Catalyst

Catalysts are divided into homogeneous and heterogeneous . Homogeneous catalyst is in the same phase with the reacting substances, heterogeneous - forms an independent phase, separated by an interface from the phase in which the reacting substances are located ^[1] . Typical homogeneous catalysts are acids and bases. Metals, their oxides and sulfides are used as heterogeneous catalysts.

Reactions of the same type can proceed with both homogeneous and heterogeneous catalysts. So, along with acid solutions, solid acidic Al ₂ O ₃ , TiO ₂ , ThO ₂ , aluminosilicates, and zeolites are used that have acidic properties. Heterogeneous catalysts with basic properties: CaO , BaO , MgO ^[1] .

Heterogeneous catalysts have, as a rule, a highly developed surface, for which they are distributed on an inert carrier ( silica gel , aluminum oxide , activated carbon , etc.).

For each type of reaction, only certain catalysts are effective. In addition to the already mentioned acid-base , there are oxidation-reduction catalysts; they are characterized by the presence of a transition metal or its compound ( Co ⁺³ , V ₂ O ₅ + MoO ₃ ). In this case, catalysis is carried out by changing the oxidation state of the transition metal.

Many reactions are carried out with the help of catalysts that act through the coordination of reagents at the atom or ion of the transition metal ( Ti , Rh , Ni ). Such catalysis is called coordination .

If the catalyst has chiral properties, then an optically active product is obtained from an optically inactive substrate.

In modern science and technology, systems of several catalysts are often used, each of which accelerates different stages of the reaction ^{[2] [3]} . The catalyst can also increase the speed of one of the stages of the catalytic cycle carried out by the other catalyst. Here there is a "catalysis catalysis", or catalysis of the second level ^[2] .

In biochemical reactions, the role of catalysts is played by enzymes .

Catalysts should be distinguished from initiators. For example, peroxides decompose into free radicals that can initiate radical chain reactions . The initiators are consumed during the reaction, so they cannot be considered as catalysts.

Inhibitors are sometimes mistakenly considered negative catalysts. But inhibitors, for example, chain radical reactions, react with free radicals and, unlike catalysts, do not persist. Other inhibitors (catalytic poisons) bind to the catalyst and deactivate it; there is suppression of catalysis, and not negative catalysis. Negative catalysis is basically impossible: it would provide a slower way for the reaction, but the reaction, naturally, would follow a faster, in this case, non-catalyzed, way.

The task of the automotive catalyst is to reduce the amount of harmful substances in the exhaust gases. Among them:

• carbon monoxide (CO) - poisonous gas without color and smell
• hydrocarbons , also known as volatile organic compounds - one of the main components of smog , is formed due to incomplete combustion of fuel
• Nitrogen oxides (NO and NO ₂ , which are often combined under the designation NO _x ) are also a component of smog , as well as acid rain , affect the human mucosa . ^[four]

To date, automotive catalysts are actively buying catalyst receiving points for recycling and processing throughout Russia. This is due to the deposition of the platinum group of metals in automotive catalysts.

• Inhibitor
• Enzyme inhibitor
• Promoter (catalysis)
• Activation energy
• Catalysis
• Supramolecular catalysis

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Stepan Mysyachenko. History of the Catalyst - Recycling of Automotive Catalysts (rus.) // Katoff : Blog. - Moscow: Catalystoff, 2019.