Surface oxidation issues in
CO
oxidation bimetallic surfaces studied by
NAP‐XPS
Authors: Ehret Eric , Mun Bongjin Simon , Languille Marie‐Angélique , Gallet Jean‐Jacques , Bournel Fabrice , Dupont Céline , Loffreda David , Santos Aires Francisco José Cadete 
Published: December 2016

Near‐ambient pressure X‐ray photoemission spectroscopy (NAP‐XPS) [1] 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 H
2
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 Pd
70
Au
30
(110) and Pt
3
Sn(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 Pd
70
Au
30
(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 [2] 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 Pt
3
Sn(111) we studied the two terminations (2x2) 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 Pt
3
Sn(111)‐(√3×√3)R30° than with Pt
3
Sn(111)‐(2x2). 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 [3]. 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).