Achieving Atomic Dispersion of Highly Loaded Transition Metals in Small‐Pore Zeolite SSZ‐13: High‐Capacity and High‐Efficiency Low‐Temperature CO and Passive NO x Adsorbers

Single Ru(II) Ions on Ceria as a Highly Active Catalyst for Abatement of NO

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 a 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. 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.

Exceptionally high and reversible NOx uptake by hollow Mn-Fe composite nanocubes derived from Prussian blue analogues

Noble metal-based catalysts are widely used as passive NO_x adsorbers (PNA) for cold-start NO_x emissions; however, efficient porous materials as an alternative have great development potential. Herein, porous Mn-Fe composites with hollow nanocubes derived from Prussian blue analogue (PBA) precursors were used as PNA. The effects of O₂, the molar ratio of Mn/Fe, calcination temperature and reaction temperature on their adsorption capacity were explored. The physicochemical properties of the obtained catalysts were systematically characterized by XRD, SEM, BET surface area, TGA, XPS and DRIFT techniques. The developed Mn₁Fe₂-450 presented excellent NO_x uptake (more than 2.16 mmol g^-1 at 200 °C). Moreover, a high NO_x adsorption performance was also retained in the presence of 10% water vapor. The existing Mn³⁺ and Fe²⁺ species could contribute to the NO_x adsorption and gaseous O₂ can accelerate NO activation to form more easily adsorbed NO₂. Surface NO₂ is further diffused and stored into the bulk of the Mn-Fe composite in the form of nitrite and nitrate. This work revealed a novel candidate for PNA catalysts, which might provide inspiration for the design of new adsorbent materials.

Biomimetic CO oxidation below -100 °C by a nitrate-containing metal-free microporous system

CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO2. Among them, few systems, such as Au/TiO2, can perform CO oxidation at temperatures as low as -70 °C. Living (an)aerobic organisms perform CO oxidation with nitrate using complex enzymes under ambient temperatures representing an essential pathway for life, which enables respiration in the absence of oxygen and leads to carbonate mineral formation. Herein, we report that CO can be oxidized to CO2 by nitrate at -140 °C within an inorganic, nonmetallic zeolitic system. The transformation of NOx and CO species in zeolite as well as the origin of this unique activity is clarified using a joint spectroscopic and computational approach.

Pd/FER vs Pd/SSZ-13 Passive NOx Adsorbers: Adsorbate-controlled Location of Atomically Dispersed Pd(II) in FER Determines High Activity and Stability

Pd-loaded FER and SSZ-13 zeolites as low-temperature passive NOx adsorbers (PNA) are compared under practical conditions. Vehicle cold-start exposes the material to CO under a range of concentrations, necessitating a systematic exploration of the effect of CO on the performance of isolated Pd ions in PNA. NO release temperature of both adsorbers decreases gradually with the increase of CO concentration from a few hundred to a few thousand ppm. This beneficial effect results from local nano-"hot spot" formation during CO oxidation. Dissimilar to Pd/SSZ-13, increasing the CO concentration above ~1,000 ppm improves the NOx storage significantly for Pd/FER, attributed to the presence of Pd ions in FER γ-site that is shielded from NOx. CO mobilizes this Pd atom to the NOx accessible position where it becomes active for PNA. This behavior explains the very high resistance of Pd/FER to hydrothermal aging: Pd/FER materials survive hydrothermal aging at 8000C in 10% H2O vapor for 16 hours with no deterioration in NOx uptake/release behavior. Thus, by allocating Pd ions to the specific microporous pockets in FER, we have produced (hydro)thermally stable and active PNA materials.

Effects of CO and H2O Co-Feed on the Adsorption and Oxidation Properties of a Pd/BEA Hydrocarbon Trap

Hydrocarbon traps for exhaust emissions control adsorb hydrocarbons in low temperature exhaust and release them as the exhaust warms up. In this work, a Pd/BEA hydrocarbon trap was tested under lean exhaust conditions using ethylene and dodecane as model hydrocarbons. Ethylene uptake was partially inhibited by CO and H2O when fed separately. When both were added, the loss in ethylene uptake was 90% relative to the condition with no H2O or CO. Dodecane uptake was unchanged under all conditions tested. During a temperature ramp, ethylene desorbed and was combusted to CO2 and H2O over active Pd centers. Further, oxidation light-off of dodecane generated an exotherm which caused rapid desorption of the remaining hydrocarbon species from the zeolite. For both hydrocarbons, CO co-feed led to a decreased oxidation light-off temperature, and therefore lower desorption temperature. By pretreating the catalyst in CO and H2O at 80 °C, and even after removing CO from the feed, the enhanced oxidation light-off behavior was observed. DRIFTS characterization shows that some form of oxidized Pd was reducible to Pd0 by CO at 80 °C only in the presence of H2O. Further, this reduction appears reversible by high temperature oxygen treatment. We speculate that this reduced Pd phase serves as the active site for low temperature hydrocarbon oxidation.

Economizing on Precious Metals in Three-Way Catalysts: Thermally Stable and Highly Active Single-Atom Rhodium on Ceria for NO Abatement under Dry and Industrially Relevant Conditions*

We show for the first time that atomically dispersed Rh cations on ceria, prepared by a high-temperature atom-trapping synthesis, are the active species for the (CO+NO) reaction. This provides a direct link with the organometallic homogeneous Rh^I complexes capable of catalyzing the dry (CO+NO) reaction. The thermally stable Rh cations in 0.1 wt % Rh₁ /CeO₂ achieve full NO conversion with a turn-over-frequency (TOF) of around 330 h^-1 per Rh atom at 120 °C. Under dry conditions, the main product above 100 °C is N₂ with N₂ O being the minor product. The presence of water promotes low-temperature activity of 0.1 wt % Rh₁ /CeO₂ . In the wet stream, ammonia and nitrogen are the main products above 120 °C. The uniformity of Rh ions on the support, allows us to detect the intermediates of (CO+NO) reaction via IR measurements on Rh cations on zeolite and ceria. We also show that NH₃ formation correlates with the water gas shift (WGS) activity of the material and detect the formation of Rh hydride species spectroscopically.

Investigation of Various Pd Species in Pd/BEA for Cold Start Application

A series of Pd/BEA catalysts with various Pd loadings were synthesized. Two active Pd2+ species, Z−-Pd2+-Z− and Z−-Pd(OH)+, on exchanged sites of zeolites, were identified by in situ FTIR using CO and NH3 respectively. Higher NOx storage capacity of Z−-Pd2+-Z− was demonstrated compared with that of Z−-Pd(OH)+, which was caused by the different resistance to H2O. Besides, lower Pd loading led to a sharp decline of Z−-Pd(OH)+, which was attributed to the ‘exchange preference’ for Z−-Pd2+-Z− in BEA. Based on this research, the atom utilization of Pd can be improved by decreasing Pd loading.

Comparative study of the mobility of Pd species in SSZ-13 and ZSM-5, and its implication for their activity as passive NOx adsorbers (PNAs) after hydro-thermal aging

PNA ability and hydrothermal stability of Pd/ZSM-5 and Pd/SSZ-13 catalysts were comparatively studied.