Kinetics of reductive adsorption of CO2 on smooth Pt electrodes

Summary The formation kinetics of the adsorbed species “reduced CO2” (“CO2”) have been studied with Pt electrodes in 1 M H2SO4 at 40°. Electrochemical transient techniques have been used for characterization of the adsorbate both during its formation and at steady state. Steady-state coverage is high (∼0.7 monolayers, 220 μC/real cm2 of oxidizable charge) and potential-independent between 0.00 and 0.25 V vs. RHE. The coverage falls rapidly above 0.25 V, becoming zero at 0.30 V. The implication of these results for the role of “CO2” in the overall fuel→CO2 reaction is discussed. The steady-state “CO2” adsorbate has the same composition over the whole potential range investigated and comprises two parts. During the approach to the steady state at any potential, the relative amounts of the two components of the adsorbate change. The major component is found at all coverages and comprises the total adsorbate for organic coverage (θorg) For θorg>0.5, a second, more highly reduced species is found. This is suggested to have the structure. Initial adsorption rates are first-order in CO2 and adsorbed hydrogen and zero-order in free surface. Between 0.15 and 0.30 V, this rate increases semilogarithmically with decrease in potential (80 mV/decade). The reaction is proposed to be rate-limiting in this potential region for θorg≤0.5. Above this coverage, the subsequent reaction is thought to limit the rate of formation of “CO2”. At low potentials (

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