Catalysts in chemistry. What are they? What are their types?

What substances can be catalysts?

Chemical processes and reactions that are catalysed occur in the presence of certain specific substances called catalysts. Their primary task in systems is to reduce the activation energy, which directly increases the speed of the process. The choice of a catalyst is a key issue on which, for example, process efficiency depends. Specific chemical compounds or systems of the core-shell type are usually used as catalysts.

Basic features of catalysts and their functions in systems:

• The presence of a catalyst in a reaction is not included in the molecular equation of a chemical reaction because it does not react with the substrates or products.
• Once the reaction is over, the catalyst is recovered. Thus, the catalytic reaction can be described as cyclic.
• The catalyst should be easily separable from the obtained products of the chemical reaction.
• The catalyst does not affect the equilibrium state of the reaction in any way and therefore does not change its thermodynamics.
• Catalysts must have three basic characteristics: high activity, high selectivity, and stability over time.
• Catalysts must meet a number of basic assumptions about their constitution, including the right pore size, crystalline phase, crushing strength, degree of reduction, fluidisation properties, wearability, average chemical composition, effective surface area, grain size, and others.

Read also: catalysis.

Examples of catalysts

Metals

Metals are very good catalysts that are readily used in industry. Transition metals attracts particular interest because they can exist in two or more oxidation states, e.g., iron in iron(II) oxide or iron(III) oxide. These metals have incompletely filled d orbitals, which allow them to easily donate to and accept electrons from other molecules. In recent years, catalysts formed on the basis of metallic nanoparticles have become increasingly important due to their unique properties.

Platinum – A metal used, e.g., in functional group hydrogenation or dehydrogenation processes in organic synthesis. The substance is chemically inert and stable in oxidising environments and has a high moisture content. At temperatures above 450 ᵒC, a film of platinum dioxide forms on its surface. Platinum in compounds occurs in several oxidation states , but usually, as a catalyst, takes the values of II or IV. In addition to its use in chemical technology, platinum is also used in automotive catalytic converters. It has the ability to bind oxygen atoms to the toxic carbon(II) oxide in vehicle exhaust. This produces significantly less harmful carbon dioxide.

Palladium – Palladium catalysts are involved in a range of organic reactions, such as cyclisation, hydrogenation, oxidation, isomerisation, radical reactions, and others. They show a high tolerance of different functional groups and are often able to provide excellent stereoselectivity, helping to avoid the need for specific protecting groups. In addition, palladium catalysts are particularly effective, e.g., in selective hydrogenation, making it possible to obtain the desired products in a single reaction cycle.

Nickel – As a catalyst, nickel plays a key role in many organic transformations, such as oxidation, reduction, cyclisation, carbon-heteroatom bond formation, and others. It occurs in several oxidation states in compounds II, III and IV. Nickel is a relatively reactive element, while also exhibiting high chemical stability. This metal has a major advantage –it is cheaper than other transition metal catalysts, which is why it is often used as an alternative to palladium ones, e.g., in coupling reactions.

Gold – Some catalytic reactions are carried out in the presence of gold. Its catalytic activity is strongly dependent on the size and structure of the crystallites. Their effect also depends on the method of preparation. Gold catalysts are usually conglomerates of this element together with a suitable carrier, which, for example, supplies sufficient oxygen to further increase the activity of gold. Complexes of this metal are very good catalysts for carbon-carbon, carbon-nitrogen or carbon-oxygen bond-forming reactions, as they can easily activate double and triple bonds, e.g., in carbon chains. Examples of reactions catalysed by gold include the oxidation of carbon(II) oxide, the oxidation of alcohols and aldehydes, epoxidation reactions, hydrogenation of aldehydes and others.

Inorganic compounds

Inorganic compounds, in particular metal and non-metal oxides, selected salts, and acids are examples of inorganic catalysts. Typically, these substances are deposited on special carriers, which are porous materials (e.g., carbon, silica or alumina) that support their catalytic properties (the larger the surface area of the carrier, the greater the contact area between the reactants). An important aspect in selecting an inorganic compound as a catalyst is to be guided by the number of active centres it has. The presence of a large number of active centres, to which the reactants involved in the catalysed reaction bind, increases the yield of the reaction.

Vanadium(V) oxide – Catalysts with V₂O₅ as their main component are effective in almost all oxidation reactions. They play an important role in today’s chemical industry. One of the most important applications of these catalysts is the production of sulfuric acid. Vanadium(V) oxide catalyses the reaction of oxidation of sulfur(IV) oxide to sulfur(VI) oxide, which is then absorbed into sulfuric acid. In these processes, the vanadium catalyst is referred to as the so-called contact, as it is in a different phase to the other reactants. In industry, it is usually used in the form of a carrier with an active phase applied to its surface. Its main advantages include a low flash point, stability during the process or a high dust absorption coefficient. In addition to the production of sulfuric acid, vanadium(V) oxide is also used as a catalyst in rubber production, oil cracking and the synthesis of some high-molecular-weight compounds.

Aluminium chloride – The most common use of aluminium chloride as a catalyst in organic synthesis is the Friedel-Crafts alkylation reaction. AlCl₃ is in a different state of aggregation (solid phase) than the other reactants, so in this case it is heterogeneous catalysis. Its catalytic properties are mainly based on the fact that it is a so-called Lewis acid as regards its chemical structure and properties. Its main feature is its ability to accept electron pairs from Lewis bases. Aluminium chloride, as a catalyst and Lewis acid, combines with selected molecules or their fragments, after which transition complexes are formed and then break down to carbocations.

Sulfuric acid – Sulfuric acid exhibits catalytic properties even in small amounts for selected chemical reactions. Examples of such a transformation include the esterification reaction of acetic acid with ethanol or the nitration reaction of aromatic compounds. The acid then acts as a homogeneous catalyst and is therefore in the system, in the same phase as the other reactants. As a very strong acid, when introduced into the reaction environment, it releases hydrogen ions that propel the processes. Furthermore, an additional property of sulfuric acid is its hygroscopicity. The water molecules formed in the esterification process are bound by the acid and this shifts the equilibrium whereby more products are formed. Remember, however, that this does not result from catalysing the reaction, but it is only due to the shift in the equilibrium state.

Biocatalysts

Biocatalysts are chemical compounds that catalyse reactions that occur and arise within the human body. They are crucial elements in all biochemical transformations. They not only accelerate such transformations, but also exhibit a certain selectivity in catalysing selected reactions. By far the largest group of biocatalysts are enzymes, including non-protein catalysts, namely ribozymes. Their specific quality is the autocatalysis ability.

Enzymes – These are highly selective catalysts that significantly increase both the speed and selectivity of metabolic reactions. They are involved in all chemical reactions of the organism. As organic catalysts (or otherwise biocatalysts), enzymes are produced by cells. These can be simple proteins as well as complex proteins. They are characterised by the presence of two groups: the prosthetic group and the aoferment. Enzymes catalyse the reactions of oxidation and reduction of complex organic compounds, the transfer of functional groups, the hydrolysis of bonds of various types, the destruction of chemical bonds, the change of isomerisation of molecules or the formation of new covalent bonds. Their role in the human body cannot be overestimated. They take part in virtually all vital processes, both anabolic and catabolic. By catalysing selected reactions, they significantly influence the direction of metabolic pathways in the body.