Adsorption and Reaction of O_2 and H_2 on Rh Crystallites Investigated by Field Ion Microscopy and Pulsed Field Desorption Mass Spectrometry

摘要

The catalytic oxidation of hydrogen on Rh tips has been investigated using field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS). The reaction was carried out in the 10~(-5) mbar range at T = 400-600 K. In a first series of experiments, emphasis was laid on revealing tip morphologies during the ongoing water formation. At a reaction temperature of 500 K, remarkable similarities with the previously studied pure oxygen case were found to exist. Accordingly, a polyhedral tip shape with missing-row type reconstruction forms on {011} and {113} planes [1] were observed. At 550 K and 600 K. this transformation towards an equilibrium shape turned out to be irreversible. In a second series of experiments, the gas mixture was varied to demonstrate the occurrence of local structural changes as well as multistability in the surface reaction at temperatures between 400 and 500 K and viewing fields of 10 V/nm. A phase diagram was established within this temperature range. At 550 K and 9 V/nm, self-sustained kinetic oscillations with cycles of ～40 s could be observed in a continuous flow of O_2-H_2 (P_(O_2) = 1,0·10~(-3) Pa and P_(H_2) = 1,3·10~(-3) Pa). Using PFDMS, ～400 atomic sites close to the central (001) pole of the Rh tip were probed with varying pulse heights. During imaging in a wide range of H_2 + O_2 gas mixtures, it was found that water ionization causes image formation, leading to the conclusion that brightness analysis is suitable for measuring the catalytic activity. On the oxygen side of the phase diagram, high amounts of water were detected and moderate intensities were also found for RhO~(2+), RhO~+ and Rh_2O~+. On the other hand, by increasing the H_2 pressure, the surface oxide was reacted off within ～0.1 second thereby leaving a relatively dark surface (H-side of the diagram). Chemical probing in the latter case revealed lower amounts of water but nearly no oxygen-containing species were found in the mass spectra. A model of rate oscillations within the frame of a mean-field approximation will be presented.