Q6(a)(i). Essential characteristics of catalysis; surface catalysis

Characteristics of every catalyst

1. New path, lower Eₐ: the catalyst provides a mechanism of lower activation energy; the rate rises because a much larger fraction of molecules can cross the barrier.
2. Unchanged at the end: it is recovered with the same mass and chemical composition (its physical form may change).
3. No change in equilibrium: it lowers the barrier for the forward and the reverse reaction equally, so K is unchanged; equilibrium is reached sooner. It cannot make a reaction with Δ G>0 go.
4. Small amount, specific: a little catalyst converts much reactant; a given catalyst acts on one reaction or a class of reactions (selectivity: CO + H₂ gives CH₃OH on Cu/ZnO but CH₄ on Ni).

Special features of surface (heterogeneous) catalysis

1. Steps: diffusion to the surface → adsorption (usually chemisorption) → reaction between adsorbed species on active sites (Langmuir–Hinshelwood) → desorption of products → diffusion away.
2. Surface area and active sites: activity grows with surface area (finely divided metals, porous supports); edges, steps and defects are the active sites.
3. Optimum adsorption strength: adsorption must be strong enough to weaken bonds in the reactant but weak enough for the product to leave. Hence the best catalysts are the d-metals in the middle of a series (volcano curve).
4. Promoters and poisons: promoters raise activity (K₂O and Al₂O₃ on Fe in the Haber process); poisons block active sites (H₂S, CO, As on Pt and Fe).

Application. iron (promoted) for N₂ + 3H₂ → 2NH₃, V₂O₅ for SO₂ → SO₃ in the contact process, Ni for hydrogenation of oils, and Pt/Rh in the car exhaust converter are all surface catalysts.