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 bars. 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 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, oxygen was only found adsorbed on the surface of Pd nanoparticles and hydrogen was found dissolved in bulk palladium hydride (α-phase) indicating a reaction of surface oxygen with lattice hydrogen to form hydrogen peroxide. Under hydrogen rich conditions, formation of β-phase palladium hydride was found to coincide with zero H2O2 yield. This constitutes an operando study of direct H2O2 synthesis under elevated partial pressures of H2 and O2 in continuous flow. The results obtained will aid in rational design of future catalysts and optimization of process parameters, bringing the concept of a viable, efficient process for H2O2 synthesis one step closer to reality.

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The structure of mono- and bimetallic supported Pd, PdSn, and PdIn NPs was monitored with a combination of techniques during continuous H2O2 synthesis with H2O2 production rates up to 580 mmolH2O2 gcat−1 h−1.

Determining local concentrations of the analytes in state of the art microreactors is essential for the development of optimized and safe processes. However, the selective, parallel monitoring of all relevant reactants and products in a multianalyte environment is challenging. Electrochemical microsensors can provide unique information on the reaction kinetics and overall performance of the hydrogen peroxide synthesis process in microreactors, thanks to their high spatial and temporal resolution and their ability to measure in situ, in contrast to other techniques. We present a chronoamperometric approach which allows the selective detection of the dissolved gases hydrogen and oxygen and their reaction product hydrogen peroxide on the same platinum microelectrode in an aqueous electrolyte. The method enables us to obtain the concentration of each analyte using three specific potentials and to subtract interfering currents from the mixed signal. While hydrogen can be detected independently, no potentials can be found for a direct, selective measurement of oxygen and hydrogen peroxide. Instead, it was found that for combined signals, the individual contribution of all analytes superimposes linearly additive. We showed that the concentrations determined from the subtracted signals correlate very well with results obtained without interfering analytes present. For the first time, this approach allowed the mapping of the distribution of the analytes hydrogen, oxygen, and hydrogen peroxide inside a multiphase membrane microreactor, paving the way for online process control.