Catalytic combustion of butane on Ru/γ-Al2O3 catalysts

Abstract

Ru/γ-Al2O3 catalysts were prepared by incipient wetness method from RuCl3 precursor and their performance in oxidation of iso- and n-butane was investigated. The catalytic activity was examined on the calcined–reduced and on the directly reduced catalysts, with and after removing Cl ions, as well as after the oxidation at 250°C. All catalysts have been characterized by various methods such as BET, XRD, TEM, XPS, H2 chemisorption and O2 uptake in order to correlate their performance with their physiochemical properties. It was found that pre-treatment procedure has a significant influence on activity of the Ru/γ-Al2O3 catalysts. The considerable contamination of Ru particles by chlorine ions results in lowering activity of the Ru/γ-Al2O3 catalysts. It was established that reduced nearly Cl-free Ru/γ-Al2O3 catalysts exhibited higher activity than catalysts oxidized at 250°C. It was shown by XRD, TEM and O2 uptake data that at this temperature, very small Ru particles were transformed completely into RuO2, while the large Ru particles were partly covered by a thin RuO2 film (1.6nm). Catalytic and characterization results revealed that the most active sites in the butane oxidation reaction, consist probably of a few layers of thick surface oxide on the large Ru particles and small RuxOy clusters without well-defined stoichiometry. Such surface species were formed at 100–200°C in all catalysts and in the most active reduced Cl-free 4.6% Ru catalyst which reached 100% butane conversion below 200°C. The catalytic performance of Ru declined as the catalysts were oxidized at higher temperature. The activity loss was attributed to the formation of crystalline RuO2 phase and to some sintering of the active phase. In the used catalysts, small Ru particles were oxidized to RuO2 while the large Ru particles were covered with RuO2 layer, with a thickness of 2–3nm, as shown by TEM and XRD.