Kinetics and dynamics of gas-surface interactions: Mechanisms of dissociative chemisorption and the role of molecular intermediates.

摘要

The dissociative chemisorption of a molecule on a surface is a primary step inherent to many heterogeneous catalytic processes. Yet, models of surface chemical processes at the molecular level remain rather simplistic. Surface science, and in particular supersonic molecular beam experiments, offer promise for a more detailed understanding of the mechanisms underlying these processes.;This dissertation presents work employing molecular beam techniques for the study of the initial (in the limit of zero coverage) dissociative chemisorption of gas molecules on single-crystalline metal surfaces. In particular, studies with two systems, nitric oxide and oxygen on the (111) face of iridium, that challenge traditional models for the adsorption and dissociation of gas molecules on surfaces are presented. Conventional mechanisms of dissociative chemisorption include a trapping-mediated mechanism (a molecule "traps" as a physically adsorbed species, a precursor to dissociation) and a direct dissociation mechanism (a molecule dissociates immediately upon impact with the surface and with no prior accommodation into a molecular surface state). Recent studies have suggested that molecularly chemisorbed species may play a role in this process and that an additional type of direct mechanism may dominate--direct population of a molecularly chemisorbed species (direct molecular chemisorption) with subsequent dissociation. Moreover, the isolation and observation of such species has provided additional support for this mechanism.;Evidence for such a mechanism in the interaction of nitric oxide and oxygen with Ir (111) is presented with measurements of the initial adsorption probability as a function of incident kinetic energy, surface temperature, and angle of incidence between the molecular beam and sample normal. Additionally, a vibrational spectroscopic technique, electron energy loss spectroscopy (EELS), is used as a means of probing the chemical nature of surface states as a function of incident molecule kinetic energy. Indeed, these measurements demonstrate the first vibrational spectroscopic evidence of molecularly chemisorbed intermediates accessed with kinetic energies above 1 eV. Hence, convincing evidence for direct molecular chemisorption mechanisms is provided in these studies. Such mechanisms are likely general for the interaction of many gas-surface systems.