Structure and properties of a Mo oxide catalyst supported on hollow carbon nanofibers in selective propene oxidation

Abstract

In situ X-ray absorption spectroscopy (XAS) under reaction conditions of selective propene oxidation was employed to elucidate the local structure of as-prepared and activated molybdenum oxide supported on hollow vapor-grown carbon nanofibers (VGCNF). The local structure of as-prepared Mo x O y -VGCNF was very similar to that of hexagonal MoO 3. During heat treatment in propene- and oxygen-containing atmosphere, as-prepared Mo x O y -VGCNF transforms into activated Mo x O y -VGCNF above 623 K. The local structure around the Mo centers in activated Mo x O y -VGCNF is similar to that of α-MoO 3. Temperature- and time-dependent XAS measurements showed a rapid transformation from hex-MoO 3 to α-MoO 3 supported on VGCNF under reaction conditions. Subsequently, the resulting activated Mo x O y -VGCNF catalyst exhibited a slowly increasing average oxidation state. The latter coincided with the formation of acrylic acid, which is hardly detectable during catalysis on regular, binary α-MoO 3. Moreover, activated Mo x O y -VGCNF is much more active in the selective oxidation of propene compared to α-MoO 3. The correlation between catalytic selectivity and average oxidation state as a result of suitable reduction–oxidation kinetics corroborates the importance of structural complexity rather than chemical complexity.