Instabilities in the Catalytic Water Production From O2 and H2 on Rh :a FIM / Atom Probe Study

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 tips morphologies during the ongoing water formation. At reaction temperature of 500 K, remarkable similarities were found with earlier studies with pure oxygen [1] where missing row type reconstruction on {011} and {113} planes were observed along with the extension of {137} planes detrimental to {012}. At 550 K and 600 K, irreversible transformations towards equilibrium morphologies were observed where the apex shape evolved to a well-defined polyhedron. In a second series of experiments, variations in the gas mixture were shown to provoke inhomogeneities as well as multistability in the surface reaction which were clearly visible by FIM between 400 and 500 K at viewing field of 10 V/nm. A phase diagram was established within these temperatures. At 550 K and 9 V/nm, self-sustained kinetic oscillations with a cycle time of ~40 s could be observed in a stable O2-H2 gas mixture (PO2 = 1,0·10-3 Pa and PH2 = 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 H2+O2 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. In the oxygen side of the phase diagram, high amounts of water were detected and moderate intensities were also found for RhO2+, RhO+ and Rh2O+. On the other hand, by increasing the H2 pressure, the surface oxide was reacted off within a timescale of ~0.1 second leaving thereby 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.