Abstract
In this paper, a series of Pd/CeO2 catalysts prepared by different synthesis routes and showing different morphological and textural properties have been investigated for passive NOx adsorption (PNA) applications. The results obtained by NOx adsorption/desorption tests demonstrated that NOx storage capacity and NOx storage efficiency of Pd/CeO2 materials depend strictly on their surface area, whereas the morphology of the support and the Pd deposition method do not seem to play a key role. In contrast, the Pd deposition method does impact the dynamics of NOx desorption by affecting the amount of NOx desorbed at different temperatures. This seems to be connected to Pd–Ce interactions at the nanoscale that favor NOx desorption at higher temperatures suitable for PNA application. These findings are relevant in designing and optimizing the properties of Pd/CeO2 materials for their function as passive NOx adsorbers.
Highlights
The regulation of NOx (NO and NO2) emissions from gasoline and diesel engines is becoming more stringent due to the growing concern about their effects on the environment and human health.[1,2] Selective catalytic reduction (SCR) and lean NOx traps (LNTs) are the current technologies developed and commercialized for the NOx emission abatement on diesel vehicles
The fresh samples supported on commercial ceria powders prepared by incipient wetness (Pd(iw)Ce(A)823F and Pd(iw)Ce(B)823F) and dry milling (Pd(m)Ce(A)823F and Pd(m)Ce(B)823F) as well as those prepared on polycrystalline CeO2 (Pd(iw)Ce(HSA1)823F and Pd(m)Ce(HSA1)823F) show intermediate surface area values, followed by fresh Pd(iw)Ce(NR)823F, Pd(SCS)CeF, and Pd(iw)Ce(LA)823F
We have reported that preparing a Pd-ceria catalyst by solution combustion synthesis can give rise to an ordered Pd−O−Ce superstructure over the ceria surface[40] and, more recently, that the low-energy milling of Pd and polycrystalline CeO2 powders leads to a strong redox exchange between Pd and CeO2, resulting in highly reactive and stable Pd−O−Ce sites.[37,44]
Summary
The regulation of NOx (NO and NO2) emissions from gasoline and diesel engines is becoming more stringent due to the growing concern about their effects on the environment and human health.[1,2] Selective catalytic reduction (SCR) and lean NOx traps (LNTs) are the current technologies developed and commercialized for the NOx emission abatement on diesel vehicles. In 2001, Ford Motor Company first reported the combination of a Pt-Al2O3 passive NOx adsorber with SCR to efficiently reduce NOx emissions.[11] More recently, Honda and Johnson Matthey developed novel materials, referred to as N-TWC material and Cold Start Concept (CSC), respectively, for low-temperature HCs and/or NOx trapping based on Pd/zeolites.[12,13] In the literature, zeolites-based adsorbers and metal oxide-supported materials have been studied for low-temperature NOx storage.[9,10,14−20] Palladium is usually the metal of choice for PNAs, as it is capable of promoting NOx storage in the form of more labile nitrites that can desorb at lower temperatures, whereas Pt tends to oxidize NO giving rise to adsorbed nitrate species that are not released in the working range of PNAs.[16,21] Pd/zeolites (BEA, MFI, SSZ-13) are promising candidates for PNA applications, and in general are considered more robust with respect to oxide-based adsorbers, especially for their resistance to sulfur poisoning.[10,20] they still present some unresolved issues, such as dealumination following hightemperature hydrothermal treatments[22] and deactivation in presence of CO, which causes the agglomeration of Pd2+ into. The overall results shed some light on the understanding of the link between material properties and NOx adsorption/desorption behavior of Pd/ CeO2 materials
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