Surface composition of some selected group VIII/Au and group VIII/Sn alloys

Abstract

We have determined the surface composition of three different solid solutions (5 at.% Ni-Au, 4 at.% Pd-Au and 14 at.% Pt-Au) and two stoichiometric compounds (Ni 3Sn and Pt 3Sn), all of them selected because of their significance in current catalytic work on group VIII/“inert metal” alloys. Low-energy ion scattering (LEIS), Auger spectroscopy (AES) and selective CO chemisorption, applied to Pt 3Sn and Ni 3Sn, yield consistent results. For both alloys the three techniques indicate surface enrichment in tin, which is more pronounced for Ni 3Sn. Averaged over the three different techniques, the surface compositions work out at Pt 0.48 Sn 0.52 and Ni 0.21 Sn 0.79 respectively. When comparing these results with theoretical predictions we notice that the restricted bond-breaking theory, which takes into account the heat of sublimation of the pure elements only, predicts enrichment of the surfaces in tin. Satisfactory agreement between theory and experiments is found for Pt 3Sn. LEIS applied to 4 at.% Pd-Au and 5 at.% Ni-Au yields surface compositions of 0.8 at.% Pd and 1.0 at.% Ni respectively, in fair agreement with predictions from the bond-breaking theory. AES is not suited to detect this segregation. LEIS is found to be inapplicable for 14 at.% Pt-Au. For this alloy AES and selective CO-chemisorption yield a surface composition between 5 and 10 at.% Pt, whereas the bond-breaking theory predicts that the surface should contain as little as about 0.01 at.% Pt. A comparison of the results with contemporary theories on surface segregation is included.