Question

Explain the mechanism of heterogeneous catalysis?

Answer 1

In heterogeneous catalysis, the catalyst is present in a different phase than the reactants, typically a solid catalyst with gaseous or liquid reactants. The mechanism is explained by the adsorption theory, which states that heterogeneous catalysis occurs through the chemisorption of reactant molecules onto the surface of a solid catalyst. The surface contains active sites or free valencies where reactant molecules are adsorbed either as whole molecules or as fragments. This adsorption brings the reactant molecules close together with proper orientation, facilitating the formation of an unstable activated complex. This complex then breaks down to form the products, which desorb from the catalyst surface, freeing it for the next cycle. For example, in the contact process for the manufacture of sulphuric acid, sulphur dioxide reacts with oxygen in the presence of a solid catalyst like vanadium(V) oxide (V₂O₅) to form sulphur trioxide.

Reaction:

\[\ce{2SO2_{(g)} + O2_{(g)} -> 2SO3_{(g)}}\]

Here, SO₂ and O₂ gases are adsorbed onto the surface of V₂O₅, react to form SO₃, which then desorbs from the surface, completing the catalytic cycle.