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

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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 VOx/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 VV 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 VV and VIV. After reduction at 773 K, XPS revealed VIV and VIII. At both temperatures, EPR revealed the presence of VIV. In very dilute samples (0.12 V atoms nm–2) prepared at pH 1, EPR detected isolated mononuclear VIV(in a square-pyramidal configuration). In samples with the same V loading, but prepared at pH 2, 3 or 4, EPR showed magnetically interacting VIV, in addition to isolated mononuclear VIV. In ZV with a V content > 0.5 atoms nm–2, irrespective of the preparation pH, magnetically interacting VIV were prevalent. These VIV species arise from the reduction of surface polyoxovanadates, possibly decavanadates. In these ZV samples, the intensity ratio of the different VIV 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.

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