Homogeneous catalysis • Homogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in the same phase as the reactants. ie: solid, liquid and gas Example: Sulphuric acid in esterification: CH3COOH(l) + C2H5OH(l) <==> CH3COOC2H5(l) + H20(l)
Heterogeneous catalyst • Heterogeneous catalysis is a chemistry term which describes catalysis where the catalyst is in a different phase to the reactants. (ie. solid, liquid and gas, but also oil and water)
Mode of action of catalyst • Homogenous catalysts can be recovered and unchanged at the end of the reaction they can form intermediate compound during the reaction NO(g) + O2(g) -> NO2(g) by rodox reaction the oxidation number increase from +2 to +4.
Mode of action of catalyst(2) • To turn the NO2(g) back to NO(g) we can add in sulfur dioxide 2SO2(g) + 2NO2(g) -> 2SO3(g) + 2NO(g) because:2SO2(g) + O2(g) -> 2SO3(g)
Mode of action of catalyst(3) • S2O82-(aq) + 2Fe2+(aq) -> 2SO42-(aq) + 2Fe3+(aq) • Heterogeneous catalyst tend to function by absorbing reactant molecules onto the surface of the catalyst and bringing them into close contact with each other in the connect orientation.
Choice of catalyst • The advantage of a homogeneous catalyst compared to a heterogeneous catalyst is that all of the reactants whereas in heterogeneous catalysis the efficiency of the catalyst is dependent upon the surface area • The disadvantage of a homogenous catalyst is that it is usually harder to remove the catalyst after the reaction whereas a heterogeneous catalyst can be relatively easily remove by filtration
Choice of catalyst • Selectivity – will be catalyst produce only the desired product? • Efficiency - will the catalyst cause a considerable increase in the rate? - will it continue to work well under severe conditions, such as those experienced by catalytic converters in cars, as well as mild conditions
Choice of catalyst • Environmental impact – will it be easy to dispose of the catalyst without causing harm to the environment? Many transition metal are classed as heavy metals and can cause problems if they enter the soil or ground water • Potential for poisoning – catalysts rely on reactant occupying the active site reversibly. A poison will occupy the active site irreversibly so blocking access to reactants. Poisons include carbon monoxides, cyanide ions and sulfur.
Choice of catalyst • Cost – industry is profit –based. Many transition metal such as rhodium, platinum and palladium are expensive and the cost to benefit ratio needs to be carefully calculated.