Alloy formation and chemisorption at Cu/Pt(111) bimetallic surfaces using alkali ISS, XPD, and TPD

Highlights

• Cu/Pt surface created by vapor deposition of Cu on Pt(111) and annealing at 550 K

• Cu substitutionally incorporated into Pt surface without vertical buckling

• Reduction of CO and NO adsorption energy compared to pure Pt(111)

• No long range order observed in surface alloy

Abstract

Alloying and surface structures of Cu films evaporated onto a Pt(111) single-crystal substrate were studied by X-ray photoelectron diffraction (XPD), low-energy alkali ion scattering spectroscopy (ALISS), and low-energy electron diffraction (LEED). Alloying begins at temperatures above 500 K, and increasing the annealing temperature of deposited films to 900 K caused all Cu atoms to diffuse deep into the subsurface region of the Pt crystal. One particular Cu/Pt(111) bimetallic alloy surface was characterized in detail, and this surface was formed by depositing one monolayer of Cu onto the Pt(111) surface and then annealing to 550 K. Only a diffuse (1 × 1) LEED pattern was observed from this Cu/Pt(111) alloy, which indicates that there was no long-range, ordered intermetallic compound created at the surface for these conditions or any others that we investigated. ALISS and XPS were used to determine that the Cu concentration in the topmost, surface layer of this alloy was 7 atomic percent. XPD and ALISS give consistent results showing that Cu in this alloy was present in the first, second, and third layers at the surface, forming a surface alloy. Cu atoms in the alloy are located at Pt atom lattice sites, and are coplanar with the topmost Pt atomic layer without significant corrugation or buckling. Temperature programmed desorption (TPD) measurements showed that both CO and NO are more weakly adsorbed and have smaller desorption energies on the Cu/Pt(111) surface alloy compared to the Pt(111) surface.

Introduction

The structure and chemistry of Pt bimetallic surfaces are of fundamental interest because of the importance of Pt-based alloys as catalysts in the petrochemical industry. In particular, Cu/Pt catalysts have been investigated for their activity in oxidation of CO and hydrocarbons to CO₂ and water and hydrocarbon reforming reactions [1], [2]. Cu is less active than Pt for these reactions, but Cu/Pt catalysts have a higher activity for both CO oxidation and hydrocarbon hydrogenolysis than pure Pt catalysts [2].

Previous work showed that ordered surface alloys can be created by the annealing of Sn films deposited on Pt single crystal substrates [3], [4]. It is of interest to determine if similar ordered surface alloys can be created from Cu films deposited on Pt(111), as well as to compare the surface composition and structure of these Cu/Pt surface alloys with that of the bulk Cu–Pt alloy surfaces. From previous studies, it has been determined that the growth mode of Cu on Pt(111) is by a layer-by-layer mechanism at temperatures below 475 K [5], [6], [7], [8], [9]. Cu grows in islands within the first monolayer and these islands coalesce near monolayer coverage. No substrate reconstruction occurs during the growth of the Cu monolayer at a substrate temperature of 340 K [9]. The growth of a second layer only begins after the first Cu layer is complete. Cu/Pt surface alloys are formed at substrate temperatures above 500 K [5], [6], [8], [10], [11]. At temperatures from 500 to 1300 K, Cu diffuses from the surface into the bulk [6], [8], [10]; it was reported that all Cu had diffused deep into the bulk and no Cu AES signals were detected at 1250 K [8]. No Cu desorption was observed up to 1300 K [10], but around 1400 K, Cu atoms migrate back to the surface and desorb [8].

These observations can be compared to those from bulk phases, in which Cu and Pt form three ordered alloys or intermetallic compounds, i.e., Cu₃Pt, CuPt, CuPt₃, at low temperatures and a disordered alloy at temperatures above 850 K [12]. Surface segregation in these alloys results in different surface and bulk compositions, for example the Cu₃Pt(111) bulk alloy has an average composition of 75% Cu but has a surface composition of 80% Cu in the first layer and 69% Cu in the second layer after annealing at 850 K; this observation is in agreement with the higher surface energy of Pt (2.299 J/m²) compared to Cu (1.952 J/m²) [13]. LEED patterns suggest that these surface layers do not have long-range ordering [14], [15].

We report herein on low energy electron diffraction (LEED), X-ray photoelectron diffraction (XPD) and low-energy alkali ion scattering spectroscopy (ALISS) studies of alloy surfaces formed by annealing Cu films deposited on Pt(111). These investigations included wide-ranging attempts to create long-range, ordered Pt-Cu surface alloys, as well as determination of the vertical displacement, if any, of Cu atoms in the surface plane caused by the substitution of Cu atoms into the Pt crystal lattice. A linear relationship between the lattice mismatch and surface corrugation in a number of bimetallic surfaces with Sn alloyed into fcc(111) crystals has been observed, with the larger Sn atoms outwardly buckled in the vertical direction as a strain relief mechanism for the lattice mismatch [16], [17], [18]. The atomic radius of Cu (1.28 Å) is 8% smaller than that of Pt (1.39 Å) [5], [6], [9], and thus observation of any buckling of Cu atoms in the top Pt atomic layer would help determine the applicability of the empirical buckling relation beyond the previously studied Sn alloys. Moreover, identification of possible ordered Cu/Pt surface alloys would enable chemisorption and reaction studies of structure–reactivity relationships for the Cu–Pt bimetallic system.

Chemical characteristics of mixed and pure Cu/Pt surfaces have been modeled in using density-functional theory (DFT)[19], [20], [21], [22]. Moreover, DFT has been used to examine catalyst function with respect to factors such as alloying, surface structure, adsorbate interactions, activity and selectivity [23], [24]. Information about the structure and chemical activity of Cu/Pt surfaces is important for the verification and advancement of these models. This study presents a detailed characterization of the composition and structure of the prepared Cu/Pt surface as well as the NO and CO adsorption properties of the Cu/Pt bimetallic surface, similar to those carried out for other bimetallic Pt surface alloys [25], [26], [27], [28], [29], [30], [31], [32].