Methane combustion over bimetallic Ru-Re/γ-Al2O3 catalysts: Effect of Re and pretreatments

Abstract

In this paper, we have presented the results of the studies on methane combustion over bimetallic Ru-Re/γ-Al2O3 catalysts containing 1 or 3wt.% of Re, and their comparison with those obtained over 5% Ru/γ-Al2O3 catalyst. The catalytic activity was examined on reduced catalysts and then aged in air at 500°C under O2-rich conditions. All catalysts were characterized by various methods such as BET, XRD, TEM, HAADF-STEM, EDS and H2-TPR in order to correlate their activity with their physiochemical properties. A positive effect of the ageing treatment on the catalytic performance was observed for the both bimetallic Ru-Re catalysts but not for the Ru catalyst. The methane oxidation rate over the aged Ru-Re catalysts was about 5–10 times higher compared to that of the aged Ru catalyst. Characterization data show that at ageing temperature of 500°C small Ru or RuRe particles were transformed into larger RuO2 particles and/or highly dispersed ReOx species strongly interacting with γ-alumina. The mean crystallite size of RuO2 decreased from 25 to 14nm when the Re loading had been changed from 1 to 3wt.%, while in the monometallic Ru catalyst it was higher and amounted to 30nm. Among the reduced catalysts, the 5%Ru-1%Re catalyst exhibited the highest activity for combustion of methane. Also, for this sample apparent activation energy (104kJmol−1) was lower as compared to that of the monometallic Ru catalyst (116kJmol−1), indicating that Re slightly lowers the methane activation barrier. For the Ru as well as bimetallic Ru-Re catalysts an instability of methane conversion as a function of time (80h) at a given temperature (500°C) was observed. The instability was associated with slow formation of larger RuO2 crystallites (40–45nm) during the course of the reaction. In summary, the addition of Re improves the catalytic activity of the Ru/γ-Al2O3 catalysts for methane combustion but thermal stability of the bimetallic samples is similar to that of the unpromted Ru catalyst.