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A 3-D atom probe system for analysis of automotive exhaust catalyst surfaces

par Bagot, Paul A. J.;Visart de Bocarmé, Thierry ;Cerezo, Alfred;Smith, George D.W.
Référence International Field Emission Symposium (IFES) (49: 2004-07-11->15: Graz, Autriche)
Publication Non publié, 2004-07-12

Communication à un colloque

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Résumé :

The three-dimensional atom probe (3DAP) technique, developed at Oxford, is employed in anew catalytic atom probe (CAP) instrument dedicated to study the reaction mechanisms ofheterogeneous catalysis, specifically for catalytic processes relevant to automobile exhaustcatalytic converters.Specimens are prepared from Pt alloy wires, using compositions comparable with those in catalytic converters. The hemispherical specimen provides a good model for real catalyst particles. To simulate engine exhaust environments, this instrument features a reaction cell for specimen heating (up to 873K). This is connected to gas lines (including CO, NO, SO2, O2), which can supply single or multiple gases in any chosen ratio for treatments. The specimens are analysed post-exposure to determine structural and chemical changes. Work presented will cover results on metal atom surface segregation of Pt/Rh, Pt/Rh/Ir and Pt/Rh/Ru specimens, illustrating that the nature of segregation is not only gas-specific but also surface-specific. For example, oxidising gases such as NO were thought to induce Rh enrichment in Pt/Rh, but we have discovered this is only true on atomic step edges. On flat planes surprisingly, Rh depletion is observed, a result which suggests segregation occurs not only perpendicular to the surface but also laterally. Such results have important implications for achieving maximum catalyst efficiency. Finally, initial work is underway to use the 3DAP to investigate reaction kinetics, dosing gas in-situ to the AP chamber. This technique is similar to pulsed-field desorption mass spectrometry (PFDMS), although the 3DAP can reveal reaction kinetics specific to individual metallic surfaces. Preliminary results from these studies will be outlined in addition to a wide range of results exploring surface segregation under different gases and temperatures