Application of precious metals

A precious metal catalyst is a precious metal material that can change the speed of a chemical reaction without itself participating in the final product of the reaction. Almost all precious metals can be used as catalysts, but platinum, palladium, rhodium, silver, ruthenium, etc. are commonly used, among which platinum and rhodium are the most widely used. Their d-electron orbitals are not filled, the surface is easy to absorb reactants, and the strength is moderate, which is conducive to the formation of intermediate "active compounds". It has high catalytic activity. At the same time, it also has comprehensive excellent characteristics such as high temperature resistance, oxidation resistance, and corrosion resistance, making it the most important catalyst material. Precious metal catalysts are closely related to the country's economic construction, the environment, and public health. For example, the development of the chemical industry and the petroleum processing industry is based on catalytic reactions. More than 85% of the world's chemical industry is related to catalytic reactions. From 1930 to early 1980. 63 major products and 34 process innovations in the chemical industry in the United States were brought about by the chemical industry, of which more than 60% of the products and 90% of the processes were based on catalytic processes. Commercialization of a new catalytic process requires a large investment, which lasts for 10-15 years. Catalyst research has minimized this time lag. The public is increasingly concerned about the environmental pollution of chemicals and industrial emissions and the status of living space. Many modern low-cost and energy-saving environmental technologies are related to catalytic technologies. Vehicle exhaust emission control is an international strategic issue. This catalyst has been well developed and applied in the United States and some European countries. In addition, the biodegradation of organic waste, the treatment of soil, sewage and groundwater pollutants, and the purification of petroleum pollutants are all inseparable from precious metal catalysts. The three major strategies for reducing the environmental damage of chemicals in modern times are: minimizing waste, reducing exhaust emissions, and remediation measures. Precious metal catalysts will play a huge role in them. Brief history In 1831, Phillips of the United Kingdom proposed the contact method using platinum as a catalyst to produce sulfuric acid. By 1875, the method was industrialized, which was the earliest industrial application of precious metal catalysts. Since then, the industrial application of precious metal catalysts has emerged one after another. In 1913, platinum net catalysts were used for ammonia oxidation to nitric acid; in 1937, Ag/Al2O3 catalysts were used for ethylene oxidation to ethylene oxide; in 1949, Pt/Al2O3 catalysts were used for petroleum reforming to produce high-quality gasoline; in 1959, PdCl _ 2-CuCl _ 2 catalysts were used for ethylene oxidation to acetaldehyde; by the end of the 1960s, rhodium complex catalysts for methanol low-pressure carbonyl synthesis of acetic acid appeared. Since 1974, precious metal catalysts (mainly platinum, supplemented by palladium and rhodium) for automobile exhaust purification have been widely promoted and applied, and soon developed into the most used precious metal catalysts. Precious metal catalysts have been developed and applied for more than a hundred years (1875-1994), and their development momentum has been prosperous for a long time. New varieties, new preparation methods, and new application fields are constantly emerging, and the relevant basic theories are constantly being improved. With the continuous progress of science and technology, precious metal catalysts will continue to play an important role in some new fields. Of course, due to the scarcity of precious metal resources and high prices, people are also constantly researching and developing non-precious metal or low-content precious metal catalysts. Classification, composition and preparation methods of precious metals According to the catalytic reaction category, precious metal catalysts can be divided into two categories: homogeneous catalysis and heterogeneous catalysis. Catalysts for homogeneous catalysis are usually soluble compounds (salts or complexes), such as palladium chloride, rhodium chloride, palladium acetate, carbonyl rhodium, triphenylphosphonyl carbonyl rhodium, etc. The catalysts for heterogeneous catalysis are insoluble solids, and their main forms are metal mesh state and porous inorganic support metal state. The application range and dosage of metal mesh catalysts (such as platinum mesh and silver mesh) are limited. Most heterogeneous catalysts are carrier-supported noble metal types, such as Pt/A12O3, Pd/C, Ag/Al2O3, Rh/SiO2, Pt-Pd/Al2O3, etc. In the whole catalytic reaction process, the heterogeneous catalytic reaction accounts for 80% to 90%. According to the shape of the support, the supported catalysts can be divided into particulate, spherical, columnar and honeycomb. The composition of homogeneous catalysts is relatively simple, usually a certain compound. The composition of supported catalysts for heterogeneous catalysis is more complex, usually composed of active metal components, cocatalysts and supports. Cocatalysts are small amounts of substances added to catalysts. They are inactive or have little activity themselves, but can improve the performance of catalysts. Supporters are dispersants or supports of active components of catalysts. The main function of supports is to increase the effective surface of catalysts, provide a suitable pore structure, and ensure sufficient mechanical strength and thermal stability. Commonly used catalyst supports are Al2O3, SiO2, porous ceramics, activated carbon, etc. Different types of catalysts have different preparation methods. The preparation of catalysts for homogeneous catalysis is mainly to obtain the desired compounds and organic complexes by chemical methods. The preparation of unsupported catalysts for heterogeneous catalysis is first melted by fire to make alloys, and then brushed and woven. The preparation of support catalysts is more complicated. Generally, the support raw materials are processed into a certain shape (such as spherical, columnar, honeycomb) through batching, forming, firing and other processes, and then the active components of precious metals and cocatalysts are loaded by impregnation method, and finally reduced and calcined. Catalytic properties of precious metals Precious metal catalysts are used in many chemical reactions, especially platinum group metals have high catalytic activity. In the 16th Organic Reaction Catalyst Conference held in Atlanta in 1996, the papers on platinum group metal catalysts accounted for 70% _8J. Why do precious metal catalysts have such good catalytic effect? Precious metals are transition metal elements with empty d-band orbitals, which have properties such as electrophilicity, nucleophilicity, and redox ability to reactive molecules. Due to their diversity, variability, and high activity, they are activated or activated by ligands or other metal ions. Therefore, transition metals are catalysts.