(488e) Initiation of Autocatalytic Surface Explosion Reactions

Autocatalytic reaction mechanisms are observed in a range of important chemical processes including catalysis, radical-mediated explosions, and biosynthesis.  Because of their complexity, the microscopic details of autocatalytic reaction mechanisms have been difficult to study on surfaces and heterogeneous catalysts.  On surfaces, these types of reactions were originally discovered during studies of formic acid decomposition on Ni(110) and the mechanism has often been described as a vacancy-mediated surface explosion.  Autocatalytic decomposition reactions of tartaric acid (TA) adsorbed on Cu(110) offer molecular-level insight into aspects of these processes, which until now, were largely a matter of speculation.  The decomposition of TA/Cu(110) is initiated by a slow, irreversible process that forms vacancies in the adsorbed TA layer, followed by a vacancy-mediated, explosive decomposition process that yields CO₂ and small hydrocarbon products.  Initiation of the explosive decomposition of TA/Cu(110) has been studied by measurement of the reaction kinetics, time-resolved low energy electron diffraction (LEED), and time-resolved scanning tunneling microscopy (STM).  Initiation results in a decrease in the local coverage of TA and a concomitant increase in the areal vacancy concentration.  Observations of explosive TA decomposition on the Cu(651)^S surface suggest that initiation does not occur at structural defects in the surface, as has been suggested in the past.  Once the vacancy concentration reaches a critical value, the explosive, autocatalytic decomposition step dominates the TA decomposition rate.  The onset of the explosive decomposition of TA on Cu(110) is accompanied by the extraction of Cu atoms from the surface to form a (6,7;-2,1) overlayer that is readily observable using LEED and STM.  The explosive decomposition step is second-order in vacancy concentration and accelerates with increasing extent of reaction.