Surface species are central to the mechanism of catalysis. They facilitate the conversion of reactants to products through several steps: 1. Adsorption: Reactants are adsorbed onto the surface. 2. Activation: Adsorbed reactants are activated, often through bond weakening or breaking. 3. Reaction: Activated species react to form intermediate compounds. 4. Desorption: Reaction products desorb from the surface, making the active sites available for new reactants.