Effect of redox thermodynamics on activity and deactivation of supported and mixed-oxide catalysts.

摘要

In this dissertation, the effect of redox thermodynamics on the activity and deactivation of various morphologies of heterogeneous catalysts was investigated. It was observed that metal-support interactions can influence the redox thermodynamics of catalysts, and consequently a core-shell morphology was developed in this work, where the interaction between the active metal core and oxide support was enhanced.;The selection of a proper support with the appropriate redox for a specific catalytic application was demonstrated in several examples. Pd-promoted PrO x and CePrOx mixed oxides were examined whether their easily reducible oxygen would be beneficial in the water-gas-shift reaction. The catalysts deactivated on-stream, with the oxygen being too weakly bound and unable to be reoxidized by steam. In another example, the redox properties of vanadium phosphorus oxide catalyst for the partial oxidation of butane to maleic anhydride were examined and were found to be significantly easier to reduce than any of the previously studied vanadium-containing catalysts, suggesting that the weakly bound oxygen are likely important for the high catalytic activity.;The effect of redox thermodynamics on the reported deactivation process of supported Co nanoparticles for the Fischer-Tropsch synthesis reaction was also examined. The thermodynamic properties of the cobalt nanoparticles was found to be similar to those of bulk cobalt, while low loadings of cobalt supported on zirconia shifted the transition of cobalt redox, CoO↔Co, to a lower P(O2) than that of bulk cobalt, suggesting the presence of strong metal-support interactions.;To further investigate these enhanced metal-support interactions, the core-shell morphology composed of a metal core surrounded by an oxide support was developed. Specifically, Pd and Pt cores with CeO2, TiO 2 and ZrO2 shells were synthesized, supported on Al 2O3, and evaluated for the water-gas-shift reaction. Pd@CeO 2/Al2O3 and Pd@TiO2/Al2O 3 catalysts exhibited transient deactivation, compared to Pd@ZrO 2/Al2O3 catalyst which deactivated slower. To understand this behavior, the reducibility of Pd@CeO2/Al2 O3 was compared to Pd/CeO2. Under water-gas-shift conditions, the CeO2 shell of Pd@CeO2/Al2O 3 was found to be reduced while the CeO2 in Pd/CeO2 remained oxidized. This reduction of the ceria shell in the core-shell catalyst appeared to affect the CO accessibility of the core following catalyst reduction.