Surface composition determination of Pt–Sn alloys by chemical titration with carbon monoxide

Abstract

The use of chemical titration with carbon monoxide to determine the surface composition of Pt–Sn alloys was studied using Pt3Sn single crystals of known surface composition. The surface composition of the (111) and (100) faces were determined independently by the combination of low-energy electron diffraction (LEED) crystallography and low-energy ion scattering (LEIS). CO was adsorbed on these surfaces to saturation at 250 K and thermally desorbed into a mass spectrometer. The area under the thermal desorption spectroscopy curve for the alloy surface was compared to the area under the curve for the pure Pt surface of the same orientation. The ratios were 0.5±0.05 for Pt3Sn(100) and 0.7±0.05 for Pt3Sn(111), in excellent agreement with the 50% Pt and 75% Pt surface compositions derived from LEED and LEIS. The success of the titration method in this case appears to be due to (a) selective adsorption of CO on Pt atoms and (b) the relatively weak effect of the Pt–Sn intermetallic bond on the Pt–CO bonding.