11.11: Heterogeneous Catalysis

Heterogeneous catalysis involves a catalyst in a different phase from the reactants. It is a process where the catalyst and the reactants are in distinct phases, typically solid and gas or liquid.

Most heterogeneous catalysts are metals, metal oxides, or acids. The list includes transition metals like iron (Fe), cobalt (Co), nickel (Ni), palladium (Pd), platinum (Pt), chromium (Cr), manganese (Mn), tungsten (W), silver (Ag), and copper (Cu). These metals possess partially vacant d orbitals that can engage in bonding with chemisorbed species. Metal oxide catalysts such as Al₂O₃, Cr₂O₃, V₂O₅, ZnO, NiO, and Fe₂O₃ are also commonly used. Acids like phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄) serve as effective catalysts, too.

The effectiveness of a catalyst is determined by the enthalpy of adsorption of the reactants. If this value is too small, the reaction will be slow due to insufficient adsorption. Conversely, if it is too large, the reactants will be held tightly at their adsorption sites, limiting their tendency to react.

Many catalysts operate through co-adsorption, a process where a second species modifies the surface of the metal's electronic structure. These modifiers can act as ‘promoters’, enhancing the catalysts' action, or as ‘poisons’ by inhibiting it. Interestingly, the amount of poison needed to eliminate a catalyst's activity is much less than that required to cover the catalyst’s surface completely.

This observation points to the existence of 'active sites' or 'active centers' on the catalyst's surface, where most of the catalytic activity occurs. The surface of a metal catalyst is not smooth but features steplike jumps joining relatively smooth planes; it is at these steps where hydrocarbon bonds primarily break.

Heterogeneous catalysis typically involves the chemisorption of one or more reactants, transforming them into a form ready for reaction, followed by the desorption of the products.

Shape-selective catalysts like zeolites distinguish between different molecular shapes and sizes due to their high internal specific surface areas. According to the Langmuir-Hinshelwood (LH) mechanism, the reaction occurs via interactions between molecules adsorbed on the catalyst's surface.

The reaction rate between two species is first order in the fractional coverage of each and second order overall. For instance, the catalytic oxidation of CO to CO₂ is believed to occur through this mechanism.

In contrast, the Eley-Rideal (ER) mechanism describes a surface-catalyzed reaction where a non-adsorbed molecule collides with a molecule already adsorbed on the surface. This leads to a reaction rate proportional to the partial pressure of the non-adsorbed gas and the fractional surface coverage of the adsorbed gas.

While most surface-catalyzed reactions follow the LH mechanism, several reactions following the ER mechanism have been identified. However, it's crucial to note that real-world reactions likely exhibit characteristics of both mechanisms, falling somewhere between these two theoretical extremes.