Metal catalyst is an important industrial catalyst and a solid catalyst with metal as the main active component. Mainly precious metals and transition elements, such as iron, cobalt and nickel. Metal catalysts mainly include bulk catalysts (such as electrolytic silver catalyst, molten iron catalyst, platinum mesh catalyst, etc.). ); Dispersed or supported metal catalyst; Intermetallic catalyst; Alloy catalyst (such as copper-nickel alloy hydrogenation catalyst); Metal cluster catalyst.
2. What kinds of metal catalysts are there?
1, classified according to whether the active components of the catalyst are loaded on the carrier.
(1) unsupported metal catalyst
Refers to the unsupported metal catalyst, which can be divided into single metal and alloy according to its composition. It is usually used in the form of skeleton metal catalyst metal, wire mesh, metal powder, metal particles, metal fragments and metal evaporation film. Skeleton metal catalyst is to alloy the metal with catalytic activity with aluminum or silicon, and then dissolve aluminum or silicon with sodium hydroxide solution to form a metal skeleton. The most commonly used skeleton catalyst in industry is skeleton nickel, which was invented by M. Rainey of the United States in 1925, so it is also called raney nickel. Skeleton nickel catalyst is widely used in hydrogenation reaction. Other skeleton catalysts include skeleton cobalt, skeleton copper and skeleton iron. Typical mesh catalysts are platinum mesh and platinum-rhodium alloy mesh, which are used in the process of ammoniation and oxidation to produce nitration.
(2) Supported metal catalyst
A catalyst with a metal component supported on a carrier is used to improve the dispersibility and thermal stability of the metal component and make the catalyst have a suitable pore structure, shape and mechanical strength. Most supported metal catalysts are prepared by impregnating a metal salt solution on a carrier and then reducing it after precipitation conversion or thermal decomposition. One of the keys to prepare supported metal catalysts is to control the heat treatment and reduction conditions.
2. According to the fact that the active component of the catalyst is one or more metal elements.
(1) single metal catalyst
Monometal catalyst refers to a catalyst with only one metal component. For example, the platinum reforming catalyst was first used in industry in 1949, and the active component was a single metal platinum supported on η-alumina containing fluorine or chlorine. Uses: Rhodium is used as a catalyst to control the exhaust emission of automobile industry, and is also used for the synthesis, hydrogenation and hydroformylation of phosphorus complexes. Platinum catalyst is used to produce sulfuric acid by contact method, and platinum mesh catalyst is used to produce nitric acid by ammonia oxidation method.
(2) Polymetallic catalyst
The components in the catalyst are composed of two or more metals. Such as a platinum-rhenium bimetallic reforming catalyst supported on chlorine-containing gamma alumina. They have better performance than the above-mentioned reforming catalysts containing only platinum. In this kind of catalyst, various metals loaded on the carrier can form binary or multicomponent metal clusters, which greatly improves the effective dispersion of active components. The concept of metal clusters originated from complex catalysts. When applied to solid metal catalysts, it can be considered that several, dozens or more metal atoms are gathered on the metal surface.
Third, the advantages and disadvantages of metal catalysts
The advantages of noble metal catalysts are obvious, mainly including the following points:
1, wide application range: noble metal catalysts can be widely used in chemical pharmacy, petrochemical industry, hydrogenation, dehydrogenation, oxidation, reduction, isomerization, aromatization, cracking, synthesis and other reactions, as well as chemical industry, oil refining, petrochemical industry, medicine, environmental protection, new energy and other fields.
2. Good stability: the noble metal catalyst can stably maintain its catalytic characteristics in various environments, and also has comprehensive excellent characteristics such as high temperature resistance, oxidation resistance and corrosion resistance.
3. Good selectivity: For different application scenarios, noble metal catalysts usually only act on a certain chemical reaction and will not affect other reactions, thus reducing the interference to the whole chemical reaction.
Although precious metal catalysts have many advantages, their disadvantages are also caused by the use conditions and chemical properties in the actual application process, mainly including:
1, the high price of precious metals leads to high application cost: precious metals such as platinum, palladium and rhodium are usually used as precious metal catalysts. Because it is scarce, the price is high. For example, the market price of platinum is about 260 yuan/gram, the market price of palladium is about 560 yuan/gram, and the price of rhodium is about 4300 yuan/gram, which also leads to the high cost of using precious metal catalysts.
2. Improper use can easily lead to catalyst poisoning. Catalyst poisoning refers to the phenomenon that a small amount of impurities contained in the catalyst are strongly adsorbed (mostly chemically adsorbed) on the active center, or react with the active center to become other substances, resulting in catalyst deactivation. If used improperly, it will easily lead to deactivation of the catalyst.