Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H$_{2}$O$_{2}$ 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. ... mehrAll 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.