XPS study of oxidation of rhenium metal on γ-Al2O3 support

The oxidation of Re/γ-Al2O3 catalysts, containing 1 and 10 wt% of rhenium, sintered in hydrogen was examined in the temperature range of 20–800 °C. The structures of the catalysts were investigated by XPS spectroscopy, TEM, and O2 uptake measurements. The low-loaded catalyst comprises metallic particles with sizes of 1–4 nm (dav=2.1 nm), while the high-loaded catalyst comprises particles with sizes of 1–9 nm (dav=4.9 nm). Even short exposure to air at room temperature causes complete oxidation of small clusters of metallic Re, while larger particles are covered with very thin ReOx skin (undetected by TEM). XPS shows that the high-loaded catalyst still contains 94.5% of metallic Re, while the low-loaded catalyst contains only 60.5%. The remaining part of the Re is oxidized to Re4+, Re6+, and Re7+ species. Oxidation at 150 °C causes enhanced formation of Re4+–Re7+ species and the amount of metallic Re quickly decreases to 33 and 2% for high- and low-loaded catalysts, respectively. This indicates a high affinity of the highly dispersed Re to oxygen. At this temperature, the Re/Al atomic ratio increases 2–4 times, indicating a large spreading of the oxide species on the support surface. Simultaneously, the average size of Re particles decreased as determined by TEM. At 300 °C, whole Re was oxidized mainly to Re2O7, though some amount to Re4+ and Re6+ species remained. The O2 uptake measurements confirm oxidation of rhenium particles. For the high-loaded catalyst O2 uptake attained a maximum level (O/Re=3.3) already at 300 °C, while for the low-loaded catalyst even at 500 °C the uptake (O/Re=2.98) is below the maximum level. XPS data showed, however, that at 500 °C, oxidation of rhenium to Re2O7 occurs for both catalysts. The Re/Al atomic ratio remains nearly constant after oxidation of both catalysts at 300–800 °C, indicating that Re7+ species are firmly bonded to alumina surfaces even at 800 °C. A detailed mechanism of oxidation of Re particles with different sizes is proposed based on a quantitative analysis of the XPS, O2 uptake, TEM, and previous Raman results.