessary

to inhibit the formation of carbon deposition by mobilizing the oxygen-exchange

capacity [2, 36]. Two spent catalysts of Ni/SiO2 and Ni/9CS were selected for

scanning electron microscopy analysis, as shown in Fig. 14. As it can be observed,

carbonaceous species (coke) concealed some parts of the catalyst surface. The carbon

formation appeared worse in used Ni/SiO2 (Fig. 14a) than in the used Ni/9CS

catalyst (Fig.15b).

For quantitative determination of carbon deposits, thermal gravimetry

analysis (TGA) was conducted by flowing oxygen with a temperature elevation. Fig.

15 shows the TGA profile by means of mass loss percentage. The mass loss was

regarded as the carbon deposits burned by O2 and released as CO2. Used Ni/SiO2

showed the highest amount of mass loss, which indicated the most carbon deposition.

Among the Ni/xCS catalysts, the amount of carbon deposits increased with

increasing of CeO2 addition. It could be related to the activities of each catalyst. The

accumulation of carbon deposits 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 than that of Ni/SiO2 and Ni/CeO2.

These observed results were consistent with the other spent catalyst analyses.

4. Conclusions

The influence of ceria addition to binary CeO2-SiO2 synthesized via

deposition precipitation for nickel catalysts was investigated. Ni catalysts supported

on CeO2-SiO2 (Ni/xCS) showed better properties than Ni/SiO2 and Ni/CeO2 in terms

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of reducibility, basicity, Ni particle size and metal dispersion. These characteristics

eventually affected the catalytic performance, stability and carbon formation of the

catalysts. The catalytic-performance results showed that all the Ni/xCS catalysts gave

high activity. In addition, Ni/SiO2 gave the highest amount of carbon deposits, which

was confirmed using XRD, TEM, SEM-EDX and TGA. Binary CeO2-SiO2 support

with 9 wt.% ceria addition was suggested to be the best composition for Ni/CeO2-

SiO2 catalysts.

Acknowledgment

Financial support from Malaysian Ministry of Science, Technology and Innovation is

gratefully acknowledged.

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Figure Caption:Fig. 1: Scanning electron micrographs of selected supports and catalysts

(a) SiO2; (b) Ni/SiO2; (c) 9CS; (d) Ni/9CS

Fig. 2: XRD patterns of supports

Fig. 3: XRD patterns of supported nickel catalysts

Fig. 4: XRD patterns of reduced catalysts

Fig. 5: TEM images of reduced catalysts; (a) Ni/SiO2; (b) Used Ni/9CS; (c) Ni/18CS; (d) Ni/30CS; (e)

Ni/CeO2

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

Fig. 7: H2-TPR profiles of catalystsFig. 8: CO2-TPD profiles of catalystsFig. 9: The effect of reaction temperatures on (a) CH4 and (b) CO2 conversions

over the catalystsFig. 10: Stability tests of (a) CH4 and (b) CO2 conversions over the catalysts as a

function of time (reaction temperature at 800 oC, GHSV 15000 ml g-1 h-

1)

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

as a function of time

Fig. 12: XRD patterns of used catalysts

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

Fig. 13: TEM images of used catalyst

(a) U Ni/SiO2; (b) U Ni/9CS; (c) U Ni/18CS; (d) Ni/30CS; (e)

Ni/CeO2

Fig. 14: SEM micrographs of selected used catalysts (a) Ni/SiO2; (b) Ni/9CS

Fig. 15: TGA profiles of used catalysts

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Table Caption:

Table 1: Textural properties of supports and catalysts

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

Table 3: Catalytic performance of catalysts, amount of carbon deposit and

surface area decrement of used catalysts

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Table 1

Textural properties of supports and catalysts Textural properties of supports and catalysts

Metal Loading (wt. %)

Chemical Compositiona

Support/Catalysts

Ce* Ni** Si Ce Ni

Surface Area

(m2/g)b

Cumulative Pore volume

(cm3/g)c

Average pore size

(nm)d

Bulk Density(g/ml)

SiO2 - - - - - 408 0.99 6.44 0.473C- S 3 - - - - 233 1.51 8.66 0.359C- S 9 - - - - 232 0.87 9.82 0.36

18C- S 18 - - - - 221 0.71 9.62 0.3830C- S 30 - - - - 221 0.65 10.40 0.44CeO2 - - - - - 63 0.22 10.66 0.89

Ni/SiO2 - 5 67.97 - 30.61 294 0.87 7.58 0.40Ni/3C-S - 5 63.21 7.325 28.42 250 0.86 10.32 0.31Ni/9C-S - 5 56.24 17.93 25.03 241 0.77 9.68 0.35Ni/18C-S - 5 46.26 30.36 22.67 220 0.73 9.90 0.37Ni/30C- S - 5 38.32 41.99 19.07 214 0.61 10.70 0.38Ni/CeO2 - 5 - - - 67 - -

* wt. % of CeO2 on silica ** wt. % of Ni on support

a. a. Investigated by EDX (surface analysis)b. b. Calculated by BET equationc. c. BJH desorption total pore volumed. d BJH desorption average pore diameter

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

H2-TPR CO2-TPD

Catalysts T peak(°C)

H2

Consumed(µmol/g)

Total T peak(°C)

CO2

uptake(µmol/g)

Ni size (nm)*

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/18CS413749

1340.14264.06

1604.20 600 750.03 11.75

Ni/30CS402778

1483.19518.61

2001.80 591 483.76 11.22

Ni/CeO2

230306393876

523.83933.21

1307.32764.33

3528.69 602 577.88 N.D.

* Determined by Debye-Scherer equation based on XRD pattern of reduced catalysts (plane 111)N.D. = not defined

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41

Table 31Catalytic performance of catalysts, amount of carbon deposit and surface area 2decrement of used catalysts 3

4

CH4 Conversion

(%)

CO2 Conversion

(%)

CO Selectivity

(%)H2/COCatalysts

Initial* Final** Initial Final Initial Final Initial Final

BET

Surface

Area***

(m2/g )

Decrease

of Surface

Area

(%)

Ni/SiO2 95.5 96.6 93.8 91.8 76.4 73.6 1.14 1.22 174 40.82

Ni/3CS 97.1 94.3 96.5 93.2 76.1 78.3 1.20 1.09 133 46.80

Ni/9CS 97.3 97.0 96.7 96.5 75.5 76.7 1.22 1.16 168 30.30

Ni/18CS 98.1 97.4 96.9 97.3 72.6 75.5 1.21 1.14 112 49.00

Ni/30CS 91.0 92.8 88.71 90.8 78.6 78.8 1.03 1.07 81 62.15

Ni/CeO2 97.8 90.7 96.45 86.7 73.5 78.0 1.25 1.02 25 62.70

5* Initial time of reaction at 800 °C (after 5 min)6** Final time of reaction at 800 °C (after 580 min)7*** Surface area of used catalysts8

91011

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42

20 30 40 50 60 70

NiO JCPDS-065-2901CeO

2JCPDS-043-1002

= CeO2

= NiO = SiO

2

Ni/CeO2

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO2

Degree (2)

Inte

nsity

(a.u

.)

1XRD patterns of supported nickel catalysts2

3

Graphical Abstract4

5

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43

Research Highlights1Dry reforming of methane (DRM) reaction to produce syngas.2

Development of supported nickel catalysts on binary CeO2-SiO2.3

H2/CO ratio for all the catalysts was slightly higher than unity (H2/CO > 1).4

9 wt.% ceria addition was the best composition for Ni/CeO2-SiO2 catalysts.5

678

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Fig. 1. Scanning electron micrographs of selected supports and catalysts

(a) SiO2; (b) Ni/SiO2; (c) 9CS; (d) Ni/9CS

c d

a b

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Fig. 2. XRD patterns of various binary CeO2-SiO2 oxides (catalyst supports)

20 30 40 50 60 70

(311)(220)

(200)

(111)

CeO2 JCPDS -043-1002

= CeO2

= SiO2

CeO2

SiO2

30CS

18CS

9CS

3CS

Degree (2)

Inte

nsity

(a.

u.)

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Fig. 3. XRD patterns of supported nickel catalysts

20 30 40 50 60 70

NiO JCPDS-065-2901 CeO

2 JCPDS-043-1002

= CeO2

= NiO = SiO

2

Ni/CeO2

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO2

Degree (2)

Inte

nsi

ty (

a.u

.)

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Fig. 4. XRD patterns of reduced catalysts

20 30 40 50 60 70

Ni JCPDF 004-0850

Red Ni/CeO2

= CeO2

= Ni= SiO2

Red Ni/30CS

Red Ni/18CS

Red Ni/9CS

Red Ni/3CS

Red Ni/SiO2

Degree (2)

Inte

nsi

ty (

a.u

.)

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Fig. 5. TEM images of reduced catalysts;

(a) Ni/SiO2; (b) Used Ni/9CS; (c) Ni/18CS; (d) Ni/30CS; (e) Ni/CeO2

a b

c d

c2

e

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

200 300 400 500 600 700 800 900

806oC

753oC534

oC

510oC

392oC

CeO2

9CS

SiO2

Temperature (oC)

H2

Consum

ption (

a.u

.)

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Fig. 7. H2-TPR profiles of catalysts

200 300 400 500 600 700 800 900

876oC

778oC

749oC

393oC

306oC

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

.)

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Fig. 8. CO2-TPD profiles of catalysts

300 400 500 600 700

602oC

591oC

600oC

597oC

581oC

344oC

Ni/CeO2

Ni/30CS

Ni/18CS

Ni/9CS

Ni/3CS

Ni/SiO2

Temperature (oC)

CO

2 des

orbe

d (a

.u)

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

catalysts

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

CH

4 C

on

ver

sio

n (

%)

Temperature (oC)

Ni/SiO2

Ni/3CS

Ni/9CS

Ni/18CS

Ni/30CS

Ni/CeO2

a

0

10

20

30

40

50

60

70

80

90

100

300 350 400 450 500 550 600 650 700 750 800

CO

2 C

onver

sion (

%)

Temperature (oC)

Ni/SiO2

Ni/3CS

Ni/9CS

Ni/18CS

Ni/30CS

Ni/CeO2

b

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Fig. 10: Stability tests of (a) CH4 and (b) CO2 conversions over the catalysts as a function of

time (reaction temperature at 800 oC, GHSV 15000 ml g

-1 h

-1)

50

55

60

65

70

75

80

85

90

95

100

0 50 100 150 200 250 300 350 400 450 500 550 600

CH

4 C

on

ver

sio

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 50 100 150 200 250 300 350 400 450 500 550 600

CO

2 C

onver

sion (

%)

Time on stream (min)

Ni/SiO2 Ni/3CS Ni/9CS

Ni/18CS Ni/30CS Ni/CeO2

b

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

of time

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300 350 400 450 500 550 600

CO

Sel

ecti

vit

y (

%)

Time onstream (min)

Ni/SiO2 Ni/3CS Ni/9CS

Ni/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 50 100 150 200 250 300 350 400 450 500 550 600

H2 /

CO

rat

io

Time on stream (min)

Ni/SiO2 Ni/3CS Ni/9CS

Ni/18CS Ni/30CS Ni/CeO2

b

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Fig. 12. XRD patterns of used catalysts

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

20 30 40 50

= SiO2

= Ni

= Graphite (Coke)

Degree (2)

Intensity (a.u.)

20 30 40 50 60 70

= SiO2

= CeO= Ni = Graphite (Coke)

Ni/CeO2

Ni/SiO2

Ni/3CS

Ni/9CS

Ni/18CS

Ni/30CS

Degree (2)

Inte

nsity (

a.u

.)

a b

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Fig. 13. TEM images of used catalyst

(a) U Ni/SiO2; (b) U Ni/9CS; (c) U Ni/18CS; (d) Ni/30CS; (e) Ni/CeO2

a b

c d

c2

e

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Fig. 14. SEM micrographs of selected used catalysts (a) Ni/SiO2; (b) Ni/9CS

b b

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Fig. 15. TGA profiles of used catalysts

20

30

40

50

60

70

80

90

100

100.00 200.00 300.00 400.00 500.00 600.00 700.00 800.00 900.00

Mas

s lo

ss (

%)

Temperature (oC)

Ni/SiO2

Ni/3CS

Ni/9CS

Ni/18CS

Ni/30CS

Ni/CeO2