Effects of the Facet Orientation of γ‐Al2O3 Support on the Direct Synthesis of H2O2 Catalyzed by Pd Nanoparticles

Abstract

Two kinds of γ‐Al2O3 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 H2O2 from H2 and O2. 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‐Al2O3 (γ‐Al2O3‐C), while extra (111) facets appear on the surface of nanoplated γ‐Al2O3 (γ‐Al2O3‐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‐Al2O3‐P, rather than on γ‐Al2O3‐C, which has been ascribed to the stabilization of high‐energy Pd (100) by dense –OH groups on (111) surfaces of γ‐Al2O3‐P. Meanwhile, less PdO species has been found to be present on the Pd/γ‐Al2O3‐P catalyst, in comparison with that on the Pd/γ‐Al2O3‐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‐Al2O3 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 H2O2 direct synthesis.