(619c) Stability of Metal Oxides on Metal Nanoparticles and Its Impact on Oxygen Reduction Reaction.

Fuel cells are emerging as an important technology in energy conversion and storage processes. Although they have high theoretical efficiency, their performance is limited by slow kinetics and high precious metal requirements for the cathode, where oxygen is reduced to water. Currently, Pt-containing alloys such as Pt₃Ni, Pt₃Cu and Pt₃Co are considered the best catalysts for the oxygen reduction reaction (ORR)under acidic conditions. However, they degrade under ORR operating conditions, as the non-precious metals continuously leach out in the form of ions. Recently, researchers showed that doping PtNi with group 6 elements increased both the stability and reactivity of this catalyst. Molybdenum doping yielded the highest specific activity, which was attributed to the formation of MoO₃. However, the exact nature of the active site, and the mechanism by which the alloy is stabilized, are unknown.

To elucidate the effects described above, we use periodic Density Functional Theory calculations, together with surface science experiments, to systematically study the molecular-level underpinnings of metal oxide doping on alloy catalysts. We identified elements which from stable oxy-hydroxy moieties on metal surfaces under ORR conditions. Remarkably, elements like Cr, Mo and Ir can form stable hydroxide 0d and 2d structures on Pt under oxidizing conditions and can resist dissolution. These nanoscale structures exhibit properties different from their bulk counterparts and can effectively tune the reactivity of the Pt surface. The oxide structures preferentially cover Pt edge and kink sites which are otherwise susceptible to dissolution. The metal oxide dopants thereby stabilize the alloy nanoparticle, but the calculations suggest that they do not interact directly with ORR intermediates. We therefore propose a strain-based model to explain the experimentally observed increase in specific activity. The molecular-level understanding of metal oxide doped metal nanoparticles developed in this study will help develop better catalysts for reactions beyond ORR.