Unraveling the Surface Chemistry of the Heterogeneous Catalytic Decomposition of O3 for Selectivity Concerning O2 or HO• Formation

Nowadays, the O3 decomposition on heterogeneous catalysts is highly
relevant for atmospheric applications where oxygen is typically formed, and wastewater treatment where the interest is to form hydroxyl radicals (advanced oxidation processes). Although both reaction mechanisms appear not to share a relationship, the surface chemistry and phenomena arising on the catalysts seem to indicate the opposite. Here, experimental and theoretical studies proposed in the literature are revisited to determine these characteristics influencing the catalytic O3 decomposition on different materials. It is systematically found that strong Lewis sites via a chemisorption process induces oxygen formation, while physisorption involving weak Lewis sites on surface hydroxyl groups decomposes ozone into hydroxyl radicals. Oxygen vacancies could act as active sites in both cases, and the point-of-zero-charge (PZC) is crucial to preserve surface hydroxyl groups at positive pH values slightly above PZC. The regeneration of active sites depends on the multi-valent oxidation state of
the surface metal. Thus, the acid character of the catalyst (Lewis), the adsorption
energy of O3, HO• and other oxygenated surface groups, and the charge transfer rate of the metallic redox atoms on the surface can be regarded as molecular descriptors for the design of novel catalysts for either atmospheric or wastewater treatment applications.