Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H2_{2}O2_{2} synthesis

The direct synthesis of hydrogen peroxide over TiO$_{2}$-supported mono- and bimetallic Pd, PdSn, and PdIn nanoparticles (NPs) was performed in a continuous plug-flow reactor at 80 bar in ethanol with H$_{2}$ : O$_{2}$ ratios varied from 10 : 1 to 1 : 10. At the same time the catalysts were monitored by operando X-ray absorption spectroscopy (XAS). The setup optimized for XAS allowed productivities that are among the highest reported up to now. A rate of up to 580 mmol$_{H_{2}O_{2}}$ gcat$^{-1}$ h$^{-1}$ and a H$_{2}$O$_{2}$ concentration of 80 mmol l$^{-1}$ were obtained which were only limited by the supply of reactants. During H$_{2}$O$_{2}$ synthesis, the studied NPs revealed a face centered cubic (fcc) Pd(Sn/In) metal (alloy) structure at H$_{2}$ : O$_{2}$ ratios equal to or smaller than 1 and the corresponding β-hydride structure at H2 : O2 > 1. Under all conditions, additional SnO$_{2}$/In$_{2}$O$_{3}$ species were observed for the bimetallic catalysts. XAS supported by DFT calculations showed that alloying Pd with In or Sn limited the H$_{2}$ uptake capacity and the corresponding lattice expansion of the bimetallic NPs. Different catalysts performed best at different H$_{2}$ : O$_{2}$ ratios. All catalysts were stable at H$_{2}$ : O$_{2}$ > 1. Significant leaching of the active Pd and PdIn species could be observed for H$_{2}$ : O$_{2}$ ≤ 1 (quantified by XAS), while PdSn was relatively stable under these conditions. The higher stability of PdSn NPs is proposed to be due to a SnO$_{2}$ shell providing strong bonding between the NPs and the titania support

Investigation of mass transport processes in a microstructured membrane reactor for the direct synthesis of hydrogen peroxide

Microstructured membrane reactors present a promising approach to master the productivity and safety challenges during the direct synthesis of hydrogen peroxide. However, various mass transport processes occur in this complex system. In order to gain a deeper understanding of these processes, the saturation and desaturation behaviour of the liquid reaction medium with the gaseous reactants is investigated experimentally to examine possible cross-contamination. Moreover, the employed PDMS membrane’s permeances to hydrogen and oxygen are researched at different pressures, by using a variable-pressure/constant-volume setup for the behaviour at ambient pressure and a constant-pressure/variable-volume setup for the behaviour at elevated pressures. A mathematical model in MATLAB is applied to simulate the results. It is shown that a certain desaturation of the gasses through the membrane occurs, and the results are underlined by the modelled ones using a solution-diffusion model in MATLAB. Thus a constant flushing of the gas channels of the reactor is required for safety reasons. Moreover, the measured permeance values indicate that the species transport is mainly limited by the diffusion in the liquid phase and not the membrane resistance

Electrochemical Multisensor System for Monitoring the Hydrogen Peroxide Direct Synthesis in Microreactors

We present an electrochemical sensor system for the detection of hydrogen peroxide inside a direct synthesis microreactor. The setup allows the online, in situ measurement of high reactant concentrations by amperometric detection across the micro channel width and length. The robust integration of the electrochemical cell in the microreactor was demonstrated. Hydrogen peroxide was detected under reaction conditions (pH 3–4, presence of bromide) showing linear behaviour up to 2 mM with high sensitivity and excellent stability. The linear range was increased up to 10 mM by applying a diffusion limiting pHEMA layer to the electrode surface

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

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, 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