Hexagonal YFe_1−xPd_xO_3−δ: Nonperovskite Host Compounds for Pd²⁺ and Their Catalytic Activity for CO Oxidation

Abstract

Metastable YFeO 3 with the hexagonal YAlO 3 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, YFe 1-x Pd x O 3-δ (0 < x e 0.1) compositions were also prepared. The substitution of Fe by Pd stabilizes the YAlO 3 structure at higher temperatures. The crystal structures of YFe 1-x Pd x O 3-δ (0 e x e 0.1) were refined by Rietveld analysis of X-ray and neutron powder diffraction data. The parent hexagonal YFeO 3 (x ) 0) crystallizes in the space group P6 3 /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 Pd 2+ -containing materials showed significant low-temperature (near 100 °C) catalytic activity for CO oxidation, comparable to that of highly dispersed PdO/Al 2 O 3 , despite their relatively low surface areas.

Dynamic Behavior of Pd Species in an LaFe0.95Pd0.05O3 Perovskite

“…On reduction with hydrogen, Pd particles segregated from the perovskite lattice, as confirmed by the H 2 -TPR, XPS, and HRTEM measurements, and were more active for CO oxidation than were the PdO species within the perovskite lattice, lowering the T 50 by 40 K. A similar phenomenon has been observed for LaCo 0.95 Pd 0.05 O 3 : reduction with hydrogen at 453 K lowered the T 50 by 43 K in CO oxidation [6]. In contrast, the activity for CO oxidation decreased on a BaCe 0.90 Pd 0.10 O 3 perovskite [9] and was almost unaffected on a YFe 0.90 Pd 0.10 O 3 perovskite [10] on reduction with hydrogen at high temperatures. For comparison, CO oxidation was tested on the reference Pd/ LaFeO 3 catalyst.…”

“…The magnified XRD patterns of the most intense (121) reflections showed that the 2h positions in the range 32.08-32.10°remained unchanged from 473 to 1,073 K, suggesting that Pd segregation from the perovskite framework did not obviously alter the perovskite structure. [10], and LaFePd 0.05 O 3 [11]. The Pd species located at the perovskite lattice [9,10] or dispersed on the perovskite surface [11] were considered to be the active sites for CO oxidation.…”

“…[10], and LaFePd 0.05 O 3 [11]. The Pd species located at the perovskite lattice [9,10] or dispersed on the perovskite surface [11] were considered to be the active sites for CO oxidation. The Pd species located at the perovskite lattice were responsible for the activity improvement in CO oxidation.…”

“…In most cases, the addition of Pd to a perovskite, either on the surface or within the perovskite lattice, enhances the catalytic activity towards CO oxidation, as demonstrated by Pd doping of BaCeO 3 [9], YFeO 3 [10], and LaFeO 3 [11]. However, it was also occasionally observed that Pd incorporation into LaCoO 3 resulted in inferior activity for CO oxidation [6].…”

Combustion activities of the Ru catalysts supported on hexagonal YbFeO3

“…Li et al have studied the CO oxidation activity of h-YFe 1¹x Pd x O 3¹¤ obtained by calcination of the precursor synthesized by a sol gel process. 13) In a previous paper, we compared the catalytic activities of o-and h-YbFeO 3 phases and found that the h-YbFeO 3 sample, which had a high surface area (29 m 2 g ¹1 ) even after calcination at 800°C, had a higher catalytic activity for methane combustion than the o-YbFeO 3 sample calcined at 800°C. 14) In this work, properties of h-and o-YbFeO 3 samples for the supports of the noble metal catalysts were examined: combustion reactions of propane or propene on these catalysts were investigated.…”