Surface Reactivity of Titania\u2013Vanadia Mixed Oxides Under Oxidizing Conditions

Abstract

The surface structure and reactivity of TiO2(110) with 1–15% of admixed vanadium were studied using thermal desorption spectroscopy, scanning tunneling microscopy, low-energy electron diffraction, and X-ray photoelectron spectroscopy with the methanol partial oxidation as the reactivity test reaction. Prior to the experiments the sample was oxidized at elevated temperatures with O2 of different pressures up to 10−6 mbar. As shown in a preceding publication (Song et al. in Surf Sci 653:181–186, 2016), vanadium leads to an increased reducibility of the mixed oxide, which resulted in the simultaneous presence of reduced and oxidized structural elements at the surface. At low vanadium concentrations (a few percent) small vanadia clusters with V4+ form above the fivefold Ti rows at the surface together with short vanadium decorated strands along [001]. These types of structure promote the partial oxidation of methanol towards formaldehyde at 550 K. At higher vanadium concentrations the vanadia aggregates at the surface contain V5+ in addition to V4+. They produce formaldehyde already at 480 K and below. The oxidized TiO2(110) layer with admixed vanadium releases QMS-detectable amounts of O2 already at a temperature of about 450 K, which is about 80 K below the corresponding temperature for TiO2(110) without vanadium. This is attributed to the increased reducibility of rutile with admixed vanadium.