Initiation of Vacancy-Mediated, Surface Explosion Reactions: Tartaric and Aspartic Acid on Cu Surfaces

Abstract

Surface explosion reactions have highly nonlinear reaction kinetics that exhibit autoacceleration under isothermal conditions. These can lead to phenomena such as oscillatory surface reaction rates and to highly enantiospecific reactions of chiral adsorbates on chiral surfaces. Tartaric acid (TA) decomposes on Cu surfaces by an explosion mechanism that is propagated by vacancies, empty adsorption sites that self-replicate autocatalytically during TA decomposition. Surface explosion kinetics result from chain-branching steps in which one vacancy decomposes an adsorbate to yield two vacancies. In the absence of vacancies, surface explosions cannot occur; they require some initiation step that creates vacancies. By comparison with the chain-branching explosion step, little is known about the processes that initiate or nucleate surface explosion reactions. Time-resolved XPS measurements during the early stages of explosion initiation of TA/Cu(hkl) reveal a process that involves direct loss of TA from the surface to create the initial vacancies. In the presence of a gas phase flux to the surface, such vacancy nuclei can be repopulated to suppress the onset of explosion. Measurements on 18 different Cu(hkl) surface orientations demonstrate that the kinetics of the initiation process are structure-insensitive. This implies that the highly enantiospecific TA decomposition kinetics observed on chiral Cu(hkl) surfaces must arise from the structure sensitivity of the chain-branching explosion kinetics.