A transient in situ infrared spectroscopy study on methane oxidation over supported Pt catalysts

Abstract

Catalysts with platinum dispersed on alumina, ceria and mixed alumina–ceria have been prepared by incipient wetness impregnation, characterized with transmission electron microscopy and X-ray diffraction, and evaluated for total oxidation of methane under both stationary and transient gas compositions (oxygen pulsing). Further, in situ diffuse reflectance Fourier transformed infrared spectroscopy has been used to monitor the evolution of surface species during precise transient gas composition changes using high-speed gas switching valves. The results show that platinum has a sufficiently strong interaction with all the supports so as to form small platinum particles. The smallest sizes are observed for the Pt/Ce sample. The alumina containing samples show, in contrast to the Pt/Ce sample, a decreased methane conversion with increasing oxygen concentration and a clear kinetic bistability between increasing and decreasing oxygen concentrations. The bistable kinetics is likely connected to oxidation and reduction of platinum occurring for different stoichiometric gas mixtures depending on the history of the system, for which an oxidation of the platinum particles effectively inhibits the dissociative adsorption of methane leading to a low reaction rate. It is shown for the alumina containing samples that the adverse effects of oxygen excess can be circumvented by the use of periodic operation so that the average methane conversion is improved. Further, the Pt/Ce sample seems to exhibit additional active sites at the platinum–ceria interface explaining the generally higher conversion of methane for this sample.