# Effects of Metal-Support Interactions and Interfaces on Catalytic Performance over M2O3- (M=La, Al-) Supported Ni Catalysts

Authors: Zhourong Xiao; Peng Li; Senlin Zhang; Jianmin Gu; Desong Wang
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The metal-support interactions and their interfaces have showed great influence on catalytic activity and stability. Herein, different support of M2O3- (M=La, Al-) supported Ni catalysts was prepared by a citric acid-assisted sol-gel method. The physical structure and chemical properties of the As-prepared catalysts were systematically characterized by various technologies. The catalytic performance was evaluated for hydrogen or syngas production by ethanol steam reforming and methane dry (CO2) reforming, respectively. The results showed that compared with Al2O3 support, the nickel supported on La2O3 possessed a smaller particle size of nickel even after high-temperature reduction. In addition, the La2O3-supported nickel catalyst had stronger metal-support interaction and higher nickel electron density as a result of higher ethanol conversion activity and stability. The ethanol conversion was maintained at 87.8% after a 3000-minute test, and the hydrogen production was as high as 6500 μmol/min. Moreover, the Ni-La2O3 catalyst also showed good activity for methane dry reforming. The initial conversion of methane and carbon dioxide was close to 90%, and the ratio of H2/CO reached 0.94. The better catalytic performance of the Ni-La2O3 catalyst was ascribed to smaller particle size of Ni, rich metal-support interfaces, more nickel electron densities, abundant strong basic sites, and strong metal-support interactions.


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## Body

## 1. Introduction

Nowadays, the hypersonic aircraft has become a hot spot in the field of aerospace development. However, it will generate a lot of pneumatic heat in flight, which may cause the surface temperature of aircraft higher than the material tolerance and will lead to decay of the life-span or even destroy the material [1]. Therefore, controlling the surface temperature lower than the material tolerance is highly desirable for dealing with the problem of vehicle cooling with liquid hydrocarbon fuel [2]. Due to the high temperature of hypersonic flight vehicle cooling requirements and the shortcomings of liquid hydrogen fuel such as low density and low boiling point, endothermic hydrocarbon fuel is a good way for response to problems of hypersonic flight vehicles [3]. For endothermic hydrocarbon fuel process, three aspects including steam reforming, catalytic dehydrogenation, and cracking have been extensively investigated [4, 5]. Compared with the cracking or dehydrogenation process, fuel steam reforming process can provide a higher heat sink and can meet the actual requirements of hypersonic missiles with flight speed up to Mach 10 [3]. Adding water/ethanol to fuel was based on a two-step reaction involving the low-temperature prereforming of fuel to produce methane and carbon dioxide, and then the methane and carbon dioxide (dry) reforming under high-temperature conditions with strong endothermic effect [6]. Therefore, fuel steam reforming such as ethanol steam reforming and methane carbon dioxide reforming is expected to provide a fundamental data for improving the heat absorption capacity of fuel. Furthermore, ethanol steam reforming and methane carbon dioxide reforming are important reactions for production of hydrogen and syngas, which have become one of the hot spots [7–12].Whether ethanol steam reforming or methane carbon dioxide reforming, the key to achieve high conversion is the design high-efficiency catalysts [13, 14]. Among them, nickel-based catalysts were regarded as the most promising catalysts for reforming reactions because of the high activity, abundant resource, and low price [15]. However, there are still some shortcomings for nickel-based catalysts, such as easy carbon deposition on the surface of nickel, which reduces the activity and stability of the catalysts [16]. Moreover, due to the low Taman temperature of nickel metal, it tends to agglomerate under high-temperature reaction conditions, which further greatly reduces the activity and stability of the catalyst. Therefore, it is an urgent problem to improve the carbon deposition resistance and sintering resistance of nickel-based catalysts [17]. Based on previous report, increasing the dispersion of nickel, reducing the particle size of nickel, and enhancing the interaction strength between nickel metal and support are efficient methods to improve the activity and stability for nickel-based catalysts [18, 19]. Xiao et al. [20] reported that the Ni-CePrx catalyst prepared by the sol-gel method achieves stable hydrogen production by ethanol steam reforming. The catalyst prepared by the sol-gel method has higher activity and anticarbon deposition performance than the catalyst prepared by the impregnation method, which was attributed to the strong interaction between nickel metal and support and the high nickel dispersion. Further, they also reported that Ni-CeO2 catalysts modified by valence states of different doping elements can achieve stable hydrogen production by ethanol steam reforming [21]. In particular, the La-doped catalyst has higher activity and stability, which was attributed to the greater nickel dispersion and oxygen vacancy concentration of the trivalent La-doped catalyst. La2O3 and Al2O3 were excellent support for supporting nickel catalysts, which have strong interaction with metal nickel and can form high-temperature sintering-resistant perovskite [22] or nickel-aluminum spinel structure [23].Herein, La2O3- and Al2O3-supported nickel catalysts were synthesized by a sol-gel method. The structures and properties of the two catalysts were characterized by wide-angle X-ray diffraction (XRD), nitrogen adsorption and desorption isotherm, transmission electron microscopy (TEM), CO2 temperature-programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS), and a hydrogen temperature-programmed reduction (H2-TPR). The results showed that the nickel catalysts supported on the two kinds of support have different performances for ethanol steam reforming and methane dry reforming. Furthermore, TEM and thermogravimetric analysis (TG) were performed on the spent catalyst. The La2O3-supported nickel catalyst showed better catalytic activity and stability as well as anticarbon deposition performance on account of its strong metal-support interaction, rich metal-support interaction interface, and smaller particle size of nickel.


## 2. Experimental Materials and Methods

### 2.1. Reagents and Materials

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, Beijing Huawei Ruike, AR), lanthanum nitrate hexahydrate (La(NO3)2·6H2O, Tianjin Guangfu Fine Chemical Research Institute, AR), aluminum nitrate ninehydrate (Al(NO3)3)·9H2O, Tianjin Guangfu Fine Chemical Industry Research Institute, AR), absolute ethanol (C2H5OH, Tianjin Guangfu Fine Chemical Industry Research Institute, AR), methane carbon dioxide mixture (CH4:CO2:N2=1:1:2, Tianjin Taiya Gas Company), nitrogen (high purity), and helium (high purity) were purchased from Six Party Gas Company, deionized water.


### 2.2. Preparation of Catalyst

The catalyst was prepared by a sol-gel method as reported by our previous work [24, 25]. For the synthesis of Ni-La2O3, a certain amount of lanthanum nitrate hexahydrate and nickel nitrate hexahydrate were weighed, then dissolved in 17 mL deionized water. Then, citric acid with molar ratio to metal ion set at 2 : 1 was added into the above solution. After stirring for 6 h, the above mixed solution was transferred into oven under 120°C for 12 h. The obtained solid was ground to a powder and then transferred to a muffle furnace at 600°C for 4 h. To prepare the reduced catalyst, a certain amount of calcined catalyst was put in a quartz ark and reduced at 780°C for 1 h under 10% vol H2/Ar mixture gas. The reduced catalyst was prepared, named as Ni-La2O3, and the weighted percentage of Ni was kept at 10 wt%. Similarly, the Ni-Al2O3 catalyst was prepared by the same method, and the precursor of aluminum used was aluminum nitrate nonahydrate.


### 2.3. Characterization

The phase structure of the prepared catalyst and the reduced catalyst was analyzed by the Rigaku D8 Focus X-ray diffractometer (XRD). The test conditions of XRD were as follows: Cu target, Kα ray (λ=0.154nm), tube voltage of 40 kV, tube current of 200 mA, scanning speed set at 8°/min, and scanning range from 10° to 80°. The instrument JEM-F200 was used to observe the nickel particles and carbon deposition of the prepared catalyst. At the same time, selected electron diffraction and crystal structure analysis could be provided, and the surface distribution analysis of the sample elements could be analyzed with the energy spectrometer. The test conditions were as follows: acceleration voltage of 200 kV, pointresolution≤0.19nm, and STEMresolution≤0.20nm. Micromeritics Tristar 3000 was used to analyze the pore structure of the material under nitrogen atmosphere at -196°C. The samples were needed to be degassed and dehydrated for 3 h at 300°C before the test. H2-TPR was carried out on an AMI-300. Firstly, a certain amount of catalyst samples were weighed and pretreated at 400°C in Ar atmosphere for 1 h. After cooling to 50°C, 10% vol H2-Ar-mixed gas was added and the temperature was increased from 50°C to 800°C with the heating rate of 10°C/min. CO2-TPD was carried out on an AMI-300. A powder sample (100 mg) was pretreated at 300°C with Ar under a flow of 30 mL/min for 1 h and then cooled to 100°C. CO2 was introduced (30 mL/min) for 0.5 h at 100°C; and then, He stream was fed until the completely remove physically adsorbed CO2. The chemically adsorbed CO2 was determined by increasing the temperature up to 800°C with a heating rate of 10°C/min. The carbon deposition of the spent catalyst was characterized by HCT-1 comprehensive thermal analyzer. About 10 mg of the sample was weighed and placed in a porcelain crucible, kept at 30°C for 20 minutes, and then the temperature was increased from 30°C to 800°C in the air atmosphere with a heating rate of 10°C/min. The weight loss curve of the sample was obtained, and DTG data was obtained by first-order differentiation of the weight loss curve.


### 2.4. Catalyst Evaluation

Hydrogen production by ethanol steam reforming was carried out in a stainless-steel tubular-fixed bed reactor. Before reaction, the calcined catalyst was pressed into tablets with the pressure of 10 MPa, and then was sieved into mesh number of 20~40. For ethanol steam reforming, 0.25 g of catalyst and 1.2 g of quartz sand were mixed uniformly and then filled in a stainless-steel tubular with a diameter of 6 mm. The temperature was raised to 780°C in nitrogen, then the gas was switched to 10% vol H2/Ar. After reducing for 1 h, the temperature was stabilized to the reaction temperature (600°C) by nitrogen, and the reaction pressure was set at 0.1 MPa. A constant quantity of ethanol aqueous solution (0.255 mL/min) was introduced into the gasifier by a high-pressure metering pump, then gasification at 300°C with nitrogen (60 mL/min). The partial pressure of ethanol vapor and nitrogen in the mixture were 15.73 vol% and 21.33 vol%, respectively. During the reaction, the gas hour space velocity (GHSV) of ethanol-water vapor was 55920 mL/gcat·h. After the reaction, the gas product was condensed, dried, and passed through Micro GC 490 (Agilent) for online analysis. The gas chromatographic with a TCD detector was equipped with three columns: an activated alumina column for the detection of hydrocarbon molecules C3 and above, a PPU column for the detection of CO2, ethane, and ethylene, and a 5 Å MS column for the detection of H2, N2, CH4, and CO. The conversion of ethanol was calculated according to Equation (1). The gas product rate and the composition of gas were calculated based on Equations (2) and (3). Similar to ethanol steam reforming, methane dry reforming was carried out in the same reactor. The 0.15 g of catalyst and 1.2 g of quartz sand were evenly mixed and loaded into a tube reactor, and then prereduced in 10% vol H2/Ar mixture at 780°C for 1 h. The temperature was stabilized to 700°C under a nitrogen atmosphere. Then the mixture of methane, carbon dioxide, and nitrogen gas with a ratio of 1 : 1 : 2 was passed into the reaction tube; and nitrogen was used as the carrier gas and internal standard gas. The gas phase products were analyzed by GC490, and the gas production rate, reactant conversion, and H2/CO molar ratio were calculated by using Equations (2), (4), and (6). FCOout, FCO2out, and FCH4out represent the total number of C1 products at the outlet of the reaction tube (mol/min). For Equation (2), RN2=60mL/min, namely, nitrogen flow rate, AH2 and fH2 represent the peak area and response factor of H2, respectively. AN2 and fN2 represent the peak area and response factor of N2, respectively. Here, Equation (3), V% (H2) (excluding nitrogen) was the percentage of hydrogen in the dry gas (without nitrogen).
(1)Ethanolconversionrate%=FCOout+FCO2out+FCH4outFethanolin×100,(2)Gasproductionrateμmolmin=RH2=AH2·fH2AN2·fN2×RN2,(3)V%H2=RH2∑RH2+CO+CO2+CH4×100,(4)Methaneconversion%=FCH4in−FCH4outFCH4in×100,(5)Carbondioxideconversion%=FCO2in−FCO2outFCO2in×100,(6)H2CO=FH2outFCOout.


## 2.1. Reagents and Materials

Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, Beijing Huawei Ruike, AR), lanthanum nitrate hexahydrate (La(NO3)2·6H2O, Tianjin Guangfu Fine Chemical Research Institute, AR), aluminum nitrate ninehydrate (Al(NO3)3)·9H2O, Tianjin Guangfu Fine Chemical Industry Research Institute, AR), absolute ethanol (C2H5OH, Tianjin Guangfu Fine Chemical Industry Research Institute, AR), methane carbon dioxide mixture (CH4:CO2:N2=1:1:2, Tianjin Taiya Gas Company), nitrogen (high purity), and helium (high purity) were purchased from Six Party Gas Company, deionized water.


## 2.2. Preparation of Catalyst

The catalyst was prepared by a sol-gel method as reported by our previous work [24, 25]. For the synthesis of Ni-La2O3, a certain amount of lanthanum nitrate hexahydrate and nickel nitrate hexahydrate were weighed, then dissolved in 17 mL deionized water. Then, citric acid with molar ratio to metal ion set at 2 : 1 was added into the above solution. After stirring for 6 h, the above mixed solution was transferred into oven under 120°C for 12 h. The obtained solid was ground to a powder and then transferred to a muffle furnace at 600°C for 4 h. To prepare the reduced catalyst, a certain amount of calcined catalyst was put in a quartz ark and reduced at 780°C for 1 h under 10% vol H2/Ar mixture gas. The reduced catalyst was prepared, named as Ni-La2O3, and the weighted percentage of Ni was kept at 10 wt%. Similarly, the Ni-Al2O3 catalyst was prepared by the same method, and the precursor of aluminum used was aluminum nitrate nonahydrate.


## 2.3. Characterization

The phase structure of the prepared catalyst and the reduced catalyst was analyzed by the Rigaku D8 Focus X-ray diffractometer (XRD). The test conditions of XRD were as follows: Cu target, Kα ray (λ=0.154nm), tube voltage of 40 kV, tube current of 200 mA, scanning speed set at 8°/min, and scanning range from 10° to 80°. The instrument JEM-F200 was used to observe the nickel particles and carbon deposition of the prepared catalyst. At the same time, selected electron diffraction and crystal structure analysis could be provided, and the surface distribution analysis of the sample elements could be analyzed with the energy spectrometer. The test conditions were as follows: acceleration voltage of 200 kV, pointresolution≤0.19nm, and STEMresolution≤0.20nm. Micromeritics Tristar 3000 was used to analyze the pore structure of the material under nitrogen atmosphere at -196°C. The samples were needed to be degassed and dehydrated for 3 h at 300°C before the test. H2-TPR was carried out on an AMI-300. Firstly, a certain amount of catalyst samples were weighed and pretreated at 400°C in Ar atmosphere for 1 h. After cooling to 50°C, 10% vol H2-Ar-mixed gas was added and the temperature was increased from 50°C to 800°C with the heating rate of 10°C/min. CO2-TPD was carried out on an AMI-300. A powder sample (100 mg) was pretreated at 300°C with Ar under a flow of 30 mL/min for 1 h and then cooled to 100°C. CO2 was introduced (30 mL/min) for 0.5 h at 100°C; and then, He stream was fed until the completely remove physically adsorbed CO2. The chemically adsorbed CO2 was determined by increasing the temperature up to 800°C with a heating rate of 10°C/min. The carbon deposition of the spent catalyst was characterized by HCT-1 comprehensive thermal analyzer. About 10 mg of the sample was weighed and placed in a porcelain crucible, kept at 30°C for 20 minutes, and then the temperature was increased from 30°C to 800°C in the air atmosphere with a heating rate of 10°C/min. The weight loss curve of the sample was obtained, and DTG data was obtained by first-order differentiation of the weight loss curve.


## 2.4. Catalyst Evaluation

Hydrogen production by ethanol steam reforming was carried out in a stainless-steel tubular-fixed bed reactor. Before reaction, the calcined catalyst was pressed into tablets with the pressure of 10 MPa, and then was sieved into mesh number of 20~40. For ethanol steam reforming, 0.25 g of catalyst and 1.2 g of quartz sand were mixed uniformly and then filled in a stainless-steel tubular with a diameter of 6 mm. The temperature was raised to 780°C in nitrogen, then the gas was switched to 10% vol H2/Ar. After reducing for 1 h, the temperature was stabilized to the reaction temperature (600°C) by nitrogen, and the reaction pressure was set at 0.1 MPa. A constant quantity of ethanol aqueous solution (0.255 mL/min) was introduced into the gasifier by a high-pressure metering pump, then gasification at 300°C with nitrogen (60 mL/min). The partial pressure of ethanol vapor and nitrogen in the mixture were 15.73 vol% and 21.33 vol%, respectively. During the reaction, the gas hour space velocity (GHSV) of ethanol-water vapor was 55920 mL/gcat·h. After the reaction, the gas product was condensed, dried, and passed through Micro GC 490 (Agilent) for online analysis. The gas chromatographic with a TCD detector was equipped with three columns: an activated alumina column for the detection of hydrocarbon molecules C3 and above, a PPU column for the detection of CO2, ethane, and ethylene, and a 5 Å MS column for the detection of H2, N2, CH4, and CO. The conversion of ethanol was calculated according to Equation (1). The gas product rate and the composition of gas were calculated based on Equations (2) and (3). Similar to ethanol steam reforming, methane dry reforming was carried out in the same reactor. The 0.15 g of catalyst and 1.2 g of quartz sand were evenly mixed and loaded into a tube reactor, and then prereduced in 10% vol H2/Ar mixture at 780°C for 1 h. The temperature was stabilized to 700°C under a nitrogen atmosphere. Then the mixture of methane, carbon dioxide, and nitrogen gas with a ratio of 1 : 1 : 2 was passed into the reaction tube; and nitrogen was used as the carrier gas and internal standard gas. The gas phase products were analyzed by GC490, and the gas production rate, reactant conversion, and H2/CO molar ratio were calculated by using Equations (2), (4), and (6). FCOout, FCO2out, and FCH4out represent the total number of C1 products at the outlet of the reaction tube (mol/min). For Equation (2), RN2=60mL/min, namely, nitrogen flow rate, AH2 and fH2 represent the peak area and response factor of H2, respectively. AN2 and fN2 represent the peak area and response factor of N2, respectively. Here, Equation (3), V% (H2) (excluding nitrogen) was the percentage of hydrogen in the dry gas (without nitrogen).
(1)Ethanolconversionrate%=FCOout+FCO2out+FCH4outFethanolin×100,(2)Gasproductionrateμmolmin=RH2=AH2·fH2AN2·fN2×RN2,(3)V%H2=RH2∑RH2+CO+CO2+CH4×100,(4)Methaneconversion%=FCH4in−FCH4outFCH4in×100,(5)Carbondioxideconversion%=FCO2in−FCO2outFCO2in×100,(6)H2CO=FH2outFCOout.


## 3. Results and Discussion

### 3.1. Characterization of Structure and Metal-Support Interaction

The XRD patterns of the As-reduced catalysts are shown in Figure1(a). For the Ni-Al2O3 catalyst, no obvious peak of Al2O3 was observed, which could be due to its amorphous structure [19]. The diffraction peaks with strong intensity appear at 44.5o, 51.9o, and 76.4o were attributed to the characteristic diffraction peaks of nickel (JCPDF#01-1260), indicating that the high crystallinity of nickel in the reduced catalyst with the uneven distribution of large particles size of nickel. For the Ni-La2O3 catalyst, the peaks of La2O3 (JCPDF#83-1344) was mainly observed. There was no obvious characteristic peak of nickel, illustrating that the small particle size of nickel with uniform distribution. These results will be confirmed by H2-TPR as shown in Figure 1(b). All the As-prepared catalysts have three reduction peaks. For the Ni-Al2O3 catalyst, the reduction peak of 391.1°C was attributed to the reduction of NiO with large particles or weak interaction with the support, and 484.1°C was assigned to the reduction of NiO with small particles or strong interaction with the support, and 695.4°C was attributed to the reduction of nickel-aluminum spinel [19]. For the Ni-La2O3 catalyst, the first peak appeared at 416.9°C, which can be attributed to the decrease of Ni2+ species on the surface. The second peak appeared at 609.3°C, possibly because LaNiO3 was reduced to La2Ni2O5 [26]. The third reduction peak was located at 767.7°C, which corresponds to the reduction of Ni2+ in LaNiO3 to metal Ni0 [27]. The higher reduction temperature of the Ni-La2O3 catalyst indicates that it possessed a strong metal-support interaction, which is beneficial to inhibit the sintering of nickel and maintain the small particle size of nickel during the high-temperature reduction process.Figure 1
(a) XRD patterns of the reduced catalysts, ♦ and ♥ stand for Ni and La2O3, respectively. (b) H2-TPR profiles of the As-prepared catalysts. (c) N2 adsorption-desorption isotherms (inset is pore size distributions) of the samples. (d) The CO2-TPD for the Ni-La2O3 and Ni-Al2O3.
(a)(b)(c)(d)The nitrogen adsorption-desorption isotherms of the calcined catalyst are shown in Figure1(c) with hysteresis loop indicating that the prepared catalyst was a mesoporous material. With the increase of the relative pressure, the adsorption capacity also increases, indicating that there was a certain amount of large pores in the material. The pore size showed double pore size distribution. The mesoporous pore was centered at 4 nm and the macroporous was around 60 nm. From Table 1, the specific surface area of the Ni-Al2O3 catalyst was 51.3 m2/g, the pore volume was 0.11 m3/g, and the average pore size was 7.8 nm (inset Figure 1(c)). For the Ni-La2O3 catalyst, the surface area and pore volume were decreased to 23.3 m2/g and 0.04 m3/g, respectively. The results indicate that the prepared catalyst has a porous structure and its specific surface area was relatively small.Table 1
Textural properties of the As-prepared samples.
SampleSBet (m2/g)VTotal (m3/g)D (nm)Ni-La2O323.30.048.8Ni-La2O351.30.117.8In order to illustrate the alkaline site on different supports, the data of CO2-TPD was collected as shown in Figure 1(d). For Ni-La2O3, the high intensity of CO2 desorption peak was mainly centered on 782°C, which illustrates abundant and strong basic sites, while for the Ni-Al2O3, the main CO2 desorption temperature was located at 167°C and 473°C, which means weak and few basic sites. The strong basic sites for Ni-La2O3 may play a positive influence on CO2 adsorption during the methane dry reforming, thus exhibiting good activity and stability for methane dry reforming [17].


### 3.2. XPS Analysis of the Reduced Catalyst

XPS of the reduced catalyst is shown in Figure2. For the Ni-Al2O3 catalyst, Al 2p3/2 was detected, indicating that the Al element was trivalent. Similarly, La 3d3/2 was detected on the Ni-La2O3 catalyst, indicating that the La element was also trivalent. For the Ni 2p, three peaks at 852, 856, and 861 eV could be assigned to Ni0, Ni2+, and satellite peak (Ni2+), respectively. Compared with Ni-Al2O3, the binding energy of Ni0 for Ni-La2O3 shifts towards low binding energy, indicating the increased electron density of Ni0, which may be explained by the enhanced metal-support interaction as verified by Figure 1(b). Jing et al. [28] pointed out that the enhanced electron density of nickel enhances the activation and dissociation of CO2 and balances the formation rates of C and O species on the catalyst. In addition, the Ni0/(Ni0 + Ni2+) ratio was also calculated. The Ni0 ratios of Ni-La2O3 and Ni-Al2O3 were 59.9% and 56.9%, respectively, indicating that the ratio of nickel for two catalysts was similar.Figure 2
XPS profiles of the As-prepared catalysts.
(a)(b)(c)(d)


### 3.3. TEM Analysis of Reduced Catalyst

Apart from binding energy of Ni, the particle size of nickel is also play vital important role in catalytic performance. The TEM images of the reduced catalyst are shown in Figure3. The distribution of nickel particles in the Ni-La2O3 catalyst was relatively uniform, and the average particle size was 13.5 nm (inset Figure 3(a)). It was found that some nickel particles were embedded in the support and showed strong interaction with the support and more metal-support interfaces, which was consistent with the results of H2-TPR (Figure 1(b)). Moreover, according to HR-TEM, the lattice parameter of nickel particles was 0.201 nm, which corresponds to the Ni (111) crystal plane. The interaction interface between nickel particles and La2O3 support was large (Figure 3(b)). For the Ni-Al2O3 catalyst, agglomeration of nickel particles was observed and the average particle size was 45.2 nm (inset Figure 3(c)), indicating the contact interface between nickel particles and the support was small (Figure 3(d)). These results were consistent with the previous XRD and TPR results. According to literature, nickel particles have strong metal-support interaction, which makes them smaller particle sizes and rich interaction interfaces, leading to the better catalytic activity and sintering resistance [23].Figure 3
TEM images of the catalysts after H2 reduction: (a, b) Ni-La2O3 and (c, d) Ni-Al2O3. The inset figure is the Ni particle size distribution.
(a)(b)(c)(d)The element distribution of reduced Ni-La2O3 and Ni-Al2O3 catalyst are analyzed by the TEM-mapping technique. As shown in Figure 4(a), the high-angle dark field images showed that Ni nanoparticles were uniformly distributed on the La2O3 support (Figures 4(b)–4(e)). However, for the Ni-Al2O3 catalyst (Figures 5(a)–5(e)), the nickel particles were agglomerated, which was consistent with the previous TEM and XRD results. In high temperature (780°C) hydrogen atmosphere, the nickel particles of Ni-La2O3 catalysts remain uniformly dispersed, due to the strong metal-support interaction which help to inhibit the agglomeration of Ni particles.Figure 4
(a) HAADF-STEM images; (b–d) TEM mapping of Ni, La, and O elements; and (e) overlap of elements for reduced Ni-La2O3.
(a)(b)(c)(d)(e)Figure 5
(a) HAADF-STEM images; (b–d) TEM mapping of Ni, Al, and O elements; and (e) overlap of elements for reduced Ni-Al2O3.
(a)(b)(c)(d)(e)


### 3.4. Catalytic Performance

The stability of ethanol steam reforming was studied at 600°C, the molar ratio of H2O/C2H5OH was 4 : 1, and the GHSV of aqueous ethanol vapor was 55920 mL/gcat∙h, under atmospheric pressure, as shown in Figure 6. Ni-La2O3 catalyst has higher ethanol steam reforming activity and stability than Ni-Al2O3 (Figure 6(a)). For Ni-Al2O3, the conversion rate of ethanol decreased rapidly from 50.2% to 15.1% within 360 min, and the conversion decreased by nearly 70% (Figure 6(b)). Similarly, the hydrogen production rate decreased from 4529.9 μmol/min to 1659.3 μmol/min. The rapid deactivation of Ni-Al2O3 in the steam reforming of ethanol may be related to weak metal-support interaction, which cause the serious metal sintering and carbon deposition. For Ni-La2O3, the conversion of ethanol increased to 93.9% and the hydrogen production rate was 6913.6 μmol/min after 75 min. After 3000 min, the conversion still remained at 87.8%, and the hydrogen production rate remained as high as 6579.4 μmol/min. The ethanol conversion rate of the Ni-La2O3 catalyst decreased by only 6.1%, which was much smaller than that of Ni-Al2O3. Meanwhile, the gas production composition (Figures 6(c) and 6(d)) were closed to balanced composition [20, 21].Figure 6
The conversion of ethanol and gas product formation rate with time on stream at the GHSV of 55920 mL/gcat·h for C2H5OH solution, reaction conditions: T=600°C, H2O/C2H5OH=4, P=0.1MPa, N2=60mL/min for (a) Ni-La2O3 and (b) Ni-Al2O3 for the gas product composition for the catalysts of (c) Ni-La2O3 and (d) Ni-Al2O3.
(a)(b)(c)(d)The high activity and stability of the Ni-La2O3 catalyst can be attributed to its strong metal-support interaction, rich metal-support interface, abundant nickel electron density, and small particle size of nickel [20, 21]. The relative low decrease of the ethanol conversion of Ni-La2O3 catalyst can be attributed to the enhancement of anticarbon deposition performance, which will be confirmed by the following XRD and TG results. Also, the catalyst performance was compared with these previous reported (Ref [29–38]) as summarized in Table 2. The Ni-La2O3 catalyst designed in this study has higher ethanol steam reforming activity, greater stability, and good carbon deposition resistance and will be used in future.Table 2
Comparison of the catalytic performances with previously reported catalysts.
ReferencesCatalyst and its specific surface area (m2/g)Temperature of reaction (°C)/molar ratio of water to carbonGHSV ethanol-water vapor (mL/gcat·h)Inactivation rate of activity (%)a/time on stream (h)Rate of carbon deposit (mg/gcat·h)[29]Ni15LaSn950 (62)650/4.5410001/800.78[30]1CeNi/SBA-15 (538)650/45700010/501.6[31]Ni-1Ce/MMT (67.2)650/2—8.9/50—[19]Meso-3LaNiAl (202)600/2570000/502.28[32]Ni-0.25Sn/CeO2 (7.3)600/5570005/20—[33]Ni/Ce0.8Zr0.2 (85)600/2.56000010/1027[34]Ni/La2O3-α-Al2O3 (33.9)700/6138000/42.6[35]Ir/CeO2 (148)650/1.51800035/300—[36]Ni-Ce-La (76)550/3774210/601[37]Ni/10 mg-ATP (356.7)700/1.5192002/506.1[38]Ni/MgO (28)650/4.28000042/242.4[20]Ni-CeO2 (29.4)600/25592017.1/502.6This workNi-La2O3 (23.3)600/2559206.1/503.0Furthermore, our Ni-La2O3 can be used to convert methane and carbon dioxide under harsh reaction condition as shown in Figure 7. Under the same reaction condition, Ni-La2O3 catalyst showed higher activity and stability of methane dry gas reforming than that of Ni-Al2O3. The initial conversion rates of methane and carbon dioxide over Ni-La2O3 catalyst were 92.6% and 92.5%, respectively (Figures 7(a) and 7(b)). While, the initial conversion rates of methane and carbon dioxide over Ni-Al2O3 catalyst were 76.3% and 86.5%, respectively. After 140 min reaction, the conversion rates of methane and carbon dioxide were 84.4% and 82.2% for Ni-La2O3 catalyst, which were higher than those of Ni-Al2O3 catalyst. In addition, the H2/CO ratio of the Ni-La2O3 catalyst remained at 0.94, while for the Ni-Al2O3 catalyst decreased to 0.85 after 140 min of reaction (Figure 7(c)). The hydrogen production rate (Figure 7(d)) of the Ni-La2O3 catalyst was maintained at 1.49 mmol/min, which was 1.37 times higher than that of the Ni-Al2O3 catalyst (1.09 mmol/min). These results indicate that Ni-La2O3 has higher catalytic activity and stability than Ni-Al2O3 during the methane dry reforming.Figure 7
(a) CH4 conversion, (b) CO2 conversion, (c) the ratio of H2/CO, and (d) hydrogen production rate over different catalysts. Reaction conditions: CH4/CO2/N2=1:1:2,mcat=0.15g, GHSV=40000mL/gcat·h, T=700°C, P=0.1MPa, and N2=50mL/min.
(a)(b)(c)(d)


### 3.5. XRD and TEM Analyses of Spent Catalyst

In order to illustrate the big difference of two catalysts in catalytic performance, the powder XRD results of catalysts after ethanol steam reforming and methane dry reforming are shown in Figures8(a) and 8(b). The structure of the catalyst after the reaction was similar to that of the catalyst before the reaction. For the Ni-Al2O3 catalyst after reaction, a very strong diffraction peak of nickel could be observed, while the characteristic peak of nickel was not observed for the Ni-La2O3 catalyst. At the same time, a new diffraction peak appeared in the catalyst after the reaction at 26.1o, which was attributed to the diffraction peak of graphite-carbon [39]. The intensity of the graphitic carbon diffraction peak of the Ni-Al2O3 catalyst was stronger than that of the Ni-La2O3 catalyst, which indicates that Ni-La2O3 has litter carbon deposition and better anticarbon deposition performance. This may be due to the Ni-La2O3 catalyst possessed smaller nickel particles size, rich electron density, and abundant strong basic sites.Figure 8
XRD analysis of the spent catalyst for (a) ethanol steam reforming and (b) methane dry reforming.
(a)(b)The TEM images for the spent catalysts were obtained. For Ni-La2O3, the average size of nickel particles was about 15.5 nm (inset Figure 9(a)), and no obvious carbon deposition was observed (Figure 9(b)). After the reaction, the average size of nickel particles in the Ni-Al2O3 catalyst was about 68.5 nm (inset Figures 9(c) and 9(d)), which was increased to a large extent compared with fresh sample, and the larger size of nickel particles reduces its catalytic activity [19]. This characterization result directly explains the reason for the significant reduction of ethanol reforming performance.Figure 9
The TEM images of spent catalysts after steam reforming of ethanol (a, b) for Ni-La2O3 and (c, d) for Ni-Al2O3. The inset is the Ni particle size distribution.
(a)(b)(c)(d)


### 3.6. Analysis of Carbon Deposition on the Spent Catalyst

The carbon deposition on the catalyst surface after the reaction were analyzed by TG and DTA, and the results are shown in Figures10(a) and 10(b). The weight loss percentage of Ni-Al2O3 after ethanol steam reforming reaction was 21.7%, indicating that the proportion of carbon deposition in the catalyst after the reaction was 21.7%. For the spent Ni-La2O3 catalyst, the weight loss rate was significantly reduced. The better anticarbon deposition performance of the Ni-La2O3 catalyst is attributed to the smaller size of Ni particles, the abundant metal-support interface, and the abundant strong basic sites, which is beneficial to the inhibition of carbon deposition.Figure 10
The TG and DTG analyses of the spent catalysts (a) after ethanol steam reforming and (b) after methane dry reforming.
(a)(b)In addition, the differential thermal analysis of the catalyst after the reaction is shown in Figure10(a). The temperature for oxidation of carbon on Ni-Al2O3 catalyst was concentrated at 594.9°C, which corresponds to the oxidation of the encapsulated carbon on the surface of nickel metal. For spent La2O3, the elimination temperature of carbon deposition on nickel surface moves to the low temperature (592°C) zone and the peak area was greatly reduced. The main carbon deposition oxidation temperature was 761.8°C, which was attributed to the oxidation of carbon nanotubes [40]. The activity and stability of the Ni-Al2O3 catalyst are reduced due to a large amount of carbon deposition on the surface of nickel, while the stability of the Ni-La2O3 catalyst was well maintained due to the small amount of carbon deposition on the surface of the nickel. The oxidation of carbon deposits after methane dry reforming exhibited a similar rule (Figure 10(b)). Large amounts of carbon deposition on the surface of nickel metal cause bad stability for Ni-Al2O3 which was consistent with the TEM results above (Figure 9).


## 3.1. Characterization of Structure and Metal-Support Interaction

The XRD patterns of the As-reduced catalysts are shown in Figure1(a). For the Ni-Al2O3 catalyst, no obvious peak of Al2O3 was observed, which could be due to its amorphous structure [19]. The diffraction peaks with strong intensity appear at 44.5o, 51.9o, and 76.4o were attributed to the characteristic diffraction peaks of nickel (JCPDF#01-1260), indicating that the high crystallinity of nickel in the reduced catalyst with the uneven distribution of large particles size of nickel. For the Ni-La2O3 catalyst, the peaks of La2O3 (JCPDF#83-1344) was mainly observed. There was no obvious characteristic peak of nickel, illustrating that the small particle size of nickel with uniform distribution. These results will be confirmed by H2-TPR as shown in Figure 1(b). All the As-prepared catalysts have three reduction peaks. For the Ni-Al2O3 catalyst, the reduction peak of 391.1°C was attributed to the reduction of NiO with large particles or weak interaction with the support, and 484.1°C was assigned to the reduction of NiO with small particles or strong interaction with the support, and 695.4°C was attributed to the reduction of nickel-aluminum spinel [19]. For the Ni-La2O3 catalyst, the first peak appeared at 416.9°C, which can be attributed to the decrease of Ni2+ species on the surface. The second peak appeared at 609.3°C, possibly because LaNiO3 was reduced to La2Ni2O5 [26]. The third reduction peak was located at 767.7°C, which corresponds to the reduction of Ni2+ in LaNiO3 to metal Ni0 [27]. The higher reduction temperature of the Ni-La2O3 catalyst indicates that it possessed a strong metal-support interaction, which is beneficial to inhibit the sintering of nickel and maintain the small particle size of nickel during the high-temperature reduction process.Figure 1
(a) XRD patterns of the reduced catalysts, ♦ and ♥ stand for Ni and La2O3, respectively. (b) H2-TPR profiles of the As-prepared catalysts. (c) N2 adsorption-desorption isotherms (inset is pore size distributions) of the samples. (d) The CO2-TPD for the Ni-La2O3 and Ni-Al2O3.
(a)(b)(c)(d)The nitrogen adsorption-desorption isotherms of the calcined catalyst are shown in Figure1(c) with hysteresis loop indicating that the prepared catalyst was a mesoporous material. With the increase of the relative pressure, the adsorption capacity also increases, indicating that there was a certain amount of large pores in the material. The pore size showed double pore size distribution. The mesoporous pore was centered at 4 nm and the macroporous was around 60 nm. From Table 1, the specific surface area of the Ni-Al2O3 catalyst was 51.3 m2/g, the pore volume was 0.11 m3/g, and the average pore size was 7.8 nm (inset Figure 1(c)). For the Ni-La2O3 catalyst, the surface area and pore volume were decreased to 23.3 m2/g and 0.04 m3/g, respectively. The results indicate that the prepared catalyst has a porous structure and its specific surface area was relatively small.Table 1
Textural properties of the As-prepared samples.
SampleSBet (m2/g)VTotal (m3/g)D (nm)Ni-La2O323.30.048.8Ni-La2O351.30.117.8In order to illustrate the alkaline site on different supports, the data of CO2-TPD was collected as shown in Figure 1(d). For Ni-La2O3, the high intensity of CO2 desorption peak was mainly centered on 782°C, which illustrates abundant and strong basic sites, while for the Ni-Al2O3, the main CO2 desorption temperature was located at 167°C and 473°C, which means weak and few basic sites. The strong basic sites for Ni-La2O3 may play a positive influence on CO2 adsorption during the methane dry reforming, thus exhibiting good activity and stability for methane dry reforming [17].


## 3.2. XPS Analysis of the Reduced Catalyst

XPS of the reduced catalyst is shown in Figure2. For the Ni-Al2O3 catalyst, Al 2p3/2 was detected, indicating that the Al element was trivalent. Similarly, La 3d3/2 was detected on the Ni-La2O3 catalyst, indicating that the La element was also trivalent. For the Ni 2p, three peaks at 852, 856, and 861 eV could be assigned to Ni0, Ni2+, and satellite peak (Ni2+), respectively. Compared with Ni-Al2O3, the binding energy of Ni0 for Ni-La2O3 shifts towards low binding energy, indicating the increased electron density of Ni0, which may be explained by the enhanced metal-support interaction as verified by Figure 1(b). Jing et al. [28] pointed out that the enhanced electron density of nickel enhances the activation and dissociation of CO2 and balances the formation rates of C and O species on the catalyst. In addition, the Ni0/(Ni0 + Ni2+) ratio was also calculated. The Ni0 ratios of Ni-La2O3 and Ni-Al2O3 were 59.9% and 56.9%, respectively, indicating that the ratio of nickel for two catalysts was similar.Figure 2
XPS profiles of the As-prepared catalysts.
(a)(b)(c)(d)


## 3.3. TEM Analysis of Reduced Catalyst

Apart from binding energy of Ni, the particle size of nickel is also play vital important role in catalytic performance. The TEM images of the reduced catalyst are shown in Figure3. The distribution of nickel particles in the Ni-La2O3 catalyst was relatively uniform, and the average particle size was 13.5 nm (inset Figure 3(a)). It was found that some nickel particles were embedded in the support and showed strong interaction with the support and more metal-support interfaces, which was consistent with the results of H2-TPR (Figure 1(b)). Moreover, according to HR-TEM, the lattice parameter of nickel particles was 0.201 nm, which corresponds to the Ni (111) crystal plane. The interaction interface between nickel particles and La2O3 support was large (Figure 3(b)). For the Ni-Al2O3 catalyst, agglomeration of nickel particles was observed and the average particle size was 45.2 nm (inset Figure 3(c)), indicating the contact interface between nickel particles and the support was small (Figure 3(d)). These results were consistent with the previous XRD and TPR results. According to literature, nickel particles have strong metal-support interaction, which makes them smaller particle sizes and rich interaction interfaces, leading to the better catalytic activity and sintering resistance [23].Figure 3
TEM images of the catalysts after H2 reduction: (a, b) Ni-La2O3 and (c, d) Ni-Al2O3. The inset figure is the Ni particle size distribution.
(a)(b)(c)(d)The element distribution of reduced Ni-La2O3 and Ni-Al2O3 catalyst are analyzed by the TEM-mapping technique. As shown in Figure 4(a), the high-angle dark field images showed that Ni nanoparticles were uniformly distributed on the La2O3 support (Figures 4(b)–4(e)). However, for the Ni-Al2O3 catalyst (Figures 5(a)–5(e)), the nickel particles were agglomerated, which was consistent with the previous TEM and XRD results. In high temperature (780°C) hydrogen atmosphere, the nickel particles of Ni-La2O3 catalysts remain uniformly dispersed, due to the strong metal-support interaction which help to inhibit the agglomeration of Ni particles.Figure 4
(a) HAADF-STEM images; (b–d) TEM mapping of Ni, La, and O elements; and (e) overlap of elements for reduced Ni-La2O3.
(a)(b)(c)(d)(e)Figure 5
(a) HAADF-STEM images; (b–d) TEM mapping of Ni, Al, and O elements; and (e) overlap of elements for reduced Ni-Al2O3.
(a)(b)(c)(d)(e)


## 3.4. Catalytic Performance

The stability of ethanol steam reforming was studied at 600°C, the molar ratio of H2O/C2H5OH was 4 : 1, and the GHSV of aqueous ethanol vapor was 55920 mL/gcat∙h, under atmospheric pressure, as shown in Figure 6. Ni-La2O3 catalyst has higher ethanol steam reforming activity and stability than Ni-Al2O3 (Figure 6(a)). For Ni-Al2O3, the conversion rate of ethanol decreased rapidly from 50.2% to 15.1% within 360 min, and the conversion decreased by nearly 70% (Figure 6(b)). Similarly, the hydrogen production rate decreased from 4529.9 μmol/min to 1659.3 μmol/min. The rapid deactivation of Ni-Al2O3 in the steam reforming of ethanol may be related to weak metal-support interaction, which cause the serious metal sintering and carbon deposition. For Ni-La2O3, the conversion of ethanol increased to 93.9% and the hydrogen production rate was 6913.6 μmol/min after 75 min. After 3000 min, the conversion still remained at 87.8%, and the hydrogen production rate remained as high as 6579.4 μmol/min. The ethanol conversion rate of the Ni-La2O3 catalyst decreased by only 6.1%, which was much smaller than that of Ni-Al2O3. Meanwhile, the gas production composition (Figures 6(c) and 6(d)) were closed to balanced composition [20, 21].Figure 6
The conversion of ethanol and gas product formation rate with time on stream at the GHSV of 55920 mL/gcat·h for C2H5OH solution, reaction conditions: T=600°C, H2O/C2H5OH=4, P=0.1MPa, N2=60mL/min for (a) Ni-La2O3 and (b) Ni-Al2O3 for the gas product composition for the catalysts of (c) Ni-La2O3 and (d) Ni-Al2O3.
(a)(b)(c)(d)The high activity and stability of the Ni-La2O3 catalyst can be attributed to its strong metal-support interaction, rich metal-support interface, abundant nickel electron density, and small particle size of nickel [20, 21]. The relative low decrease of the ethanol conversion of Ni-La2O3 catalyst can be attributed to the enhancement of anticarbon deposition performance, which will be confirmed by the following XRD and TG results. Also, the catalyst performance was compared with these previous reported (Ref [29–38]) as summarized in Table 2. The Ni-La2O3 catalyst designed in this study has higher ethanol steam reforming activity, greater stability, and good carbon deposition resistance and will be used in future.Table 2
Comparison of the catalytic performances with previously reported catalysts.
ReferencesCatalyst and its specific surface area (m2/g)Temperature of reaction (°C)/molar ratio of water to carbonGHSV ethanol-water vapor (mL/gcat·h)Inactivation rate of activity (%)a/time on stream (h)Rate of carbon deposit (mg/gcat·h)[29]Ni15LaSn950 (62)650/4.5410001/800.78[30]1CeNi/SBA-15 (538)650/45700010/501.6[31]Ni-1Ce/MMT (67.2)650/2—8.9/50—[19]Meso-3LaNiAl (202)600/2570000/502.28[32]Ni-0.25Sn/CeO2 (7.3)600/5570005/20—[33]Ni/Ce0.8Zr0.2 (85)600/2.56000010/1027[34]Ni/La2O3-α-Al2O3 (33.9)700/6138000/42.6[35]Ir/CeO2 (148)650/1.51800035/300—[36]Ni-Ce-La (76)550/3774210/601[37]Ni/10 mg-ATP (356.7)700/1.5192002/506.1[38]Ni/MgO (28)650/4.28000042/242.4[20]Ni-CeO2 (29.4)600/25592017.1/502.6This workNi-La2O3 (23.3)600/2559206.1/503.0Furthermore, our Ni-La2O3 can be used to convert methane and carbon dioxide under harsh reaction condition as shown in Figure 7. Under the same reaction condition, Ni-La2O3 catalyst showed higher activity and stability of methane dry gas reforming than that of Ni-Al2O3. The initial conversion rates of methane and carbon dioxide over Ni-La2O3 catalyst were 92.6% and 92.5%, respectively (Figures 7(a) and 7(b)). While, the initial conversion rates of methane and carbon dioxide over Ni-Al2O3 catalyst were 76.3% and 86.5%, respectively. After 140 min reaction, the conversion rates of methane and carbon dioxide were 84.4% and 82.2% for Ni-La2O3 catalyst, which were higher than those of Ni-Al2O3 catalyst. In addition, the H2/CO ratio of the Ni-La2O3 catalyst remained at 0.94, while for the Ni-Al2O3 catalyst decreased to 0.85 after 140 min of reaction (Figure 7(c)). The hydrogen production rate (Figure 7(d)) of the Ni-La2O3 catalyst was maintained at 1.49 mmol/min, which was 1.37 times higher than that of the Ni-Al2O3 catalyst (1.09 mmol/min). These results indicate that Ni-La2O3 has higher catalytic activity and stability than Ni-Al2O3 during the methane dry reforming.Figure 7
(a) CH4 conversion, (b) CO2 conversion, (c) the ratio of H2/CO, and (d) hydrogen production rate over different catalysts. Reaction conditions: CH4/CO2/N2=1:1:2,mcat=0.15g, GHSV=40000mL/gcat·h, T=700°C, P=0.1MPa, and N2=50mL/min.
(a)(b)(c)(d)


## 3.5. XRD and TEM Analyses of Spent Catalyst

In order to illustrate the big difference of two catalysts in catalytic performance, the powder XRD results of catalysts after ethanol steam reforming and methane dry reforming are shown in Figures8(a) and 8(b). The structure of the catalyst after the reaction was similar to that of the catalyst before the reaction. For the Ni-Al2O3 catalyst after reaction, a very strong diffraction peak of nickel could be observed, while the characteristic peak of nickel was not observed for the Ni-La2O3 catalyst. At the same time, a new diffraction peak appeared in the catalyst after the reaction at 26.1o, which was attributed to the diffraction peak of graphite-carbon [39]. The intensity of the graphitic carbon diffraction peak of the Ni-Al2O3 catalyst was stronger than that of the Ni-La2O3 catalyst, which indicates that Ni-La2O3 has litter carbon deposition and better anticarbon deposition performance. This may be due to the Ni-La2O3 catalyst possessed smaller nickel particles size, rich electron density, and abundant strong basic sites.Figure 8
XRD analysis of the spent catalyst for (a) ethanol steam reforming and (b) methane dry reforming.
(a)(b)The TEM images for the spent catalysts were obtained. For Ni-La2O3, the average size of nickel particles was about 15.5 nm (inset Figure 9(a)), and no obvious carbon deposition was observed (Figure 9(b)). After the reaction, the average size of nickel particles in the Ni-Al2O3 catalyst was about 68.5 nm (inset Figures 9(c) and 9(d)), which was increased to a large extent compared with fresh sample, and the larger size of nickel particles reduces its catalytic activity [19]. This characterization result directly explains the reason for the significant reduction of ethanol reforming performance.Figure 9
The TEM images of spent catalysts after steam reforming of ethanol (a, b) for Ni-La2O3 and (c, d) for Ni-Al2O3. The inset is the Ni particle size distribution.
(a)(b)(c)(d)


## 3.6. Analysis of Carbon Deposition on the Spent Catalyst

The carbon deposition on the catalyst surface after the reaction were analyzed by TG and DTA, and the results are shown in Figures10(a) and 10(b). The weight loss percentage of Ni-Al2O3 after ethanol steam reforming reaction was 21.7%, indicating that the proportion of carbon deposition in the catalyst after the reaction was 21.7%. For the spent Ni-La2O3 catalyst, the weight loss rate was significantly reduced. The better anticarbon deposition performance of the Ni-La2O3 catalyst is attributed to the smaller size of Ni particles, the abundant metal-support interface, and the abundant strong basic sites, which is beneficial to the inhibition of carbon deposition.Figure 10
The TG and DTG analyses of the spent catalysts (a) after ethanol steam reforming and (b) after methane dry reforming.
(a)(b)In addition, the differential thermal analysis of the catalyst after the reaction is shown in Figure10(a). The temperature for oxidation of carbon on Ni-Al2O3 catalyst was concentrated at 594.9°C, which corresponds to the oxidation of the encapsulated carbon on the surface of nickel metal. For spent La2O3, the elimination temperature of carbon deposition on nickel surface moves to the low temperature (592°C) zone and the peak area was greatly reduced. The main carbon deposition oxidation temperature was 761.8°C, which was attributed to the oxidation of carbon nanotubes [40]. The activity and stability of the Ni-Al2O3 catalyst are reduced due to a large amount of carbon deposition on the surface of nickel, while the stability of the Ni-La2O3 catalyst was well maintained due to the small amount of carbon deposition on the surface of the nickel. The oxidation of carbon deposits after methane dry reforming exhibited a similar rule (Figure 10(b)). Large amounts of carbon deposition on the surface of nickel metal cause bad stability for Ni-Al2O3 which was consistent with the TEM results above (Figure 9).


## 4. Conclusions

In a word, La2O3- and Al2O3-supported Ni catalysts were prepared by the citric acid-assisted sol-gel method, and their structures, properties, and reactivity were systematically compared. Ni-La2O3 with small particle size of nickel, strong metal-support interaction, rich metal-support interface, and abundant strong basic sites were achieved as verified by XRD, H2-TPR, TEM, and CO2-TPD. The catalytic performance showed that compared with Ni-Al2O3, Ni-La2O3 catalyst had higher ethanol steam reforming activity, stability, and hydrogen production rate. Under the condition of 55920 mL/gcat∙h of ethanol-water vapor at 600°C, the ethanol conversion remained at 87.8% after 3000 min of reaction. The hydrogen production rate was greater than 6500 μmol/min, and the resistance to carbon deposition was greatly improved. In addition, the Ni-La2O3 catalyst showed good activity in methane dry reforming with the initial conversion rate of methane and carbon dioxide was greater than 90%, and the H2/CO ratio was close to 0.94. The better catalytic performance for Ni-La2O3 catalyst was attributed to the smaller size of metal nanoparticles, more metal-support interfaces, rich nickel electron density, abundant strong basic sites, and strong metal-support interaction.



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*Source: 6504914-2023-02-09.xml*