In situ characterization of the highly dispersed Mo6+-oxide species supported onto various oxides and their photocatalytic reactivities

Highly dispersed Mo6+-oxide catalysts were supported onto various oxides (SiO2, Al2O3, and SiO2–Al2O3) by an impregnation method and their photocatalytic reactivities were investigated for the decomposition of NO with CO as well as the preferential oxidation of CO with O2 in the presence of excess H2 (photo-PROX). Mo/SiO2 was found to show the highest photocatalytic activity for these reactions. UV–vis and XAFS investigations revealed that isolated tetrahedral Mo6+-oxide species are the main Mo-oxide moieties on these catalysts. Furthermore, the distortion of the coordination sphere of tetrahedral Mo6+-oxide species was observed to increase in the following order: Mo/Al2O3 < Mo/SiO2–Al2O3 < Mo/SiO2, showing that the distorted tetrahedral Mo6+-oxide species having two short MoO double bonds is the active species for these reactions. Photoluminescence and FT-IR investigations indicated that the photo-exited Mo6+-oxide species (Mo5+–O−)* efficiently reacted with CO to form CO2 and the Mo4+-carbonyl species which is easily re-oxidized into the original Mo6+-oxide species (Mo6+O2−) through reactions with NO, N2O and O2 under dark conditions. Such unique redox properties of the Mo6+-oxide species were found to play a significant role in the photocatalytic reactions.