Effect of Sr Substitution on the Novel La_1-xSr_xCo_0.5Ni_0.5O_3-δ Perovskite-Type Catalysts for Hydrogen Production in Ethanol Steam Reforming

Abstract

La_1-xSr_xCo_0.5Ni_0.5O_3-δ perovskites with different substitution of La by Sr have been firstly synthesized and applied as catalysts for ethanol steam reforming (ESR). The effect of partial Sr substitution on the performance and stability of La_1-xSr_xCo_0.5Ni_0.5O_3-δ for ESR was investigated. The synthesized samples were characterized by XRD, SEM, BET, and EDS techniques. The results show that with the increase of substitution of La by Sr, the pore size and surface area of La_1-xSr_xCo_0.5Ni_0.5O_3-δ perovskite increase. The doping of Sr causes changes in the specific surface area, pore volume, and pore size of the catalyst, which in turn causes changes in its catalytic activity. Meanwhile, La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ with a highest S_BET presents the highest ethanol conversion rate among all the samples. The modifications of catalyst characteristics caused by the Sr substitution in La_1-xSr_xCo_0.5Ni_0.5O_3-δ directly affect its catalytic performance in the ESR. What is more, the influences of operating parameters on the catalytic activity of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ were studied in detail, and the optimum conditions were determined and applied for stability experiments. When the temperature is lower than 500°C, the selectivity of gas products is most likely affected by the dehydrogenation of ethanol and the decomposition of acetaldehyde. Meanwhile, the improving hydrogen selectivity and reducing the formation of by-products can be achieved by increasing the reaction temperature. The best catalytic performance for hydrogen production was achieved with La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ which presented the highest ethanol conversion (98.7%) under the optimal reaction conditions. La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ perovskites with higher strontium degree of substitution exhibited an excellent activity and stability of the catalysts derived.

1. Introduction

With the rapid development of industry, hydrogen energy is one of the attractive measures for solving the energy supply security and the greenhouse gas emission. It can be stored and transported conveniently and utilized in multiple ways. Currently, as a kind of energy carrier with high calorific value and high environmentally friendly, hydrogen has been universally acknowledged as one of the potential fuels to substitute for traditional fossil fuels [1, 2]. Pashchenko concluded that thermochemical recuperation can be used as an on-board hydrogen production technology and conducted a series of studies on the effects of operating parameters such as temperature, pressure, and steam ethanol ratio on the heat recovery rate [3]. In addition, they also studied the feasibility of using the heat contained in the outlet flue gas for ethanol steam conversion [4].

In recent years, the ethanol steam reforming (ESR) technology is known as the hot spot in the field of research. Ethanol, as a new fuel, can reduce the greenhouse gas emissions by 70% to 90% compared to fossil energy [5]. In the current situation of increasingly scarce energy, the use of ethanol new energy technology has important social and economic significance [6]. Anil et al. reviewed the nickel-based catalysts for hydrogen production from ethanol steam reforming since 2000 and elucidated the importance of preparation methods and carrier modifiers to the morphology of the catalysts and the reduction of carbon deposition [7]. In addition, the role of operating conditions such as water and ethanol feed ratio and temperature with carbon generation were interrelated, which is one of the focuses of this work [8].

Bioethanol fuel is widely concerned in the application of resources because of its liquid nature, safety, small volume, and other advantages [9]. At present, H₂ production from ethanol steam reforming is favored by the Proton Exchange Membrane Fuel Cell (PEMFC) field because of its reasonable economy and simple technology.

The development of catalysts for H₂ production by ethanol steam reforming mainly focuses on the unique structure of catalysts [10–12]. The catalysts applied to the ethanol steam reforming can be divided into two categories: noble metal catalysts represented by platinum and nonnoble metal catalysts dominated by transition metals.

An et al. prepared a nickel-aluminum-based catalyst for ethanol steam reforming to produce hydrogen by coprecipitation [13]. They found that after the addition of Cu, due to the synergistic effect of Ni-Cu, the conversion of ethanol in different temperature ranges increased, and ethanol can be completely converted at 500°C. Cortazar et al. showed that 10 wt% NiO/CaO bifunctional catalyst performs well in adsorption enhanced ethanol steam reforming at 600-750°C [14]. Martinelli et al. discussed the effect of sodium loading on Pt/ZrO₂ during ethanol steam reforming [15]. Grzybek et al. revealed the key importance of the precise optimization of the K loading and dispersion on the Co|α-Al₂O₃ catalyst for efficient hydrogen production via the ethanol steam reforming process [16]. The Ni/SiC_xO_y catalyst, synthesized by Guo et al., showed that the ethanol conversion and selectivity of H₂ were still up to 91% and 72% after 210 min, respectively [17]. Niazi et al. compared the effects of copper, magnesium, and cobalt on Ni-Ce supported catalysts for ethanol steam reforming [18]. They found that the nature of second metal has a strong influence on the catalyst selectivity for H₂ production. Among them, supported nickel-based catalysts are the most widely developed and applied, which have high H₂ selectivity and catalytic activity [19]. However, supported catalysts have some unavoidable disadvantages, such as uneven distribution of active components, easy poisoning, and short service life [20, 21].

Perovskite metal oxides have stable structures and flexible tunability of physicochemical properties, making them one of the suitable choices for hydrogen production catalysts [22]. For a typical ABO₃ perovskite-type structure, the A-site ion is generally an alkaline earth element ion with a larger ion radius; the B-site ion is generally a transition metal element ion with a smaller ion radius, and the transition metal ion forms an octahedral coordination with six oxygen ions. Perovskite-type catalysts have high mid-to-high temperature activity and excellent thermal stability. Studies have shown that if the A/B element of the perovskite-type metal oxide is partially substituted, its crystal structure remains basically unchanged, while some of its physical and chemical properties will be adjusted.

Alkaline earth metals such as La, Sr, and Ba are used as the A-site metal, or transition metal elements such as Ni and Co are used as B-site metals to adjust the physicochemical properties of the perovskite catalyst, which is one of the main ways to improve the stability and carbon resistance of perovskite catalyst for H₂ production by ethanol steam reforming [23, 24]. Marinho et al. studied the performance of nickel-based catalysts derived from LaNiO₃ and LaNiO₃ perovskite-type oxides supported by CeSiO₂ in ESR [25]. The results showed that the initial ethanol conversion of LaNiO₃ reached 97%, while that of LaNiO₃/CeSiO₂ was only 85%. However, LaNiO₃/CeSiO₂ exhibits better stability because the oxygen in cerium is transferred to metal particles through the metal carrier interface and reacts with carbon, which inhibits the formation of carbon to a certain extent. Therefore, if the lanthanide nickel-based perovskite catalyst can overcome the deactivation, it will have greater application potential. Wang et al. developed the ESR perovskite-type metal oxide catalyst, La_1-xCa_xFe_1-xCo_xO₃ (, 0.5), which has high hydrogen selectivity and thermal stability [26]. Ma et al. studied the effect of A-site substitution on the catalytic performance and stability of LaCoO₃ perovskite catalyst in ESR reaction. The experimental results indicate that the Sr-doped catalyst exhibits excellent catalytic activity and thermal stability compared to the other element-doped catalysts studied [27]. Esposito indicated that supporting perovskite LaNiO₃ on SBA-15 resulted in reduced carbon deposition. The physical and chemical properties of synthetic compounds can be controlled by carefully changing the parameters affecting different synthetic steps, which is a unique feature of sol-gel method [28]. It is precisely because of the characteristics of the sol-gel method that it is possible to accurately control the proportion of A/B doping elements in perovskite.

LaCoO₃ perovskite catalyst has been used in ESR; however, its catalytic activity and stability need to be further improved. Therefore, appropriate substitution of the A- or B-sites is one of the effective ways to improve its catalytic performance [29]. Perovskite powder samples, with a homogeneous distribution of components on the atomic scale through a technology of low temperature synthesis and with full control of the finite product microstructure, were synthesized by sol-gel method. The purpose of this work is to study the performance of La-Co-Ni-based perovskite catalysts, namely, La_1-xSr_xCo_0.5Ni_0.5O_3-δ (, 0.1, 0.5, 0.9, designated as LSCN), in which the A-site is partially substituted by Sr, for hydrogen production by steam reforming of ethanol. The influence of Sr substitution on the microscopic characteristics of the catalyst on the catalytic performance was studied by means such as XRD, SEM, BET, and EDS characterization. In addition, the effects of reaction temperature, ratio of S/C, and steam flow rate on ESR performance were investigated and optimized. Furthermore, the stability test of the optimal catalyst for ESR process was applied.

2. Material and Methods

2.1. Catalyst Synthesis

LSCN was synthesized by the sol-gel method in this study. Analytical pure La (NO₃)₃·6H₂O, Sr (NO₃)₂, Ni (NO₃)₂, and Co (NO₃)₂·6H₂O are used as metal nitrate raw materials; citric acid and ethylenediaminetetraacetic acid (EDTA) are used as complexing agents. For the specific preparation steps, refer to our previous research [30].

2.2. Catalyst Characterization

The surface morphology and chemical composition of the LSCN were performed through scanning electron microscopy (SUPRA 55 SAPPHIRE, ZEISS, Germany). The XRD patterns of samples were obtained by a Rigaku D/MAX-Ultima⁺ diffractometer operated at 40 kV and 40 mA with Co Kα radiation. The specific surface areas and pore properties of samples were calculated by Brunauer-Emmett-Teller (BET, WBL-8XX) and Barrett-Joyner-Halenda. Specifically, the specific surface area was determined by the BET multipoint method and degassed at 100°C for 120 minutes, of which the physical adsorption method covered 6 points.

2.3. Ethanol Steam Reforming

The catalyst activity was evaluated in the ESR performance test system including the mass flow controller, tube furnaces, quartz reactor, gas analyzers, and data acquisition systems. The length of the quartz tube is 50 cm, the outer diameter is 12 mm, and the inside diameter is 10 mm. 0.2 g of the synthesized catalyst sample was placed in a quartz tube reactor. The experimental setup for ESR is shown in Figure 1.

Figure 1 

Experimental setup for ESR.

The system was purged with N₂ at a flow rate of 30 mL/min for 15 minutes before each ESR test. The LSCN catalysts were reduced under 300 mL/min of 10 vol% H₂-N₂ gas at 500°C for 90 min, followed by purging at the same temperature with 30 mL/min of N₂ during 15 min. Subsequently, nitrogen as a carrier gas delivered the vapor of the ethanol aqueous solution mixture into the tube furnace reactor at a flow rate of 30 mL/min. The volumetric concentrations of H₂, CO₂, CH₄, and CO in the gas products were measured by the gas analyzer.

The conversion of ethanol and the yield of each component in the gas product were estimated as

where and are the molar amount of carbon in the feed and the molar amount of carbon in the reformed gas, respectively; is the conversion of ethanol; is the yield of each gas component in the reformed gas (, CO, CO₂, and CH₄); and is the molar amount of ethanol in the feed.

3. Results and Discussion

3.1. Effect of Sr Doping on Catalytic Properties of La_1-xSr_xCo_0.5Ni_0.5O_3-δ

The catalytic performance of perovskite is significantly affected by A-site doping. The results of ethanol conversion and product yields over La_1-xSr_xCo_0.5Ni_0.5O_3-δ in the ESR are shown in Figure 2. The ethanol conversion can reach 80%, 81%, 90%, and 92%, respectively, when the ESR reaction is stable. As shown in Figure 3(a), the highest ethanol conversion of 92% was achieved. The high ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ may be related to the high activity caused by its higher specific surface area, because the large specific surface area means that more active sites are exposed [31]. From the BET data of the LSCN samples, the doping of Sr causes changes in the specific surface area, pore volume, and pore size of the catalyst, which in turn causes changes in its catalytic activity. The highest activity of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ is represented by the highest ethanol conversion and the highest yield of hydrogen. In general, the yield of carbon-containing gas products (such as CO) is low, which is beneficial for the application of this technology in the field of hydrogen fuel cells. In addition, the experimental results show that La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ can produce 4.5 H₂ mol/(C₂H₅OH mol).

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Figure 2 

Product yield over La_1-xSr_xCo_0.5Ni_0.5O_3-δ for ethanol steam reforming: (a) CO yield; (b) CO₂ yield; (c) CH₄ yield; (d) H₂ yield.

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Figure 3 

(a) Comparison of ethanol conversion over La_1-xSr_xCo_0.5Ni_0.5O_3-δ for ethanol steam reforming. (b) Product selectivity of La_1-xSr_xCo_0.5Ni_0.5O_3-δ catalysts at different Sr doping levels.

The gas product selectivity of different samples is shown in Figure 3(b). The catalyst samples with different strontium content showed significant differences in ESR reaction. Similar to the effect of Sr on the yield of gas products, it can be observed that La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ perovskite catalyst has the highest H₂ selectivity (71.29%), and the selectivity of CO, CO₂, and CH₄ is 13.56%, 12.31%, and 2.86%, respectively. Based on the above experimental results, it can be concluded that the doping of Sr plays a positive role in the production of high-quality hydrogen over the catalyst during the ESR process. Compared with the catalyst without Sr doping, the presence of strontium improves the ethanol conversion by 2%~12%. In addition, the CH₄ selectivity of the Sr-substituted samples was significantly decreased, which means that the side reactions were inhibited in the ESR reaction. In turn, the production of CO is reduced, which, to some extent, prevents catalyst deactivation and carbon deposition. La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ has the highest hydrogen yield (4.6 mol/(mol ethanol)) and ethanol conversion (92%), so the La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ was used as candidate for the following research.

3.2. Effect of the Reaction Temperature of ESR

Temperature is a crucial influencing factor for the catalytic reaction. Figures 4 and 5 compare the curves of the ethanol conversion and the yield of each gas product of La_0.1Sr_0.9Co_0.5 Ni_0.5O_3-δ at 500°C, 550°C, 600°C, and 650°C, respectively. The results show that the hydrogen production of La_0.1Sr_0.9Co_0.5 Ni_0.5O_3-δ is directly proportional to the reaction temperature and reaches the maximum at 600°C. The ethanol steam reforming reaction is an endothermic reaction, so the medium and high temperature environment is conducive to the forward reaction [32]. In addition, too low temperature will also lead to the complexity of the reaction, which is shown by the increase of by-products. The continuous increase of temperature, for example, will lead to the decrease of hydrogen production, which may be because the excessive temperature leads to the sintering and carbon deposition of the catalyst, resulting in the decrease of catalytic activity. At low temperatures such as 500°C, the presence of carbon-containing by-products such as carbon monoxide, carbon dioxide, and methane in the gas product indicates that a chain reaction has occurred in the steam reforming reaction of ethanol. In this case, as shown in equations (3) and (4), ethanol dehydrogenates to acetaldehyde, hydrogen and acetone, carbon monoxide, and hydrogen, respectively [33]. The occurrence of acetaldehyde decomposition reaction as shown in equation (5) may be another reason for the increase of CH₄ content. In addition, the reduction of hydrogen yield is caused by the decomposition of ethylene into carbon (equation (6)) around 500°C, which also leads to the reduction or even deactivation of the catalyst.

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Figure 4 

Product yield of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different reaction temperatures: (a) CO yield; (b) CO₂ yield; (c) CH₄ yield; (d) H₂ yield.

Figure 5 

Ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different reaction temperatures.

It can be observed that the yield of H₂ is proportional to temperature, while the yield of CH₄ and CO is inversely proportional to temperature. When the temperature is higher than 600°C, the yield of H₂ and CO₂ show a downward trend, while the yield of CO gradually increases. This means that improving hydrogen selectivity and reducing the formation of by-products can be achieved by increasing the reaction temperature. In addition, ESR is considered as a carbon deposition reaction, and higher temperatures will reduce carbon deposition [34], which also explains the phenomenon as shown in Figure 5 that the catalytic performance of La_0.1Sr_0.9Co_0.5 Ni_0.5O_3-δ is the best at 600°C.

3.3. Effect of the Ratio of S/C

The comparison of the gas product yield and ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ under different S/C conditions is shown in Figures 6 and 7. As a general trend, when the ratio of S/C was increased to 4 : 1, the content of gas products such as hydrogen, carbon dioxide, carbon monoxide, and methane increased significantly, which indicates that an appropriate S/C ratio is conducive to promote the overall ESR reaction [35]. Figure 6(a) shows CO flow as function of S/C ratio. With the increase of the ratio of water to alcohol, the amount of CO production increased firstly and then decreased. Thus, it shows that the production of CO can be inhibited. The production of CO is stabilized at 0.7 mol/mol ethanol.

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Figure 6 

Product yield of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different S/C ratio: (a) CO yield; (b) CO₂ yield; (c) CH₄ yield; (d) H₂ yield.

Figure 7 

Ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different S/C ratio.

On the other hand, if the water content increases to a certain extent, its influence on the chemical balance will be weakened and the ESR reaction will need to be supplied with more energy. In order to increase the H₂ production rate as much as possible, it is essential to increase the H₂ production rate by ensuring a sufficient supply of raw water while avoiding excessive water to promote the dehydration and decomposition of ethanol. Therefore, increasing S/C ratio is an important way to inhibit CO yield. In conclusion, the S/C ratio of 5 : 1 was taken as the best reaction condition of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ in the following experiments.

3.4. Effect of the LHSV

The comparison of the gas product yield and ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ under different LHSV conditions is shown in Figures 8 and 9. It can be seen that when the LHSV is increased from 16.5 h^-1 to 19.5 h^-1, the reaction products (CO, CO₂, CH₄, and H₂) are significantly increased; at this time, the yield of H₂ is about 4.7 mol/(mol ethanol), and the product yields of CO, CO₂, and CH₄ are 0.88 mol/(mol ethanol), 0.92 mol/(mol ethanol), and 0.24 mol/(mol ethanol), respectively. With the increase of LHSV, the ethanol conversion was increased and then decreased. The ethanol conversion reached the highest value of 19.5 h^-1 and the ethanol conversion was 98.7%. According to the chemical equilibrium movement principle, an increase of the gas reaction pressure is beneficial to the direction of the decrease of the number of moles of the ESR reaction. This phenomenon can be understood as a certain number of active sites of the catalyst are a certain number, which is gradually from the under-load state to the process of full-load state. Therefore, the increase of LHSV to a certain value is not conducive to the conversion of ethanol and water.

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Figure 8 

Product yield of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different LHSV: (a) CO yield; (b) CO₂ yield; (c) CH₄ yield; (d) H₂ yield.

Figure 9 

Ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ for ethanol steam reforming at different LHSV.

3.5. Stability Test

Excellent thermal stability, i.e., service life, is the key to the practical application of the catalyst in ESR. In this study, the stability test experiment of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ was carried out for up to 40 h under the optimal conditions. Based on these experiments, it can be obtained that the optimal reforming temperature is 600°C, the optimal S/C ratio is 5 : 1, and the optimal LHSV is 19.5 h^-1. As shown in Figure 10(a), the deactivation of the La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ catalyst sample after 40 hours of ethanol steam reforming reaction was not observed. The ethanol conversion was finally stabilized at 98%, and the yield of each gas product did not drop significantly. The excellent thermal stability is due to the solid cubic structure of perovskite-type metal oxides.

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Figure 10 

(a) Stability test of the La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ perovskite structured sample at optimal conditions: (yellow diamond) CO; (red circle) CO₂; (blue up-pointing triangle) CH₄; (green down-pointing triangle) H₂; (gray square) ethanol conversion). (b) Ethanol conversions of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ and LaCo_0.5Ni_0.5O_3-δ.

In addition, the ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ and LaCo_0.5Ni_0.5O_3-δ in ESR was compared at 600°C, and the results are shown in Figure 10(b). It can be significantly observed that the ethanol conversion of the La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ catalyst is higher than that of the LaCo_0.5Ni_0.5O_3-δ. The ethanol conversion of LaCo_0.5Ni_0.5O_3-δ is only 89.4%, while the ethanol conversion of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ is close to complete conversion. As shown by the BET results we tested, the difference in performance between the two is also reflected in the difference in their BET. A catalyst with a larger specific surface area will show better catalytic activity. The addition of strontium increases the specific surface area of the catalyst, thus exposing more active sites. It is concluded that La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ has excellent catalytic performance and can produce relatively good quality H₂.

3.6. Catalyst Characterizations

The XRD pattern of the La_1-xSr_xCo_0.5Ni_0.5O_3-δ (, 0.5, 0.9) perovskite samples are shown in Figure 11. The strong peaks occurred at 27°, 38.6°, 55.7°, and 70.2° (JCPDS No. 82-0228) [36], which indicated that the lanthanide cobalt-based perovskite metal oxide has been successfully synthesized. The phase is basically the same as perovskite, except for the newly formed secondary phase LSCN. In addition, there are sharp characteristic peaks unique to perovskite on the catalyst sample, indicating that the crystal structure of the prepared catalyst sample is intact. In particular, it can be found that as the doping amount of Sr element increases, the characteristic peak shifts to a low angle, and the peak shows a trend of sharper and narrower peak width (as shown in Figure 11(b)), indicating that the performance of perovskite catalyst samples becomes better. The broadening degree of characteristic peak in XRD pattern is inversely proportional to the crystal size. In other words, when the characteristic peak is sharper, the crystal structure will be better. Figure 12 shows a comparison of SEM images of fresh La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ sample powders and La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ reduced by H₂. It can be observed that the morphology of fresh La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ samples is porous and rough. As shown in Figure 12, the fresh La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ and La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ reduced by H₂ showed similar surface morphology, but the La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ sample reduced by hydrogen showed a denser porous network structure. However, the denser surface structure of fresh La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ results in a smaller specific surface area. These results can be confirmed by XRD analysis and SEM observation. Therefore, the La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ perovskite showed a better crystal structure.

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Figure 11 

XRD patterns of (a) La_1-xSr_xCo_0.5Ni_0.5O_3-δ, (b) along with the magnified patterns for 2θ from 32° to 38°.

Figure 12 

SEM images of La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ catalysts: (a) fresh La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ sample; (b) La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ sample reduced by H₂.

Table 1 displays the specific surface area, pore volume, and pore size of the fresh samples and samples after hydrogen reduction. The specific surface area of the fresh samples is smaller because they are all obtained by high temperature calcination [37]. The small specific area is a characteristic of perovskite powders. Among these perovskites with different Sr doping amounts, La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ obtained the maximum specific surface area of 3.7714 m²/g. The specific surface area, pore volume, and pore diameter of La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ after hydrogen reduction were significantly increased, which were 7.5648 m²/g, 0.0017 mL/g, and 13.8460 nm, respectively. After 40 hours of continuous experiments, the specific surface area, pore volume, and pore diameter of La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ remain unchanged; this also proves that La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ shown in Figure 12 has excellent thermal stability.

Table 1 

S_BET, pore volume, and pore size of the fresh samples and samples after hydrogen reduction.

Figures 13(a) and 13(b) show the adsorption isotherm and pore size distribution of La_1-xSr_xNi_0.5Co_0.5O_3-δ (, 0.1, 0.5, 0.9) perovskite catalysts, respectively. The synthesized catalysts exhibit the typical type III isotherms of perovskite as exhibited in Figure 13(a). It can be seen from the adsorption isotherm curve that the content of Sr element is generally positively correlated with the adsorption capacity of perovskite samples, especially the adsorption capacity of La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ after H₂ reduction is the strongest. When , the La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ treated with hydrogen reduction can reach 16.18 mL/g, and the adsorption capacity is increased by 45% compared with the La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ before reduction. In addition, the adsorption capacity of the sample (9.51 mL/g, ) was also the highest compared with the other samples in the 40 h stability test. Figure 13(a) also shows that the adsorption capacities of LaNi_0.5Co_0.5O_3-δ, La_0.9Sr_0.1Ni_0.5Co_0.5O_3-δ, and La_0.5Sr_0.5Ni_0.5Co_0.5O_3-δ were 2.77 mL/g (), 4.68 mL/g (), and 5.48 mL/g (), respectively. The pore size distribution of the La_1-xSr_xNi_0.5Co_0.5O_3-δ perovskite catalysts is mainly concentrated in the 0-10 nm range (as shown in Figure 13(b)). The pore size of La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ reduced by H₂ is (mL·nm^-1·g^-1), which means that the La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ perovskite catalyst will have better stability and catalytic ability in the ethanol reforming process.

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Figure 13 

(a) The adsorption isotherm of La_1-xSr_xCo_0.5Ni_0.5O_3-δ perovskite oxide catalysts. (b) Pore size distribution of La_1-xSr_xCo_0.5Ni_0.5O_3-δ perovskite oxide catalysts.

Figure 14 and Table 2 display the EDS analysis results of the catalyst sample La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ with the best catalytic effect. In the selected mapping area, it can be confirmed that all the elements in the catalyst are La, Sr, Co, Ni, and O. According to the element statistics, La: Sr: Co: Ni is in good agreement with the theoretical value of 1 : 9 : 5 : 5.

Figure 14 

EDS mapping image of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ catalyst.

Table 2 

Elemental composition of synthesized La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ catalyst.

4. Conclusions

The present work shows that La_1-xSr_xCo_0.5Ni_0.5O_3-δ (, 0.1, 0.5, 0.9) perovskite is a promising precursor of ESR hydrogen production catalyst. The physicochemical properties of the synthesized catalyst samples were studied by XRD, SEM, EDS, and BET. In addition, the effects of experimental operating parameters on the catalytic ethanol reforming reaction for hydrogen production are described in detail. The main conclusions are as follows: (1)The addition of Sr at A-site changed the characteristics of the catalyst and improved its catalytic performance, which showed that the pore volume and pore diameter of La_1-xSr_xCo_0.5Ni_0.5O_3-δ perovskite gradually increased with the increase of strontium, and the reduction of hydrogen further promotes this trend(2)XRD confirmed that the substitution of Sr element will increase the number of oxygen vacancies in perovskite metal oxides, and the oxygen vacancies on the surface can adsorb oxygen-containing species, which will promote the contact between the reactants and the catalyst. The specific surface area of porous and rough La_0.1Sr_0.9Ni_0.5Co_0.5O_3-δ samples is the largest (3.7714 m²/g) among perovskite with different Sr doping amount(3)The optimal ESR operating conditions for La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ are determined: the optimal reaction temperature is 600°C, the optimal S/C is 5 : 1, and the optimal LHSV is 19.5 h^-1. After 40 hours of stability testing under optimal operating conditions, the deactivation of La_0.1Sr_0.9Co_0.5Ni_0.5O_3-δ is not observed, which is attributed to the stability of its surface properties (such as specific surface area, pore volume, pore size) and the stable structure of perovskite. In the next experiment, we will try to reduce the working temperature of the catalyst by loading the support and doping elements, which will make it more practical

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The author(s) declare(s) that they have no conflicts of interest.

Acknowledgments

This work was supported by the China Postdoctoral Science Foundation (No. 2019M651094) and the Science and Technology Innovation Foundation of Dalian (2021JJ11CG004).