Selective hydrogenation of acrolein over supported silver and silver alloy catalysts.

摘要

This thesis aims to investigate multiple critical properties of catalysts which can affect the activity and selectivity greatly in heterogeneous catalysis. Two reactions which attract wide and extensive attention were chosen as test reactions for this study: the selective hydrogenation of α,β-unsaturated aldehyde to α,β-unsaturated alcohol (using acrolein as model reactant) and the water gas shift reaction. The former was studied experimentally and the latter was studied theoretically.;For the selective hydrogenation of acrolein to allyl alcohol , it is a particularly difficult reaction to obtain satisfying selectivity towards C=O bond hydrogenation thermodynamically and kinetically. The hydrogenation of C=C bond is about 35 kJ/mol easier than that of C=O bond. In the particular case of acrolein, the lack of substituents at C=C bond makes it especially vulnerable to hydrogenation. Previous research has made great progress in understanding some of the factors (choice of metal, process condition) that may improve selectivity. But no catalyst exists with both high activity and selectivity. Thus we studied this reaction systematically from four aspects: 1) particle size effects; 2) support effects; 3) utilizing single atom alloy to improve the performance of catalysts; 4) employing single site heterogeneous catalysts. Some of the aspects were well understood through our study and others results were preliminary but very intriguing and promising.;For the water gas shift reaction, the effect of alloying was studied for a particular example: PdZn. Previous research showed that Pd-Zn alloy was a promising catalyst for the related steam reforming of methanol and that its electronic structure closely mimics that of Cu, the commercial WGS catalyst. Three mechanisms of water gas shift were investigated on PdZn(111) and NiZn(111) surfaces. NiZn was expected to be a cheaper substitution for PdZn. But despite its close similarly, NiZn actually shifts the primary mechanism from the carboxyl route to one where both the redox route and the carboxyl route are important.