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.
Low coordinated sites on catalytic surfaces often enhance reactivity, but the underlying dynamical processes are poorly understood. Using two independent approaches, we investigate the reactivity of O2impinging onto platinum and resolve how step edges on (111) terraces enhance sticking. At low incident energy, the linear dependence on step density, independence of step type, and insensitivity to O2’s molecular alignment show that trapping into a physisorbed state precedes molecular chemisorption and dissociation. At higher impact energies, direct molecular chemisorption occurs in parallel on steps and terraces. While terraces are insensitive to alignment of the molecule within the (111) plane, steps favor molecules impacting with their internuclear axis parallel to the edge. Stereodynamical filtering thus controls sticking and dissociation of O2on Pt with a twofold role of steps.
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