Supported metal cluster catalysts

Recent Progress on Covalent Organic Frameworks Supporting Metal Nanoparticles as Promising Materials for Nitrophenol Reduction

With the utilization of nitrophenols in manufacturing various materials and the expansion of industry, nitrophenols have emerged as water pollutants that pose significant risks to both humans and the environment. Therefore, it is imperative to convert nitrophenols into aminophenols, which are less toxic. This conversion process is achieved through the use of noble metal nanoparticles, such as gold, silver, copper, and palladium. The primary challenge with noble metal nanoparticles lies in their accumulation and deactivation, leading to a decrease in catalyst activity. Covalent organic frameworks (COFs) are materials characterized by a crystalline structure, good stability, and high porosity with active sites. These properties make them ideal substrates for noble metal nanoparticles, enhancing catalytic activity. This overview explores various articles that focus on the synthesis of catalysts containing noble metal nanoparticles attached to COFs as substrates to reduce nitrophenols to aminophenols.

Enhanced photocatalytic performance of coaxially electrospun titania nanofibers comprising yolk-shell particles

The present paper reports the fabrication of novel types of hybrid fibrous photocatalysts by combining block copolymer (BCP) templating, sol-gel processing, and coaxial electrospinning techniques. Coaxial electrospinning produces core-shell nanofibers (NFs), which are converted into hollow porous TiO2 NFs using an oxidative calcination step. Hybrid BCP micelles comprising a single plasmonic nanoparticle (NP) in their core and thereof derived silica-coated core-shell particles are utilized as precursors to generate yolk-shell type particulate inclusions in photocatalytically active NFs. The catalytic and photocatalytic activity of calcined NFs comprising different types of yolk-shell particles is systematically investigated and compared. Interestingly, calcined NFs comprising silica-coated yolk-shells demonstrate enhanced catalytic and photocatalytic performance despite the presence of silica shell separating plasmonic NP from the TiO2 matrix. Electromagnetic simulations indicate that this enhancement is caused by a localized surface plasmon resonance and a confinement effect in silica-coated yolk-shells embedded in porous TiO2 NFs. Utilization of the coaxially electrospun TiO2 NFs in combination with yolk-shells comprising plasmonic NPs reveals to be a potent method for the photocatalytic decomposition of numerous pollutants. It is worth noting that this study stands as the first occurrence of combining yolk-shells (Au@void@SiO2) with porous electrospun NFs (TiO2) for photocatalytic purposes and gaining an understanding of plasmon and confinement effects for photocatalytic performance. This approach represents a promising route for fabricating highly active and up-scalable fibrous photocatalytic systems.

Stability and Electronic Structure of Nitrogen-Doped Graphene-Supported Cun (n = 1-5) Clusters in Vacuum and under Electrochemical Conditions: Toward Sensor and Catalyst Design

Here, we present a detailed computational study of the stability and the electronic structure of nitrogen-doped graphene (N4V2) supported Cun (n = 1-5) clusters, which are promising carbon-dioxide electroreduction catalysts. The binding of the clusters to the nitrogen-doped graphene and the electronic structure of these systems were investigated under vacuum and electrochemical conditions. The stability analysis showed that among the systems, the nitrogen-doped graphene bound Cu4 is the most stable in vacuum, while in an electrolyte, and at a negative potential, the N4V2-Cu3 is energetically more favorable. The ground state electronic structure of the nitrogen-doped graphene substrate undergoes topological phase transition, from a semimetallic state, and we observed a metallic and topologically trivial state after the clusters are deposited. The electrode potential adjusts the type and density of the charge carriers in the semimetallic models, while the structures containing copper exhibit bands which are deformed and relaxed by the modified number of electrons.

Activation of Methane by Rhodium Clusters on a Model Support C20H10

Supported metals represent an important family of catalysts for the transformation of the most stable alkane, methane, under mild conditions. Here, using state-of-the-art mass spectrometry coupled with a newly designed double ion trap reactor that can run at high temperatures, we successfully immobilize a series of Rhn- (n = 4-8) cluster anions on a model support C20H10. Reactivity measurements at room temperature identify a significantly enhanced performance of large-sized Rh7,8C20H10- toward methane activation compared to that of free Rh7,8-. The "support" acting as an "electron sponge" is emphasized as the key factor to improve the reactivity of large-sized clusters, for which the high electron-withdrawing capability of C20H10 dramatically shifts the active Rh atom from the apex position in free Rh7- to the edge position in "supported" Rh7- to enhance CH4 adsorption, while the flexibility of C20H10 to release electrons further promotes subsequent C-H activation. The Rh atoms in direct contact with the support serve as electron-relay stations for electron transfer between C20H10 and the active Rh atom. This work not only establishes a novel approach to prepare and measure the reactivity of "supported" metal clusters in isolated gas phase but provides useful atomic-scale insights for understanding the chemical behavior of carbon (e.g., graphene)-supported metals in heterogeneous catalysis.

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.

CeO2(111) electronic reducibility tuned by ultra-small supported bimetallic Pt-Cu clusters

Controlling Ce4+ to Ce3+ electronic reducibility in a rare-earth binary oxide such as CeO2 has enormous applications in heterogeneous catalysis, where a profound understanding of reactivity and selectivity at the atomic level is yet to be reached. Thus, in this work we report an extensive DFT-based Basin Hopping global optimization study to find the most stable bimetallic Pt-Cu clusters supported on the CeO2(111) oxide surface, involving up to 5 atoms in size for all compositions. Our PBE+U global optimization calculations indicate a preference for Pt-Cu clusters to adopt 2D planar geometries parallel to the oxide surface, due to the formation of strong metal bonds to oxygen surface sites and charge transfer effects. The calculated adsorption energy values (Eads) for both mono- and bimetallic systems are of the order of 1.79 up to 4.07 eV, implying a strong metal cluster interaction with the oxide surface. Our calculations indicate that at such sub-nanometer sizes, the number of Ce4+ surface atoms reduced to Ce3+ cations is mediated by the amount of Cu atoms within the cluster, reaching a maximum of three Ce3+ for a supported Cu5 cluster. Our computational results have critical implications on the continuous understanding of the strong metal-support interactions over reducible oxides such as CeO2, as well as the advancement of frontier research areas such as heterogeneous single-atom catalysts (SAC) and single-cluster catalysts (SCC).

Synthesis and crystal structure of the chiral-linked tetrahedral cluster [(μ3-Se)RuCo2(CO)8]2 (DIOP)

The new chiral Ru–Co mixed-metal linked tetrahedral cluster [(μ3-Se)RuCo2(CO)8]2(DIOP) 3 has been synthesised from reaction between (μ3-Se)RuCo2(CO)9 1 and the (–)-DIOP [2 (2,2-dimethyl-1,3-dioxolane-4,5-diylbis-methylene) bisdiphenylphosphine] diphosphine compound. The structure of cluster 3 has been established by single-crystal X-ray diffraction method.

Engineering of Transition Metal Catalysts Confined in Zeolites

Transition metal-zeolite composites are versatile catalytic materials for a wide range of industrial and lab-scale processes. Significant advances in fabrication and characterization of well-defined metal centers confined in zeolite matrixes have greatly expanded the library of available materials and, accordingly, their catalytic utility. In this review, we summarize recent developments in the field from the perspective of materials chemistry, focusing on synthesis, postsynthesis modification, (operando) spectroscopy characterization, and computational modeling of transition metal-zeolite catalysts.

Probing the electronic structures of Con (n = 1–5) clusters on γ-Al2O3 surfaces using first-principles calculations

Using density functional theory calculations, we discussed the geometric and electronic structures and nucleation of small Co clusters on γ-Al₂O₃(100) and γ-Al₂O₃(110) surfaces.

The properties of Irn (n = 2–10) clusters and their nucleation on γ-Al2O3 and MgO surfaces: from ab initio studies

The adsorption and nucleation of Ir clusters on γ-Al₂O₃(001) and MgO(100) based on first-principles calculations.

Atomically precise cluster catalysis towards quantum controlled catalysts

Catalysis of atomically precise clusters supported on a substrate is reviewed in relation to the type of reactions. The catalytic activity of supported clusters has generally been discussed in terms of electronic structure. Several lines of evidence have indicated that the electronic structure of clusters and the geometry of clusters on a support, including the accompanying cluster-support interaction, are strongly correlated with catalytic activity. The electronic states of small clusters would be easily affected by cluster-support interactions. Several studies have suggested that it is possible to tune the electronic structure through atomic control of the cluster size. It is promising to tune not only the number of cluster atoms, but also the hybridization between the electronic states of the adsorbed reactant molecules and clusters in order to realize a quantum-controlled catalyst.

Dendrimer-mediated synthesis of supported rhodium nanoparticles with controlled size: effect of pH and dialysis

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.

Selective catalysis and nanoscience: an inseparable pair

Selective catalysts can be considered as nanomachines designed to perform the synthesis of molecules with high reaction activity and high selectivity. These properties arise from a precise control of the structure of the active sites, of the three-dimensional environment and of their relationship. In both homogeneous and heterogeneous catalysts the active site three-dimensional environment ensemble is always a complex structure resembling the tuneable structure of enzymes, which are the most efficient catalysts optimized by nature over billions of years. To illustrate this concept the structure of a few homogeneous and heterogeneous catalysts for alkenes hydrogenation and for olefin polymerization are chosen and discussed as examples.

Evidence of the Interaction of Evaporated Pt Nanoparticles with Variously Treated Surfaces of Highly Oriented Pyrolytic Graphite

The interactions of Pt nanoparticles, deposited by evaporation onto highly oriented pyrolytic graphite surfaces modified by kiloelectronvolt Ar+ beam treatment, have been studied by X-ray photoelectron spectroscopy core-level line shape analysis. The C1s and Pt4f7/2 peaks were each considered to be composed of one asymmetric peak, and changes in their asymmetry parameters were used to study their interfacial interactions. In addition to these changes, strong signal intensity changes with time were found for both the C1s and Pt4f peaks, indicating an initial crystalline orientational instability of the Pt nanoparticles, which is supported by time-dependent high-resolution electron microscopy studies at elevated temperatures.

Synthesis and Crystal Structure of a New Binuclear Iron Carbonyl Cluster Containing the Triphos and μ,η2-OCC6H5 Ligands

The new bimetallic cluster [Fe2(CO)4( μ,η2-OCC6H5)( μ4,η3-CH3C(CH2PPh2)2CH2PPh)] 3 has been synthesised from reaction between Fe3(CO)121 and the tripodal polyphosphine compound CH3C(CH2PPh2)3 (1,1,1-tris(diphenyl-phosphinomethyl)ethane: triphos) 2. The structure of cluster 3 has been established by single-crystal X-ray diffraction method.

Effects of Reduction Temperature and Metal−Support Interactions on the Catalytic Activity of Pt/γ-Al2O3 and Pt/TiO2 for the Oxidation of CO in the Presence and Absence of H2

TiO2- and gamma-Al2O3-supported Pt catalysts were characterized by HRTEM, XPS, EXAFS, and in situ FTIR spectroscopy after activation at various conditions, and their catalytic properties were examined for the oxidation of CO in the absence and presence of H2 (PROX). When gamma-Al2O3 was used as the support, the catalytic, electronic, and structural properties of the Pt particles formed were not affected substantially by the pretreatment conditions. In contrast, the surface properties and catalytic activity of Pt/TiO2 were strongly influenced by the pretreatment conditions. In this case, an increase in the reduction temperature led to higher electron density on Pt, altering its chemisorptive properties, weakening the Pt-CO bonds, and increasing its activity for the oxidation of CO. The in situ FTIR data suggest that both the terminal and bridging CO species adsorbed on fully reduced Pt are active for this reaction. The high activity of Pt/TiO2 for the oxidation of CO can also be attributed to the ability of TiO2 to provide or stabilize highly reactive oxygen species at the metal-support interface. However, such species appear to be more reactive toward H2 than CO. Consequently, Pt/TiO2 shows substantially lower selectivities toward CO oxidation under PROX conditions than Pt/gamma-Al2O3.

Surfactant-promoted novel reductive synthesis of supported metallic Cu nanoclusters and their catalytic performances for selective dehydrogenation of methanol

We have found a surfactant-promoted novel reductive synthesis of metallic Cu nanoclusters on metal oxides under hydrothermal synthesis conditions, which are active for the selective dehydrogenation of methanol.

Effects of Adsorbates on Supported Platinum and Iridium Clusters: Characterization in Reactive Atmospheres and during Alkene Hydrogenation Catalysis by X-ray Absorption Spectroscopy

MgO-, SiO2-, and gamma-Al2O3-supported platinum clusters and particles (with average diameters ranging from 11 to 45 A) and zeolite-supported Ir4 clusters (approximately 6 A in diameter) were characterized by extended X-ray absorption fine structure spectroscopy in the presence of H2, O2, ethene, propene, and ethane, as well as under conditions of alkene hydrogenation catalysis. The results indicate that under various atmospheres, the presence of adsorbates affects the smaller platinum clusters (11 A) on gamma-Al2O3 more substantially than the larger platinum particles (i.e., those greater than approximately 21 A in average diameter) on MgO or SiO2. When Pt/gamma-Al2O3 was exposed to H2, the platinum morphology did not change, although the Pt-Pt bond distance increased. In contrast, when the same sample was exposed to O2, complete oxidative fragmentation took place. This processes was reversed following subsequent treatment with H2. Exposure to alkenes changed both the morphology and electron density (as indicated by X-ray absorption near-edge spectra) of the gamma-Al2O3-supported platinum clusters. Under conditions of alkene hydrogenation catalysis at room temperature, the electronic properties and the structure of the platinum clusters were found to depend on the reactant composition and the nature of molecules involved in the reaction process. The effects of the reactant gases on the smaller iridium clusters (Ir4) were substantially less pronounced, apparently as a consequence of the extremely small number of atoms in each iridium cluster.

Mixed-metal cluster chemistry. 19. Crystallographic, spectroscopic, electrochemical, spectroelectrochemical, and theoretical studies of systematically varied tetrahedral group 6-iridium clusters

A systematically varied series of tetrahedral clusters involving ligand and core metal variation has been examined using crystallography, Raman spectroscopy, cyclic voltammetry, UV-vis-NIR and IR spectroelectrochemistry, and approximate density functional theory, to assess cluster rearrangement to accommodate steric crowding, the utility of metal-metal stretching vibrations in mixed-metal cluster characterization, and the possibility of tuning cluster electronic structure by systematic modification of composition, and to identify cluster species resultant upon electrochemical oxidation or reduction. The 60-electron tetrahedral clusters MIr(3)(CO)(11-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (5), C(5)HMe(4) (6), C(5)Me(5) (7); M = W, Cp = C(5)H(4)Me, x = 1 (13), x = 2 (14)] and M(2)Ir(2)(CO)(10-x)(PMe(3))(x)(eta(5)-Cp) [M = Mo, x = 0, Cp = C(5)H(4)Me (8), C(5)HMe(4) (9), C(5)Me(5) (10); M = W, Cp = C(5)H(4)Me, x = 1 (15), x = 2 (16)] have been prepared. Structural studies of 7, 10, and 13 have been undertaken; these clusters are among the most sterically encumbered, compensating by core bond lengthening and unsymmetrical carbonyl dispositions (semi-bridging, semi-face-capping). Raman spectra for 5, 8, WIr(3)(CO)(11)(eta(5)-C(5)H(4)Me) (11), and W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(4)Me)(2) (12), together with the spectrum of Ir(4)(CO)(12), have been obtained, the first Raman spectra for mixed-metal clusters. Minimal mode-mixing permits correlation between A(1) frequencies and cluster core bond strength, frequencies for the A(1) breathing mode decreasing on progressive group 6 metal incorporation, and consistent with the trend in metal-metal distances [Ir-Ir < M-Ir < M-M]. Cyclic voltammetric scans for 5-15, MoIr(3)(CO)(11)(eta(5)-C(5)H(5)) (1), and Mo(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (3) have been collected. The [MIr(3)] clusters show irreversible one-electron reduction at potentials which become negative on cyclopentadienyl alkyl introduction, replacement of molybdenum by tungsten, and replacement of carbonyl by phosphine. These clusters show two irreversible one-electron oxidation processes, the easier of which tracks with the above structural modifications; a third irreversible oxidation process is accessible for the bis-phosphine cluster 14. The [M(2)Ir(2)] clusters show irreversible two-electron reduction processes; the tungsten-containing clusters and phosphine-containing clusters are again more difficult to reduce than their molybdenum-containing or carbonyl-containing analogues. These clusters show two one-electron oxidation processes, the easier of which is reversible/quasi-reversible, and the more difficult of which is irreversible; the former occur at potentials which increase on cyclopentadienyl alkyl removal, replacement of tungsten by molybdenum, and replacement of phosphine by carbonyl. The reversible one-electron oxidation of 12 has been probed by UV-vis-NIR and IR spectroelectrochemistry. The former reveals that 12(+) has a low-energy band at 8000 cm(-1), a spectrally transparent region for 12, and the latter reveals that 12(+) exists in solution with an all-terminal carbonyl geometry, in contrast to 12 for which an isomer with bridging carbonyls is apparent in solution. Approximate density functional calculations (including ZORA scalar relativistic corrections) have been undertaken on the various charge states of W(2)Ir(2)(CO)(10)(eta(5)-C(5)H(5))(2) (4). The calculations suggest that two-electron reduction is accompanied by W-W cleavage, whereas one-electron oxidation proceeds with retention of the tetrahedral core geometry. The calculations also suggest that the low-energy NIR band of 12(+) arises from a sigma(W-W) --> sigma*(W-W) transition.