Photocatalytic reduction of carbon dioxide in alkaline medium on La modified sodium tantalate with different co-catalysts under UV–Visible radiation

• La doped NaTaO3 with different co-catalysts are active for CO2 photoreduction.
• Co-catalysts extend light absorption edge of NaTaO3 into the visible region.
• NaTaO3 on coupling with NiO/CuO facilitates photoelectron transfer to the oxides.
• Pt, Ag, Au and RuO2 act as electron traps and retard recombination rates.
• Water and alkali play multiple roles.

Lanthanum doped sodium tantalate (Na(1−x)LaxTaO(3+x)) with Pt, Ag, Au, CuO, NiO and RuO2 as co-catalysts, have been prepared and characterized by XRD, DRS, photoluminescence spectroscopy, SEM and TEM. Photocatalytic reduction of CO2 on these catalysts in alkaline medium under UV–Visible radiation yields methanol and ethanol as major products with traces of methane, ethane, ethylene, etc. Maximum apparent quantum yield (AQY) for CO2 reduction is achieved on the formulation with 0.2 wt% NiO, followed by 1 wt% CuO as co-catalysts. DRS studies reveal that the addition of co-catalysts brings about reduction in band gap energy of NaTaO3 and its light absorption onset edge is extended into the visible region. Coupling of the conduction band bottom energy levels of NiO and CuO with that of Na(1−x)LaxTaO(3+x) shows that they facilitate easy transfer of photo-generated electrons from the doped tantalate to NiO/CuO, wherein these electrons participate in simultaneous hydrogen generation by water splitting and CO2 photoreduction processes. Other co-catalysts, like, Pt, Ag, Au and RuO2 act as electron traps, resulting in charge separation, which, in turn, leads to improvement in photo-catalytic activity. Besides, significant reduction in the intensity of photoluminescence lines observed for Na(1−x)LaxTaO(3+x) with different co-catalysts also indicates longer life time of the charge carriers. Used catalysts retain the cubic perovskite structure of the fresh catalyst during the reaction for 20 h and stable activity is displayed for three cycles. The alkaline medium plays multiple roles, acting as scavenger of holes, increasing the solubility of CO2 and stabilization of NiO. NaTaO3 based catalysts could become viable alternatives to titania for this application.

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