Self-Regeneration of Pd–LaFeO₃ Catalysts: New Insight from Atomic-Resolution Electron Microscopy

Abstract

Aberration-corrected transmission electron microscopy was used to study atomic-scale processes in Pd-LaFeO(3) catalysts. Clear evidence for diffusion of Pd into LaFeO(3) and out of LaFe(0.95)Pd(0.05)O(3-δ) under high-temperature oxidizing and reducing conditions, respectively, was found, but the extent to which these processes occurred was quite limited. These observations cast doubt that such phenomena play a significant role in a postulated mechanism of self-regeneration of this system as an automotive exhaust-gas catalyst.

Structured Perovskite-Based Catalysts and Their Application as Three-Way Catalytic Converters—A Review

“…At least, the partial embedding of these particles in the support surface confirms that they strongly interact with the host structure ( Figure 7) [78]. In fact, contrary to what happens during the relatively long pretreatment procedure prior to HAADF-STEM observation (1 h under constant atmosphere [79] or alternance between reducing and oxidizing environments every 10 min [78]), one can expect that this interaction should be constantly revived under the high frequency (0.5-5 Hz) redox fluctuations of a real exhaust. This should limit the mobility of the precious metal particles on the surface of the host material and prevent their agglomeration.…”

“…Both phenomena undoubtedly play a role in the activity of the material. The formation and dissolution of noble metal clusters at the surface of the perovskite particles was found to be much more limited than expected [78]. The surface noble metal clusters formed after reduction in 10 vol% H 2 /N 2 at 800 °C for 1 h tended to coarsen rather than return into the perovskite matrix after oxidation in 20 vol% O 2 /N 2 at 800 °C for 1 h [79].…”

“…The surface noble metal clusters formed after reduction in 10 vol% H 2 /N 2 at 800 °C for 1 h tended to coarsen rather than return into the perovskite matrix after oxidation in 20 vol% O 2 /N 2 at 800 °C for 1 h [79]. At least, the partial embedding of these particles in the support surface confirms that they strongly interact with the host structure ( Figure 7) [78]. In fact, contrary to what happens during the relatively long pretreatment procedure prior to HAADF-STEM observation (1 h under constant atmosphere [79] or alternance between reducing and oxidizing environments every 10 min [78]), one can expect that this interaction should be constantly revived under the high frequency (0.5-5 Hz) redox fluctuations of a real exhaust.…”

“…The stronger metal-support interaction of the noble metal with a perovskite-type oxide support, as compared to a conventional alumina support, significantly increases its resistance to thermal sintering, allowing to enhance the catalytic performance while minimizing the noble metal content [76,77]. Recent HAADF-STEM observations on Pd-LaFeO 3 , Pt-CaTiO 3 , Rh-CaTiO 3 , and Pt-BaCeO 3 systems question the self-regenerative function [78,79]. The microscopy study evidenced that a fraction of the fully reversible formation-dissolution of noble metal particles under redox cycling predominantly takes place within the bulk of the host structure.…”

Comparison of precious metal doped and impregnated perovskite oxides for TWC application

“…We thus recently used scanning transmission electron microscopy (STEM) to gain new insight into the self-regeneration process [15,16]. Here, we focus on structure-activity relationships, comparing our initial catalytic activity results from two of the most prominent systems [9], Pd-LaFeO 3 and Rh-CaTiO 3 , using CO oxidation as a probe reaction.…”