A study of the catalytic properties of ceria-based materials in the WGS reaction and hydrocarbon oxidations.

摘要

This thesis aims at providing a better understanding of ceria-based materials for catalytic applications through the investigation of the effect of dopants. First, ceria-supported, precious-metal catalysts were examined for the water-gas shift reaction. Fe promoters had no effect on ceria-supported Pt and Rh, but could increase the activity of Pd/ceria by almost one order of magnitude. XRD measurements, performed after running the catalyst under WGS conditions, showed the formation of an Fe-Pd alloy suggesting this Fe-Pd alloy, possibly in equilibrium with Fe3O4, brings a thermodynamic driving force to accelerate the WGS reaction. Surface Mo was found to decrease the water-gas-shift (WGS) activity linearly with Mo coverage up to 1.8 Mo/nm2. TPD studies with 2-propanol on the Mo-containing ceria demonstrated a relationship between the loss of WGS activity and ceria sites that decompose the alcohol to propene and water. CO-O2 pulse measurements suggest the Mo-containing surface is much harder to reduce than pure ceria. These results imply MoOx deactivated the WGS activity by blocking the ability of ceria to undergo oxidation and reduction at the surface.; Solutions of CeO2 with Yb2O3, Y 2O3, Sm2O3, Gd2O3, La2O3, Nb2O5, Ta2O 5, and Pr6O11 were also investigated for n-butane oxidation. Each of the doped samples exhibited a much lower catalytic activity than that of pure CeO2. Further investigation on CeO2 and Ce0.8Sm0.2Ox (SDC) catalysts demonstrated the rates for methane and ethane were indistinguishable over the two catalysts, while the rates for propane and n-butane were much higher on ceria compared to SDC. The difference between n-butane oxidation is shown to result from a low-temperature rate process on ceria that is not present on SDC. Pulse studies demonstrate oxygen from the bulk becomes accessible at approximately the same temperature as that at which the high-temperature rate process becomes important. These results suggest the addition of Sm2O3 and other dopants deactivate the catalyst by suppressing the surface redox ability of ceria in the low temperature range. Through this work, a connection between the redox properties and catalytic properties of ceria-based materials has been established.