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

• Published in: ACS catalysis Vol. 8; no. 3; pp. 2546 - 2557
• Main Authors: Selinsek, Manuel, Deschner, Benedikt J, Doronkin, Dmitry E, Sheppard, Thomas L, Grunwaldt, Jan-Dierk, Dittmeyer, Roland
• Format: Journal Article
• Language: English
• Published: American Chemical Society 02.03.2018
• Subjects: direct synthesis; hydride; X-ray absorption spectroscopy; Pd; H2O2; operando
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.7b03514

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.