Types of Catalysis

In homogeneous catalysis, the catalyst and reactants are in the same phase. Example: $H^{+}$ ion (catalyst) and organic reactants (both in aqueous solution) in acid-catalyzed ester hydrolysis.

In heterogeneous catalysis, the catalyst is in a different phase from the reactants. Most industrial catalysts are solids acting on gaseous reactants. The mechanism involves five steps: (1) diffusion of reactants to surface, (2) chemisorption at active sites (reactant molecules are activated — bonds weakened), (3) surface reaction (new bonds form → product), (4) desorption of product, (5) diffusion of product away. The catalyst lowers the activation energy by providing these alternative steps via surface intermediates.

Industrial Importance

Haber process: $N_{2}$(g) + $3H_{2}$(g) → $2NH_{3}$(g). Catalyst: Fe. Promoter: Mo (increases Fe activity). This process produces most of the world's ammonia, the basis for fertilizers. Conditions: ~450°C, ~200 atm.

Contact process: $2SO_{2}$(g) + $O_{2}$(g) → $2SO_{3}$(g). Catalyst: $V_{2}O_{5}$. This produces $SO_{3}$ for manufacturing $H_{2}SO_{4}$. Conditions: ~450°C. $V_{2}O_{5}$ replaced Pt (which is more expensive and easily poisoned).

Selectivity

The same reactants can give different products depending on catalyst: CO + $H_{2}$ gives methanol ($CH_{3}OH$) with ZnO-$Cr_{2}O_{3}$, formaldehyde (HCHO) with Cu, and methane ($CH_{4}$) with Ni. Different catalysts provide different active site geometries and electronic properties, stabilizing different transition states.

Promoters and Poisons

A promoter increases catalyst activity without being a catalyst itself (e.g., Mo in Haber process). A catalyst poison blocks active sites by strong chemisorption, reducing activity (e.g., Pb poisons Pt in catalytic converters; CO, $H_{2}$S poison Fe in Haber process).

Enzyme Catalysis

Enzymes are biological protein catalysts with extraordinary specificity (lock-and-key model: enzyme's active site uniquely fits specific substrate). Michaelis-Menten kinetics: v = Vmax[S]/(Km + [S]). Km = substrate concentration at half-maximum rate (lower Km = higher affinity). Optimal activity at specific pH and temperature. Competitive inhibitors (bind active site, Km increases, Vmax unchanged, overcome by excess substrate) vs. non-competitive inhibitors (bind allosteric site, change shape, Vmax decreases, Km unchanged, cannot be overcome).