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. Their surface provides active sites where reactants can adsorb, react, and then desorb as products. These catalysts are widely used in industrial processes, such as the Haber process for ammonia production and the catalytic converters in vehicles to reduce harmful emissions. Their effectiveness can be influenced by factors like temperature, pressure, and the surface area of the catalyst.