Reactivity of Bismuth Molybdates for Selective Oxidation of Propylene Probed by Correlative Operando Spectroscopies

alpha-Bi2Mo3O12, beta-Bi2Mo2O9, and gamma-Bi2MoO6 as target bismuth molybdate phases were prepared by hydro thermal synthesis and flame spray pyrolysis and tested for their catalytic performance in the selective oxidation of propylene. Their structure and reactivity during temperature-programmed reaction (TPR) and under reaction conditions were investigated by in situ and operando X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and Raman spectroscopy. To gain insight into amorphous and crystalline structures at the same time, XAS and XRD as well as XAS and Raman spectroscopy were combined in one experiment. TPR in propylene revealed that the reduction of Mo6+ to Mo4+ occurred at lower temperatures than from Bi3+ to Bi in scheelite-structured systems. In a reaction cycle, mainly reduction molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO42- entities. Minor structural transformations were detected by XRD and Raman spectroscopy. The catalytic performance of aurivillius-structured systems was more diverse than for scheelite-based ones and ranged from highest to lowest observed acrolein yield, probably due to a synergy effect of two or more bismuth molybdate phases. For phase pure systems, bismuth was more easily reduced than molybdenum. In contrast, aurivillius structures with additional phases showed reduction and oxygen removal from both metal centers under steady-state conditions, but molybdenum was in most cases more easily reduced. A high catalytic activity mostly coincided with low reduction temperatures, except for the unselective pure gamma-Bi2MoO6 that showed a facilitated reduction of bismuth compared to molybdenum. Hence, the combination of operando methods led to an understanding of the redox behavior of bismuth and molybdenum and their influence on the catalytic performance.