Near-ambient pressure X-ray photoemission spectroscopy (NAP-XPS)  is a modification of XPS so that surfaces can be studied in presence of gases and not only in vacuum. This is necessary when you want to access chemical and electronic properties of catalytic surfaces in realistic reaction conditions (variable P, T). We have focused on the study of bimetallic catalytic surfaces and their chemical/electronic surface properties during the oxidation of carbon monoxide. Indeed, the low temperature catalytic oxidation of carbon monoxide to carbon dioxide is an important reaction used in several areas and in particular for removing the CO traces from the H2 combustible in fuel cells. Pt and Au are well known catalysts for the CO oxidation reaction, however on both metals the oxygen dissociation remains the rate determining step. Associating Pt or Au with a second metal, that facilitates the oxygen activation while keeping free sites for CO adsorption on the surface, appears as an effective way to improve the catalytic performances of these two metals. The surfaces Pd70Au30(110) and Pt3Sn(111) have been chosen on the basis of the performance of real catalysts (bimetallic nanoparticles supported on oxides).
The NAP-XPS experiments were performed at ALS beamlines 9.3.2 & 11.0.2 (Berkeley, USA) and, more recently, at SOLEIL beamline TEMPO (St Aubin, France). NAP-XPS allows a fine characterization of electronic properties of catalytic surfaces including reactants, products and inert spectator species. In addition, NAP-XPS can also detect the gas phase species near the surface and can therefore monitor the reaction rates, at least qualitatively. The inherent variable photon energy of synchrotron sources enables depth profiling measurements which can be used to explore the segregation of species into the subsurface region under gas environments.
In the case of Pd70Au30(110) we have a very rich Au-surface (~ 85% of Au as determined by LEISS) due to Au segregation to the surface at thermodynamic equilibrium. By environmental STM  we could notice that the surface reorganizes at atomic level and eventually roughens just after adsorption of the reactive gases both under oxygen and CO. This is followed by a reversal of segregation where Pd migrates to the surface as it is determined by varying the photon kinetic energy in NAP-XPS. For Pt3Sn(111) we studied the two terminations (2×2) and (√3x√3)R30° obtained at different annealing temperatures. For both surfaces we have identified the presence of bridge and top sites during adsorption of CO. Oxygen adsorption studied by PM-IRRAS shows a higher interaction of oxygen with Pt3Sn(111)-(√3×√3)R30° than with Pt3Sn(111)-(2×2). By NAP-XPS we observe, in both surfaces, the partial segregation of Sn to the surface with increasing temperature in the presence of oxygen and under reaction conditions . Finally we studied the evolution of the surfaces by NAP-XPSunder reaction conditions. A general result was observed for the studied bimetallic surfaces: the activity of the surface is higher in presence of chemisorbed oxygen (that we can compare to a“loose”oxide) but it rapidly decreaseswhen stoichiometric oxides are formed on the surface at higher temperatures (Figures 1 & 2).
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To cite this abstract:Eric Ehret, Bongjin Simon Mun, Marie-Angélique Languille, Jean-Jacques Gallet, Fabrice Bournel, Céline Dupont, David Loffreda, Francisco José Cadete Santos Aires; Surface oxidation issues in CO oxidation bimetallic surfaces studied by NAP-XPS. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/surface-oxidation-issues-in-co-oxidation-bimetallic-surfaces-studied-by-nap-xps/. Accessed: December 1, 2022
EMC Abstracts - https://emc-proceedings.com/abstract/surface-oxidation-issues-in-co-oxidation-bimetallic-surfaces-studied-by-nap-xps/