IN-SITU X-RAY-ABSORPTION SPECTROSCOPY STUDIES OF HYDRIDE AND CARBIDE FORMATION IN SUPPORTED PALLADIUM CATALYSTS

Extended X-ray absorption fine structure (EXAFS) spectroscopy as used to characterize hydride and carbide phases in supported Pd catalysts. Transmission EXAFS measurements were made at room temperature on 5% Pd/C and 5% Pd/gamma-Al2O3 catalysts. Combined EXAFS and transmission electron microscopy (TEM) results indicate that the average Pd particle diameter is 26 +/- 8 angstrom (or Pd dispersion is approximately 45%) in both catalysts. Supported Pd particles in air-exposed catalysts were found to be approximately 98% converted to a disordered Pd oxide; the remaining Pd is in metallic cores (average diameter is approximately 6 angstrom) inside the oxidized Pd particles. Catalysts were reduced in situ and cooled to 25-degrees-C in H-2 (partial pressure of 26 Torr), yielding a hydride phase with a lattice expansion of 2.2 +/- 0.2%. The stoichiometry of the hydride phase, PdH(x) (x approximately 0.44), is consistent with previous reports of decreased H-2 sorption capacity, relative to bulk Pd, in supported Pd catalysts. Decomposition of the hydride phase yielded metallic Pd particles with a first-shell Pd-Pd coordination number of 9 +/- 1. Reaction of the reduced 5% Pd/C catalyst with 1% C2H4/Ar at 150-degrees-C for 20 min generates a Pd carbide phase with an average Pd-Pd distance 1.2% larger than that in Pd metal. The PdC(x) phase has a maximum carbon content, x approximately 0.06, about half that of bulk PdC(x') x approximately 0. 1 3. The decreased carbon content of the Pd carbide phase in supported Pd catalysts is analogous to the decreased hydrogen content of the Pd hydride phase in Pd catalysts. PdC(x) can be distinguished from PdH(x) by its stability in the absence of H-2. The rate of carbidization of 5% Pd/gamma-Al2O3 was found to be slower than that of 5%Pd/C. A detailed study of the kinetics of carbidization is needed to determine the cause. There is no significant shift of the Pd K-edge absorption of either PdH(x) or PdC(x), relative to Pd metal, in these catalysts. The only noticeable difference between the near-edge spectra of interstitial phases and pure Pd is a decrease in the energy of the second resonance, a simple result of lattice expansion. The ability to detect Pd lattice expansion via the near-edge spectrum may be useful for time-resolved studies.