Definition of catalysis. Types of catalysis. Chemisorption and formation of an intermediate activated complex. Activation energy of a catalytic reaction. Catalysis and balance. application of catalysis.
Catalysis- the phenomenon of excitation of chemical reactions by special substances - catalysts. The catalyst repeatedly enters into an intermediate chemical interaction with the substances involved in the reaction and restores its composition after each cycle of intermediate interactions.
homogeneous catalysis– the reactants and the catalyst form one phase.
heterogeneous catalysis- the substances and the catalyst are in different phases.
Microheterogeneous catalysis occupies an intermediate position between homogeneous and heterogeneous catalysis. The catalyst is a large polymer molecule. For small molecules interacting on them, they are similar to heterogeneous particles, but they form one phase with the reagents. This group includes enzymatic reactions in which the catalyst (enzyme) is large protein molecules of complex composition and structure. Therefore, it is called enzymatic catalysis.
The chemical transformation proceeds through the formation of an active complex with excess energy. It is enough for the rearrangement of new substances - reaction products. Change in the energy of the system during chemical interaction along the reaction path (1).
A - activated complex
K - intermediate connection with a catalyst
E 0, E k - energies of initial substances and products
E 1, E 2 - activation energies of the transformation in the forward and reverse directions
∆H is the change in energy as a result of the transformation (thermal effect).
But not all molecules have sufficient energy to form an active complex - only those whose energy exceeds the activation energy E 1 . If the required energy E 1 is large, then the reaction practically does not proceed.
The catalyst opens up a new reaction path by reacting chemically to form an activated complex with less energy than is required to form an activated complex without a catalyst. The intermediate compound, which includes a catalyst, is further converted into products through another activated complex, but also with lower energy. After the second stage of the reaction, the catalyst restores its chemical composition and its components are not included in the composition of the products. And although the reaction path lengthens and becomes a stage, a decrease in the energy of the activated complex leads to an increase in the reaction rate (2).
Source unknown
Particular interest in catalysis manifested itself during the development of industrial chemistry, since the ability to accelerate chemical reactions in the right direction without consuming energy and essentially without consuming the catalyst substance itself gave catalysis great practical significance. With the help of catalysis, the problems facing the technology of bound nitrogen are solved, more than 80% of oil is processed using catalytic processes, it is impossible to carry out most of the processes of organic synthesis.
Catalysts- substances that, repeatedly entering into an intermediate interaction with the participants in the reaction, change its mechanism and increase the reaction rate; at the same time, they restore their chemical composition after each cycle of intermediate exposures.
The effect of a catalyst on the mechanism of a chemical reaction can be explained by a conventional example. Let a single-stage reaction proceed with an activation energy E 0:
The course of the reaction in the energy diagram of catalytic and non-catalytic reactions is shown by curve 1. In the presence of a catalyst, the reaction mechanism changes, it proceeds through several successive stages (curve 2). For example, the first stage may be the formation of an intermediate activated Akt complex:
A + Kt → AKt
The activated complex then reacts with a second reactant to form a catalyst-product complex:
ACT + V → RKt
The last stage is the decomposition of the RKt complex with the formation of product K and the release of the catalyst for a new catalytic cycle:
Rkt → R + Kt
Each of these successive stages is characterized by its activation energies E 1 , E 2 , E 3 , but, as a rule, the height of each of these potential barriers is lower than the activation energy E 0 . Thus, in the presence of a catalyst, the reaction proceeds along a more energetically favorable path, which allows the process to be carried out at a higher rate.
The initial (I) and final (II) energy states of the reaction system in the presence of a catalyst and without it remain the same; Consequently: the catalyst cannot change the state of chemical equilibrium, which is independent of the reaction path.
The role of the catalyst is only to change the rate at which the equilibrium state is reached. A catalyst can increase the rate of only those processes that are allowed thermodynamically, but cannot initiate thermodynamically impossible reactions.
Some chemical reactions without a catalyst are practically impossible due to too high an activation energy. It would seem that in order to overcome a high energy barrier, it is possible to increase the kinetic energy of molecules, that is, increase the temperature. But for many reversible exothermic reactions, an increase in temperature leads to a shift in the equilibrium in the opposite direction and makes the reaction unresolved thermodynamically. In such cases, the use of catalysts is not only justified, but also necessary. Catalyst reduces activation energy and thus allows it to be carried out at significantly lower temperatures.
Reagent molecules are adsorbed on the catalyst surface. Adsorption is a phenomenon associated with a decrease in the amount of gas when a gas (adsorbate) comes into contact with a solid (adsorbent), and consists in some compaction of the gas on the surface of the solid. Physical adsorption and chemisorption are distinguished depending on the nature of the forces that cause this concentration of adsorbate molecules at the surface of a solid. If these forces are of the same nature as the molecular action in gases, liquids and solids, then one speaks of physical adsorption. At chemisorption interaction forces of a chemical nature are manifested - the adsorbate molecules lose their individuality, forming surface compounds with the adsorbent.
In the course of catalytic processes, the main role belongs to chemisorption, or activated adsorption, which results in the formation activated adsorption complex- an unstable intermediate between the reactant and the catalyst. The stage of activated adsorption determines the specificity of the action of catalysts in relation to various reactions. If the chemical bond of the reactant with the adsorbent is too strong, the destruction of the complex formation leading to the formation of products is more difficult. If the bond between the adsorbent and the adsorbate is too weak, close in nature to physical adsorption, then there is no loosening of bonds in the adsorbate molecule, which leads to a decrease in the activation energy of the catalytic process compared to the non-catalytic one.
General chemical technology, Kutepov A.M., Moscow, Higher School, 1990, pp. 206-207, 214, 205
Chemistry is the science of substances and their transformations, as well as methods for obtaining them. Even in a regular school curriculum, such an important issue as types of reactions is considered. The classification that schoolchildren are introduced to at the basic level takes into account the change in the degree of oxidation, the phase of the course, the mechanism of the process, etc. In addition, all chemical processes are divided into non-catalytic and catalytic reactions. Examples of transformations taking place with the participation of a catalyst are encountered by a person in ordinary life: fermentation, decay. Non-catalytic transformations are much rarer for us.
What is a catalyst
This is a chemical substance that is able to change the rate of interaction, but does not itself participate in it. In the case when the process is accelerated with the help of a catalyst, we are talking about positive catalysis. In the event that a substance added to the process reduces it, it is called an inhibitor.
Types of catalysis
Homogeneous and heterogeneous catalysis differ in the phase in which the starting materials are located. If the initial components taken for interactions, including the catalyst, are in the same state of aggregation, homogeneous catalysis proceeds. In the case when substances of different phases take part in the reaction, heterogeneous catalysis occurs.
Selectivity of action
Catalysis is not just a means of increasing the productivity of equipment, it has a positive effect on the quality of the resulting products. This phenomenon can be explained by the fact that due to the selective (selective) action of most catalysts, the direct reaction is accelerated, side processes are reduced. In the end, the resulting products are of high purity, there is no need to further purify the substances. The selectivity of the catalyst action gives a real reduction in non-production costs of raw materials, a good economic benefit.
Benefits of using a catalyst in production
What else characterizes catalytic reactions? Examples from a typical high school show that the use of a catalyst allows the process to be carried out at lower temperatures. Experiments confirm that it can be used to significantly reduce energy costs. This is especially important in modern conditions, when there is a lack of energy resources in the world.
Examples of catalytic productions
What industries use catalytic reactions? Examples of such industries: the manufacture of nitric and sulfuric acids, hydrogen, ammonia, polymers. Catalysis is widely used in the production of organic acids, monohydric and phenol, synthetic resins, dyes, and medicines.
What is a catalyst
Many substances that are in the periodic table of chemical elements of Dmitry Ivanovich Mendeleev, as well as their compounds, can act as catalysts. Among the most common accelerators are: nickel, iron, platinum, cobalt, aluminosilicates, manganese oxides.
Features of catalysts
In addition to selective action, catalysts have excellent mechanical strength, they are able to withstand catalytic poisons, and are easily regenerated (recovered).
According to the phase state, catalytic is divided into gas-phase and liquid-phase.
Let us consider these types of reactions in more detail. In solutions, hydrogen cations H +, hydroxide ions of the base OH-, metal cations M + and substances that contribute to the formation of free radicals act as an accelerator of chemical transformation.
The essence of catalysis
The mechanism of catalysis in the interaction of acids and bases is that there is an exchange between the interacting substances and the catalyst with positive ions (protons). In this case, intramolecular transformations take place. These types of reactions are:
- dehydration (water detachment);
- hydration (attachment of water molecules);
- esterification (formation of an ester from alcohols and carboxylic acids);
- polycondensation (formation of a polymer with the elimination of water).
The theory of catalysis explains not only the process itself, but also possible side transformations. In the case of heterogeneous catalysis, the process accelerator forms an independent phase, some centers on the surface of the reactants have catalytic properties, or the entire surface is involved.
There is also a microheterogeneous process, which involves the presence of a catalyst in a colloidal state. This variant is a transitional state from a homogeneous to a heterogeneous type of catalysis. Most of these processes take place between gaseous substances using solid catalysts. They can be in the form of granules, tablets, grains.
Distribution of catalysis in nature
Enzymatic catalysis is quite widespread in nature. It is with the help of biocatalysts that the synthesis of protein molecules proceeds, the metabolism in living organisms is carried out. Not a single biological process that occurs with the participation of living organisms bypasses catalytic reactions. Examples of vital processes: synthesis of proteins specific to the body from amino acids; breakdown of fats, proteins, carbohydrates.
Catalysis algorithm
Consider the mechanism of catalysis. This process, which takes place on porous solid chemical interaction accelerators, includes several elementary stages:
- diffusion of interacting substances to the surface of the catalyst grains from the core of the flow;
- diffusion of reagents in the pores of the catalyst;
- chemisorption (activated adsorption) on the surface of a chemical reaction accelerator with the appearance of chemical surface substances - activated complexes "catalyst-reagents";
- rearrangement of atoms with the appearance of surface combinations "catalyst-product";
- diffusion in the pores of the product reaction accelerator;
- diffusion of the product from the grain surface of the reaction accelerator into the core of the flow.
Catalytic and non-catalytic reactions are so important that scientists have continued research in this area for many years.
With homogeneous catalysis, there is no need to construct special structures. Enzymatic catalysis in the heterogeneous version involves the use of various and specific equipment. For its flow, special contact apparatuses have been developed, subdivided according to the contact surface (in tubes, on walls, catalyst grids); with a filter layer; weighed layer; with moving pulverized catalyst.
Heat exchange in devices is realized in different ways:
- by using remote (external) heat exchangers;
- using heat exchangers built into the contact apparatus.
Analyzing formulas in chemistry, one can also find such reactions in which one of the final products, which is formed during the chemical interaction of the initial components, acts as a catalyst.
Such processes are usually called autocatalytic, the phenomenon itself in chemistry is called autocatalysis.
The rate of many interactions is associated with the presence of certain substances in the reaction mixture. Their formulas in chemistry are most often missed, replaced by the word "catalyst" or its abbreviated version. They are not included in the final stereochemical equation, since they do not change from a quantitative point of view after the completion of the interaction. In some cases, small amounts of substances are sufficient to significantly affect the speed of the process. Situations are also quite acceptable when the reaction vessel itself acts as an accelerator of chemical interaction.
The essence of the influence of the catalyst on the change in the rate of the chemical process is that this substance is included in the composition of the active complex, and therefore changes the chemical interaction.
During the decomposition of this complex, the regeneration of the catalyst is observed. The bottom line is that it will not be spent, it will remain in the same amount after the end of the interaction. It is for this reason that a small amount of the active substance is quite sufficient to carry out the reaction with the substrate (reactive substance). In reality, insignificant amounts of catalysts are still consumed during the process, since various side processes are possible: its poisoning, technological losses, and a change in the state of the surface of the solid catalyst. Chemistry formulas do not include a catalyst.
Conclusion
Reactions in which the active substance (catalyst) takes part surround a person, and they also take place in his body. Homogeneous reactions are much less common than heterogeneous interactions. In any case, the formation of intermediate complexes occurs first, which are unstable, are gradually destroyed, and regeneration (recovery) of the accelerator of the chemical process is observed. For example, when metaphosphoric acid reacts with potassium persulfate, hydroiodic acid acts as a catalyst. When it is added to the reactants, a yellow solution is formed. As you approach the end of the process, the color gradually disappears. In this case, iodine acts as an intermediate product, and the process occurs in two stages. But as soon as metaphosphoric acid is synthesized, the catalyst returns to its original state. Catalysts are indispensable in industry, they help speed up transformations and obtain high-quality reaction products. Impossible without their participation and biochemical processes in our body.
Catalysisis the process of changing the rate of a chemical reaction by catalysts- that take part in a chemical reaction, but are not included in the composition of the final products and are not consumed as a result of the reaction.
Some catalysts speed up the reaction ( positive catalysis ), others slow down ( negative catalysis ). Negative catalysis is called inhibition, and catalysts that decrease the rate of a chemical reaction inhibitors.
Distinguish between homogeneous and heterogeneous catalysis.
homogeneous catalysis.
In homogeneous (homogeneous) catalysis, the reactants and the catalyst are in the same place and there is no interface between them. An example of homogeneous catalysis is an oxidation reaction SO2 and SO 3 in the presence of a catalyst NO(the reactants and the catalyst are gases).
heterogeneous catalysis.
In the case of heterogeneous (inhomogeneous) catalysis, the reactants and the catalyst are in different states of aggregation and there is an interface (boundary) between them. Typically, the catalyst is a solid and the reactants are liquids or gases. An example of heterogeneous catalysis is oxidation NN 3 before NO in the presence Pt(catalyst is a solid).
The mechanism of action of catalysts
The action of positive catalysts is reduced to a decrease in the activation energy of the reaction E a(ref) , the action of inhibitors is the opposite.
So for the reaction 2 HI=H2+I 2 E a (ref) \u003d 184 kJ / mol. When this reaction proceeds in the presence of a catalyst Au or Pt, then E a (ref) \u003d 104 kJ / mol, respectively.
The mechanism of action of a catalyst in homogeneous catalysis is explained by the formation of intermediate compounds between the catalyst and one of the reactants. Next, the intermediate compound reacts with the second starting material, resulting in the formation of the reaction product and the catalyst in its original form. Since the rate of both intermediate processes is much higher than the rate of the direct process, the reaction with the participation of a catalyst proceeds much faster than without it.
For example, the reaction:
SO 2 +1/2O 2 =SO 3 proceeds very slowly, and if you use a catalyst NO
then the reactions NO + 1 / 2O 2 \u003dNO 2 and NO2+SO2 =SO3+NO flow quickly.
The mechanism of catalyst action in heterogeneous catalysis is different. In this case, the reaction proceeds due to adsorption molecules of reacting substances by the surface of the catalyst (the surface of the catalyst is not uniform: it has the so-called active centers , on which particles of the reacting substances are adsorbed.). An increase in the rate of a chemical reaction is achieved mainly due to a decrease in the activation energy of adsorbed molecules, and also, in part, due to an increase in the concentration of reactants in places where adsorption has occurred.
Catalytic poisons and promoters.
Some substances reduce or completely destroy the activity of the catalyst, such substances are called catalytic poisons. For example, small impurities of sulfur (0.1%) completely stop the catalytic action of the metal catalyst (sponge iron) used in the synthesis of ammonia. Substances that increase the activity of a catalyst are called promoters. For example, the catalytic activity of sponge iron increases significantly with the addition of about 2% potassium metaaluminate. KALO 2.
Application of catalysts
The action of the catalyst is selective and specific. This means that different products can be obtained from the same substances by using different catalysts. This is especially true for the reactions of organic substances. For example, in the presence of a catalyst AlO 3 dehydration of ethyl alcohol occurs, in the presence of Cu– dehydrogenation:
Biological catalysts that take part in complex chemical transformations occurring in the body are called enzymes.
Catalysts are widely used in the production of sulfuric acid, ammonia, rubber, plastics, and other substances.
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 more powerful and less risky in speeding 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, in the process of 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 crystals of potassium permanganate 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 on this new path the activation energy is lower 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 reactants 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 differ fundamentally 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(Greek κατάλυσις goes back to καταλύειν - destruction) - selective acceleration of one of the possible thermodynamically allowed directions of a chemical reaction under the action of a catalyst (s), which repeatedly enters into an intermediate chemical interaction with the participants in the reaction and restores its chemical composition after each cycle of intermediate chemical interactions. The term "catalysis" was introduced in 1835 by the Swedish scientist Jöns Jakob Berzelius. nitric acid, etc.). Most of all industrial reactions are catalytic. The case when the catalyst is one of the reaction products or its starting materials is called autocatalysis. The catalyst changes the reaction mechanism to an energetically more favorable one, that is, it reduces the activation energy. The catalyst forms an intermediate compound with a molecule of one of the reactants, in which the chemical bonds are weakened. This facilitates its reaction with the second reagent. It is important to note that catalysts accelerate reversible reactions in both the forward and reverse directions. Therefore, they do not shift the chemical equilibrium [
Application of catalysis in industry
Hydrogenation[edit | edit wiki text]
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, coal is gasified, that is, it is reacted with water vapor and oxygen and a mixture of hydrogen and carbon oxides is obtained. 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.
acid catalysis[edit | edit wiki text]
The catalytic activity of a large class of catalysts is due to their acidic properties. According to J. 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.
According to the influence on the reaction rate, catalysis is divided into positive (the reaction rate increases) and negative (the reaction rate decreases). In the latter case, an inhibition process takes place, which cannot be considered "negative catalysis", since the inhibitor is consumed during the reaction.
Catalysis happens homogeneous and heterogeneous(contact). In homogeneous catalysis, the catalyst is in the same phase as the reactants, while heterogeneous catalysts differ in phase.
homogeneous catalysis[edit | edit wiki text]
An example of homogeneous catalysis is the decomposition of hydrogen peroxide in the presence of iodine ions. The reaction proceeds in two stages:
H 2 O 2 + I → H 2 O + IO
H 2 O 2 + IO → H 2 O + O 2 + I
In homogeneous catalysis, the action of the catalyst is due to the fact that it interacts with the reactants to form intermediate compounds, which leads to a decrease in the activation energy.
heterogeneous catalysis[edit | edit wiki text]
In heterogeneous catalysis, the acceleration of the process usually occurs on the surface of a solid body - the catalyst, so the activity of the catalyst depends on the size and properties of its surface. In practice, the catalyst is usually supported on a solid porous support.
The mechanism of heterogeneous catalysis is more complicated than that of homogeneous catalysis. The mechanism of heterogeneous catalysis includes five steps, all of which are reversible.
1. Diffusion of reactants to the surface of a solid
2. Physical adsorption on the active sites of the surface of a solid substance of reacting molecules and then their chemisorption
3. Chemical reaction between reacting molecules
4. Desorption of products from the catalyst surface
5. Diffusion of the product from the catalyst surface into the general flow
An example of heterogeneous catalysis is the oxidation of SO 2 to SO 3 on a V 2 O 5 catalyst in the production of sulfuric acid (contact method).
The course of the reaction on the surface of the catalyst can be demonstrated in an experiment in which a plate of platinum is heated in the flame of a gas burner, then the flame is extinguished and a jet of gas from the burner is blown onto the plate, while the plate again becomes red-hot - methane oxidation occurs on the surface of the metal.
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