English

• INTRODUCTION

  Dry reforming of methane (DRM) has been well regarded as a promising process for the effective conversion of two greenhouse gases (CH₄ and CO₂) into synthesis gas (CO and H₂), which can be used as a versatile feedstock for value-added chemicals and fuels.^[1-4] Among various applied catalysts, Ni-based catalysts with comparable activity as noble-based catalysts have been designed and developed for highly efficient DRM.^[5-8] However, owing to the endothermic feature of DRM, the reaction is thermodynamically favorable at high temperature, where sintering of Ni particles easily occurs and causes the decline of the catalytic performance. Furthermore, coke deposition by the presence of large amounts of carbon filaments from CH₄ and CO dissociation is the main origin for the deactivation of the catalysts.

  A potential solution to the sintering and coking issues is to modify the structures of Ni-based catalysts by building strong metal-support interaction (SMSI) with plentiful interfacial sites for the reaction.^[9-12] Among them, industrially and widely used Al₂O₃ support Ni catalysts have gained much attention. SMSI effect on Ni/Al₂O₃ facilitated the generation of small and homogeneous Ni particles on Al₂O₃ support as originated from the formation of active NiAl₂O₄ spinel.^[13] However, if the metal-support interaction on Ni/Al₂O₃ is too strong, inactive NiAl₂O₄ spinel would be produced, which would be difficult to reduce to Ni particles at a moderate temperature and high temperature reduction from NiAl₂O₄ to Ni particles would result in the growth of large Ni particles.^{[14, 15]}

  Generally, introducing CeO₂ into Ni/Al₂O₃ with the formation of Ni/CeO₂-Al₂O₃ is able to improve Ni reducibility at low temperature with a high dispersion of Ni particles, owing to the fact that CeO₂ exhibits remarkable oxygen mobility and excellent redox properties (Ce⁴⁺/Ce³⁺).^{[16, 17]} The SMSI effect between metal and support in Ni/CeO₂-Al₂O₃ catalysts provides fruitful benefits and enhancements towards heterogeneous catalysis, including in steam or dry reforming of methane, catalytic biomass gasification, CO₂ metha-nation, reverse water-gas shift and methane catalytic decomposition.^[18-22] Specifically, the addition of CeO₂ in Ni/Al₂O₃ is able to change the metal-support interaction with different reductive behaviors, which plays an important role in controlling particle size, tuning electronic structure of Ni, providing oxygen species and stabilizing active phase of Al₂O₃.^[20-22] However, the changes of interaction and interface structures in Ni/CeO₂-Al₂O₃ usually occur simultaneously. Therefore, it is critical to investigate the accurate role of interaction or interface in Ni/CeO₂-Al₂O₃ to boost catalytic performance.

  In this work, two strategies of interpreting the effects of SMSI and interface in Ni/CeO₂-Al₂O₃ on catalytic performance were proposed. One involved manipulating the same metal-support interaction with different interface structures by comparing Ni/CeO₂-Al₂O₃ with Ni/Al₂O₃, and the other included controlling the same interface structure but different interaction of two Ni/CeO₂-Al₂O₃ catalysts. Specifically, pseudobohemite (AlOOH), being extensively used as binder and alumina precursor in industry, was first treated with nitric acid under microwave irradiation to generate aluminum gel. Afterwards, the obtained gel, rather than aluminum isopropoxide as a classic aluminium source, was blended with the solution of Ni²⁺ and/or Ce³⁺ ions and placed on a Petri dish for a modified evaporation-induced self-assembly (mEISA) method.^[23] The resultant Ni/CeO₂-Al₂O₃ catalyst presented porous structure with a high surface area, and most importantly, the SMSI effect on Ni/CeO₂-Al₂O₃ with tailored interface in Ni-CeO₂ promoted the catalytic activity and stability in DRM with enhanced resistance of coke deposition and sintering of Ni particles.

  RESULTS

  Characterizations of NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃. Aluminum gel with white translucent feature was originally generated from industrial pseudo-bohemite (AlOOH) by nitric acid treatment under microwave irradiation for 1 h. The gel was then fully mixed with ethanol solution containing Ni²⁺ and/or Ce³⁺ ions. The solution in the mixture was evaporated in an oven and the fresh samples were produced by thermal calcination of the dried mixture at 600 ℃ under air. The resultant samples, including NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃, exhibit light blue and bluish-green colors, respectively. Ni content is 8.9% for the former and 9.1% for the latter, separately, and Ce loading is 9.6% for NiO/CeO₂-Al₂O₃ according to the results of inductively coupled-plasma atomic emission spectroscopy (ICP-AES) measurements. NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃ possessed large BET surface areas of 171 and 197 m² g^-1, respectively, and have similar pore diameter distribution of approximate 16 nm (Table S1 and Figure S1).

  The powder X-ray diffraction (PXRD) patterns of NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃ are shown in Figure 1a. Both samples show characteristic diffraction peaks for Al₂O₃ (circle mark) and NiAl₂O₄ (diamond mark). Besides, two peaks indexed to (111) and (311) planes for CeO₂ (star mark) appeared on NiO/CeO₂-Al₂O₃. The reductive behaviors of NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃ were studied by temperature programmed reduction with H₂ (H₂-TPR), as shown in Figure 1b. Three deconvoluted peaks can be detected at 550, 680, and 750 ℃ on the two samples. The high temperature peak originated from the reduction of crystalline spinel (NiO-γ) and the first two peaks corresponded to the formation of NiO (NiO-β1 and NiO-β2) considerably interacted with support.^{[24, 25]} The ratio of NiO-β1:NiO-β2:NiO-γ was 6.1:34.4:59.5 for NiO/Al₂O₃ and 4.8:32.0:63.2 for NiO/CeO₂-Al₂O₃, respectively, which indicates that the two samples possessed the same metal-support interaction with 40% of NiO-β species. Compared with previous work, ^{[16, 17]} the addition of CeO₂ into NiO/Al₂O₃ hardly affected the reductive behaviors of NiO/CeO₂-Al₂O₃, which was probably owing to the different method and low CeO₂ loading content in this work.

  Figure 1

  

  Figure 1.  (a) XRD patterns, (b) H₂-TPR profiles and (c, d) Ni 2p and Ce 3d XPS spectra of NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃.

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  Figure 1c shows the X-ray photoelectron spectra for Ni 2p_3/2 on NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃. Two peaks of nickel in +2 state exist, which are attributed to the presence of NiO and NiAl₂O₄ on the surface of both samples.^{[26, 27]} Therefore, the two samples possess strong metal-support interaction between Ni and support as well as similar surface amount of NiO (37.1% for NiO/Al₂O₃ and 38.8% for NiO/CeO₂-Al₂O₃) and NiAl₂O₄ (62.9% for NiO/Al₂O₃ and 61.2% for NiO/CeO₂-Al₂O₃). In addition, the XPS spectrum of Ce 3d of NiO/CeO₂-Al₂O₃ presents the generation of Ce³⁺ and Ce⁴⁺ species, where the former accounted about 32% of the total amount of surface cerium.

  Characterizations of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ Catalysts. NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃ were reduced at 550 ℃ under H₂ flow to generate Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ catalysts, respectively. Because the Ni characteristic diffraction peaks of XRD patterns for reduced catalysts are not apparent, H₂ pulse chemisorption was carried out to characterize the accessible nickel on the catalysts (Table S2). Both catalysts have similar active Ni surface area of 55 m² g^-1 with Ni dispersion of 6.5%, and the particle sizes of Ni particles were calculated to be 7.8 nm for Ni/Al₂O₃ and 6.5 nm for Ni/CeO₂-Al₂O₃, respectively.

  The morphological observation of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ was studied by transmission electron microscope (TEM) mea-surements, as shown in Figure 2. Both catalysts showed rod-like structure with the length of about 30 nm and width of 10 nm (Figure 2a and c). No aggregation of Ni particles can be observed on the two catalysts. The dominantly exposed plane of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ presented the lattice spacing of 0.203 nm, which originated to the (111) plane of Ni (Figure 2b and d). Furthermore, Ni/CeO₂-Al₂O₃ exhibited lattice spacing of 0.312 and 0.271 nm, corresponding to the (111) and (200) planes of CeO₂. It should be noted that Ni particles were closely interacted and intimately contacted with CeO₂, leading to the formation of interface structure between metal and support. This can be further illustrated by energy-dispersive X-ray (EDX) mapping analysis in Figure 2e, where green nickel element finely scattered between the blue cerium element and homogeneously dispersed in the yellow aluminum element.

  Figure 2

  

  Figure 2.  TEM images of (a, b) Ni/Al₂O₃ and (c, d) Ni/CeO₂-Al₂O₃. (e) HADDF STEM-EDX mapping of Ni/CeO₂-Al₂O₃.

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  Catalytic Performances of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM Reaction. The catalytic performances of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ for DRM reaction were carried out from 400 to 700 ℃ under a GHSV of 24000 mL g^-1 h^-1 with a CH₄: CO₂ feed ratio of 1:1. As shown in Figure 3a and b, the conversion of CO₂ is higher than that of CH₄ under the same temperature, owing to the occurrence of reverse water gas shift (RWGS) reaction. For the catalytic behavior, Ni/CeO₂-Al₂O₃ exhibits superior activity compared to Ni/Al₂O₃ in the whole temperature region. For instance, the highest CH₄ and CO₂ conversion is 71.4% and 82.1% on Ni/CeO₂-Al₂O₃, respectively, which is higher than 64.3% and 75.6% on Ni/Al₂O₃ at 700 ℃. Meanwhile, H₂/CO ratio reaches 0.88 and 0.92 on Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ at 700 ℃, respectively. The higher H₂/CO ratio suggests the less favorability for RWGS reaction on the latter in comparison to the former. Therefore, the addition of CeO₂ to Ni/Al₂O₃ obviously promoted the catalytic performances in DRM reaction.

  Figure 3

  

  Figure 3.  (a) CH₄ and (b) CO₂ conversion as well as (c) H₂/CO ratio on Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM.

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  Stability Investigation of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM Reaction. The long-term activities of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ catalysts were studied and the time on stream of conversions for CH₄ and CO₂ on the two catalysts during DRM at 500 ℃ is shown in Figure 4a and b. CH₄ conversion of Ni/Al₂O₃ dramatically declined from 11.7% to 6.1% during the whole test of period (Figure 4a). Meanwhile, CO₂ conversion of Ni/Al₂O₃ decreased from 16.2% to 9.2% with 30 h on stream (Figure 4b). Ni/CeO₂-Al₂O₃ exhibited distinct behaviors and CH₄ conversion mainly stabilized at 12.1% up to 50 h with only a slight decline in the first 5 h. CO₂ conversion of the catalyst slightly decreased at the first 5 h and reached a stable value of 16.1% with 50 h on stream. The time dependent H₂/CO ratios of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ are presented in Figure 4c. H₂/CO ratio of Ni/Al₂O₃ dropped consistently during the stability test; whereas the ratio of Ni/CeO₂-Al₂O₃ decreased firstly and finally stabilized to around 0.35 after 40 h. Therefore, Ni/CeO₂-Al₂O₃ possessed higher catalytic long-term stability than Ni/Al₂O₃.

  Figure 4

  

  Figure 4.  Stability test of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM at 500 ℃.

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  Characterizations of the Used Catalysts after Stability Test. The used Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ after stability test were characterized by O₂-TPO measurement to determine the structure of carbonaceous species (Figure 5). It has been widely accepted that the deposited carbon species on Ni catalysts could be classified into three types, active carbon atom (C_α), less active (C_β) and inactive coke (C_γ), on the basis of burning-off temperature.^[28-30] Active C_α species were generated on the used Ni/CeO₂-Al₂O₃, as evidenced by the presence of oxidation peak at 170 ℃ on the O₂-TPO curve, which was the origin for the efficient and stable catalytic behavior. However, only C_β and C_γ species existed on the used Ni/Al₂O₃, which was confirmed by the high oxidation peaks at the temperature of 470 and 580 ℃ on the O₂-TPO curve, thus resulting in the loss of catalytic performance. The amounts of deposited carbon on the two used catalysts were further determined by automatic carbon element analyzer. The accurate amounts of carbon deposition on the used Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ are 1.25% and 0.85%, respectively. The coke deposition rate on the latter (0.42 mg_c g_cat^-1 h^-1) is slower than that on the former (0.17 mg_c g_cat^-1 h^-1). Based on XPS result, the redox cycles (Ce⁴⁺/Ce³⁺) are associated with the presence of oxygen vacancies on Ni/CeO₂-Al₂O₃, which is beneficial for the removal of carbonaceous species during DRM.

  Figure 5

  

  Figure 5.  O₂-TPO profiles of the used Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ after stability test.

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  To digger deeply into the morphological structure of the deposited carbon species, TEM measurements were further conducted and the results are illustrated in Figure 6. No obvious carbon filaments were observed on the used catalysts in Figure 6a and c by low resolution TEM analyses. However, encapsulated carbon, as carbon onion spheres, with Ni NPs inside existed on the used Ni/Al₂O₃ (Figure 6b). The graphitic carbon hardly presented on the used Ni/CeO₂-Al₂O₃ by careful detection (Figure 6d). Moreover, the formation and existence of interface structure between Ni and CeO₂ can be clearly seen, indicating the stable interface structure on Ni/CeO₂-Al₂O₃, which is originated from the SMSI effect, as shown from the H₂-TPR and TEM analyses. Meanwhile, this SMSI on Ni/CeO₂-Al₂O₃ well restricted the growth of Ni particles, as evidenced by the homogeneous Ni particles in Figure 6c.

  Figure 6

  

  Figure 6.  TEM and high resolution TEM images of the used (a, b) Ni/Al₂O₃ and (c, d) Ni/CeO₂-Al₂O₃.

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  DISCUSSIONS

  SMSI Effect on Ni Catalysts for DRM Reaction. Since Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ possessed large surface area with almost the same particle size of Ni NPs, it is clearly safe to reveal the relationship between interaction together with interface and their catalytic behaviors during DRM reaction. The interaction between metal and support is crucial for Ni/CeO₂-Al₂O₃ catalyst to achieve stable catalytic performance. This is evidenced by the fact that the control Ni/CeO₂-Al₂O₃ catalyst with weak metal-support interaction (calcinated at 400 ℃ for 3 h) exhibited poor stability in DRM at 500 ℃. The H₂-TPR profile of the control NiO/CeO₂-Al₂O₃ in Figure 7a exhibited three peaks at 550, 680, and 750 ℃, which are lower than those of NiO/CeO₂-Al₂O₃. Besides, the surface of the control NiO/CeO₂-Al₂O₃ was composed of NiO and NiAl₂O₄ hardly appeared (Figure 7b). The results are originated from the presence of more Ni species weakly interacted with support on the control NiO/CeO₂-Al₂O₃.

  Figure 7

  

  Figure 7.  Characterizations and stability test of the control NiO/CeO₂-Al₂O₃ with weak metal-support interaction. (a) H₂-TPR profile and (b) Ni 2p_3/2 XPS spectrum of the control NiO/CeO₂-Al₂O₃. (c) The stability of the control Ni/CeO₂-Al₂O₃ catalyst and (d) O₂-TPO profile of the used catalyst after stability test.

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  The stability test of the control Ni/CeO₂-Al₂O₃ catalyst after reduction is shown in Figure 7c. Obviously, CO₂ and CH₄ conversions on the catalyst declined from 23% to 20% and 14% to 12%, separately. Simultaneously, the H₂/CO ratio decreased from 0.55 to 0.45 with 50 h on stream. The amount of deposited carbon on the used catalyst, determined by automatic carbon element analyzer, is 23.4%, corresponding to the coke deposition rate of 0.61 mg_c g_cat^-1 h^-1, which is approximately 6 times larger than that of the used Ni/CeO₂-Al₂O₃ with the same reaction condition. The deposited carbon species were originated from the oxidation of C_β and C_γ species, as evidenced by the high burning-off temperature at 470 and 510 ℃, respectively (Figure 7d). This is well illustrated by the formation of large amounts of carbon filaments on the used catalyst (Figure S2).

  For Ni/CeO₂-Al₂O₃ with the same interface structure but different interaction, the weak metal-support interaction in the control Ni/CeO₂-Al₂O₃ has lower amounts of surface Ce³⁺ species and the ratio of Ce³⁺ to Ce_total is 25% (as confirmed by the XPS analysis in Figure S3), which is lower than that of 32% of Ni/CeO₂-Al₂O₃ with strong metal-support interaction. This suggests that metal-support interaction affected the surface amount of Ce³⁺ species, which contributed to the different amounts of surface oxygen vacancies. The higher amount of Ce³⁺ species in Ni/CeO₂-Al₂O₃ with SMSI provided more active oxygen species during DRM, leading to an enhanced gasification rate of carbon species from CH₄ dissociation and an improved catalytic stability with less coke formation. It can also be clearly observed that Ni/CeO₂-Al₂O₃ with SMSI possessed relatively higher activity and lower coke deposition rate in comparison with other catalysts in the literature (Table S3).

  Importance of Ni-CeO₂ Interface for CO₂ Activation on Ni/CeO₂-Al₂O₃. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were carried out to monitor the intermediates on Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ catalysts during DRM at 400 ℃ (Figure 8). Besides the peaks at 3016/1304, 2300-2400 and 2130/2010 cm^-1 for CH₄, CO₂ and CO, respectively, three reaction intermediates were observed on the two catalysts.^[31] The first type of intermediate was identified as monodentate carbonate species (m-CO₃^2-) and bidentate carbonate species (b-CO₃^2-), as evidenced by the peaks at 1520-1540 and 1625-1765 cm^-1, respectively.^{[32, 33]} The second kind of intermediate with the peaks at 1228 and 1440 cm^-1 was diagnostic of the formation of bicarbonate species (HCO₃^-).^[34] The third intermediates, identified from the peaks of 1391 and 1592 cm^-1, corresponded to the formation of formate species (HCOO^-).^{[22, 35]} Generally, Ni-based catalysts possessed bi-functional mechanism, where CH₄ and CO₂ separately activated on Ni particles and support.^{[22, 36, 37]} During the DRIFTs study, carbonate and formate species were detected as intermediates during DRM. The formation of carbonate species was attributed to the reaction of CO₂ + *O.^{[34, 38]} The generation of formate species was ascribed to the reaction of *CO_x + *H or CH_x + *O from CO₂ and CH₄ dissociation.^[39] Therefore, compared to carbonate species, more formate species indicated the enhanced CO₂ and CH₄ activation in DRM.

  Figure 8

  

  Figure 8.  DRIFTS of CH₄ + CO₂ reaction on Ni/Al₂O₃ (blue line) and Ni/CeO₂-Al₂O₃ (green line) at 400 ℃.

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  It should be noted that carbonate and bicarbonate species were the principal reaction intermediates on Ni/Al₂O₃ and formate species accounted for a minor part. However, the major portion of intermediates for Ni/CeO₂-Al₂O₃ was dominated by the generation of formate species, associated with a relatively small part of carbonate and bicarbonate species. This is well matched with previous work that strong metal-support interaction with abundant interfacial sites promoted surface formate species.^[40-42] Therefore, Ni/CeO₂-Al₂O₃ had distinct reaction pathway (as shown in Figure 9) for CO₂ and CH₄ activation and the beneficial effect of major formate species contributed to superior catalytic performance in DRM. Specifically, CH₄ and CO₂ activation occurred separately on Ni and support of Ni/CeO₂-Al₂O₃, which produced carbon atoms with hydrogen atoms (as evidenced from O₂-TPO results) and carbonate, bicarbonate with formate species (as confirmed by DRIFTS analysis), respectively. Owing to strong metal-support interaction and tailored interface in Ni/CeO₂-Al₂O₃, more active interfacial sites accelerated the conversion of carbon atoms and promoted the generation of more formate species as key intermediates (with less carbonate species), leading to a distinct reaction pathway and improved catalytic performance in DRM.

  Figure 9

  

  Figure 9.  Reaction pathway of Ni/CeO₂-Al₂O₃ catalyst in DRM.

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  CONCLUSIONS

  In the present work, we employed an improved evaporationinduced self-assembly method to design Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ catalysts by SMSI effect but distinct interface structure. The only difference of the resultant catalysts is the interface structure between Ni metal and CeO₂ or Al₂O₃ support. Compared to Ni/Al₂O₃, Ni particles over Ni/CeO₂-Al₂O₃ were closely interacted and intimately contacted with CeO₂, leading to the formation of interface structure between metal and support. This unique interface structure renders Ni/CeO₂-Al₂O₃ with higher catalytic activity and better stability relative to Ni/Al₂O₃. Less carbonaceous species with higher reactivity were formed on the used Ni/CeO₂-Al₂O₃, whereas more coke deposition was observed on the used Ni/Al₂O₃. However, the control Ni/CeO₂-Al₂O₃ with weak metal-support interaction possessed inferior performance and large amounts of coke deposition. The same SMSI effect with different interface structure altered the reaction intermediates during DRM, where carbonate and bicarbonate species were produced on Ni/Al₂O₃, but majority of formate species with carbonate and bicarbonate species were obtained on Ni/CeO₂-Al₂O₃.

  EXPERIMENTAL

  Preparation of the Catalysts. In a typical synthesis, 2.4 g of pseudo-boehmite powder (SINOPEC Dalian Research Institute of Petroleum and Petrochemicals) was dispersed in 30 mL of distilled water, and then 0.55 mL concentrated nitric acid (Sinopharm Chemical Reagent Co., Ltd) was dropped into the above solution with a [H⁺] to [Al³⁺] ratio of 0.2. Next, the mixture was treated under microwave irradiation at 70 ℃ for 1 h under continuous stirring to produce white semi-transparent alumina gel. Meanwhile, 5 g of triblock copolymer Pluronic P123 (Sigma Aldrich) together with 0.79 g of Ce(NO₃)₃·6H₂O and 1.26 g of Ni(NO₃)₂·6H₂O (Sinopharm Chemical Reagent Co., Ltd) was dissolved into 40 mL of ethanol (100%), and then stirred for 4 h. The alumina gel was added to the solution with stirring for another 4 h. Afterwards, the sample was placed in an oven to evaporate the solvent firstly at 60 ℃ for 48 h, and then at 100 ℃ for another 48 h. Finally, the solid powder was calcined at 600 ℃ with a heating rate of 1 ℃ min^-1 under air for 4 h, and the product was denoted as NiO/CeO₂-Al₂O₃.

  NiO/Al₂O₃ was prepared with the same method, except the only addition of 1.12 g of Ni(NO₃)₂·6H₂O to the ethanol in the second step. Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ were obtained by reducing with H₂ at 550 ℃ for 2 h.

  Characterizations. The Ni and/or Ce loading on NiO/Al₂O₃ or NiO/CeO₂-Al₂O₃ was determined by inductively coupling plasma-atomic emission spectrometry (ICP-AES) on a Thermo iCAP 6300 spectrometer. The component structure of the samples was studied by powder X-ray diffraction (XRD) measurements on a Rigaku D/Max-2500 diffractometer with a scanning region from 10° to 80° at a scanning speed of 2 ° min^-1. The texture properties of the samples were investigated by N₂ adsorption-desorption experiment at -196 ℃ on a NOVA1200e analyzer (Quantachrome), and the samples were pretreated under vacuum at 300 ℃ for 3 h before the test. Surface structure of the samples was studied by X-ray photoelectron spectroscopy (XPS) on a Thermo SCIENTIFIC ESCALAB 250XI spectrometer. The binding energies of Ni and Ce were calibrated with C 1s peak at 284.8 eV. The morphological structure of the samples was depicted by scanning electron microscopy (SEM) on a JEOL JSE-7100F apparatus and transmission electron microscope (TEM) with energy-dispersive X-ray spectroscopy (EDX) on an FEI Tecnai G20 instrument operated at 200 kV.

  The reductive behavior of the samples was investigated by H₂ temperature programmed reduction (H₂-TPR) measurement on a Micromeritics AutoChem II 2920 Chemisorption instrument. 0.1 g of the sample was firstly purged under Ar (30 mL min^-1) at 300 ℃ for 1 h, and then cooled down to 30 ℃. Next, 10% H₂/He (30 mL min^-1) was switched into the reactor and the temperature was increased to 1000 ℃ at a heating rate of 10 ℃ min^-1. The signal of H₂ consumption was recorded on a TCD detector. The surface area of the active nickel was measured by pulse H₂ chemisorption experiment on the same equipment. 0.1 g of the sample was reduced at 550 ℃ under 10% H₂/He (30 mL min^-1) for 2 h, then the temperature was declined to 50 ℃ under Ar. Next, 100 μL of 10% H₂/He was injected into the sample until it was saturated.

  The amount and property of coke deposition on the used samples were analyzed by oxidation under air from 50 to 800 ℃ on a thermal analyzer (TG, Setaram SETSYS, TGA) coupled with mass spectrometer (MS, Hiden HPR20 QIC R&D).

  The reaction intermediates and pathways on the catalysts during dry reforming of methane were monitored by in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) on an FT-IR spectrometer (Bruker Vertex 70) equipped with a liquid nitrogen cooled Mercury-Cadmium-Tellurium (MCT) detector and a diffuse reflectance accessory (Praying Mantis, Pike). The sample was firstly reduced ex-situ at 550 ℃ for 2 h under H₂ in a tube furnace. Afterwards, the powder sample was transferred into a crucible in the reaction cell and then reduced in-situ at 550 ℃ under 5% H₂/He (20 mL min^-1) for 2 h. Next, the sample was purged with He (20 mL min^-1) for 30 min and then cooled down to 400 ℃. Meanwhile, a background spectrum was recorded under He. Finally, 5% CH₄/He (10 mL min^-1) and 5% CO₂/He (10 mL min^-1) were introduced into the reaction cell and IR spectra with background spectrum subtracted (resolution of 4 cm^-1 with 128 scans) on the reduced catalysts during the reaction were collected at 400 ℃.

  Catalyst Evaluation. Dry reforming of methane (DRM) reaction was performed on a vertical fixed-bed reactor (10 mm of inner diameter and 300 mm of length) in a temperature range of 400 to 700 ℃ under atmospheric pressure. Typically, 50 mg of catalyst (20-40 mesh) diluted with 1 g of quartz was placed into the reactor. The catalyst was reduced at 550 ℃ under a H₂ flow (50 mL min^-1) for 2 h and then the temperature was decreased to 400 ℃ in an Ar flow (50 mL min^-1). Finally, a feed gas of CH₄/CO₂ (1:1) flow (20 mL min^-1) was introduced into the reactor for DRM. The emission was monitored by an online gas chromatograph (GC, Agilent 7820) equipped with a thermal conductivity detector (TCD) with a TDX-01 column (2 m). An ice trap was used to condense water during the reaction before the GC. The stability test of the catalyst was performed at 500 ℃ for 50 h with a feed gas of CH₄/CO₂/He (1:1) flow (20 mL min^-1).

  X_CH4 and X_CO2 and H₂/CO ratio were calculated based on the following equations:

  $ {\text{X}}_{{\text{CH}}_{\text{4}}}\text{=}\frac{{\text{F}}_{{\text{CH}}_{\text{4}}\text{, in}}{\text{- F}}_{{\text{CH}}_{\text{4}}\text{, out}}}{{\text{F}}_{{\text{CH}}_{\text{4}}\text{, in}}}\text{}\text{× 100%} $

  $ {\text{X}}_{{\text{CO}}_{\text{2}}}\text{=}\frac{{\text{F}}_{{\text{CO}}_{\text{2}}\text{, in}}{\text{- F}}_{{\text{CO}}_{\text{2}}\text{, out}}}{{\text{F}}_{{\text{CO}}_{\text{2}}\text{, in}}}\text{}\text{× 100%} $

  $ \frac{{\text{H}}_{\text{2}}}{\text{CO}}\text{=}\frac{{\text{F}}_{{\text{H}}_{\text{2}}}}{{\text{F}}_{\text{CO}}} $

  where F_{i, in} and F_{i, out} represent the molar flow rate of component i of the inlet and outlet, respectively.

  
  ACKNOWLEDGEMENTS: The authors acknowledge the National Natural Science Foundation of China (Nos. 22108189, 21878203), the Program for the Top Young and Middle-Aged Innovative Talents of Higher Learning Institutions of Shanxi, and the financial support by Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2021SX-TD005). The authors declare no competing interests.
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  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0113
  
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• 

  Figure 1  (a) XRD patterns, (b) H₂-TPR profiles and (c, d) Ni 2p and Ce 3d XPS spectra of NiO/Al₂O₃ and NiO/CeO₂-Al₂O₃.

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  Figure 2  TEM images of (a, b) Ni/Al₂O₃ and (c, d) Ni/CeO₂-Al₂O₃. (e) HADDF STEM-EDX mapping of Ni/CeO₂-Al₂O₃.

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  Figure 3  (a) CH₄ and (b) CO₂ conversion as well as (c) H₂/CO ratio on Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM.

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  Figure 4  Stability test of Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ in DRM at 500 ℃.

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  Figure 5  O₂-TPO profiles of the used Ni/Al₂O₃ and Ni/CeO₂-Al₂O₃ after stability test.

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  Figure 6  TEM and high resolution TEM images of the used (a, b) Ni/Al₂O₃ and (c, d) Ni/CeO₂-Al₂O₃.

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  Figure 7  Characterizations and stability test of the control NiO/CeO₂-Al₂O₃ with weak metal-support interaction. (a) H₂-TPR profile and (b) Ni 2p_3/2 XPS spectrum of the control NiO/CeO₂-Al₂O₃. (c) The stability of the control Ni/CeO₂-Al₂O₃ catalyst and (d) O₂-TPO profile of the used catalyst after stability test.

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  Figure 8  DRIFTS of CH₄ + CO₂ reaction on Ni/Al₂O₃ (blue line) and Ni/CeO₂-Al₂O₃ (green line) at 400 ℃.

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  Figure 9  Reaction pathway of Ni/CeO₂-Al₂O₃ catalyst in DRM.

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