Water formation from O2 and H2 on Rh tips: studies by field ion microscopy and pulsed field desorption mass spectrometry

The catalytic oxidation of hydrogen on Rh tips was investigated using field ion microscopy and pulsed field desorption mass spectrometry (PFDMS). The reaction was carried out in the 10-3 Pa range at 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. Accordingly, a polyhedral tip shape with missing-row-type reconstruction on {011} and {113} planes was formed. 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-1. A phase diagram was established within this temperature range. At 550 K and 9 V.nm-1, self-sustained kinetic oscillations with cycles of 40 s were observed in a continuous flow of O2+H2 (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 formation of the field ion image, leading to the conclusion that brightness analysis is suitable for measuring the local catalytic activity. On the oxygen side of the phase diagram, water molecules were formed on an oxidized surface so that (RhO2)+, RhO+ and Rh2O+ appeared in the mass spectra. On the other hand, by increasing the H2 pressure, the surface oxide was reacted off within 0.1 s, thereby leaving a relatively dark surface (H-side of the diagram). Chemical probing in the latter case demonstrated fast water formation leaving few oxygens on the surface, i.e. negligible RhxOn+ amounts appeared in the mass spectra.FLWINinfo:eu-repo/semantics/publishe

XPS structural characterization of Pd/SiO2 catalysts prepared by cogelation

Nanostructured Pd/SiO2xerogel catalysts prepared via cogelation were characterized by X-ray photo-electron spectroscopy. The preparation route allowed highly porous silica particles to be formed alongwith embedded Pd nanocrystals. After heat-treating the catalysts in vacuum, Pd was found to be in themetallic state. To obtain information on the xerogel catalyst texture and, in particular, on the size of thesilica particles, a theoretical formalism was developed based on measuring the relative intensities of Pd 3ddoublet and the associated background tail due to inelastically scattered photoelectrons. The suggestedprocedure also involved the measurement of the background tails accompanying Si 2p and O 1s spectrallines as internal standards. Using the developed formalism, the size of the silica particles in the catalystsafter different treatments was evaluated and compared with TEM data. The results obtained indicate thatthe textural properties of the sol–gel catalysts start to develop already at the level of co-condensation ofalkoxides with the network-forming reagent. Calcination causes these properties to run to completion

Immobilizing metal nanoparticles in porous silica through sol-gel process

SCOPUS: ar.kinfo:eu-repo/semantics/publishe

Cogelation: An effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides

SCOPUS: cp.kinfo:eu-repo/semantics/publishe

Cogelation: an effective sol-gel method to produce sinter-proof finely dispersed metal catalysts supported on highly porous oxides

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