Dynamic structural changes of supported Pd, PdSn, and PdIn nanoparticles during continuous flow high pressure direct H(2)O(2)synthesis

摘要

The direct synthesis of hydrogen peroxide over TiO2-supported mono- and bimetallic Pd, PdSn, and PdIn nanoparticles (NPs) was performed in a continuous plug-flow reactor at 80 bar in ethanol with H2 : O2ratios varied from 10 : 1 to 1 : 10. At the same time the catalysts were monitored byoperandoX-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 mmolH2O2gcat-1h-1and a H2O2concentration of 80 mmol l-1were obtained which were only limited by the supply of reactants. During H2O2synthesis, the studied NPs revealed a face centered cubic (fcc) Pd(Sn/In) metal (alloy) structure at H2 : O2ratios equal to or smaller than 1 and the corresponding beta-hydride structure at H2 : O2> 1. Under all conditions, additional SnO2/In2O3species were observed for the bimetallic catalysts. XAS supported by DFT calculations showed that alloying Pd with In or Sn limited the H2uptake capacity and the corresponding lattice expansion of the bimetallic NPs. Different catalysts performed best at different H2 : O2ratios. All catalysts were stable at H2 : O2> 1. Significant leaching of the active Pd and PdIn species could be observed for H2 : O22shell providing strong bonding between the NPs and the titania support.