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

Abstract

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.