Selectivity in UV Photocatalytic CO2 Conversion over Bare and Silver-Decorated Niobium-Tantalum Perovskites

The hydrothermal synthesis of the perovskites NaNbO3, NaTaO3 and the intermediate composition NaNb0.5Ta0.5O3, as CO2 conversion photocatalysts is reported. Among them, the niobate shows the most promising performance under UV irradiation not only in terms of conversion and light utilization ability, but also regarding the selectivity towards CO2 reduction against hydrogen evolution from water protons. Further modification of NaNbO3 with silver as co-catalyst results in an increase of the selectivity towards highly reduced products, primarily methanol, against the carbon monoxide production mainly observed with the bare semiconductor. A thorough structural, electronic, electrochemical characterization, together with in-situ surface analysis by APXPS, was undertaken to gain deeper insight into the reasons that account for such changes. On the one hand, for the bare semiconductors, increased light absorption and the sole presence of Nb in +4 state at the surface seem to drive the superior activity of NaNbO3. On the other hand, electronic and surface chemistry modifications induced by 0.1 wt.% silver deposition are proposed to govern the higher selectivity towards methanol. Excessive metal loading, in turn, enhances the selectivity effect but at the expense of conversion, in such a way that light utilization becomes poorer than with the bare niobate.