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Why can products that remain on a catalyst surface act as catalyst poisons?

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nanoparticles: hydrogen peroxide — catalysis Nanoparticles of a gold-palladium (yellow-blue) alloy supported on acid-treated carbon (gray) directly catalyzing hydrogen peroxide formation from hydrogen (white) and oxygen (red) while shutting off unwanted hydrogen peroxide decomposition.

In heterogeneous catalysis, a catalyst functions by providing a surface where reactants can undergo chemical transformations. For a catalyst to be efficient, it must be able to release the resulting products as readily as possible after the reaction occurs.1

If the products of a reaction remain on the catalyst surface, they act as catalyst poisons because they occupy the active sites where the reaction is intended to take place. By preferentially adsorbing onto these sites and resisting removal, these substances effectively block the reactants from accessing the surface, thereby reducing the catalyst's overall effectiveness. In essence, the accumulation of these products prevents the catalyst from facilitating further cycles of the reaction, necessitating steps to remove the poison or replenish the active surface components.21