Catalyzing the effect of changing the rate of a chemical reaction without affecting chemical equilibrium. The effect of a catalyst to change the rate of a chemical reaction is called Catalysis, which is essentially a chemical action. Catalytic Converter

Catalytic

The chemical reaction carried out with the participation of a catalyst is referred to as a catalytic reaction. Catalysis is an important phenomenon that is ubiquitous in nature, and its catalysis is almost in the entire field of chemical reactions.

Catalytic Definition

The catalysis is a reaction in which the activation free energy required for the reaction is changed by a catalyst, the chemical reaction rate of the reactant is changed, and the amount and quality of the catalyst are not changed before and after the reaction.

In order for a chemical reaction to undergo a chemical reaction, it is necessary to change its chemical bond. To change or break a chemical bond requires a certain amount of energy support.

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The minimum energy threshold required to change the chemical bond is called the activation free energy, and the catalyst changes the chemical reaction. The free energy of activation of the species, in turn, affects the rate of reaction.

A positive catalyst accelerates the reaction; a negative catalyst or inhibitor reacts with the reactants to reduce the chemical reaction. Substances that increase the activity of the catalyst are referred to as promoters; those that reduce the activity of the catalyst are referred to as catalytic poisons. catalytic converter

History of development

As early as BC, China had used koji (bio-enzyme catalyst) to make wine. In the middle of the 18th century, the use of nitrogen dioxide as a catalyst in the lead-room process sulfuric acid was the beginning of industrial use of catalysts.

The word catalysis was coined in the chemistry discipline by JJ Becherius in 1835. In 1902 W. Osterwald defined catalysis as: “Accelerating chemical reactions without affecting the chemical balance.” The large-scale production of synthetic ammonia in 1910 is a milestone in the history of catalytic processes.

Since the 20th century, the catalytic process has developed rapidly. For example, in the 1920s, the Fischer-Tropsch process, which successfully synthesized a liquid fuel from carbon monoxide and hydrogen using a cobalt catalyst, was successfully studied.

In 1955, the Ziegler-Natta catalyst was successfully used for directional polymerization of olefins; More than 90% of the production processes in the chemical and refining industries use catalytic methods.

Principle

Reduce activation energy

At least one reactant molecule must undergo some form of chemistry with the catalyst during the catalytic reaction. Due to the intervention of the catalyst, the chemical reaction changes the route of conduct, and the new reaction pathway requires less activation energy, which is why catalysis can increase the rate of the chemical reaction.

Catalytic

For example, the chemical reaction A+B→AB, the required activation energy is E, and with the participation of Catalyst C, the reaction proceeds in the following two steps:

A+C→AC, the required activation energy is E1.

AC+B→AB+C, the required activation energy is E 2.

E 1 and E 2 are both smaller than E (see figure). Catalyst C only temporarily intervenes in the chemical reaction, and after the reaction is completed, the catalyst C is regenerated.

According to the Arrhenius equation k = A e – E/RT (where k is the reaction rate constant at temperature T; A is the pre-factor, also known as the Arrhenius constant, the unit is the same as k; R It is the gas constant, kJ/mol·K; T is the thermodynamic temperature, K; E is the activation energy, kJ/ mol ), and the reaction rate expressed by the reaction rate constant k is mainly determined by the activation energy E of the reaction.

When Δ E is lowered, the reaction rate is increased by e – ΔE / RT times. The catalytic reaction generally reduces the activation energy by about 41.82 kJ/mol. If the reaction is carried out at 300 K, the reaction rate can be increased by about 1.7×10 times.

Mode of action

How the catalyst interacts with the reactant molecules is related to the nature of the catalyst and the reactant molecules themselves. Experiments have shown that the acid-catalyzed reaction of organic compounds is generally carried out by a positive carbon ion mechanism; the base-catalyzed reaction is catalyzed by anions such as OH, RO, RCOO, for example:

CH3COOC 2H5 + OH → CH3 of COOH + C2 H5 O

2 H 5 O+H 2 O→C 2 H 5 OH+OH

The role of the transition metal compound catalyst in the homogeneous catalytic reaction is complex catalysis; the sodium alkoxide catalyzed the polymerization of butadiene is carried out by a free radical mechanism, for example:

4 H 6 + NaR → R·+C 4 H 6 Na·

4 H 6 Na·+ n C 4 H 6  → Na(C 4 H 6n+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 indicates that the catalyst has a catalytic activity.

The rate of the catalytic reaction is a measure of the magnitude of catalyst activity. Activity is the most important indicator for evaluating the quality of a catalyst.

2. Selectivity: A catalyst has a significant acceleration effect on only one type of reaction, and has little or no acceleration effect on other reactions. This property is the selectivity of the catalyst. The selectivity of the catalyst determines the directionality of the catalysis. The direction of the chemical reaction can be controlled or altered by selecting different catalysts.

3. Life or stability: The stability of the catalyst is expressed in terms of a lifetime. It includes thermal stability, mechanical stability, and anti-toxic stability.

Catalytic species

Homogeneous catalysis

The catalysis of the catalyst and the reactants in the same homogeneous phase. It is homogeneously catalyzed by liquid phase and gas phase. The catalysis of a liquid acid-base catalyst, a soluble transition metal compound catalyst, and a gaseous molecular catalyst such as iodine or nitrogen monoxide belongs to this category.

The active centers of homogeneous catalysts are relatively homogeneous, with high selectivity and few side reactions. It is easy to study the action of catalysts by means of spectroscopy, spectroscopy, and isotope tracing. The reaction kinetics are generally not complicated. However, homogeneous catalysts have the disadvantage of being difficult to separate, recover and regenerate.

Heterogeneous catalytic

Heterogeneous catalysis occurs at the interface of the two phases, typically the catalyst is a porous solid and the reactants are liquid or gaseous. The heterogeneous catalytic reaction can generally be carried out in the following seven steps:

1. out-diffusion of the reactants – diffusion of the reactants to the outer surface of the catalyst

2. internal diffusion of the reactants – diffusion of reactants on the outer surface of the catalyst into the pores of the catalyst;

3. chemical adsorption of the reactants:

4. surfaces chemical reaction;

5. product desorption;

6. product diffusion;

7. product out-diffusion.

The slowest step in this series of steps is called the rate control step. Chemical adsorption is the most important step. Chemical adsorption activates the reactant molecules and reduces the activation energy of the chemical reaction.

Therefore, in order to carry out the catalytic reaction, at least one reactant molecule must be chemisorbed on the surface of the catalyst. The surface of the solid catalyst is not uniform, and only a part of the surface activates the reactant molecules, and these points are called active centers.

Complex phase catalysis

Complex phase catalysis is an independent chemical reaction. It combines the temperature of homogeneous catalysis and the speed of heterogeneous catalysis. At the same time, it has controllable directionality.

During the reaction, catalysis is carried out in all directions, resulting in a reaction speed that is thousands of times faster. Due to the doubling of catalytic capacity, it can move hydrogen and oxygen from carbohydrates, which is the scientific basis for the conversion of industrial and biological waste into one-step 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 under normal temperature and neutral conditions. High temperatures, strong acids and strong bases can deactivate the enzyme. The ex vivo enzymes are still catalytically active and can be used in a variety of enzyme preparations for medical and industrial and agricultural production.

Metal catalysis

Metal catalysts are mainly used for 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, ruthenium, and the like.

Catalytic

Metal catalysis is mainly determined by the electronic structure of metal atoms, especially the ability of d-orbital electrons and d-space orbits that do not participate in metal bonds to form adsorbed bonds with adsorbed molecules. Therefore, the chemisorption capacity and d-orbital percentage of the metal catalyst are the main factors determining the catalytic activity.

Metal oxide catalysis

Mainly referred to as transition metal oxide catalysis, non-transition metal oxide catalysis has been classified as acid-base catalysis. Transition metal oxide catalysts are widely used in oxidation, hydrogenation, dehydrogenation, polymerization, addition and the like.

Useful metal oxide catalysts are often mixtures of multi-component oxides. Many metal oxide catalysts are semiconductors, the chemical composition of which is mostly non- stoichiometric, and therefore, the catalyst components are complex.

The conductivity and work function of the metal oxide catalyst, the d- electron configuration of the metal ion, the lattice oxygen property in the oxide, the semiconductor electron energy band, and the adsorption capacity of the catalyst surface are all related to the catalytic activity of the catalyst.

Coordination (complexation) catalysis

Metals, especially transition metals and their compounds, have a strong complexing ability to form various types of complexes. When a certain molecule is complexed with a metal (or metal ion), it is easy to carry out a specific reaction called coordination (complexation) catalytic reaction and the metal or it’s compound functions as a complex catalyst.

Transition metal complex catalysts have been studied and applied as homogeneous catalysts in solution. The transition metal complex catalysis is generally a coordination (complexation) catalysis whereby the catalyst complexes the activated reactant molecules in its empty coordination.

The complex catalysts are generally metal complexes or compounds such as complexes of palladium, rhodium, titanium, cobalt, and the like.

Acid-base catalysis

The catalysis of Arrhenius acid, Brenda citrate, and Lewis acid-base (see acid-base theory) is 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., are often catalyzed by acid-base homogeneous. Heterogeneous catalysis represented by a solid acid catalyst is widely used for catalytic cracking, isomerization, alkylation, dehydration, hydrogen transfer, disproportionation, polymerization, and the like.

Catalytic Application

Industrial application

The great achievements of the modern chemical industry are inseparable from the use of catalysts. About 90% of chemical industry products are produced by means of a catalytic process.

For example, the synthesis of basic organic materials such as methanol, ethanol, acetone, butanol from coal and petroleum resources 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 humans.

Dependence on wood. The production of synthetic rubber, fertilizers, medicines, synthetic foods, and condiments is inseparable from the use of catalysts.

Catalytic

For example, the production of sulfuric acid, compared with the lead chamber method using nitrogen dioxide as a catalyst, the product concentration is low, the impurities are large, and the yield is small; and the concentration of the sulfuric acid product is more than 98% by using platinum as a catalyst, and fuming sulfuric acid can be obtained.

After using vanadium as a catalyst, the quality of the product is greatly improved and the cost is greatly reduced. Another example is catalytic cracking in the refining industry. After replacing the amorphous silica-alumina catalyst with a molecular sieve catalyst, the distribution of the cracked product is changed due to the shape-selective action of the molecular sieve, and a high-quality product is obtained.

Ecological application

Handle all types of waste.

Carbon dioxide + waste plastic tires → gasoline and diesel + combustible gas + carbon black, not only solve the air environment blockage, but also convert the ground waste into energy; coal + ground agriculture, forestry, animal husbandry, urban living waste, urban industrial waste → Gasoline and diesel + combustible gas + carbon black not only solve the pollution problem of the ground, the blockage of the ground ecological channel, and the CO2 problem of coal discharge, but also convert the coal and ground waste into the much needed steam and diesel base oil. Low carbon emissions from flammable gases and natural gas are a level: flammable gases emitted, 16% carbon emissions, and 12% carbon emissions from natural gas.

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Optimize the industrial structure of fossil energy.

Using advanced catalytic technology and biomimetic energy processes, the refining industry is transformed into a resource-saving industrial structure. Oil → gasoline and diesel + combustible gas + carbon black, with high-tech means, break the monopoly, form a resource-saving industry, and reduce the cost of underground fossil energy.

Compared to traditional refining, the equipment cost is (1/5) and the production cost is (1/2), and more output comes from biomass in petroleum.