Ambient Pressure PES Study of CeO2 Supported Catalysts for Water Gas Shift Reaction

Ceria (CeO2) supported Pd/Fe catalysts for water-gas shift (WGS) reaction were analyzed in situ under 0.5mbar pressure using ambient pressure synchrotron-based Photoemission Spectroscopy (PES). High-resolution core-level spectra were obtained both under reducing atmosphere of H2 at 300oC and under the WGS reaction mixture consisted of 7% CO, 2% H2O, 37% H2, 9% CO2 at 250oC at total pressure of 0.5mbar. To obtain depth profile, the photon energy of the exciting radiation was varied as to have the kinetic energy of photoelectrons ranging from 120eV to 420eV. The oxidation state of metals under different atmospheres and change of the composition as a function of depth were investigated. Pd/CeO2 catalyst is promoted by iron. Fe is not much reduced neither under WGS nor reduction environment. Ceria on the surface reduces to Ce2O3 both under the reducing and WGS reaction environment. The ratio of the reduced ceria decreases as the sampling depth is increased. Pt concentration on the surface stays around the same value under both environments. The WGS rate of the Fe promoted sample is 6.6 x10-2 and that of un-promoted catalyst is 2.7 x10-2 mole H2 mole Pt-1 s-1 at 250oC under our standard conditions*, a promotion of a factor of around 2.4, when compared with the un-promoted Pd catalyst. Considering the fact that the ceria supported Pd has the highest rate among different supports (Al2O3, ZrO2), an additional increase of the rate is worth mentioning. Additional systems studied in this work are the ceria supported Pt catalyst promoted with iron and zirconia supported Pd promoted with iron. Based on the findings, models describing the structure of catalysts were developed and compared to images obtained by High-Resolution Transmission Electron Microscopy (HR-TEM).