Heterogeneous Catalysis

A heterogeneous catalyst exists in a different phase from the reactants. Typically, the catalyst is a solid while reactants are gases or liquids. This phase difference allows the reaction to occur at the catalyst's surface, where reactants adsorb, transform, and then desorb as products.

Catalyst effectiveness depends critically on the enthalpy of adsorption of reactants. If adsorption enthalpy is too low, reactants bind weakly and the reaction proceeds slowly. If too high, reactants bind too tightly and cannot react easily, reducing catalytic efficiency.

Promoters enhance catalytic activity by preventing large crystal formation on the catalyst surface, preserving active surface area. Poisons bind strongly to active sites and inhibit the catalyst. Remarkably, only a small amount of poison is needed to eliminate catalytic activity compared to the surface coverage required.

Heterogeneous catalysis typically involves three steps: chemisorption of one or more reactants onto the catalyst surface, transformation of reactants into a reactive form, and desorption of products. This cycle allows the catalyst to regenerate and process additional reactant molecules.

The Langmuir-Hinshelwood mechanism involves reactions between molecules already adsorbed on the catalyst surface, with reaction rate first order in each species' fractional coverage. The Eley-Rideal mechanism involves a gas-phase molecule colliding with an adsorbed molecule, with rate proportional to gas partial pressure and surface coverage.

Common heterogeneous catalysts include transition metals like iron, cobalt, nickel, palladium, and platinum, which possess partially vacant d orbitals for bonding with chemisorbed species. Metal oxides such as Al2O3, Cr2O3, and Fe2O3, and acids like phosphoric and sulfuric acid are also effective catalysts.

Most catalytic activity occurs at active sites or active centers on the catalyst surface. Metal catalyst surfaces are not smooth but feature steplike jumps between relatively smooth planes. Hydrocarbon bonds primarily break at these surface steps, making them the primary locations for catalytic reactions.