Quantum Nature of Hydrogen on Metals: Dissociative Adsorption

When hydrogen is adsorbed on a Pd(111) surface, very simple vacancy defects -- which form quite commonly -- exhibit fundamentally quantum wavelike behavior. We show that a full quantum treatment of the hydrogen nuclear motion, in addition to electron motion, predicts that the dissociative adsorption of H2 will be strongly suppressed at divacancies---in violation of the standard description from classical Langmuir adsorption kinetics---but will easily occur at trivacancies. Recent experiments confirm that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation, while divacancies are inactive [Mitsui et al, Nature 422, 705 (2003)]. Our findings indicate that traditional classical explanations for such observations require revisiting, and suggestthat the quantum nature of hydrogen may play a surprisingly prominent role in future hydrogen technologies.