Direct Synthesis of H₂O₂on AgPt Octahedra: The Importance of Ag–Pt Coordination for High H₂O₂Selectivity

Abstract

H2O2 production by direct synthesis (H2 + O2 → H2O2) is a promising alternative to the energy-intensive anthraquinone oxidation process and to the use of chlorine for oxidation chemistry. Steady-state H2O2 selectivities are approximately 10-fold greater on AgPt octahedra (50%) than on Pt nanoparticles of similar size (6%). Moreover, the initial H2O2 formation rates and selectivities are sensitive to the fractional coverage of Pt atoms and their location on the surfaces of AgPt octahedra, which can be controlled by exposing these catalysts to either CO or inert gases at 373 K to produce Pt-rich (16% initial H2O2 selectivity) or Pt-poor (36% initial H2O2 selectivity) surfaces. Increasing the coordination of Pt to Ag significantly modifies the electronic structure of Pt active sites, which is reflected by a shift in the ν(C=O) singleton frequency in 13CO from 2016 cm–1 on Pt to ∼1975 cm–1 on AgPt. These bimetallic AgPt catalysts present lower activation enthalpies (ΔH⧧) for H2O2 formation (29 kJ mol–1 on Pt to 5 kJ mol–1 on AgPt) but a lesser decrease for H2O formation (26 kJ mol–1 on Pt to 16 kJ mol–1 on AgPt). Comparisons of H2O2 selectivities, ΔH⧧ values, and differences among the 13CO singleton frequencies show that a combination of coordinating Ag to Pt and inducing strain modifies the electronic structure of individual Pt atoms, causing them to bind η1-species (e.g., CO) more strongly than on Pt nanoparticles. Yet the dramatic increase in the number of isolated Pt atoms increases H2O2 selectivities by decreasing the number of Pt atom ensembles of sufficient size to cleave O–O bonds and form H2O.