Electrochemical Surface Stress Development during CO and NO Oxidation on Pt

The adsorbate-induced surface stress during the electrochemical oxidation of CO and NO on Pt is studied with in situ surface stress measurements and density functional theory (DFT) calculations. The changes in the surface stress response, Delta stress, demonstrate the interplay between the adsorbed species during the oxidation process, which is determined by the coverage and the nature of the adsorbates. The oxidation of adsorbed CO, COads, shows a nonlinear surface stress response in both acidic and alkaline electrolytes, with the greatest tensile Delta stress observed in the beginning of the oxidation where the CO coverage is the highest. Once a significant amount of CO is removed, OH starts to populate the surface and the Delta stress becomes compressive. This surface stress development profile-the nonlinear stress development at high COads coverages and the inflection point due to coadsorption of CO and OH-is further interrogated by DFT calculations. While a tensile to compressive switch in Delta stress is observed during CO oxidation, the oxidation of another strongly bound diatomic adsorbate, NOads, shows a continuous compressive Delta stress. DFT calculations show that this behavior is attributed to the adsorption of the oxidation product, NO3-, which induces a similar magnitude of compressive Delta stress compared to that of NOads. Hence, the compressive Delta stress from the oxide and hydroxide on the surface governs the surface stress response.