1We are addressing the fundamental question, if semiconductor physics concepts can 2 be applied to describe the working mode of heterogeneous oxidation catalysts and if 3 they can be even used to discriminate between selective and unselective reaction path-4 ways. By the application of near-ambient pressure X-ray photoelectron spectroscopy 5 it could be shown exemplarily for the oxidation of n-butane to maleic anhydride on the 6 *

A description of the VPP sample preparation can be found elsewhere. 1 The catalyst was pelletized with 1 wt-% graphite and activated in an n-butane oxidation gas feed (2% n-butane / 3% H2O in air, 1 ppm triethylphosphate) for about 500 h after a thermal treatment (in air and in N2/H2O 1:1), see Eichelbaum et al. 1 In the operando MCPT measurements a sieve fraction of 100 to 200 µm of the crushed catalyst pellets was investigated. X-ray fluorescence (XRF) measurements were performed on a Bruker Pioneer S4 Spectrometer. The V2O5 sample (SN 22054) was found to consist of 100 wt-% V2O5 and the VPP sample (SN 12831) showed 56.4 wt-% V2O5 and 43.6 wt-% P2O5 (P/V atomic ratio of 1.0). X-ray powder diffraction (XRPD) measurements were conducted on the Bruker AXS D8 Advance II theta/theta diffractometer (Ni filtered Cu Kα radiation). The powder diffractograms

Complex Mo, V based mixed oxides that crystallize in the orthorhombic M1-type structure are promising candidates for the selective oxidation of small alkanes. The oxygen sub-lattice of such a complex oxide has been studied by annular bright field scanning transmission electron microscopy. The recorded micrographs directly display the local distortion in the metal oxygen octahedra. From the degree of distortion we are able to draw conclusions about the distribution of oxidation states in the cation columns at different sites. The results are supported by X-ray diffraction and electron paramagnetic resonance measurements that provide integral details about the crystal structure and spin coupling, respectively.

Isostructural orthorhombic oxides of the general formula (Mo,V,Te,Sb,Nb,Ta)O x are an important class of solids, which are interesting as catalysts for oxidation of light alkanes. We investigated relations between the electronic properties of MoV-oxide (orthorhombic M1 phase) and its catalytic performance in the oxidation of ethane, propane, and n-butane. Operando conductivity and permittivity measurements were performed and complemented by near-ambient-pressure X-ray photoelectron spectroscopy. In contrast to the n-type MoVTeNb-oxide, MoV-oxide showed p-type semiconducting behavior. The conductivity of the sample adapted sensitively to the surrounding atmosphere, not only to alkane chain lengths but also to reactant conversion levels. However, no measurable change in band bending depending on the alkane chain length was observed, indicating that the gas-phase-dependent surface potential barrier, which controls the charge transfer between reactants and catalyst, is less pronounced or missing in dry alkane oxidation feeds. The addition of steam in propane oxidation led to a decrease of its conductivity and work function. Steam significantly influenced the surface layer on MoV-oxide, resulting in an enrichment of covalently bonded V 5+ species and surface hydroxylation. A small change in the surface potential barrier induced by wet propane oxidation feed can contribute to a modification of the bulk−surface charge transfer and improved selectivity to acrylic acid.

The cooperation of metal oxide subunits in complex mixed metal oxide catalysts for selective oxidation of alkanes still needs deeper understanding to allow for a rational tuning of catalyst performance. Herein we analyze the interaction between vanadium and molybdenum oxide species in a monolayer supported on mesoporous silica SBA-15. Catalysts with variable Mo/V ratio between 10 and 1 were studied in the oxidation of propane and characterized by FTIR, Raman, and EPR spectroscopies, temperature-programmed reduction, UV/vis spectroscopy in combination with DFT calculations, and time-resolved experiments to analyze the redox properties of the catalysts. Molybdenum oxide (sub)­monolayers on silica contain mainly dioxo (Si–O−)2Mo­(O)2 species. Dilution of silica-supported vanadium oxide species by (Si–O−)2Mo­(O)2 prevents the formation of V–O–V bonds, which are abundant in the pure vanadium oxide catalyst that predominantly contains two-dimensional vanadium oxide oligomers. Existing single vanadyl (Si–O−)3V­(O) sites and neighboring (Si–O−)2Mo­(O)2 sites do not strongly interact. The rates of reduction in propane and of oxidation in oxygen are lower for single metal oxide sites compared to those for oligomers. The rate of propane oxidation correlates with the overall redox rates of the catalysts but not linearly with the chemical composition. Retarded redox behavior facilitates selectivity toward acrolein on single-site catalysts. The abundance of M–O–M bonds is more important in terms of activity and selectivity compared to the nature of the central atom (molybdenum versus vanadium).