High Selectivity toward Photoreduction of Carbon Dioxide to Methane over Copper-Doped Titania: Investigations Using Operando Techniques

The photocatalytic reduction of CO2 to CH4 is a highly beneficial option from the perspective of both energy and the environment. Several promising photocatalysts that selectively produce CH4 (C1) mainly comprise Cu in different oxidation states. Since the CO2 reduction to CH4 is an 8 electron reaction, the formation of the different intermediates upon adsorption and photocatalytic reaction had to be discerned individually. Earlier, it has been demonstrated by our group that the Cu-doped TiO2 photocatalyst is able to selectively reduce CO2 to CH4 with very good yield. The present article reports detailed in situ FT-IR studies to delineate the different intermediates formed by exposure to CO2 and moisture over the Cu-doped TiO2 catalysts like bidentate carbonates, monodentate carbonates, bicarbonates, and carboxylates identified over different Cu-doped TiO2 surfaces. These intermediates were correlated with the differential electron densities over Cu, Ti, and O vacancies ascertained from XANES/EXAFS (XAS) studies. These catalysts were then selectively reduced within the in situ FT-IR chamber to further ascertain the role of the Cu1+, Ti3+, and O vacancies to rationalize the effect of different Lewis base sites in the photocatalysts. In situ XAS studies in correlation with the in situ FT-IR studies lead to a generic understanding of different adsorbate species formed over the Cu surfaces at the molecular level. The time-dependent XAS studies used for the photoreduction of CO2 over the Cu-doped photocatalysts were undertaken to establish the precise role of Cu and to delineate the alteration in the bond distances on the photocatalytic reduction of CO2. This study principally delineates the effect of Cu sites for the photocatalytic CO2 reduction process in the presence of moisture as compared to the other probable active sites present in the photocatalyst. The effect of oxidation states and the formation of different intermediates for reactive adsorption and, in turn, their effect on photocatalysis will pave the way to formulate further better photocatalysts for the photoreduction of CO2.