Hydrogen activation, uptake and reaction are important phenomena in heterogeneous catalysis, fuel cells, hydrogen storage devices, materials processing, and sensing. Much attention has been devoted to materials that exhibit facile activation and weak binding of hydrogen, as these properties lead to the best energy landscape for storage or chemical reactivity. Spillover is a common method by which a reagent can be activated at one location and then reacted at another, and it is commonly invoked to explain the synergistic relationship between metals in an alloy or metal/metal oxide mixtures. The traditional view of these processes involves molecules adsorbing somewhat uniformly over a surface, migrating to preferred/active sites, followed by reaction or desorption. For example, in heterogeneous catalysis hydrogen spillover from metal particles to reducible oxide supports is implicated as an important step in a variety of reactions including hydrogenations, hydroisomerizations, and methanol synthesis. Hydrogen spillover has also been shown to significantly enhance the performance of hydrogen storage materials such as metal organic frameworks, zeolites and many carbon-based nanostructures. Despite these advances, the mechanism of spillover in most systems remains poorly understood, and with the exception of hydrogen bridges in storage systems, methods for mediating the spillover pathway do not exist.We discovered a system in which the uptake and release of hydrogen from a copper surface occurs solely through 1% of the surface sites which are individual, isolated, catalytically active palladium atoms. The surface can either be kept free of hydrogen by pre-adsorption of carbon monoxide or that hydrogen can be trapped on the surface by post-adsorption. In this way the coverage of the surface as a whole can be controlled by the addition of a single molecule to a minority atomic-scale site which we term a molecular cork effect. This non-equilibrium effect can be used to produce superheated hydrogen atoms, a more reactive form than regular surface bound hydrogen. We also demonstrate in a model system that the molecular cork effect could offer a new method for mediating the kinetics of hydrogen uptake and release from storage materials that rely on spillover from small metal particles.

New method for controlling hydrogen spillover, reaction, and storage: The molecular cork effect

247th National Meeting (2014)

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