Pt-Sn bimetallic catalysts serve important applications, including reforming and cracking, selective hydrogenation, and direct oxidation in methanol fuel cells. We have employed parahydrogen enhanced NMR techniques to gain insight into the active sites, interactions and surface processes associated with hydrogenation on Pt(111), Pt3Sn(111) and PtSn(110) intermetallic nanoparticle catalysts. Alloying Sn into the Pt(111) surface is known to produce electronic and geometric changes that affect the adsorption of substrates and diffusion of hydrogen on the surface, as has been extensively studied. While diffusion of hydrogen on Pt(111) is very rapid with a barrier of only 5 kJ/mol, the calculated diffusion barrier on Pt3Sn(111) is 41 kJ/mol. Unfavorable interactions with tin have the effect of blocking H ad-atom diffusion. This lack of hydrogen mobility has profound implications for the kinetics of hydrogenation reactions on the surface and in the mechanism, which we studied using parahydrogen enhanced NMR techniques. The availability of Pt, Pt3Sn and PtSn nanoparticle catalysts that mimic the model surfaces with three-fold Pt sites that are contiguous, isolated, or absent, provided the opportunity for us to systematically investigate the dependence of parahydrogen induced polarization on these surface structures. While the activity for hydrogenation of propene follows the trend Pt&gt;Pt3Sn&gt;PtSn, the pairwise selectivity of hydrogenation (i.e. the fraction of the products formed by transfer of two protons originating from the same H2 molecule) exhibits the opposite trend. Moreover, the neat solvents such as water, methanol and ethanol are also hyperpolarized under certain conditions. The results are interpreted in terms of the competing effects of activation of dissociation versus restriction of diffusion, which protects the incipient singlet-state spin order of parahydrogen against statistical randomization.

Parahydrogen spin labelling studies of hydrogenation catalysis over silica-encapsulated Pt-Sn intermetallic nanoparticles

255th National Meeting (2018)

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