Effect of Selectivity Enhancers on the Structure of Palladium during High-Pressure Continuous-Flow Direct Synthesis of Hydrogen Peroxide in Ethanol

A potentially green process to produce hydrogen peroxide (H2O2) is its direct synthesis from molecular hydrogen and oxygen. Still, the lack of mechanistic understanding of the reaction impedes a knowledge-based catalyst design for improved selectivity and stability. In this study, we employed X-ray absorption spectroscopy (XAS) to obtain structural information on a titania-supported palladium (Pd) catalyst under H2O2 synthesis conditions. The study focuses on the effect of the liquid-phase H-2:O-2 ratio between 0.67 and 3 and solvent composition under industrially relevant conditions, that is ethanol with and without H2SO4 and NaBr as selectivity enhancers at 40 bar. The X-ray near-edge structure of the absorption spectra (XANES) and the extended X-ray absorption fine structure (EXAFS) revealed that ethanol fully reduces Pd nanoparticles, even when saturated with oxygen. Oxygen presence at the Pd surface could only be suspected based on its small contribution to the EXAFS signal. A palladium hydride phase is detected under stoichiometric conditions due to lattice expansion. Addition of H2SO4 causes Pd lattice expansion and an increased yield at stoichiometric conditions. Further addition of bromide did not lead to any significant change in catalyst state and activity.