6 Search Results

Synthesis of robust and hydrothermally stable PGM/ceria materials for NO, CO, and hydrocarbon abatement remains a formidable challenge, as ceria and PGMs are known to sinter severely >800 °C under hydrothermal conditions, leading to irreversible activity loss. In this work, we tackle this challenge by synthesizing well-defined catalysts with atomically dispersed rhodium supported on ceria with varying abundance of (100), (101), and (111) facets. Evaluation of these catalysts for NO reduction by CO as well as CO and propylene oxidation under model and industrially relevant conditions reveals pronounced reactivity and stability differences. Different modes of interaction of Rh ions withmore » the ceria facets and their facile reducibility were shown to be the crucial parameters controlling reactivity, resulting in pronounced activity and stability variations. Facet-dependent poisoning of surfaces by nitrites was identified as the main reason for deactivation of the catalysts at low temperature, which is mitigated for (111) ceria facets. (111)-enriched ceria nanoparticles survive very harsh hydrothermal aging at 950 °C by maintaining and preserving (111) facets, unlike other ceria nanoparticles which sinter into poorly defined shapes. Thus, putting atomically dispersed PGM sites on (111) ceria facets lead to the catalytic material with the highest activity and stability for all studied reactions, providing the pathway to catalysts that can endure extremely harsh hydrothermal aging conditions.« less

Nitrous oxide N₂O reduction is a big challenge due to high global warming potential of N₂O. (~300 times higher compared with CO₂). The best known catalysts, such as Rh/ceria, require relatively high temperatures for N₂O decomposition. Herein, we report that Ru/ceria catalysts with low Ru loading of ~0.25 wt% efficiently catalyze low temperature N₂O reduction by CO starting at 100 °C (full N₂O conversion below 200 °C) under industrially relevant flow rates and gas concentrations. Further, this remarkable performance stems from maintaining isolated Ru cations even on reduced ceria surface and, simultaneously, the propensity of Ru to affect ceria surfacemore » to form labile surface oxygen thereby creating large number of oxygen vacancies (Ce⁺³ cations) in the presence of CO. In contrast, for Rh/CeO₂ catalysts with equivalent metal loading, the activity is much lower because atomically dispersed Rh sinters into metallic clusters at the onset reaction temperature (~200 °C): these clusters are much less effective than isolated single Ru ions, with lower Ce⁺³ concentration maintained on reduced Rh/CeO₂ catalyst. Our study highlights the benefits of gaining molecular-level insight into the dynamic nature of catalytically active sites under reaction conditions for preparing catalysts containing low loading of precious metals with unsurpassed low temperature activity.« less

Ultra small Pd₄ clusters form in the micropores of FER zeolite during low temperature treatment (100 °C) in the presence of humid CO gas. They effectively catalyze CO oxidation below 100°C, whereas Pd nanoparticles are not active as they are poisoned by CO. Using catalytic measurements, infrared (IR) spectroscopy, X-ray absorption spectroscopy (EXAFS), microscopy, and density functional theory calculations we provide the molecular level insight into this previously unreported phenomenon. Pd nanoparticles get covered with CO at low temperatures which effectively blocks O₂ activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in themore » presence of CO, which significantly increases their affinity for binding O₂. In conclusion, our study shows a pathway for achieving low temperature CO oxidation activity on the basis of well-defined Pd/zeolite system.« less

Moisture contained in vehicle exhaust gas normally degrades the capacity and efficiency of Pd ion-exchanged zeolites as NO_x adsorbents by competitive adsorption on active sites. Here, we report a counterexample to this general proposition, in which moisture facilitates the storage of NO as a nitrosyl complex on hydrated Pd ions in high temperature calcined FER-type zeolites. The divalent Pd²⁺ cations upon elevated temperature (>800 °C) calcination occupy cationic position that render them fully coordinated by oxygen ions of the zeolite framework, and become inactive for the adsorption of probe molecules such as NO or CO. These ‘hidden’ Pd ions, however,more » are accessible by NO when the zeolite is hydrated, but readily release NO at around 200°C as dehydration proceeds. Herein, by combining systematic in situ infra-red data with X-ray diffraction Rietveld analyses, we revealed that the high temperature-induced relocation of Pd ions to more stable cationic positions located near 6-membered ring of the ferrierite cage is responsible for this anomalous behavior. This discovery constitutes a notable advance in understanding coordination chemistry of cations in zeolites.« less

Atom trapping leads to catalysts with atomically dispersed Ru₁O₅ sites on (100) facets of ceria, as identified by spectroscopy and DFT calculations. This is a new class of ceria-based materials with Ru properties drastically different from the known M/ceria materials. They show excellent activity in catalytic NO oxidation, a critical step that requires use of large loadings of expensive noble metals in diesel aftertreatment systems. Ru₁/CeO₂ is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru₁/CeO₂ shows very high NO_x storage properties due to formation of stable Ru–NO complexes as well as amore » high spill-over rate of NO_x onto CeO₂. Only ~0.05 wt % of Ru is required for excellent NO_x storage. Ru₁O₅ sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO₂ nanoparticles. Here we clarify the location of Ru(II) ions on the ceria surface and experimentally identify the mechanism of NO storage and oxidation using DFT calculations and in situ DRIFTS/mass spectroscopy. Moreover, we show excellent reactivity of Ru₁/CeO₂ for NO reduction by CO at low temperatures: only 0.1–0.5 wt % of Ru is sufficient to achieve high activity. Modulation-excitation in situ infrared and XPS measurements reveal the individual elementary steps of NO reduction by CO on an atomically dispersed Ru ceria catalyst, highlighting unique properties of Ru₁/CeO₂ and its propensity to form oxygen vacancies/Ce⁺³ sites that are critical for NO reduction, even at low Ru loadings. Our study highlights the applicability of novel ceria-based single-atom catalysts to NO and CO abatement.« less

Pd ion exchanged zeolites emerged as promising materials for the adsorption and oxidation of air pollutants. For low-temperature vehicle exhaust, dispersed Pd ions are able to adsorb NO_x even in H₂O-rich exhaust in the presence of carbon monoxide. In order to understand this phenomenon, changes in Pd ligand environment have to be monitored in-situ. Herein, we directly observe the activation of hydrated Pd ion shielded by H₂O into a carbonyl-nitrosyl complex Pd²⁺(NO)(CO) in SSZ-13 zeolite. The subsequent thermal desorption of ligands on Pd²⁺(NO)(CO) complex proceeds to nitrosyl Pd²⁺ rather than to carbonyl Pd²⁺ under various conditions. Thus, CO molecules actmore » as additional ligands to provide alternative pathway through Pd²⁺(NO)(CO) complex with lower energy barrier for accelerating NO adsorption on hydrated Pd²⁺ ion, which is kinetically limited in the absence of CO. We further demonstrate that hydration of Pd ions in the zeolite is a prerequisite for CO-induced reduction of Pd ions to metallic Pd. The reduction of Pd ions by CO is limited under dry conditions even at a high temperature of 500°C, while water makes it possible at near RT. However, the primary NO adsorption sites are Pd²⁺ ions even in gases containing CO and water. These findings clarify additional mechanistic aspects of the passive NO_x adsorption (PNA) process and will help extend the NOx adsorption chemistry in zeolite-based adsorbers to practical applications.« less