Effect of Temperature on Surface Processes at the Pt(111)-Liquid Interface: Hydrogen Adsorption, Oxide Formation, and CO Oxidation N. M. Markovic ´ ,* ,² T. J. Schmidt, ‡ B. N. Grgur, ² H. A. Gasteiger, ‡ R. J. Behm, ‡ and P. N. Ross ² Materials Sciences DiVision, Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720, and Abteilung Oberflachenchemie und Katalyse, UniVersitat Ulm, D-89069 Ulm, Germany ReceiVed: June 3, 1999; In Final Form: July 20, 1999 The variation of the adsorption pseudocapacitance with temperature is used to obtain the enthalpy, entropy, and free energies of adsorption of H upd and OH ad on Pt(111) as a function of pH and nature of the anion of the supporting electrolyte. It is shown that the heat (enthalpy) of adsorption of hydrogen on Pt(111) at the electrochemical interface is essentially independent of either the pH of the electrolyte or the nature of the supporting anion. The heat of adsorption has a linear decrease with Θ Hupd , from ∼42 kJ/mol at Θ Hupd ) 0 ML to ∼24 kJ/mol at Θ Hupd ) 0.66 ML. The heat of adsorption of OH ad is more sensitive to the nature of the anion in the supporting electrolyte. This is presumably due to coadsorption of the anion and OH ad in electrolytes other than the simple alkali bases. From the isosteric heat of adsorption of OH ad in alkaline solution (ca. ∼200 kJ/mol) and the enthalpy of formation of OH • we estimated the Pt(111)-OH ad bond energy of 136 kJ/mol. This value is much smaller than the Pt-O ad bond energy at a gas-solid interface (∼350 kJ/mol). In basic solution the electrooxidation of CO proceeds at low overpotentials (<0.2 V) between the adsorbed states of CO ad and OH ad , the latter forming at low overpotentials selectively at defect sites. In acid solution, however, these sites are not active because they are blocked by specific adsorption of anions of the supporting electrolyte. 1. Introduction The past 2 decades have witnessed substantial advances in our knowledge of the electrocatalytic properties of well-defined platinum single-crystal surfaces. With the advent of surface structural probes such as ex situ analyses of emersed surfaces by low-energy electron diffraction (LEED), 1 in situ surface X-ray scattering (SXS), and in situ scanning tunneling micros- copy (STM), 2 complemented by the development of the rotating ring-disk electrode technique for use of Pt(hkl) electrodes, 3 it became possible to establish the relationship between the surface structure and the electrocatalytic activity of Pt(hkl) electrodes. These methodologies present the ability to correlate the kinetics of the hydrogen and the oxygen electrode reactions with the structural sensitive adsorption of the reaction intermediates, such as underpotentially deposited hydrogen (hereafter, denoted as H upd ) and anions. 3-6 Measurements of the thermodynamic state functions for the H upd state on Pt(111) 5,7 were recently obtained from the temperature dependence of voltammetric profiles of the Pt(111) surface in sulfuric acid solution. However, these experiments offered very little information about the oxygenated species, such as OH ad or surface “oxide” because their formation on the Pt(111) surface is almost completely inhibited by the strong adsorption of bisulfate anions. To learn more about the kinetic and thermodynamic aspects of oxygenated species on Pt(111), the adsorption of these species should be studied in solutions with weakly adsorbing anions. Measurements in alkaline solution and perchloric acid solution would be of particular interest, since in these electrolytes the initial stage of the reversible surface adsorption of OH ad has been clearly separated from an irreversible “oxide” formation. Much discus- sion has centered on the distinction between these two oxygen- ated species, 8 but there is general uncertainty about what physical processes are associated either with adsorption of OH ad or with the “oxide” formation. In this communication, we present voltammetric profiles of the Pt(111) surface in 0.1 M HClO 4 and 0.1 M NaOH obtained at different temperatures with the objective of observing the effect of temperature on the reversible adsorption of both the H upd state and the OH ad state, from which thermodynamic state functions may be obtained, and on the irreversible “oxide” formation process. The enthalpy of formation of the H upd state on Pt(111) in 0.1 M HClO 4 and 0.1 M NaOH is derived from H upd isotherms at variable temperatures. The chemisorption bond energy of the OH ad state is estimated from the temperature dependence of the peak potential for OH ad layer formation with respect to the O 2 /H 2 O reversible potential. The potential for incipient formation of the OH ad state is estimated by titration using the electrooxidation of dissolved CO (denoted as CO b ). From this potential, the Pt(111)-OH ad bond is estimated to be ca. -136 kJ/mol. This incipient adsorption of OH ad is hypoth- esized as forming at steplike imperfections in the (111) surface. 2. Experimental Section The pretreatment and assembly of the Pt(hkl) single crystals (0.283 cm 2 ) in a rotating disk electrode (RDE) configuration were fully described in our previous paper. 3 Following flame annealing, the single crystal was mounted in the disk position * Corresponding author. E-mail: nmmarkovic@lbl.gov. ² University of California, Berkeley. ‡ Universitat Ulm. 8568 J. Phys. Chem. B 1999, 103, 8568-8577 10.1021/jp991826u CCC: $18.00 © 1999 American Chemical Society Published on Web 09/16/1999