Facet-Dependent Enhancement in the Activity of Bismuth Vanadate Microcrystals for the Photocatalytic Conversion of Methane to Methanol

Photocatalysis provides a route to convert methane into an energy-dense, liquid fuel, such as methanol, using only natural gas, sunlight, water (or oxygen), and the catalyst. In this report, we compare the photocatalytic activity and selectivity for bismuth vanadate (BiVO₄) microcrystals with different morphologies to partially oxidize methane to methanol. Bipyramidal BiVO₄ microcrystals comprising {102} and {012} surface facets were found to be both more active and more selective for methane to methanol conversion compared to platelet microcrystals that expose {001} facets as their top and bottom surface. The selectivity of the bipyramidal BiVO₄ microcrystals for methanol production was over 85% for reaction times between 60 and 120 min with mass activity between 112 and 134 μmol h^-1 g^-1 during this period. These activities are among the highest reported for photocatalytic methane to methanol conversion using illumination conditions comparable to solar irradiation and without the need for sacrificial reagents. Photochemical deposition of metal salts indicates that photoexcited electrons and holes react selectivity at different facets of the platelet and bipyramidal BiVO₄ microcrystals. Combining the photodeposition results with surface energy calculations, we propose that the high selectivity for methanol observed using bipyramidal BiVO₄ microcrystals arises due to efficient extraction of photoexcited holes from surfaces that have intermediate reactivity for oxidation.