Isotope effects in hydrogen adsorption on Ni(111): Direct observation of a molecular precursor state

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Abstract
The nature of the interaction of hydrogen with Ni(111) is probed by comparing the adsorption and desorption kinetic behavior of H2 and D2. Pure H2 and pure D2 adsorbed on Ni(111) at 140 K exhibit identical desorption behavior. The adsorption rate law depends upon (1‐θ) rather than (1‐θ)2, suggesting that the rate determining step involves the interaction of an H2 molecule with a single Ni site. The temperature programmed desorption spectra show two desorption features, β1 and β2, which have maxima in desorption rate at 290 and 370 K. When either pure H2 and pure D2 or a mixture of H2 and D2 interact with the Ni(111) surface at 87 K, evidence for kinetic retardation of D2adsorption is observed. When the H2:D2 mixture is exposed to the Ni(111) surface, held at 87 K, an additional desorption feature, α, is uniquely observed at 100 K for D2. α‐D2desorbs with first order kinetics exhibiting E d =11.1 kJ/mol and a preexponential factor of 1.2×105 s− 1. It may be due to desorption from an intrinsic molecular precursor state. The observed differences between hydrogen and deuterium adsorption indicate the presence of an activation energy barrier for dissociative chemisorption, and a sensitivity of the adsorption process to differences in zero point vibrational energies. A molecular precursor state is proposed to describe the H2+Ni(111) interaction. Dissociative adsorption of hydrogen on intentionally produced surface defects has been shown to destabilize α‐D2.