Work Function, Band Bending, and Microwave Conductivity Studies on the Selective Alkane Oxidation Catalyst MoVTeNb Oxide (Orthorhombic M1 Phase) under Operation Conditions

Abstract

A systematic gas phase-dependence of the electron work function, electron affinity, band bending and the high frequency electrical conductivity of the prospective oxidation catalyst MoVTeNbO x with orthorhombic M1 structure was identified under selective alkane oxidation conditions. The conductivity measured in a fixed bed flow reactor at 1 bar with a noncontact microwave technique and the surface electronic properties studied by in situ X-ray photoelectron spectroscopy at 0.25 mbar were determined at 400 °C in 2:1 mixtures of oxygen and the alkanes ethane, propane, and n-butane, respectively. The observed modulation of the surface electron affinity is explained by a gas phase dependent modification of the dipolar structure of the active surface, while the band alignment is interpreted in terms of the formation and modification of the space charge region due to pinning of the Fermi energy to the surface state energy as defined by the V 4+ /V 5+ oxide surface layer. The thus changed charge carrier density in the space charge region gives rise to the observed conductivity response. Consequently, the catalytic system and its working mode can be described as a semiconductor heterostructure comprising the semiconducting bulk phase, a V 4+ /V 5+ oxide termination layer, and the reactive gas phase modulating the Fermi energy of the whole system.

Heterogeneous Catalysis

“…In the form of semiconductor properties from many active phases, these have an important and not yet fully understood effect80c, 204b in catalysis because they directly influence the degree of coverage of the reacting surface. This effect cannot be considered to be independent because it is influenced in many ways by the other factors of catalyst function; it acts rather in a combined51i, 204a fashion with the other elements of dynamics. In metals the situation is somewhat easier as the bath of free electrons at the Fermi level can be assumed to be a reservoir for redox equivalents required for surface chemical reactions.…”

Atomic‐Scale Determination of Active Facets on the MoVTeNb Oxide M1 Phase and Their Intrinsic Catalytic Activity for Ethane Oxidative Dehydrogenation

“…Surface changes of MoVTeNb oxides when exposed to reaction conditions have been recently detected by different in situ techniques . [13] In consequence,models proposing some identifiable arrangement of M1 lattice sites as active centers, as discussed here,have been called into question. Thepresent results show unequivocally and quantitatively that specific features in M1 phase MoVTeNb oxide are associated with the C À Ha ctivation.…”

The Electronic Factor in Alkane Oxidation Catalysis

“…In addition, we investigated an alternative selective nbutane oxidation catalyst, the orthorhombic MoVTeNbO x M1 phase, with a maleic anhydride selectivity of more than 40 %. [31] The conditions were shifted to 1:2 mixtures of ethane/ O 2 and n-butane/O 2 to mimic a more oxidizing and reducing atmosphere, respectively, since the catalyst is not stable in pure O 2 or n-butane at reduced pressures. The vanadium oxidation state changed (reversibly) from about 4.6 to 4.5 (oxidation states of the other metal ions remained constant); [31] a gas-phase-dependent surface potential barrier and a change of the surface electron affinity by about 130 meV.…”

“…[31] The conditions were shifted to 1:2 mixtures of ethane/ O 2 and n-butane/O 2 to mimic a more oxidizing and reducing atmosphere, respectively, since the catalyst is not stable in pure O 2 or n-butane at reduced pressures. The vanadium oxidation state changed (reversibly) from about 4.6 to 4.5 (oxidation states of the other metal ions remained constant); [31] a gas-phase-dependent surface potential barrier and a change of the surface electron affinity by about 130 meV. Since the shifts are significantly larger than observed for V 2 O 5 , despite the more moderate changes in the oxidation/reduction conditions, these results are in line with the intermediate selectivity of this catalyst and support the concept of a surface-barrier-mediated bulk-surface charge transfer influencing the catalytic selectivity.…”