Catalyst– a substance that changes the rate of a chemical reaction but is not consumed. Catalysts are either accelerating or decelerating.
Catalysis– the phenomenon of a change in the reaction rate in the presence of catalysts.
catalytic reactions– reactions that take place with the participation of catalysts.
If the catalyst is one of the reaction products, then the reaction is called autocatalytic, and the phenomenon itself autocatalysis.
Inhibitor– a catalyst that slows down the reaction.
An example of positive catalysts is water in the interaction of aluminum powder with iodine.
Enzymes– biological catalysts of protein nature.
Enzymes are present in all living cells. It is customary to divide enzymes into simple and complex, or one-component or two-component. Simple enzymes consist only of protein, complex enzymes consist of protein and a non-protein part, which is called coenzyme.
Enzymes are characterized by high catalytic activity and selectivity. In terms of catalytic activity, they are significantly superior to inorganic catalysts. For example, 1 mole of catalase at 0 degrees decomposes 200,000 moles of H 2 O 2 in one second, and 1 mole of platinum at 20 degrees decomposes from 10 to 80 moles of hydrogen peroxide in one second.
Such accelerations of the reaction are due to the fact that enzymes sharply reduce energy barriers in the reaction path. For example, the activation energy for the decomposition reaction of H 2 O 2 under the action of an iron (II) ion and catalase molecules, respectively, is 42 and 7.1 kJ / mol; for the hydrolysis of urea by acid and urease, 103 and 28 kJ/mol, respectively.
Enzymes are very specific compared to inorganic catalysts. For example, amylase, contained in saliva, easily and quickly breaks down starch, but does not catalyze the process of sugar breakdown. Urease is extremely effective in catalyzing the hydrolysis of urea, but has no effect on its derivatives. This feature of enzymes allows living organisms, having an appropriate set of enzymes, to actively respond to external influences. For example, it has been noticed that in stressful situations our body shows amazing capabilities. A fact is described when a weak woman lifted a car by the bumper and held it while people came to the rescue to free a child who fell under it; a person pursued by an angry animal easily overcomes obstacles that are insurmountable for him in his usual state; in important competitions, athletes lose several kilograms in weight during the period of performance.
All that has been said about the remarkable properties of enzymes is explained by the fact that the selectivity of action (selectivity) and activity are interrelated: the higher the selectivity, the higher its activity. Enzymes have a unique selectivity, and therefore their activity is the highest.
The rates of chemical reactions can increase dramatically in the presence of various substances that are not reactants and are not part of the reaction products. This remarkable phenomenon is called catalysis(from the Greek "katalysis" - destruction). A substance that increases the rate of a reaction in a mixture is called catalyst. Its amount before and after the reaction remains unchanged. Catalysts do not represent any special class of substances. In various reactions, metals, oxides, acids, salts, and complex compounds can exhibit a catalytic effect. Chemical reactions in living cells proceed under the control of catalytic proteins called enzymes. Catalysis should be considered as a true chemical factor in increasing the rates of chemical reactions, since the catalyst is directly involved in the reaction. Catalysis is often a more powerful and less risky way to speed up a reaction than raising the temperature. This is clearly manifested in the example of chemical reactions in living organisms. Reactions, such as the hydrolysis of proteins, which in laboratories have to be carried out with prolonged heating to the boiling point, during digestion proceed without heating at body temperature.
For the first time, the phenomenon of catalysis was observed by the French chemist L. J. Tenard (1777-1857) in 1818. He discovered that oxides of certain metals, when hydrogen peroxide is added to a solution, cause its decomposition. Such an experience is easy to reproduce by adding potassium permanganate crystals to a 3% hydrogen peroxide solution. Salt KMp0 4 turns into Mn0 2, and oxygen is quickly released from the solution under the action of oxide:
The direct effect of the catalyst on the reaction rate is associated with a decrease in the activation energy. At normal temperature decrease? and by 20 kJ/mol increases the rate constant by approximately 3000 times. downgrade E L may be much stronger. However, the decrease in the activation energy is an external manifestation of the action of the catalyst. The reaction is characterized by a certain value E. v which can only change if the reaction itself changes. Giving the same products, the reaction with the participation of the added substance proceeds along a different path, through different stages and with a different activation energy. If the activation energy is lower on this new path and the reaction is correspondingly faster, then we say that this substance is a catalyst.
The catalyst interacts with one of the reactants, forming some intermediate compound. At one of the subsequent stages of the reaction, the catalyst is regenerated - it leaves the reaction in its original form. Reagents, participating in a catalytic reaction, continue to interact with each other and along a slow path without the participation of a catalyst. Therefore, catalytic reactions belong to a variety of complex reactions called series-parallel. On fig. 11.8 shows the dependence of the rate constant on the concentration of the catalyst. The dependence graph does not pass through zero, since in the absence of a catalyst, the reaction does not stop.
Rice. 11.8.
observable constant k expressed as a sum k u+ & k c(k)
Example 11.5. At a temperature of -500 °C, the oxidation reaction of sulfur oxide (IV)
which is one of the stages of industrial production of sulfuric acid, is very slow. A further increase in temperature is unacceptable, since the equilibrium shifts to the left (exothermic reaction) and the product yield drops too much. But this reaction is accelerated by various catalysts, one of which may be nitric oxide (II). First, the catalyst reacts with oxygen: 
and then transfers an oxygen atom to sulfur oxide (IV):
Thus, the final product of the reaction is formed and the catalyst is regenerated. For the reaction, the possibility of flowing along a new path was opened, in which the rate constants increased significantly:
The diagram below shows both pathways of the S0 2 oxidation process. In the absence of a catalyst, the reaction proceeds only along the slow path, and in the presence of a catalyst, along both.
There are two types of catalysis - homogeneous And heterogeneous. In the first case, the catalyst and reagents form a homogeneous system in the form of a gas mixture or solution. An example of sulfur oxide oxidation is homogeneous catalysis. The rate of a homogeneous catalytic reaction depends on both the concentrations of the reactants and the concentration of the catalyst.
In heterogeneous catalysis, the catalyst is a solid in pure form or supported on carrier. For example, platinum as a catalyst can be fixed on asbestos, alumina, etc. Reagent molecules are adsorbed (absorbed) from a gas or solution at specific points on the catalyst surface - active centers and are activated at the same time. After the chemical transformation, the resulting product molecules are desorbed from the catalyst surface. Acts of particle transformation are repeated at active centers. Among other factors, the rate of a heterogeneous catalytic reaction depends on the surface area of the catalytic material.
Heterogeneous catalysis is especially widely used in industry. This is due to the ease of carrying out a continuous catalytic process with the passage of a mixture of reagents through a contact apparatus with a catalyst.
Catalysts act selectively, accelerating a very specific type of reaction or even a single reaction without affecting others. This makes it possible to use catalysts not only to speed up reactions, but also to purposefully convert starting materials into desired products. Methane and water at 450 ° C on the Fe 2 0 3 catalyst are converted into carbon dioxide and hydrogen:
The same substances at 850 °C react on the nickel surface to form carbon monoxide (II) and hydrogen:
Catalysis belongs to those areas of chemistry in which it is not yet possible to make accurate theoretical predictions. All industrial catalysts for the processing of petroleum products, natural gas, ammonia production and many others have been developed on the basis of laborious and lengthy experimental studies.
The ability to control the speed of chemical processes is of inestimable importance in human economic activity. In the industrial production of chemical products, it is usually necessary to increase the rates of technological chemical processes, and in the storage of products, it is required to reduce the rate of decomposition or exposure to oxygen, water, etc. Known substances that can slow down chemical reactions. They're called inhibitors, or negative catalysts. Inhibitors are fundamentally different from real catalysts in that they react with active species (free radicals) that, for one reason or another, arise in a substance or its environment and cause valuable decomposition and oxidation reactions. Inhibitors are gradually consumed, ending their protective action. The most important type of inhibitors are antioxidants, which protect various materials from the effects of oxygen.
It should also be reminded of what cannot be achieved with the help of catalysts. They are capable of accelerating only spontaneous reactions. If the reaction does not proceed spontaneously, then the catalyst will not be able to accelerate it. For example, no catalyst can cause water to decompose into hydrogen and oxygen. This process can be carried out only by electrolysis, while spending electrical work.
Catalysts can also activate unwanted processes. In recent decades, there has been a gradual destruction of the ozone layer of the atmosphere at an altitude of 20-25 km. It is assumed that some substances are involved in the decay of ozone, for example, halogenated hydrocarbons emitted into the atmosphere by industrial enterprises, as well as used for domestic purposes.
Catalysis has found wide application in the chemical industry, in particular, in the technology of inorganic substances. Catalysis- the excitation of chemical reactions or a change in their speed under the influence of substances - catalysts that repeatedly enter into chemical interaction with the participants in the reaction and recover their chemical composition after each cycle of interaction. There are substances that reduce the rate of a reaction, which are called inhibitors or negative catalysts. Catalysts do not change the state of equilibrium in the system, but only facilitate its achievement. A catalyst can simultaneously accelerate both the forward and reverse reactions, but the equilibrium constant remains constant. In other words, the catalyst cannot change the equilibrium of thermodynamically unfavorable reversible reactions in which the equilibrium is shifted towards the starting materials.
The essence of the accelerating action of catalysts is to lower the activation energy E a of a chemical reaction by changing the reaction path in the presence of a catalyst. For the reaction of converting A to B, the reaction path can be represented as follows:
A + K AK
VK V + K
As can be seen from Figure 1, the second stage of the mechanism is limiting, since it has the highest activation energy Еcat, but significantly lower than for the non-catalytic process Еnecat. The decrease in the activation energy occurs due to the compensation of the energy of breaking the bonds of the reacting molecules with the energy of the formation of new bonds with the catalyst. A quantitative characteristic of the decrease in the activation energy, and hence the efficiency of the catalyst, can be the degree of compensation for the energy of broken bonds Di:
\u003d (Di - E cat) / Di (1)
The lower the activation energy of the catalytic process, the higher the degree of compensation.
Simultaneously with a decrease in the activation energy, in many cases there is a decrease in the order of the reaction. The decrease in the reaction order is explained by the fact that in the presence of a catalyst, the reactions proceed through several elementary stages, the order of which may be less than the order of non-catalytic reactions.
Types of catalysis
According to the phase state of the reactants and the catalyst, catalytic processes are divided into homogeneous and heterogeneous. In homogeneous catalysis, the catalyst and the reactants are in the same phase (gas or liquid), while in heterogeneous catalysis they are in different phases. Often, the reacting system of a heterogeneous catalytic process consists of three phases in various combinations, for example, the reactants can be in the gas and liquid phases, and the catalyst can be in the solid.
Enzymatic (biological) catalytic processes, common in nature and used in industry for the production of feed proteins, organic acids, alcohols, as well as in the treatment of wastewater, are distinguished into a special group.
According to the types of reactions, catalysis is divided into redox and acid-base. In reactions proceeding according to the redox mechanism, the intermediate interaction with the catalyst is accompanied by the homolytic breaking of two-electron bonds in the reactants and the formation of bonds with the catalyst at the site of the unpaired electrons of the latter. Typical catalysts for redox interactions are metals or oxides of variable valence.
Acid-base catalytic reactions proceed as a result of an intermediate protolytic interaction of reactants with a catalyst or interaction involving a lone pair of electrons (heterolytic) catalysis. Heterolytic catalysis proceeds with such a rupture of the covalent bond, in which, in contrast to homolytic reactions, the electron pair that makes the bond remains wholly or partially at one of the atoms or a group of atoms. The catalytic activity depends on the ease with which a proton is transferred to a reactant (acid catalysis) or a proton is abstracted from a reactant (basic catalysis) in the first act of catalysis. Catalytic reactions of hydrolysis, hydration and dehydration, polymerization, polycondensation, alkylation, isomerization, etc. proceed according to the acid-base mechanism. Compounds of boron, fluorine, silicon, aluminum, sulfur and other elements with acidic properties, or compounds of the elements of the first and the second group of the periodic system, with basic properties. Ethylene hydration according to the acid-base mechanism with the participation of the HA acid catalyst is carried out as follows: at the first stage, the catalyst serves as a proton donor
CH 2 \u003d CH 2 + HA CH 3 -CH 2 + + A -
the second stage is the actual hydration
CH 3 -CH 2 + + HOH CH 3 CH 2 OH + H +
third stage - catalyst regeneration
H + + A - ON.
Redox and acid-base reactions can be considered according to the radical mechanism, according to which a strong molecule–catalyst lattice bond formed during chemisorption facilitates the dissociation of the reacting molecules into radicals. In heterogeneous catalysis, free radicals migrate over the catalyst surface to form neutral product molecules, which are desorbed.
There is also photocatalysis, when the process is initiated by the action of light.
Since heterogeneous catalysis on solid catalysts is the most common in inorganic chemistry, we will dwell on it in more detail. The process can be divided into several stages:
1) external diffusion of reactants from the core of the flow to the surface of the catalyst; in industrial devices, turbulent (convective) diffusion usually prevails over molecular diffusion;
2) internal diffusion in the pores of the catalyst grain, depending on the size of the pores of the catalyst and the size of the molecules of the reagents, diffusion can occur according to the molecular mechanism or according to the Knudsen mechanism (with constrained movement);
3) activated (chemical) adsorption of one or more reactants on the catalyst surface with the formation of a surface chemical compound;
4) rearrangement of atoms with the formation of a surface product-catalyst complex;
5) desorption of the catalysis product and regeneration of the active center of the catalyst; for a number of catalysts, not the entire surface of the catalyst is active, but separate areas - active centers;
6) diffusion of the product in the pores of the catalyst;
7) diffusion of the product from the surface of the catalyst grain into the gas flow.
The overall rate of a heterogeneous catalytic process is determined by the rates of individual stages and is limited by the slowest of them. Speaking about the stage that limits the process, it is assumed that the remaining stages proceed so quickly that in each of them an equilibrium is practically reached. The speeds of the individual stages are determined by the parameters of the technological process. According to the mechanism of the process as a whole, including the catalytic reaction itself and the diffusion stages of substance transfer, processes occurring in the kinetic, external diffusion and intradiffusion regions are distinguished. The process speed is generally determined by the expression:
d/d = k c (2)
where c is the driving force of the process, equal to the product of the effective concentrations of the reactants, for a process occurring in the gas phase, the driving force is expressed in partial pressures of the reactants р; k is the rate constant.
In general, the rate constant depends on many factors:
k \u003d f (k 1, k 2, k pob, ..... D and, D and /, D p, ....) (3)
where k 1 , k 2 , k pob - rate constants of direct, reverse and side reactions; D and, D and / , D p - diffusion coefficients of the initial substances and the product, which determine the value of k in the external or internal diffusion areas of the process.
IN kinetic area k does not depend on diffusion coefficients. The general kinetic equation for the rate of the gas catalytic process, taking into account the effect on the rate of the main parameters of the technological regime:
u = kvpP n 0 = k 0 e -Ea/RT vpP n 0 (4)
where v is the gas flow rate, p is the driving force of the process at Р0.1 MPa (1 atm), P is the ratio of the working pressure to normal atmospheric pressure, that is, a dimensionless value, 0 is the conversion factor to normal pressure and temperature, n is the order of the reaction.
The mechanism of chemical steps is determined by the nature of the reactants and the catalyst. The process can be limited by chemisorption of one of the reactants by the catalyst surface or by desorption of the reaction products. The reaction rate can be controlled by the formation of a charged activated complex. In these cases, the charging of the catalyst surface under the action of some factors has a significant effect on the course of the reaction. In the kinetic region, processes mainly take place on low-activity fine-grained catalysts with large pores at a turbulent flow of reactants, as well as at low temperatures close to the ignition temperatures of the catalyst. For reactions in liquids, the transition to the kinetic region can also occur with an increase in temperature due to a decrease in the viscosity of the liquid and, consequently, an acceleration of diffusion. With an increase in temperature, the degree of association, solvation, and hydration of reagent molecules in solutions decreases, which leads to an increase in diffusion coefficients and, accordingly, a transition from the diffusion region to the kinetic region. For reactions whose overall order is higher than unity, a transition from the diffusion region to the kinetic region is characteristic with a significant decrease in the concentration of the initial reagents. The transition of the process from the kinetic region to the external diffusion region can occur with a decrease in the flow rate, an increase in concentration, an increase in temperature.
In external diffusion region First of all, processes proceed on highly active catalysts, which ensure a fast reaction and a sufficient yield of the product during the contact time of the reagents with the catalysts, measured in fractions of a second. The very fast reaction takes place almost entirely on the outer surface of the catalyst. In this case, it is impractical to use porous grains with a highly developed inner surface, and it is necessary to strive to develop the outer surface of the catalyst. So, in the oxidation of ammonia on platinum, the latter is used in the form of the thinnest grids containing thousands of weaves of platinum wire. The most effective means of accelerating the processes occurring in the region of external diffusion is the mixing of the reagents, which is often achieved by increasing the linear velocity of the reagents. Strong flow turbulence leads to the transition of the process from the external diffusion region to the intradiffusion region (for coarse-grained finely porous catalysts) or to the kinetic region.
where G is the amount of substance transferred over time in the x direction, perpendicular to the surface of the catalyst grain at a concentration of the diffusing component in the core of the reactant flow, S is the free outer surface of the catalyst, dc/dx is the concentration gradient.
A large number of methods and equations have been proposed for determining the diffusion coefficients of substances in various media. For a binary mixture of substances A and B according to Arnold
where T - temperature, K; M A, M B - molar masses of substances A and B, g / mol; v A, v B - molar volumes of substances; P - total pressure (0.1 M Pa); C A+B is Sutherland's constant.
The Sutherland constant is:
C A + B \u003d 1.47 (T A / + T B /) 0.5 (7)
G 
de T A / , T B / - boiling points of components A and B, K.
For gases A and B with similar values of molar volumes, =1 can be taken, and with a significant difference between them, 1.
The diffusion coefficient in liquid media D W can be determined by the formula
where is the viscosity of the solvent, PaС; M and v - molar mass and molar volume of the diffusing substance; x a is a parameter that takes into account the association of molecules in the solvent.
In intradiffusion region, that is, when the overall rate of the process is limited by the diffusion of reactants in the pores of the catalyst grain, there are several ways to speed up the process. It is possible to reduce the size of the catalyst grains and, accordingly, the path of the molecules to the middle of the grain, this is possible if they pass simultaneously from the filter layer to the boiling layer. It is possible to manufacture large-pore catalysts for a fixed bed without reducing the grain size in order to avoid an increase in hydraulic resistance, but in this case, the inner surface will inevitably decrease and, accordingly, the intensity of catalyst operation will decrease compared to fine-grained large-pore catalyst. An annular contact mass with a small wall thickness can be used. Finally, bidisperse or polydisperse catalysts, in which large pores are transport routes to a highly developed surface created by fine pores. In all cases, they strive to reduce the depth of penetration of reagents into pores (and products from pores) to such an extent as to eliminate intradiffusion inhibition and move to the kinetic region, when the rate of the process is determined only by the rate of the actual chemical acts of catalysis, that is, the adsorption of reagents by active centers, the formation of products and its desorption. Most of the industrial processes that take place in the filter bed are inhibited by internal diffusion, such as large-scale catalytic processes for the reforming of methane with steam, the conversion of carbon monoxide, the synthesis of ammonia, etc.
The time required for the diffusion of the component into the pores of the catalyst to a depth l can be determined by the Einstein formula:
\u003d l 2 / 2D e (10)
The effective diffusion coefficient in the pores is determined approximately depending on the ratio of the pore sizes and the mean free path of the molecules. In gaseous media, when the mean free path of the component molecule is less than the equivalent pore diameter d=2r (2r), it is assumed that normal molecular diffusion occurs in the pores D e = D, which is calculated by the formula:
In a constrained mode of movement, when 2r, determine D e \u003d D to according to the approximate Knudsen formula:
(
12)
where r is the transverse radius of the pore.
(
13)
Diffusion in the pores of the catalyst in liquid media is very difficult due to a strong increase in the viscosity of the solution in narrow channels (anomalous viscosity), therefore, dispersed catalysts, that is, small non-porous particles, are often used for catalysis in liquids. In many catalytic processes, with a change in the composition of the reaction mixture and other process parameters, the mechanism of catalysis, as well as the composition and activity of the catalyst, can change; therefore, it is necessary to take into account the possibility of changing the nature and rate of the process even with a relatively small change in its parameters.
Catalysts can increase the rate constant of a reaction indefinitely, however, unlike temperature, catalysts do not affect the rate of diffusion. Therefore, in many cases, with a significant increase in the reaction rate, the overall rate remains low due to the slow supply of components to the reaction zone.
The content of the article
CATALYSIS, acceleration of chemical reactions under the influence of small amounts of substances (catalysts), which themselves do not change during the reaction. Catalytic processes play a huge role in our life. Biological catalysts called enzymes are involved in the regulation of biochemical processes. Many industrial processes would not be possible without catalysts.
The most important property of catalysts is selectivity; the ability to increase the rate of only certain chemical reactions out of many possible. This allows reactions that are too slow under normal conditions to be of practical use, and ensures the formation of the desired products.
The use of catalysts contributed to the rapid development of the chemical industry. They are widely used in oil refining, obtaining various products, creating new materials (for example, plastics), often cheaper than those used before. Approximately 90% of modern chemical production is based on catalytic processes. Catalytic processes play a special role in environmental protection.
Most catalytic reactions are carried out at certain pressures and temperatures by passing the reaction mixture, which is in a gaseous or liquid state, through a reactor filled with catalyst particles. The following concepts are used to describe the reaction conditions and characterize the products. Space velocity is the volume of gas or liquid passing through a unit volume of a catalyst per unit time. Catalytic activity is the amount of reactants converted into products by the catalyst per unit of time. Conversion is the proportion of a substance converted in a given reaction. Selectivity is the ratio of the amount of a certain product to the total amount of products (usually expressed as a percentage). Yield is the ratio of the amount of a given product to the amount of starting material (usually expressed as a percentage). Productivity is the amount of reaction products formed per unit volume per unit time.
TYPES OF CATALYSTS
Catalysts are classified according to the nature of the reaction they promote, their chemical composition, or their physical properties. Almost all chemical elements and substances have catalytic properties to one degree or another - by themselves or, more often, in various combinations. According to their physical properties, catalysts are divided into homogeneous and heterogeneous. Heterogeneous catalysts are solids that are homogeneous and dispersed in the same gaseous or liquid medium as the reactants.
Many heterogeneous catalysts contain metals. Some metals, especially those belonging to group VIII of the periodic system of elements, have catalytic activity in themselves; a typical example is platinum. But most metals exhibit catalytic properties, being in the composition of compounds; an example is alumina (aluminum oxide Al 2 O 3).
An unusual property of many heterogeneous catalysts is their large surface area. They are permeated with numerous pores, the total area of which sometimes reaches 500 m 2 per 1 g of the catalyst. In many cases, oxides with a large surface area serve as a substrate on which metal catalyst particles are deposited in the form of small clusters. This ensures efficient interaction of the reagents in the gas or liquid phase with the catalytically active metal. Zeolites, crystalline minerals of the aluminosilicate group (silicon and aluminum compounds), constitute a special class of heterogeneous catalysts. Although many heterogeneous catalysts have a large surface area, they usually have only a small number of active sites, which account for a small part of the total surface area. Catalysts can lose their activity in the presence of small amounts of chemical compounds called catalyst poisons. These substances bind to active centers, blocking them. Determining the structure of active centers is the subject of intense research.
Homogeneous catalysts have a different chemical nature - acids (H 2 SO 4 or H 3 PO 4), bases (NaOH), organic amines, metals, most often transitional (Fe or Rh), in the form of salts, organometallic compounds or carbonyls. Catalysts also include enzymes - protein molecules that regulate biochemical reactions. The active center of some enzymes contains a metal atom (Zn, Cu, Fe or Mo). Metal-containing enzymes catalyze reactions involving small molecules (O 2 , CO 2 or N 2). Enzymes have very high activity and selectivity, but they work only under certain conditions, such as those in which reactions occur in living organisms. The industry often uses the so-called. immobilized enzymes.
HOW CATALYSTS WORK
Energy.
Any chemical reaction can proceed only if the reactants overcome the energy barrier, and for this they must acquire a certain energy. As we have already said, the X ® Y catalytic reaction consists of a series of successive stages. Each one needs energy to run. E called the activation energy. The change in energy along the reaction coordinate is shown in fig. 1.
Consider first the non-catalytic, "thermal" path. For the reaction to take place, the potential energy of the X molecules must exceed the energy barrier E t. The catalytic reaction consists of three stages. The first is the formation of the X-Cat complex. (chemisorption), the activation energy of which is E ads. The second stage is the X-Cat rearrangement. ® Y-Cat. with activation energy E cat, and finally, the third - desorption with activation energy E des; E ads, E kat and E des much smaller E m. Since the reaction rate depends exponentially on the activation energy, the catalytic reaction proceeds much faster than the thermal one at a given temperature.
A catalyst can be likened to an instructor-guide who guides climbers (reacting molecules) through a mountain range. He leads one group through the pass and then returns for the next. The path through the pass lies much lower than that which lies through the top (the thermal channel of the reaction), and the group makes the transition faster than without a conductor (catalyst). It is even possible that on their own the group would not have been able to overcome the ridge at all.
Theories of catalysis.
Three groups of theories have been proposed to explain the mechanism of catalytic reactions: geometric, electronic, and chemical. In geometric theories, the main attention is paid to the correspondence between the geometric configuration of the atoms of the active centers of the catalyst and the atoms of that part of the reacting molecules that is responsible for binding to the catalyst. Electronic theories are based on the idea that chemisorption is due to electronic interaction associated with charge transfer, i.e. these theories relate catalytic activity to the electronic properties of the catalyst. Chemical theory considers a catalyst as a chemical compound with characteristic properties that forms chemical bonds with reactants, resulting in the formation of an unstable transition complex. After the decomposition of the complex with the release of products, the catalyst returns to its original state. The latter theory is now considered the most adequate.
At the molecular level, a catalytic gas phase reaction can be represented as follows. One reacting molecule binds to the active site of the catalyst, while the other interacts with it while being directly in the gas phase. An alternative mechanism is also possible: the reacting molecules are adsorbed on neighboring active sites of the catalyst and then interact with each other. Apparently, this is how most catalytic reactions proceed.
Another concept suggests that there is a relationship between the spatial arrangement of atoms on the catalyst surface and its catalytic activity. The rate of some catalytic processes, including many hydrogenation reactions, does not depend on the mutual arrangement of catalytically active atoms on the surface; the speed of others, on the contrary, changes significantly with a change in the spatial configuration of surface atoms. An example is the isomerization of neopentane to isopentane and the simultaneous cracking of the latter to isobutane and methane on the surface of a Pt-Al 2 O 3 catalyst.
APPLICATION OF CATALYSIS IN INDUSTRY
The rapid industrial growth that we are now experiencing would not have been possible without the development of new chemical technologies. To a large extent, this progress is determined by the widespread use of catalysts, with the help of which low-grade raw materials are converted into high-value products. Figuratively speaking, the catalyst is the philosopher's stone of the modern alchemist, only it does not turn lead into gold, but raw materials into medicines, plastics, chemical reagents, fuel, fertilizers and other useful products.
Perhaps the very first catalytic process that man learned to use is fermentation. Recipes for the preparation of alcoholic beverages were known to the Sumerians as early as 3500 BC. Cm. WINE; BEER.
A significant milestone in the practical application of catalysis was the production of margarine by catalytic hydrogenation of vegetable oil. For the first time, this reaction on an industrial scale was carried out around 1900. And since the 1920s, catalytic methods have been developed one after another for the production of new organic materials, primarily plastics. The key point was the catalytic production of olefins, nitriles, esters, acids, etc. - "bricks" for the chemical "building" of plastics.
The third wave of industrial use of catalytic processes occurs in the 1930s and is associated with oil refining. In terms of volume, this production soon left all others far behind. Oil refining consists of several catalytic processes: cracking, reforming, hydrosulfonation, hydrocracking, isomerization, polymerization and alkylation.
And finally, the fourth wave in the use of catalysis is related to environmental protection. The most famous achievement in this area is the creation of a catalytic converter for automobile exhaust gases. Catalytic converters, which have been installed in cars since 1975, have played a big role in improving air quality and have saved many lives in this way.
About a dozen Nobel Prizes have been awarded for work in the field of catalysis and related fields.
The practical significance of catalytic processes is evidenced by the fact that the share of nitrogen, which is part of the nitrogen-containing compounds obtained industrially, accounts for about half of all nitrogen that is part of food products. The amount of nitrogen compounds produced naturally is limited, so that the production of dietary protein depends on the amount of nitrogen applied to the soil with fertilizers. It would be impossible to feed even half of humanity without synthetic ammonia, which is produced almost exclusively by the Haber-Bosch catalytic process.
The scope of catalysts is constantly expanding. It is also important that catalysis can significantly increase the efficiency of previously developed technologies. An example is the improvement in catalytic cracking through the use of zeolites.
Hydrogenation.
A large number of catalytic reactions are associated with the activation of a hydrogen atom and some other molecule, leading to their chemical interaction. This process is called hydrogenation and underlies many stages of oil refining and the production of liquid fuels from coal (the Bergius process).
The production of aviation gasoline and motor fuel from coal was developed in Germany during World War II, since there are no oil fields in this country. The Bergius process is the direct addition of hydrogen to carbon. Coal is heated under pressure in the presence of hydrogen and a liquid product is obtained, which is then processed into aviation gasoline and motor fuel. Iron oxide is used as a catalyst, as well as catalysts based on tin and molybdenum. During the war, approximately 1,400 tons of liquid fuel per day were obtained at 12 German factories using the Bergius process.
Another process, Fischer-Tropsch, consists of two stages. First, the coal is gasified, i.e. carry out its reaction with water vapor and oxygen and get a mixture of hydrogen and carbon oxides. This mixture is converted into liquid fuel using catalysts containing iron or cobalt. With the end of the war, the production of synthetic fuel from coal in Germany was discontinued.
As a result of the rise in oil prices that followed the oil embargo in 1973–1974, vigorous efforts were made to develop an economically viable method for producing gasoline from coal. Thus, direct liquefaction of coal can be carried out more efficiently using a two-stage process in which the coal is first contacted with an alumina-cobalt-molybdenum catalyst at a relatively low and then at a higher temperature. The cost of such synthetic gasoline is higher than that obtained from oil.
Ammonia.
One of the simplest hydrogenation processes from a chemical point of view is the synthesis of ammonia from hydrogen and nitrogen. Nitrogen is a very inert substance. To break the N–N bond in its molecule, an energy of the order of 200 kcal/mol is required. However, nitrogen binds to the surface of the iron catalyst in the atomic state, and this requires only 20 kcal/mol. Hydrogen bonds with iron even more readily. The synthesis of ammonia proceeds as follows:
This example illustrates the ability of a catalyst to accelerate both the forward and reverse reactions equally, i.e. the fact that the catalyst does not change the equilibrium position of the chemical reaction.
Hydrogenation of vegetable oil.
One of the most important hydrogenation reactions in practice is the incomplete hydrogenation of vegetable oils to margarine, cooking oil, and other food products. Vegetable oils are obtained from soybeans, cotton seeds and other crops. They include esters, namely triglycerides of fatty acids with varying degrees of unsaturation. Oleic acid CH 3 (CH 2) 7 CH \u003d CH (CH 2) 7 COOH has one double bond C \u003d C, linoleic acid has two and linolenic acid has three. The addition of hydrogen to break this bond prevents the oils from oxidizing (rancidity). This raises their melting point. The hardness of most of the products obtained depends on the degree of hydrogenation. Hydrogenation is carried out in the presence of a fine powder of nickel deposited on a substrate or Raney nickel catalyst in a highly purified hydrogen atmosphere.
Dehydrogenation.
Dehydrogenation is also an industrially important catalytic reaction, although the scale of its application is incomparably smaller. With its help, for example, styrene, an important monomer, is obtained. To do this, dehydrogenate ethylbenzene in the presence of a catalyst containing iron oxide; potassium and some structural stabilizer also contribute to the reaction. On an industrial scale, propane, butane and other alkanes are dehydrogenated. Dehydrogenation of butane in the presence of an alumina-chromium catalyst produces butenes and butadiene.
acid catalysis.
The catalytic activity of a large class of catalysts is due to their acidic properties. According to I. Bronsted and T. Lowry, an acid is a compound capable of donating a proton. Strong acids easily donate their protons to bases. The concept of acidity was further developed in the works of G. Lewis, who defined an acid as a substance capable of accepting an electron pair from a donor substance with the formation of a covalent bond due to the socialization of this electron pair. These ideas, together with ideas about reactions that form carbenium ions, helped to understand the mechanism of various catalytic reactions, especially those involving hydrocarbons.
The strength of an acid can be determined using a set of bases that change color when a proton is added. It turns out that some industrially important catalysts behave like very strong acids. These include a Friedel-Crafts catalyst such as HCl-AlCl 2 O 3 (or HAlCl 4) and aluminosilicates. The strength of an acid is a very important characteristic, since it determines the rate of protonation, a key step in the process of acid catalysis.
The activity of catalysts such as aluminosilicates used in oil cracking is determined by the presence of Bronsted and Lewis acids on their surface. Their structure is similar to the structure of silica (silicon dioxide), in which some of the Si 4+ atoms are replaced by Al 3+ atoms. The excess negative charge that arises in this case can be neutralized by the corresponding cations. If the cations are protons, then the aluminosilicate behaves like a Brønsted acid:
The activity of acid catalysts is determined by their ability to react with hydrocarbons with the formation of a carbenium ion as an intermediate product. Alkylcarbenium ions contain a positively charged carbon atom bonded to three alkyl groups and/or hydrogen atoms. They play an important role as intermediates formed in many reactions involving organic compounds. The mechanism of action of acid catalysts can be illustrated by the example of the isomerization reaction n-butane to isobutane in the presence of HCl-AlCl 3 or Pt-Cl-Al 2 O 3 . First, a small amount of C 4 H 8 olefin attaches the positively charged hydrogen ion of the acid catalyst to form a tertiary carbenium ion. Then the negatively charged hydride ion H - splits off from n-butane to form isobutane and secondary butylcarbenium ion. The latter, as a result of the rearrangement, turns into a tertiary carbenium ion. This chain can continue with the elimination of a hydride ion from the next molecule n- butane, etc.:
Significantly, tertiary carbenium ions are more stable than primary or secondary ones. As a result, they are mainly present on the catalyst surface, and therefore the main product of butane isomerization is isobutane.
Acid catalysts are widely used in oil refining - cracking, alkylation, polymerization and isomerization of hydrocarbons. The mechanism of action of carbenium ions, which play the role of catalysts in these processes, has been established. At the same time, they participate in a number of reactions, including the formation of small molecules by splitting large ones, the combination of molecules (olefin with olefin or olefin with isoparaffin), structural rearrangement by isomerization, the formation of paraffins and aromatic hydrocarbons by hydrogen transfer.
One of the latest industrial applications of acid catalysis is the production of leaded fuels by the addition of alcohols to isobutylene or isoamylene. The addition of oxygenated compounds to gasoline reduces the concentration of carbon monoxide in the exhaust gases. Methyl- tert-butyl ether (MTBE) with a blending octane number of 109 also makes it possible to obtain the high-octane fuel required for the operation of an automobile engine with a high compression ratio without resorting to the introduction of tetraethyl lead into gasoline. The production of fuels with octane numbers 102 and 111 is also organized.
main catalysis.
The activity of catalysts is determined by their basic properties. An old and well-known example of such catalysts is sodium hydroxide, which is used to hydrolyze or saponify fats in the manufacture of soap, and a recent example is the catalysts used in the production of polyurethane plastics and foams. Urethane is formed by the interaction of alcohol with isocyanate, and this reaction is accelerated in the presence of basic amines. During the reaction, the base is attached to the carbon atom in the isocyanate molecule, as a result of which a negative charge appears on the nitrogen atom and its activity with respect to alcohol increases. A particularly effective catalyst is triethylenediamine. Polyurethane plastics are obtained by reacting diisocyanates with polyols (polyalcohols). When the isocyanate reacts with water, the previously formed urethane decomposes to release CO 2 . When a mixture of polyalcohols and water reacts with diisocyanates, the resulting polyurethane foam foams with gaseous CO 2 .
Dual action catalysts.
These catalysts speed up two types of reactions and give better results than passing the reactants in series through two reactors each containing only one type of catalyst. This is due to the fact that the active sites of the double-acting catalyst are very close to each other, and the intermediate product formed on one of them immediately turns into the final product on the other.
Combining a hydrogen activating catalyst with a hydrocarbon isomerization promoting catalyst gives a good result. The activation of hydrogen is carried out by some metals, and the isomerization of hydrocarbons by acids. An effective dual-acting catalyst used in oil refining to convert naphtha to gasoline is finely dispersed platinum deposited on acid alumina. The conversion of naphtha components such as methylcyclopentane (MCP) to benzene increases the octane number of gasoline. First, the MCP is dehydrogenated on the platinum part of the catalyst into an olefin with the same carbon backbone; then the olefin passes to the acid part of the catalyst, where it isomerizes to cyclohexene. The latter passes to the platinum part and dehydrogenates to benzene and hydrogen.
Dual action catalysts significantly accelerate oil reforming. They are used to isomerize normal paraffins to isoparaffins. The latter, boiling at the same temperatures as gasoline fractions, are valuable because they have a higher octane number compared to straight hydrocarbons. In addition, the transformation n-butane to isobutane is accompanied by dehydrogenation, contributing to the production of MTBE.
stereospecific polymerization.
An important milestone in the history of catalysis was the discovery of catalytic polymerization a-olefins to form stereoregular polymers. Stereospecific polymerization catalysts were discovered by K. Ziegler when he tried to explain the unusual properties of the polymers he obtained. Another chemist, J. Natta, suggested that the uniqueness of Ziegler polymers is determined by their stereoregularity. X-ray diffraction experiments have shown that polymers prepared from propylene in the presence of Ziegler catalysts are highly crystalline and indeed have a stereoregular structure. Natta introduced the terms "isotactic" and "syndiotactic" to describe such ordered structures. In the case where there is no order, the term "atactic" is used:
A stereospecific reaction occurs on the surface of solid catalysts containing transition metals of groups IVA–VIII (such as Ti, V, Cr, Zr) in an incompletely oxidized state, and any compound containing carbon or hydrogen, which is associated with a metal from groups I–III. A classic example of such a catalyst is the precipitate formed during the interaction of TiCl 4 and Al(C 2 H 5) 3 in heptane, where titanium is reduced to the trivalent state. This extremely active system catalyzes the polymerization of propylene at normal temperature and pressure.
catalytic oxidation.
The use of catalysts to control the chemistry of oxidation processes is of great scientific and practical importance. In some cases, oxidation must be complete, for example, when neutralizing CO and hydrocarbon contaminants in car exhaust gases. However, more often it is necessary that the oxidation be incomplete, for example, in many processes widely used in industry for the conversion of hydrocarbons into valuable intermediate products containing such functional groups as -CHO, -COOH, -C-CO, -CN. In this case, both homogeneous and heterogeneous catalysts are used. An example of a homogeneous catalyst is a transition metal complex that is used to oxidize pair-xylene to terephthalic acid, the esters of which serve as the basis for the production of polyester fibers.
Catalysts for heterogeneous oxidation.
These catalysts are usually complex solid oxides. Catalytic oxidation takes place in two stages. First, the oxide oxygen is captured by a hydrocarbon molecule adsorbed on the oxide surface. The hydrocarbon is oxidized and the oxide is reduced. The reduced oxide reacts with oxygen and returns to its original state. Using a vanadium catalyst, phthalic anhydride is obtained by partial oxidation of naphthalene or butane.
Ethylene production by methane dehydrodimerization.
The synthesis of ethylene through dehydrodimerization allows natural gas to be converted into more easily transportable hydrocarbons. The reaction 2CH 4 + 2O 2 ® C 2 H 4 + 2H 2 O is carried out at 850 ° C using various catalysts; the best results are obtained with Li-MgO catalyst. Presumably, the reaction proceeds through the formation of a methyl radical by splitting off a hydrogen atom from a methane molecule. Cleavage is carried out by incompletely reduced oxygen, for example, O 2 2–. Methyl radicals in the gas phase recombine to form an ethane molecule and are converted to ethylene during subsequent dehydrogenation. Another example of incomplete oxidation is the conversion of methanol to formaldehyde in the presence of a silver or iron-molybdenum catalyst.
Zeolites.
Zeolites constitute a special class of heterogeneous catalysts. These are aluminosilicates with an ordered honeycomb structure, the cell size of which is comparable to the size of many organic molecules. They are also called molecular sieves. Of greatest interest are zeolites whose pores are formed by rings consisting of 8–12 oxygen ions (Fig. 2). Sometimes the pores overlap, as in the ZSM-5 zeolite (Fig. 3), which is used for the highly specific conversion of methanol to gasoline fraction hydrocarbons. Gasoline contains significant amounts of aromatic hydrocarbons and therefore has a high octane number. In New Zealand, for example, one third of all gasoline consumed is obtained using this technology. Methanol is obtained from imported methane.
Catalysts that make up the group of Y-zeolites significantly increase the efficiency of catalytic cracking due primarily to their unusual acidic properties. Replacing aluminosilicates with zeolites makes it possible to increase the yield of gasoline by more than 20%.
In addition, zeolites are selective with respect to the size of the reacting molecules. Their selectivity is due to the size of the pores through which molecules of only certain sizes and shapes can pass. This applies to both starting materials and reaction products. For example, due to steric restrictions pair-xylene is formed more easily than more voluminous ortho- And meta-isomers. The latter are "locked" in the pores of the zeolite (Fig. 4).
The use of zeolites has made a real revolution in some industrial technologies - the dewaxing of gas oil and engine oil, the production of chemical intermediates for the production of plastics by alkylation of aromatic compounds, xylene isomerization, the disproportionation of toluene and the catalytic cracking of oil. Zeolite ZSM-5 is especially effective here.
Catalysts and environmental protection.
The use of catalysts to reduce air pollution began in the late 1940s. In 1952, A. Hagen-Smith found that hydrocarbons and nitrogen oxides, which are part of exhaust gases, react to light to form oxidants (in particular, ozone), which irritate the eyes and give other undesirable effects. Around the same time, J. Houdry developed a method for the catalytic purification of exhaust gases by oxidizing CO and hydrocarbons to CO 2 and H 2 O. In 1970, the Clean Air Declaration was formulated (revised in 1977, expanded in 1990), according to which all new cars , starting with 1975 models, must be equipped with exhaust gas catalytic converters. Norms have been established for the composition of exhaust gases. Since lead compounds added to gasoline poison catalysts, a phase-out program has been adopted. Attention was also drawn to the need to reduce the content of nitrogen oxides.
Especially for automobile catalytic converters, catalysts have been created in which active components are deposited on a ceramic substrate with a honeycomb structure, through the cells of which exhaust gases pass. The substrate is covered with a thin layer of metal oxide, such as Al 2 O 3 , on which a catalyst is applied - platinum, palladium or rhodium. The content of nitrogen oxides formed during the combustion of natural fuels at thermal power plants can be reduced by adding small amounts of ammonia to the flue gases and passing them through a titanium-vanadium catalyst.
Enzymes.
Enzymes are natural catalysts that regulate biochemical processes in a living cell. They participate in the processes of energy exchange, the breakdown of nutrients, biosynthesis reactions. Many complex organic reactions cannot proceed without them. Enzymes function at ordinary temperature and pressure, have very high selectivity and are able to increase the rate of reactions by eight orders of magnitude. Despite these advantages, only approx. Of the 15,000 known enzymes, 20 are used on a large scale.
Man has been using enzymes for thousands of years to bake bread, produce alcoholic beverages, cheese and vinegar. Now enzymes are also used in industry: in the processing of sugar, in the production of synthetic antibiotics, amino acids and proteins. Proteolytic enzymes that accelerate hydrolysis processes are added to detergents.
With the help of bacteria Clostridium acetobutylicum H. Weizmann carried out the enzymatic conversion of starch into acetone and butyl alcohol. This method of obtaining acetone was widely used in England during the First World War, and during the Second World War, butadiene rubber was made with its help in the USSR.
An exceptionally large role was played by the use of enzymes produced by microorganisms for the synthesis of penicillin, as well as streptomycin and vitamin B 12 .
Enzymatically produced ethyl alcohol is widely used as an automotive fuel. In Brazil, more than a third of the approximately 10 million cars run on 96% ethyl alcohol derived from sugar cane, and the remainder on a mixture of gasoline and ethyl alcohol (20%). The technology for the production of fuel, which is a mixture of gasoline and alcohol, is well developed in the United States. In 1987, approx. 4 billion liters of alcohol, of which approximately 3.2 billion liters were used as fuel. Various applications are also found in the so-called. immobilized enzymes. These enzymes are associated with a solid carrier, such as silica gel, over which the reagents are passed. The advantage of this method is that it ensures efficient contact of the substrates with the enzyme, separation of products and preservation of the enzyme. One example of the industrial use of immobilized enzymes is the isomerization of D-glucose to fructose.
TECHNOLOGICAL ASPECTS
Modern technologies cannot be imagined without the use of catalysts. Catalytic reactions can proceed at temperatures up to 650°C and pressures of 100 atm or more. This makes it necessary to solve the problems associated with the contact between gaseous and solid substances and with the transfer of catalyst particles in a new way. For the process to be effective, its modeling must take into account the kinetic, thermodynamic and hydrodynamic aspects. Computer modeling is widely used here, as well as new instruments and methods for controlling technological processes.
In the 1960s significant progress was made in the production of ammonia. The use of a more active catalyst made it possible to lower the temperature of hydrogen production during the decomposition of water vapor, due to which it was possible to lower the pressure and, consequently, reduce production costs, for example, through the use of cheaper centrifugal compressors. As a result, the cost of ammonia fell by more than half, there was a huge increase in its production, and in connection with this, an increase in food production, since ammonia is a valuable fertilizer.
Methods.
Research in the field of catalysis is carried out using both traditional and special methods. Radioactive labels, X-ray, infrared and Raman (Raman) spectroscopy, electron microscopy methods are used; kinetic measurements are carried out, the influence of the methods of obtaining catalysts on their activity is studied. Of great importance is the determination of the surface area of the catalyst by the Brunauer-Emmett-Teller method (BET method), based on the measurement of nitrogen physical adsorption at different pressures. To do this, determine the amount of nitrogen required for the formation of a monolayer on the surface of the catalyst, and, knowing the diameter of the N 2 molecule, calculate the total area. In addition to determining the total surface area, chemisorption of various molecules is carried out, which makes it possible to estimate the number of active centers and obtain information about their properties.
Researchers have at their disposal various methods for studying the surface structure of catalysts at the atomic level. Unique information can be obtained using the EXAFS method. Among the spectroscopic methods, UV, X-ray and Auger photoelectron spectroscopy are increasingly being used. Of great interest is secondary ion mass spectrometry and ion scattering spectroscopy. NMR measurements are used to study the nature of catalytic complexes. The scanning tunneling microscope allows you to see the arrangement of atoms on the surface of the catalyst.
PERSPECTIVES
The scale of catalytic processes in industry is increasing every year. Catalysts are increasingly being used to neutralize environmental pollutants. The role of catalysts in the production of hydrocarbons and oxygen-containing synthetic fuels from gas and coal is growing. It seems very promising to create fuel cells for the economical conversion of fuel energy into electrical energy.
New concepts of catalysis will make it possible to obtain polymeric materials and other products with many valuable properties, improve energy production methods, increase food production, in particular by synthesizing proteins from alkanes and ammonia with the help of microorganisms. It may be possible to develop genetically engineered methods for the production of enzymes and organometallic compounds that approach natural biological catalysts in their catalytic activity and selectivity.
Literature:
Gates B.K. Chemistry of catalytic processes. M., 1981
Boreskov G.K. Catalysis. Questions of theory and practice. Novosibirsk, 1987
Gankin V.Yu., Gankin Yu.V. New general theory of catalysis. L., 1991
Tokabe K. Catalysts and catalytic processes. M., 1993
Catalysis is one of the most common methods in chemistry for accelerating chemical reactions.
catalysis called a selective change in the rate of chemical reactions in the presence of substances (catalysts), which, taking part in intermediate processes, are regenerated during the reaction and are not part of the final products.
positive catalysis, or simply catalysis, - this is a significant increase in the rate of a reaction, for example, the production of sulfuric acid or the oxidation of ammonia in the presence of platinum. negative catalysis, or inhibition, - this is a slowdown in the reaction, for example, the interaction of a sodium sulfite solution with atmospheric oxygen in the presence of ethyl alcohol or the decomposition of hydrogen peroxide at low concentrations of sulfuric acid (inhibitors, respectively, ethyl alcohol and sulfuric acid).
Reactions that take place under the action of catalysts are called catalytic.
The action of a catalyst in the process of changing a chemical system can be not only accelerating, but also guiding: if the initial chemical system can develop in several thermodynamically possible directions under given conditions, the catalyst predominantly accelerates one of them.
Catalysis changes the reaction mechanism. The catalyst and one of the starting materials form activated complex- an intermediate compound that reacts with another starting material to form reaction products and regenerate catalyst molecules.
Let some reaction A + B = AB have a very high activation energy E a and therefore proceed slowly. Its energy diagram is shown in fig. 4.4, A.
Rice. 4.4. Enthalpy change during the reaction: a - without a catalyst: b- with catalyst
If this reaction is carried out in the presence of catalyst K (Fig. 4.4, b), then it enters into a chemical interaction with one of the starting substances (for example, A), as a result of which, through the activated battery complex *, an unstable chemical compound AK is formed according to the reaction A + K = AK. The activation energy of this process E" less than that in the absence of a catalyst (E a "therefore, the reaction proceeds quickly. Further, the intermediate AK through another activated complex, AVK *, interacts with the second starting material B: AK + B \u003d AB + K; in this case, the catalyst returns to its original state. The activation energy of this process is also small (E "which causes it to proceed at a high speed. When summing up both successively occurring processes, the final equation for a fast reaction is obtained: A + B (+ K) \u003d AB (+ K). The catalyst is indicated in this equation only to emphasize the fact of its regeneration.
Common to all catalysts is that they always change the activation energy, decreasing it with positive catalysis, i.e. lowering the height of the energy barrier. In the presence of a catalyst, an activated complex is formed with a lower energy level than without it, resulting in a significant increase in the reaction rate.
According to the phase feature, homogeneous (homogeneous) and heterogeneous (non-uniform) catalysis are distinguished; enzymatic catalysis is considered separately.
At homogeneous catalysis the catalyst and reactants form one phase (gas or solution) in which there are no interfaces (phase boundaries). Gas- and liquid-phase catalytic processes are very numerous. An example of homogeneous catalysis in the gas phase is the catalytic oxidation of sulfur oxide (IV) in the chamber method for producing sulfuric acid. Oxidation of sulfur dioxide to trioxide according to the reaction:
flows slowly. The introduction of the NO catalyst changes the reaction mechanism: 
and reduces the activation energy, and therefore increases the rate of the reaction.
In homogeneous catalysis, the rate of a chemical reaction is proportional to the concentration of the catalyst. The disadvantages of homogeneous catalysis in solutions are the limited temperature range and, in some cases, the difficulty in separating the catalyst from the reaction products.
At heterogeneous catalysis the catalyst (usually a solid) is in the system as an independent phase, i.e. there is an interface between the catalyst and the reactants (gases or liquids). Thus, the oxidation of ammonia (gaseous phase) is carried out in the presence of platinum (solid phase), and the decomposition of hydrogen peroxide (liquid phase) is accelerated by coal or manganese (IV) oxide, present in the form of a solid phase:
In heterogeneous catalysis, all reactions proceed at the phase boundary, i.e. on the surface of the catalyst, the activity of which depends on the properties of its surface - the size of the area, chemical composition, defectiveness of the structure and state. Features of the kinetics of processes are determined by diffusion and adsorption.
The surface of the catalyst (adsorbent) is physically inhomogeneous and has the so-called active centers, on which catalytic reactions mainly occur due to the adsorption of reactants (adsorbates) on these centers and an increase in their concentration on the catalyst surface. This partly speeds up the reaction. However, the main reason for the increase in the reaction rate is a significant increase in the chemical activity of adsorbed molecules, in which bonds between atoms are weakened under the action of a catalyst, which makes these molecules more reactive. The acceleration of the reaction in this case also occurs as a result of a decrease in the activation energy, to which the formation of surface intermediates also makes a certain contribution.
Substances poisoning the solid catalyst, i.e. reducing or completely destroying its activity are called catalytic poisons. For example, compounds of arsenic, mercury, lead, cyanides poison platinum catalysts, which must be regenerated under production conditions in this case.
Substances that enhance the action of catalysts for a given reaction, but are not themselves catalysts, are called promoters. It is known, for example, to promote platinum catalysts with additions of iron, aluminum, etc.
Selectivity (selectivity) of action The use of catalysts manifests itself, in particular, in the fact that with the help of different catalysts it is possible to obtain different products from the same substance. So, in the presence of a catalyst Al 2 O e at 300 ° C, water and ethylene are obtained from ethyl alcohol:
But if copper powder is used as a catalyst at the same temperature, then ethyl alcohol decomposes into hydrogen and acetaldehyde:
Thus, for each reaction there is a catalyst.
With the participation of biological catalysts, enzymes complex chemical processes take place in plant and animal organisms. For example, saliva contains the enzyme ptyalin, which catalyzes the conversion of starch into sugar, and pepsin, which is present in gastric juice, promotes the breakdown of proteins. There are about 3,000 different enzymes in the human body, each of which is an effective catalyst for the corresponding reaction.
Many catalysts, especially enzymes, have purely individual catalytic action, which is why they are called individually specific.
