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

摘要

The partial oxidation of methane over well-defined Ru nanoparticles supported on alumina was investigated in the 350-650 deg C temperature range.A 12% Ru/Al_2O_3 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 nanoaprticles,as well as on the nature of the oxidizing agent (i.e.,O_2 and NO).The formation of an aboundant RuO_2 phase in the reaction mixture (CH_4/O_2=1.8),which is favored in the low-temeprature region,was found to be responsible for the conversion of methane to total oxidation products.The RuO_2<-> Ru equilibrium is shifted to the formation of Ru metal for reaction temperatures hgiher than 450 deg C.At that moment,the reaction became ignited because of a change in the reaction mechanism.Above the ignition tmeperature,the reaction rate increased ocnsiderably,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 production distribution.A relationship between the reaction temperature and the ratio between RuO_2 and total amount of Ru was quantitatively determined.At low temperatures,the alumina-supported Ru nanoaprticles (initial size approx = 5.8 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 deg C were 72.1 and 67.6%,respectively.The idea that the high catalytic performance of Ru nanoaprticles 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.