The preferential oxidation of CO in the presence of hydrogen, or PrOx, is an important potential purification step in the production of hydrogen. The PrOx reaction essentially consists of two competing reactions: H2 and CO oxidation. Our group has studied PrOx on supported Au nanoparticle catalysts and shown that adding water is a co-catalyst in oxygen activation, providing a proton during oxygen binding. Incorporating water into the PrOx reaction dramatically improves activity and selectivity for CO oxidation. In this presentation, we report our most recent studies designed to improve our understanding of the role of H2O in H2 oxidation. Kinetic studies were performed on Au/TiO2 catalysts, carefully controlling the support water coverage and H2 conversion to keep conversions below ca. 15%. Water was found to be a strong inhibitor of H2 oxidation; this clearly contributes to the improved selectivity in PrOx. DFT calculations suggested that H2 activation occurs by heterolytic H-H cleavage, resulting in a formal Au hydride and a proton on either the support or adsorbed water. The DFT predictions also suggested significantly different activation barriers for these two subtly different different reaction pathways. In support of these calculations, we determined experimental apparent activation energies under two constant coverage conditions. The first was under constant coverage where there was always less than one monolayer of water present on the support. The second was also under constant coverage, but at greater than one monolayer of water. The two experimentally determined apparent activation barriers differed by about a factor of 2 (~30 kJ/mole for less than 1 ML water and ~70 kJ/mole for greater than 1 ML water). These values agreed well with the DFT calculations of 37 and 73 kJ/mole, respectively. The experimental results support the prediction that H2 activation occurs through a "proton-hydride" mechanism over Au. Further, this mechanism appears to be surprisingly sensitive to the Bronsted base that serves as the proton acceptor; this may account for some of the strong support effects reported in the literature.

Kinetic investigation into the mechanisim for H2 activation on supported Au nanoparticles

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.