Engineering the Interface and Interaction Structure on Highly Coke-Resistant Ni/CeO2-Al2O3 Catalyst for Dry Reforming of Methane

Sha Li Xin Wang Min Cao Jingjun Lu Li Qiu Xiaoliang Yan

Citation:  Sha Li, Xin Wang, Min Cao, Jingjun Lu, Li Qiu, Xiaoliang Yan. Engineering the Interface and Interaction Structure on Highly Coke-Resistant Ni/CeO2-Al2O3 Catalyst for Dry Reforming of Methane[J]. Chinese Journal of Structural Chemistry, 2022, 41(12): 221200. doi: 10.14102/j.cnki.0254-5861.2022-0113 shu

Engineering the Interface and Interaction Structure on Highly Coke-Resistant Ni/CeO2-Al2O3 Catalyst for Dry Reforming of Methane


  • 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.

    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.

    Figure 1c shows the X-ray photoelectron spectra for Ni 2p3/2 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.

    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.

    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 ℃.

    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 (Cα), less active (Cβ) and inactive coke (Cγ), on the basis of burning-off temperature.[28-30] Active Cα 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 Cβ and Cγ 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 mgc gcat-1 h-1) is slower than that on the former (0.17 mgc gcat-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.

    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.

    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 2p3/2 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.

    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 mgc gcat-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 Cβ and Cγ 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 Cetotal 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-CO32-) and bidentate carbonate species (b-CO32-), 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 *COx + *H or CHx + *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 ℃.

    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.

    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.

    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).

    XCH4 and XCO2 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 Fi, in and Fi, 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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  • 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.

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

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

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

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

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

    Figure 7  Characterizations and stability test of the control NiO/CeO2-Al2O3 with weak metal-support interaction. (a) H2-TPR profile and (b) Ni 2p3/2 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.

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

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

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  • 发布日期:  2022-12-02
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