The relevance of Ru nanoparticles morphology and oxidation state to the partial oxidation of methane

The partial oxidation of methane over well-defined Ru nanoparticles supported on alumina was investigated in the 350-650 degreesC temperature range. A 12% Ru/Al2O3 catalyst was prepared by deposition of relatively monodispersed colloidal Ru nanoparticles of about 5.8 nm on alumina. The evolution of the chemical state and the morphology of Ru nanoparticles under the reaction conditions were followed by various techniques (TEM, H-2 chemisorption, XRD, TPR, and TPO). The experimental results suggest that the mechanism of partial oxidation of methane over the catalyst is related to the morphology (size) and chemical state of the supported Ru nanoparticles, as well as on the nature of the oxidizing agent (i.e., O-2 and NO). The formation of an abundant RuO2 phase in the reaction mixture (CH4/O-2 = 1.8), which is favored in the low-temperature region, was found to be responsible for the conversion of methane to total oxidation products. The RuO2 double left right arrow Ru equilibrium is shifted to the formation of Ru metal for reaction temperatures higher than 450 degreesC. At that moment, the reaction became ignited because of a change in the reaction mechanism. Above the ignition temperature, the reaction rate increased considerably, and CO and H-2 were simultaneously produced. In the high-temperature region the mass transport phenomena affects the reaction rate as well as the product distribution. A relationship between the reaction temperature and the ratio between RuO2 and total amount of Ru was quantitatively determined. At low temperatures, the alumina-supported Ru nanoparticles (initial size approximate to5.8 nm) show significantly higher catalytic activity and selectivity to CO and H-2, than the conventionally prepared catalysts. For example, the yields to CO and H-2 at 650 degreesC were 72.1 and 67.6%, respectively. The idea that the high catalytic performance of Ru nanoparticles can be ascribed to the better preservation of the metallic character under the reaction conditions as compared to the well-dispersed Ru particles is advanced. A brief reaction mechanism is proposed in light of the experimental results. (C) 2003 Elsevier Inc. All rights reserved.