Brief description of activated alumina catalyst types in exhaust gas treatment
Date: 2019-12-16 10:40:09
There are many types of activated alumina catalysts in exhaust gas treatment, and the classification methods are also different. According to the big aspects, it can be divided into acid-base catalysts, metal catalysts, semiconductor catalysts and molecular sieve catalysts. Their common feature is that they can produce different degrees of chemical adsorption on reactants. Therefore, catalysis is inseparable from adsorption, and the general catalytic process starts with adsorption.
1. Acid-base catalysts referred to here are acids and bases in a broad sense, that is, Lewis acids and Lewis bases. Both of them can provide acid-base active adsorption centers for chemisorption of reactants, thereby promoting chemical reactions.
Such as activated clay, aluminum silicate, aluminum oxide and oxides of some metals, especially oxides of transition metals or their salts.
2. Metal catalyst Metal adsorption capacity depends on the molecular structure and adsorption conditions of the metal and gas. It was found through experiments that metal elements with d-electron empty orbits have different chemical adsorption capacities for some representative gases.
Except for Ca, Sr, and Ba, most of these metals are transition metals. They rely on electrons or unbound electrons that do not participate in the hybrid orbitals of the metal bond to form adsorption bonds with the adsorbent molecules, which catalyzes the interaction between them Reaction.
3. Semiconductor catalysts are mainly some semiconductor-type transition metal oxides. They are divided into n-type semiconductors and p-type semiconductors in order to provide quasi-free electrons or quasi-free holes.
The n-type semiconductor catalyst relies on its quasi-free electrons to form adsorption bonds with the reactants; the p-type semiconductor catalyst relies on its quasi-free holes to form adsorption bonds with the reactants. Due to the formation of adsorption bonds, the conductivity of the semiconductor is changed, which is one of the main factors affecting the activity of the catalyst.
In fact, the formation of adsorption bonds between gas molecules and semiconductor catalysts is a very complicated process. In the study of the catalytic mechanism of semiconductors, it was also found that the energy bands due to electronic transitions play an important role in the formation of adsorption bonds. effect. Therefore, it cannot be simply assumed that a reactant molecule capable of donating an electron can only form an adsorption bond with a p-type semiconductor catalyst.
4. Zeolite molecular sieve catalyst is widely used as an adsorbent in drying, purification, separation and other processes. It began to make its appearance in the application of catalysts and catalyst carriers in the 1960s.
Zeolite refers to the natural crystalline aluminosilicate, which has the same diameter micropores, so it is also called molecular sieve. At present, there are more than hundreds of species, and many important industrial catalytic reactions are inseparable from molecular sieve catalysts.
The catalysis of molecular sieve also relies on acidic centers on its surface to form adsorption bonds. However, it is more selective than acid-base catalysts because it can reject molecules with a larger pore size from entering the inner surface. At the same time, the acidity and alkalinity on the surface of the molecular sieve can also be adjusted artificially by means of ion exchange, which has better performance than ordinary acid-base catalysts.
In recent years, a kind of non-silicon-aluminum-based synthetic molecular sieve has been developed and has been widely used in the field of catalysis. It can be seen that molecular sieve has its special status and role in the field of catalysis.