On the kinetics of oscillating reactions: H2/O2 and H2/H2O on platinum

Field ion microscopic and spectroscopic analyses have been carried out for individual, catalytically active sites of a Pt tip exposed to a continuous flow of H2-O2 and H2-H2O gas mixtures. Product ion yields (H2O+, H3O+) are measured as a function of H2 pressure showing hysteresis and bistability; transitions between different reactive phases at distinct critical H2 pressures are observed. For H2/O2/Pt, results agree qualitatively with previous (field free) experimental data and recent theoretical predictions thus indicating a minor influence of the applied electric field. For the oscillating H2/H2O reaction system the kinetics differs markedly. Local fluctuations and oscillating reaction diffusion fronts are imaged by probe hole FIM and are analyzed simultaneously using field ion appearance energy spectroscopy. Different surface-reaction pathways are found in different kinetic phases.