Optimally designed synthesis of advanced Pd‐Rh bimetallic three‐way catalyst

Abstract

For Pd‐Rh bimetallic three‐way catalysts, appropriate Pd‐Rh interaction is of significant importance to the catalytic activity. In this work, an optimally designed approach was utilized to synthesize Pd‐Rh bimetallic catalyst supported on CeO2‐ZrO2‐Al2O3 (Rh/CZ/Pd/A), which was compared with the conventional catalysts prepared by mechanical mixing method and co‐impregnation technique. In Rh/CZ/Pd/A, Rh species are distributed on the external surface of CZ, while Pd species are distributed in the inner layer. Upon the application of the hydrothermal aging treatment, Pd species undergo migration to the outer sphere of CZ grains. Meanwhile, modified Pd‐Rh interaction and metal‐support interaction are obtained, which are efficient for stabilizing PdO and Rh2O3 at high temperatures, as well as suppressing the deactivation of Rh species by diffusing into the support. Overall, smaller size, better dispersion, along with the higher oxidation state of Pd species and the more reducible oxidation state of Rh species are maintained for Rh/CZ/Pd/A‐a. Consequently, improved three‐way catalytic activity is achieved for Rh/CZ/Pd/A especially after the hydrothermal aging treatment.

Effects of Feed Gas Composition on Fresh and Aged TWC-Coated GPFs Loaded with Real Soot

“…Samples treated at 650 °C presented a larger recovery of the catalytic activity compared to the ones subjected at 950 °C. A better dispersion (smaller particle sizes), along with a higher oxidation state of Pd species, 31 helped achieve a high conversion efficiency for Pd650 and PdRh650 samples after the soot regeneration, even though the soot oxidation was not complete (Figure 12). In contrast, Pd950 and PdRh950 presented larger catalytic degradation probably associated with the soot remaining on the catalysts, limiting the diffusion of species toward the active sites.…”

“…The Pd-Rh alloy may be formed as a result of the thermal treatment to which the samples were exposed. This observation basically involves the samples subjected to 800 and 950 °C, since, according to the literature, the interaction between metals is promoted at temperatures above 800 °C. − This interaction forms a core–shell-structured alloy with Rh covered by Pd . According to Vedyagin et al, the Pd-Rh alloy hinders both the diffusion of Rh into the support bulk and the surface migration of Pd, increasing the thermal stability of the system and preventing its deactivation.…”

Superior Pd–Rh Three-Way Catalyst: Modulating the Surface Composition by Introducing Ceria-Zirconia with Partial κ-Ce2Zr2O8 Structure as Support

“…According to the literature, 5 the Rh 3d 5/2 peak at 307.0− 307.3 eV is attributed to metallic Rh (Rh 0 ), the region of 308.6−309.4 eV belongs to Rh 2 O 3 (Rh 3+ ), and the peak referring to RhO 2 (Rh 4+ ) is located at 309.6−310.0 eV. Figure 9 shows the Rh 3d 5/2 bands at 307.0 and 309.0 eV, implying the formation of Rh 0 and Rh 3+ without Rh 4+ both in Rh/κCZ and Rh/CZ.…”

“…2,3 To weaken the deactivation induced by the formation of Pd−Rh alloys, there are three main strategies: one is to physically separate the metals, namely, Pd and Rh are separately deposited on different washcoats which are then physically mixed or used by layer-by-layer coating on the cordierite monolith, obviating the phenomenon of the Pd-rich surface. 4 Lan et al 5 reported an optimized layer-by-layer Pd− Rh catalyst (Rh/CZ/Pd/A) for TWC, in which the Pd-layer was placed at the bottom layer (Pd was supported on Al 2 O 3 ) and the Rh-layer (Rh was on CZ) was delivered to the top layer, showing positive results in TWC performance than the traditional Pd−Rh catalyst because the formation of the Pd− Rh alloy was inhibited. Specifically, the optimized catalyst could achieve complete conversion for NO and HC at 440 °C, whereas the traditional catalyst could not.…”

Recent progress of Rh‐based three‐way catalysts

“…The positions of Rh and Pd were controlled by adjusting the impregnation sequence (Figure 9b). [73] When Rh and Pd were separately impregnated onto two different supports, both Rh and Pd showed severe aggregation and generated inactive oxidation states, when Rh and Pd were co‐impregnated on two different physically mixed supports, Rh and Pd interacted with each other the most, which was detrimental to the three‐way catalysts, and the large alloy particles were easily formed during aging. During hydrothermal aging, the Pd species in the inner layer of Rh/CZ/Pd/A moved outward to the CZ layer, and the CZ layer on Al 2 O 3 maintained a high specific surface area, which enhanced metal‐support interactions of Pd, and hydrothermal stability of Rh.…”