Metal Ensembles Research Articles

Controlling noble metal ensembles on pure-silica zeolites towards selective catalysis

Controlling noble metal ensembles on pure-silica zeolites towards selective catalysis

Low-nuclearity CuZn ensembles on ZnZrOx catalyze methanol synthesis from CO2

Metal promotion could unlock high performance in zinc-zirconium catalysts, ZnZrOx, for CO2 hydrogenation to methanol. Still, with most efforts devoted to costly palladium, the optimal metal choice and necessary atomic-level architecture remain unclear. Herein, we investigate the promotion of ZnZrOx catalysts with small amounts (0.5 mol%) of diverse hydrogenation metals (Re, Co, Au, Ni, Rh, Ag, Ir, Ru, Pt, Pd, and Cu) prepared via a standardized flame spray pyrolysis approach. Cu emerges as the most effective promoter, doubling methanol productivity. Operando X-ray absorption, infrared, and electron paramagnetic resonance spectroscopic analyses and density functional theory simulations reveal that Cu0 species form Zn-rich low-nuclearity CuZn clusters on the ZrO2 surface during reaction, which correlates with the generation of oxygen vacancies in their vicinity. Mechanistic studies demonstrate that this catalytic ensemble promotes the rapid hydrogenation of intermediate formate into methanol while effectively suppressing CO production, showcasing the potential of low-nuclearity metal ensembles in CO2-based methanol synthesis.

Electronic and structural engineering of supported single atomic layer, low-nuclearity palladium catalysts for conversion of levulinic acid to 1,4-pentanediol

The size and geometry of supported metal ensembles have a substantial influence on their catalytic efficacy and are pivotal in the design of effective heterogeneous catalysts. Here we developed a straightforward electronic and structural engineering strategy to create supported single atomic-layered, low-nuclearity palladium catalysts. This atomically dispersed Pd catalyst possesses unique synergistic geometric and electronic effects and nearly full metal availability to the reactant, exhibiting excellent catalytic activity (turnover frequency of 12480 h−1) and yield (>99%) in the hydrogenation of levulinic acid to 1,4-pentanediol under mild conditions, a reaction of importance for conversion of biomass to renewable chemicals. Theoretical calculations reveal that the high catalytic activity results from the cooperation of adjacent Pd atoms, high d-band center, and strong electronic metal-support interactions, thus guaranteeing efficient activation of reactant and facilitating the subsequent ring-opening step. The present work sheds light on the elegant design of low-nuclearity metal cluster catalysts with structure sensitivity to maximize the catalytic efficiency at the atomic scale.

Enhancing the activity of Pd ensembles on graphene by manipulating coordination environment

Atomic dispersion of metal catalysts on a substrate accounts for the increased atomic efficiency of single-atom catalysts (SACs) in various catalytic schemes compared to the nanoparticle counterparts. However, lacking neighboring metal sites has been shown to deteriorate the catalytic performance of SACs in a few industrially important reactions, such as dehalogenation, CO oxidation, and hydrogenation. Metal ensemble catalysts (Mn), an extended concept to SACs, have emerged as a promising alternative to overcome such limitation. Inspired by the fact that the performance of fully isolated SACs can be enhanced by tailoring their coordination environment (CE), we here evaluate whether the CE of Mn can also be manipulated in order to enhance their catalytic activity. We synthesized a set of Pd ensembles (Pdn) on doped graphene supports (Pdn/X-graphene where X = O, S, B, and N). We found that introducing S and N onto oxidized graphene modifies the firstshell of Pdn converting Pd-O to Pd-S and Pd-N, respectively. We further found that the B dopant significantly affected the electronic structure of Pdn by serving as an electron donor in the secondshell. We examined the performance of Pdn/X-graphene toward selective reductive catalysis, such as bromate reduction, brominated organic hydrogenation, and aqueous-phase CO2 reduction. We observed that Pdn/N-graphene exhibited superior performance by lowering the activation energy of the rate-limiting step,i.e., H2 dissociation into atomic hydrogen. The results collectively suggest controlling the CE of SACs in an ensemble configuration is a viable strategy to optimize and enhance their catalytic performance.

Atomically precise bimetallic metal ensembles with tailorable synergistic effects

The large-scale synthesis of supported multinuclear catalysts with controllable metal nuclearity and constituent composition remains a formidable challenge. We report the stepwise assembly of supported atom-precise bimetallic ligand-mediated metal ensembles (LMMEs) by exploiting the underlying principles of coordination chemistry and solid-state chemistry. Lewis di-basic 2-methylimidazole is used to bridge multiple Cu2+ and M2+ (M = Co, Ni, Cu, and Zn) ions within ZSM-5 zeolites. We observe the metal constituent composition of the LMMEs by mass spectroscopy. The adjacent metal nuclei in the LMMEs offer substantial synergistic effects that enhance the catalytic performance by at least an order of magnitude in the model catalytic “click” reaction. It is envisaged that this stepwise assembly approach to develop supported multinuclear catalysts with atom precision could effectively bridge homogeneous and heterogeneous catalysis.

Interface-directed epitaxially growing nickle ensembles as efficient catalysts in dry reforming of methane

Supported nickel catalysts are promising candidates for dry reforming of methane, but agglomeration of Ni0 and coke deposition hinder the industrial applications. Herein, we report a novel interface-directed synthetic approach to construct distinct metal ensembles by carefully tuning the compositions of the carriers. A Zr-Mn-Zn ternary oxide-supported Ni catalyst, together with the respective binary oxide-supported analogues, was synthesized by adopting a sequential co-precipitation and wetness impregnation method. Combined characterization techniques identify distinct catalyst models, including (i) conventional NiO nanoparticles with different sizes on Zr-Mn and Zr-Zn, and (ii) epitaxially growing NiO ensembles of a few nanometers thickness at the periphery of MnZnOx particles. These catalysts exhibit divergent responses in the catalytic testing, with the ternary oxide system significantly outperforming the binary analogues. The strong electronic interactions between Mn-Ni increase Ni dispersion and the activity while the stability is strengthened upon Zn addition. Both high activity, high selectivity, and remarkable stability are attained upon co-adding Mn and Zn. The interfaces between Ni and Zr-Mn-Zn rather than the physical contacts of individual oxide-supported analogues through mechanical mixing are keys for the outstanding performance.

Collapse-Induced Multimer Formation of Self-Assembled Nanoparticles for Surface Enhanced Raman Scattering

Metallic nanoparticle ensemble, with narrow inter-particle distance, is a useful element for diverse optical devices due to highly enhanced electric field intensity at the gap. Self-assembly of block copolymer (BCP) can provide the versatile solution to fabricate precise nanostructures, but this methodology has the intrinsic limitation to realize optically coupled metallic multimer geometry with narrow inter-particle distance. This is because BCP-based nanotemplate possesses a minimum size limit for interparticle distance imposed by its thermodynamic restriction. Herein, we investigate the facile formation of metallic multimer with scalability and area-selectivity through the collapse of self-assembled BCP nanopattern. The capillary-force-induced collapse phenomenon enables a spatial transformation of lateral regular ordering in metallic nanoparticle array and enhances electric field intensity. The fabrication of this metallic nanoparticle ensemble from BCP lithography is successfully utilized for surface enhanced Raman scattering (SERS). The enhancement factor of metal nanoparticle multimer is calculated as ~6.74 × 105 at 1000 cm−1, 2.04 × 106 at 1022 cm−1, and 6.11 × 106 at 1580 cm−1, respectively.

Investigating the Effect of Several Model Configurations on the Transient Response of Gas-Insulated Substation during Fault Events Using an Electromagnetic Field Theory Approach

Assessment of very fast transient overvoltage (VFTO) requires good knowledge of the behavior of gas-insulated substation when subjected to very high frequencies. The international standards and guidelines generically present only recommendations regarding the VFTO suppression without a technical and mathematical background. Therefore, to provide an accurate image regarding the critical locations across a gas-insulated substation (GIS) from a transient response point of view, a suitable modeling technique has to be identified and developed for the substation. The paper aimed to provide an accurate assessment of the GIS holistic transient response through an electromagnetic field theory (EMF) approach. This modeling technique has always been a difficult task when it came to gas-insulated substations. However, recent studies have shown that through suitable Computer-aided design models, representing the GIS metallic ensemble, accurate results can be obtained. The paper investigated several simplifications of the computational domain considering different gas-insulated substation configurations in order to identify a suitable modeling approach without any unnecessary computational effort. The analysis was performed by adopting the partial equivalent element circuit (PEEC) approach embedded into XGSLab software package. Obtained results could provide useful hints for grounding grid designers regarding the proper development and implementation of transient ground potential rise (TGPR) mitigation techniques across a gas-insulated substation.

Interfacial Active Sites for CO2 Assisted Selective Cleavage of C–C/C–H Bonds in Ethane

Summary Selective upgrading of underutilized ethane in shale gas with greenhouse gas CO2 can produce syngas via dry reforming (C–C bond scission) or ethylene via oxidative dehydrogenation (C–H bond scission). However, it remains challenging to identify active sites responsible for the selective bond cleavage in ethane due to the complexity of supported catalysts. Herein, the ethane-CO2 reaction over CeO2-supported catalysts was investigated to unravel the functions of distinct interfacial sites by combining kinetic measurements with in situ characterizations and calculations: the Pd/CeOx interface is responsible for supplying reactive oxygen species, electron-deficient oxygen species on Pd surface boosts the non-selective bond scission to produce syngas, electron-enriched oxygen in the FeOx/Pd interface enhances the selective scission of C–H bond to yield ethylene, and the FeOx/CeO2 interaction mediates oxygen supply and confines metal ensembles. The current work identifies opportunities for using different interfacial structures in upgrading abundant shale gas and CO2.

Highly durable metal ensemble catalysts with full dispersion for automotive applications beyond single-atom catalysts

Reducing the size of metal nanoparticles down to the single-atom level has been actively pursued to maximize the use of precious metals. Recently, single-atom catalysts, in which all the metal atoms are isolated on a support with 100% dispersion, have received much attention. However, the lack of ensemble sites prevents valuable surface reactions that require metal proximity to occur. Here, we present metal (Pt, Pd and Rh) ensemble catalysts with 100% dispersion and a reduced metallic surface state. More specifically, nanoceria particles were anchored on Al3+penta sites of activated γ-alumina, and then metal was deposited and reduced. The ensemble catalysts are highly durable: their structure was maintained even after hydrothermal ageing at 900 °C for 24 h or after long-term reaction. These catalysts have superior activity and durability for three-way catalytic reactions and can provide insights beyond single-atom catalysts for heterogeneous catalysis. Supported single atoms can minimize metal utilization in catalysis, although reactivity restrictions exist. Here, fully exposed Pt, Pd and Rh ensembles localized on CeO2 islands anchored onto partially reduced γ-Al2O3 are introduced as a superior and durable alternative for three-way catalysis.

On the mechanism of H2 activation over single-atom catalyst: An understanding of Pt1/WOx in the hydrogenolysis reaction

Owing to the atomic dispersion of active sites via electronic interaction with supports, single-atom catalysts (SACs) grant maximum utilization of metals with unique activity and/or selectivity in various catalytic processes. However, the stability of single atoms under oxygen-poor conditions, and the mechanism of hydrogen activation on SACs remain elusive. Here, through a combination of theoretical calculation and experiments, the stabilization of metal single atoms on tungsten oxide and its catalytic properties in H2 activation are investigated. Our calculation results indicate that the oxygen defects on the WO3(001) surface play a vital role in the stabilization of single metal atoms through electron transfer from the oxygen vacancies to the metal atoms. In comparison with Pd and Au, Pt single atoms possess greatly enhanced stability on the WOx(001) surface and carry negative charge, facilitating the dissociation of H2 to metal−H species (Hδ−) via homolytic cleavage of H2 similar to that occurring in metal ensembles. More importantly, the facile diffusion of Pt−H to the WOx support results in the formation of Brønsted acid sites (Hδ+), imparting bifunctionality to Pt1/WOx. The dynamic formation of Brønsted acid sites in hydrogen atmosphere proved to be the key to chemoselective hydrogenolysis of glycerol into 1,3-propanediol, which was experimentally demonstrated on the Pt1/WOx catalyst.

Pyrene-imidazole conjugate as a fluorescent sensor for the sequential detection of iron(III) and histidine in aqueous solution.

We developed PIM, a pyrene-based fluorescence sensor bearing an imidazole moiety and a carbonyl group as the binding sites for Fe3+ ions. The pyrene-based control compounds 1 and 2 were synthesized to demonstrate the structure-activity relationships. Compound 1, which contained a thiazoline moiety and a carbonyl group, displayed high selectivity for Cu2+ ions. This property indicated that heterocycles play an important role in the metal ion selectivity modulation. Compound 2, which lacked a carbonyl group, did not display metal ion selectivity. This characteristic demonstrated that introducing an additional recognition unit (cooperative recognition strategy) should be an effective way to improve metal ion selectivity. Furthermore, the PIM-Fe3+ ensemble can serve as a fluorescent sensor for histidine (His) detection via the removal of Fe3+ from the ensemble by His and the release of PIM. The sequential detection of Fe3+ and His exhibited on-off-on phenomenon, and the Fe3+ and His detection limits were 0.11 and 3.06 μM, respectively. These results will help in the further enhancement or modulation of metal ion selectivity in the development of fluorescent sensor systems. Moreover, the organic-metal ensemble provides an effective platform for detecting amino acids through the displacement strategy.

AIEE Active Supramolecular Metallic Ensemble as “Turn-On” Platform for Detection of Creatinine in Human Blood Serum at Picomolar Level

Supramolecular metallic ensemble (3-Fe2+) has been developed, which exhibited highly sensitive “on–on” response toward creatinine in human blood serum with a detection limit in picomolar range. The...

Enhanced methane activation on diluted metal-metal ensembles under an electric field: breakthrough in alloy catalysis.

Methane activation on diluted metal ensembles is a challenging task in the field of alloy chemistry. This report describes that synergy between an electric field and Pd-Zn alloy allows improved catalytic activities in the steam reforming of methane. Because of surface protonics, Pd-Pd ensembles are no longer needed. Ligand effects facilitate methane conversion.

Formation of Rhodium Metal Ensembles that Facilitate Nitric Oxide Reduction over Rhodium/Ceria in a Stoichiometric Nitric Oxide–Carbon Monoxide–Propene–Oxygen Reaction

AbstractA series of ceria‐supported Rh nanoparticles (NPs) that have various particle sizes of 2–7 nm was prepared to investigate the size effect of Rh NPs on NO reduction in NO–CO–C3H6–O2 under stoichiometric conditions. The turnover frequency (TOF) for NO reduction on Rh NPs increased drastically according to their particle size. The strong size dependence of the TOF was attributed to the oxidation states of Rh NPs but not to particle geometries, which include fractions of surface local sites (e.g., corner, edge, plane) and the surface crystal structures (e.g., Rh(1 1 1), (1 0 0)). The variation of the TOF with the Rh metal fraction suggested that Rh metal ensembles are highly active species for NO reduction.

Aspects Regarding Repetitive Maintenance Concept in Push Blades for Loading Equipment

Preventive repeated maintenance concept provides elaborating, developing and reconditioning, repeatedly, in economic efficient conditions, of the pushing blades that equip bulk loading machines, namely leveling them. Applying the solution implies developing and elaborating active areas in direct contact with processed materials by loading through welding. To this end the base material of the blades is chosen to comply with the addition material afferent to the selected process, for predetermined circle tries, approx. 5 in the present moment.Research conducted to validate the loading through welding technology had primary objectives to obtain material couples base/addition that have a high resistance to intense bending demands, under load, combined with a good abrasion usage resistance or high pressure. The chemical composition of the basic materials in the pushing blades, is afferent to high resistance steels, with a maximum of1.6%Mn, max. 1.5%Cr, max.2%Ni, max. 0.7%Mo and max. 0.005%B. The chemical composition of the deposited material, through welding, falls into steel prescription, type Fe-25%Cr-5%W-Nb-Ti-B, with metallographic structures that have a high content of complex carbides and hardness of minimum 60 HRC. Sclerometric tried and metallographic research of specific area of the deposited metal/base metal ensemble did not highlight any imperfections such as cracks or white sports in the thermal influenced areas.Research regarding exploitation behaviour, of basaltic aggregates in loading conditions or cereals, have shown a good resistance at wear.

Chemosensing Ensembles of 2-(2-Thiazolylazo)-p-cresol with Metal Ions in Colorimetric Detection of Anions.

This paper reports that changing metal ions (Co2+, Ni2+, Cu2+, or Cd2+) in the 2-(2-thiazolylazo)-p-cresol (TAC)-metal ensemble may generate varied optical responses to anions. The TAC and Ni2+ ensemble can detect and quantify CN- in a highly sensitive and selective manner. CN- competes for the Ni2+ present in the ensemble during recognition events, which thereby triggers colorimetric and absorption spectral changes. CN- and S2- decomposed TAC-Cu2+ ensemble by forming [Cu(CN)x]n- species and CuS, respectively; however, the discrimination of CN- and S2- was not achieved. The TAC-Co2+ ensemble exhibited discriminated interaction with CN- through the absorption channel, but CN- was not quantitatively determined. Although the TAC-Cd2+ ensemble responded to different anions, it did not recognize each anion selectively. These results demonstrated that metal ions can powerfully modulate anion identification to some extent, which can be an effective strategy to achieve selectivity of certain anions by varying the metal ions in the ensemble.

Radiation losses and dark mode at light guiding by a linear chain of nanoparticles.

A new general formula is presented for a collective extinction cross section of a dielectric or a metallic nanoparticle ensemble in terms of incident electric field work on currents excited inside particles. The formula is obtained by identical transformation of the well-known expression for the summing power of electromagnetic field energy losses caused by particle ensemble scattering and absorption. The derived formula is applied to the problem of radiation losses at electromagnetic excitation transfer along a straight chain of particles. Our general formula predicts a zero collective extinction cross section for an infinite straight chain of nonabsorbing dielectric particles providing that the projection of the wave vector of an incident electromagnetic wave on the chain axis does not coincide with its counterpart of the Bloch wave vector of propagating excitation. In another case of a finite chain of particles, with only the first particle of the chain irradiated by an incident narrow electromagnetic wave beam, the derived formula shows that only the irradiated particle directly contributes to the collective extinction cross section despite how large the total number of particles can be, which makes a direct summing contribution of all other particles to wave scattering as if they were unviewed (dark mode). Using a recently developed quasi-separable T-scattering operator approach that leads to the equation system for self-consistent currents excited inside particles by an incident electromagnetic wave field and restricting ourselves to the electric dipole single scattering and neighbor coupling approximation, we revealed a few gigahertz transparency band in the terahertz frequency range (orange color) in the spectra of a straight chain of closely spaced gold nanospheres of a certain radius and a length of a few millimeters. A resonant mechanism of filtering the dark mode from radiation losses established in this work allowed us to reveal a few-fold-more narrow passband in the spectra of a longer gold particle chain with the full length of a few centimeters.

Active Sites in Ni2P/USY Catalysts for the Hydrodeoxygenation of 2-Methyltetrahydrofuran

A series of nickel phosphide catalysts supported on ultrastable Y zeolites USY (Si/Al = 40) with microporous/mesoporous structure were prepared by impregnation and temperature-programmed reduction and were studied for the hydrodeoxygenation (HDO) of 2-methyltetrahydrofuran (2-MTHF). The loading of the active phase was varied from 0.58, 1.16, and 1.74 to 2.3 mmol (g support)−1 and the corresponding samples were denoted as 0.5, 1.0, 1.5, and 2.0 Ni2P/USY. The two lowest loading supports did not show X-ray diffraction (XRD) lines, but X-ray absorption fine-structure spectroscopy (XAFS) indicated the formation of a Ni2P phase, with low Ni–Ni coordination, consistent with high dispersion. The two highest loading supports showed XRD patterns typical of Ni2P, and XAFS indicated similar bond distances to bulk Ni2P and high Ni–Ni coordination. Furthermore, the XAFS data indicated that the low-loading samples had shorter bond distances and more Ni in square-pyramidal coordination compared to the high loading samples and the reference Ni2P material, suggesting that there were differences in structural properties in the samples. This likely was due to preferred termination of the small crystallites with pyramidal Ni sites. The HDO of 2-MTHF was studied at 0.5 MPa and 513–593 K and the main products were n-pentane and n-butane for all catalysts. The low-loading samples showed higher turnover frequency (based on sites titrated by CO chemisorption), and this was attributed to the higher intrinsic activity of the pyramidal Ni sites. In addition, the low-loading samples showed higher selectivity to n-pentane, and this was attributed to lower C–C hydrogenolysis type reactions, which are favored by metallic ensembles as found in the high-loading samples.

Catalytic dehydrogenation of isobutane over Co-based catalysts

In this work, to optimize the catalytic performance of Co-based catalysts for isobutane dehydrogenation, the effects of support, calcination temperature and some promoters were investigated systematically. Results of activity tests and catalyst characterization jointly indicated that both the support and calcination temperature influenced Co dispersion, catalyst acid properties and the interaction between support and Co species significantly. Consequently, the adsorption–desorption behaviors for isobutane and isobutene as well as the dehydrogenation performance were further affected. Sulfided Co/SiO2, with no acidity, exhibited the best performance. Although high calcination temperature was beneficial for achieving a high selectivity to isobutene over Al2O3 and MgAl2O4 supported Co-based catalysts, the inevitable formation of Co2SiO4 over Co/SiO2 at high temperature led to reduced active sites and dehydrogenation activity. In addition, by comparing the different effects of the separate introduction of S, Sn, Cu and Cl, it was concluded that an efficient promoter should present the following characteristics: inhibition of the formation of metal ensembles, and strong binding affinity to two hydrogen atoms of one additive atom.