Controlling a spillover pathway with the molecular cork effect

Abstract

Spillover of reactants from one active site to another is important in heterogeneous catalysis and has recently been shown to enhance hydrogen storage in a variety of materials [1][2][3][4][5][6][7] . The spillover of hydrogen is notoriously hard to detect or control 1,2,4-6 . We report herein that the hydrogen spillover pathway on a Pd/Cu alloy can be controlled by reversible adsorption of a spectator molecule. Pd atoms in the Cu surface serve as hydrogen dissociation sites from which H atoms can spillover onto surrounding Cu regions. Selective adsorption of CO at these atomic Pd sites is shown to either prevent the uptake of hydrogen on, or inhibit its desorption from, the surface. In this way, the hydrogen coverage on the whole surface can be controlled by molecular adsorption at a minority site, which we term a "molecular cork" effect. We show that the molecular cork effect is present during a surface catalyzed hydrogenation reaction and illustrate how it can be used as a method for controlling uptake and release of hydrogen in a model storage system 1,2,[4][5][6]8 .Hydrogen activation, uptake, and reaction are important phenomena in heterogeneous catalysis, fuel cells, hydrogen storage devices, materials processing and sensing [1][2][3][4][5][6][7][8][9][10][11] . 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 [12][13][14] . 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 1,[3][4][5][6][7]12,15 . 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 1,3,6 . 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 2,4-6 . In these cases, the addition of small metal particles, typically Pt or Pd, promotes uptake by activating molecular H 2 and facilitating spillover of hydrogen atoms (H a ) onto the support. Despite these advances, the mechanism of spillover in most systems remains poorly understood, and with the exception of hydrogen bridges 16 in storage systems, methods for mediating the spillover pathway do not exist. In this paper we describe how the hydrogen spillover pathway on the Pd/Cu alloy system can be controlled via the reversible adsorption of a spectator molecule (CO) at minority Pd atom sites. The use of a model system amenable to study by scanning tunnelling microscopy (STM) was critical in order to monitor the detailed distribution of Pd atoms, H a and CO molecules, all of which are distributed heterogeneously at the atomic-scale. This information ca...

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