SURFACE-STRUCTURE AND REACTIVITY OF V-TI-O CATALYSTS PREPARED BY SOLID-STATE REACTION .1. FORMATION OF A VIV-INTERACTING LAYER

The solid-state interaction between V2O5 and TiO2 in the 700-800 K range of temperatures gives rise to the formation of VIV sites even in the absence of reducing agents. A VIV interacting layer covering the entire surface of TiO2 anatase may be created in the absence of any indication of partial transformation to the rutile phase. The nature, amount, and distribution of these VIV sites are characterized by means of titration combined with selective extraction, reactivity measurements in o-xylene oxidation, evaluation of redox properties, and by XRD, XPS, and ESR analyses. The amount of VIV depends on the crystallographic nature (anatase or rutile) and surface area of the TiO2 and on the conditions (temperature, time, and type of atmosphere) of the heat treatment. In the anatase sample the VIV sites can be reduced to VIII, but not oxidized to VV due to the strong interaction with the titania surface. In rutile samples part of the VIV may be reduced to VIII, but also oxidized to VV. The remaining VIV sites are present in solid solution in the rutile matrix and are not accessible to redox reagents. The model of a VIV-modified TiO2 (anatase) surface is discussed with reference to the problem of surface diffusion of vanadium species on the anatase surface. In TiO2 (rutile)-based samples, due to the competition of the migration of vanadium ions toward the bulk of the rutile with respect to surface diffusion, V2O4-like islands form that are coherently intergrown into the main rutile TiO2 matrix. © 1991.