Structure and reactivity of Ru nanoparticles supported on modified graphite surfaces: a study of the model catalysts for ammonia synthesis

Cs-Ba-Ru/La2O3-MgO Catalysts from Microdrop-Confined Pyrolysis for Superior NH3 Synthesis under Mild Conditions

Support modification is an important way to improve the activity of the Ru/MgO-based NH3 synthesis catalyst. This work synthesized 24 composite MgO-based supports of MOx-MgO (M = La, Zn, etc.) via a microdrop-confined pyrolysis (MDCPy) method and then prepared the corresponding Cs-Ba-Ru/MOx-MgO catalysts using the impregnation method. Under the same NH3 synthesis conditions, the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of 17 elements such as Pr was significantly lower than that of Cs-Ba-Ru/MgO, while the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of four elements such as Nd was comparable to that of Cs-Ba-Ru/MgO, and the activity of the Cs-Ba-Ru/MOx-MgO catalysts composed of three elements such as La was significantly higher than that of Cs-Ba-Ru/MgO. Among them, it is clear that La2O3-MgO composite supports have the most significant effect on the improvement of NH3 synthesis performance. Through the optimization of the La/Mg ratio and loading of Ru, the optimal activities achieved ∼7 mol(NH3)/g(Ru)/h with corresponding Ru loading of 0.25-0.5 wt % under mild conditions as follows: H2/N2 ratio of 3:1, temperature of 400 °C, pressure of 3 MPa and a weight hourly space velocity (WHSV) of 96.0 L/gcat/h. In addition, the CsBa-Ru/La2O3-MgO catalyst showed good stability.

Ru3@Mo2CO2 MXene single-cluster catalyst for highly efficient N2-to-NH3 conversion

Single-cluster catalysts (SCCs) representing structurally well-defined metal clusters anchored on support tend to exhibit tunable catalytic performance for complex redox reactions in heterogeneous catalysis. Here we report a theoretical study on an SCC of Ru3@Mo2CO2 MXene for N2-to-NH3 thermal conversion. Our results show that Ru3@Mo2CO2 can effectively activate N2 and promotes its conversion to NH3 through an association mechanism, in which the rate-determining step of NH2* + H* → NH3* has a low energy barrier of 1.29 eV. Notably, with the assistance of Mo2CO2 support, the positively charged Ru3 cluster active site can effectively adsorb and activate N2, leading to 0.74 |e| charge transfer from Ru3@Mo2CO2 to the adsorbed N2. The supported Ru3 also acts as an electron reservoir to regulate the charge transfer for various intermediate steps of ammonia synthesis. Microkinetic analysis shows that the turnover frequency of the N2-to-NH3 conversion on Ru3@Mo2CO2 is as high as 1.45 × 10-2 s-1 site-1 at a selected thermodynamic condition of 48 bar and 700 K, the performance of which even surpasses that of the Ru B5 site and Fe3/θ-Al2O3(010) reported before. Our work provides a theoretical understanding of the high stability and catalytic mechanism of Ru3@Mo2CO2 and guidance for further designing and fabricating MXene-based metal SCCs for ammonia synthesis under mild conditions.

Enhancement of the ammonia synthesis activity of a Cs- or Ba-promoted ruthenium catalyst supported on barium niobate

Barium niobates with different crystalline structures and morphologies were prepared via a hydrothermal method and applied as a support for a ruthenium catalyst in ammonia synthesis. The sample synthesized with a nominal Ba/Nb ratio = 2.0, having a pure Ba5Nb4O15 crystalline phase and uniform flake-like structure, exhibited the best performance as a support in ammonia synthesis. The flake-like substrate favored the uniform distribution of ruthenium on its surface, which could promote ruthenium to expose more B5 sites. Addition of a Ba- or Cs-promoter enhanced the activity of the Ru/Ba5Nb4O15 catalyst markedly. The highest rate of ammonia synthesis over 2Cs- and 1Ba-4 wt% Ru/Ba5Nb4O15 was 4900 and 3720 (μmol g-1 cat h-1) at 0.1 MPa and 623 K, respectively. Both catalysts were stable during the reaction for 72 h at 673 K and 0.1 MPa. Thus, the synthesized Ba5Nb4O15 is expected to be a promising oxide support for ruthenium catalysts for ammonia synthesis.

Regulating oxygen vacancies by Zn atom doping to anchor and disperse promoter Ba on MgO support to improve Ru-based catalysts activity for ammonia synthesis

In heterogeneous catalysis, surface defects are widely regarded as an effective means to enhance the catalytic performance of catalysts. In this study, the oxygen vacancy-rich Mg(1-X)ZnXO solid solution support was successfully prepared by doping a small amount of Zn into MgO nanocrystals. Based on this support, Ru/Ba-Mg(1-X)ZnXO catalyst for ammonia synthesis was prepared. Characterization using TEM, EPR, XPS, and DFT calculations confirmed the successful substitution of Zn atoms for Mg atoms leading to the formation of more oxygen vacancies (OVs). N2-TPD, SEM and TEM analyses revealed that a small amount of Zn had minimal influence on the surface morphology and the size of Ru nanoparticles. The abundance of OVs in the support was identified as the primary factor enhancing the catalytic activity. XPS, H2-TPD and kinetics experiment studies further elucidated the mechanism by which OVs promote the reaction, with OVs serving as an anchor point for the promoter Ba on the MgO support and promoted the dispersion of Ba. This anchoring effect not only enhanced the electron density on Ru, favoring the dissociation of the N[triple bond, length as m-dash]N bond, but also mitigated hydrogen poisoning. As a result,the ammonia synthesis rate reached 1.73 mmol g-1 h-1. Furthermore, the CO2-TPD and H2-TPR analyses indicated that Zn doping effectively promotes the metal-support interaction (MSI) and surface alkalinity. The findings of this study offers valuable insights for the design of defective modified catalyst supports.

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.

Plasma-Promoted Ammonia Decomposition over Supported Ruthenium Catalysts for COx -Free H2 Production

The efficient decomposition of ammonia to produce COx -free hydrogen at low temperatures has been extensively investigated as a potential method for supplying hydrogen to mobile devices based on fuel cells. In this study, we employed dielectric barrier discharge (DBD) plasma, a non-thermal plasma, to enhance the catalytic ammonia decomposition over supported Ru catalysts (Ru/Y2 O3 , Ru/La2 O3 , Ru/CeO2 and Ru/SiO2 ). The plasma-catalytic reactivity of Ru/La2 O3 was found to be superior to that of the other three catalysts. It was observed that both the physicochemical properties of the catalyst (such as support acidity) and the plasma discharge behaviours exerted significant influence on plasma-catalytic reactivity. Combining plasma with a Ru catalyst significantly enhanced ammonia conversion at low temperatures, achieving near complete NH3 conversion over the 1.5 %-Ru/La2 O3 catalyst at temperatures as low as 380 °C. Under a weight gas hourly space velocity of 2400 mL gcat -1  h-1 and an AC supply power of 20 W, the H2 formation rate and energy efficiency achieved were 10.7 mol gRu -1  h-1 and 535 mol gRu -1 (kWh)-1 , respectively, using a 1.5 %-Ru/La2 O3 catalyst.

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

Ammonia is regarded as an energy vector for hydrogen storage, transport and utilization, which links to usage of renewable energies. However, efficient catalysts for ammonia decomposition and their underlying mechanism yet remain obscure. Here we report that atomically-dispersed Ru atoms on MgO support on its polar (111) facets {denoted as MgO(111)} show the highest rate of ammonia decomposition, as far as we are aware, than all catalysts reported in literature due to the strong metal-support interaction and efficient surface coupling reaction. We have carefully investigated the loading effect of Ru from atomic form to cluster/nanoparticle on MgO(111). Progressive increase of surface Ru concentration, correlated with increase in specific activity per metal site, clearly indicates synergistic metal sites in close proximity, akin to those bimetallic N2 complexes in solution are required for the stepwise dehydrogenation of ammonia to N2/H2, as also supported by DFT modelling. Whereas, beyond surface doping, the specific activity drops substantially upon the formation of Ru cluster/nanoparticle, which challenges the classical view of allegorically higher activity of coordinated Ru atoms in cluster form (B5 sites) than isolated sites.

Catalysis of dinitrogen activation and reduction by a single Fe13 cluster and its doped systems

Catalyzing N₂ reduction to ammonia under ambient conditions is known to be significant both in the fertilizer industry and life sciences. To unveil the synergy of multiple sites, here, we have studied the catalysis of ammonia synthesis using a typical Fe₁₃ cluster and its doped systems, Fe₁₂X (X = V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, Ru, and Rh). The energetics analysis showed that center substitution (X@Fe₁₂) was favored while doping single V, Cr, Co, and Mo atoms, whereas Mn, Ni, Cu, Zn, Nb, Ru, and Rh tended to form shell-doped structures (Fe₁₂X). Among all the 13 clusters, Fe₁₂Nb exhibited the lowest activation energy for N₂ dissociation; moreover, in the hydrogenation process, Fe₁₂Nb could convert N₂ to ammonia efficiently. We have fully illustrated the reaction dynamics and structural chemistry essence of these diverse 13-atom systems and propose Fe₁₂Nb as an ideal candidate for catalytic ammonia synthesis.

Heteroepitaxial Growth of B5 -Site-Rich Ru Nanoparticles Guided by Hexagonal Boron Nitride for Low-Temperature Ammonia Dehydrogenation

Ruthenium is one of the most active catalysts for ammonia dehydrogenation and is essential for the use of ammonia as a hydrogen storage material. The B₅ -type site on the surface of ruthenium is expected to exhibit the highest catalytic activity for ammonia dehydrogenation, but the number of these sites is typically low. Here, a B₅ -site-rich ruthenium catalyst is synthesized by exploiting the crystal symmetry of a hexagonal boron nitride support. In the prepared ruthenium catalyst, ruthenium nanoparticles are formed epitaxially on hexagonal boron nitride sheets with hexagonal planar morphologies, in which the B₅ sites predominate along the nanoparticle edges. By activating the catalyst under the reaction condition, the population of B₅ sites further increases as the facets of the ruthenium nanoparticles develop. The electron density of the Ru nanoparticles also increases during catalyst activation. The synthesized catalyst shows superior catalytic activity for ammonia dehydrogenation compared to previously reported catalysts. This work demonstrates that morphology control of a catalyst via support-driven heteroepitaxy can be exploited for synthesizing highly active heterogeneous catalysts with tailored atomic structures.

Quantitative Characterization of the Thermally Driven Alloying State in Ternary Ir-Pd-Ru Nanoparticles

Compositional and structural arrangements of constituent elements, especially those at the surface and near-surface layers, are known to greatly influence the catalytic performance of alloyed nanoparticles (NPs). Although much research effort often focuses on the ability to tailor these important aspects in the design stage, their stability under realistic operating conditions remains a major technical challenge. Here, the compositional stability and associated structural evolution of a ternary iridium-palladium-ruthenium (Ir-Pd-Ru) nanoalloy at elevated temperatures have been studied using interrupted in situ scanning transmission electron microscopy and theoretical modeling. The results are based on a combinatory approach of statistical sampling at the sub-nanometer scale for large groups of NPs as well as tracking individual NPs. We find that the solid solution Ir-Pd-Ru NPs (∼5.6 nm) evolved into a Pd-enriched shell supported on an alloyed Ir-Ru-rich core, most notably when the temperature exceeds 500 °C, concurrently with the development of expansive atomic strain in the outer surface and subsurface layers with respect to the core regions. Theoretically, we identify the weak interatomic bonds, low surface energy, and large atomic sizes associated with Pd as the key factors responsible for such observed features.

Regulating the interaction of Ru nanoparticles and an Eu2O3 support achieves enhanced activity for ammonia synthesis

A Ru/Eu2O3 catalyst with enhanced metal–support interaction is successfully prepared for highly efficient ammonia synthesis from nitrogen and hydrogen.

Ruthenium nanoparticles canopied by heptagon-containing saddle-shaped nanographenes as efficient aromatic hydrogenation catalysts

Ruthenium nanoparticles stabilized with non-planar polycyclic aromatic hydrocarbons (PAHs) are active catalysts in the hydrogenation of aromatic substrates under mild conditions.

Defect-assisted electronic metal-support interactions: tuning the interplay between Ru nanoparticles and CuO supports for pH-neutral oxygen evolution

Electronic metal-support interactions (EMSIs) comprise an area of intense study, the manipulation of which is of paramount importance in the improvement of heterogeneous metal nanoparticle (NP) supported catalysts. EMSI is the transfer of charge from the support to NP, enabling more effective adsorption and interaction of reactants during catalysis. Ru NPs on CuO supports show different levels of EMSI (via charge transfer) depending on their crystal structure, with multiple twinned NPs showing greater potential for EMSI. We use magnetron-assisted gas phase aggregation for the synthesis of batches of Ru NPs with different populations of single crystal and multiple twinned nanoparticles, which were deposited on CuO nanowires (NWs). The surface charging of the Ru-CuO catalysts was investigated by Kelvin probe force microscopy (KPFM) and X-ray photoelectron spectroscopy (XPS). By doubling the population of multiple twinned NPs, the surface potential of the Ru-CuO catalysts increases roughly 4 times, coinciding with a similar increase in the amount of Ru⁴⁺. Therefore, tuning the amount of EMSI in a catalyst is possible through changing the population of multiple twinned Ru NPs in the catalyst. Increasing the amount of multiple twin NPs resulted in improved activity in the oxygen evolution reaction (a roughly 2.5 times decrease in the overpotentials when the population of multiple twinned NPs is increased) and better catalyst stability. This improvement is attributed to the fact that the multiple twin NPs maintained a metallic character under oxidation conditions (unlike single crystal NPs) due to the EMSI between the NP and support.

Applications of rare earth oxides in catalytic ammonia synthesis and decomposition

Due to their unique structural and electronic properties, rare earth oxides have been widely applied as supports and promoters in catalytic ammonia synthesis and decomposition.

Surface reactions of ammonia on ruthenium nanoparticles revealed by 15N and 13C solid-state NMR

Ruthenium nanoparticles (Ru NPs) stabilized by bis-diphenylphosphinobutane (dppb) and surface-saturated with hydrogen have been exposed to gaseous ¹⁵NH₃ and ¹³CO and studied using solid-state NMR and DFT calculations.

Metal Clusters on Semiconductor Surfaces and Application in Catalysis with a Focus on Au and Ru

Metal clusters typically consist of two to a few hundred atoms and have unique properties that change with the type and number of atoms that form the cluster. Metal clusters can be generated with a precise number of atoms, and therefore have specific size, shape, and electronic structures. When metal clusters are deposited onto a substrate, their shape and electronic structure depend on the interaction with the substrate surface and thus depend on the properties of both the clusters and those of the substrate. Deposited metal clusters have discrete, individual electron energy levels that differ from the electron energy levels in the constituting individual atoms, isolated clusters, and the respective bulk material. The properties of clusters with a focus on Au and Ru, the methods to generate metal clusters, and the methods of deposition of clusters onto substrate surfaces are covered. The properties of cluster-modified surfaces are important for their application. The main application covered here is catalysis, and the methods for characterization of the cluster-modified surfaces are described.

Transition Metal-Free Alkyne Hydrogenation Catalysis with BaGa2, a Hydrogen Absorbing Layered Zintl Phase

Inexpensive, transition metal-free intermetallic compounds have received almost no attention as heterogeneous catalysts. Here, we show that BaGa₂, a Zintl-Klemm compound composed of honeycomb sheets of Ga^- anions separated by Ba²⁺ cations and known to react with H₂ under moderate conditions to form a layered polyanionic hydride BaGa₂H₂, effectively catalyzes the hydrogenation of phenylacetylene into styrene and ethylbenzene under modest conditions (1-50 bar H₂, 40-100 °C). Remarkably, the catalytic activity of BaGa₂ (surface specific activities up to 8390 h^-1) is on the same order of magnitude as commercial Pd-based catalysts. In contrast, BaGa₂H₂ shows negligible catalytic activity, thereby indicating that the unsaturated Ga^- framework is necessary for phenylacetylene and styrene adsorption. These findings open up future explorations of utilizing and optimizing the long-term stability of transition metal-free intermetallic hydrogen absorbing compounds for hydrogen-based catalysis.

Acid-durable electride with layered ruthenium for ammonia synthesis: boosting the activity via selective etching

We reported LnRuSi as a B₅-site-free Ru catalyst for ammonia synthesis, and its activity enhanced 2–4-fold by selective etching with EDTA-2Na.

Ruthenium (Ru) loaded catalysts are of significant interest for ammonia synthesis under mild reaction conditions. The B₅ sites have been reported as the active sites for ammonia formation, i.e., Ru with other coordinations were inactive, which has limited the utilization efficiency of Ru metal. The implantation of Ru into intermetallic compounds is considered to be a promising approach to tune the catalytic activity and utilization efficiency of Ru. Here we report an acid-durable electride, LnRuSi (Ln = La, Ce, Pr and Nd), as a B₅-site-free Ru catalyst. The active Ru plane with a negative charge is selectively exposed by chemical etching using disodium dihydrogen ethylenediaminetetraacetate (EDTA-2Na) acid, which leads to 2–4-fold enhancement in the ammonia formation rate compared with that of the original catalyst. The turnover frequency (TOF) of LnRuSi is estimated to be approximately 0.06 s^–1, which is 600 times higher than that of pure Ru powder. Density functional theory (DFT) calculations revealed that the dissociation of N₂ occurs easily on the exposed Ru plane of LaRuSi. This systematic study provides firm evidence that layered Ru with a negative charge in LnRuSi is a new type of active site that differs significantly from B₅ sites.

Bimetallic Ru-Co Clusters Derived from a Confined Alloying Process within Zeolite-Imidazolate Frameworks for Efficient NH3 Decomposition and Synthesis

Herein, a series of carbocatalysts containing Ru-based clusters have been prepared by the assistance of zeolite-imidazolate frameworks (ZIFs). The introduction of Ru is based on the adsorption of well-defined Ru₃(CO)₁₂ within the cavity of ZIFs following decomposition at 900 °C. Moreover, without breaking the skeleton and porosity of ZIFs, the as-generated Ru species would bond with the Co nodes in situ to form bimetallic Ru-Co clusters if the Co-bearing metal-organic frameworks were utilized as the host. Within the confined space of ZIFs, the assembly of Ru and Co could be rationally designed, and their structures could be sophisticatedly controlled at the atomic scale. Among these Ru-based compositions, the Ru-Co clusters@N-C exhibited remarkable catalytic activity for the NH₃ decomposition to H₂ and NH₃ synthesis versus Ru-Co NPs@N-C, Ru clusters@N-C, and Ru NPs@N-C. This study may open up a new routine to synthesize metallic clusters or other subnano structures by the confinement of ZIFs.

New insights into the support morphology-dependent ammonia synthesis activity of Ru/CeO2catalysts

Different morphologies ceria (nanocubes, nanorods and nanoparticles) were synthesized and exhibited significant support-morphology-dependent catalytic activity towards ammonia synthesis.

A low-crystalline ruthenium nano-layer supported on praseodymium oxide as an active catalyst for ammonia synthesis

Low-crystalline Ru nano-layers and the strong basicity of Ru/Pr₂O₃ synergistically accelerated the rate-determining step of ammonia synthesis.

Ammonia is a crucial chemical feedstock for fertilizer production and is a potential energy carrier. However, the current method of synthesizing ammonia, the Haber–Bosch process, consumes a great deal of energy. To reduce energy consumption, a process and a substance that can catalyze ammonia synthesis under mild conditions (low temperature and low pressure) are strongly needed. Here we show that Ru/Pr₂O₃ without any dopant catalyzes ammonia synthesis under mild conditions at 1.8 times the rates reported with other highly active catalysts. Scanning transmission electron micrograph observations and energy dispersive X-ray analyses revealed the formation of low-crystalline nano-layers of ruthenium on the surface of Pr₂O₃. Furthermore, CO₂ temperature-programmed desorption revealed that the catalyst was strongly basic. These unique structural and electronic characteristics are considered to synergistically accelerate the rate-determining step of NH₃ synthesis, cleavage of the N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N bond. We expect that the use of this catalyst will be a starting point for achieving efficient ammonia synthesis.

Recent progress in the structure control of Pd-Ru bimetallic nanomaterials

Pd and Ru are two key elements of the platinum-group metals that are invaluable to areas such as catalysis and energy storage/transfer. To maximize the potential of the Pd and Ru elements, significant effort has been devoted to synthesizing Pd-Ru bimetallic materials. However, most of the reports dealing with this subject describe phase-separated structures such as near-surface alloys and physical mixtures of monometallic nanoparticles (NPs). Pd-Ru alloys with homogenous structure and arbitrary metallic ratio are highly desired for basic scientific research and commercial material design. In the past several years, with the development of nanoscience, Pd-Ru bimetallic alloys with different architectures including heterostructure, core-shell structure and solid-solution alloy were successfully synthesized. In particular, we have now reached the stage of being able to obtain Pd-Ru solid-solution alloy NPs over the whole composition range. These Pd-Ru bimetallic alloys are better catalysts than their parent metal NPs in many catalytic reactions, because the electronic structures of Pd and Ru are modified by alloying. In this review, we describe the recent development in the structure control of Pd-Ru bimetallic nanomaterials. Aiming for a better understanding of the synthesis strategies, some fundamental details including fabrication methods and formation mechanisms are discussed. We stress that the modification of electronic structure, originating from different nanoscale geometry and chemical composition, profoundly affects material properties. Finally, we discuss open issues in this field.

Platinum-ruthenium bimetallic clusters on graphite: a comparison of vapor deposition and electroless deposition methods

Bimetallic Pt-Ru clusters have been grown on highly ordered pyrolytic graphite (HOPG) surfaces by vapor deposition and by electroless deposition. These studies help to bridge the material gap between well-characterized vapor deposited clusters and electrolessly deposited clusters, which are better suited for industrial catalyst preparation. In the vapor deposition experiments, bimetallic clusters were formed by the sequential deposition of Pt on Ru or Ru on Pt. Seed clusters of the first metal were grown on HOPG surfaces that were sputtered with Ar(+) to introduce defects, which act as nucleation sites for Pt or Ru. On the unmodified HOPG surface, both Pt and Ru clusters preferentially nucleated at the step edges, whereas on the sputtered surface, clusters with relatively uniform sizes and spatial distributions were formed. Low energy ion scattering experiments showed that the surface compositions of the bimetallic clusters are Pt-rich, regardless of the order of deposition, indicating that the interdiffusion of metals within the clusters is facile at room temperature. Bimetallic clusters on sputtered HOPG were prepared by the electroless deposition of Pt on Ru seed clusters from a Pt(+2) solution using dimethylamine borane as the reducing agent at pH 11 and 40 °C. After exposure to the electroless deposition bath, Pt was selectively deposited on Ru, as demonstrated by the detection of Pt on the surface by XPS, and the increase in the average cluster height without an increase in the number of clusters, indicating that Pt atoms are incorporated into the Ru seed clusters. Electroless deposition of Ru on Pt seed clusters was also achieved, but it should be noted that this deposition method is extremely sensitive to the presence of other metal ions in solution that have a higher reduction potential than the metal ion targeted for deposition.

Enabling electrochemical reduction of nitrogen to ammonia at ambient conditions through rational catalyst design

Investigation of transition metal nitrides reveals extremely promising electrocatalysts for high-yield ammonia production in aqueous electrolytes under ambient conditions.