Atomically Dispersed Ru Catalyst for Low-Temperature Nitrogen Activation to Ammonia via an Associative Mechanism

Evolution of Oxygen Vacancy Sites in Ceria-Based High-Entropy Oxides and Their Role in N2 Activation

In this work, a relatively new class of materials, rare earth (RE) based high entropy oxides (HEO) are discussed in terms of the evolution of the oxygen vacant sites (Ov) content in their structure as the composition changes from binary to HEO using both experimental and computational tools; the composition of HEO under focus is the CeLaPrSmGdO due to the importance of ceria-related (fluorite) materials to catalysis. To unveil key features of quinary HEO structure, ceria-based binary CePrO and CeLaO compositions as well as SiO2, the latter as representative nonreducible oxide, were used and compared as supports for Ru (6 wt % loading). The role of the Ov in the HEO is highlighted for the ammonia production with particular emphasis on the N2 dissociation step (N2(ads) → Nads) over a HEO; the latter step is considered the rate controlling one in the ammonia production. Density functional theory (DFT) calculations and 18O2 transient isotopic experiments were used to probe the energy of formation, the population, and the easiness of formation for the Ov at 650 and 800 °C, whereas Synchrotron EXAFS, Raman, EPR, and XPS probed the Ce-O chemical environment at different length scales. In particular, it was found that the particular HEO composition eases the Ov formation in bulk, in medium (Raman), and in short (localized) order (EPR); more Ov population was found on the surface of the HEO compared to the binary reference oxide (CePrO). Additionally, HEO gives rise to smaller and less sharp faceted Ru particles, yet in stronger interaction with the HEO support and abundance of Ru-O-Ce entities (Raman and XPS). Ammonia production reaction at 400 °C and in the 10-50 bar range was performed over Ru/HEO, Ru/CePrO, Ru/CeLaO, and Ru/SiO2 catalysts; the Ru/HEO had superior performance at 10 bar compared to the rest of catalysts. The best performing Ru/HEO catalyst was activated under different temperatures (650 vs 800 °C) so to adjust the Ov population with the lower temperature maintaining better performance for the catalyst. DFT calculations showed that the HEO active site for N adsorption involves the Ov site adjacent to the adsorption event.

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Single-atom and cluster catalysts for thermocatalytic ammonia synthesis at mild conditions

Ammonia (NH3) is closely related to the fields of food and energy that humans depend on. The exploitation of advanced catalysts for NH3 synthesis has been a research hotspot for more than one hundred years. Previous studies have shown that the Ru B5 sites (step sites on the Ru (0001) surface uniquely arranged with five Ru atoms) and Fe C7 sites (iron atoms with seven nearest neighbors) over nanoparticle catalysts are highly reactive for N2-to-NH3 conversion. In recent years, single-atom and cluster catalysts, where the B5 sites and C7 sites are absent, have emerged as promising catalysts for efficient NH3 synthesis. In this review, we focus on the recent advances in single-atom and cluster catalysts, including single-atom catalysts (SACs), single-cluster catalysts (SCCs), and bimetallic-cluster catalysts (BCCs), for thermocatalytic NH3 synthesis at mild conditions. In addition, we discussed and summarized the unique structural properties and reaction performance as well as reaction mechanisms over single-atom and cluster catalysts in comparison with traditional nanoparticle catalysts. Finally, the challenges and prospects in the rational design of efficient single-atom and cluster catalysts for NH3 synthesis were provided.

A single site ruthenium catalyst for robust soot oxidation without platinum or palladium

The quest for efficient non-Pt/Pd catalysts has proved to be a formidable challenge for auto-exhaust purification. Herein, we present an approach to construct a robust catalyst by embedding single-atom Ru sites onto the surface of CeO2 through a gas bubbling-assisted membrane deposition method. The formed single-atom Ru sites, which occupy surface lattice sites of CeO2, can improve activation efficiency for NO and O2. Remarkably, the Ru1/CeO2 catalyst exhibits exceptional catalytic performance and stability during auto-exhaust carbon particle oxidation (soot), rivaling commercial Pt-based catalysts. The turnover frequency (0.218 h-1) is a nine-fold increase relative to the Ru nanoparticle catalyst. We further show that the strong interfacial charge transfer within the atomically dispersed Ru active site greatly enhances the rate-determining step of NO oxidation, resulting in a substantial reduction of the apparent activation energy during soot oxidation. The single-atom Ru catalyst represents a step toward reducing dependence on Pt/Pd-based catalysts.

Isolating Single and Few Atoms for Enhanced Catalysis

Atomically dispersed metal catalysts have triggered great interest in the field of catalysis owing to their unique features. Isolated single or few metal atoms can be anchored on substrates via chemical bonding or space confinement to maximize atom utilization efficiency. The key challenge lies in precisely regulating the geometric and electronic structure of the active metal centers, thus significantly influencing the catalytic properties. Although several reviews have been published on the preparation, characterization, and application of single-atom catalysts (SACs), the comprehensive understanding of SACs, dual-atom catalysts (DACs), and atomic clusters has never been systematically summarized. Here, recent advances in the engineering of local environments of state-of-the-art SACs, DACs, and atomic clusters for enhanced catalytic performance are highlighted. Firstly, various synthesis approaches for SACs, DACs, and atomic clusters are presented. Then, special attention is focused on the elucidation of local environments in terms of electronic state and coordination structure. Furthermore, a comprehensive summary of isolated single and few atoms for the applications of thermocatalysis, electrocatalysis, and photocatalysis is provided. Finally, the potential challenges and future opportunities in this emerging field are presented. This review will pave the way to regulate the microenvironment of the active site for boosting catalytic processes.

Elucidating the role of confinement and shielding effect over zeolite enveloped Ru catalysts for propane low temperature degradation

Volatile organic compounds (VOCs) are the main precursor for ozone formation and hazardous to human health. Light alkane as one of the typical VOCs is difficult to degrade to CO₂ and H₂O by catalytic degradation method due to its strong C-H bond. Herein, a series of ultrafine Ru nanoclusters (<0.95 nm) enveloped in silicalite-1 (S-1) zeolite catalysts were designed and prepared by a simple one-pot method and applied for catalytic degradation of propane. The results demonstrate that the enveloped Ru₁@S-1 catalyst has excellent propane degradation performance. Its T₉₅ is as low as 294 °C with moisture, and the turnover frequency (TOF) value is up to 5.07 × 10^-3 s^-1, evidently higher than that of the comparison supported catalyst (Ru₁/S-1). Importantly, Ru₁@S-1 exhibits superior thermal stability, water resistance and recyclability, which should be attributed to the confinement and shielding effect of the S-1 shell. The in-situ DRIFTS result reveals that the propane degradation over Ru₁@S-1 follows the Mars-van-Krevelen (MvK) mechanism, where the hydroxy from the framework of zeolite can provide the active oxygen species. Our work provides a new candidate and guideline for an efficient and stable catalyst for the low-temperature degradation of the light alkane VOCs.

Unveiling the Ir single atoms as selective active species for the partial hydrogenation of butadiene by operando XAS

Single-atom catalysts represent an intense topic of research due to their interesting catalytic properties for a wide range of reactions. Clarifying the nature of the active sites of single-atom catalysts under realistic working conditions is of paramount importance for the design of performant materials. We have prepared an Ir single-atom catalyst supported on a nitrogen-rich carbon substrate that has proven to exhibit substantial activity toward the hydrogenation of butadiene with nearly 100% selectivity to butenes even at full conversion. We evidence here, by quantitative operando X-ray absorption spectroscopy, that the initial Ir single atoms are coordinated with four light atoms i.e., Ir-X₄ (X = C/N/O) with an oxidation state of +3.2. During pre-treatment under hydrogen flow at 250 °C, the Ir atom loses one neighbour (possibly oxygen) and partially reduces to an oxidation state of around +2.0. We clearly demonstrate that Ir-X₃ (X = C/N/O) is an active species with very good stability under reactive conditions. Moreover, Ir single atoms remain isolated under a reducing atmosphere at a temperature as high as 400 °C.

Interfacial Electron Regulation of Rh Atomic Layer-Decorated SnO2 Heterostructures for Enhancing Electrocatalytic Nitrogen Reduction

Ammonia (NH₃), which serves as a fertilizer supply, is struggling to satisfy the ever-growing population requirements over the world. The electrocatalytic nitrogen reduction to NH₃ production is highly desired but shows the extremely poor activity and selectivity of reported electrocatalysts. In this work, we rationally design a novel Rh atomic layer-decorated SnO₂ heterostructure catalyst through the interfacial engineering strategy, simultaneously achieving the highest NH₃ yield rate (149 μg h^-1 mg_cat^-1) and Faradaic efficiency (11.69%) at -0.35 V vs the reversible hydrogen electrode. This result is superior to the optimum response of previously reported SnO₂- or Rh-based catalysts for electrochemical nitrogen reduction. Both X-ray absorption spectra characterization and density functional theory calculations reveal the strong electron interaction between the Rh atomic layer and the SnO₂ heterostructure, which effectively regulated the interfacial electron transfer and d-band center. The downshift of the d-band center results in the greatly reduced H adsorption energy and the highly accelerated reaction kinetics for nitrogen reduction. This work endows a new insight into the interfacial electron regulation for weakening H adsorption and further enhancing the electrocatalytic N₂ reduction.

Single-Atomic Ruthenium Active Sites on Ti3 C2 MXene with Oxygen-Terminated Surface Synchronize Enhanced Activity and Selectivity for Electrocatalytic Nitrogen Reduction to Ammonia

Downsizing the catalyst to atom scale offers an effective way to maximize the atom utilization efficiency for electrocatalytic nitrogen reduction reaction (NRR). Herein, single-atomic ruthenium (Ru) anchored on a chemically activated Ti₃ C₂ with O-terminated groups (Ti₃ C₂ O) was designed to catalyze the NRR process. The catalyst achieved a superior activity and selectivity with ammonia yield rate of 27.56 μg h^-1  mg^-1 and faradaic efficiency of 23.3 % at a low potential of -0.20 V versus the reversible hydrogen electrode. According to the atomic resolution images from aberration-corrected scanning transmission electron microscopy, Ru sites on Ti₃ C₂ O achieved good dispersion on atomic scale. X-ray photoelectron spectroscopy analysis further demonstrated that the O-termination groups were successfully activated. Density functional theory calculations combined with experiments revealed that single Ru sites binding to four oxygen were the main reaction centers that permitted the hydrogenation of *NNH₂ to *NHNH₂ in a novel distal/alternating hybrid path while reducing the energy barrier of the potential-limiting step to 0.78 eV from 0.96 eV in the distal path alone or 1.18 eV in the alternating path alone, thereby significantly promoting the NRR dynamics.

Single-Atom Ruthenium Biomimetic Enzyme for Simultaneous Electrochemical Detection of Dopamine and Uric Acid

Single-atom catalysts have attracted numerous attention due to the high utilization of metallic atoms, abundant active sites, and highly catalytic activities. Herein, a single-atom ruthenium biomimetic enzyme (Ru-Ala-C₃N₄) is prepared by dispersing Ru atoms on a carbon nitride support for the simultaneous electrochemical detection of dopamine (DA) and uric acid (UA), which are coexisting important biological molecules involving in many physiological and pathological aspects. The morphology and elemental states of the single-atom Ru catalyst are studied by transmission electron microscopy, energy dispersive X-ray elemental mapping, high-angle annular dark field-scanning transmission electron microscopy, and high-resolution X-ray photoelectron spectroscopy. Results show that Ru atoms atomically disperse throughout the C₃N₄ support by Ru-N chemical bonds. The electrochemical characterizations indicate that the Ru-Ala-C₃N₄ biosensor can simultaneously detect the oxidation of DA and UA with a separation of peak potential of 180 mV with high sensitivity and excellent selectivity. The calibration curves for DA and UA range from 0.06 to 490 and 0.5 to 2135 μM with detection limits of 20 and 170 nM, respectively. Moreover, the biosensor was applied to detect DA and UA in real biological serum samples using the standard addition method with satisfactory results.

Improved ammonia synthesis activity of Ce doped barium tantalate supported Ru catalysts

Ce doping could increase the specific surface area and reducibility of the barium tantalate carrier, which is beneficial to Ru based ammonia synthesis.