Preparation of nano-sized palladium particles anchored onto Ln₂O₃ grains with Ln=La, Nd, Sm, Eu or Gd

Abstract

Nano-sized Pd-particles anchored onto Ln₂O₃ particles with Ln=La, Nd, Sm, Eu or Gd have been prepared by reduction of the Ln₄PdO₇ precursor materials in a flowing gas mixture of CO and an inert gas (He or Ar). The reductions have been monitored by theromogravimetric (TG) and differential scanning calorimetric (DSC) measurements. The TG experiments show that the onset temperatures of the reduction varies from 450 to 575 °C with the lanthanoid used and Ln=Sm or Eu exhibit the lowest reduction temperatures. The air re-oxidation of the reduced materials back to their oxidised formed, e.g. Ln₄PdO₇, has also been studied by TG and this reaction starts at 280-340 °C depending on the lanthanoid used; the re-oxidation process occurs in two steps for all lanthanoids except Sm and Gd. The formation of Ln₄PdO₇ is observed to be very fast. The prepared and reduced materials have been characterised by their X-ray powder diffraction (XRPD) patterns. The DSC studies and XRPD patterns of quenched samples suggest that the reduction process occurs in at least two steps. The reduced materials have also been characterised by transmission electron microscopy (TEM/EDS) studies and a wet-chemistry method for selective dissolution of Ln₂O₃ has been used to extract the formed Pd nano-particles. Line-broadening analysis of the XRPD pattern of extracted particles suggests that the Pd particles are nanosized single crystals as the calculated particle size (20 nm) agrees fairly well with corresponding data from the TEM studies (5-20 nm). Reduced Ln₄PdO₇ materials have been shown to exhibit three way catalytic activity for car exhaust clean-up (i.e. simultaneous oxidation of CO and hydrocarbons and reduction of NO) with T₅₀ values around 450 °C.

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