Site-specific reactivity of molecules with surface defects—the case of H2 dissociation on Pt

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Science  11 Jan 2019:
Vol. 363, Issue 6423, pp. 155-157
DOI: 10.1126/science.aau6716

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Broken on impact

Two competing models have been proposed for the adsorption of molecular hydrogen on platinum surfaces. Both invoke dissociation at surface defects but differ on whether hydrogen molecules diffuse along the surface before encountering a defect or adsorb only if they initially impact a defect site. Van Lent et al. studied the sticking of hydrogen molecules from a molecular beam scanned across a curved platinum single-crystal surface that varied in the density and type of defects exposed. Modeling of the results was consistent only with the second model invoking direct impact.

Science, this issue p. 155


The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.

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