High-rate solar-light photoconversion of CO2 to fuel: controllable transformation from C1 to C2 products

Abstract

The production of solar fuels offers a viable pathway for reducing atmospheric CO₂ concentrations and the storage and transport of solar energy. While photoconversion of CO₂ into C₁ hydrocarbon products, notably methane (CH₄), is known, the ability to directly achieve significant quantities of higher-order hydrocarbons represents an important step towards practical implementation of solar fuel technologies. We describe an efficient, stable, and readily synthesized CO₂-reduction photocatalyst, Pt-sensitized graphene-wrapped defect-induced blue-coloured titania, that produces a record high combined photocatalytic yield of ethane (C₂H₆) and methane. For the first time, a systematic ultraviolet photoelectron spectroscopy study on the mechanism underlying ethane formation indicates that the process is dependent upon upward band bending at the reduced blue-titania/graphene interface. Furthermore, transient absorption spectroscopy indicates photogenerated holes move into the graphene while electrons accumulate on the Ti³⁺ sites, a phenomenon contradicting prior assumptions that graphene acts as an electron extractor. We find that both mechanisms serve to enhance multielectron transfer processes that generate ˙CH₃. Utilizing a continuous flow-through (CO₂, H₂O) photoreactor, over the course of multiple 7 h runs approximate totals of 77 μmol g⁻¹ C₂H₆ and 259 μmol g⁻¹ CH₄ are obtained under one sun AM 1.5G illumination. The photocatalyst exhibits an apparent quantum yield of 7.9%, 5.2% CH₄ and 2.7% C₂H₆, and stable photocatalytic performance over the test duration of 42 h. The carbon source for both products is verified using ¹³CO₂ isotopic experiments.