Model Catalyst Studies on Vanadia Particles Deposited onto a Thin-Film Alumina Support. 1. Structural Characterization

Abstract

A model system of alumina-supported vanadia particles, representing catalysts of the type oxide 1 /oxide 2 , was prepared under ultrahigh vacuum (UHV) conditions and characterized regarding its structural and electronic properties. As supporting oxide we used a thin, well-ordered alumina film grown on NiAl(110), which allows the application of scanning tunneling microscopy (STM), infrared reflection-absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) without charging effects. Vanadium oxide particles were prepared via metal evaporation in an oxygen ambient, leading to the growth of small, roundish oxide particles with vanadium in the +3 oxidation state. The particles are shown to interact strongly with the alumina support, resulting in an increased alumina film thickness and a distortion of the alumina film structure. IR absorption signals of the deposits could be successfully assigned to specific V-containing species, thus providing insight into the inner structure of the particles. The species identified are surface-localized vanadyl groups (VdO), interface-localized vibrations involving V, O, and Al ions, and lattice structures typical of bulk V 2 O 3 .

Relating methanol oxidation to the structure of ceria-supported vanadia monolayer catalysts

“…Such morphology, where vanadia wets the ceria surface, is different from that previously observed for vanadia supported on alumina and silica films [38,62,63] , where primarily threedimensional nanoparticles were formed from the onset. This comparison again highlights the strong interaction of vanadia with ceria.…”

Towards an atomic level understanding of niobia based catalysts and catalysis by combining the science of catalysis with surface science

“…Based on Raman and IR spectroscopy results, the peak at 1045 cm -1 has been assigned to the stretching of vanadyl (V=O) groups. Meanwhile, the broad signal at ∼ 950 cm -1 is associated with vibrations involving V-O-Al linkage at the vanadia/alumina interface as predicted by theoretical calculations (Magg et al 2003b(Magg et al , 2002. Therefore, turning back to the niobia/alumina system, the peak at 986 cm -1 must be assigned to the niobyl (Nb=O) stretching, and the signal at 890 cm -1 must be assigned to Nb-O-Al interface vibrations, which should show lower frequencies as compared to vanadia/alumina, based solely on mass considerations (Nb atom is heavier than V).…”

“…At the coverage of 1Å, a band centered at 986 cm -1 emerges, which grows in intensity with the coverage. The assignment of these spectral features is facilitated by comparison with the results on a supported vanadia system previously studied in detail (Magg et al 2003a(Magg et al , 2002(Magg et al , 2004. For convenience, the bottom curve in Figure 12a shows the spectrum obtained for the similarly prepared 3.5Å vanadia/alumina sample in the same experimental setup.…”

Selectivity in Methanol Oxidation as Studied on Model Systems Involving Vanadium Oxides

“…10 where the vanadia is prepared by physical vapor deposition of vanadium in an oxygen ambient onto the surface of a silica film. The morphology of the film is granular indicating threedimensional growth of the vanadium oxide particles, which have been proven to be mainly V 2 O 3 particles exposing vanadyl groups at their surface [8][9][10][11]. In this sense they are similar to the single crystal V 2 O 3 (0001) surface.…”

“…To obtain a more direct insight into structure-reactivity relations supported vanadia systems have been prepared under ultrahigh vacuum conditions on silica, alumina, and ceria thin film supports [8][9][10][11][12][13][14][15][16]. Scanning tunneling microscopy has been used to show that under such conditions the vanadia does not wet the surfaces of silica and alumina even if those surfaces had been exposed to water beforehand.…”