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Research Progress on Carbon Monoxide Shift Catalysts by Shi Yushan [Abstract] This article presents the research status and development trends of carbon monoxide shift catalysts. It focuses on introducing the research progress of high-temperature shift catalysts, low-temperature shift catalysts, and new types of catalysts. It discusses the industrial application prospects of the aforementioned catalysts. [keywords] transformation ； High temperature ； Low temperature ； catalyst ； Over 90% of modern chemical and refining processes are catalytic processes. Driven by strong economic pressures, catalytic technology has developed rapidly, being primarily applied in areas such as chemical production, petroleum refining, and pollution control. It not only improves the utilization rate of raw materials but also reduces the total investment and operating costs of facilities. China is rich in coal resources, and currently most large-scale coal chemical projects in the country use coal as a raw material for gas production, shift reaction, and synthesis processes; as a result, there is a very high demand for shift catalysts, and this demand continues to grow. However, coal has a high sulfur content; therefore, it is very important to develop new sulfur-resistant shift catalysts with high activity and stability. 1 High-temperature shift catalyst 1.1 Traditional high-temperature shift catalysts The traditional high-temperature shift catalysts are iron-chromium-based catalysts, with Cr2O3 serving as the main structural additive and Fe3O4, which has a spinel structure, acting as the active phase. Ferrochromium catalysts exhibit high catalytic activity at 350–450°C, possess high mechanical strength, and have a certain degree of resistance to poisons and heat. However, iron-chromium catalysts also have drawbacks: (1) Traditional high-temperature shift catalysts are not suitable for low steam-to-gas ratios ； (2) Chromium oxide (CrO3) is costly and causes severe environmental pollution. 1.2 High-temperature shift catalysts for low steam-to-gas ratios: High-temperature shift catalysts suitable for shift processes with low steam-to-gas ratios have been developed; one type of such catalyst is a copper (manganese) iron-based improved high-temperature shift catalyst ； Another category is copper-based high-temperature shift catalysts that do not contain iron or chromium. The SK-201 iron-based high-temperature shift catalyst Cu-Fe-Cr developed by Topsoe in Denmark enhances the activity of the shift reaction while suppressing side reactions; copper salts help prevent the dissociation of CO, avoid carbon deposition on the catalyst surface, and reduce the F-T and dissociation reactions under conditions of low gas-to-vapor ratios. ICI is researching Fe-Cr-based improved catalysts with copper additives. Both catalysts are suitable for operation at low gas-to-vapor ratios, enabling the amount of F-T by-products to be reduced to less than 10% of the original amount produced. BASF’s improved ferrochrome high-activity catalyst K6-11 features high selectivity and can be used in operations with a low gas-to-vapor ratio. This catalyst has high activity and produces few synthesis by-products. The Siid-Chemie Group has developed the C12-4/G-3C high-shift catalyst with copper as a co-catalyst, based on the C12-3/G-3 catalyst; it operates at an inlet temperature of 320°C and achieves a high CO conversion rate. At a vapor/vapor ratio of 0.4–0.6 and a CO/CO2 ratio of around 2, the F-T reaction can be suppressed by 100%. 2 Low-temperature shift catalysts Copper-based CO low-temperature shift catalysts can be divided into two main categories: the Cu-Zn-Al series and the Cu-Zn-Cr series. Due to the low cost of aluminum-based catalysts and the absence of Cr contamination during production and use, aluminum-based low-activation catalysts are currently used in most cases. Copper-based catalysts exhibit high activity and selectivity, and can achieve high conversion rates at relatively low temperatures (180–240°C). However, they have poor thermal stability, are highly sensitive to sulfides and chlorides, and can become deactivated due to poisoning. Therefore, research and development companies have been constantly working to improve the performance of their low-volatility catalysts. 2.1 Improvement of low-temperature shift catalysts: To enhance the activity of low-temperature shift catalysts, it is necessary not only to determine the optimal ratio of CuO to the carrier (with an appropriate copper content of 30%–40%), but also to improve their preparation method in order to achieve the best microcrystalline size. For low-temperature shift catalysts with the same components. There is a significant difference in catalyst performance between those prepared by physical mixing and those prepared by coprecipitation. The low-temperature shift catalyst prepared by the coprecipitation method yields copper crystals of 10-6 cm in size; the copper grains are evenly separated from the ZnO and Al2O3 grains, which increases the available specific surface area of copper. This in turn enhances the catalyst’s activity and prevents loss of activity due to thermal sintering of the copper grains. Most of the sulfur in the feed gas exists in the form of H2S, and it can be preferentially adsorbed by the ZnO component in the catalyst and converted into ZnS. When the free ZnO in the catalyst is exhausted, the active component Cu undergoes thermal sintering and its activity declines rapidly. Therefore, during the catalyst preparation process, altering the pore structure and size of the catalyst as well as increasing the content of free ZnO can all enhance the catalyst’s resistance to poisoning. In the low-temperature shift process, as catalyst activity increases, the acidity of the catalyst carrier rises, the H2O/C ratio decreases, the residence time lengthens, and the reaction temperature and pressure increase, the tendency to form by-products grows. A way to address this increased side reaction is to modify the catalyst preparation process by adding alkali metal additives to reduce the number of acidic centers, which can improve the selectivity of copper-based low-temperature shift catalysts. 2.2 New directions in the development of low-temperature shift catalysts: Low-temperature shift catalysts have new applications in the treatment of CO components in hydrogen-rich gases, as well as in the development of new types of water-gas shift catalysts. Copper possesses good low-temperature activity and is inexpensive and readily available; compared to catalysts loaded with precious metals, it reduces the cost of the catalysts, facilitating large-scale application. Domestic studies have found that as the copper loading increases from 5% to 40%, the catalyst activity remains unchanged, indicating that the addition of a small amount of Cu can alter the CO conversion activity of CeO2. 2.3 Co-Mo series wide-temperature sulfur-resistant shift catalysts: The Co-Mo series wide-temperature sulfur-resistant shift catalysts possess a wide operating temperature range (160–500°C), have no upper limit regarding sulfur resistance, and feature high strength. The main industrial sulfur-resistant shift catalysts abroad are K8-11 developed by the German company BASF, which uses magnesium-aluminum spinel as a carrier and features high activity as well as strong resistance to poisons. The SSK catalyst from the Danish company TOPSΦE contains the promoter K2CO3, which grants it high activity at low temperatures and makes it insensitive to poisons; however, potassium tends to leach away. 3 New types of CO conversion catalysts. In recent years, research into replacing internal combustion engines with proton exchange membrane fuel cells has been on the rise. The thermodynamic efficiency of exchange membrane fuel cells is much higher than that of heat engines; they produce fewer by-products, only water and heat, and are not constrained by the Carnot cycle. Only by removing CO, which is toxic to the counter electrode, can a hydrogen-rich gas source meeting the requirements of fuel cells be obtained. The CO conversion reaction has attracted considerable attention due to its ability to reduce CO in the reformed gas, and many new types of highly active conversion catalysts have become a focus of research. 3.1 Catalysts loaded with precious metals Many transformation catalysts in which precious metals (Pt, Rh, Ru, Pd, Au) are deposited on reducible supports (CeO2, TiO2, ZrO2) have been reported in the literature. These catalysts are expected to become conversion catalysts for fuel cells. Because they exhibit relatively high activity at temperatures ranging from 250 to 400°C, and also possess a certain degree of antioxidant properties. Factors that have a significant impact on the activity and stability of catalysts loaded with precious metals include many aspects such as the metal, the metal support, the methods for preparing the metal and the support, and the types of metal precursors. The impregnation method is a commonly used approach for preparing traditional gold catalysts; however, it fails to exhibit excellent catalytic activity due to the inability to produce highly active gold ultramicroparticles. The method for obtaining highly dispersed gold ultraparticles with a uniform distribution is called the deposition-precipitation method. These small particles can easily adsorb simple molecules. Studies using MS and FTIR techniques on the reaction mechanisms of Au/α-Fe2O3 and Au/TiO2 have shown that at room temperature, H2 dissociates and adsorbs onto the Au sites of these two catalysts as hydrogen atoms. Ndreeva used this method to prepare gold catalysts. The catalysts prepared using RuCl3•nH2O with Fe2O3 as a carrier exhibit relatively high activity. However, the presence of chloride ions in this catalyst may affect its performance. Therefore, Basin ska et al. used Ru3(CO)12 to prepare the catalyst. The results show that, due to the absence of Cl- ions, the catalyst prepared from Ru3(CO)12 exhibits good activity, and the ruthenium metal formed by its thermal decomposition has a high degree of dispersion. 3.2 As an important component of three-way catalysts, ceria-based conversion catalysts possess a high oxygen storage capacity; they can also maintain highly dispersed metal particles. Moreover, the metals loaded on these catalysts enhance the formation of oxygen vacancies in CeO2 as well as its degree of reduction, thereby facilitating the CO conversion reaction. Furthermore, certain dopants such as cations in low-valent states (Ga3+, Sm3+, etc.) can cause charge redistribution and lead to lattice distortion in cerium oxide, as well as enhance the formation of oxygen vacancies. 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