par Chau, Thoi-Dai 
;Visart de Bocarmé, Thierry ;Kruse, Norbert
Référence International Field Emission Symposium (IFES) (49: 2004-07-11->15: Graz, Autriche)
Publication Non publié, 2004-07-12
;Visart de Bocarmé, Thierry ;Kruse, Norbert Référence International Field Emission Symposium (IFES) (49: 2004-07-11->15: Graz, Autriche)
Publication Non publié, 2004-07-12
Communication à un colloque
| Résumé : | Gold metal is known to be chemically quite inert. On the other hand, it has been demonstrated that it may be highly active as a catalyst for CO oxidation provided it is prepared in the form of small particles dispersed on a metal-oxide support. The catalytic mechanisms responsible for the activation of the reactants are, however, largely unknown. While some authors suggest step sites of Au particles to be able to activate the molecules others advocate the occurrence of metal-support interactions. In the present study we have investigated the nitric oxide (NO) decomposition on Au tips using Pulsed Field Desorption Mass Spectrometry (PFDMS). Moreover, we present first results of a video-FIM study of the NO reduction with hydrogen at 300K. Probing several surface areas (up to about 400 atomic sites) of a clean (001)- or (111)- oriented Au tip during continuous supply of NO gas (p=10-6 mbar, 300 K) results in the detection of various ionic species including NO+, N2O+ and (NO)2 + under conditions of mere field pulses and in the absence of steady electric fields. The qualitative nature of the detected species suggests a reaction mechanism involving dimerization of NO molecules in step sites. At low pulse amplitudes much less O+ than N2O+ ions are found in the time-of-flight mass spectra. This causes the oxygen coverage to increase and the relative abundance of the ionic species to change drastically with time. On a strongly Oad-covered surface only NO+ and NO2 + ionic species are detected. An Oad-covered Au surface may be imaged by video-FIM using NO molecules as imaging gas. The respective micrographs appear rather uniform (i.e. barely influenced by the atomic structure of the Au tip) and moderately bright. Subsequent supply of hydrogen gas causes the Oad layer to react slowly. This reaction starts in the outskirts and leads to the formation of water which desorbs thermally. The continuous reaction is associated with a shrinking of the Oad layer, i.e. water formation occurs in the interface between Oad (or OHad) and Had. The pattern evolution with time is used to calculate a rate constant for the reaction |
