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Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low-surface-area α-Al 2 O 3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ-Al 2 O 3 to α-Al 2 O 3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation., (© 2024 Wiley‐VCH GmbH.)

Ultrasmall Pd 4 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 O 2 activation until CO desorption occurs. Small Pd clusters in zeolites, in contrast, demonstrate fluxional behavior in the presence of CO, which significantly increases the affinity for binding O 2 . Our study provides a pathway to achieve low-temperature CO oxidation activity on the basis of a well-defined Pd/zeolite system.

In this study, we utilized an ultramicroporous metal-organic framework (MOF) named [Ni 3 (pzdc) 2 (ade) 2 (H 2 O) 4 ]·2.18H 2 O (where H 3 pzdc represents pyrazole-3,5-dicarboxylic acid and ade represents adenine) for hydrogen (H 2 ) adsorption. Upon activation, [Ni 3 (pzdc) 2 (ade) 2 ] was obtained, and in situ carbon monoxide loading by transmission infrared spectroscopy revealed the generation of open Ni(II) sites. The MOF displayed a Brunauer-Emmett-Teller (BET) surface area of 160 m 2 /g and a pore size of 0.67 nm. Hydrogen adsorption measurements conducted on this MOF at 77 K showed a steep increase in uptake (up to 1.93 mmol/g at 0.04 bar) at low pressure, reaching a H 2 uptake saturation at 2.11 mmol/g at ∼0.15 bar. The affinity of this MOF for H 2 was determined to be 9.7 ± 1.0 kJ/mol. In situ H 2 loading experiments supported by molecular simulations confirmed that H 2 does not bind to the open Ni(II) sites of [Ni 3 (pzdc) 2 (ade) 2 ], and the high affinity of the MOF for H 2 is attributed to the interplay of pore size, shape, and functionality.

Atom trapping leads to catalysts with atomically dispersed Ru 1 O 5 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. Ru 1 /CeO 2 is stable during continuous cycling, ramping, and cooling as well as the presence of moisture. Furthermore, Ru 1 /CeO 2 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 CeO 2 . Only ∼0.05 wt % of Ru is required for excellent NO x storage. Ru 1 O 5 sites exhibit much higher stability during calcination in air/steam up to 750 °C in contrast to RuO 2 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 Ru 1 /CeO 2 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 Ru 1 /CeO 2 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.

Pd-based catalysts are widely used in the oxidation of CH 4 and have a significant impact on global warming. However, understanding their active sites remains controversial, because interconversion between Pd and PdO occurs consecutively during the reaction. Understanding the intrinsic active sites under reaction conditions is critical for developing highly active and selective catalysts. In this study, we demonstrated that partially oxidized palladium (PdO x ) on the surface plays an important role for CH 4 oxidation. Regardless of whether the initial state of Pd corresponds to oxides or metallic clusters, the topmost surface is PdO x , which is formed during CH4 oxidation. A quantitative analysis using CO titration, diffuse reflectance infrared Fourier-transform spectroscopy, X-ray diffraction, and scanning transmission electron microscopy demonstrated that a surface PdO layer was formed on top of the metallic Pd clusters during the CH 4 oxidation reaction. Furthermore, the time-on-stream test of CH 4 oxidation revealed that the presence of the PdO layer on top of the metallic Pd clusters improves the catalytic activity. Our periodic density functional theory (DFT) calculations with a PdO x slab and nanoparticle models aided the elucidation of the structure of the experimental PdO particles, as well as the experimental C-O bands. The DFT results also revealed the formation of a PdO layer on the metallic Pd clusters. This study helps achieve a fundamental understanding of the active sites of Pd and PdO for CH 4 oxidation and provides insights into the development of active and durable Pd-based catalysts through molecular-level design.

Pd-based catalysts are the most widely used for CO oxidation because of their outstanding catalytic activity and thermal stability. However, fundamental understanding of the detailed catalytic processes occurring on Pd-based catalysts under realistic conditions is still lacking. In this study, we investigated CO oxidation on metallic Pd clusters supported on Al 2 O 3 and SiO 2 . High-angle annular dark-field scanning transmission electron microscopy revealed the formation of similar-sized Pd clusters on Al 2 O 3 and SiO 2 . In contrast, CO chemisorption analysis indicated a gradual change in the dispersion of Pd (from 0.79 to 0.2) on Pd/Al 2 O 3 and a marginal change in the dispersion (from 0.4 to 0.24) on Pd/SiO 2 as the Pd loading increased from 0.27 to 5.5 wt %; these changes were attributed to differences in the metal-support interactions. Diffuse reflectance infrared Fourier-transform spectroscopy revealed that fewer a-top CO species were present in Pd supported on Al 2 O 3 than those in Pd supported on SiO 2 , which is related to the morphological differences in the metallic Pd clusters on these two supports. Despite the different dispersion profiles and surface characteristics of Pd, O 2 titration demonstrated that linearly bound CO (with an infrared signal at 2090 cm -1 ) reacted first with oxygen in the case of CO-saturated Pd on Al 2 O 3 and SiO 2 , which suggests that a-top CO on the terrace site plays an important role in CO oxidation. The experimental observations were corroborated by periodic density functional calculations, which confirmed that CO oxidation on the (111) terrace sites is most plausible, both kinetically and thermodynamically, compared to that on the edge or corner sites. This study will deepen the fundamental understanding of the effect of Pd clusters on CO oxidation under reaction conditions., (© 2022 Wiley-VCH GmbH.)

Cu/zeolites efficiently catalyze selective reduction of environmentally harmful nitric oxide with ammonia. Despite over a decade of research, the exact NO reduction steps remain unknown. Herein, using a combined spectroscopic, catalytic and DFT approach, we show that nitrosyl ions (NO + ) in zeolitic micropores are the key intermediates for NO reduction. Remarkably, they react with ammonia even below room temperature producing molecular nitrogen (the reaction central to turning the NO pollutant to benign nitrogen) through the intermediacy of the diazo N 2 H + cation. Experiments with isotopically labeled N-compounds confirm our proposed reaction path. No copper is required for N 2 formation to occur during this step. However, at temperatures below 100 °C, when NO + reacts with NH 3 , the bare Brønsted acid site becomes occupied by NH 3 to form strongly bound NH 4 + , and consequently, this stops the catalytic cycle, because NO + cannot form on NH 4 -zeolites when their H + sites are already occupied by NH 4 + . On the other hand, we show that the reaction becomes catalytic on H-zeolites at temperatures when some ammonia desorption can occur (>120 °C). We suggest that the role of Cu(ii) ions in Cu/zeolite catalysts for low-temperature NO reduction is to produce abundant NO + by the reaction: Cu(ii) + NO → Cu(i)⋯NO + . NO + then reacts with ammonia to produce nitrogen and water. Furthermore, when Cu(i) gets re-oxidized, the catalytic cycle can then continue. Our findings provide novel understanding of the hitherto unknown steps of the SCR mechanism pertinent to N-N coupling. The observed chemistry of Cu ions in zeolites bears striking resemblance to the copper-containing denitrification and annamox enzymes, which catalyze transformation of NO x species to N 2 , via di-azo compounds., Competing Interests: The authors have no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Steamed zeolites exhibit improved catalytic properties for hydrocarbon activation (alkane cracking and dehydrogenation). The nature of this practically important phenomenon has remained a mystery for the last six decades and was suggested to be related to the increased strength of zeolitic Bronsted acid sites after dealumination. We now utilize state-of-the-art infrared spectroscopy measurements and prove that during steaming, aluminum oxide clusters evolve (due to hydrolysis of Al out of framework positions with the following clustering) in the zeolitic micropores with properties very similar to (nano) facets of hydroxylated transition alumina surfaces. The Bronsted acidity of the zeolite does not increase and the total number of Bronsted acid sites decreases during steaming. O 5 Al(VI)-OH surface sites of alumina clusters dehydroxylate at elevated temperatures to form penta-coordinate Al 1 O 5 sites that are capable of initiating alkane cracking by breaking the first C-H bond very effectively with much lower barriers (at lower temperatures) than for protolytic C-H bond activation, with the following reaction steps catalyzed by nearby zeolitic Bronsted acid sites. This explains the underlying mechanism behind the improved alkane cracking and alkane dehydrogenation activity of steamed zeolites: heterolytic C-H bond breaking occurs on Al-O sites of aluminum oxide clusters confined in zeolitic pores. Our findings explain the origin of enhanced activity of steamed zeolites at the molecular level and provide the missing understanding of the nature of extra-framework Al species formed in steamed/dealuminated zeolites.

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. The NO release temperature of both adsorbers decreases gradually with an increase in 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 ≈1000 ppm improves the NOx storage significantly for Pd/FER, which was attributed to the presence of Pd ions in FER sites that are 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 800 °C in 10 % H 2 O 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., (© 2021 Wiley-VCH GmbH.)

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) 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., (© 2021 Wiley-VCH GmbH.)

CO oxidation is of importance both for inorganic and living systems. Transition and precious metals supported on various materials can oxidize CO to CO 2 . Among them, few systems, such as Au/TiO 2 , 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 CO 2 by nitrate at -140 °C within an inorganic, nonmetallic zeolitic system. The transformation of NO x and CO species in zeolite as well as the origin of this unique activity is clarified using a joint spectroscopic and computational approach., (© 2021. The Author(s).)

The analogy between single-atom catalysts (SACs) and molecular catalysts predicts that the specific catalytic activity of these systems is constant. We provide evidence that this prediction is not necessarily true. As a case in point, we show that the specific activity over ceria-supported single Pd atoms linearly increases with metal atom density, originating from the cumulative enhancement of CeO 2 reducibility. The long-range electrostatic footprints (≈1.5 nm) around each Pd site overlap with each other as surface Pd density increases, resulting in an observed deviation from constant specific activity. These cooperative effects exhaust previously active O atoms above a certain Pd density, leading to their permanent removal and a consequent drop in reaction rate. The findings of our combined experimental and computational study show that the specific catalytic activity of reducible oxide-supported single-atom catalysts can be tuned by varying the surface density of single metal atoms., (© 2021 Wiley-VCH GmbH.)

γ-alumina is one of the oldest and most important commercial catalytic materials with high surface area and stability. These attributes enabled its use as the first commercial large-scale heterogeneous catalyst for ethanol dehydration. Despite progress in materials characterization the nature of the specific sites on the surface of γ-alumina that are responsible for its unique catalytic properties has remained obscure and controversial. By using combined infrared spectroscopy, electron microscopy and solid-state nuclear magnetic resonance measurements we identify the octahedral, amphoteric (O) 5 Al(VI)-OH sites on the (100) segments of massively restructured (110) facets on typical rhombus-platelet γ-alumina as well as the (100) segments of irrational surfaces (invariably always present in all γ-alumina samples) responsible for its unique catalytic activity. Such (O) 5 Al(VI)-OH sites are also present on the macroscopically defined (100) facets of γ-alumina with elongated/rod-like geometry. The mechanism by which these sites lose -OH groups upon thermal dehydroxylation resulting in coordinatively unsaturated penta-coordinate Al +3 O 5 sites is clarified. These coordinatively unsaturated penta-coordinate Al sites produce well-defined thermally stable Al-carbonyl complexes. Our findings contribute to the understanding of the nature of coordinatively unsaturated Al sites on the surface of γ-alumina and their role as catalytically active sites., (© 2021 Wiley-VCH GmbH.)

Rh(C 2 H 4 ) 2 species grafted on the HY zeolite framework significantly enhance the activation of H 2 that reacts with C 2 H 4 ligands to form C 2 H 6 . While in this case, the simultaneous activation of C 2 H 4 and H 2 and the reaction between these species on zeolite-loaded Rh cations is a legitimate hydrogenation pathway yielding C 2 H 6 , the results obtained for Rh(CO)(C 2 H 4 )/HY materials exposed to H 2 convincingly show that the support-assisted C 2 H 4 hydrogenation pathway also exists. This additional and previously unrecognized hydrogenation pathway couples with the conversion of C 2 H 4 ligands on Rh sites and contributes significantly to the overall hydrogenation activity. This pathway does not require simultaneous activation of reactants on the same metal center and, therefore, is mechanistically different from hydrogenation chemistry exhibited by molecular organometallic complexes. We also demonstrate that the conversion of zeolite-supported Rh(CO) 2 complexes into Rh(CO)(C 2 H 4 ) species under ambient conditions is not a simple CO/C 2 H 4 ligand exchange reaction on Rh sites, as this process also involves the conversion of C 2 H 4 into C 4 hydrocarbons, among which 1,3-butadiene is the main product formed with the initial selectivity exceeding 98% and the turnover frequency of 8.9 × 10 -3  s -1 . Thus, the primary role of zeolite-supported Rh species is not limited to the activation of H 2 , as these species significantly accelerate the formation of the C 4 hydrocarbons from C 2 H 4 even without the presence of H 2 in the feed. Using periodic density functional theory calculations, we examined several catalytic pathways that can lead to the conversion of C 2 H 4 into 1,3-butadiene over these materials and identified the reaction route via intermediate formation of rhodacyclopentane.

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-temperature 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., (© 2020 Wiley-VCH GmbH.)

The majority of harmful atmospheric CO and NO x emissions are from vehicle exhausts. Although there has been success addressing NO x emissions at temperatures above 250 °C with selective catalytic reduction technology, emissions during vehicle cold start (when the temperature is below 150 °C), are a major challenge. Herein, we show we can completely eliminate both CO and NO x emissions simultaneously under realistic exhaust flow, using a highly loaded (2 wt %) atomically dispersed palladium in the extra-framework positions of the small-pore chabazite material as a CO and passive NO x adsorber. Until now, atomically dispersed highly loaded (>0.3 wt %) transition-metal/SSZ-13 materials have not been known. We devised a general, simple, and scalable route to prepare such materials for Pt II and Pd II . Through spectroscopy and materials testing we show that both CO and NO x can be simultaneously completely abated with 100 % efficiency by the formation of mixed carbonyl-nitrosyl palladium complex in chabazite micropore., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A low-cost poly(ethyleneimine)-cobalt (PEI-Co) complex solution with high O2 absorption capacity (as high as 1.5 L O2 (STP)/L solution) was developed and used in a bio-mimetic system to produce high-purity O2 from air. An oxygen concentration greater than 99.6% was demonstrated.

Rh-dendrimer nanocomposites were synthesized in solution under different conditions and were subsequently used as precursors for the preparation of ZrO2-supported Rh nanoparticles. Elemental analysis, UV-vis, XPS, and STEM measurements were used to estimate the extent of the Rh-dendrimer interactions and to illustrate how the solution pH and dialysis affect the number of Rh atoms complexed with each dendrimer molecule, as well as the final size of the ZrO2-supported Rh particles. When the solution acidity was not controlled and the solution was not purified by dialysis, Rh particles with sizes in the 1-6 nm range were formed on the ZrO2 support. In contrast, the formation of nearly uniform Rh particles was observed when the synthesis was performed under controlled pH and dialysis conditions. Furthermore, the size of these Rh particles can be regulated by controlling the Rh/dendrimer ratio in the original solution., (Copyright © 2013 Elsevier Inc. All rights reserved.)