1060
J. Phys. Chem. C 2008, 112, 1060-1063
Surface Stress Observations during the Adsorption and Electrochemical Oxidation of CO on Pt{111} L. Mickelson, Th. Heaton, and C. Friesen* School of Materials, Arizona State UniVersity, Tempe, Arizona, 85287 ReceiVed: August 13, 2007; In Final Form: October 22, 2007
The adsorbate-induced surface stress was studied for the system CO/Pt{111}-textured thin films in COsaturated 0.1 M HClO4 and characterized through the techniques of cyclic voltammetry and chronoamperometry. A compressive stress of -1.1 N/m was measured for a saturation coverage. It is shown that CO blocks the electrode both electrochemically (charge transfer) and physically (surface stress changes). Additionally, it is demonstrated with surface stress measurements that CO does not desorb with the removal of CO from the electrolyte. The adsorption kinetics of CO are shown to be slow, with a saturation coverage taking more than 65 s to acquire. When driven electrochemically, CO oxidation kinetics are fast, occurring in much less than 1 s.
Introduction Surface stress measurements have recently become recognized as a robust way to interrogate surfaces in ultrahigh vacuum (UHV) and electrochemical environments. Changes in thermodynamic surface stress yield information about surface reconstruction,1,2 adsorption,3-7 thin film growth,8-12 catalysis,13,14 strain-charge relationships of nanoporous metals,15 and the structure of the electrode-electrolyte interphase.16 Adsorbate-induced surface stresses have been widely studied in the UHV environment for a variety of adsorbates and substrates. One of the best studied adsorption systems is CO/ Pt{111} and its oxidation to CO2 by the Langmuir-Hinshelwood reaction, and some surface stress measurements have been made on this system.17,18 In addition, much work has been done electrochemically to characterize the charge displaced during adsorption of CO, the role of CO adsorption in blocking the surface from adsorption of ions, the diminished capacitance of the double-layer with a CO-saturated surface, and coverage measurements through CO oxidation charges.19,20 However, until now no work has been done to characterize surface stress changes during these processes in an electrolyte. The adsorption of CO is a displacive reaction; hence, the charge measured during CO adsorption is due to desorption and double-layer charging processes, not charge transfer from CO to the electrode. Thus, it is difficult to define the coverage of CO dynamically, during the adsorption process, since the charge displaced per CO molecule is unknown and is likely a function of adsorption potential. Final saturation coverages can be calculated by measuring the charge during oxidation of CO to CO2, a two-electron process, and correctly accounting for double-layer charging. Reported saturation coverages of CO on single-crystal Pt (111) in 0.1 M HClO4 range from θ ) 0.60 to 0.74 ML.21,22 In this paper we present results for the adsorbate-induced surface stress changes observed during the adsorption and oxidation of CO on Pt{111} in an aqueous electrolyte. It is shown that no appreciable decrease in CO coverage occurs upon * Corresponding author. E-mail:
[email protected].
deaerating CO from the electrolyte. After oxidative stripping of CO from the electrode, the electrode’s chemical activity is recovered. Experimental Section The surface stress monitoring was performed using a cantilevered sample technique described elsewhere.16 The apparatus consisted of an electrochemical cell, a thin film sample, and the stress monitor sensor. The stress monitor utilizes a capacitive method to measure
