Various samples of vanadia supported on zirconia [as prepared (AP), after heating in O-2 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 (less than or equal to 5 mmol l(-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 l(-1). This pH value did not lead to precipitation. On AP samples and after heating in O-2 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 (less than or equal to 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 O-2.
Characterisation of VOx/ZrO2 catalysts by electron paramagnetic resonance and X-ray photoelectron spectroscopy
Roberto Dragone;
1996
Abstract
Various samples of vanadia supported on zirconia [as prepared (AP), after heating in O-2 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 (less than or equal to 5 mmol l(-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 l(-1). This pH value did not lead to precipitation. On AP samples and after heating in O-2 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 (less than or equal to 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 O-2.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
