Reaction characteristics of catalysts

Chemical reactions that take place in the presence of a catalyst are called catalytic reactions. In a chemical reaction, some of the original chemical bonds of the reacting molecules must dissociate and form new ones, which requires a certain activation energy. In some systems where chemical reactions are difficult to occur, the addition of a third substance (catalyst) that helps rearrange the chemical bonds of the reacting molecules acts to reduce the activation energy of the reaction, thus accelerating the chemical reaction and controlling the selectivity and steric regularity of the products.

A catalyst can only selectively accelerate a particular reaction, thus potentially allowing the chemical reaction to proceed in one of several thermodynamically possible directions. Catalysts in the same phase as the reactants are called Homogeneous Catalytic Reaction (HPCR), while those in different phases are called Heterogeneous Catalytic Reaction (HPCR). The reactions in which biocatalysts-enzymes participate are called Enzymic Catalytic Reaction. Most industrial reactions are catalytic reactions. Metabolism, nutrient and energy conversion in living organisms are enzymatic catalytic reaction processes.

Compared to uncatalyzed reactions, catalyzed reactions have lower activation energy at the same temperature. Catalysts can also produce unnatural reaction intermediates, such as osmium tetroxide catalyzing the dihydroxylation of olefins to produce osmiumate esters, or catalysts can cause the cleavage of reactants, such as monatomic hydrogen in hydrogen production.

There are four basic characteristics of catalytic reactions that can be derived from the definitions and are important for understanding the function of catalysts.

1. Catalysts can only accelerate reactions that are thermodynamically possible. The requirement to develop a new catalyst for a chemical reaction starts with a thermodynamic analysis of the reaction to see if it is a thermodynamically feasible reaction.

2. Catalysts can only accelerate the reaction to equilibrium, they cannot change the equilibrium position (equilibrium constant) of the reaction.

3. The catalyst is selective to the reaction, when the reaction may have more than one different direction, the catalyst only accelerates one of them, promoting the reaction rate and selectivity is unified.

4. The lifetime of catalyst. Catalyst can change the rate of chemical reaction, it does not enter the reaction itself, in the ideal situation catalyst does not change for the reaction. However, in the actual reaction process, the catalyst is subjected to heat and chemical action for a long time, and also some irreversible physicochemical changes will occur.

According to the definition and characterization of catalysts, there are three important catalyst indicators: activity, selectivity, and stability.

Classified by reaction mechanism, catalytic reactions can be divided into acid-base catalytic reactions and oxidation-reduction catalytic reactions; classified by reaction type, they can be divided into hydrogenation, dehydrogenation, oxidation, carbonylation, polymerization, halogenation, cracking, hydration, alkylation, isomerization, etc.