Combined Experimental and Computational Study of the Role of Promoters in Selective Hydrogenation
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This thesis provides a systematic insight into the observed catalytic performance of the Pd promoted Ag nanoparticles on SiO2 support for α,β-unsaturated aldehyde hydrogenation to allyl alcohol, and of V promoted Rh surfaces for CO hydrogenation to C2+ oxygenates. It addresses a fundamentally important and interesting problems of regioselectivity with respect to selective hydrogenation of C=O bond vs. C=C bond in unsaturated aldehydes, and metal-oxide interface mediated H-assisted vs. Direct CO dissociation mechanism. Multiple critical properties affecting the activity and selectivity of heterogeneous catalyst were investigated. Heterogeneity is usually present due to a broad size distribution of the supported metal particles with irregular morphology, various alloy phases in bimetallic alloys, and multiple types of active sites. As such, elucidation of reaction mechanisms is rarely straightforward. Experimental study of a well-dispersed, single atom alloyed Pd-Ag/SiO2 catalysts on a selective hydrogenation of acrolein to allyl alcohol, showed that the activity of the catalyst is a function of particle size, with minimal dependence on the amount of Pd. The selectivity to allyl alcohol of the Ag/SiO2 catalyst cannot be improved on, as it appears to be limited by the intrinsic property of the silver on a silica support. The performance of these catalysts appears to be a delicate balance between the surface morphology and alloying effect. The origin of this morphology-dependence might be related to the activation energy for the formation of the reaction intermediates on silver surface planes. Varying Pd to Ag ratio causes a change to H2 reaction order corresponding to the observed change in the rate-determining step in the reaction mechanism In the computational study of vanadium promoted Rh surfaces, metallic V promotion induced a charge transferred from each adjacent Rh to V in the RhV structure, creating a more reactive Rh site necessary for C-O bond breaking reactions. Promotion via surface oxide clusters (V6O12) was shown to be an example of a bifunctional catalyst, where the presence of both elements (V and Rh) is necessary for the proposed V3+/V4+ oxidation/reduction cycle. Metal-oxide interface can provide oxygen “vacancy” sites for CO adsorption and highly favorable C-O bond dissociation.
single site alloys