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Catalyst changes reaction rate without being consumed. Key facts: lowers activation energy, provides alternative pathway, does NOT change delta_G, delta_H, K_eq, or equilibrium position. Only accelerates attainment of equilibrium. Types: Positive (increases rate) vs negative/inhibitor (decreases rate). Homogeneous (same phase): mineral acid catalysis, enzyme catalysis in solution. Heterogeneous (different phase): solid catalyst with gaseous reactants. Mechanism: diffusion -> adsorption -> surface reaction -> desorption -> diffusion. Zeolites: shape-selective catalysts (ZSM-5 converts alcohols to gasoline). Enzymes: biological catalysts, highly specific, optimal pH and temperature. Michaelis-Menten kinetics: v = v_max[S]/(K_m + [S]). At low [S]: first order. At high [S]: zero order (saturation). Catalyst poison: blocks active sites (As on Pt). Promoter: enhances activity (Mo on Fe). Auto-catalysis: product acts as catalyst (MnO4- + C2O4^2-: Mn^2+ produced acts as catalyst).