10 Search Results

Abstract Low‐loaded platinum‐group single‐atom catalysts on CeO ₂ (M ₁ /CeO ₂ ) were synthesized via high‐temperature atom trapping (AT) and tested for the NO+CO reaction under dry and wet conditions. The activity of these catalysts for NO+CO reaction follows the order Rh>Pd≈Ru>Pt>Ir. For Rh, Ru, and Pd single‐atom catalysts, the N ₂ O byproduct is formed but not clearly observed in Ir and Pt cases, which may result from the higher reaction temperature (>200 °C) required for Pt and Ir catalysts. The presence of water can promote the activity of these M ₁ /CeO ₂ catalysts for the NO+CO reaction.more » Under wet conditions, significant NH ₃ formation occurred during the reaction, which is due to the co‐existence of water‐gas‐shift reaction on these catalysts. Compared with Pt, Pd and Ir, the Rh and Ru single‐atom catalysts show higher selectivity to NH ₃ species, resulting from the hydride species on the surface. Among all tested catalysts, Ru ₁ /CeO ₂ shows the highest production of ammonia and highest CO conversion due to excellent water‐gas‐shift activity, whereas Pd ₁ /CeO ₂ shows lowest ammonia production. Rh ₁ /CeO ₂ shows the best low temperature NO reduction activity among all tested catalysts.« less

Abstract Single atoms of platinum group metals on CeO ₂ represent a potential approach to lower precious metal requirements for automobile exhaust treatment catalysts. Here we show the dynamic evolution of two types of single-atom Pt (Pt ₁ ) on CeO ₂ , i.e., adsorbed Pt ₁ in Pt/CeO ₂ and square planar Pt ₁ in Pt _AT CeO ₂ , fabricated at 500 °C and by atom-trapping method at 800 °C, respectively. Adsorbed Pt ₁ in Pt/CeO ₂ is mobile with the in situ formation of few-atom Pt clusters during CO oxidation, contributing to high reactivity with near-zero reaction order inmore » CO. In contrast, square planar Pt ₁ in Pt _AT CeO ₂ is strongly anchored to the support during CO oxidation leading to relatively low reactivity with a positive reaction order in CO. Reduction of both Pt/CeO ₂ and Pt _AT CeO ₂ in CO transforms Pt ₁ to Pt nanoparticles. However, both catalysts retain the memory of their initial Pt ₁ state after reoxidative treatments, which illustrates the importance of the initial single-atom structure in practical applications.« less

Boron-based catalysts have been shown to be both active and selective for driving the oxidative dehydrogenation of propane (ODHP) without the use of precious metals. This reaction occurs at temperatures that melt the oxide catalyst which challenges our ability to identify the liquid structures of the boron oxide phase under reaction conditions, hindering the understanding of its active sites and reaction mechanism. By combining ab initio molecular dynamics simulation, in-situ Raman characterization, and microkinetic modeling, we propose that the di-coordinated boron sites (BO2) in liquid boron oxide are the active species for O2 activation under reaction conditions. The formed peroxy-likemore » species (>B-O-O-B<) can be viewed as a moderate oxidant for ODHP. The dynamical >B-O* dangling bond originated from >B-O-O-B< site as well as the liquid B2O3 structure itself, plays a critical role in the abstraction of H atoms from propane (C3H7 radical formation). Microkinetic modeling reveals C3H7 radical formation to be the main rate controlling step (~75% degree of rate control) with the dehydration of boron hydroxyls (B-OHs) to recover the di-coordinated boron active sites controlling the remainder of the rate (~25% degree of rate control). Moreover, the activation barriers are found to strongly depend upon the surface B-OH concentration. These findings provide significant insights into the active site and reaction mechanisms on boron-based catalysts for ODHP and underlie the importance of understanding the liquid nature of the catalyst to account for the catalytic activity.« 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

Zeolite-confined metal nanoparticles (NPs) have attracted much attention owing to their superior sintering resistance and broad applications for thermal and environmental catalytic reactions. However, the pore size of the conventional zeolites is usually below 2 nm, and reactants are easily blocked to access the active sites. Herein, a facile in situ mesoporogen-free strategy is developed to design and synthesize palladium (Pd) NPs enveloped in a single-crystalline zeolite (silicalite-1, S-1) with intra-mesopores (termed Pd@IM-S-1). Pd@IM-S-1 exhibited remarkable light alkanes deep oxidation performances, and it should be attributed to the confinement and guarding effect of the zeolite shell and the improvement inmore » mass-transfer efficiency and active metal sites accessibility. The Pd–PdO interfaces as a new active site can provide active oxygen species to the first C–H cleavage of light alkanes. This work exemplifies a promising strategy to design other high-performance intra-crystalline mesoporous zeolite-confined metal/metal oxide catalysts for high-temperature industrial thermal catalysis.« less

A single-atom Pt₁/CeO₂ catalyst formed by atom trapping (AT, 800 degrees C in air) shows excellent thermal stability but is inactive for CO oxidation at low temperatures owing to over-stabilization of Pt²⁺ in a highly symmetric square-planar Pt₁O₄ coordination environment. Reductive activation to form Pt nanoparticles (NPs) results in enhanced activity; however, the NPs are easily oxidized, leading to drastic activity loss. In this work, we show that tailoring the local environment of isolated Pt²⁺ by thermal-shock (TS) synthesis leads to a highly active and thermally stable Pt₁/CeO₂ catalyst. Ultrafast shockwaves (>1200 degrees C) in an inert atmosphere induced surfacemore » reconstruction of CeO₂ to generate Pt single atoms in an asymmetric Pt₁O₄ configuration. Owing to this unique coordination, Pt₁^δ+ in a partially reduced state dynamically evolves during CO oxidation, resulting in exceptional low-temperature performance. CO oxidation reactivity on the Pt₁/CeO₂_TS catalyst was retained under oxidizing conditions.« less

Abstract A single‐atom Pt ₁ /CeO ₂ catalyst formed by atom trapping (AT, 800 °C in air) shows excellent thermal stability but is inactive for CO oxidation at low temperatures owing to over‐stabilization of Pt ²⁺ in a highly symmetric square‐planar Pt ₁ O ₄ coordination environment. Reductive activation to form Pt nanoparticles (NPs) results in enhanced activity; however, the NPs are easily oxidized, leading to drastic activity loss. Herein we show that tailoring the local environment of isolated Pt ²⁺ by thermal‐shock (TS) synthesis leads to a highly active and thermally stable Pt ₁ /CeO ₂ catalyst. Ultrafast shockwaves (>1200 °C)more » in an inert atmosphere induced surface reconstruction of CeO ₂ to generate Pt single atoms in an asymmetric Pt ₁ O ₄ configuration. Owing to this unique coordination, Pt ₁ ^δ+ in a partially reduced state dynamically evolves during CO oxidation, resulting in exceptional low‐temperature performance. CO oxidation reactivity on the Pt ₁ /CeO ₂ _TS catalyst was retained under oxidizing conditions.« less

Abstract 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 % Rh ₁ /CeO ₂ 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 N ₂ with N ₂ O being the minor product. The presence of water promotes low‐temperaturemore » activity of 0.1 wt % Rh ₁ /CeO ₂ . 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 NH ₃ formation correlates with the water gas shift (WGS) activity of the material and detect the formation of Rh hydride species spectroscopically.« less

Abstract 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 % Rh ₁ /CeO ₂ 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 N ₂ with N ₂ O being the minor product. The presence of water promotes low‐temperaturemore » activity of 0.1 wt % Rh ₁ /CeO ₂ . 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 NH ₃ formation correlates with the water gas shift (WGS) activity of the material and detect the formation of Rh hydride species spectroscopically.« less

Hexagonal boron nitride (h-BN) with high thermal conductivity is potentially an effective catalyst for highly exothermic propane oxidative dehydrogenation (ODH) reaction. Here, we report our experimental and theoretic studies of such a catalyst for propane ODH in a fixed-bed reactor. Based on the computational fluid dynamics calculation (CFD) results, the catalyst bed temperature increases by less than 1°C in the h-BN catalyst bed which is much smaller than that (8°C) in the VO_x/γ-Al₂O₃ catalyst bed at a similar propane conversion (25%) using a micro-tubular reactor with a diameter of 6 mm. Even in an industrially relevant reactor with an innermore » diameter of 60 mm, a uniform temperature profile can still be maintained using the h-BN catalyst bed due to its excellent thermal conductivity as opposed to a temperature gradient of 47°C in the VO_x/γ-Al₂O₃ catalyst bed. The results reported here provide useful information for potential application of h-BN catalyst in propane ODH.« less