Oxygen vacancies role in thermally driven and photon driven catalytic reactions

Abstract

Surface and subsurface oxygen vacancies play a crucial role in catalysis. Their formation requires considerable energy, which is mostly provided by reducing chemical compounds such as CO or hydrocarbons in catalytic oxidation and water gas shift reactions. This article covers three main points related to oxygen vacancies in catalysis by metal oxides: their formation, their stability, and the way their effect can be studied. Most of the information given is from spectroscopic and computation results on well-defined oxides and metal-metal oxides interfaces. The intention is to share these results with researchers working on applied systems such as water splitting to H 2 and O 2 and CO 2 reduction to CO. An emphasis on the catalytic cycle is considered because their role in photo- and photo-electrocatalytic reactions, in these two sought-after reactions, has been invoked recently. Deviation from the catalytic cycle would result in materials instability owing to corrosion, driven by system thermodynamics. • Surface oxygen vacancies are now attracting increasing interest by researchers working on water splitting and CO 2 reduction. This perspective's focus is in on our state of knowledge on the formation, stability, and regeneration of these vacancies under thermal and photo-excitation conditions. • An emphasis on surface studies of model oxides and metal/oxides of these vacancies, both at the experimental and theoretical levels. The most important challenge in the case of the water splitting and CO 2 reduction is the regeneration of these vacancies, because sun light (excluding heat) does not have enough energy to directly reduce metal oxides. • This is often not the case for thermally driven catalytic reactions, such as in the thermochemical water splitting. Care needs to be taken in particular for photo-stimulated catalytic reactions to ensure that indeed the catalytic cycle is maintained. Oxygen vacancies are of considerable importance in catalytic reactions of oxides. While in a thermally driven reaction of oxygen containing compounds, heat and/or chemical energy provide the needed energy to make them again, photons with 2–3 eV energy cannot reduce back the oxide. Therefore, the prior made vacancies of oxide catalysts would be removed irreversibly. Ensuring that the reaction is indeed catalytic is a requirement for CO 2 reduction and pure water splitting.