Heterogeneous catalysts are substances that speed up chemical reactions without being consumed in the process, and they exist in a different phase than the reactants. Typically, these catalysts are solid materials that facilitate reactions involving gases or liquids, allowing for easier separation and recovery after the reaction. These catalysts work by providing a surface for the reactants to interact, lowering the energy barrier needed for the reaction to occur. Common examples include metals like platinum and palladium, which are often used in processes such as catalytic converters in vehicles to reduce harmful emissions.