Chapter 2 CATALYSIS
Course Syllabus Definitions Characteristics of Catalytic Materials Types of Catalysis Homogeneous Catalysis Theories of Acid – Base Catalysts Heterogeneous Catalysis Different Theories of Heterogeneous Catalytic Reactions
• CATALYSIS comes from two Greek words, the prefix CATA- meaning down and the verb LYSEIN, meaning to split or break. “A catalyst breaks down the normal forces which inhibit the reactions of molecules”
Catalysis & Catalysts Facts a Fa s and Fi Figures a abou bout Ca t Catal talysi sis Life cycle on the earth Catalysts (enzyme) participates most part of life cycle (e.g. forming, growing, decaying) Catalysis contributes great part in the processes of converting sun energy to various other forms of energies Catalysis plays a key role in maintaining our environment (e.g. photosynthesis by plant) CO 2 + H 2 O=HC + O 2 Chemical Industry ca. $2 bn annual sale of catalysts ca. $200 bn annual sale of the chemicals that are related products 90% of chemical industry has catalysis-related processes Catalysts contributes ca. 2% of total investment in a chemical process
What is Catalysis: Definitions Catalyst Is a substance which accelerates the rate of approach to equilibrium of a chemical reaction without being substantially consumed in the reaction. Catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction. A catalyst is defined as a substance which alters the rate of a chemical reaction, itself remaining chemically unchanged at the end of the reaction. The process is called Catalysis. A catalyst which enhances the rate of a reaction is called a Positive catalyst and the process Positive catalysis or simply Catalysis. A catalyst which retards the rate of a reaction is called a Negative catalyst and the process Negative catalysis. Inhibitor Is a substance that decreases the rate of the reaction; it was formerly called a negative catalyst .
Threekeyas pectso fcatalys tactio n T aking part in the reaction it will change itself during the process by interacting with other reactant/product molecules A ltering the rates of reactions in most cases the rates of reactions are increased by the action of catalysts; however, in some situations the rates of undesired reactions are selectively suppressed R eturning to its original form After reaction cycles a catalyst with exactly the same nature is ‘reborn’ In practice a catalyst has its lifespan - it deactivates gradually during use
appliCation of Catalysis Industrial applications Almost all chemical industries have one or more steps employing catalysts Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals, ….. Advantages of catalytic processes Achieving better process economics and productivity Increase reaction rates - fast Simplify the reaction steps - low investment cost Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption Reducing wastes Improving selectivity toward desired products - less raw materials required, less unwanted wastes Replacing harmful/toxic materials with readily available ones Producing certain products that may not be possible without catalysts Having better control of process (safety, flexible etc.) Encouraging application and advancement of new technologies and materials And many more …
Environmental applications Pollution controls in combination with industrial processes Pre-treatment - reduce the amount waste/change the composition of emissions Post-treatments - once formed, reduce and convert emissions Using alternative materials Pollution reduction gas - converting harmful gases to non-harmful ones liquid - de-pollution, de-odder, de-colour etc solid - landfill, factory wastes And many more … Other applications Catalysis and catalysts play one of the key roles in new technology development.
General CharaCteristiCs of Catalysts 1. The Catalyst is unchanged chemically at the end of the reaction (Stability) The amount of the catalyst should not be changed at the end of the process, unchanged chemically although it frequently changes physically. Example: Manganese dioxide catalyzes the potassium chlorate decomposition; changes from large crystals to fine powder (Physical change). A good catalyst should resist to deactivation, caused by the presence of impurities in feed (e.g. lead in petrol poison TWC. thermal deterioration, volatility and hydrolysis of active components attrition due to mechanical movement or pressure shock
2. A small amount of catalyst is often sufficient to bring about a considerable extent of reaction A small amount of catalyst will often cause large quantities of material to react. Example: Cupric ions (Cu 2+ ) at a concentration of 1g ion in 109 liters accelerate the oxidation of sodium sulphite solution by atmospheric oxygen. Example: Some catalysts need to be present in relatively large amount to be effective. (Friedel-Crafts reaction), + → + AlCl C H C H Cl C H C H HCl 3 6 6 2 5 6 5 2 5 anhydrous AlCl 3 functions as a catalyst effectively when present to the extent of 30% of the mass of benzene.
3. A catalyst does not affect the final position of equilibrium, although it shortens the time required to establish the equilibrium o A catalyst does not alter the position of equilibrium for a reversible process. o It must affect the direct and reverse reactions to the same extent. o The ratio of the rates of two opposing reactions i.e. , the equilibrium constant, remains unchanged. Example: Sulphur trioxide vapor decomposes readily in the presence of Example: Haber Process platinum, which is also the better + ← → Fe N 3 H 2 NH catalyst for the combination of 2 2 3 sulphur dioxide and oxygen.
4. The catalyst is specific in its action While a particular catalyst works for one reaction, it will not necessarily work for another reaction. Different catalysts, moreover, can bring about completely different reactions for the same substance. + → + Ni CO 3 H CH H O 2 4 2 Example 1 + → Cu / ZnO CO 2 H CH OH 2 3 Example 2 → = + hot Al O C H OH CH CH H O (dehydrati on) 2 3 2 5 2 2 2 → + hot Cu C H OH CH CHO H (dehydroge nation) 2 5 3 2
5. A catalyst provides an alternative route for the reaction with lower activation energy "A catalyst provides an alternative route for the reaction with lower activation energy“ It does not "lower the activation energy of the reaction"
Catalysis Action: Reaction Kinetics Without cat Energy and Mechanism With cat Catalyst action leads to change of the rate of the reaction by E A A)Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that • The reaction activation energy is altered • The intermediates formed are E A ,cat different from those formed in non-catalytic reaction • The rates of reactions are React altered (both desired and X undesired ones) B) Reactions proceed under less Product demanding conditions Y . Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials Reaction coordinate
6. A catalyst is more effective when finely divided A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure. In heterogeneous catalysis, the solid catalyst is more effective when in a state of fine subdivision than it is used in bulk. A lump ﺔﻌﻄﻗةﺮﯿﺒﻛ of platinum will have much less catalytic activity than colloidal or platinised asbestos. Finely divided nickel is a better catalyst than lumps of solid nickel.
Typ ypes o es of Ca Catalyst CLASSIFICATION BASED CLASSIFICATION BASED ON THE SUBSTANCES FROM ON THE PHYSICAL STATE WHICH A CATALYST IS MADE A catalyst can be gas, liquid or solid Organic (organic Inorganic (gases, metals, acids, enzymes etc.) metal oxides, inorganic acids, bases etc.) CLASSIFICATION BASED ON THE CATALYSTS ACTION Electrocatalysts etc. Acid – base Catalysts Photocatalysts Enzymatic Catalysts
Classification of Catalytic Reactions Classification based on the ways catalysts work Homogeneous Heterogeneous No phase boundary exists Phase Boundary separates both catalyst and all catalyst from the reactants reactants / products Enzymatic reaction system involves multi- are in the same phase (catalysts + reactants / phase (gas or liq) Complex organic products) molecules, usually protein, which form a lyophilic colloid
Recommend
More recommend
