Hexagonal YFe_1−<i>x</i>Pd_<i>x</i>O_3−δ: Nonperovskite Host Compounds for Pd²⁺ and Their Catalytic Activity for CO Oxidation

Abstract

Metastable YFeO3 with the hexagonal YAlO3 structure was obtained by a sol−gel process at 700 °C, using metal nitrate precursors with pH control and the appropriate citric acid to nitrate ratio. Under similar conditions, YFe1−xPdxO3−δ (0 < x ≤ 0.1) compositions were also prepared. The substitution of Fe by Pd stabilizes the YAlO3 structure at higher temperatures. The crystal structures of YFe1−xPdxO3−δ (0 ≤ x ≤ 0.1) were refined by Rietveld analysis of X-ray and neutron powder diffraction data. The parent hexagonal YFeO3 (x = 0) crystallizes in the space group P63/mmc with a = 3.5099(3) Å and c = 11.759(2) Å. The redox-driven mobility of Pd to integrate into the oxide host as ions and to dissociate from it as fcc-Pd nanoparticles was monitored by a combination of X-ray diffraction and X-ray photoelectron spectroscopy. Pd nanoparticles in the reduced samples were detected by scanning backscattered electron microscopy and transmission electron microscopy. The Pd2+-containing materials showed significant low-temperature (near 100 °C) catalytic activity for CO oxidation, comparable to that of highly dispersed PdO/Al2O3, despite their relatively low surface areas.