Ag on Pt(111): Changes in Electronic and CO Adsorption Properties upon PtAg/Pt(111) Monolayer Surface Alloy Formation

Abstract

The electronic and chemical (adsorption) properties of bimetallic Ag/Pt(111) surfaces and their modification upon surface alloy formation, that is, during intermixing of Ag and Pt atoms in the top atomic layer upon annealing, were studied by X-ray photoelectron spectroscopy (XPS) and, using CO as probe molecule, by temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS), respectively. The surface alloys are prepared by deposition of sub-monolayer Ag amounts on a Pt(111) surface at room temperature, leading to extended Ag monolayer islands on the substrate, and subsequent annealing of these surfaces. Surface alloy formation starts at ≈600-650 K, which is evidenced by core-level shifts (CLSs) of the Ag(3d5/2 ) signal. A distinct change of the CO adsorption properties is observed when going to the intermixed PtAg surface alloys. Most prominently, we find the growth of a new desorption feature at higher temperature (≈550 K) in the TPD spectra upon surface alloy formation. This goes along with a shift of the COad -related IR bands to lower wave number. Surface alloy formation is almost completed after heating to 700 K.

Reactivity of CO and C2H4 on Bimetallic PtxAg1‐x/Pt(111) Surface Alloys Investigated by High‐Resolution X‐ray Photoelectron Spectroscopy

“…Notably, we find occupied bridge sites for all alloys, even for those with the highest Ag content. This is in contrast to earlier IRAS results,, where bridge CO was only detected for alloys with an Ag content below ∼30 %. The authors proposed that at higher Ag coverages large continuous Pt surface areas are missing and that it is not possible to form long‐range ordered c(4×2)‐adsorbate layers that allow for CO adsorbed on bridge sites.…”

“…Upon increasing the amount of silver, the Ag 3d peaks shift to higher binding energies, finally reaching 367.88 eV for 68 % Ag. This shift of ∼90 meV fits very well to the shift of ∼140 meV found by Diemant et al . after annealing, when increasing the Ag content from 20 % to 80 %, and is somewhat smaller than the value of ∼180 meV observed by Strüber et al .…”

“…These results were extended by the XPS studies of Diemant et al. who also found downshifts of the Ag 3d 5/2 peak for the annealed surfaces (up to 0.4 eV, depending on Ag coverage) . By LEED specular beam measurements, Tsay et al.…”

“…We first discuss the formation of the Ag x Pt 1‐x /Pt(111) surface alloys that were prepared for the CO and C 2 H 4 experiments. This bimetallic system is well‐known in literature and characterized with several techniques including XPS, TPD, STM, LEED and DFT ,,. Annealing submonolayer coverages of Ag adsorbed on Pt(111) to above ∼600 K leads to the formation of a 2D surface alloy.…”

“…By varying the evaporation time it was possible to achieve the desired Ag amounts on the sample, which were in line with the intensity of the corresponding Ag 3d XP spectra. The sample temperature was kept below 200 K during evaporation and afterwards the Ag x Pt 1‐x /Pt(111) surface alloys were prepared by annealing at 800 K for 1 minute ,…”