Observation of Strained PdO in an Aged Pd/Ceria-Zirconia Catalyst

Abstract

A model automotive-exhaust catalyst, Pd on Zr-rich ceria–zirconia, was characterized by X-ray diffraction, optical and electron microscopies, and micro-Raman spectroscopy following high-temperature aging. A broad bimodal distribution of Pd, introduced amongst the 10-μ m spherical particles of support material during catalyst preparation, was found to persist upon aging, and strained PdO was detected predominantly in those particles containing the higher concentration of Pd. The strain, approximately -1% by volume, was determined from the shift of the strong PdO Raman line at 650 cm -1 and the Grüneisen parameter, which was measured in a separate experiment in a diamond-anvil cell. The origin of the compressive stress that gives rise to this strain is believed to be the same as in the previously known phenomenon of Pd–metal encapsulation, but the Pd particles involved here are apparently not highly constrained within the sintered ceria–zirconia matrix prior to their oxidation.