Probing Hydrogen Spillover in Spatially Proximal Bimetallic Catalysts

Abstract

Hydrogen spillover is a phenomenon that happens over heterogenous catalyst surfaces and may be described as the H2 dissociation over an initial metal surface and migration of the resulting adsorbed hydrogen (Hads) to or across the catalyst support surface. The transferred Hads can promote less active catalytic sites (including other metals that are unable to promote enough H2 dissociation during hydrogenation reactions). These processes are hotly debated in the literature and hard to demonstrate. This thesis showcases the development and application of a well-defined catalyst material containing two spatially separate but proximal metal sites for hydrogen spillover. This work shows hydrogen spillover occurs even with non-reducible support, and the locations of H2 dissociation and Hads sites can also affect the reaction pathway, opening up opportunities for novel, efficient, and selective heterogeneous catalyst design, such as in multistep catalytic systems.
Hollow silicalite-1 samples, with a microporous zeolite shell and a large cavity, were used as the primary support to encapsulate the hydrogen dissociation site, Pd nanoparticles. Ultra-small Cu nanoparticles (2 – 5 nm) were dispersed over the silicalite-1 shell as the secondary active site. The preparation and optimal synthesis of these two components were thoroughly investigated, along with their catalytic performance in their own right. When testing the proximal PdCu catalyst in the vapour phase furfural hydrogenation, the activity of external Cu particles was found to be improved by the internal Pd participating. The enhanced furfuryl alcohol pathway %yield of the proximal PdCu catalyst suggests that the Cu species is promoted when part of the proximal bimetallic catalyst. An in-situ TPR/XANES study over the proximal bimetallic catalyst demonstrates a lower reduction temperature for Cu in the proximal bimetallic than the analogous Cu-only system, indicating the presence of hydrogen spillover on a ~20 nm length scale (minimum), even though the Pd and Cu are spatially separated. The success of the designed proximal bimetallic catalyst in hydrogen spillover provides a motivation for seeking to applying hydrogen spillover for real industrial selective hydrogenation.