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Classification and mechanism of metal catalysts

Add time：2022-04-22 16:13 Views：
Metal catalyst is an important class of industrial catalysts. It mainly includes bulk catalysts, such as electrolytic silver catalysts, molten iron catalysts, platinum mesh catalysts, etc.; dispersed or supported metal catalysts, such as Pt-Re/-Al2O3 reforming catalysts, Ni/Al2O3 hydrogenation catalysts, etc.

Classification of common precious metal catalysts:
1.1 Heterogeneous catalysis
Heterogeneous catalytic reactions account for four-fifths of all catalytic reactions. Most of them are insoluble solids, and most heterogeneous catalysts are carrier-supported noble metal types, such as Pt-Rh/Al2O3, Pt-Pd/Al2O3, etc.

1.2 Homogeneous catalysis
Usually soluble compounds (salts or complexes), such as palladium chloride, rhodium chloride, triphenylphosphine carbonyl rhodium, etc.

1.3 Others
According to the shape of the carrier, the supported catalyst can be divided into spherical, particulate, honeycomb and columnar. Of course, it can also be classified according to the main active metals in the catalyst, commonly used are: platinum catalyst, palladium catalyst and silver catalyst.

The mechanism action of metal catalysts
1. Adsorption function of metal catalysts
Adsorption is an important link in the process of heterogeneous catalysis. Transition metals can adsorb O and other gases. The strong chemical adsorption capacity is related to the characteristics of transition metals, because transition metals have d empty orbitals or unpaired d electrons in the outermost electron layer. , It is easy to form chemical adsorption bonds with gas molecules, the adsorption activation energy is small, and it can adsorb most gases. The most important thing is that the d orbital is half or fully full, which is relatively stable and is not easy to form chemical adsorption bonds with gas molecules.

In the catalytic reaction, the metal catalyst first adsorbs one or more reactant molecules, so that the latter can react chemically on the metal surface. relative stability of the intermediates.

Under normal circumstances, molecules in a medium-strength chemical adsorption state will have the greatest catalytic activity, because too weak adsorption prevents the chemical bonds of reactant molecules from being relaxed or broken, and is not easy to participate in the reaction; while too strong adsorption will generate stable The intermediate compound covers the catalyst surface and is not conducive to desorption.

2. Interaction between metal-support
Two major categories of factors that induce metal-support interactions are electronic interactions and chemical interactions. For different metal catalyst systems, various factors have different effects on the metal-substrate interaction, and which factor dominates mainly depends on the properties of the metal catalyst itself and the reaction conditions.

Electronic interaction means that when the metal is in contact with the carrier, the energy is kept to a minimum and the solid potential is continuous, and the redistribution of charges will occur at the metal/carrier interface. The influence range is divided into local charge transfer and long-range charge transfer. The main factor responsible for the local charge transfer is the mutual polarization of electron orbitals caused by weak van der Waals forces.

The long-range charge transfer is due to the fact that when the metal is in contact with the oxide, the Fermi energy level at the interface between the two phases must be consistent, and the charge is transferred. At the interface of metal-carrier contact, the carrier has a large number of surface states, which have an important influence on the formation of the potential barrier for free electron transfer. Taking carrier semiconductors as an example, if the work functions of the metal and the carrier are different, they form On contact, charge transfer occurs.

Chemical interaction refers to the material transport process between the metal and the carrier. The material transport process includes the diffusion of metal on the surface of the carrier, the diffusion of metal or carrier atoms at the interface, and the occurrence of interfacial reactions (oxidation-reduction, alloying, carrier coating, interdiffusion).

Precious metal catalysts are highly valued for their excellent activity, selectivity, stability and synergistic effect, and are widely used in oxidation, reduction, hydrogenation, dehydrogenation, cracking, synthesis, isomerization, aromatization and other reactions. It plays a very important role in the fields of chemical industry, medicine, environmental protection and new energy.
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