Effects of the Facet Orientation of γ‐Al 2 O 3 Support on the Direct Synthesis of H 2 O 2 Catalyzed by Pd Nanoparticles

Two kinds of γ‐Al 2 O 3 materials, with different facet proportions, were carefully prepared by using a template‐free solvothermal method. They were used to support Pd catalysts, with varying Pd particle sizes in the range of 0.6–2.6 nm, for the direct synthesis of H 2 O 2 from H 2 and O 2 . The evaluation of the catalytic activity and selectivity has demonstrated significant support effects and Pd particle‐size effects. The (110) facets predominate the surface of clustered γ‐Al 2 O 3 (γ‐Al 2 O 3 ‐C), while extra (111) facets appear on the surface of nanoplated γ‐Al 2 O 3 (γ‐Al 2 O 3 ‐P), occupying around 30 % of the area, in addition to (110) facets. More Pd (100) domains, besides Pd (111), have been disclosed to constitute the surface of small Pd particles deposited on γ‐Al 2 O 3 ‐P, rather than on γ‐Al 2 O 3 ‐C, which has been ascribed to the stabilization of high‐energy Pd (100) by dense –OH groups on (111) surfaces of γ‐Al 2 O 3 ‐P. Meanwhile, less PdO species has been found to be present on the Pd/γ‐Al 2 O 3 ‐P catalyst, in comparison with that on the Pd/γ‐Al 2 O 3 ‐C catalyst having parallel Pd particle sizes, suggesting the Pd (100) domains are more resistant than Pd (111) to oxidization. A conceptual illustration has been proposed to explain the effects of the facet orientation of the γ‐Al 2 O 3 support on both the topography of the Pd nanoparticles and the subsequent oxidative species, which are decisive factors for the catalytic activity and selectivity during H 2 O 2 direct synthesis.