Direct synthesis of H₂O₂ in AuPd coated micro channels: An in-situ X-Ray absorption spectroscopic study

For the first time, we perform in-situ X-Ray Absorption Spectroscopy on both Au L₃ and Pd K edges during the direct synthesis of hydrogen peroxide in a high-pressure gas-liquid-solid microreactor using SiO₂ supported Pd, Au and AuPd catalysts under relevant reaction conditions. The catalysts were coated on the walls of 320 µm ID micro capillaries, used in this work both as microreactors and as spectroscopic cells, ensuring safety of operation, ease of portability and absence of reactor effects during the measurements. XANES analysis revealed that the fresh Pd and AuPd catalysts contained a mix of Pd/PdO phases which was readily reduced in H₂ atmosphere. The presence of only metallic Pd under reaction conditions for both catalysts proved that metallic Pd is the active phase for this reaction. Au was present in its metallic form in both Au and AuPd under all conditions tested. Unlike the AuPd, monometallic Pd showed an increase in the Pd-Pd distance under reaction conditions, attributed to the formation of Pd-hydride. We propose that the absence of Pd-hydride in the AuPd catalysts reduces further hydrogenation of H₂O₂ to water, thereby explaining the increased selectivity to H₂O₂ observed with AuPd catalysts.