Phase development in the catalytic system V 2O 5/TiO 2 under oxidising conditions

Abstract

The target of this work was to investigate phase development in the catalyst system consisting of TiO 2 (anatase) and V 2O 5 (Shcherbinaite). Thus, a set of V 2O 5/TiO 2 specimens was prepared by ball milling and exposed to subsequent annealing in air in the temperature range from 400 to 700 °C. The XRD-results showed the presence of anatase and shcherbinaite as the only phases up to 525 °C. For temperatures above 525 °C the peak intensities were diminishing and rutile as a new TiO 2-phase occurred. Peak intensities and positions were shifted. No loss of oxygen or vanadium was detected. The reaction involves the formation of a rutile solid solution containing VO x species. XPS studies showed an oxidation state of 4.75 for V in the rutile solid solution as compared to 4.65 in the shcherbinaite. A rutile solid solution once formed could not be re-transformed. The rutile solid solution was first found at 525 °C < T < 550 °C for compositions of 3 mol% < V 2O 5 < 5 mol%. The phase field for rutile solid solutions extends to 10 mol% < V 2O 5 < 12.5 mol% at 675 °C. For very high V 2O 5 concentrations (95 mol% V 2O 5) a eutectic reaction was found at 631 °C. The DTA runs showed a widened endothermic melting peak and a very sharp crystallization peak on cooling. A shcherbinaite structure remained with shifted peak intensities and positions due to the alloying of Ti-ions. SEM inspections showed that the rutile formation and the eutectic reaction both cause a substantial grain growth and a loss of surface area. The catalytic activity is entirely lost when the rutile formation occurs. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.