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Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies

Abstract

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, ‘spectators’ that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2—a central step in many industrially important catalytic processes—is generally assumed1 to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation2,3,4,5,6, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking1, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

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Figure 1: STM images of hydrogen atoms on Pd(111).
Figure 2: STM images from a movie showing the formation, separation and annihilation of H-vacancy clusters.
Figure 3: STM images from a movie showing the formation and annihilation of a four-vacancy (4V) cluster.
Figure 4: Lifetimes of three-vacancy clusters.

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Acknowledgements

This work was supported by the Office of Basic Energy Science of the US Department of Energy.

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Correspondence to M. Salmeron.

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Supplementary information

41586_2003_BFnature01557_MOESM1_ESM.mpg

Supplementary Movie: Hydrogen vacancy diffusion, 180 seconds. In this movie, vacancies appear as bright spots surrounded by dark rings, while the periodic background represents the 1×1 H adlayer. (MPG 552 kb)

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Mitsui, T., Rose, M., Fomin, E. et al. Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies. Nature 422, 705–707 (2003). https://doi.org/10.1038/nature01557

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