摘要
H 2 O 2 production by direct synthesis (H 2 + O 2 → H 2 O 2 ) is a promising alternative to the energy-intensive anthraquinone oxidation process and to the use of chlorine for oxidation chemistry. Steady-state H 2 O 2 selectivities are approximately 10-fold greater on AgPt octahedra (50%) than on Pt nanoparticles of similar size (6%). Moreover, the initial H 2 O 2 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 H 2 O 2 selectivity) or Pt-poor (36% initial H 2 O 2 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 13 CO from 2016 cm -1 on Pt to ∼1975 cm -1 on AgPt. These bimetallic AgPt catalysts present lower activation enthalpies (ΔH ‡ ) for H 2 O 2 formation (29 kJ mol -1 on Pt to 5 kJ mol -1 on AgPt) but a lesser decrease for H 2 O formation (26 kJ mol -1 on Pt to 16 kJ mol -1 on AgPt). Comparisons of H 2 O 2 selectivities, ΔH ‡ values, and differences among the 13 CO 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 η 1 -species (e.g., CO) more strongly than on Pt nanoparticles. Yet the dramatic increase in the number of isolated Pt atoms increases H 2 O 2 selectivities by decreasing the number of Pt atom ensembles of sufficient size to cleave O-O bonds and form H 2 O.