Chemistry of fast electrons

A chemicurrent is a flux of fast (kinetic energy greater than or similar to 0.5 - 1.3 eV) metal electrons caused by moderately exothermic (1 - 3 eV) chemical reactions over high work function (4 - 6 eV) metal surfaces. In this report, the relation between chemicurrent and surface chemistry is elucidated with a combination of top-down phenomenology and bottom-up atomic-scale modeling. Examination of catalytic CO oxidation, an example which exhibits a chemicurrent, reveals 3 constituents of this relation: The localization of some conduction electrons to the surface via a reduction reaction, 0.5 O-2 + delta e(-) -> O delta- (Red); the delocalization of some surface electrons into a conduction band in an oxidation reaction, O delta- + CO -> CO2 delta- -> CO2 + delta e(-) (Ox); and relaxation without charge transfer (Rel). Juxtaposition of Red, Ox, and Rel produces a daunting variety of metal electronic excitations, but only those that originate from CO2 reactive desorption are long-range and fast enough to dominate the chemicurrent. The chemicurrent yield depends on the universality class of the desorption process and the distribution of the desorption thresholds. This analysis implies a power-law relation with exponent 2.66 between the chemicurrent and the heat of adsorption, which is consistent with experimental findings for a range of systems. This picture also applies to other oxidation-reduction reactions over high work function metal surfaces.