Oxidation state and surface segregation of Mo ions in titania catalysts evidenced by ESR, UV and ESCA techniques

Abstract

Incorporation of Mo ions into a TiO 2 matrix has been studied as a function of preparation procedure and heat treatment conditions, using ESCA, UV and ESR spectroscopies. Oxidation state, coordination geometry and incorporation of Mo ions into the TiO 2 particles have been considered. For anastase samples, UV and ESCA techniques show that surface and bulk concentrations of Mo are about identical, with the majority of Mo being in the form of Mo 6+ and a significant quantity existing as Mo 5+ in distorted octahedral symmetries. ESR spectroscopy only detects substitutional ions designated Mo s 5+ in very small amounts (∼1% of the 0.5% Mo introduced). For rutile, heated at 800°C in air, ESCA technique shows that the surface concentration is enriched in Mo in the +6 oxidation state by a factor 10 compared to the bulk, whereas UV spectroscopy shows that Mo is mainly incorporated in the bulk in the form of Mo 5+ presenting two different environments of the distorted octahedral type. ESR techniques detect substitutional Mos in very small amounts (∼1% of the 0.5% Mo introduced). For rutile samples, directly prepared by the flame method, the surface concentration of Mo is about twice that of the bulk, Mo ions being mainly in a +6 oxidation state at the surface and +5 in the bulk. Strong surface enrichment (× 10–20) occurs after heating anastase or rutile at 800°C in air. Rutile samples turn out to be composed of MoO 3 islands at the surface of TiO 2 doped with Mo(V) ions in Mo ensembles or clusters occluded in the TiO 2.