Auger spectroscopic study of the surface composition of Pt/Sn alloys in ultrahigh vacuum and in the presence of oxygen and hydrogen |
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Authors: | R. Bouwman L.H. Toneman A.A. Holscher |
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Affiliation: | Koninklijke/Shell-Laboratorium, Amsterdam (Shell Research B.V.), Amsterdam, The Netherlands |
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Abstract: | The surface composition of two Pt/Sn alloys, viz. PtSn and Pt3Sn, has been followed by means of AES, as a function of annealing in ultrahigh vacuum, oxygen chemisorption and reduction with hydrogen.The results, which were quantitatively interpreted with the aid of a novel calibration technique, reveal the following features: - The surface of PtSn and Pt3Sn becomes enriched with tin by annealing in vacuum. Ultimate values of 68±5 at% Sn for PtSn and 41±5 at% Sn for Pt3Sn were attained after annealing at 500°C. - The adsorption of oxygen on the annealed surface of PtSn and Pt3Sn causes a further enrichment with tin, while severe oxidation of PtSn at 500°C leads to complete disappearance of Pt from the surface. - Oxygen is more strongly and differently bound on a surface containing about 40 at% Sn than on a surface containing about 70 at% Sn. Activated adsorption of oxygen takes place only on the latter. The results suggest the formation of SnO2 surface complexes on the exposed surface of Pt3Sn. - Reduction of the alloys at 500°C carries the excess of tin into the bulk and reduces its surface concentration to 35±5 at% for Pt3Sn and 64±5 at% for PtSn, which is an enrichment of the surface with platinum relative to the annealed state. |
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