Amorphous Silica Surface Research Articles

Application of density functional theory to equilibrium adsorption of argon and nitrogen on amorphous silica surface

We present a new version of non-local density functional theory (NL-DFT) adapted to description of vapor adsorption isotherms on amorphous materials like non-porous silica. The novel feature of this approach is that it accounts for the roughness of adsorbent surface. The solid–fluid interaction is described in the same framework as in the case of fluid–fluid interactions, using the Weeks–Chandler–Andersen (WCA) scheme and the Carnahan–Starling (CS) equation for attractive and repulsive parts of the Helmholtz free energy, respectively. Application to nitrogen and argon adsorption isotherms on non-porous silica LiChrospher Si-1000 at their boiling points, recently published by Jaroniec and co-workers, has shown an excellent correlative ability of our approach over the complete range of pressures, which suggests that the surface roughness is mostly the reason for the observed behavior of adsorption isotherms. From the analysis of these data, we found that in the case of nitrogen adsorption short-range interactions between oxygen atoms on the silica surface and quadrupole of nitrogen molecules play an important role. The approach presented in this paper may be further used in quantitative analysis of adsorption and desorption isotherms in cylindrical pores such as MCM-41 and carbon nanotubes.

A Comparison of Polyvinylpolypyrrolidone (PVPP), Silica Xerogel and a Polyvinylpyrrolidone (PVP)-Silica Co-Product for Their Ability to Remove Polyphenols from Beer

Proanthocyanidins have been characterized and levels compared in an ale and a lager using normal-phase liquid chromatography-mass spectroscopy (LC/MS) after a pre-concentration step employing Sephadex LH-20 chromatography. Twenty proanthocyanidins were identified including both non-galloylated and galloylated forms of the monomers, dimers and trimers. Oligomers greater than trimer were not detected. A proportion of all classes of proanthocyanidin adsorbed non-specifically to a silica xerogel product, but polyvinylpolypyrrolidone (PVPP) and a product that comprises polyvinylpyrrolidone (PVP) on a surface of amorphous silica displayed greater capability (i.e. specificity) for binding these materials. Of the two specific adsorbents, it was the PVPP that had the greater binding capability compared to the PVP-silica co-product.

Amorphous silica as a versatile supermolecular ligand for Ni(II) amine complexes: toward interfacial molecular recognition.

Selective adsorption of Ni(II) amine complexes used as precursors for supported catalysts was studied on amorphous silica surfaces. The nature of the adsorption sites was probed by [Ni(en)(dien) (H2O)]2+, [Ni(en)2(H2O)2]2+, and [Ni(dien)(H2O)3]2+ (en = ethylenediamine, dien = diethylenetriamine), which respectively contain one, two, and three labile aqua ligands. The silica surface acts as a mono- or polydentate ligand that can substitute the aqua ligands of the Ni(II) complexes in an inner-sphere adsorption mechanism. Room-temperature adsorption isotherms indicate that each nickel complex selects a limited number of adsorption sites; different sites are recognised by the three complexes, even though they have the same charge and comparable sizes. Several spectroscopic techniques (UV/Vis/NIR, EXAFS, and 29Si NMR) were used to confirm the selective character of the interaction of Ni(II) amine complexes with the silica surface. The specific sites include both silanol/silanolate groups in the same number as the original labile ligands and other surface groups that probably act as hydrogen-bond acceptors. These two types of groups cooperate to result in interfacial molecular-recognition phenomena with interactional complementarity.

Surface Modification of Inorganic Oxide Particles with a Carbene Complex of Palladium: A Recyclable Catalyst for the Suzuki Reaction

The reaction of 1-(2,4,6-trimethylbenzyl)-3-(propyltriethoxysilane)imidazolidinium chloride, with the Pd(OAc)2 complex proceeded smoothly in DMSO to give the bis[1-(2,4,6-trimethylbenzyl)-3-(propyltriethoxysilane)imidazolidin-2-ylidene]dichloropalladium(II) complex that was grafted onto the surface of amorphous silica. The effects of various factors, such as reaction temperature and time, on the grafting efficiency was evaluated. Inorganic oxide particles that were modified with a carbene complex of palladium(II) were tested for catalytic activity in the Suzuki coupling reaction between phenylboronic acid and several aryl halides.

Sodium diffusion through amorphous silica surfaces: a molecular dynamics study.

We have studied the diffusion inside the silica network of sodium atoms initially located outside the surfaces of an amorphous silica film. We have focused our attention on structural and dynamical quantities, and we have found that the local environment of the sodium atoms is close to the local environment of the sodium atoms inside bulk sodo-silicate glasses obtained by quench. This is in agreement with recent experimental results.

Paramagnetic species on catalytic surfaces—DFT investigations into structure sensitivity of the hyperfine coupling constants

Structure sensitivity of the hyperfine coupling constants was investigated by means of DFT calculations for selected surface paramagnetic species. A CH 2OH radical trapped on silica and intrazeolite copper nitrosyl adducts encaged in ZSM-5 were taken as the examples. The surface of amorphous silica was modeled with a [Si 5O 8H 10] cluster, whereas the zeolite hosting sites were epitomized by [Si 4AlO 5(OH) 10] − cluster. Three different coordination modes of the CH 2OH radical were considered and the isotropic 13 C and 1 H hyperfine constants of the resultant van der Waals complexes, calculated with B3LYP/6-311G(d), were discussed in terms of the angular deformations caused by hydrogen bonds with the cluster. The magnetic parameters of the η 1-N{CuNO} 11 and η 1-O{CuNO} 11 linkage isomers were calculated at the BPW91/LanL2DZ and 6-311G(df) level. For the most stable η 1-N adduct a clear dependence of the spin density distribution within the CuNO moiety on changes in the CuNO angle and the CuN bond distance was observed and accounted for by varying spin polarization and delocalization contributions.

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.

Ni and CO Used as Probes of the Amorphous Silica Surface: IR and Theoretical Studies of Dicarbonyl NiII Complexes

This paper is the continuation of a study of NiII−(CO)n complexes formed on amorphous silica. Both Ni and CO are used as successive probes of the local structure and reactivity of the silica surface. An isolated tricoordinated NiII3c ion in a single oxidation state is characterized with CO as a probe of the coordination vacancies of the transition metal. An experimental FTIR study shows two main types of NiII dicarbonyl with equivalent and nonequivalent COs formed from the NiII monocarbonyl previously characterized in J. Am. Chem. Soc. 2002, 24, 7210−7217, whereas no tricarbonyl NiII complex is observed. Theoretical models are tested using experimental CO frequencies: DFT calculations with the cluster approach already used for the NiII monocarbonyl complex allow us to gain valuable information on the local environment of the NiII ion. For NiII dicarbonyls, information can be obtained on the second coordination sphere of NiII (namely, Si atoms) whereas for monocarbonyls, only first oxygen neighbors were c...

The effects of Mg 2+ and H + on apatite nucleation at silica surfaces

Apatite (Ca 10[PO 4] 6[OH] 2) precipitation from aqueous solutions involves multiple steps, such as nucleation of a precursor calcium phosphate phase, cluster aggregation, crystal growth, and transformation to apatite, which are affected by the presence of other ions in solution. I report the mechanisms by which two ions common in natural solutions, Mg 2+ and H +, affect heterogeneous calcium phosphate nucleation on the amorphous silica surface.

Chemisorption of HSi(OH) 3 on silica surfaces: an ab initio periodic study

The study of the grafting of trialkoxysilane R′Si(OR) 3 molecules on amorphous silica has been undertaken at the Hartree–Fock level using a biperiodic model for the surface. Different types of slab cut out from the model system Edingtonite (a tetragonal silica structure with five SiO 2 groups per unit cell) have been used to simulate isolated and interacting silanol sites at the amorphous silica surface, while only the simple case of HSi(OH) 3 has been considered for the interacting molecule. In a first step, for each type of surface the geometrical parameters have been optimised and the surface formation energy determined. The geometrical structure of the grafted molecule is compared with that of the isolated one. The geometrical strains of the surfaces with either isolated or interacting silanols are also compared before and after the grafting reactions. The calculated values of the chemisorption energies show that the grafting process is favored on isolated silanols only if correlation effects are included.

Ab initio simulations of hydroxylation anddehydroxylation reactions at surfaces: amorphous silica andbrucite

Hydroxylation and dehydroxylation reactions at the surfaces ofamorphous silica and Mg(OH)2 have been studied by ab initioCar-Parrinello molecular dynamics simulations. A particulartopological defect on the amorphous silica surface, thetwo-membered (2M) silicon ring, is shown to react fast with waterthus confirming the experimental assignment of the most reactivesurface sites. The mechanism of the chemisorption of bothwater and ammonia on the 2M ring has been identified.Ab initio simulations on Mg(OH)2 have provided information onthe structural properties of the low-indices surfaces (0001),(11̄00) and (1000) of brucite. Comparison of the calculateddehydroxylation energy at surfaces and in the bulk suggests thatthe decomposition of brucite into MgO and water wouldpreferentially occur at surfaces.

Classical and ab-initio molecular dynamic simulation of an amorphous silica surface

We present the results of a classical molecular dynamic simulation as well as of an ab initio molecular dynamic simulation of an amorphous silica surface. In the case of the classical simulation we use the potential proposed by van Beest et al. (BKS) whereas the ab initio simulation is done with a Car-Parrinello method (CPMD). We find that the surfaces generated by BKS have a higher concentration of defects (e.g. concentration of two-membered rings) than those generated with CPMD. In addition also the distribution functions of the angles and of the distances are different for the short rings. Hence we conclude that whereas the BKS potential is able to reproduce correctly the surface on the length scale beyond approx 5 Angstroems, it is necessary to use an ab initio method to predict reliably the structure at small scales.

Structural properties of silicon dioxide thin films densified by medium-energy particles

Classical molecular-dynamics simulations have been carried out to investigate densification mechanisms in silicon dioxide thin films deposited on an amorphous silica surface, according to a simplified ion-beam assisted deposition scenario. We compare the structures resulting from the deposition of near-thermal (1 eV) ${\mathrm{SiO}}_{2}$ particles to those obtained with increasing fraction of 30 eV ${\mathrm{SiO}}_{2}$ particles. Our results show that there is an energy interval---between 12 and 15 eV per condensing ${\mathrm{SiO}}_{2}$ unit, on average---for which the growth leads to a dense, low-stress amorphous structure, in satisfactory agreement with the results of low-energy ion-beam experiments. We also find that the crossover between low- and high-density films is associated with a tensile-to-compressive stress transition, and a simultaneous healing of structural defects of the $a\ensuremath{-}{\mathrm{SiO}}_{2}$ network, namely, threefold and fourfold rings. It is observed, finally, that densification proceeds through significant changes at intermediate length scales $(4--10 \mathrm{\AA{}}\mathrm{}),$ leaving essentially unchanged the ``building blocks'' of the network, viz. the $\mathrm{Si}({\mathrm{O}}_{1/2}{)}_{4}$ tetrahedra. This latter result is in qualitative agreement with the mechanism proposed to explain the irreversible densification of amorphous silica recovered from high pressures $(\ensuremath{\sim}15--20 \mathrm{GPa}).$

Simulations of the structure and properties of amorphous silica surfaces

The structure and transport properties of solid surfaces have been described using models of varying complexity and rigor without systematic comparisons among available methods. Here, we describe the surface of amorphous silica using four techniques: (1) an ordered surface created by cutting the structure of a known silica polymorph ( α-cristobalite); (2) an unrelaxed amorphous surface obtained by cutting bulk amorphous silica structures created by molecular dynamics methods; (3) a relaxed amorphous surface created by relaxing the amorphous surface; and (4) a random surface created by Monte Carlo sphere packing methods. Calculations of the adsorption potential surface and simulation of the surface diffusion of weakly bound adsorbates (N 2, Ar, CH 4) interacting via Lennard–Jones potentials with these surfaces were used to compare surface models and to judge their fidelity by comparisons with available experimental values. Similar heats of adsorption were obtained on the relaxed, unrelaxed, and random surfaces ( ±0.5 kJ/mol ), but the relaxed surface showed greater heterogeneity with a wider distribution of adsorption energies. Surface diffusion on the relaxed surface was slower than on the other surfaces, with slightly higher activation energies (0.5– 1.0 kJ/mol ). Rigorous comparisons between simulated and experimental surface diffusivities are not possible, because of scarce surface diffusion data on well-characterized surfaces. The values obtained from simulations on silica were similar to experimental surface diffusivities reported on borosilicate glasses.

Modeling of hydrophilic wafer bonding by molecular dynamics simulations

The role of moisture in hydrophilic wafer bonding was modeled using molecular dynamics computer simulations of interface formation between amorphous silica surfaces. Three different surface treatments were used in order to determine the effect of moisture on the formation of siloxane (Si–O–Si) bridges across the interface at two temperatures. The three surface conditions that were studied were: (a) wet interfaces containing 1 monolayer of water adsorbed at the interface (based on the room temperature bulk density of water), (b) hydroxylated interfaces with concentrations of 3–5 silanols/nm2 on each surface and no excess water molecules initially in the system, and (c) pristine interfaces that had only Si and O and no water or H present. The surfaces were slowly brought together and siloxane bond formation was monitored. In the pristine interfaces, siloxane bridges formed across the interface by the coalescence of various defect species in each surface. A bimodal distribution of siloxane bond angles formed during the first 2.5 Å of approach after the first siloxane bond was formed. These bond angles were much lower than and higher than the bulk average, indicating the formation of less stable bonds. The hydroxylated (with no excess water) and wet surfaces showed a more uniform distribution of siloxane bond angles, with no highly reactive small bond angles forming. The presence of water molecules enhanced H-bond formation across the interface, but trapped water molecules inhibited formation of the strong siloxane bridges across the interface. In real systems, high temperatures are required to remove this trapped moisture.

Structure and dynamics of amorphous silica surfaces

We use molecular dynamics computer simulations to study the equilibrium properties of the surface of amorphous silica. Two types of geometries are investigated: (i) clusters with different diameters (13.5, 19, and 26.5 Å) and (ii) a thin film with thickness 29 Å. We find that the shape of the clusters is independent of temperature and that it becomes more spherical with increasing size. The surface energy is in qualitative agreement with the experimental value for the surface tension. The density distribution function shows a small peak just below the surface, the origin of which is traced back to a local chemical ordering at the surface. Close to the surface the partial radial distribution functions as well as the distributions of the bond–bond angles show features which are not observed in the interior of the systems. By calculating the distribution of the length of the Si–O rings we can show that these additional features are related to the presence of two-membered rings at the surface. The surface density of these structures is around 0.6/nm2, in good agreement with experimental estimates. From the behavior of the mean-squared displacement at low temperatures, we conclude that at the surface the cage of the particles is larger than the one in the bulk. Close to the surface the diffusion constant is somewhat larger than the one in the bulk and with decreasing temperature the relative difference grows. The total vibrational density of states at the surface is similar to the one in the bulk. However, if only the one for the silicon atoms is considered, significant differences are found.

Photoabsorption of dioxasilyrane and silanone groups at the surface of silica

The lowest singlet→singlet and singlet→triplet transitions of two point defects observed at the surface of amorphous silica, the dioxasilyrane group, =Si(O2), and the silanone group, =Si=O, have been computed by means of first-principles quantum chemical techniques using cluster models of various sizes. The lowest allowed transitions of the dioxasilyrane ring are found at ≈3 and ≈5 eV; the first excitation in silanone is computed at 5.7 eV. These values are in quantitative agreement with the observed bands and allow the unambiguous assignment of the observed optical transitions in the surface of activated silica to the =Si(O2) and =Si=O point defects.

Surface modification of mesoporous, macroporous, and amorphous silica with catalytically active polyoxometalate clusters.

Transition-metal-substituted polyoxometalates (TMSP) of the type [MII(H2O)PW11O39]5- (M = Co, Zn) and [SiW9O37(CoII(H2O))3]10- have been chemically anchored to modified macroporous (400 nm pores), mesoporous (2.8 nm pores), and amorphous silica surfaces. The materials were characterized by solid-state 31P MAS NMR, UV-vis, FT-IR spectroscopy, and N2 adsorption experiments to verify cluster attachment and the structure of the TMSP on the support. On the basis of the spectroscopic evidence, clusters were attached datively to the surface, and the integrity of the [CoPW11] cluster was maintained for nonaqueous impregnation with TBA5[CoPW11]; partial degradation of the cluster occurred when it was impregnated from aqueous solution using the K5[CoPW11] salt. Both the amine surface groups and the cobalt centers in the clusters were found to be necessary to prevent cluster loss during washing or reaction processes. The catalytic activities of the supported TMSP clusters were tested by the epoxidation of cyclohexene to cyclohexene oxide in the presence of isobutyraldehyde. The percent conversion of the substrate and the amount of product formed per unit time were similar for [CoPW11] clusters on each of the three silica supports, but slightly lower than for purely homogeneous reactions. [SiW9Co3] clusters with three available cobalt centers exhibited higher catalytic activity with nearly identical performance on a silica support or in homogeneous solution.

Hydrolysis of the amorphous silica surface. II. Calculation of activation barriers and mechanisms

Using a previously derived model of the dry, amorphous, hydrophilic SiO2 surface, the reactivity of generic defect sites on the surface with respect to water, and the local network rearrangement that accompanies hydrolysis at these sites, is investigated using cluster models. Ab initio methods are used to calculate reaction barriers and reaction pathways. Consequences of the various types of hydrolysis product found are discussed with reference to potential sites for polymer chemisorption on the hydrolyzed, amorphous SiO2 surface.

Hydrolysis of the amorphous silica surface. I. Structure and dynamics of the dry surface

Amorphous SiO2 surfaces are generated from bulk–liquid configurations using simulations employing a polarizable-ion model. The surfaces are characterized in terms of the ion environments as a function of depth into the surface. Comparison is made to previous simulation studies and subtle differences are highlighted and attributed to differences in the potential models. The connectivity of the surface sites is established with a view to investigating the hydrolysis of this surface. Dynamical properties are calculated using a simple projected velocity time correlation function and normal mode analysis and compared to the simulated bulk and experimental bulk and surface spectra.