CO-H2O interactions with Au and segregation effects on Au38-Pd62 alloy investigated by tomographic atom probe

A number of catalysts used in industry or in automotive pollution control are conditioned as a dispersion of small catalytic particles on a support of high specific area. Assuming a similar bulk composition, the extremity of a sharp metallic tip can mimic one of these catalytic particles. Field ion microscopy (FIM) is used to characterize these surfaces at the atomic scale at cryogenic temperatures and subsequently to image catalytic surface reactions at temperatures where they usually occur. As an example, interactions of CO and H2O gases over gold tips have been followed by FIM. Chemical waves are observed at room temperature without indication of surface-sensitivity. The combination of FIM with time-of-flight mass spectrometry, the atom probe, allows a local chemical determination of the surface during the ongoing phenomenon and reveals here drastic changes in the surface composition before and after the travelling of the waves on the probed area. However, there is no direct proof of the occurrence of the water gas shift reaction (WGS :CO+H2O→CO2+H2) hitherto since possibly desorbed neutral CO2 species cannot be detected by atom probe.Au-62at.%Pd alloy has been investigated by 3D atom-probe (3DAP) so as to reveal surface enrichment by gold or by palladium depending on the environmental conditions of the tip sample. While thermodynamics forecasts Au enrichment, Pd segregation is observed in the presence of NO gas and is attributed to the chemical pumping effect by Oad species following NO dissociation.