The rate of chemical reactions can increase significantly under the influence of substances called catalysts.
The phenomenon of changing the rate of a reaction in the presence of catalysts is called catalysis, and reactions with their participation are called catalytic.
A catalyst is a simple or complex substance that takes part in a chemical reaction and changes its rate, but in the end remains in a chemically unchanged state. If the rate of a chemical reaction increases under the influence of a catalyst, then such catalysis is called positive, and if it decreases, then negative.
Transition metals and their compounds exhibit catalytic properties - oxides, hydroxides, sulfides, amines, amino acids, etc. They are capable of not only significantly accelerating reactions, but also changing their mechanism. For example, during the interaction of carbon monoxide (P) and hydrogen, depending on the nature of the catalyst, different products are formed - methane or methanol.
In the process of oxidation of methane with atmospheric oxygen, in the presence of different catalysts, methanol, formaldehyde or formic acid can be obtained.
Catalysts are widely used in the production of ammonia, sulfuric, nitric, acetic acids, rubber, in oil cracking processes, in the synthesis of certain medications, and the like. The reactions of polymerization, hydrogenation and dehydrogenation, the production of alcohols, aldehydes, and carboxylic acids at a rate sufficient for technical needs occur only in the presence of catalysts.
Substances that slow down the rate of chemical reactions are called inhibitors.
Inhibitors are also widely used in technology. Their name is associated with the chemical or biochemical process that they slow down. In particular, substances that reduce the rate of metal corrosion are called corrosion inhibitors, and substances that inhibit any processes of oxidation of various substrates with molecular oxygen - antioxidants.
Catalysts are evaluated according to certain criteria, among which the most important are: activity, specificity, resistance to aging and poisoning.
Activity is determined by the ratio of the rates of catalytic and non-catalytic reactions. The more active a catalyst is, the more it reduces the activation energy of a reaction.
Specificity (selectivity) is the ability of a catalyst to increase the rate of only one reaction.
The activity of catalysts is significantly affected by impurities. Some of them can enhance, while others can slow down the action of catalysts. Substances that do not themselves have catalytic properties, but enhance the action of catalysts, are called promoters or activators.
It is also known that some chemicals have a negative effect on the activity of catalysts, the so-called catalytic poisons. These compounds partially or completely reduce the activity of catalysts.
One of the important types of catalytic processes is enzyme catalysis, which occurs under the influence of protein catalysts: so-called enzymes, or enzymes.
Enzymes as biological catalysts
All chemical processes in the physiological environment of the body (hydrolysis, protolysis, phosphorelation, complex formation, redox reactions) can only occur with the participation of catalysts, which are called enzymes, or enzymes.
Enzymes are protein substances that are produced by the cells of living organisms and significantly increase the speed of biochemical processes.
More than 1,800 enzymes are now known, many of which have been isolated in pure crystalline form. It is believed that a cell contains about 10 thousand molecules of various enzymes, which accelerate over 2 thousand reactions. A quarter of the currently studied enzymes contain ions of different metals and are therefore called metalloenzymes.
Both enzymes and inorganic catalysts obey the general laws of catalysis and are characterized by a number of common features, that is, they:
catalyze only those reactions that are energetically possible;
do not change the direction of reactions;
reduce the activation energy of reactions, thereby accelerating them;
are not consumed during the reaction.
However, enzymes are also characterized by special features that make it possible to distinguish them from conventional inorganic catalysts. These differences are associated with the structural features of enzymes, which are complex macromolecules of a protein nature.
Lecture 7
Catalysis
Catalysis has found wide application in the chemical industry, in particular in the technology of inorganic substances. Catalysis– excitation of chemical reactions or changes in their speed under the influence of substances - catalysts, which repeatedly enter into chemical interaction with reaction participants and restore their chemical composition after each cycle of interaction. There are substances that reduce the rate of reaction, 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 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 substances.
The essence of the accelerating effect of catalysts is to reduce the activation energy Ea of a chemical reaction by changing the reaction path in the presence of a catalyst. For the reaction of converting A into 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 E cat, but significantly lower than for the non-catalytic process E ne cat. The activation energy decreases 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 activation energy, and therefore the efficiency of the catalyst, can be the degree of compensation for the energy of broken bonds Di:
= (Di – E cat)/Di (1)
The lower the activation energy of the catalytic process, the higher the degree of compensation.
Simultaneously with the decrease in 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 reaction proceeds through several elementary stages, the order of which may be less than the order of non-catalytic reactions.
Types of catalysis
Based on the phase state of the reagents and catalyst, catalytic processes are divided into homogeneous and heterogeneous. In homogeneous catalysis, the catalyst and reactants are in the same phase (gas or liquid); 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 phase.
A special group includes enzymatic (biological) catalytic processes, common in nature and used in industry for the production of feed proteins, organic acids, alcohols, as well as for wastewater treatment.
Based on the types of reactions, catalysis is divided into redox and acid-base. In reactions proceeding according to the redox mechanism, intermediate interaction with the catalyst is accompanied by homolytic cleavage of two-electron bonds in the reacting substances and the formation of bonds with the catalyst at the site of the latter's unpaired electrons. Typical catalysts for redox reactions are metals or oxides of variable valence.
Acid-base catalytic reactions occur as a result of intermediate protolytic interaction of the reactants with the catalyst or interaction involving a lone pair of electrons (heterolytic) catalysis. Heterolytic catalysis proceeds with a rupture of the covalent bond in which, unlike homolytic reactions, the electron pair performing the bond remains in whole or in part with one of the atoms or a group of atoms. Catalytic activity depends on the ease of transfer of a proton to the reagent (acid catalysis) or abstraction of a proton from the reagent (base catalysis) in the first act of catalysis. According to the acid-base mechanism, catalytic reactions of hydrolysis, hydration and dehydration, polymerization, polycondensation, alkylation, isomerization, etc. occur. Active catalysts are compounds of boron, fluorine, silicon, aluminum, sulfur and other elements with acidic properties, or compounds of elements of the first and the second groups of the periodic table, which have basic properties. The hydration of ethylene by the acid-base mechanism with the participation of the acid catalyst NA is carried out as follows: in the first stage, the catalyst serves as a proton donor
CH 2 =CH 2 + HA CH 3 -CH 2 + + A -
the second stage is the actual hydration
CH 3 -CH 2 + + HONCH 3 CH 2 OH + H +
third stage – catalyst regeneration
N + + A - NA.
Redox and acid-base reactions can be considered according to the radical mechanism, according to which the strong molecule-catalyst lattice bond formed during chemisorption promotes the dissociation of the reacting molecules into radicals. In heterogeneous catalysis, free radicals, migrating over the surface of the catalyst, form neutral product molecules, which are desorbed.
There is also photocatalysis, where the process is initiated by exposure to light.
Since heterogeneous catalysis on solid catalysts is 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 reacting substances from the core of the flow to the surface of the catalyst; in industrial devices, turbulent (convective) diffusion usually predominates over molecular;
2) internal diffusion in the pores of the catalyst grain, depending on the size of the catalyst pores and the size of the reagent molecules, diffusion can occur by the molecular mechanism or by the Knudsen mechanism (with constrained movement);
3) activated (chemical) adsorption of one or more reactants on the surface of the catalyst with the formation of a surface chemical compound;
4) rearrangement of atoms to form 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 its entire surface is active, but individual 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 limiting the process, it is assumed that the remaining stages proceed so quickly that in each of them equilibrium is practically achieved. The speeds of individual stages are determined by the parameters of the technological process. Based on 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 internal diffusion regions are distinguished. The speed of the process in the general case is 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 reacting substances; for a process occurring in the gas phase, the driving force is expressed in partial pressures of the reacting substances p; k is the rate constant.
In general, the rate constant depends on many factors:
k = f (k 1 , k 2 , k sub, …..D and, D and / ,D p, ….) (3)
where k 1 , k 2 , k ab are the rate constants of the direct, reverse and side reactions; D and, D and / , D p are the diffusion coefficients of the starting substances and the product, which determine the value of k in the external or internal diffusion regions of the process.
IN kinetic region k does not depend on diffusion coefficients. The general kinetic equation for the rate of a gas catalytic process, taking into account the influence of the main parameters of the technological regime on the rate:
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 P0.1 MPa (1 at), P is the ratio of operating pressure to normal atmospheric pressure, that is, a dimensionless quantity, 0 is the conversion factor to normal pressure and temperature, n - reaction order.
The mechanism of chemical stages is determined by the nature of the reactants and catalyst. The process can be limited by the chemisorption of one of the reactants by the surface of the catalyst or the desorption of reaction products. The rate of the reaction can be controlled by the formation of a charged activated complex. In these cases, charging the catalyst surface under the influence of some factors has a significant impact on the course of the reaction. In the kinetic region, processes occur mainly on low-activity, fine-grained catalysts with large pores in a turbulent flow of reagents, 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 increasing 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. Reactions whose overall order is higher than unity are characterized by a transition from the diffusion region to the kinetic region 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, and an increase in temperature.
In external diffusion region First of all, processes take place on highly active catalysts, which provide a fast reaction and sufficient product yield 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 not advisable to use porous grains with a highly developed internal surface, but one must strive to develop the outer surface of the catalyst. Thus, when oxidizing ammonia on platinum, the latter is used in the form of very fine meshes containing thousands of interweavings of platinum wire. The most effective means of accelerating processes occurring in the region of external diffusion is mixing of reagents, which is often achieved by increasing the linear speed of the reagents. Strong turbulization of the flow leads to a transition of the process from the external diffusion region to the internal diffusion region (with 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 concentration c of the diffusing component in the core of the reagent 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 is 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 the Sutherland constant.
The Sutherland constant is:
C A+B = 1.47(T A / +T B /) 0.5 (7)
G 
de T A /, T B / - boiling temperatures of components A and B, K.
For gases A and B with close values of molar volumes, we can take =1, and with a significant difference between them, 1.
The diffusion coefficient in liquid media D g can be determined by the formula
where is the viscosity of the solvent, PaC; M andv - molar mass and molar volume of the diffusing substance; xa 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 reagents in the pores of the catalyst grain, there are several ways to accelerate 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 move simultaneously from the filter layer to the boiling one. It is possible to produce large-porous catalysts for a fixed layer without reducing the grain size to avoid an increase in hydraulic resistance, but this will inevitably reduce the internal surface and, accordingly, reduce the intensity of the catalyst compared to a fine-grained, large-porous catalyst. You can use a ring-shaped contact mass with a small wall thickness. Finally, bidisperse or polydisperse catalysts, in which large pores are transport routes to the highly developed surface created by thin pores. In all cases, they strive to reduce the depth of penetration of reagents into the pores (and products from the pores) so much as to eliminate intra-diffusion inhibition and move into 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 industrial processes occurring in the filter bed are inhibited by internal diffusion, for example large-scale catalytic processes of methane-steam reforming, carbon monoxide conversion, ammonia synthesis, etc.
The time required for the diffusion of a component into the pores of the catalyst to a depth l can be determined using the Einstein formula:
= l 2 /2D e (10)
The effective diffusion coefficient in pores is determined approximately depending on the ratio of pore sizes and the free path of molecules. In gaseous media, when the mean free path of a 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, D e =D k is determined using the approximate Knudsen formula:
(
12)
where r is the transverse radius of the pore.
(
13)
Diffusion in the pores of a catalyst in liquid media is very difficult due to a strong increase in the viscosity of the solution in narrow channels (abnormal viscosity), therefore, dispersed catalysts, that is, small non-porous particles, are often used for catalysis in liquids. In many catalytic processes, with changes 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, so it is necessary to take into account the possibility of changing the nature and speed of the process even with a relatively small change in its parameters.
Catalysts can increase the reaction rate constant indefinitely, but 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.
Structure and composition of catalysts
Industrial catalysts are often multicomponent systems. Catalyst components can be in various forms: in the form of elementary compounds (metals, coals), oxides, sulfides, halides, as well as complex compounds (enzymes, complexes of metals with organic ligands). The complexity of the composition of catalysts is due to the fact that the catalytic activity of two or more compounds is not additive, but takes on an extreme value, the so-called “synergistic effect.” One of the ways to increase the activity of a catalyst is to promote it - adding a substance to the catalyst ( promoter), which in itself does not have catalytic properties, but increases the activity of the catalyst. There are two types of promoters: electronic and structural.
Electronic promoters
The mechanism of their action is reduced to a change in the electronic states in the catalyst crystals and a decrease in the electron work function. Electronic promoters change the structure and chemical composition of the active phase, forming active centers of a new chemical nature on the surface of the catalyst, and therefore the nature and speed of the elementary stages of catalytic processes, and sometimes a change in selectivity, change. For example, the addition of K 2 O to an ammonia synthesis catalyst promotes the desorption of ammonia, which leads to an increase in the specific catalytic activity of the catalyst.
Structural promoters
They stabilize the active phase of the catalyst in relation to sintering, mechanical or chemical destruction. For example, aluminum oxide, when added to an iron catalyst for the synthesis of ammonia, interacts with Fe 3 O 4 forming a spinel crystal lattice FeAl 2 O 4, thereby preventing the recrystallization process. In addition, the addition of 8-10% Al 2 O 3 leads to an increase in the specific surface area of the iron catalyst from 1 to 25-30 m 2 /g. It should be noted that, depending on the amount of promoter, it can have both a promoting and a poisoning effect on the catalyst.
Most adsorbents and catalysts can be divided into two types based on the nature of their macrostructure: spongy and xerogels. Sponge catalysts are a solid solid body penetrated by conical, cylindrical and bottle-shaped pores formed when volatile or soluble products are released from this body as a result of drying or treatment with aggressive liquids and gases (leaching, reduction, roasting). The porous structure of xerogels is described by the globular model, according to which a solid substance consists of contacting or fused particles, the pores are the voids between them. Depending on the production method, catalysts can be mixed or applied.
Mixed catalysts
In mixed catalysts, the components are introduced in comparable quantities and each of them is catalytically active in relation to a given reaction. Mixed catalysts are obtained either by mechanical mixing of active components with or without subsequent heat treatment, or by coprecipitation of intermediates followed by calcination, for example, when using oxides as catalysts. An increase in the activity of a mixed catalyst may be due to the fact that during its preparation the components react with each other to form a new, more active compound. For example, an iron-molybdenum catalyst for the oxidation of methyl alcohol to formaldehyde is iron molybdate, obtained with a ratio of molybdenum and iron oxides in a ratio of 1.5: 1. A catalyst containing a different oxide ratio will be less active due to the existence of two phases: iron molybdate and excess MoO oxides 3 and Fe 2 O 3 . An increase in activity may be a consequence of the formation of a solid solution of one component in another or their alloy. For example, the introduction of zirconium oxide into cerium oxide, which is a catalyst for soot oxidation, leads to an improvement in the thermal stability of the catalyst and an increase in activity due to an increase in the mobility of lattice oxygen.
Supported catalysts are the most common type of complex contact masses. In them, the active component is applied in one way or another (impregnation, spraying, etc.) onto a porous substrate - carrier. Most often, the carrier is inert for a given process and, unlike promoters, constitutes a large part of it, however, carriers that have catalytic properties in the processes being carried out are often used. By using a carrier, the working surface of the catalyst is increased and its cost is reduced. The carrier must have the following properties: high melting point, heat resistance, strength, developed porous structure, specific surface area of more than 100 m 2 /g. In some cases, the carrier interacts with the active component, increasing its activity. The most common carriers are: zeolites, oxides of aluminum, silicon, titanium, coals.
The required composition of the contact mass is largely determined by the conditions of the catalytic process, the composition of the initial mixture, in particular humidity, the presence of foreign inert or toxic impurities, temperature and hydrodynamics of the process.
Catalyst properties:
1. Catalyst activity
As a measure of activity, the difference in the rates of a chemical reaction in the presence of a catalyst and without it is used, taking into account the proportion of the reaction space occupied by the catalyst and inaccessible to the reacting substances:
A 1 = cat - (1- cat) (14)
this expression can only be used with a constant driving force of the process с.
It is more convenient to use the ratio of constant rates of catalytic and non-catalytic processes as a measure of catalyst activity:
A 2 = k cat / k = k 0 cat e -Ea cat /RT / k 0 e -Ea/RT = (k 0 cat /k 0) e - Ea/RT (15)
Active catalysts provide high intensity of the process (a significant degree of conversion at high volumetric flow rates). Activity is characterized by a reaction rate constant depending on the specific catalytic activity k beats (per 1 m 2 surface), which reflects the chemical nature of the catalyst, the internal specific surface area of the catalyst S beats (m 2 /g) and the degree of its use :
k = k beat S beat (16)
To compare the activity of a catalyst in a reaction under different conditions, or to compare several catalysts, the ratio of the amount of product obtained per 1 hour of operation per unit volume of catalyst is used as a measure of activity.
A = G/ V(17)
2. Selectivity (selectivity)
The selectivity of a catalyst can be characterized as the ratio of the rate of formation of the target product to the total rate of transformation of the main reactant in all directions (from differential kinetics data), or as the ratio of the amount of the main substance converted into the target product to its total amount that entered into all reactions ( from integrated kinetic data). High selectivity values are achieved by using catalysts of a certain chemical composition and porous structure, ensuring optimal shapes and sizes of grains, as well as hydrodynamic conditions in the reactor. Thus, from a mixture of CO and H 2 (water gas), depending on the catalyst and synthesis conditions, various products can be formed. Methane is formed above metallic nickel at temperature; methyl alcohol is formed on copper at elevated pressure.
3. Mechanical strength
The strength of the catalyst and their activity are often inversely proportional. An acceptable way to increase the strength of catalysts is to use various binders (inorganic adhesives) that do not have a negative effect on their activity.
4. Heat resistance
The resistance of catalysts to overheating is important for high-temperature processes. During the process of thermal recrystallization of the catalyst or its support, the specific surface area, and therefore the activity, decreases. Typically, a catalyst is characterized by the maximum temperature at which its activity remains for a long time, or the value of the relative loss of activity during operation under more severe conditions is given.
5. Specific surface area
The specific surface area of various catalysts ranges from a few square meters per gram to hundreds. The specific surface of the catalyst is determined, on the one hand, by the particle size of the substance, and on the other hand, by its porosity. Transition pores are most preferable for catalysis, since they make the main contribution to the specific surface area. In microporous materials, the diffusion of reactants to the catalyst surface and reaction products from it is difficult. Catalysts that have a bidisperse (biporous) structure have high activity, when large globules, between which transport pores are formed, are in turn formed from small globules, creating a high specific surface area. To increase the activity of the catalyst, it is advisable to use smaller grains, which makes it possible to increase the degree of grain utilization.
6. Resistance to harmful impurities (poison resistance)
Catalytic poisons are compounds whose presence in the reaction mixture can reduce or completely suppress the activity of the catalyst, causing “poisoning” of the catalyst. In case of poisoning, the active centers of the catalyst are blocked. Poisoning can be reversible or irreversible. Thus, a platinum catalyst is poisoned by CO and CS 2, however, when it is added to a pure mixture of starting substances, the poisons are desorption and activity is restored. In case of poisoning with H 2 S and PH 3, platinum is completely deactivated, here the poison interacts with the catalyst to form stable compounds.
7. Low hydraulic resistance of the layer.
The disadvantage of using small granules is the high hydraulic resistance. The means to reduce it is to increase the proportion of free volume (porosity) of the layer. For this purpose, catalysts of special shapes are used (hollow cylinders, stars, honeycomb-shaped catalysts). With the same activity as for a layer of conventional solid cylinders, these forms make it possible to reduce hydraulic resistance by 1.5 times and reduce the mass loading of the catalyst by the same amount. However, the mechanical strength of such catalysts is less, and abrasion is greater. Therefore, the desire for thin-walled new forms has severe limitations in terms of strength, since the fines formed during the destruction of the catalyst fill the space between the grains, which sharply increases the hydraulic resistance of the layer.
One of the most effective influences on chemical reactions is the use of a catalyst. Catalysts are substances that speed up chemical reactions. The presence of catalysts changes the reaction rate by thousands and even millions of times. Catalysts actively participate in a chemical reaction, but unlike reagents, they remain unchanged at the end of the reaction.
- these are substances that change the rate of a reaction, but are not themselves consumed during the reaction and are not part of the final products.
An important characteristic of a catalytic reaction (catalysis) is the homogeneity or heterogeneity of the catalyst and reactants. There are homogeneous and heterogeneous catalytic processes. In homogeneous (homogeneous) catalysis, there is no interface between the reactants and the catalyst. In this case, catalysis occurs through the formation of unstable intermediate products.
For example, substance A must react with substance B. However, high heat is required to initiate the reaction, and the reaction then proceeds slowly. Then the catalyst is selected in such a way that it forms an active intermediate compound with substance A, which can then react vigorously with substance B:
A+ Cat. = A ∙ Cat.
A ∙ Cat. + B = AB ∙ Cat.
Cat.
A+B=AB
Processes in which the catalyst and catalyzed substances are in different states of aggregation are referred to as heterogeneous (non-uniform) catalysis. When gaseous or liquid reagents are adsorbed on the catalyst surface, chemical bonds are weakened and the ability of these substances to interact increases.
The accelerating effect of the catalyst is to lower the activation energy of the main reaction. Each of the intermediate processes involving a catalyst occurs with a lower activation energy than a non-catalyzed reaction. Catalysis opens up a different path for a chemical reaction to proceed from starting materials to reaction products.
Experience shows that catalysts are strictly specific for specific reactions. For example, in react:
N 2 + 3H 2 = Fe 2NH 3
The catalyst is metallic iron, and in the reaction of sulfur(IV) oxide oxidation into sulfur(VI) oxide, the catalyst is vanadium(V) oxide V 2 O 5 . Platinum, nickel, palladium, and aluminum oxide are often used as catalysts. To accelerate the process of decomposition of hydrogen peroxide, manganese(IV) oxide is used as a catalyst. If you add a little manganese(IV) oxide to a glass with a solution of hydrogen peroxide, violent foaming of the liquid immediately occurs as a result of the release of oxygen.
The catalyst for the reaction between aluminum and iodine is ordinary water. If water is added to a mixture of aluminum and iodine, the substances in the mixture react violently.
There are substances that can slow down a chemical reaction - carry out so-called negative catalysis. They are called inhibitors. Such substances are used when necessary to slow down some processes, for example, corrosion of metals, oxidation of sulfides during storage, etc.
You need to enable JavaScript to voteInitiating chemical reactions due to intermediate chemical interactions with reaction participants and restoration of their chemical composition after each cycle of such intermediate interactions (see the article Catalysis). Based on the method of organization and phase composition of the reaction system, it is customary to distinguish between heterogeneous and homogeneous catalysts, as well as catalysts of biological origin - enzymes. In heterogeneous catalysis, catalysts are sometimes called contacts.
In general, the carrier of the catalytic activity of catalysts (see the articles Heterogeneous catalysis, Homogeneous catalysis) is usually a substance that directly enters into chemical interaction with at least one of the starting reagents with the formation of unstable (under the conditions of the catalytic reaction) chemical compounds - active component of the catalyst (for solid heterogeneous catalysts, often the catalytic active phase). The mechanisms of action of catalysts are quite diverse and depend on the type of catalytic reaction being carried out and the nature of the substance of the active component of the catalyst; the chemical nature of the active component of catalysts can also be very diverse. The mass fraction of the active component in catalysts can vary from 100% to very small values (tenths of a percent).
The main characteristics of catalysts are catalytic activity, selectivity with respect to the target products of catalytic transformations, specificity with respect to the reagents of catalytic reactions, stability, resistance to the action of catalytic poisons; for industrial catalysts there is also productivity (the amount of the target product obtained per unit of time per unit volume or mass of the catalyst).
Typically, catalysts are divided according to the types of catalytic processes: deep and partial (selective) oxidation, hydrogenation, polymerization, oil refining processes, organic synthesis, etc. Typical catalysts for redox reactions (oxidation, hydrogenation, etc.) are transition elements in metallic form, and also their salts, complex compounds, oxides and sulfides. Typical catalysts for acid-base reactions (hydration, dehydration, alkylation, polymerization, cracking, etc.) are liquid and solid mineral and organic acids and bases, acid salts, aluminosilicates, zeolites, etc.
In industry, they prefer to use solid heterogeneous catalysts due to the ease of their separation from the reaction medium and the ability to operate at elevated temperatures. The active component (catalytically active phase) of many industrial heterogeneous catalysts is highly dispersed and often supported on a durable porous support (usually highly porous carbon, oxide of a non-transition element, for example, silicon, aluminum, titanium, zirconium, etc.). To increase catalytic activity, selectivity, chemical stability and thermal stability, a small amount of a promoter (or activator) is sometimes introduced into the catalysts - a substance that may not have independent catalytic activity.
Solid industrial catalysts must have high catalytic activity, specificity with respect to a given reaction, selectivity with respect to the target product, mechanical strength, heat resistance, and a certain thermal conductivity. Industrial catalysts must also be resistant to deactivation - a decrease or complete suppression of their catalytic activity. Deactivation of catalysts can occur due to sintering or mechanical destruction (for example, abrasion) of the active component and/or carrier substance, blocking of active centers by by-products of the process - dense carbon deposits (coke), resinous substances, etc., poisoning by catalytic poisons. The effect of catalytic poisons is usually due to the blocking of the most active sites of the active component of the catalysts due to strong chemisorption and therefore manifests itself even in the presence of small amounts of poisons. Typical catalytic poisons are compounds of sulfur, nitrogen, phosphorus, arsenic, lead, mercury, cyanide compounds, oxygen, carbon monoxide, acetylene derivatives, sometimes water, etc. In industry, to prevent poisoning of catalysts, deep preliminary purification of reacting substances from catalytic poisons is carried out. In industrial catalytic processes, to restore catalytic activity, catalysts are regenerated after their deactivation. Regeneration of catalysts is carried out, for example, by burning off coke and resinous substances, washing with water or specially selected solvents.
The catalytic activity of a solid catalyst depends on the size and condition of the catalyst surface accessible to reagents, the shape, size and pore profile of the catalyst (that is, its texture), which is determined by the method of preparing the catalyst and its pre-treatment. In the absence of diffusion restrictions, the activity of a solid catalyst is directly proportional to this surface area. Therefore, most industrial heterogeneous catalysts have a developed specific surface area, up to several hundred m2 per 1 g of catalyst. The most common methods for producing active solid catalysts are the precipitation of metal hydroxides and carbonates from solutions of salts or complex compounds, followed by thermal decomposition of the precipitate to oxides, decomposition of other compounds in air to oxides, alloying of several substances with subsequent leaching of one of them (the so-called alloys, or “skeletal” catalysts), as well as applying the active component of the catalyst to a carrier by impregnation or from the gas phase with subsequent activation of the catalyst. Typical procedures for activating catalysts are their reduction with hydrogen, sulfidation with various sulfur-containing compounds, etc.; For some types of catalysts, thermal activation is used, which is carried out by heating the catalyst to the temperature of formation of the active phase. Mechanically strong catalysts are manufactured in the form of pressed tablets, as well as granules, balls, solid and hollow cylinders (Raschig rings), various kinds of extrudates obtained by special methods. In some cases, to reduce the aero- or hydrodynamic resistance of the catalyst layer, they are given more specific forms. For example, catalytic converters for automobile exhaust are usually made in the form of ceramic or metal "honeycomb" blocks with many parallel channels along the flow of the gas being purified. Industry also uses suspensions of catalysts in the liquid phase (suspension process) and dust-like catalysts, which during the reaction are suspended in vapors of reaction components (the so-called fluid process).
The cost of the catalyst depends on its chemical composition, method of preparation and varies from 0.5 to several thousand US dollars per 1 kg of catalyst. However, in the cost of finished products obtained using industrial catalysts, the cost of the catalyst is usually no more than 0.1-1%.
Industrial heterogeneous catalysts are low- or medium-volume products. The total volume of their annual consumption in Russia is about 100 thousand tons.
See the literature under the article Catalysis.
Catalysts provide a faster outcome to any chemical reaction. By reacting with the starting materials of the reaction, the catalyst forms an intermediate compound with them, after which this compound undergoes transformation and ultimately decomposes into the necessary final reaction product, as well as into the unchanged catalyst. After decomposition and formation of the desired product, the catalyst again reacts with the original reagents, forming an increasing amount of the original substance. This cycle can be repeated millions of times, and if the catalyst is removed from a group of reagents, the reaction can last hundreds or thousands of times slower.
Catalysts are heterogeneous and homogeneous. Heterogeneous catalysts during a chemical reaction form an independent phase, which is separated by a dividing boundary from the phase of the initial reagents. Homogeneous catalysts, in contrast, are part of the same phase as the starting reactants.
There are catalysts of organic origin that are involved in fermentation and ripening, they are called enzymes. Without their direct participation, humanity would not be able to obtain most of the alcoholic beverages, lactic acid products, dough products, as well as honey and. Without the participation of enzymes, metabolism in living organisms would be impossible.
Requirements for catalyst substances
Catalysts, which are widely used in industrial production, must have a number of properties necessary for the successful completion of the reaction. Catalysts must be highly active, selective, mechanically strong and heat-resistant. They must have a long-lasting effect, easy regeneration, resistance to catalytic poisons, hydrodynamic properties, and also a low price.
Modern Applications of Industrial Catalysts
In current high-tech production, catalysts are used in the cracking of petroleum products, the production of aromatic hydrocarbons and high-octane gas, the production of pure hydrogen, oxygen or inert gases, the synthesis of ammonia, and the production of sulfuric acid at no additional cost. Catalysts are also widely used for the production of nitric acid, phthalic anhydride, methyl alcohol and acetaldehyde. The most widely used catalysts are platinum metal, vanadium, nickel, chromium, iron, zinc, silver, aluminum and palladium. Some salts of these metals are also used quite often.
