Investigation of reaction mechanisms on surfaces with Scanning Tunneling Microscopy

During the catalyzed oxidation of hydrogen on a Pt(111) surface, reaction fronts develop below the desorption temperature of water (170 K). The reaction fronts travel at constant velocity over the surface; their width (from 10 nm till 100 nm) is in the mesoscopic range. The spatiotemporal evolution of the fronts was investigated by means of scanning tunneling microscopy (STM) at temperatures between 108 K and 134 K. The reaction was modeled by a reaction diffusion system, which contains the autocatalytic reaction system (OH + H -> H2O; 2 H2O + O -> 3 OH + H) and the diffusion of water. The numeric solutions of the kinetic equations qualitatively reproduce the experimental findings. Circular reaction fronts develop from a starting water nucleus and travel over the surface in the form of an OH maximum, thereby transforming the oxygen to water. A lower limit for the front velocity was derived by an analysis of the differential equations. For a quantitative comparison between experiment and theory the rate constants of the involved reactions and the diffusion coefficient of H2O on the oxygen-covered Pt(111) surface were determined by additional STM and LEED experiments. The comparison with the experimentally determined front velocities and widths reveal a good agreement for the velocities. For the front width the agreement between theory and experiment is much less good. It is shown that these deviations result from the inability of the reaction-diffusion system to describe reaction fronts on the mesoscopic scale. In RD systems interactions of the diffusing water molecules with each other and with OH islands and the complex chemical processes in the front are not included. The transferability of the reaction model for the low temperature mechanism of the hydrogen oxidation to other transition metal surfaces was tested on the Rh(111) surface. STM data of the oxygen phases on Rh(111) confirm the existence of a (2x1)O structure, which has not been observed by STM so far. STM and LEED experiments show that in contrast to Pt(111) at 145 K on Rh(111) no reaction of oxygen with hydrogen occurs. It is suggested that the higher binding energy of oxygen to Rh(111) compared to Pt(111) leads to an increase for the onset temperature of the involved reactions; therefore, the low temperature mechanism for the oxidation of hydrogen fails on rhodium