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Abstract

A series of mixed Mo/V oxide materials was treated under varying carbon-rich,
reducing conditions in order to model conditions found when high-performance multimetal
oxide catalysts are operated at high conversion in selective oxidation reactions.
The objectives were twofold: to better understand the mechanism of reductioncarburisation
in the bimetallic oxide system, and to determine the influence of
chemical composition and oxidation state on the reactivity in propane activation.
Bimetallic carbides of the form (Mo1-xVx)2C were synthesised by carbothermal
reduction of the corresponding h-Mo1-xVxO3 oxide precursors. The reduction was
followed by in situ X-ray diffraction and the mechanism was investigated using solidstate
kinetic modeling techniques. The reactivity of the resultant carbides for the
catalytic dehydrogenation of propane was measured under various conditions
including CO2- and H2O-containing feeds. To probe the surface structure-activity
relationships, in situ reaction methods were applied as characterisation tools in
addition to the standard structural and surface analysis techniques.
The kinetic and structural analysis of carbothermal reduction revealed a strong
stabilising influence of the vanadium dopant via modification of the oxygen mobility
in the lattice. With respect to the catalytic dehydrogenation of propane, it was
observed that vanadium doping increased the initial conversion of propane but
decreased selectivity to propylene. In soft-oxidative feeds (CO2, H2O) the carbide
surface is oxidised, promoting the dehydrogenation reaction. Notably, Mo2C is an
excellent catalyst for the hydrogenation of CO2 via reverse water-gas-shift reaction. In
situ XPS experiments correlate propane dehydrogenation activity with an oxycarbide
surface modification and alkyl-recombinative products are observed in concert with
clear changes in the valence band spectra. Post-mortem analyses of the catalysts by
transmission electron microscopy reveal significant surface and bulk restructuring as
well as the recrystallisation of metastable phases and inhomogeneous bloating of the
structure by aliphatic carbon.
Vanadium is proposed to exert a stabilising influence due to its strongly
oxophilic nature, resulting in the formation of V=O moieties which kinetically hinder
the mobility of oxygen and carbon through the oxide/carbide lattice. The resulting
higher initial activity for propane activation is attributed to the increased activity of
sites selective to hydrogenolysis. Hexagonal (Mo/V)2C is proposed to be a reasonable
model for bronze-like multi-metal oxide catalysts in propane activation due to the
recrystallisation of the metastable fcc carbide structure observed under reaction
conditions.