Catalysis Subhrangsu Sekhar Dey M.SC –Applied Chemistry

TOPICS Introduction Catalysis Activation Energy Types of Catalysis Homogeneous Catalysis Heterogeneous Catalysis Adsorption Phase-Transfer Catalysis Intermolecular Catalysis Enzyme Catalysis Photocatalysis Semi-conductor Catalysis Promoter and Stabilizers

Definition of catalysis INTRODUCTION Definition of catalysis A substance which changes the speed of a reaction without being used itself is called a catalyst. The phenomenon of increasing the rate of reaction by the use of catalyst is called catalysis. If а catalyst increases (accelerates) the speed of а reaction, it is called а positive catalyst and the phenomenon is called positive catalysis. On the other hand, if а catalyst decreases (retards) the speed of а reaction, it is called а negative catalyst and the phenomenon is called negative catalysis. 1

Catalysis Catalysts increase reaction rate without themselves being changed Can accelerate a reaction in both directions Do not affect the state of equilibrium of reaction simply allow equilibrium to be reached faster

Activation Energy Catalyst Affect Catalyst lowers the activation energy for both forward and reverse reactions.

4 Types of Catalysis Homogeneous Catalysis Heterogeneous Catalysis

Homogeneous Catalysts If the catalyst is present in the same phase as the reactants, it is called а homogeneous catalyst and this type of catalysis is called homogeneous catalysis. NO(g) 2 SO2(g) + О2(g) ===== SO3(g) Н+ (aq) CH3COOC2H5(l) + Н2О(1) ===== СНЗСООН(1) + C2H5OH(1) С12Н22О11 (aq) + Н2О (1) ====== С6Н12О6 (aq) + С6Н12О6 (aq) Sucrose Glucose Fructose These reactions can be explained on the basis of intermediate compound formation. According to this theory, the catalyst combines with one of the reactants to form an intermediate. Intermediate compound being unstable combines with the other reactant to form product. For example, the combination of SO2 and O2 to form SO3 is a slow process. However, in the presence of NO (catalyst), the reaction becomes fast.

Heterogeneous catalysts If the catalyst is present in а different phase than the reactants, it is called а heterogeneous catalyst and this type of catalysis is called heterogeneous catalysis. Pt, 8000С 4NH3 + 5O2 ======== 4NO + 6Н2O The heterogeneous catalysis is a surface phenomenon. It involves the following steps: Diffusion of the reactants at the surface of the catalyst. Adsorption of the molecules of the reactants at the active sites. Occurrence of the chemical reactions on the surface of the catalyst. Desorption of products molecules from the surface. Diffusion of products away from the surface of the catalyst.

Adsorption Adsorption is commonly an essential first step in heterogeneous catalysis. Adsorption is when a molecule in the gas phase or in solution binds to atoms on the solid or liquid surface. The molecule that is binding is called the adsorbate, and the surface to which it binds is the adsorbent. The process of the adsorbate binding to the adsorbent is called adsorption. The reverse of this process (the adsorbate splitting from adsorbent) is called desorption. In terms of catalyst support, the catalyst is the adsorbate and the support is the adsorbent. Two types of adsorption, Physisorption and Chemisorption The role of adsorption of reactants on the surface to helps the reaction in the following ways: 1. Adsorption increases the concentration of reactant on the surface of the catalyst. Due to increased concentration of the reactants, the reactions proceed rapidly. 2. Adsorbed molecules get dissociated to form active species like free radicals which react faster than molecules. 3. The adsorbed molecules are not free to move about and therefore, they collide with other molecules on the surface. 4. The heat of adsorption evolved acts as energy of activation for the reaction.

Phase-Transfer Catalyst a phase-transfer catalyst or PTC is a catalyst that facilitates the migration of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic reactants are often soluble in an aqueous phase but insoluble in an organic phase in the absence of the phase-transfer catalyst. The catalyst functions like a detergent for solubilizing the salts into the organic phase. Phase-transfer catalysis refers to the acceleration of the reaction upon the addition of the phase-transfer catalyst. Examples C8H17Br(org) + NaCN(aq) → C8H17CN(org) + NaBr(aq) (catalyzed by a R4P+Cl− PTC) Via the quaternary phosphonium cation, cyanide ions are "ferried" from the aqueous phase into the organic phase

Intermolecular Catalysis The acceleration of a chemical transformation at one site of a molecular entity through the involvement of another functional ('catalytic') group in the same molecular entity, without that group appearing to have undergone change in the reaction product. The use of the term should be restricted to cases for which analogous intermolecular catalysis by chemical species bearing that catalytic group is observable. Intramolecular catalysis can be detected and expressed in quantitative form by a comparison of the reaction rate with that of a comparable model compound in which the catalytic group is absent, or by measurement of the effective molarity of the catalytic group.

Enzyme Catalysis Enzyme catalysis is the increase in the rate of a chemical reaction by the active site of a protein. The protein catalyst(enzyme) may be part of a multi-subunit complex, and/or may transiently or permanently associate with a Cofactor (e.g.adenosine triphosphate). Catalysis of biochemical reactions in the cell is vital due to the very low reaction rates of the uncatalysed reactions. A key driver of protein evolution is the optimization of such catalytic activities via protein dynamics

Enzyme-substrate complex Step 1: (All of these steps are in equilibrium) Enzyme and substrate combine to form complex E + S ES Enzyme Substrate Complex Step 2: An enzyme-product complex is formed. ES EP The enzyme and product separate EP E + P

Photocatalysis In chemistry, Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generate free radicals (e.g. hydroxyl radicals: •OH) able to undergo secondary reactions. Its practical application was made possible by the discovery of water electrolysis by means of titanium dioxide. The commercially used process is called the advanced oxidation process (AOP). There are several ways the AOP can be carried out; these may (but do not necessarily) involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl radical. Two types of Photocatalysis 1.Homogeneous Photocatalysis The mechanism of hydroxyl radical production by ozone O3 + hν → O2 + O(1D) (? O3 "-" hν → O2 + O(1D) ?) O(1D) + H2O → •OH + •OH O(1D) + H2O → H2O2 H2O2 + hν → •OH + •OH

Oxidative reactions due to photocatalytic effect: 2. Heterogeneous catalysis Heterogeneous catalysis has the catalyst in a different phase from the reactants. Heterogeneous photocatalysis is a discipline which includes a large variety of reactions: mild or total oxidations, dehydrogenation, hydrogen transfer, 18O2–16O2 and deuterium-alkane isotopic exchange, metal deposition, water detoxification, gaseous pollutant removal. Oxidative reactions due to photocatalytic effect: UV + MO → MO (h + e−) Here MO stands for metal oxide --- h+ + H2O → H+ + •OH 2 h+ + 2 H2O → 2 H+ + H2O2 H2O2→ HO• + •OH The reductive reaction due to photocatalytic effect: e− + O2 → •O2− •O2− + HO•2 + H+ → H2O2 + O2 HOOH → HO• + •OH

Semi-conductor Catalysis Most common heterogeneous photocatalyts are transition metal oxides and semiconductors, which have unique characteristics. Unlike the metals which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The void region, which extends from the top of the filled valence band to the bottom of the vacant conduction band, is called the band gap. When a photon with energy equal to or greater than the materials band gap is absorbed by the semiconductor, an electron is excited from the valence band to the conduction band, generating a positive hole in the valence band. The excited electron and hole can recombine and release the energy gained from the excitation of the electron as heat. Recombination is undesirable and leads to an inefficient photocatalyst. The ultimate goal of the process is to have a reaction between the excited electrons with an oxidant to produce a reduced product, and also a reaction between the generated holes with a reductant to produce an oxidized product. Due to the generation of positive holes and electrons, oxidation-reduction reactions take place at the surface of semiconductors

Promoter and Stabilizer Promoter,  in chemistry, substance added to a solid catalyst to improve its performance in a chemical reaction. By itself the promoter has little or no catalytic effect. Some promoters interact with active components of catalysts and thereby alter their chemical effect on the catalyzed substance. The interaction may cause changes in the electronic or crystal structures of the active solid component. Commonly used promoters are metallic ions incorporated into metals and metallic oxide catalysts, reducing and oxidizing gases or liquids, and acids and bases added during the reaction or to the catalysts before being used. In chemistry Stabilizer is a chemical which tends to inhibit the reaction between two or more other chemicals. It can be thought of as the antonym to a catalyst. The term can also refer to a chemical that inhibits separation of suspensions, emulsions, and foams.Heat and light stabilizers are added to plastics and elastomers because they ensure safe processing and protect products against premature aging and weathering. The trend is towards fluid systems, pellets, and increased use of masterbatches. There are monofunctional, bifunctional, and polyfunctional stabilizers. In economic terms the most important product groups on the market for stabilizers are compounds based on calcium (calcium-zinc and organo-calcium), lead, and tin stabilizers as well as liquid and light stabilizers (HALS, benzophenone, benzotriazole). Cadmium-based stabilizers largely vanished in the last years due to health and environmental concerns.

Thank You