In situ characterization of the highly dispersed Mo 6+-oxide species supported onto various oxides and their photocatalytic reactivities

Abstract

Highly dispersed Mo 6+-oxide catalysts were supported onto various oxides (SiO 2, Al 2O 3, and SiO 2–Al 2O 3) 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 O 2 in the presence of excess H 2 (photo-PROX). Mo/SiO 2 was found to show the highest photocatalytic activity for these reactions. UV–vis and XAFS investigations revealed that isolated tetrahedral Mo 6+-oxide species are the main Mo-oxide moieties on these catalysts. Furthermore, the distortion of the coordination sphere of tetrahedral Mo 6+-oxide species was observed to increase in the following order: Mo/Al 2O 3 < Mo/SiO 2–Al 2O 3 < Mo/SiO 2, showing that the distorted tetrahedral Mo 6+-oxide species having two short Mo O double bonds is the active species for these reactions. Photoluminescence and FT-IR investigations indicated that the photo-exited Mo 6+-oxide species (Mo 5+–O −) * efficiently reacted with CO to form CO 2 and the Mo 4+-carbonyl species which is easily re-oxidized into the original Mo 6+-oxide species (Mo 6+ O 2−) through reactions with NO, N 2O and O 2 under dark conditions. Such unique redox properties of the Mo 6+-oxide species were found to play a significant role in the photocatalytic reactions.