Pt Acts As a Catalyst to Activate RuO2: The Active Sites of RuO2 Co-Catalyst for the CO Oxidation

Abstract

RuO2 has been suggested to act as stable co-catalysts for CO oxidation on Pt. However, little is known on how the oxide functions to oxidize nearby CO. In this work, we use model rotating disk electrodes composed of RuO2 nanosheets (RuO2(ns)) deposited on the surface of polycrystalline Pt, polycrystalline Au and glassy carbon. A new co-catalytic mechanism regarding the Pt/RuO2 interface towards the electro-oxidation of CO is proposed. Two distinct synergetic effects were identified, decreasing the bulk CO electro-oxidation onset potential compared to bare Pt by 50 mV (the ‘high potential’ synergetic effect at 0.85 V vs. RHE) and 250 mV (the ‘low potential’ synergetic effect at 0.65 V vs. RHE). We discuss the later with a new mechanism, involving an activated surface of RuO2(ns) generated by a partial reduction catalyzed by Pt. The peculiar ‘low potential’ synergetic effect decays upon potential cycling in CO-saturated electrolyte where Pt is poisoned with COad, but can be reactivated by COad-free Pt in HUPD potential region. These results were resolved considering the formation of ‘Ru metal-like’ sites on RuO2(ns) which are generated from a partial reduction of the RuO2(ns) surface catalyzed by a Pt surface that is not poisoned by CO (spill-over hydrogen). Therefore, this ‘low potential’ synergetic effect interestingly differs from the customary bifunctional mechanism and shows a highly interdependent relationship where Pt-catalyzed activated surface of RuO2(ns) provides oxygenated species for the removal of CO adsorbed on Pt. Acknowledgments This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan.