High rate CO2 photoreduction using flame annealed TiO2 nanotubes

Abstract The photocatalytic reduction of CO 2 into light hydrocarbons using sunlight and water is a challenging reaction involving eight electron transfer steps; nevertheless, it has great potential to address the problem of rising anthropogenic carbon emissions and enable the use of fossil fuels in a sustainable way. Several decades after its first use, TiO 2 remains one of the best performing and most durable photocatalysts for CO 2 reduction albeit with a poor visible light absorption capacity. We have used flame annealing to improve the response of TiO 2 to visible photons and engineered a nanotubular morphology with square-shaped cross-sections in flame-annealed nanotubes. An enhanced CH 4 yield was achieved in the photoreduction of CO 2 using flame annealed TiO 2 nanotubes, and isotope labeled experiments confirmed the reaction products to originate from the CO 2 reactant. Flame-annealed TiO 2 nanotubes formed in aqueous electrolyte (FANT-aq) yielded 156.5 μmol g catalyst –1 .hr –1 of CH 4 , which is in the top tier of reported performance values achieved using TiO 2 as a stand-alone photocatalyst. This performance resulted because appreciable amounts of CH 4 were generated under visible light illumination as well. TiO 2 nanotubes exhibited CO 2 photoreduction activity up to a wavelength of 620 nm with visible light driven photocatalytic activity peaking at 450 nm for flame annealed TiO 2 nanotubes. Isotope labelling studies, using GC–MS and gas-phase FTIR, indicated photoreduction of 13 CO 2 to 13 CH 4 . The detection of 13 CO in the product mixture, and the absence of HCHO and HCOOH provides strong support for the photoreduction proceeding along a carbene pathway. The enhanced CO 2 photoreduction performance of FANT-aq is attributed to increased visible light absorption, square morphology, and the presence of rutile as the only crystalline phase with (110) as the dominant plane.