Entropy Effects in Atom Distribution and Electrochemical Properties of Au_xPt_1−x/Pt(111) Surface Alloys

Abstract

We report on the structural and electrochemical properties of Au(x)Pt(1-x) surface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for Pd(x)Ru(1-x)/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). H(ad) and OH(ad) related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but H(ad) requires smaller ensembles of Pt atoms than OH(ad). The onset potential for H(ad)-formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption.

Oxygen Reduction Reaction Activities for Pt/Au(hkl) Bimetallic Surfaces Prepared by Molecular Beam Epitaxy

“…6d), the extension of the topmost Pt(100) lattice might also contribute to the ORR activity enhancement. Similar ECSA-dependent ORR activity enhancements are reported for annealed Au/Pt(111) by Bergbriter et al 29 and MBE-prepared Pt/Au(111) reported by us. 30 Therefore, ORR appears to require extended Pt lattices at the topmost surfaces of Pt/Au(hkl).…”

Promotion Effect of Platinum on Gold’s Reactivity: A High-Resolution Photoelectron Spectroscopy Study

“…The extent of ensemble effect on Au/Pt(111) has been the subject of study of various publications 27,[44][45] . In these reports it is demonstrated that the process a surface atomic exchange may induce a modification on the surface atomic distribution that will have a direct impact on the reactivity of the surface as a whole.…”

Oxygen reduction reaction activities of Pt/Au(111) surfaces prepared by molecular beam epitaxy

“…the 0.3-nm-thick Pt layer almost covers the Au(1 1 1) substrate surface. Hoster et al [39] reported the electrochemical properties of Pt-Au alloys prepared through Au depositions on clean Pt(1 1 1) and subsequent thermal annealing in UHV. They showed that Q OHad decreased steeply with decreasing Pt ratio and deduced that the geometric influence of surface Au atoms is significant for OH ad/desorption behavior.…”

“…Thus, well-defined Pt-Au bimetallic alloys fabricated in UHV should shed some light on the understanding of electrode catalysis of the surfaces. Bergbriter et al fabricated Pt-Au surface alloys through Au deposition on a Pt(1 1 1) substrate and subsequent annealing in UHV conditions and elucidated the surface geometric effects on the electrochemical properties of the surface Pt atoms [39]. On the basis of scanning tunnelling microscopy (STM) and cyclic voltammetry (CV) measurements, they showed that electrochemical properties of Pt-Au bimetallic alloys are dominated by the surface atomic ratios of Pt; the charges for hydrogen and hydroxyl-related adsorptions (Q Had and Q OHad ) decrease with decreasing Pt content, although Q OHad is more sensitive than Q Had .…”