Catalysts

Learning Objectives Catalysts can be classified as either heterogeneous or homogeneous. Catalysts can be classified as either heterogeneous or homogeneous. Heterogeneous catalysts work by the adsorption of reactant molecules. Heterogeneous catalysts work by the adsorption of reactant molecules. The surface activity of a catalyst can be reduced by poisoning. The surface activity of a catalyst can be reduced by poisoning. Impurities in the reactants result in the industrial catalysts having to be regenerated or renewed. Impurities in the reactants result in the industrial catalysts having to be regenerated or renewed. Catalytic converters are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen. Catalytic converters are fitted to cars to catalyse the conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen.

Definition of Catalysts Speed up the rate of a chemical reaction Speed up the rate of a chemical reaction Are not chemically changed or used up in the reaction Are not chemically changed or used up in the reaction May be specific for a particular reaction May be specific for a particular reaction

Types of catalysts Homogeneous catalysts – reactants and catalyst in same state Homogeneous catalysts – reactants and catalyst in same state Heterogeneous catalysts – reactants and catalysts in different states Heterogeneous catalysts – reactants and catalysts in different states

How do heterogeneous catalysts work? Catalyst is usually solid Catalyst is usually solid Reactants are adsorbed onto surface Reactants are adsorbed onto surface Adsorb – formation of weak bonds Adsorb – formation of weak bonds

Catalyst

Catalyst Reactant A

Catalyst Adsorption occurs at active site Adsorption occurs at active site

Catalyst

Catalyst Reactant B

Catalyst

Catalyst

Catalyst

Catalyst

Catalyst Products

H H C C H H H E.g Hydrogenating alkenes (turning oils into fats) H

Bonds within reactant molecules are weakened and break Bonds within reactant molecules are weakened and break H H C C H H H

New bonds form between the reactants New bonds form between the reactants H H C C H H H

H H H C C H H New bonds form between the reactant molecules H H

The product(s) leave the catalyst surface The product(s) leave the catalyst surface H H C C H H H H H H

Catalysts NUMBER OF MOLECUES WITH A PARTICULAR ENERGY MOLECULAR ENERGY EaEa NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER

Catalysts Catalysts lower the activation energy for a reaction Catalysts lower the activation energy for a reaction EaEa EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER NUMBER OF MOLECUES WITH A PARTICULAR ENERGY MOLECULAR ENERGY

EXAMPLES OF CATALYSTS Metals Ni, Pthydrogenation reactionsMetals Ni, Pthydrogenation reactions FeHaber Process FeHaber Process Rh, Pdcatalytic converters Rh, Pdcatalytic converters Format is FINELY DIVIDEDFormat is FINELY DIVIDED This increases the surface area and provides more active sitesThis increases the surface area and provides more active sites

Catalyst poisoning In heterogeneous catalysis Catalyst poisoned by a chemical binding to surface In heterogeneous catalysis Catalyst poisoned by a chemical binding to surface Poison bonds irreversibly with catalyst Poison bonds irreversibly with catalyst Sulphur and lead are two examples of catalyst poisons Sulphur and lead are two examples of catalyst poisons

Poisoning Catalyst

Poisoning Catalyst

Poisoning Active site taken up by poison Active site taken up by poison Effectiveness of catalyst reduced. Effectiveness of catalyst reduced. Catalyst

Cleaning a catalyst Catalysts can sometimes be regenerated by blowing hot air over catalyst – oxidises chemicals on surface Catalysts can sometimes be regenerated by blowing hot air over catalyst – oxidises chemicals on surface Used to remove carbon (soot) after catalytic cracking Used to remove carbon (soot) after catalytic cracking

Catalytic Converters Found in car exhaust systems Found in car exhaust systems Reduce levels of pollutant gases Reduce levels of pollutant gases Pollutants react in exhaust to form less harmful gases. Pollutants react in exhaust to form less harmful gases.