(704d) Effect of Pd Coordination and Isolation on the Reduction of O2 to H2O2 over PdAu Bimetallic Catalysts

The direct synthesis of hydrogen peroxide (H­₂ + O₂ -> H₂O₂) offers a greener and low-cost alternative for H₂O₂ production in contrast to the commercial anthraquinone auto-oxidation process. Direct synthesis is however not economical due to lower selectivity towards H₂O₂ versus the thermodynamically favored side product H₂O. Experimental results with PdAu_x nanoparticles (0 ≤ x ≤ 100) show that catalysts with high Au to Pd ratios (>50) give steady-state H₂O₂ selectivities that approach 100%. Although previous investigations show that PdAu nanoparticles provide greater H₂O₂ selectivities than Pd, the reasons for these differences remain unclear. Herein we use ab-initio molecular dynamics (AIMD) and density functional theory (DFT) methods to simulate the reactivity and calculate reaction barriers for O₂ hydrogenation on surfaces with varying Pd surface concentrations to explain the improvement in the selectivity towards H₂O₂ with Pd isolation. The simulations carried out in an explicit aqueous solution show that the hydrogenation of O₂^* involves direct participation of H₂O molecules and occurs via a proton coupled electron transfer (PCET) mechanism involving the formation of H₃O⁺ species. Investigation of different catalytic surfaces with decreasing oligomer Pd cluster size, monomer Pd atoms with no nearest neighbors but with varying number of Pd as next nearest neighbors and completely isolated monomers show that the enthalpic barriers for H₂O₂ formation increase as Pd becomes isolated. The increase in the barrier for O-O bond splitting and H₂O formation, however, is increased much higher as Pd is isolated on the catalytic surface. This results in increasing H₂O₂ selectivity as suggested by experiments. The greater barriers for H₂O formation for more isolated Pd catalysts appear to be the result of differences in the interaction of the transition states for H₂O and H₂O₂ product formation with the Pd atoms on the surface.