(41d) Catalytic Effects of Metal Coordination in Dilute Bimetallic Alloy Nanoparticles

Alloy nanoparticles offer advantages for multiple classes of catalytic reactions. Single atom alloy (SAA) catalysts represent a unique class of materials in which a matrix of an unreactive element (e.g., Au) contains isolated atoms of a reactive element (e.g., Pd, Pt). The absence of homometal coordination provides emergent catalytic properties and escape from linear free energy relationships that govern monometallic catalysts. These properties give remarkable turnover rates and selectivities for reactions that involve O₂ and H₂. MAu_x catalysts provide measurable rates and selectivities that depend strongly on the atomic ratio of Au to M and the identity of the dilute metal (M). Selectivities to H₂O₂ increase systematically with the ratio of Au to M for PtAu_x (Pt to PtAu₂₀₀) and PdAu_x (Pd to PdAu₂₂₀) catalysts. All catalysts give rates for H₂O₂ and H₂O formation that show consistent and molecularly interpretable dependencies on H₂ and O₂ pressures and require protic solvents signifying similar mechanisms dominate. Differences in rates and selectivities therefore reflect metal identity and coordination. In situ XAS shows that catalysts with the greatest H₂O₂ selectivities do not possess detectable Pd-Pd coordination. Analysis of dipole coupling measured by infrared of mixed ¹²CO-¹³CO adlayers distinguishes Pd site separation even among materials without Pd-Pd nearest neighbors. Comparisons between catalytic results and surface characterization demonstrate that single M-atoms in Au provide significantly greater selectivities than bulk M for all M-Au combinations, but forming these species depends strongly upon mixing enthalpies and nanoparticle size. For all bimetallic combinations examined (PdAu, PtAu, RhAu, IrAu), M single-atoms in Au selectively destabilize the multidentate transition states which cleave O-O bonds to form H₂O, and consequently, such materials offer the greatest selectivities to H₂O₂.