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.