Passivation occurs primarily due to the chemisorption of reactive species on the surface of the catalyst. These species can form a monolayer or multilayer that blocks the active sites, hindering the adsorption of reactants. Factors such as temperature, pressure, and the presence of impurities can accelerate this process. For example, in heterogeneous catalysis, the exposure of a metal catalyst to air can lead to the formation of an oxide layer, rendering the catalyst inactive.