Electronic Effects in CO Chemisorption on Pt−Pb Intermetallic Surfaces: A Theoretical Study

Abstract

The chemisorption of CO on surfaces of Pt-Pb intermetallic compounds, found to be useful as fuel cell electrocatalysts, was analyzed theoretically. Specifically, density-functional theory and extended Huckel- based calculations on CO adsorption on Pt(111), Pt3Pb(111), and PtPb(0001) surfaces are reported. Binding energies on Pt3Pb(111) are computed to be generally smaller than binding energies on Pt(111). The binding energies at the 2- and 3-fold sites on Pt 3Pb(111) increase if there is a Pb atom underneath the site in the second surface layer. The binding energies on PtPb(0001) are much higher than those on the other surfaces. These trends have been analyzed with crystal overlap Hamilton population (COHP)-based energy partitioning. The most stabilizing interaction in chemisorption is the Pt -adsorbate bond formation; the surface and the adsorbate are internally destabilized. The major surface effects are pretty much restricted to the top two layers. The binding energy trend for the top site chemisorption follows the Pt-adsorbate interaction term (most stabilizing interaction term in chemisorption). This surface Pt -adsorbate interaction term, for top site chemisorption, has been analyzed further with a Frontier molecular orbital formalism based on the extended Huckel calculations. Electron donation from Pb atoms to Pt atoms plays an important role in distinguishing chemisorption on these surfaces. The higher Fermi energy of the Pt-Pb intermetallic surfaces, relative to Pt(111) surface, leads to a weaker Pt-adsorbate interaction, which correlates well with the lower binding energy on Pt-Pb intermetallic surfaces when compared to Pt(111). The variation of the binding energy within the 2- and 3-fold sites on Pt3Pb(111) cannot be explained by the Pt-adsorbate interaction term alone. From a detailed COHP analysis of the surface and adsorbate, we find that the adsorption site affects the electron movements (transfer of electrons) in the surface slab upon chemisorption and through them the overall binding energy of the adsorbate. The difference in binding energies between the Pt3Pb(111) hcp and fcc sites can be explained this way.