Scanning electrochemical microscopy of hydrogen electro-oxidation. Rate constant measurements and carbon monoxide poisoning on platinum

We describe an application of the scanning electrochemical microscope that uses tip-sample feedback to characterize the electro-oxidation of hydrogen on a polycrystalline platinum electrode in sulfuric acid solutions in the p resence and absence of adsorbed carbon monoxide. The hydrogen oxidation rea ction is probed by reducing protons at a diffusion-limited rate at the micr oscope's tip electrode while it is positioned near a platinum substrate. A series of approach curves measured as a function of the substrate potential provides hydrogen oxidation rate constant values over a wide range of subs trate conditions. In the absence of CO, the rate of hydrogen oxidation exce eds 1 cm s(-1) at potentials within the hydrogen adsorption and double laye r charging regions. A Tafel slope of similar to 30 mV per decade is determi ned near the reversible potential. At increasingly positive substrate poten tials, the hydrogen oxidation rate decreases exponentially with increasing potential as the surface is covered with an oxide layer. The adsorption of solution-phase carbon monoxide completely deactivates the platinum substrat e towards steady-state hydrogen oxidation over a large range of substrate p otentials. Approach curves indicate a near-zero rate constant for hydrogen oxidation on CO-covered platinum at potentials below oxide formation. An in crease in the hydrogen oxidation rate is seen at potentials sufficiently po sitive that CO fails to adsorb and the platinum oxide forms. In comparison, dynamic tip-substrate voltammetry depicts a complex substrate response whe reby the adsorbed carbon monoxide layer transforms from a weakly adsorbed s tate at low potentials to a strongly adsorbed state at high potentials. Alt hough steady-state approach curve measurements depict the complete deactiva tion of catalytic activity at these potentials, a significant hydrogen oxid ation current is observed during the potential-induced transformation betwe en these weakly and strongly adsorbed CO states. The rate of hydrogen oxida tion approaches that of a pristine platinum surface during this surface tra nsformation before returning to the poisoned state. (C) 2001 Elsevier Scien ce B.V. All rights reserved.