Phase Relations in the System TiO 2 -V 2 O x under Oxidizing and Reducing Conditions
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Section I: Basic and Applied Research
Phase Relations in the System TiO 2 -V 2 O x under Oxidizing and Reducing Conditions D. Habel, O. Goerke, M. Tovar, E. Kondratenko, and H. Schubert
(Submitted February 7, 2008; in revised form June 16, 2008) The target of this work was to investigate the phase development in the catalyst system consisting of TiO2 (Anatase) and V2O5 (Shcherbinaite) under several gas atmospheres. Thus a set of V2O5/ TiO2 specimens was prepared by ball milling and exposed to subsequent annealing in air and feed gas in the temperature range from 400 to 700 °C. The XRD-results showed that the initial phases Anatase and Shcherbinaite remain stable for all atmospheres containing oxygen. In the temperature range above 525 °C the formation of a Rutile solid solution (Rutile-ss) containing VOx species takes place. However, under reducing conditions (lower oxygen partial pressure) the reduction of V2O5 to V2O3 was found by X-ray diffraction measurements. There is no miscibility up to 1300 °C followed by the formation of V2TiO5 (Berdesinskiite). SEM images underline the reduction by monitoring the change in morphology with respect to the V-containing phases. TiO2 remains without much alteration. The two phases V2Ti7O17 and V2Ti3O9 (Schreyerite) as described in mineralogy have not been observed in these experiments. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.
Keywords
Anatase, catalysis, phase diagram, Rutile, Shcherbinaite, TiO2, V2O3, V2O5
1. Introduction Polyolefins are a class of substances from petrol industry and basis of a large number of chemical products. A route to produce propene at low temperatures is the oxidative dehydrogenation of propane (ODP). One of the key steps of the ODP reaction is the induction of double bonds which is associated by removal of two hydrogen species. This hydrogen reacts at the surface with an oxygen species donated by the catalyst. Experiments with isotope labeled gases[1] proved that the oxygen does not come from the feed gas but from the catalyst. The partial oxidation may be considered as a key step in olefine processing and also as a model reaction. The common reaction path is discussed in literature as a Mars-van Krevelen mechanism.[2,3] It has been found that vanadium based systems are the most selective catalysts for the oxidation of hydrocarbons.[4] There have been reported numerous applications of the TiO2-V2O5 system.[5-10] Especially the Anatase-Rutile transformation was focused by many authors because it is considered to represent the major factor in catalyst D. Habel, O. Goerke, and H. Schubert, Institute of Material Science and Technologies, TU Berlin, Englische Straße 20, D-10587 Berlin, Germany; M. Tovar, Division Structure Research, Hahn-MeitnerInstitute, Glienicker Straße 100, D-14109 Berlin, Germany; E. Kondratenko, Leibniz Institute for Catalysis, University Rostock, Branch Berlin, Richard-Willsta¨tter-Straße 12, D-12489 Berlin, Germany. Contact e-mail: habe
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