Effect of molybdenum carbide concentration on the Ni/ZrO₂ catalysts for steam-CO₂ bi-reforming of methane

Abstract

The effect of the molybdenum carbide concentration, in the range of 0.2–3.0 wt% (nominal loading), on modified supported nickel catalysts on ZrO₂ (Mo₂C–Ni/ZrO₂) for steam-CO₂ bi-reforming of methane was investigated by correlating the various characterization results, including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM), N₂ physisorption (BET), H₂ temperature-programmed reduction (H₂-TPR), inductively coupled plasma atomic emission spectroscopy (ICP-AES) and thermogravimetric analysis (TGA), to the catalytic performance. It was found that the Ni dispersion increased with an increase in Mo₂C loading, which might lead to the strong Ni–ZrO₂ interactions confirmed by XPS. However, an appropriate Mo₂C loading is required to obtain a high surface Mo(II) content that may promote the reforming reaction by serving as another active species besides Ni. The optimized modified Ni/ZrO₂ with 0.5 wt% nominal Mo₂C loading exhibits higher catalytic activity than the others for steam-CO₂ bi-reforming of methane, which is ascribed to an increased Ni dispersion and a higher Mo(II) content. Moreover, the developed 0.5 wt% Mo₂C–10 wt% Ni/ZrO₂ catalyst shows higher catalytic stability in comparison with the unmodified 10 wt% Ni/ZrO₂ catalyst, which is ascribed to the different coke types caused by the diverse strength of the Ni–ZrO₂ interactions for the modified and unmodified catalysts.

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1. Introduction

Methane is one of the most significant C1 resources from natural gas hydrates, natural gas, coalbed gas, shale gas, methane hydrates, livestock excretion, fermented wastes, and other sources, but it is also an important greenhouse gas besides CO₂. Therefore, much attention has been focused on CO₂ reforming of methane for synthesis gas production owing to both resource utilization and environmental issues. In particular, due to depleted crude oil resources, the gas to liquid (GTL) process has been considered as an alternative technology to produce clean synthesized fuel and other chemicals from methane via the Fischer–Tropsch synthesis in large-scale production.^1–8 For the GTL process, it’s highly desirable to develop an economic and efficient method for synthesis gas production since it accounts for about 60% of the total capital cost. Moreover, it is required to produce synthesis gas with variable H₂/CO ratios according to the target process, such as Fischer–Tropsch or methanol synthesis.^9–11

The commonly used methane transformation technologies for synthesis gas production, steam reforming, dry reforming, or partial oxidation, produce a synthesis gas mixture composed of a H₂/CO ratio close to 3, 1 or 2, respectively. Additional steps are required to adjust the H₂/CO ratio to satisfy the specific requirement for the downstream transformation of the synthesis gas. Therefore, it’s highly desirable to develop an efficient synthesis gas production method to overcome the drawbacks of the conventional reforming processes. Recently, combined reforming technologies including bi and tri-reforming of methane have attracted extensive interest owing to the elimination of the additional process of adjusting the H₂/CO ratio.^12,13 Partial oxidation of methane is an energy-saving exothermic reaction and it has attracted much attention.^14–20 However, many issues such as difficult to control, easily-produced hot spots and the associated dangers of explosion limit the large-scale application of combined reforming processes involving partial oxidation reforming. Steam-CO₂ bi-reforming of methane has been considered as a judicious approach for potential industrial application in synthesis gas production as the H₂/CO ratio can be easily adjusted by controlling the H₂O/CO₂ ratio in the feed and the catalyst has an improved coke-resistance due to the introduction of steam.^{2–9,21–23} Thus it is essential to develop a robust mixed reforming catalyst with high catalytic activity and good coke resistance. In the bi-reforming process, Ni-based catalysts have been extensively employed as promising catalysts owing to their low cost but high activity, comparable to noble metal catalysts for reforming of methane.^{2,3,8,24–27}

However, Ni-based catalysts are faced with some barriers, including quick deactivation owing to metal sintering and coke deposition in the severe reforming operating conditions. Therefore, many efforts have been focused on ways to inhibit the metal sintering and coke deposition. Some interesting and efficient approaches such as employing diverse preparation methods, steam treatment, changing the support type and introducing additives have been performed to improve the catalytic stability by minimizing coke formation and inhibiting the thermal sintering of the Ni metal.^2,8,28,29 In particular, recent studies demonstrated that the catalytic activity and stability of Ni-based catalysts can be significantly improved through the introduction of a small amount of noble metals, such as Rh and Pt, resulting from the improved dispersion and reducibility of NiO, enhanced sintering-resistance, and suppressed coke deposition.^21,25–29 The noble metal modification of Ni-based catalysts could be considered as a fascinating approach to improve the catalytic performance, especially the stability, of Ni-based reforming catalysts. However, this approach is limited in its large scale application owing to the high cost and the poor availability of noble metals. There is an urgent need to develop efficient but low cost alternatives to noble metals for improving the catalytic stability of Ni-based catalysts in the reforming process.

Fortunately, non-precious transition-metal carbides, such molybdenum carbide, as Pt-like metals, have attracted much attention as promising materials due to their outstanding bulk and surface physicochemical properties.^25,26 In previous reports, metal carbides, as alternatives to noble metals, have been demonstrated as efficient catalysts and have been widely used in many reactions such as the decomposition of hydrazine, ammonia and methanol, Fischer–Tropsch synthesis, hydrogenolysis, water–gas shift, reduction of nitroarenes, electrocatalysis and reforming of diverse substrates.^30,31 According to the literature, we found that the studies focused on the use of carbides as the main active component, and the added Ni just acts as a promoter to modify the catalyst structure and catalytic performance, or the use of small amounts of metal oxide to modify the Ni-based catalyst and decrease the rate of coking in dry or steam methane reforming.^32–37 However, there is no report on the use of small amounts of metal carbide in Ni-based catalysts to improve their stability in steam-CO₂ bi-reforming of methane. Based on the excellent thermal and chemical stability, high oxygen mobilization, basicity and redox properties, ZrO₂ has been extensively used as a promising support for reforming catalysts.¹⁸

Herein, we first report the molybdenum carbide modified Ni/ZrO₂ catalyst for steam-CO₂ bi-reforming of methane (SCRM). Through adding the appropriate amount of molybdenum carbide to the supported Ni catalyst, the catalytic activity and stability of Ni-based catalysts can be significantly improved. Various characterization techniques such as XRD, TEM, BET, ICP-AES, and TGA were employed to investigate the effect of the Mo₂C concentration on the Ni/ZrO₂ catalysts for synthesis gas production. Results show that the developed molybdenum carbide modified supported nickel catalyst, prepared by the urea-assisted hydrothermal process, incipient wetness impregnation, and subsequent reduction carburization approach in this work, shows much superior catalytic stability for steam-CO₂ bi-reforming of methane, which makes it a promising candidate for syngas production via steam-CO₂ bi-reforming of methane, a key process in the gas to liquid process. Moreover, it was also found that the addition of a small amount of Mo₂C into the supported Ni catalyst significantly improved the catalytic stability, which may be ascribed to the improved NiO dispersion and suppression of carbon shell formation resulting from the strong Ni–Mo₂C interactions.

2. Experimental

2.1. Catalyst preparation

The ZrO₂ support was synthesized using a modified hydrothermal method.³⁸ In a typical synthesis, 13.7 g of Zr(NO₃)₄·5H₂O (Sinopharm Chemical Reagent Co., Ltd) and 19.2 g of urea were dissolved in deionized water. The solution was vigorously stirred for 1 h at room temperature, and was then transferred into a Teflon-lined stainless-steel autoclave, maintained at 180 °C for 20 h, and subsequently cooled to room temperature. The resulting precipitate was collected and then washed with deionized water until the filtrate became neutral. Then, the sample was dried at 105 °C for 12 h, following calcination at 400 °C for 4 h, and then the ZrO₂ support was obtained. The as-synthesized ZrO₂ support with a 141.2 m² g⁻¹ BET surface area was obtained.

The series of modified 10 wt% Ni/ZrO₂ catalysts with diverse Mo₂C loadings from 0.2 to 3 wt% (nominal content, calculated on the amount of Mo₂C added) were prepared by the incipient wetness impregnation and subsequent reduction carburization method (IWI–RC). For comparison, the unmodified Ni/ZrO₂ catalyst was prepared by the same procedure but in the absence of ammonia heptamolybdate ((NH₄)₆Mo₇O₂₄·4H₂O) in the IWI process. Generally, the aqueous solution of nickel nitrate (Ni(NO₃)₂·6H₂O) and (NH₄)₆Mo₇O₂₄·4H₂O were impregnated onto the ZrO₂ support. The obtained sample was dried at 105 °C for 12 h and subsequently calcined at 550 °C for 6 h in a muffle furnace to prepare NiO–MoO_x/ZrO₂. The resultant NiO–MoO_x/ZrO₂ was subjected to the reduction carburization process in CH₄–H₂ (20 vol% CH₄) with the heating process as follows: from room temperature to 300 °C at a ramp rate of 5 °C min⁻¹, then from 300 °C to 850 °C at a ramp rate of 2 °C min⁻¹, and maintaining this temperature for 30 min. The resulting Mo₂C–Ni/ZrO₂ samples were passivated in a mixture of 1% O₂/Ar for 12 h, and the series of Mo₂C modified Ni/ZrO₂ catalysts was obtained. As is seen from Table 1, a smaller specific surface area of the Mo₂C modified Ni/ZrO₂ in comparison to the unmodified catalyst can be observed, although the unmodified sample was also subjected to the RC process. The increase in the nominal Mo₂C loading doesn’t lead to a visible change in the surface area.

Table 1 Physical and chemical properties of the low-content molybdenum carbide modified Ni/ZrO₂ catalysts with different nominal Mo₂C loadings (x)

x (%)	SBET^a (m^2 g^−1)	CSNi^b		WCCP^c		NXPS^d
		Fresh	Spent	Ni (wt%)	Mo (wt%)	Ni/Zr	Mo/Zr
a BET specific surface area from N2 physisorption.b The average crystallite sizes of Ni estimated from the Scherrer equation according to the Ni (111) plane.c Weight content from ICP-AES measurements.d Surface atomic ratio from XPS analysis.
0	57	16.8	24.3	7.53	—	0.21	—
0.2	37	16.5	—	7.51	0.14	0.23	0.0025
0.5	39	14.8	22.1	7.49	0.33	0.22	0.0067
1.0	38	14.4	—	7.50	0.71	0.22	0.0124
2.0	38	13.5	—	7.52	1.44	0.23	0.0263
3.0	39	13.3	—	7.51	2.11	0.22	0.0396

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2.2. Catalyst characterization

The XRD patterns were collected from 10 to 80° at a step width of 0.02° using a Rigaku Automatic X-ray Diffractometer (D/Max 2400) equipped with a Cu Kα source (λ = 1.5406 Å). The average crystallite sizes were estimated on the basis of the Scherrer formula over the multiple characteristic diffraction peaks by the MDI Jade 5 software. The XPS experiments were carried out on an ESCALAB 250 XPS system with a monochromatized Al Kα X-ray source (15 kV, 150 W, 500 μm, pass energy = 50 eV). The binding energy was calibrated according to the C 1s photoelectron peak at 284.6 eV, and the binding energy of Mo 3d was analyzed at 220–240 eV using the Shirley baseline-correction method. HRTEM images were obtained using a Tecnai F30 HRTEM instrument (FEI Corp.) at an acceleration voltage of 300 kV. The impregnation of the catalyst was investigated by measuring the Ni and Mo contents (Table 2) using the ICP-AES technique on the ICP spectrometer (Optima 2000DV, Perkin Elmer). H₂-TPR experiments were performed in an in-house constructed system equipped with a TCD (thermal conductivity detector) to measure H₂ consumption. A quartz tube was loaded with 50 mg of catalyst pretreated by calcination in Ar at 300 °C for 30 min and was then cooled down to ambient temperature in Ar. After that, it was reduced with a 10 vol% H₂–Ar mixture (30 ml min⁻¹) by heating up to 850 °C at a ramp rate of 10 °C min⁻¹. TGA analysis was conducted to study the amount of coke deposited on the spent LA-Ni/ZrO₂ and Ni/ZrO₂ catalysts using a Perkin-Elmer STA 6000 instrument at a heating rate of 10 °C min⁻¹ from 30 to 800 °C in an air stream.

Table 2 Relative integrated intensity of deconvoluted Mo 3d and Zr 3d XPS spectra for the low-content molybdenum carbide modified Ni/ZrO₂ catalysts with different nominal Mo₂C loadings (x)

x (%)	Mo(II)^a (%)	Mo(IV)^a (%)	Mo(V)^a (%)	Mo(VI)^a (%)	NMo(II)/Zr^b	Zr(IV)^c (%)	Zr(III)^c (%)	NNi(0)/Ni^d (%)
a Percentage of diverse Mo species in the total Mo content.b The atomic ratio of Mo(II) to total Zr from XPS, calculated by the Mo(II) percentage in the total Mo content multiplied by the atomic ratio of surface Mo to Zr calculated from XPS.c Percentage of diverse Zr species in the total Zr content.d The atomic ratio of Ni(0) to total Ni calculated from XPS.
0	—	—	—	—	—	65.7	34.3	51.7
0.2	100	—	—	—	0.0025	62.7	37.3	49.9
0.5	85.2	—	—	14.8	0.0057	56.4	43.6	41.0
1.0	41.6	—	12.5	45.9	0.0054	53.4	46.6	34.9
2.0	19.4	25.3	11.8	43.5	0.0051	50.0	50.0	31.9
3.0	12.3	25.0	6.8	55.5	0.0049	48.0	52.0	29.9

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2.3. Catalytic performance measurement

The catalytic performance measurements were performed in a quartz tube fixed-bed continuous reactor (6 mm O.D.) at atmospheric pressure. In each test, 50 mg catalyst with a 60–80 mesh particle size was loaded between two quartz wool plugs. The temperatures were measured using K-type thermocouples and controlled by a PID controller (Xiamen Yudian). Before the reaction, the catalyst was reduced at 700 °C for 2 h under a mixture of 20% H₂ in N₂ at a flow rate of 30 ml min⁻¹. The reaction feed was controlled by the mass flow controllers (Huibolong Co.). The reactions over the diverse modified Ni/ZrO₂ catalysts were performed at 700 °C, and the feed (1.0 : 0.4 : 0.8 : 1.0 of CH₄/CO₂/H₂O/N₂) was introduced into the reactor at 36 000 ml h⁻¹ g⁻¹ gas hourly space velocity (GHSV). N₂ was used as an internal standard to calculate the CH₄ and CO₂ conversion. The effluent gas was passed through a trap to condense the residual steam and then analyzed using a gas chromatograph on-line with a molecular sieve column and a Porapaq Q column. The data were taken at 30 min for the initial performance measurement. For the stability test, the reaction conditions are as follows: 50 mg catalyst, 1.0 : 0.4 : 0.8 : 1.6 of CH₄/CO₂/H₂O/N₂ and 60 000 ml h⁻¹ g⁻¹ GHSV at 850 °C.

The CH₄ conversion, CO₂ conversion, and H₂/CO molar ratio were calculated on the basis of the following equations:

CH₄ conversion

CO₂ conversion

H₂/CO molar ratio

where the F_CH4,in, F_CH4,out, F_CO2,in, F_CO2,out, F_H2,out and F_CO,out are the flow rate of CH₄, CO₂, CO and H₂ in the feed (in) and effluent (out), respectively.

3. Results and discussion

3.1. Characterization of catalysts

The surface components and chemical state of the Mo₂C modified and unmodified Ni/ZrO₂ were measured by XPS analysis. The XPS spectra are presented in Fig. 1 and S1,† and the quantitative analytical results are shown in Tables 1 and 2. From Table 1, a diverse loading of Mo was successfully introduced into the modified catalysts. From ref. 39–45 the Mo 3d region was deconvoluted to analyze the distribution of the molybdenum oxide states, and the peaks at approximately 228.2, 229.0, 231.4, and 234.0 eV can be assigned to Mo(II), Mo(IV), Mo(V), and Mo(VI) of the Mo 3d_5/2 region. No XPS peak corresponding to metallic Mo appearing at about 227.0 eV (Mo 3d_5/2) can be detected for any samples. From Table 2, the molar ratio of Mo(II) to Zr (N_Mo(II)/Zr, denoted as surface Mo(II) content) increases as the nominal Mo loading rises, and reaches a maximum for the 0.5 wt% Mo₂C–Ni/ZrO₂ sample, but it decreases as the nominal Mo loading is further increased from 0.5 to 3.0 wt%. Mo(II) corresponds to Mo₂C,⁴⁶ which could be an active species for reforming of methane. The Zr 3d was fitted into four peaks at approximately 182.7, 183.8, 185.1, and 186.4 eV. The former two peaks can be assigned to Zr(III) and Zr(IV) of Zr 3d_5/2 (the latter two are in the Zr 3d_3/2 region). From Table 2, the Zr(III) percentage in the total Zr containing Zr(III) and Zr(IV) monotonously increases with the increase in the introduced Mo₂C, and as a corresponding result, the molar ratio of Ni(0) to total Ni decreases. The formation of Zr(III) could result from two possible processes: one is the reduction of Zr(IV) under the reduction atmosphere in the RC process for the preparation of Mo₂C–Ni/ZrO₂; the other is the solid state reaction of Ni and ZrO₂ in the RC process.⁴⁷ From Fig. S2,† the ZrO₂ was hardly reduced by hydrogen even when the temperature was increased up to 850 °C. That is to say, only a very small amount of Zr(IV) can be reduced to Zr(III) in the above mentioned RC process and the large amount of Zr(III) on the Mo₂C modified Ni/ZrO₂ catalysts confirmed by the XPS analysis (Table 2) must result from the solid state reaction of the reduced Ni and Zr(IV). The continuous increase in Zr(III) content with the increase in Mo content suggests that the introduced Mo can enhance this solid state reaction, which might be ascribed to the intensified Ni–ZrO₂ interaction. Furthermore, from Table 2, the amount of surface Mo(II), denoted as Mo₂C, reaches a maximum on 0.5 wt% Mo₂C–Ni/ZrO₂, and then decreases as the Mo content is further increased. However, the percentage of Zr(III) continuously increases with the increase of Mo content, suggesting that all Mo species can enhance the solid state reaction between Ni and Zr(IV), and not just Mo₂C. The above analysis presents evidence that the introduction of Mo can facilitate the Ni–ZrO₂ interaction. Then why does the Mo introduction lead to stronger Ni–ZrO₂ interactions? The XRD results provide one of the reasons.

Fig. 1 Mo 3d (a) and Zr 3d (b) regions of XPS spectra for the molybdenum carbide modified Ni/ZrO₂ catalysts with different nominal Mo₂C loadings (x); a: x = 0, b: x = 0.2%, c: x = 0.5%, d: x = 1.0%, e: x = 2.0%, and f: x = 3.0%.

From Fig. 2, the metallic Ni phase is identified (JCPDS no. 03-1051), and the average crystalline size of Ni was estimated by the Scherrer equation (Table 1). However, no XRD peak corresponding to the Mo₂C phase can be found, which is ascribed to the very low content of Mo₂C in the catalysts (generally it is invisible when the content is not more than 4%) or the possible insertion of Mo₂C into Ni or ZrO₂. From Table 1, the average crystalline size decreases from 16.8 to 13.3 nm as the nominal Mo₂C content is increased from 0 to 3.0 wt%. The decrease in Ni crystalline size may strengthen the interactions of Ni with the ZrO₂ support, which is consistent with the results from XPS analysis.

Fig. 2 XRD patterns of the low-content molybdenum carbide modified Ni/ZrO₂ catalysts with different nominal Mo₂C loadings with the XRD patterns for Ni and bare ZrO₂ included for comparison; a: Bulk Ni, b: ZrO₂, c: 0.5% Mo₂C/ZrO₂, d: Ni/ZrO₂, e: 0.2% Mo₂C–Ni/ZrO₂, f: 0.5% Mo₂C–Ni/ZrO₂, g: 1.0% Mo₂C–Ni/ZrO₂, h: 2.0% Mo₂C–Ni/ZrO₂, and i: 3.0% Mo₂C–Ni/ZrO₂.

3.2. Catalytic activity for steam-CO₂ reforming of methane

Fig. 3 displays the catalytic activity of the Mo₂C modified and unmodified Ni/ZrO₂ for the SCRM reaction. The catalytic activity increases as the nominal Mo₂C loading is increased, and reaches the maximum conversion for both CH₄ and CO₂ when 0.5 wt% Mo₂C is used. However, a further increase in Mo₂C leads to a decreased catalytic activity. Correlating the reaction results to the catalyst’s nature, the promoting effect of the addition of an appropriate amount of Mo₂C onto the catalytic performance of the supported Ni catalysts is independent of the surface area since the change in Mo loading doesn’t vary the surface area. Moreover, it was found that Mo(II) can efficiently enhance the SCRM reaction. For one thing, the decrease in Ni crystalline size with the increase in Mo loading, confirmed by XRD (Table 1), favors the reaction. However, many inactive MoO_x species may cover the active sites.^46,48 In addition, Mo(II) (Mo₂C) may also provide active sites for the reforming reaction as Mo₂C has Pt-like electronic properties and Pt is active for reforming. As a consequence, 0.5 wt% nominal Mo₂C loading is required for higher catalytic activity.

Fig. 3 CH₄ and CO₂ conversion over the Mo₂C modified Ni/ZrO₂ catalysts with different nominal Mo₂C loadings. The unmodified sample is included for comparison. Reaction conditions: m_cat = 50 mg, CH₄ : CO₂ : H₂O : N₂ = 1.0 : 0.4 : 0.8 : 1.0, GHSV = 36 000 ml h⁻¹ g⁻¹, performed at atmospheric pressure.

3.3. Stability for steam-CO₂ reforming of methane

The development of stable SCRM catalysts is a major concern for their industrial application. Herein, the effect of Mo₂C addition on the catalytic stability of the Ni/ZrO₂ catalysts for the SCRM reaction was investigated, and the results are displayed in Fig. 4. The optimized catalyst (0.5 wt% Mo₂C–10 wt% Ni/ZrO₂) exhibits higher catalytic stability than the unmodified Ni/ZrO₂, and no obvious decrease in the CH₄ and CO₂ conversions can be observed as the time on stream is extended to 20 h. This makes it a potential candidate for use in synthesis gas production via the SCRM reaction.

Fig. 4 Catalytic stability test of the optimized Mo₂C–Ni/ZrO₂ and the Ni/ZrO₂ catalysts for steam-CO₂ reforming of methane at 850 °C. (a) CH₄ conversion, (b) CO₂ conversion and (c) H₂/CO ratio. Reaction conditions: m_cat = 50 mg, CH₄ : CO₂ : H₂O : N₂ = 1.0 : 0.4 : 0.8 : 1.6, GHSV = 60 000 ml h⁻¹ g⁻¹, performed at atmospheric pressure.

Generally, the sintering of metallic particles and carbon deposition on the catalysts are responsible for catalyst deactivation in methane reforming.²⁴ XRD, TEM, and TGA were thus employed to reveal the reason why the addition of Mo₂C can improve the catalyst’s stability for the SCRM reaction.

Fig. 5 presents the XRD patterns of the fresh and spent supported Ni catalysts on ZrO₂ with and without Mo₂C modification, and the average crystalline size of Ni was estimated on the basis of the Ni (111) plane by the Scherrer equation (Table 1). From Fig. 5 and Table 1, the sintering phenomenon exists on both the modified and unmodified Ni/ZrO₂ after they are subjected to the high temperature SCRM reaction process, and the addition of Mo₂C doesn’t cause a visible sintering-resistance. This is different from the effect of adding precious metals that can inhibit Ni sintering,²¹ although Mo₂C is a precious metal-like material. Thus the superior stability may result from the unique coke behaviour. Fig. 6 presents the TGA/DSC profiles of the two spent catalysts. From Fig. 6, the amount of coke deposition on the modified catalyst is not less than that on the unmodified one, and therefore the higher stability of the modified Ni/ZrO₂ catalyst may result from the coke properties. Furthermore, from the DTG curves, the coke on both of the spent samples is of two chemical structures: one is amorphous carbon (α-carbon, at around 300 °C) and the other is graphitized (γ-carbon, at around 600 °C), which is consistent with the reported results.⁴⁹ It’s worth mentioning that the peak corresponding to γ-carbon over the modified catalyst shifts to a lower temperature in comparison with that over the unmodified one, suggesting a lower degree of graphitization. This confirms that the coke on the two spent catalysts is of different chemical structures, which may be responsible for the different catalytic stability. Although the amount of coke on the developed catalyst is not less than on the unmodified one, the modified catalyst shows superior catalytic stability as shown in Fig. 4.

Fig. 5 XRD patterns of fresh and spent supported Ni catalysts on ZrO₂ with and without Mo₂C modification.

Fig. 6 TGA/DTG profiles of spent supported Ni catalysts on ZrO₂ with (a) and without (b) Mo₂C modification.

The difference in the coke deposited on the two spent catalysts was further confirmed by TEM analysis. Fig. 7 presents the TEM images of the spent Ni/ZrO₂ catalysts with and without modification. From Fig. 6, the coke deposited on the spent Mo₂C–Ni/ZrO₂ catalyst is completely different from that on the unmodified one. It can be observed that two kinds of carbon are deposited: whisker-like carbon on the modified catalyst and shell-like coke encapsulating the Ni particles. It was previously reported⁵⁰ that the different strengths of Ni-support interactions can lead to different coke morphologies, and a weak Ni-support interaction can result in shell-like coke encapsulating Ni, owing to the Ni being lifted up by the coke species. However, for the supported Ni catalysts with strong metal–support interactions, the Ni cannot detach from the support, which results in whisker-like coke formation. In this work, whisker-like coke is deposited on the Mo₂C modified Ni/ZrO₂ catalyst while shell-like coke is observed on the unmodified catalyst, which is ascribed to the different strength of the Ni–ZrO₂ interactions as confirmed by the above XPS analysis.

Fig. 7 TEM images of coke on the spent Mo₂C–Ni/ZrO₂ (a) and Ni/ZrO₂ (b) catalysts. Inset in Fig. 7b is magnified shell-like carbon.

Scheme 1 illustrates the proposed mechanism for the effect of the coke morphology on the SCRM reaction. For the whisker-like coke on the modified Ni/ZrO₂ catalyst, the Ni active sites are still accessible to the reaction mixture (Scheme 1). However, the shell-like coke encapsulating the Ni active sites of the unmodified Ni/ZrO₂ catalyst would hinder the access of the reaction feed to the active sites. As a consequence, the modified Ni/ZrO₂ catalyst exhibited much higher catalytic stability for the SCRM reaction in comparison to the unmodified catalyst.

Scheme 1 Schematic illustration for steam-CO₂ bi-reforming over Mo₂C modified (a) and unmodified (b) supported Ni catalysts on ZrO₂.

4. Conclusions

In summary, the effect of the addition of low-content Mo₂C on the catalyst nature and catalytic performance in SCRM was investigated. The Mo₂C modified Ni/ZrO₂ exhibits superior catalytic activity and stability for the SCRM reaction compared to the unmodified one, which is ascribed to the catalysis of Mo₂C, improved Ni dispersion and the different coke morphologies caused by the change in the Ni–ZrO₂ interactions. An appropriate Mo₂C loading is required. Moreover, it was found that the promoting effect of the addition of an appropriate amount of Mo₂C on the catalytic performance of the supported Ni catalysts is independent of the surface area since the change in Mo loading doesn’t vary the surface area. The improving effect of adding Mo₂C to the SCRM catalyst, with low cost in comparison with precious metals, makes it a promising approach for preparing excellent SCRM catalysts with industrial prospects.

Acknowledgements

This work is financially supported by the Joint Fund of Coal, set up by National Natural Science Foundation of China and Shenhua Co., Ltd. (grant no. U1261104), and the National Natural Science Foundation of China (grant no. 21276041), and also sponsored by the Chinese Ministry of Education via the Program for New Century Excellent Talents in University (grant no. NCET-12-0079), the Natural Science Foundation of Liaoning Province (grant no. 2015020200) and the Fundamental Research Funds for the Central Universities (grant no. DUT15LK41).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22237k

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