Dispersed surface Ru ensembles on MgO(111) for catalytic ammonia decomposition

Heterogeneous Frustrated Lewis Pair Catalysts: Rational Structure Design and Mechanistic Elucidation Based on Intrinsic Properties of Supports

ConspectusThe discovery of reversible hydrogenation using metal-free phosphoborate species in 2006 marked the official advent of frustrated Lewis pair (FLP) chemistry. This breakthrough revolutionized homogeneous catalysis approaches and paved the way for innovative catalytic strategies. The unique reactivity of FLPs is attributed to the Lewis base (LB) and Lewis acid (LA) sites either in spatial separation or in equilibrium, which actively react with molecules. Since 2010, heterogeneous FLP catalysts have gained increasing attention for their ability to enhance catalytic performance through tailored surface designs and improved recyclability, making them promising for industrial applications. Over the past 5 years, our group has focused on investigating and strategically modifying various types of solid catalysts with FLPs that are unique from classic solid FLPs. We have explored systematic characterization techniques to unravel the underlying mechanisms between the active sites and reactants. Additionally, we have demonstrated the critical role of catalysts' intrinsic electronic and geometric properties in promoting FLP formation and stimulating synergistic effects. The characterization of FLP catalysts has been greatly enhanced by the use of advanced techniques such as synchrotron X-ray diffraction, neutron powder diffraction, X-ray photoelectron spectroscopy, extended X-ray absorption fine structure, elemental mapping in scanning transmission electron microscopy, electron paramagnetic resonance spectroscopy, diffuse-reflectance infrared Fourier transform spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. These techniques have provided deeper insights into the structural and electronic properties of FLP systems for the future design of catalysts.Understanding electron distribution in the overlapping orbitals of LA and LB pairs is essential for inducing FLPs in operando in heterogeneous catalysts through target electron reallocation by external stimuli. For instance, in silicoaluminophosphate-type zeolites with weak orbital overlap, the adsorption of polar gas molecules leads to heterolytic cleavage of the Alδ+-Oδ- bond, creating unquenched LA-LB pairs. In a Ru-doped metal-organic framework, the Ru-N bond can be polarized through metal-ligand charge transfer under light, forming Ru+-N- pairs. This activation of FLP sites from the framework represents a groundbreaking innovation that expands the catalytic potential of existing materials. For catalysts already employing FLP chemistry to dynamically generate products from substrates, a complete mechanistic interpretation requires a thorough examination of the surface electronic properties and the surrounding environment. The hydrogen spillover ability on the Ru-doped FLP surfaces improves conversion efficiency by suppressing hydrogen poisoning at metal sites. In situ H2-H2O conditions enable the production of organic chemicals with excellent activity and selectivity by creating new bifunctional sites via FLP chemistry. By highlighting the novel FLP systems featuring FLP induction and synergistic effects and the selection of advanced characterization techniques to elucidate reaction mechanisms, we hope that this Account will offer innovative strategies for designing and characterizing FLP chemistry in heterogeneous catalysts to the research community.

Single-Atom Barium Promoter Enormously Enhanced Non-Noble Metal Catalyst for Ammonia Decomposition

As a well-established topic, single-atom catalyst has drawn growing interest for its high utilization of metal. However, researchers prefer to develop various active metals with single-atom form, the intrinsic roles of single-atom promoters are usually underrated, which are significant in boosting reaction activity. In this work, Ba single atoms were in situ prepared in the Co-Ba/Y2O3 catalyst with crystallized BaCO3 as the precursor under the ammonia decomposition reaction condition. The optimized Co-Ba/Y2O3 catalyst achieves extremely high H2 production rate of 138.3 mmolH2 ⋅ gcat -1 ⋅ min-1 at very low temperature (500 °C, GHSV=840,000 mL ⋅ g-1 ⋅ h-1) and Co-Ba/Y2O3 exhibits excellent durability during the 350 h test, which realizes the highest activity among all non-noble catalysts, and reaches or even exceeds numerous reported Ru-based catalysts. Both Y2O3 and Co demonstrate positive interactions with Ba, which significantly facilitates the dispersion of Ba species at high temperatures (≥600 °C). Ba single atoms significantly enhance the charge density of Co and form additionally active Co-O-Ba-Y2O3 interfacial sites, which alleviates hydrogen poisoning and decreases the reaction barrier of the N-H bond activation of *NH. The exploration of atomically dispersed promoters is groundbreaking in heterogeneous catalysis, which opens up a whole new domain of catalytic material.

Optimizing surface active sites via burying single atom into subsurface lattice for boosted methanol electrooxidation

The precise fabrication and regulation of the stable catalysts with desired performance still challengeable for single atom catalysts. Here, the Ru single atoms with different coordination environment in Ni3FeN lattice are synthesized and studied as a typical case over alkaline methanol electrooxidation. The Ni3FeN with buried Ru atoms in subsurface lattice (Ni3FeN-Ruburied) exhibits high selectivity and Faradaic efficiency of methanol to formate conversion. Meanwhile, operando spectroscopies reveal that the Ni3FeN-Ruburied exhibits an optimized adsorption of reactants along with an inhibited surface structural reconstruction. Additional theoretical simulations demonstrate that the Ni3FeN-Ruburied displays a regulated local electronic states of surface metal atoms with an optimized adsorption of reactants and reduced energy barrier of potential determining step. This work not only reports a high-efficient catalyst for methanol to formate conversion in alkaline condition, but also offers the insight into the rational design of single atom catalysts with more accessible surficial active sites.

Metal-Independent Correlations for Site-Specific Binding Energies of Relevant Catalytic Intermediates

Establishing energy correlations among different metals can accelerate the discovery of efficient and cost-effective catalysts for complex reactions. Using a recently introduced coordination-based model, we can predict site-specific metal binding energies (ΔE M) that can be used as a descriptor for chemical reactions. In this study, we have examined a range of metals including Ag, Au, Co, Cu, Ir, Ni, Os, Pd, Pt, Rh, and Ru and found linear correlations between predicted ΔE M and adsorption energies of CH and O (ΔE CH and ΔE O) at various coordination environments for all the considered metals. Interestingly, all the metals correlate with one another under specific surface site coordination, indicating that different metals are interrelated in a particular coordination environment. Furthermore, we have tested and verified for PtPd- and PtIr-based alloys that they follow a similar behavior. Moreover, we have expanded the metal space by taking some early transition metals along with a few s-block metals and shown a cyclic behavior of the adsorbate binding energy (ΔE A) versus ΔE M. Therefore, ΔE CH and ΔE O can be efficiently interpolated between metals, alloys, and intermetallics based on information related to one metal only. This simplifies the process of screening new metal catalyst formulations and their reaction energies.

Electrified Dynamically Responsive Ammonia Decomposition to Hydrogen Based on Magnetic Heating of a Ru Nanocatalyst

Storing and transporting pressurized or liquid hydrogen is expensive and hazardous. As a result, safer methods, such as chemical storage in ammonia, are becoming increasingly important. However, the instantaneous start of a conventionally heated decomposition reactor is challenging. Here we report on the electrified and dynamically responsive decomposition of ammonia as a means of releasing on-demand chemically bonded hydrogen based on the rapid magnetic heating of a well-designed Ru-based nanocomposite catalyst. Under relatively mild conditions (400 °C, 1 bar) a rapid decomposition rate of 5.33 molNH3 gRu -1 h-1 was achieved. Experimental observations under non-isothermal, dynamic conditions coupled with modelling at the level of density functional theory and micro-kinetic modeling confirmed the minute-scale response of the H2 release. The rapid response of our catalytic system would, at least in principle, enable the utilization of intermittent, renewable electricity and a tunable H2/NH3 ratio in the reactor's effluent.

Formation and Segregation of the Ru- and Rh-MgO Solid Solutions

Structural changes in Ru- and Rh-loaded magnesium oxide (MgO) subjected to thermal treatment were investigated by using X-ray absorption spectroscopy. The thermal treatment of the MgO-loaded Ru and Rh nanoparticles led to the formation of Ru-MgO and Rh-MgO solid solutions, respectively. The valences of Ru and Rh in the solutions were 4+ and 3+, respectively, as determined by the Ru and Rh K-edge X-ray absorption near-edge structure (XANES). The degree of solid solution formation was the highest at 873 and 1073 K in Ru-MgO and Rh-MgO, respectively, as observed in the extended X-ray absorption fine structure and XANES analyses. The nearest neighboring Ru-O and Rh-O bond distances were shorter than the Mg-O bond in MgO. The dispersion of Ru and Rh on the MgO surface, measured by CO adsorption, increased for samples thermally heated at 1273 K, suggesting that the segregation of Ru or Rh from the solid solutions occurred at this temperature. Consequently, a relatively high dispersion was realized in the sample thermally heated to 1273 K. A good correlation was found between the dispersion value of Ru in Ru/MgO and the NH3 decomposition activity.

Utilizing full-spectrum sunlight for ammonia decomposition to hydrogen over GaN nanowires-supported Ru nanoparticles on silicon

Photo-thermal-coupling ammonia decomposition presents a promising strategy for utilizing the full-spectrum to address the H2 storage and transportation issues. Herein, we exhibit a photo-thermal-catalytic architecture by assembling gallium nitride nanowires-supported ruthenium nanoparticles on a silicon for extracting hydrogen from ammonia aqueous solution in a batch reactor with only sunlight input. The photoexcited charge carriers make a predomination contribution on H2 activity with the assistance of the photothermal effect. Upon concentrated light illumination, the architecture significantly reduces the activation energy barrier from 1.08 to 0.22 eV. As a result, a high turnover number of 3,400,750 is reported during 400 h of continuous light illumination, and the H2 activity per hour  is nearly 1000 times higher than that under the pure thermo-catalytic conditions. The reaction mechanism is extensively studied by coordinating experiments, spectroscopic characterizations, and density functional theory calculation. Outdoor tests validate the viability of such a multifunctional architecture for ammonia decomposition toward H2 under natural sunlight.

Active nitrogen sites on nitrogen doped carbon for highly efficient associative ammonia decomposition

Nitrogen doped carbon materials have been studied as catalyst support for ammonia decomposition. There are 4 different types of nitrogen environments (graphitic, pyrrolic, pyridinic and nitrogen oxide) on the amorphous support identified. In this paper, we report a 5%Ru on MgCO3 pre-treated nitrogen doped carbon catalyst with high content of edge nitrogen-containing sites which displays an ammonia conversion rate of over 90% at 500°C and WHSV = 30,000 mL gcat -1 h-1. It also gives an impressive hydrogen production rate of 31.3 mmol/(min gcat) with low apparent activation energy of 43 kJ mol-1. Fundamental studies indicate that the distinct average Ru-N4 coordination site on edge regions is responsible for such high catalytic activity. Ammonia is stepwise decomposed via a Ru-N(H)-N(H)-Ru intermediate. This associative mechanism circumvents the direct cleavage of energetic surface nitrogen from metal to form N2 hence lowering the activation barrier for the decomposition over this catalyst.

Hydrogen Production from Ammonia Decomposition: A Mini-Review of Metal Oxide-Based Catalysts

Efficient hydrogen storage and transportation are crucial for the sustainable development of human society. Ammonia, with a hydrogen storage density of up to 17.6 wt%, is considered an ideal energy carrier for large-scale hydrogen storage and has great potential for development and application in the "hydrogen economy". However, achieving ammonia decomposition to hydrogen under mild conditions is challenging, and therefore, the development of suitable catalysts is essential. Metal oxide-based catalysts are commonly used in the industry. This paper presents a comprehensive review of single and composite metal oxide catalysts for ammonia decomposition catalysis. The focus is on analyzing the conformational relationships and interactions between metal oxide carriers and active metal sites. The aim is to develop new and efficient metal oxide-based catalysts for large-scale green ammonia decomposition.

Inverse kinetic isotope effect of ammonia decomposition over Ru/CeO2 using deuterated ammonia

This study investigated the ammonia decomposition mechanism over Ru/CeO2. Isotopic tests using ND3 revealed that the rate-determining step involves adsorbed nitrogen atoms on Ru. Moreover, an inverse kinetic isotope effect where ND3 decomposition was faster than NH3 was clearly observed. The origin of the inverse effect was explained by the lower D coverage on the catalyst surface compared to H coverage for mitigating the inhibition of ND3 activation.

Liberating C-H Bond Activation: Achieving 56% Quantum Efficiency in Photocatalytic Cyclohexane Dehydrogenation

The technology of liquid organic hydrogen carriers presents great promise for large-scale hydrogen storage. Nevertheless, the activation of inert C(sp3)-H bonds in hydrocarbon carriers poses formidable challenges, resulting in a sluggish dehydrogenation process and necessitating high operating temperatures. Here, we break the shackles of C-H bond activation under visible light irradiation by fabricating subnanometer Pt clusters on defective Ce-Zr solid solutions. We achieved an unprecedented hydrogen production rate of 2601 mmol gcat.-1 h-1 (turnover frequency >50,000 molH2 molPt-1 h-1) from cyclohexane, surpassing the most advanced thermo- and photocatalysts. By optimizing the temperature-dominated hydrogen transfer process, achievable by harnessing hitherto wasted infrared light in sunlight, an astonishing 56% apparent quantum efficiency and a 5.2% solar-to-hydrogen efficiency are attained at 353 K. Our research stands as one of the most effective photocatalytic processes to date, holding profound practical significance in the utilization of solar energy and the exploitation of alkanes.

Schiff-base Polymer Immobilized Ruthenium for Efficient Catalytic Cross-coupling of Secondary Alcohols with 2-amino- and γ-aminobenzyl Alcohols to Give Quinolines and Pyridines

A Schiff-base porous polymer has been impregnated with ruthenium trichloride for acceptor-free dehydrogenation coupling (ADC) of secondary alcohols with γ-amino- and 2-aminobenzyl alcohols to give pyridines and quinolines. This heterogenous catalyst exhibited high catalytic efficiency over repeated cycles with wide functional group tolerance.

Biomimetic Design of a Dynamic M-O-V Pyramid Electron Bridge for Enhanced Nitrogen Electroreduction

Electrochemical nitrogen reduction reaction (eNRR) offers a sustainable route for ammonia synthesis; however, current electrocatalysts are limited in achieving optimal performance within narrow potential windows. Herein, inspired by the heliotropism of sunflowers, we present a biomimetic design of Ru-VOH electrocatalyst, featuring a dynamic Ru-O-V pyramid electron bridge for eNRR within a wide potential range. In situ spectroscopy and theoretical investigations unravel the fact that the electrons are donated from Ru to V at lower overpotentials and retrieved at higher overpotentials, maintaining a delicate balance between N2 activation and proton hydrogenation. Moreover, N2 adsorption and activation were found to be enhanced by the Ru-O-V moiety. The catalyst showcases an outstanding Faradaic efficiency of 51.48% at -0.2 V (vs RHE) with an NH3 yield rate exceeding 115 μg h-1 mg-1 across the range of -0.2 to -0.4 V (vs RHE), along with impressive durability of over 100 cycles. This dynamic M-O-V pyramid electron bridge is also applicable to other metals (M = Pt, Rh, and Pd).

Unveiling the Morphology of Carbon-Supported Ru Nanoparticles by Multiscale Modeling

Simulating the behavior of metal nanoparticles on supports is crucial for boosting their catalytic performance and various nanotechnology applications; however, such simulations are limited by the conflicts between accuracy and efficiency. Herein, we introduce a multiscale modeling strategy to unveil the morphology of Ru supported on pristine and N-doped graphene. Our multiscale modeling started with the electronic structures of a supported Ru single atom, revealing the strong metal-support interaction around pyridinic nitrogen sites. To determine the stable configurations of Ru2-13 clusters on three different graphene supports, global energy minimum searches were performed. The sintering of the global minimum Ru13 clusters on supports was further simulated by ab initio molecular dynamics (AIMD). The AIMD data set was then collected for deep potential molecular dynamics to study the melting of Ru nanoparticles. This study presents comprehensive descriptions of carbon-supported Ru and develops modeling approaches that bridge different scales and can be applied to various supported nanoparticle systems.