The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase)

Abstract

The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam significantly increases both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V(5+) species to the extent of Mo(6+), a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. Furthermore we could prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure are induced by the reaction mixture, which is however not dependent on the steam content.

Constructing A Rational Kinetic Model of the Selective Propane Oxidation Over A Mixed Metal Oxide Catalyst

“…The extension of band bending was not affected by the addition of steam, hence no affect on the number of charge carriers in this space charge region. Near-edge X-ray absorption fine structure (NEXAFS) investigations point to a significantly modified electronic structure of the surface, especially the vanadyl bond motif (suggesting an enrichment of covalently bonded V 5+ ) [33].…”

Selective Oxidation: From a Still Immature Technology to the Roots of Catalysis Science

“…The concepts 2, 3, 4 can be summarized into one concept of selecting the best electronic structure. The respective concepts follow from each other as bulk structural motifs, covalence of metal-oxygen bonds and structural motifs are all consequences of the combined local and extended electronic structure enabling redox properties and defect dynamics [49,[55][56][57] at fixed chemical potentials. The operation of a selective oxidation catalyst can be described in the framework of the thermodynamics and kinetics of oxygen anion [58][59][60][61] conductors.…”

Characterization of MoVTeNbOx Catalysts during Oxidation Reactions Using In Situ/Operando Techniques: A Review

“…The effect of steam on the electronic and surface structure of the MoVTeNbOx catalyst was studied by Heine at al using operando microwave conductivity, X-ray photoelectron, soft X-ray absorption, and resonant photoelectron spectroscopy under reaction conditions at 350 • C [66]. As shown in Figure 5, steam increases both activity and acrylic acid selectivity.…”

“…This result is explained by the fact that steam causes the decrease of electrical conductivity, the increase of the concentration of V +5 and Te +6 and the decrease of Mo +6 on the surface. The catalyst can, therefore, be concluded to act like a semiconducting gas sensor where the bulk conductivity can be used to detect electronic changes on the surface [66]. Extensive in situ/operando XPS and XRD studies were performed by the same group in other papers to analyze the surface and bulk catalyst compositions/structures during the propane oxidation reaction [17,75,76].…”

“…Acrylic acid, C 3 H 4 O 2 , with an annual production of 4.7 million tons, is a very important chemical intermediate with a major application in surface coatings, and its global market size is estimated to reach $13 billion USD by 2020 [66,67]. It is currently produced in a two-step process with the oxidation of propylene to acrolein, subsequently followed by oxidation to acrylic acid [68].…”