Characterisation of VOx/ZrO2 catalysts by electron paramagnetic resonance and X-ray photoelectron spectroscopy

Abstract

Various samples of vanadia supported on zirconia [as prepared (AP), after heating in O₂ at 773 K (HO), and after reduction with CO at 400–923 K], have been characterised by means of XPS and EPR spectroscopy. The VO_x/ZrO₂(ZV) samples were prepared by adsorption from ammonium metavanadate solutions (AV) at pH 1, 2, 3 or 4. With dilute AV solutions (⩽5 mmol 1^–1) at pH 2, 3 or 4, adsorption led to a plateau of about 3 V atoms nm^–2. More concentrated solutions induced precipitation. At pH 1, the V uptake increased with the AV concentration, reaching 2.4 V atoms nm^–2 at [AV]= 26 mmol 1^–1. This pH value did not lead to precipitation. On AP samples and after heating in O₂ at 773 K, XPS showed the presence of V^V only, uniformly spread on the zirconia surface. No EPR signals were detected on either the AP or HO samples. The reduction extent (e/V, electrons per V atom determined from the CO consumed or CO₂ produced) was equal to 1 at 500 K and 1.5 at 800 K. After reduction at 473 K, XPS showed the presence of V^V and V^IV. After reduction at 773 K, XPS revealed V^IV and V^III. At both temperatures, EPR revealed the presence of V^IV. In very dilute samples (⩽0.12 V atoms nm^–2) prepared at pH 1, EPR detected isolated mononuclear V^IV(in a square-pyramidal configuration). In samples with the same V loading, but prepared at pH 2, 3 or 4, EPR showed magnetically interacting V^IV, in addition to isolated mononuclear V^IV. In ZV with a V content > 0.5 atoms nm^–2, irrespective of the preparation pH, magnetically interacting V^IV were prevalent. These V^IV species arise from the reduction of surface polyoxovanadates, possibly decavanadates. In these ZV samples, the intensity ratio of the different V^IV species, (isolated):(magnetically interacting), does not depend on the pH of the preparation. All catalysts showed reversible behaviour in the redox cycles with CO and O₂.