Q3(a)(i). Adsorption theory of catalysis; catalyst and equilibrium

The theory

1. A solid catalyst has active sites on its surface: atoms with unused valencies (edges, corners, defects). Gas molecules are held there by adsorption.
2. The reaction passes through five steps: diffusion of reactants to the surface; adsorption on active sites; reaction between neighbouring adsorbed species; desorption of products; diffusion of products away.
3. Adsorption raises the local concentration of reactants on the surface and holds them close together in a favourable orientation.
4. Chemisorption forms new bonds to the surface atoms, so the bonds inside the reactant are weakened or broken (H₂ is adsorbed as H atoms). The surface complex is a new activated complex of lower activation energy.
5. The products are held less strongly and desorb, releasing the sites for fresh molecules: the catalyst is regenerated. The heat of adsorption must be moderate; a reactant held too strongly blocks the sites (as a poison does).

Example and type of adsorption

Hydrogenation of ethene on finely divided nickel: CH₂=CH₂ + H₂ →CH₃-CH₃

1. H₂ is adsorbed dissociatively as two Ni–H bonds; ethene is bonded through its π electrons. Two H atoms add in turn to the adsorbed ethene, and ethane, which bonds only weakly, leaves the surface.
2. The adsorption is chemisorption: enthalpy of chemisorption of C₂H₄ on Ni is -243 kJ mol⁻¹, against about -20 kJ mol⁻¹ for physical adsorption. Only chemisorption is strong enough to break the H–H bond.

Effect of a catalyst on an equilibrium

1. The catalyst lowers the barrier of the forward and the reverse reaction by the same amount, so both rates rise by the same factor.
2. K=k_f/kᵣ is unchanged, and so is Δ G°=-RTln K: the catalyst is not used up and the end states are the same. The position of equilibrium and the yield are not altered.
3. Equilibrium is only reached sooner. In the Haber process the iron catalyst does not raise the percentage of NH₃ at a given T and p; it allows that yield to be reached quickly at a workable temperature.

Application. the same steps explain promoters (Mo in the Haber iron catalyst) and poisons (As₂O₃ on Pt in the contact process), and the catalytic converter of a car, where CO and NO are chemisorbed on Pt–Rh.