A study of effect of particle size on the oxidation of rhenium in the Re/γ-Al 2O 3 catalysts

Abstract

The oxidation of the reduced Re/γ-Al 2O 3 catalysts with dispersion of Re from 0.25 to 0.74 and metal particle sizes of 1–9 nm, was examined over a temperature range 20–500 °C. The structure of the catalysts was characterised using H 2 chemisorption, O 2 uptake, BET and XPS and Raman spectroscopy. The O 2 uptake and XPS results showed that very small Re particles were in direct and strong contact with the oxygen atoms of the support. As a result, the O 2 uptakes were low at room temperature and even at 500 °C were lower than expected for oxidation of Re 0 to R 2O 7 oxide. On the other hand, XPS data indicated that at room temperature clusters and small Re particles were oxidised to Re 4+, Re 6+ and Re 7+ species. The O/Re ratios at temperatures up to 200 °C increased with the Re dispersion what implies additionally, high affinity of the highly dispersed Re to oxygen. Fraction of rhenium strongly interacting with the support amounts to 6 or 19% of total Re in the catalysts with average particle sizes of about 5 or 1.5 nm, respectively. At higher temperatures the process of oxidation of rhenium accelerates, with instantaneous sublimation of Re 2O 7 oxide and in accord to Raman data, its simultaneous adsorption as the ReO 4 species on γ-alumina. The O 2 uptake at 500 °C by the supported Re phase depends on the size of rhenium particles. Small Re particles could be oxidised to an oxidation level lower than heptvalent, while large particles to Re 2O 7. XPS and Raman data showed, however, that after treatment at 500 °C, independent on the particle size of the Re and its interaction with the support, whole rhenium was oxidised to Re 7+ species. This species forms some kind of surface complex with an Al O ReO 3 or Al (O ReO 3) 3 structure, which inhibits the surface migration of rhenium and its loss from the oxidised catalysts.