Non-aqueous sol–gel synthesis, characterization and catalytic properties of metal fluoride supported palladium nanoparticles
High surface area (HS) metal fluoride supported palladium catalysts (Pd0/MFx) (MFx = AlF3, MgF2, CaF2 and KMgF3) together with the respective Pd-free HS-MFx catalysts have been prepared via a novel sol–gel synthesis. In a first step, the reaction of anhydrous hydrogen fluoride (aHF) with the respective solutions of the metal alkoxides containing up to 10% Pd(acac)2 yielded the catalyst precursors as HS-gels. In a second step, gas phase fluorination followed by H2-treatment was employed to obtain the final Pd0/MFx catalysts. X-ray diffraction (XRD) indicated the formation of mainly amorphous AlF3 and Pd0/AlF3, whereas bulk MgF2, CaF2 and KMgF3 and the respective Pd-loaded phases were partially crystalline. The XRD patterns of Pd0/MFx revealed besides the occurrence of broad MFx-reflections only very weak reflections for Pd0 supporting a uniform dispersion of Pd nanoparticles inside the highly distorted MFx structures as it was independently reconfirmed by TEM-investigations. FTIR photo-acoustic spectroscopy with pyridine chemisorption (FTIR-PAS) and dismutation activities showed that AlF3 and MgF2 bulk as well as the supported Pd samples possess strong to moderate Lewis acidities. CaF2 and KMgF3 bulk as well as the respective Pd samples were found to be neutral. BET specific surface areas of MFx and Pd0/MFx are very high, up to ten times higher than known from literature. TEM showed homogeneously dispersed Pd nanoparticles on MFx matrices with a mean particle size of ∼5 to 8 nm. Catalytic activities of the samples were tested for the conversion of CHClF2 into valuable compounds by gas phase dismutation, hydrodehalogenation and pyrolysis, in presence as well as absence of hydrogen. Based on the obtained results and D/H-isotope exchange measurements, a carbene mechanism involving competitive dismutation coupled with consecutive reactions is proposed for the hydrodehalogenation process.
Journal: Applied Catalysis B: Environmental - Volume 78, Issues 1–2, 17 January 2008, Pages 80–91