Oxygen vacancies–elusive but common catalytic key defects for thermal upgrading of CO2 to CH4, CO and CH3OH

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Single carbon products (C1 compounds) are simple but important chemicals in the road towards energy transition. Catalytic conversion of CO2 with H2 (desirably renewable) can be performed over reducible oxides supporting transition metals to obtain products such as CH4, CO and MeOH. Oxygen vacancies (O-vacancies), which are inherent defects of reducible metal oxides, play an enormous role in driving the catalytic performance (activity, selectivity, stability) for the desired reactions. Yet, the assessment of O-defects at realistic conditions is often complex. Only few techniques can provide direct evidence for their existence and influence in CO2 activation. Among them, electron paramagnetic spectroscopy (EPR), Raman spectroscopy, scanning probe microscopies (SPM) and environmental transmission electron microscopy (ETEM) are nowadays the most informative. In most cases, however, the measurements require reaction conditions far away from CO2 valorization applications. Although great efforts have been fruitful in explaining and demonstrating the huge importance of O-vacancies in CO2 catalysis, still ambiguous or erroneous interpretations about structure-function correlations involving O-vacancies are found in literature, especially, when information is not properly gathered, e.g., by O 1s ex-situ X-ray photon spectroscopy (XPS). Moreover, despite the recognized importance of O-vacancies for CO2 valorization, critical literature compilations about their effects in thermal processes are scarce. Herein, we attempt to contribute in closing this gap by integrally encompassing representative investigations on the thermo-catalytic production of CH4, CO and MeOH. Particularly, we emphasize in the proper selection of assessment tools (direct/indirect) to unambiguously establish structure-function relationships to design optimized O-defective catalysts for the targeted compounds.