Photocatalyzed Conversion of CO2 to CH4: An Excited-State Acid-Base Mechanism

Photoinduced reduction of CO2 by H2O occurs in nanoporous Vycor glass doped with tungsten oxides derived from 312 nm photolysis of physisorbed W(CO)(6). In polished forms of Vycor, a hydrated precursor to monoclinic WO3 is formed, whereas in the unpolished glass, the WO3 precursor and mixed valence tungsten oxides that exhibit lower energy intervalence charge transfer (IVCT) transitions are formed. The relative amounts of the different tungsten oxide are attributed to structural differences between the polished and unpolished forms of PVG. Chemisorption converts CO2 to a formic acid-like species, and population of the conduction band of the WO3 precursor with 312 nm light or population of the IVCT state of the mixed valence oxide with >= 437 nm light photocatalyzes the reduction of chemisorbed by coadsorbed water yielding CH4 and O-2. Regardless of the excitation wavelength, electronic spectra and the dependence of methane yield on absorbed energy and the energetics of the conversions implies that neither metal oxide acts as a source of reducing equivalents. Instead, excitation of the metal oxide induces a charge polarization that creates local acidic and basic regions about the oxide in which the reduction of chemisorbed CO2 and oxidation of chemisorbed H2O occur exergonically. The dependence of methane yield on surface pH, and the pH dependencies of the respective half reactions show that the pH gradient needed for the reactions to occur spontaneously fall within the range of known photoinduced changes in acid-base properties. Within this excited-state acid-base mechanism, the metal oxide is not a source of electrons, but a conduit of electrons and protons between two exergonic processes.