Three-coordinated Sites Research Articles

Aluminum Impregnation into Mesoporous Silica Molecular Sieves for Catalytic Application to Friedel–Crafts Alkylation

The catalytic activity of mesoporous molecular sieves for Friedel–Crafts alkylation of benzene, toluene, and m-xylene with benzyl alcohol was measured with three different types of materials, KIT-1 (disordered three-dimensional channel network), MCM-41 (hexagonal packing of one-dimensional channels), and MCM-48 (cubic Ia3d structure composed of two independent three-dimensional channel systems). Aluminum was incorporated into the mesoporous materials by various routes such as direct syntheses, grafting of the Al species with anhydrous AlCl3, and impregnation with an aqueous solution of AlCl3. The catalytic activity was analyzed with respect to the effects of the preparation methods, channel structures, and Al content. It turned out that postsynthesis impregnation was the most effective means for increasing the catalytic activity. The three types of mesoporous materials exhibited approximately the same surface character with regard to the catalytic reaction, independent of the different structures and synthesis conditions. Magic angle spinning 27Al NMR spectroscopy of the impregnated samples indicated that the coordination of the Al sites thus generated lost the tetrahedral symmetry upon complete dehydration, supporting the possibility of three-coordinated catalytic sites.

The structure of oxygen adsorbed on Mo(100) studied by high-reolution electron energy-loss spectroscopy

Oxygen-modified Mo(100) surfaces with coverages from 0.8 to 1.4 monolayers (ML), prepared by high-temperature annealing, have been investigated using high-resolution electron energy loss spectroscopy (HREELS). The surface with 0.8 ML coverage, a ( 5 × 5 )- R26°33′ low-energy electron diffraction (LEED) pattern, produces a three-peak pattern associated with modes derived from oxygen located in a three-coordinate site. With increasing oxygen coverage up to 1.1 ML and a change in the LEED pattern to (2×1), the three-peak pattern persists which indicates that the three-coordinate O/Mo structural motif is preserved. As the oxygen coverage exceeds 1 ML, an additional loss peak due to oxygen located on an atop site appears at 985 cm −1. At 1.4 ML the HREELS spectrum is very similar to the vibrational spectrum of MoO 2, consistent with the formation of this oxide phase. Time-dependent HREELS measurements at 300 K and coverages ≤1 ML reveal a gradual clustering of adsorbed oxygen and relocation from three-coordinate sites to atop sites.

Identification of Sn variants of theE′center in Sn-dopedSiO2

Electron paramagnetic resonance data on x-ray irradiated Sn-doped ${\mathrm{SiO}}_{2}$ samples produced by the sol-gel method are presented. Two variants of Sn-related sites were identified, an orthorhombic one with ${g}_{1}=1.994,$ ${g}_{2}=1.986,$ and ${g}_{3}=1.975$ and an axial one with ${g}_{\ensuremath{\parallel}}=1.994$ and ${g}_{\ensuremath{\perp}}=1.977.$ A relation between g anisotropy and the spin-orbit coupling constant along the isoelectronic Si-Ge-Sn series strongly supports the attribution to ${E}^{\ensuremath{'}}\ensuremath{-}\mathrm{Sn}$ centers consisting of unpaired spins in ${\mathrm{sp}}^{3}$ orbitals of three-coordinated Sn sites. An asymmetric 215 mT doublet due to hyperfine interaction with Sn isotopes with non-null nuclear spin confirms this assignment.

Unimportance of the Surrounding Lattice in the Adsorption of CO on Low-Coordinated Mg Sites of MgO

The frequency shifts and binding energies of CO adsorbed on three-coordinated (3c) and four-coordinated (4c) Mg sites of MgO via the C-end are computed using bare, stoichiometric cluster models of progressively increasing size: CO·MgnOn, n = 4, 10, 13, 19 for the Mg3c site and n = 6, 10, 12, 20 for the Mg4c site. Calculations are performed by the SCF and DFT methods with a combined basis set which effectively provides the 6-311+G(2df) level of treatment. No marked dependence of the calculated properties on the model size is found. All the bare cluster models reproduce the related experimental frequency shifts and binding energies of CO on MgO to about the same accuracy as the embedded cluster models based on the total ion ab initio model potential (AIMP) formalism [Nygren, M. A.; Pettersson, L. G. M. J. Chem. Phys. 1996, 105, 9339]. Unlike the common assumption that the surrounding lattice makes a significant contribution to the adsorption characteristics of CO on Mg3c and Mg4c sites of MgO, these results suggest its role to be unimportant. The analysis of the electric field (EF) in the adsorption region of these sites of the bare MgO clusters allows us to conclude that, due to the very steep decrease of the EF on moving away from the surface, both the frequency shift and binding energy of CO are mainly defined by the EF in the CO lone pair subregion nearby the adsorbing Mg. This finding justifies the adequacy of the bare cluster models: as the EF in the vicinity of the adsorbing Mg should be dominated by the smallest stoichiometric cluster including the Mg and its nearest surroundings, the effect of the residual lattice is insignificant.

H2O Dissociation at Low-Coordinated Sites on (MgO)n Clusters, n = 4, 8

We invest the interactions on one and two H{sub 2}O molecules at three-coordinated sites of (MgO){sub 4} and (MgO){sub 8} clusters. Energies of adsorption are calculated and compared to similar work involving periodic and embedded cluster systems. The cluster model results predict that H{sub 2}O chemidissociates without a barrier directly onto adjacent three-coordinated sites. The chemidissociation energy per water molecule is observed to be greater upon the dissociation of two neighboring water molecules compared to the dissociation of a single water molecule: a positive cooperativity effect. Correlation energy treated at the MP2 level of theory reduces the predicted chemidissociation energy by approximately 10 kcal/mol, and it is found that the MP2 single-point energies evaluated at the RHF-optimized geometries, MP2//RHF, differ from the energy of the MP2-optimized structures by approximately 1 kcal/mol. 26 refs., 3 figs., 3 tabs.

Network Modification in Potassium Borate Glasses: Structural Studies with NMR and Raman Spectroscopies

We have examined the structural changes in the boron−oxygen network that result from chemical modification of glassy boron oxide by potassium oxide. By means of boron-11 dynamic angle spinning (DAS) NMR and Raman spectroscopies, we have identified changes in the structure resulting from conversion of three-coordinate boron to four-coordinate species. We observe coordination changes in two different 3-fold coordinate boron sites: those in six-membered boroxol rings and those in nonring BO3 units. The DAS NMR data show that the fraction of boron in nonring three-coordinate sites decreases as modifier is added. Modification of the boroxol rings is also evident in the Raman data. Assignment of the NMR resonances is made with the help of comparison to spectra of related crystalline potassium borates. Using 11B DAS NMR and 17O magic angle spinning (MAS) NMR, we do not detect significant formation of nonbridging oxygen through 35% added modifier. The changes in the boroxol ring breathing mode in the Raman spect...

Tensor LEED analyses for the (√3 × √3)R30° and c(4 × 2) structures formed by sulphur chemisorbed on the (111) surface of rhodium

Tensor LEED analyses have been made for the Rh(111)-(√3 × √3)R30°-S and Rh(111)-c(4 × 2)-S surface structures chemisorbed at 1 3 and 1 2 monolayer coverages respectively. For the lower-coverage form, S adsorbs on the regular three-coordi sites which continue the fcc packing sequence; the S-Rh bond lengths are indicated to equal 2.23 Å, and relaxations in the metallic structure are negligible. In the c(4 × 2) form, the adsorption occurs equally on both types of three-coordinate site (fcc and hcp), although some surface Rh atoms bond to two S atoms while others bond to just one, and this sets up some interesting relaxations. Specifically, the S atoms displace laterally from the centre of the three-fold sites by 0.20 to 0.29 Å, and the first metal layer is buckled by about 0.23 Å. The first-to-second interlayer spacing in the metal expands to 2.26 Å from the bulk value of 2.20 Å. The average S-Rh bond lengths equal 2.22 Å, and so they are not significantly changed from that in the low-coverage form. The structural evolution for S chemisorbed on the (111) surface of rhodium with increasing coverage is compared with the corresponding evolution on the Rh(110) surface.

Characterization of Metal Binding by a Designed Protein: Single Ligand Substitutions at a Tetrahedral Cys2His2 Site

The tetrahedral Cys2His2 Zn(II)-binding site in the de novo designed protein Z alpha 4 [Regan, L., & Clarke, N. D. (1990) Biochemistry 29, 10878] has been studied by independently mutating each of the metal-binding ligands to alanine. The contribution of each ligand to the geometry and affinity of metal binding has been characterized using Co(II), Zn(II), and Cd(II). The results indicate that all four ligands contribute to high-affinity metal binding in Z alpha 4. Two of the four metal-site mutants retain the tetrahedral Zn(II)-binding geometry of Z alpha 4, with one water molecule presumed to bind in the vacant ligand position. These mutants provide the first examples of a demonstrated de novo tetrahedral three-coordinate site designed into a protein and as such are a first step toward the design of catalytic rather than structural Zn(II) sites. One of the metal-site mutants binds Zn(II) with either tetrahedral four-coordinate or five-coordinate geometry, while the last ligand-to-alanine substitution abolishes tetrahedral binding. The importance of ligand type for metal-binding in Z alpha 4 was investigated by characterizing two ligand-swap mutants in which a cysteine residue was replaced with a histidine. In both cases, tetrahedral metal binding was lost. Collectively, these results affirm the strategy used to design Z alpha 4 by showing that all designed liganding residues are participating in metal binding, and by suggesting that the tetrahedral geometry of the binding site is perturbed when the designed side chain ligands are replaced with alternate ligands.

Location of an O atom in the Pd(110)c(2×4)-O structure. An EELS study

The Pd(110)c(2×4)-O surface has been investigated mainly by high-resolution electron energy loss spectroscopy. Two losses have been observed at 62 and 68 meV for the c(2×4) surface, which is consistent with the structure model involving substrate reconstruction into the (1×2) missing-row structure where the O atoms are located in the three-coordinated sites of the (111) microfacets to form zig-zag chains along the [110] Pd rows. The losses are attributed to the hindered motions of an O atom perpendicular and parallel to the (111) microfacet, respectively. A calculation of the vibrational energies of a Pd 3O cluster based on Wilson's GF matrix technique suggests that the first interlayer spacing is expanded compared to that of the bulk.

Chemisorption of CO on defect sites of MgO

Chemisorption of a CO molecule on regular and defect sites of the MgO(100) surface has been investigated by means of cluster model calculations. At all sites studied, CO bonds at the cation with the C atom closest to the surface. The bonding is considerably larger at a three-coordinated corner site than for a regular five-coordinated surface site. A blueshift in the C−O stretching frequency, ω e, of adsorbed CO compared to free CO is found; the shift is much higher for a corner than for a surface site because of the larger local electric field for low-coordinated cations. Both the bond strength and the ω shift are largely due to electrostatic effects and not to the formation of a dative σ-bond with the surface. Surface relaxation effects have also been considered.

Adsorption of methane on magnesium oxide surfaces under ultraviolet irradiation

CH4 adsorption on MgO under the influence of UV-irradiation near room temperature was studied, mainly using temperature-programmed desorption. In addition to an adsorbed species (type M1) observed in the dark adsorption, an adsorbed species (type IM), which was desorbed at a higher temperature range of 350–800 K than M1 in a TPD run, was found. The active site IM was formed by pretreatment of MgO above 973 K in vacuo, and the number of sites increased remarkably with increase in pretreatment temperature, although the maximum number of sites observed was still very small (1015 site m–2). Many similarities were found between M1 and IM adsorptions and both types of active site are believed to involve Mg2+LC and O2–LC ions, where LC (low coordination) refers mainly to 3C and partly 4C (three-coordinated and four-coordinated sites), on which CH4 is heterolytically dissociated into CH–3 and H+. However, the locations of both the LC ions are different for the two types of site; Mg2+LC and O2–LC are separately located in IM while they constitute the nearest lattice pair in M1. Therefore, the IM site can adsorb CH4 only under more severe adsorption condition; UV-irradiation causes activation of the site to produce O–LC from O2–LC prior to the adsorption process.

Electronic structure of dangling and floating bonds in amorphous silicon.

The electronic structure of amorphous silicon models has been calculated with the tight-binding parameters of Chadi. An energy gap of {similar to}1.0 eV is obtained for a fully four-coordinated model, whereas gap states occur for models with coordination defects. Dangling-bond gap states are well localized on the three-coordinated site. Floating bonds lead to considerably less localized gap states with wave-function amplitudes on the neighbors of the five-coordinated atom. The hyperfine splittings of these coordination defects are calculated and compared to experiment.

Model Hamiltonians in the study of chemisorption and catalysis

The MNDO model Hamiltonian can be derived from the exact non-relativistic one by a number of approximations which retain a mathematical and physical coherence. On this ground we have studied by the MNDO method some key surface problems, namely the surface reconstruction of diamond-like solids and the chemisorption of hydrogen on graphite, beryllium, diamond, and silicon. In the case of surface relaxation numerical results are in agreement with previous studies for the small X 4H 9, clusters and indicate that the Haneman model of 2 × 1 reconstruction could be equally valid for Si and diamond, but in the latter case the differences in vertical shifts are too small to be detected experimentally. The same good agreement with ab initio results is obtained for the chemisorption of hydrogen on graphite, beryllium, and the top sites of diamond and silicon. The only significant shortcoming occurs in the case of chemisorption on three-coordinated sites of silicon.

Neutron-scattering and volumetric study of hydrogen adsorbed and absorbed on Raney palladium

The adsorption and absorption of hydrogen on Raney palladium has been studied by inelastic neutron scattering and by volumetry. At low hydrogen pressure (2 mbar) several species are observed by neutron scattering: hydrogen adsorbed on a three-coordinated site, subsurface species and also the α- and β-hydrides. At higher hydrogen pressures (400 mbar) the intensity is dominated by scattering from the α- and β-hydrides. The ease of hydride formation on Raney palladium at low hydrogen pressure is accounted for by the residual aluminium, which acts as an alloy component. On the other hand, the solubility decreases at high hydrogen pressure, compared with pure palladium.

Hydrogen adsorption on platinum

The amplitude weighted vibrational density of states of hydrogen adsorbed at two- and three-coordinated sites on Pt(111) have been computed. Electron energy loss spectroscopy shows peaks at 550, 860 and 1230 cm −1 [6]. The peaks at 550 and 1230 cm −1 are assigned to vibrations of hydrogen at a two-coordinated site, with nearest neighbour distance R (1) HPt = 1.93 Å and force constant k (1) HPt = 0.8631×10 5 dyn cm −1. The value R (1) HPt =1.93 Å compares with the value R (1) HNi =1.84(±0.06) A determined by LEED for hydrogen on Ni(111), and the force constant is in good agreement with the average value k (1) HPt =1.08 × 10 5 dyn cm −1 obtained for bridged metal hydrides. Inelastic neutron scattering shows, in addition to modes at 500 and 1296 cm −1, peaks at 856 and 936 cm −1 corresponding to the weak feature observed at 860 cm −1 with EELS. These vibrations are assigned to hydrogen motion perpendicular and parallel to the surface at a three-coordinated site with bond length R (1) HPt =1.905 Å and force constant k (1) HPt =0.4866 ×10 5 dyn cm −1. This bond length compares with the value R (1) HPt =1.9 Å obtained from helium diffraction data for hydrogen at a three-coordinated site on Pt(111).

The vibration spectrum of hydrogen bound by nickel catalysts

The amplitude-weighted vibrational density of states of hydrogen adsorbed on nickel (100) and (111) surfaces is computed and compared with inelastic neutron scattering data on nickel catalysts. Local modes at 113 meV and 140 meV are interpreted as being due to vibrations of hydrogen parallel and perpendicular to the surface at a three-coordinated site respectively. A peak at 80 meV is interpreted as arising from vibrations of hydrogen perpendicular to the surface at a four-coordinated site. The relative intensities of the local modes in the data suggest that the three-coordinated site is the preferred site for adsorption on nickel catalysts. However the measured density of states below 300 cm-1 indicates some occupancy of four-coordinated sites in agreement with the existence of a peak at 80 meV.

Defective non-planar surfaces of MgO

Defect lattice and ab initio SCF calculations are reported for defective non-planar surfaces of MgO. It is suggested that the concentration of free vacancies and Ca 2+ will be much lower at non-planar irregularities than at planar surfaces and in the bulk, whereas the level of Li + and trapped holes will be higher. The binding of H atoms and CO to three-coordinated comer sites is found to be appreciably higher than to the five-coordinated {001} surface. Thus it is concluded that calcium doping of MgO will affect neither CO chemisorption nor the dissociative chemisorption of hydrogen whereas lithium doping will influence both.

Defective non-planar surfaces of MgO

Abstract Defect lattice and ab initio SCF calculations are reported for defective non-planar surfaces of MgO. It is suggested that the concentration of free vacancies and Ca2+ will be much lower at non-planar irregularities than at planar surfaces and in the bulk, whereas the level of Li+ and trapped holes will be higher. The binding of H atoms and CO to three-coordinated comer sites is found to be appreciably higher than to the five-coordinated {001} surface. Thus it is concluded that calcium doping of MgO will affect neither CO chemisorption nor the dissociative chemisorption of hydrogen whereas lithium doping will influence both.

Hydrogen chemisorption on Ni(110) by high-resolution electron energy loss spectroscopy

High-resolution electron energy loss spectra of hydrogen-covered Ni(110) surfaces both at 100 and 300 K are presented. The adsorbed sites of hydrogen atoms are discussed. High-resolution electron energy loss spectra of hydrogen covered Ni(110) surfaces have been studied. Tentative models for the adsorbed sites of hydrogen atoms are as follows: (1) For the (2 × 1)-H surface, hydrogen is adsorbed in the three-coordinated sites of the rudimentary (111) face of the unreconstructed Ni(110) substrate. (2) For the low-temperature (1 × 2)-H surface, hydrogen is adsorbed in the three-coordinated sites and, probably, in the two-fold hollow sites of the distorted Ni(110) substrate. (3) For the room-temperature (1 × 2)-H surface, hydrogen is disorderedly adsorbed in the three-coordinated, two-fold hollow and short-bridge sites and, possibly, in the octahedral sites of the distorted Ni(110) substrate. Some of the unresolved problems in the above assignments are summarized: (1) Strictly, the three-coordinated sites above are somewhat different from those discussed in the molecular-beam diffraction study [5]. (2) For the low-temperature (1 × 2)-H surface, the loss associated with hydrogen in the two-fold hollow sites is apparently not observed. (3) Intensity changes of the three losses for the room-temperature (1 × 2)-H surface with increasing hydrogen pressure (Fig. 2) are not well understood.

Mobile ion distribution and anharmonic thermal motion in fast ion conducting Cu2S

We have determined a unique model for the mobile copper ion disorder in hexagonal Cu2S between 120° and 325°C via single crystal neutron diffraction. The copper ions partially occupy two sets of three-coordinated sites within the HCP sulfur array and display anharmonic thermal motion. The results suggest that the conductivity is two dimensional in nature.