Classification of metal catalysts

Unsupported and supported metal catalysts

According to whether the active components of the catalyst are supported on the carrier or not:

Unsupported metal catalyst

Refers to metal catalysts without a carrier, which can be divided into two types: single metal and alloy according to their composition. Usually used in the form of framework metal, metal wire mesh, metal powder, metal particles, metal chips and metal evaporation film. The framework metal catalyst is to make an alloy with catalytically active metal and aluminum or silicon, and then use sodium hydroxide solution to dissolve the aluminum or silicon to form a metal framework. The most commonly used skeletal catalyst in industry is skeletal nickel, which was invented by M. Raney of the United States in 1925, so it is also called Raney nickel. Skeletal nickel catalysts are widely used in hydrogenation reactions. Other framework catalysts include framework cobalt, framework copper and framework iron. Typical metal wire mesh catalysts are platinum mesh (see picture) and platinum-rhodium alloy mesh, which are used in the process of ammoxidation to produce nitric acid.

Supported metal catalyst

The catalyst in which the metal component is supported on the carrier is used to improve the dispersion and thermal stability of the metal component, so that the catalyst has a suitable pore structure, shape and mechanical strength. Most supported metal catalysts are prepared by impregnating the metal salt solution on the carrier, and reducing it after precipitation transformation or thermal decomposition. One of the keys to the preparation of supported metal catalysts is to control the heat treatment and reduction conditions.

Single metal and multi-metal catalysts

According to the catalyst active component is one or more metal elements classification:

Single metal catalyst

Refers to a catalyst with only one metal component. For example, in the platinum reforming catalyst first used in industry in 1949, the active component is a single metal platinum supported on η-alumina containing fluorine or chlorine.

Multimetallic catalyst

The components in the catalyst are composed of two or more metals. For example, platinum-rhenium and other double (multiple) metal reforming catalysts supported on chlorine-containing γ-alumina. They have better performance than the aforementioned reforming catalysts containing only platinum. In this type of catalyst, a variety of metals supported on the carrier can form binary or multi-element metal clusters, so that the effective dispersion of the active components is greatly improved. improve. The concept of metal cluster compounds was first derived from complex catalysts. When applied to solid metal catalysts, it can be considered that there are several, dozens or more metal atoms clustered on the metal surface. Since the 1970s, based on this concept, a model of the active center of metal clusters has been proposed to explain the mechanism of some reactions. In supported and unsupported multimetal catalysts, if an alloy is formed between the metal components, it is called an alloy catalyst. The most researched and applied are binary alloy catalysts, such as copper-nickel, copper-palladium, palladium-silver, palladium-gold, platinum-gold, platinum-copper, platinum-rhodium, etc. The activity of the catalyst can be adjusted by adjusting the composition of the alloy. Some alloy catalysts have obvious differences in the composition of the surface and the bulk phase. For example, after adding a small amount of copper to the nickel catalyst, the original surface structure of the nickel catalyst is changed due to the enrichment of copper on the surface, thereby hydrogenating ethane. The lysis activity decreases rapidly. Alloy catalysts have applications in hydrogenation, dehydrogenation, oxidation, etc.