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

摘要

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 Ml 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 photoelectrpn 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 heterostmcture 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.