Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct HO 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 g cat −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 H 2 : O 2 > 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. The structure of mono- and bimetallic supported Pd, PdSn, and PdIn NPs was monitored with a combination of techniques during continuous H 2 O 2 synthesis with H 2 O 2 production rates up to 580 mmol H 2 O 2 g cat −1 h −1 .