Stability and performance of cation vacant Fe3-x-yVx□yO4 spinel phase catalysts in methanol oxidation
Summary, in English
A series of spinel type Fe-V-O phases were prepared with V/Fe atomic ratios ranging from 0 to 1 and were used for methanol oxidation to formaldehyde. X-ray powder diffraction shows that the basic spinel-type structure is retained after use of the samples in methanol oxidation. Compared with the freshly prepared samples, in and ex situ analyses of the samples with XANES show that both V and Fe are oxidized under influence of methanol oxidation, where octahedrally coordinated V3+ partially is oxidized to V4+. Simultaneously, partial oxidation occurs of Fe2+ in tretrahedral and octahedral coordinations to form Fe3+ species. XPS reveals that in general the surfaces with predominantly V5+ and Fe3+ species are more oxidized compared to the bulk. Besides XRD, HRTEM imaging confirms that the basic rutile-type structure is stable in methanol oxidation. Consequently, the structure is very flexible allowing the cations to change oxidation state by forming cation vacancies without structural breakdown. All preparations show activity of similar magnitude although great differences in selectivity to formaldehyde. The best performing spinel catalyst is poor in vanadium with a V/Fe ratio of 1/14, indicating that isolated vanadia moieties perform better than polymeric vanadia structures. The stable and flexible structure makes the spinel-type Fe-V-O catalysts an interesting alternative to the presently used industrial catalyst consisting of a mixture of MoO3 and ferric molybdate. Opposed to the industrial catalyst, which suffers from Mo volatilization, the spinel samples show no measurable volatilization of vanadium.