Five steps: (1) Diffusion of reactants to catalyst surface. (2) Adsorption of reactants on active sites. (3) Surface reaction: bonds weaken, new bonds form. (4) Desorption of products from surface. (5) Diffusion of products away. The surface reaction is usually the rate-determining step. Active sites: specific locations on the catalyst surface where reaction occurs. Catalyst poison: substance that blocks active sites (e.g., CO poisons Pt). Catalyst promoter: enhances catalyst activity (e.g., Mo promotes Fe in Haber process). Catalyst support: increases surface area (e.g., alumina).