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

Abstract

MoO 6 as target bismuth molybdate phases were prepared by hydrothermal 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 Mo 6+ to Mo 4+ occurred at lower temperatures than from Bi 3+ to Bi 0 in scheelite-structured systems. In a reaction cycle, mainly reduction of molybdenum was observed and EXAFS fitting confirmed the removal of oxygen from MoO 4 2− 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 γ-Bi 2 MoO 6 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.

Chemical Imaging of Mixed Metal Oxide Catalysts for Propylene Oxidation: From Model Binary Systems to Complex Multicomponent Systems

“…The preparation of the two-component catalyst system HS: α-Bi 2 Mo 3 O 12 is described elsewhere. [44]…”

“…The setup for evaluation of the catalytic performance is described in detail in the literature. [44] The hydrothermally-prepared and subsequently calcined catalyst HS: BiÀ MoÀ CoÀ Fe oxide was ground, pressed and sieved to give a fraction of 300-450 μm. 800 mg of the catalyst were placed in a quartz tubular reactor (6 mm inner diameter).…”

Structural Evolution of Highly Active Multicomponent Catalysts for Selective Propylene Oxidation

“…Catalysts used for this process are typically multicomponent systems based on bismuth molybdate phases [3][4][5][6]. Recent studies have shown that such bismuth molybdates are accessible through a variety of novel synthesis methods [7][8][9][10][11][12], the choice of which can exert a strong influence on catalyst activity and performance [13]. In this context, flame spray pyrolysis (FSP) has proven to be an attractive method for the synthesis of metal oxide catalysts [14][15][16].…”

ROCK: A Beamline Tailored for Catalysis and Energy-Related Materials from ms Time Resolution to µm Spatial Resolution

“…The complete change in beamline configuration to jump from one edge to another one takes one minute in the worst case and 30 s generally. This "edge jumping" feature is highly demanded [4][5][6][7] as it provides multiple edge characterization on a material evolving under working conditions (temperature, pressure, or gas composition for catalysis, applied current or potential for energy related materials) in a single experiment. In this way, the experimentalist has a true depiction of the concomitant temporal evolutions of the different active centers subjected to exactly the same operating conditions.…”

“…As a result, the photons density impinging the sample can be adjusted by a factor 100, which is of paramount importance for samples suffering from radiation damage, without altering the signal-tonoise ratio since the photon flux is preserved [3]. This versatility in beam size also allows accommodating different sample environments, such as large Lytle-type cells [2] or thin capillary reactors [7].…”