Structure and activity of flame made ceria supported Rh and Pt water gas shift catalysts

Abstract

A series of ceria-supported Rh and Pt catalysts was synthesized by flame spray pyrolysis for high temperature water gas shift (WGS) reactions in a membrane reactor. The structural and catalytic properties were compared with a Pt/CeO2 catalyst prepared conventionally by incipient wetness impregnation. BET results show that compared to conventionally prepared catalysts the specific surface area of the flame made catalysts is larger and decreases with increasing metal content. This is in line with X-ray diffraction (XRD) results uncovering a high nanocrystallinity of the ceria, whose crystallite size slightly increases with higher noble metal loading. Both Pt and Rh particles are not detected by XRD due to the small crystallite size. In fact, transmission electron microscopy (TEM) images of the flame made catalysts with high loading showed Pt and Rh particles with diameters of ca. 3nm which remained stable after catalytic testing. X-ray absorption near edge spectra (XANES) at the Rh K- and Pt L3-edge showed that both Pt and Rh were in an oxidized state after preparation. In situ XANES during temperature programmed reduction in 5% H2/He indicated a lower reduction temperature than for the conventional Pt/CeO2 catalyst. Hence, Rh and Pt species are located on the surface of the CeO2 particles. Extended X-ray absorption fine structure (EXAFS) spectra provide further evidence for oxidized Rh, Pt before reduction and full reduction after activation in hydrogen, and small particle sizes in agreement with the TEM and XRD results. The catalytic activity measurements were performed at atmospheric pressure in two different gas mixtures by measuring CO conversion as a function of temperature. In all cases the Pt/CeO2 catalysts prepared by flame spray pyrolysis showed high catalytic activity and much higher selectivity than the Rh/CeO2 catalyst, where methane formation is observed above 300°C. Furthermore, the reaction rate in terms of turnover frequencies is much higher on the Pt-based catalysts, which makes them promising candidates for high temperature WGS catalysts in membrane and microreactors.