Shape-selective synthesis of intermetallic Pd3Pb nanocrystals and enhanced catalytic properties in the direct synthesis of hydrogen peroxide

Abstract

Hydrogen peroxide production by direct synthesis (H2 + O2 → H2O2) is a promising alternative to the commercialized indirect process involving sequential hydrogenation and oxidation of anthraquinones. Metal dopants are known to enhance the performance of Pd-based catalysts in this reaction by increasing H2O2 rates and selectivity. Recently, binary and ternary Pd-based alloys with Pb have been proposed as catalysts by theoretical studies, but these compositions lack experimental proof. Herein, shape-selective Pd3Pb nanocrystals were created to produce catalysts where the active and doping metal are colocalized to a fine extent. This strategy enables us to study the effects of both Pb doping and nanocrystal shape on the catalytic performance in direct H2O2 synthesis. In order to achieve these goals, we developed a procedure for the shape-controlled synthesis of Pb-doped nanocrystals with phase-pure, intermetallic Pd3Pb composition. By a change of the ligands, uniform Pd3Pb nanocrystals with cubic, cuboctahedral, and spherical shapes as well as flowerlike aggregates were obtained, which were supported on acid-treated TiO2. We show that the catalytic efficiency in direct H2O2 synthesis not only is influenced by the nanocrystal composition but also depends on the particle shape. Pd3Pb cubes, predominately terminated by their (200) facets, outperformed not only the monometallic Pd reference catalyst but also Pd3Pb nanocrystals with other shapes. Further DFT calculations and surface studies indicated not only the electronic modification of Pd surface atoms with a higher barrier for O2 dissociation on Pd3Pb but also a lack of larger Pd ensembles in Pd3Pb cubes which are known to cleave O.O bonds and form water. © 2021 American Chemical Society.