With the rapid development of social economy, the supply of fossil energy is becoming increasingly tight, and the environmental pollution caused by carbon based energy is also getting more and more attention. As the leader of clean energy in the 21st century, hydrogen energy has attracted people's attention because of its clean, efficient and high energy density. At present, hydrogen energy utilization technology is gradually mature, fuel cells using hydrogen as fuel begin to be practical, and green products such as hydrogen cars and hydrogen turbines have been put on the market. Therefore, the development of green hydrogen energy has also been selected as one of the top ten emerging technologies in 2020, and has been written in the government work report, which is one of the important ways to achieve carbon neutrality. However, due to multiple limitations such as immature hydrogen production technology, high production cost and imperfect infrastructure, hydrogen energy is still unable to be widely used. At present, hydrogen production methods mainly include chemical reforming hydrogen production, hydrolysis hydrogen production and biological hydrogen production. Among them, chemical reforming hydrogen production technology is relatively mature, with low energy consumption cost and high hydrogen production, which is one of the important hydrogen production methods. However, in the research of hydrogen production by chemical reforming, the catalyst system with stable catalytic performance and high hydrogen production efficiency is not perfect. At present, many researches have improved the activity of the catalyst system by modifying the hydrogen production catalyst and developing new catalyst supports, or improved the hydrogen production efficiency by optimizing the hydrogen production process by reforming.
Rare earth elements have special 4f electronic structure and lanthanide shrinkage phenomenon. Therefore, the oxidation and reduction ability of the catalyst can be further improved, the formation of solid carbon and acid sites can be inhibited, and the overall catalytic activity of the catalyst can be enhanced by doping rare earth metals into the modified reforming hydrogen production catalyst. The commonly used rare earth modified catalysts are La, Ce and Pr, among which, La can reduce the problem of carbon deposition in the sintering of hydrogen production catalysts at high temperatures; Ce can adsorb lattice oxygen, increase catalyst dispersion, and then promote hydrogen production by reforming; Pr can improve the anti deactivation ability of the catalyst and reduce the reaction temperature of hydrogen production. In the aspect of modifying the support, rare earth can reduce the acidic sites of the support, improve the specific surface area of the support and the dispersion of the catalyst supported on its surface, and prevent its coalescence, carbon deposition, etc. Therefore, reasonable use of China's rich rare earth resources to modify the catalytic system for hydrogen production by reforming plays an important role in promoting the development and utilization of hydrogen energy. Jiangxi Province is a big province of rare earth. In order to expand the green mining and efficient utilization of rare earth resources in the province and the expansion of rare earth industry chain, Jiangxi Rare Earth Research Institute of University of Science and Technology of China will be located in Ganzhou, the Kingdom of Jiangxi Rare Earth in 2020. In this context, this paper focuses on the research status and problems of rare earth modified reforming hydrogen production catalyst and support in recent years, and looks forward to the research direction of rare earth modified reforming hydrogen production catalyst.
1. Rare earth La modification
At present, Ni based, Cu based and Co based catalysts are mostly used for reforming hydrogen production. Rare earth modification can effectively improve the shortcomings of Ni based catalysts, such as easy coke deposition at high temperature, easy agglomeration of Cu based catalysts, high reaction temperature and many side reactions of Co based catalysts. Among them, Ni based catalysts are cheap, widely available, and have excellent C-C bond breaking ability, which can effectively reduce the selectivity of CO and CH4 in reforming hydrogen production reaction. It is found that rare earth La can improve the water adsorption capacity of Ni based catalyst for hydrogen production, promote the gasification of coke precursor, reduce the metal particle size of the catalyst, expand the catalytic active area, thus avoiding coke deposition and high-temperature sintering of the catalyst. MA and others successfully synthesized ordered mesoporous 3% La Ni/Al2O3 catalyst. It was found that La can enhance alkalinity by reducing the acidic sites of the catalytic system, and inhibit the production of C2 compounds such as coke precursor ethylene; La2O3 produced at high temperature can adsorb and activate water molecules, vaporize the formed carbon deposit, and inhibit the formation of carbon nanotubes that can rapidly inactivate Ni metal particles. This ordered mesoporous structure can increase the dispersion of Ni catalyst. At 873K, the raw material ethanol can be almost completely converted, and the H2 selectivity is about 85%.
Cu based catalysts have the advantages of low catalytic temperature and low selectivity of Co by-products, but Cu tends to agglomerate gradually in the reaction, resulting in the reduction of its effective catalytic specific surface area. In recent years, it has been found that proper Cu+/(Cu0+Cu+) ratio can improve the catalytic activity of Cu, promote the water gas reaction (CO+H2O → CO2+H2), and thus improve the H2 selectivity. Rare earth element La can effectively regulate the ratio of Cu+/(Cu0+Cu+) and inhibit Cu agglomeration. Huang et al. prepared 1La Cu/SiO2 (La/Cu mass ratio is 1) catalyst to catalyze dimethyl ether reforming to produce hydrogen. Under the control of La, the conversion rate of raw materials and H2 yield of the catalyst reached 98.6% and 97.5% respectively when the Cu+/(Cu0+Cu+) ratio was 0.5380 ℃.
The expensive Co is a kind of transition metal with variable valence. It has strong oxidation-reduction capacity and high H2 selectivity, but its catalytic temperature is higher. A series of side reactions at high temperatures have a greater impact on its catalytic performance, and the corresponding carbon deposition is also serious. Greluk et al. prepared Co La/CeO2 catalyst, and found that the formed lanthanum cerium oxide solid solution can promote the dispersion of Co, increase the specific surface area of catalyst Co, reduce the formation of graphite carbon, and inhibit the further deposition of coke; La can also inhibit the sintering of the support CeO2, so as to stabilize the catalytic hydrogen production performance of the catalytic system. At 500 ℃, the carbon generation rate of Co-2%/La/CeO2 catalyst was reduced by 4.8% after the modification of La. The raw materials were completely converted within 21 hours, and the H2 selectivity of the product was up to 94%.
In addition to being used as a promoter to modify the catalyst, La oxide can also be compounded with the support of hydrogen production catalyst. Al2O3, as one of the most commonly used supports of reforming catalysts for hydrogen production, has a high selectivity for H2, but Al2O3 is highly acidic and easy to deposit carbon on its surface, which leads to deactivation of the supported catalyst. Hernandez et al. synthesized Ni/Al La catalyst, which can stabilize the structure of Al2O3 support, promote the dispersion of NiO phase, reduce the oxidation rate of Ni at low temperature, and reduce the acid effect of Al2O3 through the modification and recombination of La2O3; The results showed that the H2 yield and ethanol conversion of Ni/Al La catalyst were higher than those of unmodified catalyst. The ethanol conversion of Ni/Al2O3-12% La catalyst reached 100% at 600 ℃, and the H2 yield of 1mol ethanol was about 3.4mol. However, Song et al. carried out more in-depth research on Ni/Al La catalysts and prepared Ni/Al2O3-La2O3 dry gel catalysts. It was found that excessive La would cause defects in Al2O3 lattice instead, weaken the affinity of the Ni catalyst supported on its surface for ethanol, and affect the effect of Ni catalytic reforming for hydrogen production. Therefore, the amount of modified rare earth should be strictly controlled.
Natural ore carrier structure has super adsorption performance, which can provide more reaction sites for hydrogen production raw materials and improve the dispersion of catalytic active metals on its surface, but it still faces serious sintering problems. For this reason, Chen et al. used La to composite natural ore carrier SEP (sepiolite), and found that La can improve its structure by interacting with SEP nanoparticles, so that its specific surface area and pore diameter are significantly increased, thereby inhibiting the sintering of Ni/SEP catalyst. At 600 ℃, the coke conversion and elimination effect of Ni/10% La SEP is the best, the CO yield is reduced by about 10%, and the H2 yield is up to 87.9%. In view of the advantages of rare earth La modification, our research group is exploring the use of rare earth La oxide's ability to adsorb water and remove carbon, combined with the ability of alkali metal support to inhibit the generation of C2 by-products, to prepare rare earth composite support supported Ni based catalyst for catalytic reforming of biological alcohol to produce hydrogen, in order to obtain a reforming hydrogen production catalytic system with stable performance and high hydrogen production rate.
2.Rare earth Ce modified rare earth
Ce modified reforming hydrogen production catalyst has been studied for a long time. Its unique oxygen storage and desorption function and adsorption capacity for lattice oxygen can not only inhibit coke formation, but also promote the gasification of coke on the surface. Amin et al. used MCM-22 molecular sieve with unique pore structure, good thermal stability, strong adsorption capacity and large specific surface area as a carrier to prepare Ce Ni/MCM-22 catalyst for hydrogen production from biomass corncob. The study found that the strong adsorption capacity of Ce on lattice oxygen and the function of storing and releasing oxygen can effectively reduce the rate and extent of Ni oxidative sintering, increase the dispersion of Ni, reduce the particle size of NiO, increase the number of active sites of NiO, and significantly improve the shortcomings of Ni based catalysts that are easy to sinter and easy to deposit carbon; Compared with the unmodified catalyst, its H2 generation rate is more than 2 times higher and the conversion rate of hydrogen production feedstock is 71%. Yang Shuqian et al. synthesized Ce Cu/Zn Al catalyst to catalyze methanol reforming for hydrogen production. The research results show that the Ce modification increases the dispersion and specific surface area of Cu, inhibits the agglomeration of Cu, reduces the particle size of CuO, and thus prevents the oxidized Cu species from entering the crystal lattice of zinc oxide or zinc aluminum oxide, making coke easier to gasify, and significantly reduces the carbon deposition. At 250 ℃, the conversion rate of raw materials is increased by nearly 40%, and the CO content is only 0.39%, At 240 ℃, the hydrogen production rate of 1kg catalyst reached 810.7mL/s.
Compared with the composite catalyst support formed by La and Al2O3, the interaction between Ce and Al2O3 is less, and Ce is more inclined to improve the active components of the catalyst through its own characteristics to promote the catalytic reaction. Isarapakdeetham et al. prepared Ni/La Ce Al catalyst. It was found that the Ce La solid solution in the oxygen carrier structure enhanced the oxygen storage capacity and the mobility of oxygen, so that the catalyst could fully react with the feed. As the oxygen carrier, Ce4+increased the dispersion of Ni, reduced the size of Ni grains, and enhanced the reducibility of the carrier; Ce can not only improve the water vapor transfer (WGS) reaction, but also adjust the acidity and alkalinity of the catalyst, thereby reducing the coke deposition on the catalyst; After the fifth operation cycle of N/3LCA (12.5% Ni/3% La2O3-7% CeO2-Al2O3), the conversion rate of raw ethanol reached 88%, and the H2 yield of 1mol ethanol reached 2.7mol. The research adopts the improved chemical cycle steam reforming hydrogen production process (CLSR), which can provide heat for the subsequent endothermic reforming reaction, and has the potential of energy self-sufficiency. It is the optimization of reforming hydrogen production process and one of the key research directions in the future.
Attapulgite carriers contain a special layer chain structure. The bar crystals formed inside are small in diameter, short in length, and rich in pores. However, the surface is acidic, which will inhibit the transfer of electrons in the reaction and is very easy to cause carbon deposition. Wang et al. prepared Ni Fe/Ce palygorskite (PG) catalyst. It was found that Ce enhanced the interaction between the active component Ni Fe alloy and the support, increased the oxygen consumption on the surface of the support, promoted the gasification reaction of coke, and effectively solved the problem of coke deposition of palygorskite. After 10 h of Ni Fe/PG catalyst deactivation, the acetic acid conversion and H2 yield of raw materials decreased to 55% and 18%, respectively, while the acetic acid conversion and H2 yield of Ni Fe/Ce PG catalyst after 10 h were stable at 95% and 87%, respectively.
Molecular sieves are widely used in the catalytic field because of their rich mesoporous structure, super adsorption capacity and stability, and are one of the popular supports for reforming hydrogen production catalysts. Guo Danyu et al. prepared Co/Ce-SBA-15 catalyst with a new ordered mesoporous silica molecular sieve SBA-15 as the support. The introduction of Ce made the SBA-15 support skeleton shrink, in which the length of Ce-O bond is longer than the original Si-O bond, thereby increasing the specific surface area of the support and improving the dispersion of the loaded Co species; The synergistic effect of Ce and Co can accelerate the decomposition of by-products such as acetaldehyde and other C2 compounds. When the ratio of Si/Ce content is 20, Co/Ce-SBA-15 can achieve 75.5% hydrogen selectivity and 96.6% ethanol conversion of raw materials, and the carbon deposition rate decreases by 10%.
3. Rare earth Pr modification
Compared with other rare earth elements, Pr has a stronger re oxidation ability and is not easy to be deactivated in the reaction. At the same time, Pr can also increase the oxygen vacancy of hydrogen production catalyst, which has a better effect in improving the stability of catalyst and reducing the reaction temperature. Barroso et al. prepared the Pr Ni/MgAl2O4 catalyst. The study found that the re oxidation ability of Pr effectively prevented the accumulation of solid carbon. The carbon deposition on the surface was in the form of loose multilayer deposition filaments. This carbon deposition structure did not affect the diffusion of reactants to the catalyst for contact reaction. However, when the amount of such filiform carbon deposition increased to a certain extent, it would also block the reforming hydrogen production reactor, affecting the hydrogen production effect. The research results show that 0.6% Pr and 1.8% Pr modified Ni/MgAl2O4 catalysts can reduce the catalytic reaction temperature, while when the content of Pr exceeds 2.6%, the reaction temperature of reforming hydrogen production increases by 14 ℃, but it can effectively and continuously catalyze for more than 40 hours, and the conversion rate of ethanol can be maintained at about 80%, with better catalytic stability. Therefore, the use of modified rare earth will also be a focus of future research.
Rare earth element Pr is widely used in carrier modification. Ryczkowski et al. prepared Ni/8% Pr Zr catalyst system supported by Ni on rare earth composite support, which was used to catalyze the reforming of lignocellulose to produce hydrogen; Under the modification of Pr, the carbon deposition on the surface of the carrier is fibrous, but this carbon deposition has little effect on the reaction; Pr promoted the formation of oxygen vacancies in the catalyst, which led to the reduction of NiO to metal Ni, improved the hydrogen selectivity, and increased the hydrogen yield by about 3mmol/g.
The addition of rare earth elements can promote the formation of rare earth and other phases that are conducive to interface bonding. Ishiyama et al. successfully prepared the Ce Zr Pr multi rare earth composite support CZP supported Ni catalyst. The catalytic activity and stability of this catalytic system are better than that of Ni/CZ (Ce and Zr composite support). It is analyzed that the introduction of Pr leads to the lattice mismatch between the rich Pr phase and the rich Ce cubic fluorite phase, thus forming a new interface, where oxygen vacancies are easily formed to enhance the catalytic activity of the catalyst; At 873K temperature and feed flow rate of feed methane of 2.5Ml/min, the conversion of CH4 and H2 yield of Ni/CZP catalyst increased by about 4.5% and 5% respectively compared with Ni/CZ catalyst.
4.Conclusion and Outlook
Under the background of carbon peaking and carbon neutralization, the use and development of green hydrogen energy will become one of the main directions of energy development in the future. The increasingly mature hydrogen utilization technology makes the development of efficient, low-cost and large-scale hydrogen production process an urgent demand in the hydrogen economy era. Among many hydrogen production processes, the reforming hydrogen production technology, which has easy access to raw materials, mature process and relatively high hydrogen production efficiency, is still facing great challenges, and the research on the modification of various supports and catalysts in its catalytic system is also crucial.
Table 1 summarizes the unique advantages and practical application effects of commonly used rare earth element modified reforming hydrogen production catalytic systems.
Table 1 Mechanism and application effect of different rare earth elements modified reforming catalysts for hydrogen production
Hydrogen energy is an important form of energy to replace conventional fossil carbon based energy. With the continuous expansion of hydrogen energy application, China has paid more and more attention to hydrogen energy. How to break the bottleneck of hydrogen energy industry and achieve new breakthroughs in hydrogen extraction technology is the focus of research, and rare earth modified reforming hydrogen production catalytic system will also become one of the research hotspots. In the future, the research work on rare earth modification can focus on the multi rare earth modified reforming hydrogen production catalyst system, the development of new composite rare earth catalyst carrier, and the optimization of rare earth modified catalyst reforming hydrogen production process, so as to obtain catalysts with good low-temperature activity, stable and efficient catalytic performance, expand the application field of rare earth metals, and realize rare earth The green and coordinated development of high-tech industrial clusters related to environment and new energy.
Fund project: Science and technology project of Jiangxi Provincial Department of Education (GJJ190109, GJJ109803); Key Project of Fuzhou Medical College of Nanchang University (CDFY-KJ1403)
Rare Metals and Cemented Carbides, Issue 2, 2022
Authors: Li Liangrong, Ding Yonghong, Deng Zhiwei