Structure of Tl Adlayers on the Pt(111) Electrode Surface: Effects of Solution pH and Bisulfate Coadsorption R. R. Adz ˇ ic ´ * and J. X. Wang Department of Applied Science, Chemical Sciences DiVision, BrookhaVen National Laboratory, Upton, New York 11973 O. M. Magnussen and B. M. Ocko Department of Physics, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: January 2, 1996; In Final Form: May 19, 1996 X The structure of Tl adlayers deposited at underpotentials on Pt(111) has been investigated in four different electrolyte solutions (HClO 4 , NaClO 4 , NaOH, and H 2 SO 4 ) with surface X-ray scattering (SXS) techniques. In all solutions investigated, Tl forms an incommensurate, aligned-hexagonal phase at the most negative potentials prior to its bulk deposition or hydrogen evolution. With decreasing potential the monolayer compresses and thus the Tl coverage increases. Under hydrogen evolution, the close-packed hexagonal Tl monolayer also exists and this reaction causes a slight lowering of the Tl coverage and a significant decrease of the in-plane ordering of the monolayer. In sulfuric acid solution, besides the close-packed hexagonal phase, a commensurate ( 3 ×  3)R30° phase with a Tl coverage of 1 / 3 monolayer is formed in coadsorption with bisulfates over a 0.22 V wide potential range. I. Introduction Electrochemical formation of metal monolayers by so-called underpotential deposition (UPD) involves deposition of up to 1 or 2 monolayers of a metal onto a different substrate at potentials more positive than the deposition potential of the bulk metal. It has been the subject of numerous recent in situ structural studies. Most of these studies involved Ag and Au as the substrate metals, such as for UPD of Tl. 1-4 Platinum, as a transition metal element, is chemically more active than Au and Ag and its modification by UPD adlayers has rather unique electrocatalytic properties, which have attracted considerable attention. 5 Distinct adlayer structures have been observed on Pt(111) compared to that on the silver and gold (111) surfaces. Lead, for instance, forms commensurate monolayers on Pt(111) 6 while it forms an incommensurate hexagonal monolayer on Ag(111) and Au(111). 7 In this work, the structure of Tl monolayers on Pt(111) is determined and compared to those on Au(111) and Ag(111). Of particular interest is that the large Tl UPD potential range overlaps with the potential region for hydrogen adsorption/ evolution on Pt in acid and neutral solutions. In the past, the effect of adsorbates on hydrogen adsorption, evolution, and absorption at metal electrodes has been extensively studied via a variety of electrochemical and spectroscopic techniques. However, in most instances the adlayer structure and coverage have not been determined under reaction conditions, although this information is very important for understanding the adlayer effects. We report here the structure and coverage of the Tl monolayer as a function of potential in acid, neutral, and alkaline solutions. The results not only provide accurate structural information but also reveal the solution pH effects on the coverage and the phase behavior of the Tl adlayers. Another interesting aspect of Tl/Pt(111) is anion coadsorption. The first observation of anion coadsorption in an UPD system was reported by Horanyi et al. for sulfate and chloride coadsorption with a copper adlayer on platinum using radiotracer techniques. 8 In electrochemical-UHV studies of copper ad- sorption on I 2 -prepared Pt(111) several Cu-I structures have been found. 9 The effects of anions on the structure of copper adlayers on Au low index surfaces 10,11 and on Pt(111) 12 and silver adlayers on Au(111) 13 have been studied by in situ scanning probes and SXS techniques. Besides copper, sulfate chemisorption was reported for the cadmium adlayer on Pt 14 and hydroxide ion coadsorption with Tl on Au(111). 3 In a recent note, pronounced effects of specifically adsorbed anions on the UPD of Tl on Pt(111) were reported. 15 In this paper, the thallium and anion coadsorbed adlayer structure observed in sulfuric acid solution are reported. II. Experimental Section A platinum single crystal, obtained from Metal Crystals and Oxides Ltd., Cambridge, England, was oriented within 0.2° of the 〈111〉 crystallographic direction and polished by using diamond paste with a final polishing using 1 micron particles. The crystal was annealed in a propane flame, cooled in a hydrogen stream, and subsequently transferred to the electro- chemical X-ray scattering cell protected by a drop of the solution. The cell was constructed from Kel-F 16 and sealed using a 4 μm thick Prolene (Chemplex Inc.) X-ray window. The solutions were prepared from Tl 2 CO 3 (Aldrich), NaClO 4 , NaOH, HClO 4 (Merck, Suprapur), and Milli-Q water (Millipore Inc.). Ultrapure nitrogen was used to deoxygenate the solutions. The counter electrode was platinum, while a reversible hydrogen electrode served as the reference. All potentials are given with respect to normal hydrogen electrode (NHE). SXS measurements were carried out at the National Syn- chrotron Light Source (NSLS) at beam line X22B and X22A with λ ) 1.54 and 1.20 Å, respectively. A full description of the electrochemical SXS technique has been presented else- where. 16 For in-plane diffraction measurements, the resolution was determined primarily by a 3 mrad Soller slit on the detector arm. This arrangement provides a 2θ resolution of 0.1° half- width at half-maximum (hwhm). For specular reflectivity X Abstract published in AdVance ACS Abstracts, August 1, 1996. 14721 J. Phys. Chem. 1996, 100, 14721-14725 S0022-3654(96)00030-5 CCC: $12.00 © 1996 American Chemical Society