catalytic reaction, Catalysis changes the rate of a chemical reaction without affecting the chemical equilibrium. The effect of a catalyst on changing the rate of a chemical reaction is called catalysis, which is essentially a chemical action.
A chemical reaction carried out with the participation of a catalyst is called a catalytic reaction. Catalysis is an important phenomenon that exists in the natural world. Catalysis affects almost the entire field of chemical reactions.
Definition of Catalysis
Catalysis is a reaction in which the free energy required for the reaction is changed by the catalyst, the chemical reaction rate of the reactants is changed, and the amount and quality of the catalyst are not changed before and after the reaction.
For a chemical reactant to undergo a chemical reaction, its chemical bond must be changed. Changing or breaking the chemical bond requires a certain amount of energy support.
The minimum energy threshold required to change the chemical bond is called activation free energy, and the catalyst changes the chemical reaction The activation free energy of the material, in turn, affects the reaction rate.
Positive catalysts can accelerate the reaction, negative catalysts or inhibitors can react with the reactants and reduce the chemical reaction. Materials that can increase the activity of a catalyst are called promoters, those that reduce the activity of a catalyst are called catalytic poisons.
Principle
Reduce activation energy
During the catalytic reaction, at least one kind of reactant molecule and the catalyst have to undergo some form of chemical interaction. Due to the intervention of the catalyst, the chemical reaction changes the way of progress, and the new reaction pathway requires lower activation energy, which is why catalysis can increase the rate of chemical reactions.
For example, the chemical reaction A + B → AB, the required activation energy is E. With the participation of catalyst C, the reaction proceeds in the following two steps:
A + C → AC, the required activation energy is E 1.
AC + B → AB + C, the required activation energy is E 2.
E1 and E2 are both smaller than E (see picture). Catalyst C only temporarily involved in the chemical reaction. After the reaction, catalyst C was regenerated.
According to the Arrhenius equation k = A e -E / RT (where k is the reaction rate constant at temperature T, A is the former factor, also known as Arrhenius constant, the unit is the same as k, R is the gas constant, kJ / mol · K, T is the thermodynamic temperature, K, E is the activation energy, kJ / mol ).
The reaction rate expressed by the reaction rate constant k is mainly determined by the reaction activation energy E. If the activation energy of the reaction is catalyzed reduction [Delta] E, the reaction rate increase i.e. E -ΔE / RT times. The catalytic reaction can generally reduce the activation energy by about 41.82 kJ / mol. If the reaction is carried out at 300K, the reaction rate can be increased by about 1.7 × 10 times.
Mode of Action
How the catalyst and reactant molecules react chemically depends on the nature of the catalyst and the reactant molecules.
Experiments have shown that acid-catalyzed reactions of organic compounds are generally carried out through the mechanism of carbocations, alkali-catalyzed reactions are catalyzed by anions such as OH, RO, and RCOO, for example:
CH3COOC2H5 + OH → CH3COOH + C2H5O
C2H5O + H2O → C2H5OH + OH
The role of transition metal compound catalysts in homogeneous catalytic reactions is complex catalysis; sodium alkoxide catalyzes the polymerization of butadiene through a free radical mechanism, such as:
C4H6 + NaR → R· + C4H6Na
C4H6Na· + nC4H6 → Na (C4H6)n + 1
Performance
① Catalytic activity: The catalyst participates in the chemical reaction, which reduces the activation energy of the chemical reaction and greatly accelerates the rate of the chemical reaction. This shows that the catalyst has a catalytic activity. The rate of the catalytic reaction is a measure of the activity of the catalyst. Activity is the most important index to evaluate the quality of the catalyst.
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② Selectivity: A catalyst only has a significant acceleration effect on one type of reaction, and it has very little acceleration effect on other reactions, even no acceleration effect. This property is catalyst selectivity. The selectivity of the catalyst determines the directivity of the catalysis. The direction of the chemical reaction can be controlled or changed by selecting different catalysts.
③ Life or stability: The stability of the catalyst is expressed by life. It includes thermal stability, mechanical stability and anti-toxic stability.
Catalytic Species
Homogeneous catalysis
The catalyst and the reactant are catalyzed in a homogeneous phase. There are liquid phase and gas phase homogeneous catalysis. The catalysis of liquid acid-base catalysts,
Soluble transition metal compound catalysts and gaseous molecular catalysts such as iodine and nitric oxide belong to this category.
Homogeneous catalysts have relatively homogeneous active centers, high selectivity, and few side reactions. It is easy to study the role of the catalyst by methods such as spectroscopy, spectroscopy, and isotope tracking. The reaction kinetics are generally not complicated.
However, homogeneous catalysts have the disadvantage of being difficult to separate, recover and regenerate.
Heterogeneous catalysis
Heterogeneous catalysis occurs at the interface of two phases. Usually, the catalyst is a porous solid and the reactants are liquid or gas. Heterogeneous catalytic reactions can generally be carried out in the following seven steps:
① external diffusion of reactants-diffusion of the reactants to the outer surface of the catalyst.
② internal diffusion of reactants-diffusion of reactants on the outer surface of the catalyst into catalyst holes
③ chemical adsorption of reactants
④ surface chemical reaction
⑤ product desorption
⑥ product internal diffusion
⑦ product external diffusion.
The slowest step in this series of steps is called the rate control step. Chemisorption is the most important step. Chemisorption activates the reactant molecules and reduces the activation energy of the chemical reaction.
Therefore, if the catalytic reaction is to proceed, at least one reactant molecule must be chemisorbed on the catalyst surface. The surface of the solid catalyst is uneven, and only a few points on the surface activate the reactant molecules. These points are called active centers.
Biphasic catalysis
Heterogeneous catalysis is an independent chemical reaction. It has both the temperature of homogeneous catalysis and the speed of heterogeneous catalysis. It also has controllable directivity.
During the reaction, catalysis is carried out in all directions, resulting in thousands of times faster reaction speed. Due to the doubling of its catalytic capacity, it can move hydrogen and oxygen from carbohydrates, which is the scientific basis for converting industrial and biological waste into a one-step process into standard gasoline and diesel.
Biocatalysis
Enzymes are biological catalysts, chemical changes in all organisms in almost all enzymatic performed under the catalysis of an enzyme called biocatalysis. The enzyme has high catalytic activity and strong selectivity.
Biocatalysis is carried out at a normal temperature and neutral conditions. High temperatures, strong acids and strong bases will cause the enzyme to lose its activity. The isolated enzyme still has catalytic activity and can be made into various enzyme preparations for medical and industrial and agricultural production.
Metal catalysis
Metal catalysts are mainly used in dehydrogenation and hydrogenation reactions. Some metals also have catalytic activity for oxidation and reforming. Metal catalysts mainly refer to certain transition metals of 4, 5, and 6 cycles, such as iron, gold, platinum, palladium, rhodium, iridium, and the like.
Metal catalysis is mainly determined by the electronic structure of metal atoms, especially the ability of d-orbital electrons and d-empty orbitals that do not participate in metal bonds to form adsorption bonds with the adsorbed molecules. Therefore, the chemical adsorption capacity and d-orbital percentage of metal catalysts are the main factors determining catalytic activity.
Metal oxide catalysts
It mainly refers to transition metal oxide catalysis, and non-transition metal oxide catalysis has been classified as acid-base catalysis. Transition metal oxide catalysts are widely used in reactions such as oxidation, hydrogenation, dehydrogenation, polymerization, and addition.
Practical metal oxide catalysts are often a mixture of multi-component oxides. Many metal oxide catalysts are semiconductors, and their chemical composition is mostly non- stoichiometric.
Therefore, the catalyst components are very complex. The conductivity and work function of metal oxide catalysts, the d- electron configuration of metal ions, the lattice oxygen characteristics in oxides, the semiconductor electronic energy band, and the surface adsorption capacity of the catalyst are all related to the catalytic activity of the catalyst.
Coordination (Complexation) catalysis
Metals, especially transition metals and their compounds, have the strong complexing ability and can form many types of complexes. Some molecules are easy to carry out a specific reaction after they are complexed with a metal (or metal ion).
This reaction is called coordination (complexation) catalytic reaction, and the metal or its compound functions as a complexation catalyst. Research and application of transition metal complex catalysts in solution as homogeneous catalysts are more.
Transition metal complex catalysis is generally coordination (complexation) catalysis, that is, the catalyst complexes the activated reactant molecules on its empty coordination. Complex catalysts are generally metal complexes or compounds, such as complexes of palladium, rhodium, titanium, and cobalt.
Acid-base catalysis
The catalysis of Arrhenius acid-base, Brunsted acid-base, and Lewis acid-base (see acid-base theory) are all acid-base catalysis. Acid-base catalysis can be divided into homogeneous catalysis and heterogeneous catalysis. Many ionic organic reactions, such as hydrolysis, hydration, dehydration, condensation, esterification, rearrangement, etc., can often be catalyzed by acid-base homogeneity.
Heterogeneous catalysis represented by solid acid catalysts is widely used in catalytic cracking, isomerization, alkylation, dehydration, hydrogen transfer, disproportionation, polymerization and other reactions.
Application
Industrial applications
The great achievements of the modern chemical industry are inseparable from the use of catalysts. About 90% of the products of the chemical industry are produced by means of catalytic processes.
For example, starting from coal and petroleum resources, basic organic raw materials such as methanol, ethanol, acetone, and butanol have been synthesized, which has changed the way of food production in the past; the production of synthetic fibers has reduced human dependence on cotton; the development of plastics has reduced human Dependence on wood.
The production of synthetic rubber, fertilizer, medicine, synthetic food, and condiments is inseparable from the use of catalysts.
For example, sulfuric acid production has lower product concentration, more impurities, and lower yield than the lead chamber method using nitrogen dioxide as a catalyst, using platinum as a catalyst can make sulphuric acid product concentrations above 98%, and fuming sulfuric acid can be produced, After using vanadium as a catalyst, the product quality is greatly improved, and the cost is greatly reduced.
Another example is the catalytic cracking in the oil refining industry. After replacing the amorphous silica-alumina catalyst with a molecular sieve catalyst, the distribution of cracked products is changed due to the shape-selective effect of the molecular sieve, and a high-quality product is obtained.
Ecological application
Dispose of all types of waste.
Carbon dioxide + waste plastic tires → gasoline and diesel + combustible gas + carbon black, which solves the air environment blockage and converts ground waste into energy, coal + ground agriculture, forestry, animal husbandry, urban domestic waste, urban industrial waste → Gasoline diesel + combustible gas + carbon black not only solves the problem of ground pollution, blockage of ecological passages on the ground, and CO2 emissions from coal but also converts coal and ground waste into urgently needed gasoline and diesel base oil.
The low carbon emissions of combustible gases and natural gas are at one level: the emitted carbon emissions are 16%, and the carbon emissions of natural gas are 12%.
Optimize the industrial structure of fossil energy.
Using advanced catalysis technology and bionic energy process methods, the refining industry is transformed into a resource-saving industrial structure. Petroleum → petrol + diesel + combustible gas + carbon black, use high-tech means to break the monopoly, form a resource-saving industry, and reduce the cost of underground fossil energy. Compared with traditional refining, the equipment cost is (1/5), the production cost is (1/2), and more output comes from the biomass in petroleum.
