The characterization and thermal stability of a cluster grafted on silica surface

Ru₃(CO)₁₂, supported on silica in the absence of oxygen, reacts with silanol groups of the surface to produce a grafted cluster

, which has been characterized by IR and Raman spectroscopy; the molecular formula of this cluster is in agreement with the stoichiometric balance of CO evolved during its formation from Ru₃(CO)₁₂. The grafted cluster is an intermediate step to produce by thermal decomposition small metallic ruthenium particles of 14 Å together with some Ru(II) carbonyl species encapsulated in the silica surface.     

Ab initio cluster calculations of silica surface sites

Silica surface sites, which can be formed in cleavage processes, and their hydrolyzed counterparts are investigated with ab initio cluster calculations. Natural Bond Orbital (NBO) theory is used to characterize bonding around silica surface sites. Higher energy lone pairs of electrons on oxygen atoms either hyperconjugate to vicinal silanol/siloxane antibonding orbitals or backdonate electron density via donor–acceptor π-type bonding with participation of pd or p hybrids on silicon atoms. Upon substitution of hydroxyl groups of orthosilicic acid with silica monomers the strength of siloxane and silanol Si–O bonding increases as energies of bonding orbitals and contributions from p-orbitals decrease. Silanone sites and a complementary pair of silyl/siloxy radical sites are found to be the most stable geminal and single non-hydrolyzed sites, respectively. Atomic charges based on natural wavefunctions and on fitting to electrostatic potential, and characteristic bands of IR spectra associated with siloxane and silanol stretching vibrations of silica surface sites are reported.     

Cu cluster embedded porous nanofibers for high-performance CO2 electroreduction

Metal-doped carbon materials, as one of the most important electrocatalytic catalysts for CO₂ reduction reaction (CO₂RR), have attracted increasing attention. Herein, a series of Cu cluster embedded highly porous nanofibers have been prepared through the carbonization of electro-spun MOF/PAN nanofibers. The obtained Cu cluster doped porous nanofibers possessed fibrous morphology, high porosity, conductivity, and uniformly dispersed Cu clusters, which could be applied as promising CO₂RR catalysts. Specifically, best of them, MCP-500 exhibited high catalytic performance for CO₂RR, in which the Faradaic efficiency of CO (FE_CO) was as high as 98% at ?0.8 V and maintained above 95% after 10 h continuous electrocatalysis. The high performance might be attributed to the synergistic effect of tremendously layered graphene skeleton and uniformly dispersed Cu clusters that could largely promote the electron conductivity, mass transfer and catalytic activity during the electrocatalytic CO₂RR process. This attempt will provide a new idea to design highly active CO₂RR electrocatalyst.     

Fractal approach to the CO oxidation on silica porous materials

We present an approach establishing a relation between the activation energy of heterogeneous catalytic processes and the fractal dimension of a catalyst. The approach is verified by experimental study of the CO oxidation on various porous silica and zeolite NaX. The fractal dimension of a catalyst (D_F) was calculated from the nitrogen adsorption isotherms. Our results indicate that the activation energy increases with increasing the fractal dimension of a catalyst. We show a good correspondence between theoretical and experimental results.     

Modeling of heterogeneous catalysts based on silica and zeolites by the hybrid quantum chemical embedded cluster method

The review is dedicated to the elaboration and application of hybrid quantum mechani-cal/molecular mechanical methods for heterogeneous catalytic systems, including single atoms and clusters of transition metals immobilized on covalent oxide supports. The following issues are considered: (1) elaboration of the hybrid covEPE method for modeling of covalent sys-tems of the zeolite and silicate types, (2) computations of the properties of atoms and small titanium, rhodium, iridium, and gold clusters localized in cavities or embedded in the zeolite framework, and (3) computations of small silver and tantalum clusters anchored at the dehydr-oxylated and hydroxylated silica surfaces. The calculations were performed by the density functional theory (DFT) with the Becke—Perdew (BP) exchange-correlation potential.     

New approach to fabricate an extremely super‐amphiphobic surface based on fluorinated silica nanoparticles

Superoleophobic surfaces possessing static contact angles greater than 140° with organic liquids are extremely rare. A simple approach has been developed to fabricate an extremely superamphiphobic coating material based on fluorinated silica nanoparticles resulting contact angles of water and diiodomethane at 167.5° and 158.6°, respectively. The contact angle of diiodomethane at 158.6° is substantially higher than the highest literature reported value we know of at 110°. In addition, this developed film also possesses extremely high contact angles with other organic liquids such as soybean oil (146.6°), decahysronaphthalene (142.5°), diesel fuel (140.4°), and xylene (140.5°). This developed superamphiphobic organic–inorganic hybrid film possesses unique liquid repellency for both water and organic liquids that can be used as functional coatings on numerous substrates by a simple coating process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1984–1990, 2008     

Polyethylenimine-impregnated mesoporous silica: effect of amine loading and surface alkyl chains on CO2 adsorption

Poly(ethyleneimine) (PEI) supported on pore-expanded MCM-41 whose surface is covered with a layer of long-alkyl chains was found to be a more efficient CO(2) adsorbent than PEI supported on the corresponding calcined silica and all PEI-impregnated materials reported in the literature. The layer of surface alkyl chains plays an important role in enhancing the dispersion of PEI, thus decreasing the diffusion resistance. It was also found that at low temperature, adsorbents with relatively low PEI contents are more efficient than their highly loaded counterparts because of the increased adsorption rate. Extensive CO(2) adsorption-desorption cycling showed that the use of humidified feed and purge gases affords materials with enhanced stability, despite limited loss due to amine evaporation.     

Hydrolysis of a two-membered silica ring on the amorphous silica surface

We have combined density functional theory (DFT) with classical interatomic potential functions to model hydrolysis of amorphous silica surfaces. The water-silica interaction is described by DFT with incorporation of a long-range elastic field described by classical interatomic potentials. Both physisorption and chemisorption of water on a surface site, known as the two-membered silica ring, are studied in detail. The hybrid quantum-mechanical and classical mechanical method enables more realistic treatment of chemical processes on an extended surface than previous methods. We have studied cooperative events in the hydrolytic reactions and discovered a new reaction pathway that involves a double proton transfer process. In addition, the evaluation of the total energy in a hybrid quantum-mechanical and classical mechanical system is discussed.     

Quantum-chemical analysis of the cluster models of coordination of imidazoline nitroxides on the silica surface

The results of ESR-spectroscopic and quantum-chemical studies of the coordination of 2,2,4,5,5-pentamethyl-3-imidazoline-N-oxyl and 2-phenyl-2,4,5,5-tetramethyl-3-imidazoline-N-oxyl with the surface acid sites (AS) of silica are analyzed. Cluster models of AS, corresponding to one- and two-point coordination and accepted in radiospectroscopy on the basis ofg-factors and constants of hyperfine splitting at the N nuclei in the resulting donor-acceptor complexes, are discussed. Within the framework of the unrestricted Hartree-Fock method using the MNDO approach, a comparative quantum-chemical analysis of the structural, spin, electrostatic, energy, and radiospectroscopic characteristics of the coordination of the model cluster AS to the imidazoline nitroxide probes is performed. For the two-center adsorption, three types of AS structures are considered, together with the paramagnetic surface complexes they form. A consistent semi-quantitative interpretation of the whole set of features found experimentally for the coordination of imidazoline nitroxides to the surface AS on silica is given. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 6, pp. 1081–1086, June, 1998.     

Interface structure of silicon nanocrystals embedded in an amorphous silica matrix

We performed molecular dynamics simulations of the atomic structure of silicon nanocrystals embedded in a stoichiometric amorphous silica matrix. The atom–atom interactions are described by a combination of well-assessed potentials for bulk silicon and SiO₂, plus a mixing term to allow adjusting the charge transfer at the interface between Si and silica. For the free-standing Si nanocrystals, we find that the spherical structure is favoured with respect to the faceted one, up to at least a diameter of 6 nm. Correspondingly, the surface layer shows a higher diffusivity than the bulk. When embedded in the silica matrix, nanocrystals are under severe mechanical stress which is released by the combined formation of porosity at the interface and of bridging Si–O–Si bonds, whose density increases with the nanocrystal size. Vibrational frequencies specific to the interface bonding are identified and discussed.     

Gold cluster formation on a fullerene surface

The growth of Au clusters on a fullerene thin film was investigated by in situ photoelectron spectroscopy in the ultraviolet (UPS) and x-ray (XPS) regime. Due to its highly corrugated surface fullerene films provide a wide range of bonding sites which could be exploited as molecular templates and serve to create a cluster superstructure. To gain insight into the fullerene-Au interaction two types of experiments were performed: (i) the deposition of Au on a fullerene surface, and (ii) the deposition of fullerenes on a Au surface. In both experiments an island growth mode is observed. The deposition of submonolayer amounts of C60 onto a gold film showed that the main interaction of the two species is due to chemisorption of the first C60 monolayer. In addition a constant band bending in the fullerene film is detected, but the UPS valence-band spectra show that there is no charge transfer from the Au to the C60 lowest unoccupied molecular orbital. In the reverse experiment, the cluster growth of Au on the corrugated C60 surface, the analysis of the Au core level does not reveal a specific bonding or nucleation site for Au atoms and clusters. This is in contrast to observations with Si clusters, which prefer to reside in the troughs between the fullerene molecules. The Au clusters grow continually from a size of about 55 atoms for the early stages of growth up to 150 atoms for the deposition of a nominal coverage of 1.5 nm. These data are derived from an analysis of the d-band splitting and the Au 4f core-level shift due to delayed photohole relaxation. The thermal stability of the Au-clusters-covered fullerene film was investigated by annealing in situ up to temperatures of 650 degrees C. For temperatures up to 450 degrees C a continuous growth of the clusters is detected, which is accompanied by a slight drop in Au concentration in the range of XPS for annealing temperatures higher than 350 degrees C. This may be due to a ripening of the clusters. The presence of Au apparently delays fullerene sublimation. The film shows a very good thermal stability and even after annealing at 650 degrees C there is still a fullerene film detectable in the photoelectron spectroscopy spectra.     

Electrochemical control of CO/NO ligand exchange in a triruthenium cluster monolayer assembled on a gold electrode surface

A highly selective ligand exchange reaction is realized in the self-assembled monolayer (SAM) of a triruthenium cluster on a gold electrode surface under precise electrochemical potential control. CO as well as NO molecules, which are known to play key roles in many chemical, biological, and environmental systems, can be efficiently introduced into the SAM by electrochemically tuning the electronic state of the Ru site. These unique surface reactions are more convenient and efficient than conventional ligand exchange reactions in solution and could be used for the elucidation of the electron-transfer mechanism in a biological system as well as in the development of molecular sensors and devices.     

Single-step dispersion of functionalities on a silica surface

A new process for coating a mesoporous silica gel with a mixture of the grafting reagents para-aminophenyltrimethoxysilane and phenyltrimethoxysilane is thoroughly analyzed. The dilution of para-aminophenylsilane with phenylsilane at different ratios allows the density of the functional amino groups present on the silica surface to be controlled, while keeping constant the overall number of grafts. Furthermore, the choice of a rigid linker prevents undesirable interactions between the active function and the inorganic support that could alter the function reactivity. This simple and new method, which results in the improvement of the dispersion of a functionality in a one-pot synthesis, could be particularly interesting in the field of supported catalysis and molecular recognition. The dispersion of the functional groups of the synthesized hybrid solids is investigated using a pyrene derivative covalently linked to the free amino groups of the para-aminophenylsilanes by analyzing the excimer and monomer fluorescence properties of the probe.     

High surface area nanoporous amorphous silica prepared by dodecanol assisted silica formate sol-gel approach

A simple polycondensation of monocarboxylic acids with silicon alkoxides led to transparent silica gels mainly comprised of silicate species of closed structures. This 'sol-gel formic acid' approach was modified by trapping an organic template (dodecanol) inside the silicate network during the polymerization process. Using this templating approach, porous silica of extremely high surface area, was produced in contrast to non-porous silica obtained by non-templating approach. The S(BET) surface areas of the template assisted samples resulting from the entire pores were found to be up to 725 m(2)/g. The total pore volumes of the samples were in the range of 0.40-0.74 cc/g in which micropore volumes were about 0.15-0.25 cc/g; the porosity depending on the reactants molar ratios of dodecanol, silicon alkoxide and formic acid.     

Molecular orbital cluster model study of bonding and vibrations of CO adsorbed on MgO surface

The interaction of CO with the MgO(100) surface has been investigated by means of all electron cluster model calculations. The CO molecule is bound on the Mg²⁺ site of MgO with a chemisorption energy of about 0.2 eV. The binding mechanism is electrostatic in nature and arises almost entirely from the interaction of the weak electric field generated by the ionic surface and the CO charge distribution, with negligible contributions from chemical effects as the CO σ donation. When CO is bound through carbon, its vibrational frequency increases with respect to the gas-phase value. This shift, Δ, has been analyzed and decomposed into the sum of different contributions. It is found that the positive Δω does not arise entirely from the field–dipole interaction but is due, in part, to the increase in Pauli repulsion occurring when the C? O molecule vibrates in the presence of the surface “wall.” A stronger electrostatic interaction, bringing the CO adsorbate closer to the surface, increases this wall effect and results in a more pronounced positive ω shift. It is also found that the two CO orientations exhibit opposite shifts in ω_e, thus, the two orientations can be distinguished, in principle, by IR spectroscopy. The analysis of our ab initio cluster wave functions gives a very different picture than the standard view of the metal–CO bond as arising from σ donation and π back donation.     

Grafting of chains organo-silane on silica surface: a quantum chemical investigation

Theoretical calculations have been carried out on the grafting of two chains organo-silane compounds on SiO₂ hydroxylated solid surfaces. Considering two different silylated coupling agents, two grafting stable complexes are obtained. These complexes are stabilized by two interactions: (i) the chain is grafted to the cluster with a covalent bond SiOSi; (ii) the chain interacts with the cluster via an hydrogen bond HOO in the other side of the chain. The electronic, geometrical and vibrational properties of these systems are analysed. These results give new insight about the grafting of long chains organo-silane on silica surfaces.     

Properties of 8-hydroxyquinoline immobilized on a silica surface

The ionization constants of benzeneazo-8-hydroxyquinoline grafted to a silica surface were determined by potentiometric titration. The spectrophotometric measurements showed that with increase in pH, deprotonation of the grafted 8-hydroxyquinoline nitrogen takes place. The dynamic capacity of the sorbent obtained with respect to the Cu²⁺, Co²⁺, and Ni²⁺ ions at various pH was determined. The immobilized benzeneazo-8-hydroxyquinoline is suitable for removing trace amounts of metal ions from solutions and for their chromatographic separation. It was found that the metal complexes of Cu²⁺ and Co²⁺ with grafted benzeneazo-8-hydroxyquinoline exhibit catalytic activity in the oxidation of hydrazine by molecular oxygen in aqueous solutions.Translated from Teoreticheskaya i Ékperimental'naya Khimiya, Vol. 25, No. 1, pp. 108–112, January–February, 1989.     

The adsorption and dissociation of Cl2 on the MgO (001) surface with vacancies: embedded cluster model study

The adsorption of Cl(2) at a low-coordinated oxygen site (edge or corner site) and vacancy site (terrace, edge, corner F, F(+), or F(2+) center) has been studied by the density functional method, in conjunction with the embedded cluster models. First, we have studied the adsorption of Cl(2) at the edge and corner oxygen sites and the results show that Cl(2), energetically, is inclined to adsorb at the corner oxygen site. Moreover, similar to the most advantageous adsorption mode for Cl(2) on the MgO (001) perfect surface, the most favorable adsorption occurs when Cl(2) approaches the corner oxygen site along the normal direction. A small amount of electrons are transferred from the substrate to the antibonding orbital of the adsorbate, leading to the Cl-Cl bond strength weakened a little. Regarding Cl(2) adsorption at the oxygen vacancy site (F, F(+), or F(2+) center), both large adsorption energies and rather much elongation of the Cl-Cl bond length have been obtained, in particular at the corner oxygen vacancy site, with concurrently large amounts of electrons transferred from the substrate to the antibonding orbital of Cl(2). It suggests, at the oxygen vacancy site, that Cl(2) prefers to dissociate into Cl subspecies. And the potential energy surface indicates that the dissociation process of molecular Cl(2) to atomic Cl is virtually barrierless.     

Simulation of water cluster assembly on a graphite surface

The assembly of small water clusters (H2O)n, n = 1-6, on a graphite surface is studied using a density functional tight-binding method complemented with an empirical van der Waals force correction, with confirmation using second-order M?ller-Plesset perturbation theory. It is shown that the optimized geometry of the water hexamer may change its original structure to an isoenergy one when interacting with a graphite surface in some specific orientation, while the smaller water cluster will maintain its cyclic or linear configurations (for the water dimer). The binding energy of water clusters interacting with graphite is dependent on the number of water molecules that form hydrogen bonds, but is independent of the water cluster size. These physically adsorbed water clusters show little change in their IR peak position and leave an almost perfect graphite surface.