Scanning electrochemical microscopy of hydrogen electro-oxidation Part II. Coverage and potential dependence of platinum deactivation by carbon monoxide

We describe measurements using the scanning electrochemical microscope (SECM) to evaluate the reactivity of polycrystalline platinum towards the hydrogen oxidation reaction in the presence and absence of adsorbed carbon monoxide (CO). Steady-state rate constant measurements for the hydrogen oxidation reaction on platinum in a CO saturated sulfuric acid solution indicate that the platinum surface is completely inactive at potentials below 0.8 V (vs. RHE) due to adsorbed CO. Rate constant measurements as a function of fractional CO coverage (theta(CO)) show that the activity of the platinum surface towards hydrogen oxidation is high for theta(CO) < 0.6 ML but drops precipitously for theta(CO) > 0.6 ML and becomes completely inactive for theta(CO) greater than or equal to 0.8 ML. Surprisingly, dynamic tip-substrate voltammetry (TSV) experiments show that CO-covered platinum surface becomes highly active for hydrogen oxidation at potentials in the pre-ignition region (0.4-0.7 V) where the surface is completely inactive under steady-state conditions, This activation is thought to occur due to a surface phase transition between a high coverage, weakly adsorbed CO (COads,w) state that forms at low potentials and a lower coverage, strongly adsorbed CO (COads,s) state that is favored at high potentials. This structural phase transition results in the creation of reactive sites of sufficient quantity that diffusion-limited oxidation rates are observed for hydrogen oxidation in the pre-ignition region. Although, this activation was found to be transient, pulsing the substrate potential between the hydrogen adsorption (similar to 0.1 V) and pre-ignition (0.4-0.7 V) regions resulted in a sustained activity for hydrogen oxidation on a nominally high CO coverage surface. (C) 2002 Elsevier Science BY. All rights reserved.