An investigation on the role of lanthanide promoters in promoted γ-alumina-supported nickel catalysts for

dry reforming of methane Mohamad Hassan Amin, James Tardio, Suresh K. Bhargava

Centre for Advanced Materials & Industrial Chemistry, School of Applied Sciences,

RMIT University, Melbourne, VIC 3001, Australia

Email: mohamadhassan.amin@rmit.edu.au

Abstract— The influence of promoters (Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm) on the catalytic performance of sol-gel prepared Ni/γγγγ-Al 2O3 catalysts in CO2 reforming of methane was investigated. The catalysts investigated were characterised using the following techniques: synchrotron X-Ray diffraction, X-ray Photoemission Spectroscopy, a Temperature Programmed Reduction technique and N2 adsorption-desorption isotherms. It was found that there was almost a linear relationship between catalyst activity and the extent to which nickel was reduced in the catalysts tested.

Keywords- CO2 reforming of methane; Ni/γγγγ-Al2O3 catalyst; Promoter

I. INTRODUCTION

Catalytic CO2/dry reforming of methane (CH4(g) +CO2(g)↔ 2CO(g) +2H2(g)) is of significant interest due to the potential environmental benefits of utilising CO2 and due to the increased demand for syngas (H2, CO), [1, 2]. Research on dry reforming of methane (DRM) has focussed mostly on the development of improved materials for catalysing this reaction. Although noble metal based catalysts are usually highly active for the DRM (and experience low carbon deposition) they have the drawbacks of high cost and limited availability, hence there has been significant interest in the development of cheaper non-noble metal based catalysts. Of the non-noble metal based catalysts Ni based catalysts in particular have received a significant amount of interest due to the promising activity and selectivity that has been achieved. Ni-based catalysts however have a larger tendency to undergo deactivation via carbon deposition and sintering, which in turn significantly reduces their stability over time [1, 2]. Therefore, significant efforts have been devoted to improvements in the aforementioned areas.

Approaches to achieve improvements in the activity and stability of Ni based catalysts for the dry reforming of methane have focussed mostly on the discovery of better supports [1-9] and suitable promoters. The types of promoters for Ni based catalysts that have been studied include alkali [10, 11], alkaline earth [10, 12], transition metal [10, 13-17] and rare earth metal oxides [7, 12, 18-24]. Some lanthanides have been used as promoters in Ni based catalysts for the dry reforming of methane, with Ce and La oxides being the most widely studied [23, 25-29]. The positive effects of lanthanides

on catalytic activity and stability of Ni based catalysts for the dry reforming of methane have been proposed to be mostly due to the following: (1) the high oxygen storage capacity of these oxides [26], (2) improved dispersion of Ni metal on the surface of supports [26], (3) formation of nanotube and reactive filamentous carbon on Ni-based catalysts[1, 30], (4) promotion of CO2 adsorption on the surface of the catalyst [30], (5) positive effects on the chemical environment and electronic state of Ni at the Ni-lanthanides interface [31], and (6) reduction of the chemical interaction between nickel and the support resulting in an increase in the extent to which Ni is formed (i.e. increase in the extent at which Ni(II) in precursor material is reduced) [32].

Previous studies have not been conducted on the effects of a full series of lanthanide doped catalysts on Ni based dry reforming catalysts. This is important however as it can allow direct comparison in both activity and stability of catalysts doped with lanthanides under the same process conditions. Hence in the work reported here, the effect of 10 of lanthanide elements on the activity of a Ni/Al2O3 catalyst for dry reforming of methane was studied and compared under the same conditions. The lanthanide-promoted Ni/Al2O3 catalyst was chosen based on previous finding’s which showed that the introduction of 2 wt% Yb as promoter to Ni/γ-Al 2O3 catalyst via sol-gel method leads to superior catalytic activities and stabilities [1, 33- 36].

II. EXPERIMENTALS

A. Catalyst preparation Based on previous observations [1, 34] that the optimal

amount of lanthanide promoter and Ni on γ-Al 2O3 under the DRM conditions selected for testing were 2 and 15 wt% respectively, Ni(15wt%)/γ-Al 2O3 containing 2wt% of lanthanide elements (Lns) including praseodymium(Pr), neodymium (Nd), samarium(Sm), europium(Eu), gadolinium(Gd), terbium(Tb), dysprosium(Dy), holmium(Ho), erbium(Er), Thulium (Tm) and non-doped catalyst were prepared using a sol-gel method as reported earlier[1]. Materials were also subjected to a reduction process to simulate that used in the activation of Ni based catalysts prior to use in catalytic dry reforming. This process involved exposing catalysts to a mixed flow of H2 and He (10:40 mL min-1) at

700°C for 2 h. Such catalysts are hereafter referred to as reduced catalysts.

B. Characterization

Synchrotron X-ray diffraction patterns were obtained using the X-ray diffraction instrument at the Australian Synchrotron using photon energy of 17.0295521 keV (0.72888 Angstroms). A Mythen microstrip detector, which required each pattern to be taken in two parts with an acquisition time of 180 s, was used. These patterns were later merged together to produce the full pattern. BET surface area was measured using a Micromeritics ASAP 2010 surface area analyser. Surface element composition was determined using a Thermo Scientific K-Alpha XPS spectrometer. To determine the absolute peak positions in XPS, the C1s peak arising from the hydrocarbons adsorbed from the residual gas onto the pure γ-alumina and Ni/γ-Al 2O3 were used for calibration. The energy of the C1s level arising from the pure γ-alumina and Ni/γ-Al 2O3 was 285.0 eV. Temperature-programmed reduction (TPR) of the calcined catalysts was undertaken in a quartz reactor (I.D. 4.5 mm), packed with 50 mg catalyst in a flow (20 mL.min-1) of H2-Ar mixture (4.14 mol% H2) from 250 oC to 880 oC at a linear heating rate of 10oC.min-1. The H2 consumed in the TPR study was measured quantitatively by TCD. Before the TPR study, the catalyst was pretreated at 500 oC for 1h under a flow (20 cm3 min-1) of argon.

C. Catalyst testing

CO2 reforming of methane was carried out at atmospheric pressure in a conventional vertical fixed-bed quartz tubular reactor. Typically, 100 mg of granulated catalyst (500-710 microns) was pre-reduced under a 10:40 mL min-1 ratio of H2:He at 700 oC for 2 h before the reaction. The dry reforming

of methane was carried out with a vol/vol ratio of 1:1:2 of CO2:CH4:He at a constant temperature of 700 oC. The time-on-stream (TOS) activity and durability of the catalyst was collected at 700 oC and a GHSV value of 52000 mL h-1g-1. The flow rates of the gases were controlled with MKS type

1479 digital mass flow controllers. The effluent gases were analysed intermittently with a Shimadzu GC-17A with an attached (TCD). A silica packed column was used for the separation of H2, CO, CH4 and CO2. Reactant gases were analysed at STP before the reactions were allowed to proceed. The catalytic performance of the catalysts was evaluated in terms of conversions of CH4 and CO2 and the yields of H2 and CO. The time-on-stream activity and durability of the catalysts were carried out at 700 oC for 20 h.

The test conditions used in this study (feed flow rate and catalyst particle size, catalyst mass) have been shown previously to not lead to any internal or external mass transfer limitations [1].

III. RESULTS AND DISCUSSIONS

A. Characterization

The textural and chemical characteristics of the prepared materials are summarized in Table 1. From the data given in Table 1 it can be seen that the surface area of the catalysts increased slightly with the introduction of the promoters investigated.

XRD patterns of the prepared materials, prior to being reduced, are shown in Figure 1. The XRD pattern of non-doped 15Ni/Al2O3 catalyst indicated the presence of γ-Al 2O3 (and/or NiAl 2O4) and another weak peak observed at d value of 2.09 angstroms was assigned to the (012) reflection of nickel oxide. With the doping of the lanthanide promoters, no lanthanides-related crystalline phases were detected. This was mostly likely due to any crystalline phases containing these elements being below the XRD detection limit. Interestingly the NiO peaks in some of the promoted catalysts became stronger. The XRD patterns of the lanthanides-doped catalysts consisted of diffractions lines of γ-Al 2O3 (and/or NiAl2O4) and NiO phases were clarified by referring to PDF cards 10-0425 and 50-0741 for the γ-Al 2O3 and 44-1159 for NiO.

TABLE 1. Textural and chemical characteristics of prepared Ni/γ-Al2O3 catalysts

Promoter BET surface area (m2g-1)A

Average pore radius (nm)B

Average Pore Volume (cm3g-1)C

M 3+ ionic radius(nm) of

lanthanide [37]

Density of basic sites (mmol m-2) of lanthanide

[37] Without promoter

124 8.17 0.24 - -

Pr 149 7.50 0.26 0.0990 9.5 Nd 155 6.37 0.23 0.0983 15.1 Sm 162 6.18 0.23 0.0958 13.5 Eu 142 7.68 0.25 0.0947 8.4 Gd 167 5.96 0.23 0.0938 10.5 Tb 144 7.37 0.24 0.0923 9.2 Dy 156 6.11 0.22 0.0912 11.2 Ho 144 7.62 0.25 0.0901 10.8 Er 144 7.45 0.26 0.0890 11.3 Tm 152 7.45 0.25 0.0880 10.8

A: Langmuir Surface Area, B: Adsorption average pore diameter (4V/A by BET), C: BJH Adsorption Cumulative Pore Volume of pores pore volume of pores

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

Inte

nsity

(a.

u.)

d (Angstroms)

Pure γγγγ-Al2O3

Un-doped Pr-doped

Nd-doped

Sm-doped

Eu-doped

Gd-doped

Tb-doped

Dy-doped

Ho-doped

Er-doped

Tm-doped

ANiO (101)NiO

(012)NiO(110)

A A A= NiAl 2O3 or /and γγγγ-Al2O3

Figure 1. Synchrotron XRD patterns of pre-reduced non-doped and Ln-doped Ni/γ-Al2O3 samples

TABLE 2. Synchrotron XRD analysis data of catalyst samples

XRD line intensity ratio

Without promoter

Pr Nd Sm Eu Gd Tb Dy Ho Er Tm

32OAl

NiOI

I−γ

1

0.72 0.77 0.87 0.84 0.87 0.96 0.90 0.96 0.96 0.94 0.94

32OAl

NiI

I−γ

2

0.83 1.08 0.93 1.27 1.04 1.52 0.93 1.41 1.02 0.96 0.89

1-32OAl

NiOI

I−γ is the relative intensity ratios of the main diffraction peaks of NiO and γ-Al2O3, dimensionless.

2-32OAl

NiI

I−γ is the relative intensity ratios of the main diffraction peaks of Ni° and γ-Al2O3, dimensionless.

The XRD patterns obtained for the non-doped and Lns-doped materials after being reduced under similar conditions to those used at the beginning of catalyst testing (in a mixed flow of H2 and He at 700 oC for 2 h) are presented in Figure 2. It can be seen from these patterns that after the reducing treatment the samples do not show any peaks for NiO. The XRD peaks observed at d values of 2.03, 1.79, 1.24 and 1.06 angstroms, correspond to the (111), (200), (220) and (311) planes of cubic Ni0. Again no diffraction lines due to lanthanides based compounds were observed in all of the materials studied. The relative intensity ratios of the main diffraction peaks of NiO (012)/γ-Al 2O3 (211) and Ni° (111)/γ-Al 2O3(211) are listed in Table 2. The data listed in Table 2 showed that these relative intensity ratios increased with introducing lanthanides as additive.

XPS was deployed to analyse the chemical state of the elements and the surface composition of the catalysts (not shown). The quantitative results derived from the XPS analysis conducted are listed in Table 3. It was found that the amount of elemental Ni present and the binding energies of

the surface elements Ni, Al and O were affected by introduction of various lanthanides. The surface composition of the non-doped Ni/Al2O3 catalyst was characteristic only of NiAl 2O4, as shown by the Ni (2p3/2) principal peak at a binding energy of 856.18 eV with an accompanying shake-up satellite peak at 862.28 eV [38]. The origin of this satellite is due to the shake-up transition d8 → d8* or exchange interaction of 2p and 3d electrons [39]. The spectrum of O(ls) is also a singlet with binding energy at 530.73 eV which can be assigned to NiAl 2O4 spinel. There is a moderate shift of BEs to lower energies for the Ni (2p3/2) and Al (2p) peaks for promoted catalysts which suggested that the addition of promoters prevents the reaction between γ-Al 2O3 and NiO and leads to the formation of separate phases of γ-Al 2O3 and NiO, as confirmed by XRD. In addition, it was found that the Ni(2p3/2) and O(ls) binding energy separation (∆E) decrease slightly with introducing promoters which indicated that introducing lanthanide promoters reduced the chemical interaction between nickel and the support. The degree of shifting and ∆E was dependent on the specific promoter.

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Inte

nsity

(a.

u.)

d (Angstroms)

Un-doped

Pr-doped

Nd-doped

Sm-doped

Eu-doped

Gd-doped

Tb-doped

Dy-doped

Ho-doped

Er -doped

Tm-doped

Ni(111)

Ni(200)Ni

(220)

AA A A

A= NiAl 2O3 or /and γγγγ-Al2O3

Figure 2. Synchrotron XRD patterns of reduced non-doped and Ln-doped Ni/γ-Al2O3 samples

TABLE 3. Data obtained from XPS analysis on the non-doped and Lns-doped Ni/γ-Al 2O3 catalysts

Promoter

Binding energy of the surface elements of the pre-reduced catalysts (eV) ANi 0/(ANi 0 +A Ni2+) in reduced

catalysts Ni (2p3/2)

Principal peak

Ni (2p3/2)

Satellite peak O (1s) Al (2p) ∆∆∆∆E

Without promoter 856.18 862.28 530.73 74.28 325.45 0.43

Pr 856.38 862.38 530.98 73.98 325.40 0.45

Nd 855.68 862.08 530.38 74.28 325.30 0.3

Sm 856.38 862.48 531.03 74.18 325.35 0.45

Eu 855.88 861.98 530.58 73.68 325.30 0.5

Gd 855.28 861.28 530.08 73.68 325.20 0.23

Tb 855.38 861.38 530.08 74.08 325.30 0.52

Dy 855.68 861.58 530.48 73.98 325.20 0.38

Ho 855.78 861.98 530.58 73.88 325.20 0.6

Er 856.18 862.28 530.93 75.28 325.25 0.57

Tm 855.28 861.48 530.03 73.88 325.25 0.56

∆E: Ni (2p3/2)→ O (1s) binding energy separation (principal peaks)

Deconvolution of the Ni 2p spectra of reduced catalysts was performed by Gaussian–Lorentzian curve-fitting to determine the peak areas and obtain relative surface concentrations of nickel species. The ratio of peak areas ANi 0/(ANi 0 +A Ni2+) is used to illustrate the degree of reducibility. As displayed in Table 3, an increase in the ANi 0/(ANi 0 +A Ni2+) ratio for promoted 15Ni/γ-Al 2O3 catalysts is seen after reduction compared to that of the un-promoted catalyst, indicative of an enhancement in the Ni(II) reducibility for some of the catalysts that contained lanthanides (Pr, Sm, Eu, Tb, Ho, Er, Tm).

The reducibility of the Ni(II) in the non-promoted and lanthanides-promoted Ni/γ-Al 2O3 catalysts were studied using temperature-programmed reduction (TPR). Figure 3 shows the TPR profiles of the catalysts. The TPR profiles of all the catalysts consisted of two clear reduction temperature zones. The low temperature zone (340 – 520oC) and high temperature zone (590 – 945oC) represent the reduction of NiO and spinel type nickel-aluminium oxides in Ni/γ-Al 2O3 respectively [1, 25, 40-41]. Based on the amount of H2 consumed in the two temperature zones the majority of the oxidised Ni in both the non-doped and doped Ni/γ-Al2O3 catalysts was present in the form of a nickel aluminate.

Based on the H2 consumption obtained from TPR analysis, the reducibility of Ni(II) in the calcined samples was calculated as shown in Table 4. The reducibility of pure NiO, which was

used as the reference material, was defined as 1 as reported earlier [1].

350 450 550 650 750 850 950

Inte

nsity

(a.

u.)

Temperature

Un-doped

Pr-doped

Nd-doped Sm-dopedEu-doped

Gd-doped

Tb-doped

Dy-dopedHo-doped

Er-doped

Tm-doped

Figure 3. TPR profile of Ni/γ-Al2O3 catalysts with different lanthanide promoter.

TABLE 4. Reducibility of 15Ni/γ-Al2O3 catalysts with different promoter

Promoter Without promoter

Pr Nd Sm Eu Gd Tb Dy Ho Er Tm

Reduction degree at the first peak

0.25 0.23 0.21 0.25 0.28 0.21 0.27 0.22 0.30 0.31 0.30

Reduction degree at the second peak

0.25 0.20 0.14 0.19 0.19 0.13 0.12 0.14 0.16 0.11 0.13

B. Catalytic activity for dry reforming of methane

The ability of the un-doped and doped Ni/γ-Al 2O3 materials to catalyse the dry reforming of methane was investigated at 700 °C using a GHSV of 5.2×104 mLg−1h−1 for a period of 20 h. The results, in terms of CH4 and CO2 conversion, from the aforementioned tests are presented in Figures 4a and b.

From the results presented in Figures 4a and b it can seen that the activity strongly depends on the effect caused by the addition of different additives to the Ni/γ-Al 2O3 catalyst. Eu,

Tb, Ho, Er and Tm show a positive effect whereas a negative effect is caused by Gd, Nd, Dy, Sm and Pr (in terms of activity at 1220 min time on stream). The activity of the catalysts investigated at 20 h time on stream was as follows (in terms of promoter used): Gd < Nd < Dy < Sm <Pr < un-doped <Tb < Ho <Eu <Er < Tm. The best activity and stability was found for thulium-doped Ni/γ-Al 2O3 catalyst, whose almost constant CH4 and CO2 conversion are the highest while on stream of 20h.

(a)

(b)

Figure 4. Effect of promoters on the conversion of (a) CH4 and (b) CO2 over Ni/γ-Al2O3 catalysts (reaction conditions: reaction temperature = 700 oC; GHSV = 5.2 × 104 mLg−1h−1; CO2/CH4 = 1.0, TOS = 20 h)

The trends observed in activity for the un-doped and doped Ni/γ-Al 2O3 materials correlated strongly with the relative intensity ratios of the main diffraction peaks of Ni and γ-Al 2O3 and the ratio of peak areas ANi0/(ANi 0 +A Ni2+) which calculated using XRD and XPS techniques, respectively, and the reduction degree of the first peaks of TPR of these

materials (Figure 5) with the degree of CH4 and CO2 conversions observed increasing with increasing the degree of reducibility of nickel oxides. Based on the results, it can be concluded that easier reduction of nickel species (nickel oxide and nickel aluminium compound) in lanthanide- doped Ni/γ-Al 2O3 catalysts to metallic state plays a key role to

improvement in catalytic activity with respect to CH4 and CO2 conversions.

The H2/CO ratio results for Ni/γ-Al 2O3 catalysts with different promoters are given in Table 5. From the data given in Table 5 it can be seen that the H2/CO ratios were less than unity for all the catalysts investigated. This was most likely due to the reverse water gas shift reaction also occurring under the conditions used [23]. With the exception of the Pr and Gd doped Ni/γ-Al 2O3 that had the lowest H2/CO ratios, the trends

observed in the H2/CO ratios for the doped Ni/γ-Al 2O3 materials were closely linked with the density of basic sites of lanthanide (Figure 6). The reduced H2/CO ratios obtained using the catalysts containing more basic promoters is most likely due to increased reverse water gas shift (RWGS) reaction occurring (CO2 + H2→ CO + H2O). This in turn may have been due to the basicity introduced by the lanthanide promoter causing an increase in CO2 affinity for the structure; where this increased attraction would result in an increase in the occurrence of the RWGS reaction.

52

56

60

64

68

0.85 1.05 1.25 1.45

CH

4co

nver

sion

(%

)

I(Ni)/I(Al 2O3) (a)

52

56

60

64

68

0.2 0.3 0.4 0.5 0.6

CH

4co

nver

sion

(%

)

ANi0/(ANi 0 +A Ni2+)

(b)

52

56

60

64

68

0.2 0.24 0.28 0.32

CH

4co

nver

sion

(%

)

Reduction degree at the first peak

(c)

Figure 5. CH4 conversion (%) versus (a) relative intensity ratios of the main diffraction peaks of Ni (111) and γ-Al 2O3(211), (b) ratio of peak areas ANi0/(ANi 0 +A Ni2+) and (c) reduction degree at the first peak in TPR profile.

TABLE 5. H2/CO ratios obtained at 20 h time on stream over Ni/γ-Al2O3 catalysts with different promoter

Promoter Without promoter

Pr Nd Sm Eu Gd Tb Dy Ho Er Tm

H2/CO 0.902 0.893 0.893 0.899 0.909 0.892 0.913 0.903 0.914 0.911 0.910

0.89

0.895

0.9

0.905

0.91

0.915

0.92

9 10 11 12 13 14 15 16

H2/

CO

mmol/m2

Figure 6. H2/CO ratios obtained at 20 h time on stream over Ni/γ-Al2O3 catalysts versus the density of basic sites of lanthanide

CONCLUSIONS

A series of sol-gel Ni-γAl 2O3 catalysts promoted with

different lanthanide elements (Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm) have been studied in the dry reforming of methane. Europium(Eu), terbium(Tb), holmium(Ho), erbium(Er) and Thulium (Tm) had a positive effect on catalytic activity when used as a promoter in Ni/γ-Al 2O3 catalysts, while the other promoters investigated, Gd, Nd, Dy, Sm and Pr, caused decreased catalyst performance (as compared to a similar non-promoted catalyst). The positive effect on the dry reforming catalyst that was observed with some of the lanthanide promoters studied was most likely due to the Ni(II) in these catalysts being more readily reduced to the active form required, (Ni). This in turn most likely led to an increased number of active catalytic sites in these promoted catalysts.

ACKNOWLEDGMENT

The authors gratefully acknowledge the facilities, and the scientific and technical assistance, of the Australian Microscopy & Microanalysis Research Facility at the RMIT Microscopy & Microanalysis Facility, at RMIT University and support from the Australian Synchrotron Research Program.

The authors thank Dr Selvakannan Periasamy and Dr. Jamie Booth for their helpful discussions, Dr. Jarrod Newnham and Dr. Samuel Ippolito for their help in automating the reactor system, and Dr. Justin Kimpton (Scientist – PD) and Moein Amin for their help acquiring the diffraction data at Australian synchrotron.

REFERENCES [1] M. H Amin, K. Mantri, J. Newnham, J. Tardio, S. K. Bhargava, Highly

stable ytterbium promoted Ni/γ-Al2O3 catalysts for carbon dioxide

reforming of methane, Appl. Catal., B, vol. 119– 120, pp. 217– 226, 2012.

[2] J. Newnham, K. Mantri, M. H Amin, J. Tardio, S. K. Bhargava, Highly stable and active Ni-mesoporous alumina catalysts for dry reforming of methane, Int. J. Hydrogen Energy, vol. 37(2), pp. 1454-1464, 2012.

[3] D. Liu, R. Lau, A. Borgna, Y. Yang, Carbon dioxide reforming of methane to synthesis gas over Ni-MCM-41 catalysts, Appl. Catal., A , vol. 358, pp. 110–118, 2009.

[4] Z. Meili, J. Shengfu, H. Linhua, Y. Fengxiang, L. Chengyue, L. Hui, Structural characterization of highly stable Ni/SBA-15 catalyst and its catalytic performance for methane reforming with CO2, Chin J Catal., vol. 27(9), pp. 777–782, 2006.

[5] D. Liu, X.-Y. Quek, H. H. A. Wah, G. Zeng, Y. Li, Y. Yang, Carbon dioxide reforming of methane over nickel-grafted SBA-15 and MCM-41 catalysts, Catal. Today, vol. 148, pp. 243–250, 2009.

[6] D. Sun, X. Li, S. Ji, L. Cao, Effect of O2 and H2O on the tri-reforming of the simulated biogas to syngas over Ni-based SBA-15 catalysts, J. Nat. Gas Chem., vol. 19, pp. 369–374, 2010.

[7] N. Wang, W. Chu, T. Zhang, X.S. Zhao, Synthesis, characterization and catalytic performances of Ce-SBA-15 supported nickel catalysts for methane dry reforming to hydrogen and syngas, Int. J. Hydrogen Energy, vol.37, pp. 19-30, 2012.

[8] A. Albarazi, P. Beaunier, P. Da Costa, Hydrogen and syngas production by methane dry reforming on SBA-15 supported nickel catalysts: On the effect of promotion by Ce0.75Zr0.25O2 mixed oxide, Int. J. Hydrogen Energy, vol. 38, pp. 127-139, 2013.

[9] Z. Liu, J. Zhou, K. Cao, W. Yang, H. Gao, Y. Wanga, H. Li, Highly dispersed nickel loaded on mesoporous silica: One-spot synthesis strategy and high performance as catalysts for methane reforming with carbon dioxide, Appl. Catal., B, vol. 125, pp. 324– 330, 2012.

[10] A. E. Castro Luna, M. E. Iriarte, Carbon dioxide reforming of methane over a metal modified Ni-Al2O3 catalyst, Appl. Catal., A , vol. 343(1-2) , pp. 10-15, 2008.

[11] M. M. Barroso-Quiroga, A. E. Castro-Luna, Catalytic activity and effect of modifiers on Ni-based catalysts for the dry reforming of methane, Int. J. Hydrogen Energy, vol. 35, pp. 6052– 6056, 2010.

[12] H. S. A. de Sousa, A. N. da Silva, A. J.R. Castro, A. Campos, J. M. Filho, A. C. Oliveira, Mesoporous catalysts for dry reforming of methane: Correlation between structure and deactivation behaviour of Ni-containing catalysts, Int. J. Hydrogen Energy, vol.37, pp. 12281-12291, 2012.

[13] S. Corthals, J. V. Nederkassel, H. De Winne, J. Geboers, P. Jacobs, B. Sels, Design of active and stable NiCeO2ZrO2MgAl2O4 dry reforming catalysts, Appl. Catal., B, vol. 105, pp. 263–275, 2011.

[14] Mun-Sing Fan, Ahmad Zuhairi Abdullah, Subhash Bhatia, Utilization of greenhouse gases through carbon dioxide reforming of methane over Ni–Co/MgO–ZrO2: Preparation, characterization and activity studies, Appl. Catal., B, vol. 100, pp. 365–377, 2010.

[15] M. Garcia-Dieguez, E. Finocchio, M. A. Larrubia, L. J. Alemany, G. Busca, Characterization of alumina-supported Pt, Ni and PtNi alloy catalysts for the dry reforming of methane, J. Catal., vol. 274, pp. 11–20, 2010.

[16] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, Nanostructured Pt- and Ni-based catalysts for CO2-reforming of methane, J. Catal., vol. 270, pp. 136–145, 2010.

[17] X. Li, Q. Hua, Y. Yanga, Y. Wanga, F. He, Studies on stability and coking resistance of Ni/BaTiO3–Al2O3 catalysts for lower temperature dry reforming of methane (LTDRM), Appl. Catal., A , vol. 413– 414, pp. 163– 169, 2012.

[18] B. Li, X. Xu, S. Zhang, Synthesis gas production in the combined CO2 reforming with partial oxidation of methane over Ce-promoted Ni/SiO2 catalysts, Int. J. Hydrogen Energy, vol. 38, pp. 890-900, 2013.

[19] A. Jahangiri, H. Pahlavanzadeh, H. Aghabozorg, Synthesis, characterization and catalytic study of Sm doped LaNiO3 nanoparticles in reforming ofmethane with CO2 and O2, Int. J. Hydrogen Energy, vol.37, pp. 9977-9984, 2013.

[20] X. Yu, N. Wang, W. Chu, M. Liu, Carbon dioxide reforming of methane for syngas production over La-promoted NiMgAl catalysts derived from hydrotalcites, Chem. Eng. J., vol. 209, pp. 623–632, 2012.

[21] J. Zhu, X. Peng, L. Yao, J. Shen, D. Tong, C. Hu, The promoting effect of La, Mg, Co and Zn on the activity and stability of Ni/SiO2 catalyst for CO2 reforming of methane, Int. J. Hydrogen Energy, vol. 36, pp. 7094-7104, 2011.

[22] O. Gamba, S. Moreno, R. Molina. Catalytic performance of Ni-Pr supported on delaminated clay in the dry reforming of methane. Int. J. Hydrogen Energy, vol. 36, pp.1540-1550, 2011.

[23] C. E. Daza, S. Moreno, R. Molina, Co-precipitated Ni-Mg-Al catalysts containing Ce for CO2 reforming of methane, Int. J. Hydrogen Energy, vol. 36 (2011) 3886-3894.

[24] H. Li, H. Xu, J. Wang, Methane reforming with CO2 to syngas over CeO2-promoted Ni/Al2O3-ZrO2 catalysts prepared via a direct sol-gel process, J. Nat. Gas Chem., vol. 20, pp.1–8, 2012.

[25] X. Junke, Z. Wei, W. Jihui, L. Zhaojing, M. Jianxin; Characterization and analysis of carbon deposited during the dry reforming of methane over Ni/La2O3/Al2O3 catalysts; Chin J Catal., vol. 30(11), pp.1076-1084, 2009.

[26] R. Yang, C. Xing, C. Lv, L. Shi, N. Tsubaki; Promotional effect of La2O3 and CeO2 on Ni/γ-Al2O3 catalysts for CO2 reforming of CH4;

Appl. Catal., A, vol. 385, pp.92–100, 2010.

[27] C. E. Daza, J. Gallego, F. Mondragón, S. Moreno, R. Molina, High stability of Ce-promoted Ni/Mg–Al catalysts derived from hydrotalcites in dry reforming of methane, Fuel 89 (3), pp.592-603, 2010.

[28] M. Ocsachoque, F. Pompeo, G. Gonzalez, Rh–Ni/CeO2–Al2O3 catalysts for methane dry reforming, Catal. Today, vol. 172(1), pp.226–231, 2011.

[29] L. Guczi, G. Stefler, O. Geszti, I. Sajo, Z. Paszti, A. Tompos, Z. Schay, Methane dry reforming with CO2: A study on surface carbon species, Appl. Catal., A, vol. 375, pp.236–246, 2010.

[30] W.D. Zhang, B.S. Liu , C. Zhu, Y.L. Tian, Preparation of La2NiO4/ZSM-5 catalyst and catalytic performance in CO2/CH4 reforming to syngas, Appl. Catal., A, vol. 292, pp.138–143, 2005.

[31] Z. W. Liu, H. S. Roh, and K.W. Jun, Carbon dioxide reforming of methane over Ni/La2O3/Al2O3, J. Ind. Eng. Chem., 9(3), pp. 267-274, 2003.

[32] A. Horvath, G. Stefler, O. Geszti, A. Kienneman, A. Pietraszek, L. Guczi, Methane dry reforming with CO2 on CeZr-oxide supported Ni, NiRh and NiCo catalysts prepared by sol–gel technique: Relationship between activity and coke formation, Catal. Today, vol. 169(1) , pp. 102-111, 2011.

[33] M. H. Amin, K. Mantri, S. K. Bhargava, Effect of ytterbium promoted Ni/γ-Al2O3 catalysts on catalytic performance in CO2 reforming of methane, Chemeca2010, September 2010, The University of Adelaide, Australia.

[34] M. H. Amin, K. Mantri, S. K. Bhargava, Activity and stability improvements of sol-gel-generated Ni-Yb/Al2O3 catalysts for CH4/CO2 reforming, Chemeca2011, September 2011, Hilton Sydney, New South Wales, Australia.

[35] M. H. Amin, J. Newnham, S. K. Bhargava, The Effect of lanthanide promotion on catalytic performance of sol-gel Ni/Al2O3 catalysts in the CO2 dry reforming of methane, Chemeca2011, September 2011, Hilton Sydney, New South Wales, Australia.

[36] M. H Amin, J. Tardio, S. K. Bhargava, An investigation on the role of ytterbium in ytterbium promoted γ-alumina-supported nickel catalysts for dry reforming of methane, Int. J. Hydrogen Energy, in press.

[37] Satoshi Sato, Ryoji Takahashi, Mika Kobune, Hiroshi Gotoh, Basic properties of rare earth oxides, Appl. Catal., A, vol. 356, pp.57–63, 2009.

[38] D. Dissanayake, M.P. Rosynek, K.C.C. Kharas, J.H. Lunsford, Partial Oxidation of Methane to Carbon Monoxide and Hydrogen over a Ni/Al 2O3 Catalyst, J. Catal. 132, pp.117–127, 1991.

[39] G. R. Gavalas, C. Phichitkul, G. E. Voecks, Structure and activity of NiO/α-Al2O3 and NiOZrO2 calcined at high temperatures: I. Structure, J. Catal. 88, pp.54-64, 1984.

[40] J.M. Rynkowski, T. Paryjczak, M. Lenik, Characterization of alumina supported nickel-ruthenium systems, Appl. Catal., A, vol. 126, pp. 257-271, 1995.

[41] B.S. Liu, C.T. Au, Carbon deposition and catalyst stability over La2NiO4/-Al2O3 during CO2 reforming of methane to syngas, Appl. Catal., A, vol. 244, pp.181–195, 2003.

[42] X.i Zhu, P. Huo, Y.P. Zhang, D.G. Cheng, C.J. Liu, Structure and reactivity of plasma treated Ni/Al2O3 catalyst for CO2 reforming of methane, Appl. Catal., B, vol. 81, pp.132–140, 2008.