Precious metal catalyst
Source: | Author:weitongch | Published time: 2019-02-15 | 554 Views | Share:
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
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 adsorb reactants, and the strength is moderate, which is conducive to the formation of intermediate "active compounds", has high catalytic activity, and also has comprehensive excellent characteristics such as high temperature resistance, oxidation resistance, and corrosion resistance. , Become the most important catalyst material.

Main performance index
(1) Activity. It is a standard to measure the efficiency of the catalyst. In industry, it is usually expressed by the number of products per unit time (or weight) of the catalyst under certain conditions per unit time.
(2) Selectivity. Refers to the specificity of the role of the catalyst, that is, under certain conditions, a certain catalyst only accelerates a certain chemical reaction. The selectivity is usually expressed as a percentage of the ratio of the moles of the expected product obtained after the reaction to the moles of the raw materials participating in the reaction.
(3) Stability. Refers to the ability of the catalyst to maintain its activity and selectivity during use, usually expressed in terms of service life. The good performance of the catalyst depends not only on the inherent characteristics of the active metal (atomic electronic structure, etc.), but also on its crystal structure, particle size, specific surface area, pore structure and dispersion state. In addition, co-catalysts and supports also have an important effect on the performance of the catalyst.

classification
According to the type of catalytic reaction, 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, rhodium carbonyl, rhodium triphenylphosphonium carbonyl, etc. The catalyst for heterogeneous catalysis is an insoluble solid, and its main forms are a wire mesh state and a porous inorganic carrier supported metal state. Metal wire mesh catalysts (such as platinum mesh and silver mesh) have limited application scope and dosage. Most heterogeneous catalysts are supported by precious metals, such as Pt/A12O3, Pd/C, Ag/Al2O3, Rh/SiO2, Pt-Pd/Al2O3, Pt-Rh/Al2O3, etc. In all catalytic reactions, heterogeneous catalytic reactions account for 80% to 90%. According to the shape of the carrier, the supported catalyst can be divided into particulate, spherical, columnar and honeycomb. According to the main active metal classification of catalysts, commonly used are: silver catalyst, platinum catalyst, palladium catalyst and rhodium catalyst.

application
Noble metal catalysts are highly valued for their excellent activity, selectivity, and stability. They are widely used in reactions such as hydrogenation, dehydrogenation, oxidation, reduction, isomerization, aromatization, cracking, and synthesis. , Petrochemistry, medicine, environmental protection and new energy play a very important role.

composition
The composition of a homogeneous catalyst is relatively simple, usually a certain compound. The composition of the supported catalyst for heterogeneous catalysis is relatively complicated, and usually consists of an active metal component, a cocatalyst and a carrier. The co-catalyst is a small amount of material added to the catalyst. It is inactive or has little activity itself, but can improve the performance of the catalyst. The carrier is a dispersant or support for the active component of the catalyst. The main function of the carrier is to increase the effective surface of the catalyst, 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.

Preparation
Different types of catalysts have different preparation methods. The preparation of catalysts for homogeneous catalysis is mainly to obtain the required compounds and organic complexes by chemical methods. The preparation of unsupported catalysts for heterogeneous catalysis (such as Pt-Rh mesh) is first made by fire smelting into alloys, and then drawn and woven mesh. The preparation of the carrier catalyst is relatively complicated. Generally, the carrier material is processed into a certain shape (such as spherical, columnar, and honeycomb) through batching, forming, and firing processes, and then the noble metal active component and the cocatalyst are loaded by the impregnation method. After reduction roasting.