Photoelectron Spectroscopy (XPS) and Thermogravimetry (TG) of Pure and Supported Rhenium Oxides. 2. Rhenium Oxides on Titanium Dioxide

Abstract

AbstractAmmonium perrhenate supported on titanium dioxide has been investigated in order to elucidate the interaction between the carrier and the dispersed species.The samples, containing up to 14% by weight of rhenium, have been prepared by impregnation with an ammonium perrhenate solution.The study, performed using X‐ray photoelectron spectroscopy and thermogravimetric techniques, demonstrates that a different situation is present depending on the starting rhenium content and gas environment. At low rhenium content (0–2.5%), rhenium is mainly atomically dispersed as per‐rhenate‐like surface compound; on heating in H2 up to 500°C, the surface compound is reduced to metal which is still in a dispersed form. In the range 2.5–10% in addition to the perrhenate‐like surface compound, small crystallites of NH4ReO4 are initially present on the surface. During the heat treatment in hydrogen these crystallites are reduced to ReO2. The incorporation of Re(IV) in the TiO2 structure preserves it from further reduction. At higher rhenium content large crystallites of ammonium perrhenate are present on the titanium dioxide surface. They show the reduction behaviour expected for this compound.The data also indicate that when metallic rhenium is heated in air, the heptoxide formed reacts with the free surface of the support and is not lost by volatilization.