Revealing the Structure and Mechanism of Palladium during Direct Synthesis of Hydrogen Peroxide in Continuous Flow Using Operando Spectroscopy

Abstract

The direct synthesis of H2O2 is a dream reaction in the field of selective oxidation and green chemical synthesis. However, the unknown active state of the catalyst and the lack of a defined catalytic mechanism preclude the design and optimization of suitable catalysts and reactor setups. Here direct synthesis of H2O2 over Pd–TiO2 in water was investigated in a continuous flow reactor setup utilizing undiluted oxygen and hydrogen to increase aqueous-phase concentrations safely at ambient temperature and a pressure of 10 bar. In this experiment operando X-ray absorption spectroscopy (XAS) and online flow injection analysis for photometric quantification of H2O2 were combined to build catalyst structure–activity relationships in direct H2O2 synthesis. XAS at the Pd K absorption edge was used to observe the oxidation state and local Pd structure together with H2O2 production for three reactant ratio (H2/O2) regimes: hydrogen rich (>2), hydrogen lean (<0.5), and balanced (0.5–2). During H2O2 production, oxyge...