New Fundamental Insights About Surface Compositions of Supported and Bulk Mixed Metal Oxide Catalysts From Ion Scattering Spectroscopy

GRUENERT, W.; LAURA ESTEFANIA BRIAND; TKACHENKO, O. P.; TOLKACHEV, N.; WACHS, I. E.

París

Congreso; 13th International Congress on Catalysis; 2004

Mixed metal oxide materials including vanadium or molybdenum oxides are important catalysts for numerous redox reactions that are extensively employed in industry. Such materials may be structured as supported metal oxide catalysts, in which one of the oxides serves as a substrate or support for a dispersed surface phase of the second oxide, or as mixed metal oxide bulk phases where the two oxides are intimately mixed in the bulk phase as a solid solution or well-defined solid compound. For supported V and Mo oxide catalysts, the ca­pacity of the oxide support substrate to accommodate the supported transition metal oxide in the form of inter­acting surface metal oxide species is usually determined by vibrational spectroscopy, in particular Raman spectroscopy, because of its excellent sensitivity for both the surface and microcrystalline phases. From such studies, it has been concluded that many oxide supports are initially covered by surface monolayers of V or Mo oxide surface species before three-dimensional aggregates are formed (provided that the materials have been prepared by appropriate methods and are characterized in controlled environments subsequent to calcination. With bulk mixed metal oxide catalytic materials, vibrational spectroscopy is unable to provide information about the surface composition of the outermost layer because the vibrational signals tend to be dominated by the strong vibrations from the bulk metal oxide phase, but chemisorption of chemical probe molecules may still be successfully employed. Using methanol chemisorption as a chemical probe for surface V or Mo oxide species and comparing turnover frequencies of methanol oxidation over bulk molybdates or vanadates and supported V or Mo oxide catalysts, Briand et al. have recently sug­gested that the surfaces of many bulk metal molyb­dates and vanadates are largely covered by surface Mo or V oxide species as in the corre­sponding supported metal oxide catalysts. This is in variance to expectations implicitly underlying many existing models and approaches for the explanation of the catalytic activity and selectivity properties of bulk mixed metal oxide catalytic materials and needs to be verified by appropriate independent physical characterization surface tech­niques. Such fundamental surface compositional issues can only be addressed by surface sensitive physical characterization techniques that only provide information about the outermost surface. Low energy Ion Scattering Spectroscopy (ISS) is the physical characterization technique of choice because of its ~0.1 nm sampling depth. However, several earlier ISS studies of supported V and Mo oxide catalysts are at vari­ance with the picture of a complete close-packed surface monolayer and reported that the oxide support cation is still ex­posed even when the surface cation density significantly exceeds the monolayer capacity . The present contribution reports low eenrgy He ISS studies of supported V and Mo oxide catalysts and the corresponding bulk vanadate and molybdate mixed metal oxide compounds. On the basis of recent insight into the surface chemistry of supported metal oxides, a new sample handling method­ology has been devel­oped for supported metal oxide cata­lysts, which has been also applied to the bulk mixed metal oxide phases. It com­prises the use of an “in situ calcination” pretreatment that avoids con­tact with the ambient atmos­phere prior to surface analysis because surface hydration by ambient moisture has been shown to alter the dispersion of oxide surface phases. Finally, sputter series are investi­gated and the ratios of elemental signals intensities are extrapolated to zero expo­sure time to correct for any surface perturbations that may result from the low energy He ions during even the first ISS scan.