Design and Investigation of Molybdenum Modified Platinum Surfaces for Modeling of CO Tolerant Electrocatalysts

Mo overlayers were prepared on smooth polycrystalline platinum and platinized platinum electrode surfaces by in situ electrochemical deposition of molybdenum oxide at potential below 500 mV for modeling Mo–Pt electrocatalysts. Correlations were found between the applied potential and the amount of deposited Mo, which never exceeded a monolayer, thus Pt–Mo bonds stabilize the deposited Mo oxide. Electrochemical measurements suggested that Mo deposited from a Mo(VI) solution was reduced to the 4+ oxidation state. In line with the ex situ XPS findings a certain part (20–25%) of the initial Mo layer remained irreversibly adsorbed on the Pt/Pt electrode even after oxidation into the 6+ state at high potentials; this fractional monolayer cannot be dissolved even by prolonged cyclic polarization up to 1000 mV. It has been demonstrated that the irreversibly bound Mo partial monolayer is enough to change significantly the CO poisoning properties of the Pt surface. On this Mo:Pt (1:4) surface CO oxidation is initiated at extremely low potentials (ca. 100 mV). Moreover, only Pt modified by Mo(IV) species is active in low-potential CO oxidation reaction as after oxidizing the irreversibly adsorbed Mo to the 6+ state, CO oxidation is no longer observable. Nevertheless, the catalyst can be reactivated by reduction of molybdenum into the 4+ oxidation state. However, this reduction requires clean, CO-free Pt surface. If Pt is largely covered by CO, reduction of Mo(VI) into Mo(IV) does not occur and thus the low potential CO oxidation remains hindered.