Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

We report the catalytic oxidation of cyclohexane to CO and CO₂ over size-selected palladium clusters (Pd _N clusters, N = 10–120) supported on graphite as a function of cluster size. The stability of the pinned clusters (nanoparticles) under reaction conditions is investigated by scanning tunnelling microscopy measurement both before and after reaction. Temperature-programmed reaction experiments at 800 Torr show that the turnover rates (per surface Pd atom) for both CO and CO₂ increase significantly as cluster size decreases and correlate with the number of Pd perimeter atoms at the graphite interface. Under oxygen-rich conditions, the activity of the clusters increases by a factor of 3 while the product ratio CO:CO₂ rises by an order of magnitude.     

Nano-assembled Pd catalysts on MgO thin films

Cluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often behave completely differently than their bulk analogues. By depositing gas phase Pd clusters on MgO thin films, model catalysts are fabricated which exhibit remarkable catalytic activity. In contrast to the high selectivity of Pd (111) surfaces for the cyclotrimerization of acetylene to benzene, small supported Pd_n clusters reveal a strongly size-dependent selectivity and catalyse the formation of benzene as well as of other hydrocarbons. The understanding at the atomistic level of the observed processes has been obtained by means of first-principles quantum–mechanical simulations. The theoretical studies have shown the importance of the surface defects of the oxide substrate in stabilizing the supported clusters but also in promoting their catalytic activity. For instance, Pd atoms bound at the regular sites of the MgO (100) terraces do not promote the acetylene to benzene conversion while they become active catalysts when bound at oxygen vacancies (F centers).     

Electrochemical Oxidation Encapsulated Ru Clusters Enable Robust Durability for Efficient Oxygen Evolution (Small 29/2023)

Electrochemical oxidization and thermodynamic instability agglomeration are a primary challenge in triggering metal-support interactions (MSIs) by immobilizing metal atoms on a carrier to achieve efficient oxygen evolution reactions (OER). Herein, Ru clusters anchored to the VS₂ surface and the VS₂ nanosheets embedded vertically in carbon cloth (Ru-VS₂@CC) are deliberately designed to realize high reactivity and exceptional durability. In situ Raman spectroscopy reveals that the Ru clusters are preferentially electro-oxidized to form RuO₂ chainmail, both affording sufficient catalytic sites and protecting the internal Ru core with VS₂ substrates for consistent MSIs. Theoretical calculations elucidate that electrons across the Ru/VS₂ interface aggregate toward the electro-oxidized Ru clusters, while the electronic coupling of Ru 3p and O 2p orbitals boosts a positive shift in the Fermi energy level of Ru, optimizing the adsorption capacity of the intermediates and diminishing the migration barriers of the rate-determining steps. Therefore, the Ru-VS₂@CC catalyst demonstrated ultra-low overpotentials of 245 mV at 50 mA cm⁻², while the zinc–air battery maintained a narrow gap (0.62 V) after 470 h of reversible operation. This work has transformed the corrupt into the miraculous and paved a new way for the development of efficient electrocatalysts.     

Enhanced ferromagnetism in ZnO nanoribbons and clusters passivated with sulfur

Inspired by recent experimental results, the electronic and magnetic properties of sulfur-passivated ZnO clusters and zigzag nanoribbons have been studied using first principles calculations in the framework of the local spin density approximation. In the case of the ZnO nanoribbons, the sulfur atoms or thiol groups were attached in different ways to the zinc or oxygen atoms located at the edges, whereas in clusters, the sulfur atoms were set on the surface, mainly interacting with atoms with low-coordinate number. After an exhaustive atomic relaxation, we found that a magnetic moment emerges in zigzag nanoribbons both with and without sulfur-passivation on the edges. However, the magnitude of the magnetic moment is very sensitive to sulfur passivation. In particular, we found that when sulfur is attached to the zinc atoms in an alternating fashion along the ribbon edges, the magnetic moment is a maximum (1.4 μ_B/unit cell). In the case of clusters, we found that the Zn₁₅O₁₅ cluster exhibits a high spin moment of 5.5 μ_B when capped with sulfur atoms. Our calculations indicate that sulfur-passivating of ZnO nanosystems could be responsible for recently observed ferromagnetic responses. This article is published with open access at Springerlink.com     

Thermodynamically and Kinetically Stabilized Pt Clusters Against Sintering on CeO2 Nanofibers Through Enclosing CeO2 Nanocubes

Sintering is a major concern for the deactivation of supported metals catalysts, which is driven by the force of decreasing the total surface energy of the entire catalytic system. In this work, a double-confinement strategy is demonstrated to stabilize 2.6 nm-Pt clusters against sintering on electrospun CeO₂ nanofibers decorated by CeO₂ nanocubes (m-CeO₂). Thermodynamically, with the aid of CeO₂-nanocubes, the intrinsically irregular surface of polycrystalline CeO₂ nanofibers becomes smooth, offering adjacent Pt clusters with decreased chemical potential differences on a relatively uniform surface. Kinetically, the Pt clusters are physically restricted on each facet of CeO₂ nanocubes in a nanosized region. In situ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation reveals that the Pt clusters can be stabilized up to 800 °C even in a high density, which is far beyond their Tammann temperature, without observable size growth or migration. Such a sinter-resistant catalytic system is endowed with boosted catalytic activity toward both the hydrogenation of p-nitrophenol after being aged at 500 °C and the sinter-promoting exothermic oxidation reactions (e.g., soot oxidation) at high temperatures over 700 °C. This work offers new opportunities for exploring sinter-resistant nanocatalysts, starting from the rational design of whole catalytic system in terms of thermodynamic and kinetic aspects.     

Helium Droplets: A Nanoscale Cryostat for High Resolution Spectroscopy and Studies of Quantized Vorticity

In this paper we discuss helium droplets as a nanoscale cryostat for a variety of fundamental experiments in condensed matter physics. Specifically, we describe our recent work on the spectroscopy of silver atoms, europium atoms and C ₆₀ in helium droplets and compare the helium droplet, as a matrix for low temperature studies of complex systems, with traditional matrix and gas phase techniques. Further, we discuss our recent work on the production of ultra-cold metal clusters of silver, indium and europium embedded in helium droplets at a temperature (T=0.37K) two orders of magnitude lower than previously achieved in beams of free metal clusters. This work opens the door to high resolution spectroscopic studies of metal clusters and, possibly, high resolution studies of the size dependence of their superconducting properties. We further speculate on a series of experiments where we plan to use standard spectroscopic methods, developed in recent years, to exploit the helium droplet for studies of the existence, stability and dynamics of quantized vortices. Helium droplets may be the ideal system for such studies due to the complete absence of pinning sites that plague many similar experiments performed in bulk helium.     

Oxygen condensation stacking faults in crystallized Co found during magnetic annealing of Co-rich amorphous alloy

Amorphous Co_75.26–x Fe_4.74(BSi)_20+x (0 < x) magnetic alloys were examined with Transmission Electron Microscopy (TEM) after magnetic field annealing. TEM analysis revealed that the crystallized Co layer under the surface oxide could be highly faulted with planar defects depending on the composition. Based on the electron diffraction, we have proposed a new form of stacking fault in which two oxygen atoms are substituted for FCC Co in the {111} planes to create randomly distributed clusters of oxygen atoms on the Co {111} planes. Such clusters would explain the absent {200} peak in the highly faulted composition of crystallized FCC Co and the streaks observed in the electron diffraction pattern of the material. The oxygen fault was also closely related to the magnitude of the induced magnetic anisotropy of the material, suggesting the Co–O bonds acting as the localized antiferromagnetic region.     

Double-Shelled Porous g-C3N4 Nanotubes Modified with Amorphous Cu-Doped FeOOH Nanoclusters as 0D/3D Non-Homogeneous Photo-Fenton Catalysts for Effective Removal of Organic Dyes

Graphite phased carbon nitride (g-C₃N₄) has attracted extensive attention attributed to its non-toxic nature, remarkable physical–chemical stability, and visible light response properties. Nevertheless, the pristine g-C₃N₄ suffers from the rapid photogenerated carrier recombination and unfavorable specific surface area, which greatly limit its catalytic performance. Herein, 0D/3D Cu-FeOOH/TCN composites are constructed as photo-Fenton catalysts by assembling amorphous Cu-FeOOH clusters on 3D double-shelled porous tubular g-C₃N₄ (TCN) fabricated through one-step calcination. Combined density functional theory (DFT) calculations, the synergistic effect between Cu and Fe species could facilitate the adsorption and activation of H₂O₂, and the separation and transfer of photogenerated charges effectively. Thus, Cu-FeOOH/TCN composites acquire a high removal efficiency of 97.8%, the mineralization rate of 85.5% and a first-order rate constant k = 0.0507 min⁻¹ for methyl orange (MO) (40 mg L⁻¹) in photo-Fenton reaction system, which is nearly 10 times and 21 times higher than those of FeOOH/TCN (k = 0.0047 min⁻¹) and TCN (k = 0.0024 min⁻¹), respectively, indicating its universal applicability and desirable cyclic stability. Overall, this work furnishes a novel strategy for developing heterogeneous photo-Fenton catalysts based on g-C₃N₄ nanotubes for practical wastewater treatment.     

Defect Effects on TiO2 Nanosheets: Stabilizing Single Atomic Site Au and Promoting Catalytic Properties

Isolated single atomic site catalysts have attracted great interest due to their remarkable catalytic properties. Because of their high surface energy, single atoms are highly mobile and tend to form aggregate during synthetic and catalytic processes. Therefore, it is a significant challenge to fabricate isolated single atomic site catalysts with good stability. Herein, a gentle method to stabilize single atomic site metal by constructing defects on the surface of supports is presented. As a proof of concept, single atomic site Au supported on defective TiO₂ nanosheets is prepared and it is discovered that (1) the surface defects on TiO₂ nanosheets can effectively stabilize Au single atomic sites through forming the Ti–Au–Ti structure; and (2) the Ti–Au–Ti structure can also promote the catalytic properties through reducing the energy barrier and relieving the competitive adsorption on isolated Au atomic sites. It is believed that this work paves a way to design stable and active single atomic site catalysts on oxide supports.     

Structure of Si/Ge nanoclusters: Kinetics and thermodynamics

The optimal structure and element distribution of Si_xGe_1?x clusters was investigated in terms of free energy. The methods employed were computational simulations based on classical molecular dynamics. Clusters obtained in our previous work were further simulated through annealing at different temperatures. In addition, a combination of molecular dynamics and a semi-grand-canonical Monte Carlo algorithm was used to find a free-energetically favorable element configuration for the clusters. The results show that annealing at conventional temperatures improves the clusters’ sphericity only slightly, and they remain much more amorphous than clusters cut out from crystalline bulk; only at extreme annealing temperatures are the sphericity and crystallinity notably improved. Furthermore, Ge atoms are found to segregate to the surface of the clusters, which greatly reduces the free energy of the clusters.     

Synergy of Ultrathin CoOx Overlayer and Nickel Single Atoms on Hematite Nanorods for Efficient Photo-Electrochemical Water Splitting

To solve surface carrier recombination and sluggish water oxidation kinetics of hematite (α-Fe₂O₃) photoanodes, herein, an attractive surface modification strategy is developed to successively deposit ultrathin CoO_x overlayer and Ni single atoms on titanium (Ti)-doped α-Fe₂O₃ (Ti:Fe₂O₃) nanorods through a two-step atomic layer deposition (ALD) and photodeposition process. The collaborative decoration of ultrathin CoO_x overlayer and Ni single atoms can trigger a big boost in photo-electrochemical (PEC) performance for water splitting over the obtained Ti:Fe₂O₃/CoO_x/Ni photoanode, with the photocurrent density reaching 1.05 mA cm⁻² at 1.23 V vs. reversible hydrogen electrode (RHE), more than three times that of Ti:Fe₂O₃ (0.326 mA cm⁻²). Electrochemical and electronic investigations reveal that the surface passivation effect of ultrathin CoO_x overlayer can reduce surface carrier recombination, while the catalysis effect of Ni single atoms can accelerate water oxidation kinetics. Moreover, theoretical calculations evidence that the synergy of ultrathin CoO_x overlayer and Ni single atoms can lower the adsorption free energy of OH* intermediates and relieve the potential-determining step (PDS) for oxygen evolution reaction (OER). This work provides an exemplary modification through rational engineering of surface electrochemical and electronic properties for the improved PEC performances, which can be applied in other metal oxide semiconductors as well.     

Superatomic Gallium Clusters in Dendrimers: Unique Rigidity and Reactivity Depending on their Atomicity

Superatoms have been investigated due to their possible substitution for other elements. The solution-phase synthesis of superatoms has attracted attention to realize the availability of superatoms. However, the previous approach is basically limited to the formation of a single cluster. Here, superatoms are investigated and the number of valence electrons in these superatoms is changed by designing the number of gallium atoms present. Based on the dendrimer template method, clusters consisting of 3, 12, 13, and other numbers of atoms have been synthesized. The halogen-like superatomic nature of Ga₁₃ is structurally and electrochemically observed as completely different to the other clusters. The gallium clusters of 13 and 3 atoms, which can fill the 2P and 1P superatomic orbitals, respectively, exhibit different reactivities. The 3-atom gallium cluster is suggested as being reduced to Ga₃H₂⁻ due to the lower shift of energy levels in the unoccupied orbitals. The results for these gallium clusters provide candidates for superatoms.     

Monte Carlo and molecular dynamics simulation of argon clusters andn-alkanes in the confined regions of zeolites

Geometry and energy of argon clusters confined in zeolite NaCaA are compared with those of free clusters. Results indicate the possible existence of magic numbers among the confined clusters. Spectra obtained from instantaneous normal mode analysis of free and confined clusters give a larger percentage of imaginary frequencies for the latter indicating that the confined cluster atoms populate the saddle points of the potential energy surface significantly. The variation of the percentage of imaginary frequencies with temperature during melting is akin to the variation of other properties. It is shown that confined clusters might exhibit inverse surface melting, unlike medium-to-large-sized free clusters that exhibit surface melting. Configurational-bias Monte Carlo (CBMC) simulations ofn-alkanes in zeolites Y and A are reported. CBMC method gives reliable estimates of the properties relating to the conformation of molecules. Changes in the conformational properties ofn-butane and other longern-alkanes such asn-hexane andn-heptane when they are confined in different zeolites are presented. The changes in the conformational properties ofn-butane andn-hexane with temperature and concentration is discussed. In general, in zeolite Y as well as A, there is significant enhancement of thegauche population as compared to the pure unconfined fluid. Contribution No. 1260 from the Solid State and Structural Chemistry Unit     

Ab initio molecular dynamics studies of metal clusters

We present results of ourab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahedral AI₁₂X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al₁₃, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al₇ and Al₁₃ clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.     

An atomic-scale study of hydrogenated silicon cluster deposition on a crystalline silicon surface

Controlled deposition of clusters on solid surfaces has attracted lots of attention in recent years, because of its potential application to tailoring the desired electronic properties of the resulting surfaces. We have carried out an atomic-scale study to understand the deposition mechanism. The molecular dynamics approach based on a modified Tersoff potential is used to simulate the deposition mechanism of hydrogenated silicon clusters on a crystalline silicon surface in detail. The important factors governing the deposition process such as impact energy and substrate temperature, are investigated for the hydrogenated silicon cluster Si₂₉H₂₄ on a H-terminated Si(100)-(2x1) surface.     

Ab initio calculations of Fe–Ni clusters

The clusters of Fe, Ni, and Fe–Ni are investigated computationally using a density functional approach. The geometries of clusters are optimized under the constraint of well-defined point group symmetries at the UB3LYP/LanL2DZ level. The equilibrium geometries and binding energies are presented and discussed, together with natural populations and natural electron configurations. In addition, the binding energies of Fe_n−xNi_x clusters are found to generally decrease by successive substitutions of Ni atoms for Fe atoms. For Fe_n−xNi_x clusters, the comparisons on total energies between isomers indicate that Ni atoms energetically prefer clustering in the mixed Fe–Ni clusters. The calculations for Fe_n−xNi_x clusters show that the clustering leads to a segregation of Ni atoms from Fe atoms.     

Novel aspects of oxygen diffusion in silicon

Normal diffusion of interstitial oxygen atoms (O_i ) accounts for the rate of oxygen aggregation in silicon for T > 500C. There is evidence for the dissociation of SiO₂ precipitates (Ostwald ripening) and the formation of self-interstitials (I-atoms) to accommodate the local increase in volume. For T < 500 C, measurements of the loss of oxygen atoms from solution indicate that O₂ dimer formation is the rate-limiting process, but dissociation of dimers must be taken into account when modelling this process. Large clusters of up to 10–20 O_i atoms, usually assigned to thermal donor (TD) defects cannot form unless dimer diffusion is much greater (by a factor of 10⁴ to 10⁷ ) than diffusion of O_i atoms and unless there is dissociation of clusters with the emission of dimers. Hydrogen impurities enhance O_i diffusion by a catalytic process and speed up donor formation. Infrared absorption measurements reveal H-O_i complexes and there is also partial passivation of TD defects to produce shallow thermal donors (STDs).     

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

The Rutherford backscattering technique utilizing 2 MeV He⁺ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta₂O₅ layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta₂O₅ films on tantalum, there is no sharp boundary between Ta and Ta₂O₅. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm^-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.     

On the mechanism of carbon monoxide oxidation on the surface of gold nanoclusters supported on titanium oxide

The process of carbon monoxide (CO) oxidation on the surface of a system comprising nanodimensional gold clusters deposited onto thin films of titanium oxide of variable stoichiometry formed on a Re(1000) single crystal surface has been studied by methods of thermodesorption, IR, and X-ray photoelectron spectroscopy. It is established that oxygen contained in titanium oxide plays an important role in the conversion of CO into CO₂. The efficiency of this process on the Au/TiO _x (x < 2) system surface is significantly higher that that on the Au/TiO₂ system.     

Density functional theory and tight binding-based dynamical studies of carbon metal systems of relevance to carbon nanotube growth

Density functional theory (DFT) and tight binding (TB) models have been used to study systems containing single-walled carbon nanotubes (SWNTs) and metal clusters that are of relevance to SWNT growth and regrowth. In particular, TB-based Monte Carlo (TBMC) simulations at 1000 or 1500 K show that Ni atoms that are initially on the surface of the SWNT or that are clustered near the SWNT end diffuse to the nanotube end so that virtually none of the Ni atoms are located inside the nanotube. This occurs, in part, due to the lowering of the Ni atom energies when they retract from the SWNT to the interior of the cluster. Aggregation of the atoms at the SWNT end does not change the chirality within the simulation time, which supports the application of SWNT regrowth (seeded growth) as a potential route for chirality-controlled SWNT production. DFT-based geometry optimisation and direct dynamics at 2000 K show that Cr and Mo atoms in Cr₅Co₅₀ and Mo₅Co₅₀ clusters prefer to be distributed in the interior of the clusters. Extension of these calculations should deepen our understanding of the role of the various alloy components in SWNT growth.