12082
J. Phys. Chem. B 2001, 105, 12082-12086
Temperature-Dependent Surface Electrochemistry on Pt Single Crystals in Alkaline Electrolyte: Part 1: CO Oxidation T. J. Schmidt, P. N. Ross, and N. M. Markovic* Materials Sciences DiVision, Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720 ReceiVed: June 25, 2001; In Final Form: September 7, 2001
In this work we studied the continuous electrooxidation of CO dissolved in 0.1 M KOH electrolyte (COb) on Pt(hkl) at 275 and 333 K, respectively. The results clearly demonstrate that Pt(hkl) electrodes in alkaline electrolyte represent a highly active system for COb oxidation. Significant reaction rates are observed even in the potential region for hydrogen underpotential deposition (Hupd). The COb oxidation on Pt(hkl) surfaces proceeds through a Langmuir-Hinshelwood type reaction between the adsorbed states of CO and OHad, the latter forming selectively at the defect/step sites. The coexistence of OHad in the Hupd potential region implies that the potential-dependent surface coverages by Hupd and OHad in alkaline solutions cannot be obtained by simple coulometry. The kinetics of CO oxidation on Pt(hkl) surfaces is found to vary with crystal face. The difference in activity is attributed to the structure-sensitive adsorption of OHad on the defect/step sites and mobility of adsorbed CO (COad).
1. Introduction Carbon monoxide is the simplest C1 molecule that can be electrochemically oxidized in a low temperature fuel cell at a reasonable (although not necessarily practical) potential. Thus, the chemistry of COad on Pt(hkl) electrodes has been a subject of extensive study as a model adsorbate applicable to many important fuel cell reactions. In previous work, electrocatalysis of CO oxidation on Pt(hkl) surfaces was mostly examined in acid solutions.1-8 However, studies of the kinetics of CO oxidation in alkaline solution are scarce. The one exception is our recent study of oxidation of dissolved CO (COb) on the Pt(111) surface in KOH solution.7 By using the rotating disk electrode (RDE) we found that the electrooxidation of CO proceeds at low overpotentials (
