Role of titanium oxidation states in polymerization activity of Ziegler-Natta catalyst: A density functional study

The role of titanium oxidation states in olefin polymerization activity for Ziegler-Natta (ZN) catalyst has been investigated using density functional calculations at B3LYP/LANL2DZ as well as extended LANL2DZ basis that includes diffuse and polarization functions for C, H and Cl. Using the simple [TiCl₂CH₃]ⁿ (n = +1, 0, −1) model catalyst systems, we could rationalize some of the well-known experimental facts with varying Ti oxidation states (+4, +3, +2) in the real ZN systems. Firstly, irrespective of Ti oxidation states, the activation barriers (E_act) for ethylene and syn propylene insertion in Ti-CH₃ bond are comparable in accordance with experimental and modeling studies. Secondly, it was observed that Ti(IV) catalyst has the lowest E_act which progressively increase in the order Ti(IV) < Ti(III) < Ti(II) high spin < Ti(II) low spin catalysts in line with experimental and several modeling results. The effect of solvation on olefin insertion barriers are seen more prominent in case of Ti(IV) systems compared to other oxidation states.     

Relative brookite and anatase content in sol-gel-synthesized titanium dioxide nanoparticles

Sol-gel synthesis of titania typically produces a mixture of brookite and anatase. Rietveld refinements were used to systematically track the brookite content and particle size as functions of synthetic variables. Results demonstrate that brookite content and anatase particle size decrease with decreasing Ti/H(2)O ratios. In syntheses at pH 3, the addition of HCl resulted in increased amorphous content compared to samples synthesized using HNO(3). Similar amorphous contents were observed for particles prepared at pH 6-9. Hydrothermal aging for 4 h at 200 degrees C of sol-gel products containing substantial amorphous titania resulted in higher brookite content than did hydrothermal aging of sol-gel products containing little to no amorphous titania. Finally, dialysis prior to hydrothermal aging appeared to inhibit phase transformation from brookite to anatase in aged materials. Results presented demonstrate that considerable control over the relative anatase and brookite contents can be achieved through control of synthetic variables.     

Small-Angle electron scattering and electron density in carbon dioxide

The total (elastic and inelastic) intensity of electrons scattered by CO₂ was measured in the s range of 1 to 12 Å^?1 and compared with the theoretical intensity calculated from the Hartree-Fock molecular wave function and those calculated for the independent-atom-model (IAM ) molecule. In the range of s ? 4 Å^?1 the electron correlation effect on the total scattered intensity was found to be represented by that for the IAM molecule.     

Time‐dependent density functional theory study on direction‐dependent electron and hole transfer processes in molecular systems

We report on real‐time time‐dependent density functional theory calculations on direction‐dependent electron and hole transfer processes in molecular systems. As a model system, we focus on α‐sulfur. It is shown that time scale of the electron transfer process from a negatively charged S₈ molecule to a neighboring neutral monomer is comparable to that of a strong infrared‐active molecular vibrations of the dimer with one negatively charged monomer. This results in a strong coupling between the electrons and the nuclei motion which eventually leads to S₈ ring opening before the electron transfer process is completed. The open‐ring structure is found to be stable. The similar infrared‐active peak in the case of hole transfer, however, is shown to be very weak and hence no significant scattering by the nuclei is possible. The presented approach to study the charge transfer processes in sulfur has direct applications in the increasingly growing research field of charge transport in molecular systems. © 2017 Wiley Periodicals, Inc.     

Non-isothermal kinetics in mixtures of barium carbonate and titanium dioxide (anatase)

The complexes AgL (L = theobromine, 1,3,8-trimethylxanthine and 3,8-dimethylxanthine) Hg₂L·NO₃ (L = 3,8-dimethylxanthine and 1,3,8-trimethylxanthine) and Hg₂C₂(NO₃)₂ (C = caffeine), were prepared in aqueous and HNO₃ medium. These complexes were characterized by IR, ¹H-NMR, TG, DTG and DSC techniques.     

Hybrid density functional study of the oxidized states of NiFe-hydrogenase

The oxidized states of NiFe-hydrogenase and their activations have been studied using hybrid DFT. For Ni-B, which is activated in seconds, there is good agreement with experiments. The structure has a bridging hydroxide, which can be removed in two ways. In one pathway an outside electron and a proton is added and water removed, while the other pathway does not involve any outside electrons but uses a hydrogen molecule instead. For Ni-A, which can take hours to activate, there appears to be a rather severe discrepancy with experiments. Even though a structure was obtained in good agreement with experiment, with a protonated peroxide side-on bonded to nickel, this is not the lowest-energy structure. Instead, a structure with a bridging, end-on, protonated peroxide is found to be much lower in energy even for the largest model used with nearly 120 atoms. A remaining deficiency in the chemical model is the most likely explanation for the error. An error of the hybrid DFT method is also possible but appears much less likely, since the agreement between hybrid DFT and non-hybrid DFT is very good. The side-on structure of Ni-A is activated by first adding an outside electron and a proton and then cleaving the O–O bond. Altogether, three outside electrons are required to remove the peroxide, which could be a reason for the slow activation of Ni-A.     

A screened hybrid density functional study on energetic complexes: Metal carbohydrazide nitrates

The molecular geometry, electronic structure and thermochemistry of a series of metal carbohydrazide nitrates were investigated using the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional. The results show that Ca, Sr, and Ba complexes have additional coordinated oxygen atoms from the nitrate ion, which differ obviously from Cu, Ni, Co, and Mg complexes in terms of the geometric structure. Detailed NBO analyses clearly indicate that the metal–ligand interactions in Cu, Ni, and Co complexes are covalent, whereas those of Mg, Ca, Sr, and Ba complexes are ionic in nature. Furthermore, the donor–acceptor interactions result in a reduction of occupancies of σ_{C? O} and σ_{N? H} orbitals. Consequently, the bond lengths increase and the bond orders decrease. Finally, the calculated heats of formation predict that the ionic alkaline‐earth metal carbohydrazide nitrates are more stable than the covalent transition metal carbohydrazide nitrates. It agrees well with the available experimental thermal stabilities, indicating that the metal–ligand bonding character plays an important role in the stabilities of these energetic complexes. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Adsorbate-substrate interaction between chlorodifluoromethane and titanium dioxide: Infrared spectroscopy and density functional theory studies

Infrared spectra of chlorodifluoromethane (CHClF₂) adsorbed on titanium dioxide (TiO₂) at room temperature have been investigated for the first time. From the comparison between the adsorption characteristics and the gas-phase spectra it can be deduced that the molecule interacts with the surface Lewis acid site (Ti⁴⁺) mainly through the Cl atom even if also the adsorption with one F atom is also observed. Moreover, the spectra show the presence of H-bonds between the CH group and the surface Lewis basic site (OH⁻ or O²⁻). In order to obtain more information on the molecule orientation and the variation of the structural parameters, a DFT-B3LYP study has been carried out considering the anatase (1 0 1) surface and evaluating the adsorption energetics in terms of interaction, distortion and binding energies. The obtained geometries confirm that both the acid-base interactions through Cl or F atoms are possible and suggest the formation of one H-bond between the CH group of the molecule and the Lewis basic site of the surface. The calculated vibrational frequencies of the adsorbed molecule have been found to be in reasonable agreement with the experimental data.     

Effect of silicon dioxide on the formation of the phase composition and pore structure of titanium dioxide with the anatase structure

The formation of the structure of titanium dioxide modified with silicon dioxide, which was introduced as tetraethyl orthosilicate, was studied. It was found that the formation of the nanocrystalline structure of TiO₂ occurred upon the modification of titanium dioxide with silicon dioxide. This nanocrystalline structure of TiO₂ was formed by highly dispersed anatase particles of size 6–10 nm stabilized by silicon oxide layers, which were formed upon the decomposition of tetraethyl orthosilicate. An increase in the modifier concentration resulted in a deceleration of the growth of anatase particles and an increase in the temperature of the phase transition of anatase to rutile. It was found that the anatase phase in the samples containing 5–15 wt % SiO₂ was stable up to 1000°C. The stabilization of highly dispersed anatase particles facilitated the retention of the developed fine-pore structure of xerogels with a pore diameter of 4 nm up to 900°C.     

A finite-temperature density functional study of electron self-trapping in 3He and 4He

We introduce a compact finite-temperature density functional model to study electron self-trapping in both liquid and vapor (3)He and (4)He. This model can quantitatively reproduce the most essential thermodynamic properties of (3)He and (4)He along their liquid-vapor coexistence lines. The structures and energetics of self-trapped electron bubbles on the 1S ground state and 1P excited state are particularly investigated. Our results show that 1S and 1P bubbles exist in liquid at any temperature, whereas 1S bubbles exist in vapor only above 1.6 K in (3)He and above 2.8 K in (4)He, 1P bubbles exist in vapor only above 2.5 K in (3)He and 4.0 K in (4)He. An initially spherical 1P bubble is unstable against deformation towards a peanut shape. In liquid, a peanut-shaped 1P bubble is held from fission by surface tension until reaching the liquid-vapor critical point, whereas in vapor it always splits into two smaller bubbles. The existence of 1P bubbles in finite-temperature liquid helium and their fission instability in helium vapor reveal interesting physics in this system.     

Synthesis of titanium dioxide nanoparticles with mesoporous anatase wall and high photocatalytic activity

Mesoporous titanium dioxide nanosized powder with high specific surface area and anatase wall was synthesized via hydrothermal process by using cetyltrimethylammonium bromide (CTAB) as surfactant-directing agent and pore-forming agent. The resulting materials were characterized by XRD, nitrogen adsorption, FESEM, TEM, and FT-IR spectroscopy. The as-synthesized mesoporous TiO2 nanoparticles have mean diameter of 17.6 nm with mean pore size of 2.1 nm. The specific surface area of the as-synthesized mesoporous nanosized TiO2 exceeded 430 m2/g and that of the samples after calcination at 600 degrees C still have 221.9 m2/g. The mesoporous TiO2 nanoparticles show significant activities on the oxidation of Rhodamine B (RB). The large surface area, small crystalline size, and well-crystallized anatase mesostructure can explain the high photocatalytic activity of mesoporous TiO2 nanoparticles calcined at 400 degrees C.     

A screened hybrid density functional study on energetic complexes: Alkaline-earth metal carbohydrazide perchlorates

The molecular geometries, electronic structures and stabilities of a series of alkaline-earth metal carbohydrazide perchlorates were investigated using the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional. The results show that Be and Mg complexes have six-coordinated octahedron features, as previously reported for the transition metal complexes. However, Ca, Sr and Ba complexes have additional coordinated oxygen atoms from the perchlorate ion. Detailed NBO analyses indicate that the metal–ligand interactions are essentially ionic and play an important role in the stabilities of these energetic complexes. The donor–acceptor interactions result in a reduction of occupancies of σ_C=O and σ_N–H bond orbitals, and also their subsequent impact on bond length and bond order.     

A comparison of the photochemical reactivity of polycrystalline (anatase), amorphous and colloidal forms of titanium dioxide

Freshly precipitated amorphous titanium dioxide and colloidal titanium dioxide are inefficient photocatalysts for hydrogen production from water and propan-2-ol and for methane and hydrogen production from acetic acid. Their reactivity, which is increased by platinization (but is less than that of platinized anatase), is attributed to the formation of titanium(III) species rather than electron—hole pairs.     

Arrangement of acid sites on the surfaces of anatase titanium dioxide nanoparticles according to cluster models

Computer-implemented cluster models have been devised for ～2-nm anatase nanoparticles with exposed (001) and (100) faces. The Lewis acid sites occurring in these faces have been characterized by calculating the enthalpy of CO adsorption. In the Ti₁₁₄O₂₂₈ and Ti₁₈₇O₃₇₆H₄ clusters, the corner oxygen atoms compensating the electric charge are bound to titanium atoms by double bonds with a length of approximately 1.7 Å, which is in agreement with experimental data. The average enthalpy of CO adsorption on the (001) and (100) faces at a zero coverage is ?87.62 and ?135.31 kJ/mol, respectively. The deviation from the average value is 20.2 and 8.8%, respectively. The average enthalpy of CO adsorption for the Ti₁₁₄O₂₂₈ cluster is ?129.40 kJ/mol, and that for the Ti₁₈₇O₃₇₆H₄ cluster is ?119.79 kJ/mol.     

Copper impurities in bulk ZnO: a hybrid density functional study

Transition metal doping of ZnO is considered as a promising way to obtain a diluted magnetic semiconducting oxide. In this work we investigate copper doping of ZnO by means of density functional theory, using a hybrid exchange-correlation functional and a periodic approach with localized atomic basis functions. Isolated copper species, such as copper substitutional to zinc, Cu(s), and Cu interstitial, Cu(i), are analyzed in terms of transition energy levels and hyperfine coupling constants with reference to available spectroscopic data. We also examine the potential magnetic interaction between copper species, their interaction with oxygen vacancies, and the possibility of copper clustering. The relative stability of the various copper impurities considered in this study is finally compared on the basis of their formation energy at different oxygen chemical potentials and Fermi level values.     

Lithium insertion into titanium dioxide (anatase) electrodes: microstructure and electrolyte effects

Insertion characteristics of anatase electrodes were studied on single-crystal and polycrystalline electrodes of different microstructures. The lithium incorporation from propylene carbonate solution containing LiClO₄ and Li(CF₃SO₂)₂N was studied by means of cyclic voltammetry (CV), the quartz crystal microbalance (QCM) and the galvanostatic intermittent titration technique (GITT). The electrode microstructure affects both the accessible coefficient x and the reversibility of the process. The highest insertion activity was observed for electrodes composed of crystals with characteristic dimensions of ∼10^–8 m. The insertion properties deteriorate for higher as well as for smaller crystal sizes. Enhanced insertion was observed in Li(CF₃SO₂)₂N-containing solutions. Lithium insertion is satisfactorily reversible for mesoscopic electrodes; the reversibility in the case of compact polycrystalline and single-crystal electrodes is poor. The reversibility of the insertion improves with increasing electrolyte concentration. The lithium diffusion coefficient decreases with increasing x and ranges between 10^–15 and 10^–18 cm² s^–1. Electronic Publication     

A selective etching phenomenon on {001} faceted anatase titanium dioxide single crystal surfaces by hydrofluoric acid

A selective etching phenomenon on {001} faceted anatase TiO(2) single crystal surfaces by HF and associated etching mechanism are reported. Density functional theory (DFT) calculations reveal that HF stabilizes the grown {001} facets at low concentrations, but selectively destroys the grown {001} facets at high concentrations.     

Foaming in chemical surfactant free aqueous dispersions of anatase (titanium dioxide) particles

Steady-state dynamic aqueous foams were generated from surfactant-free dispersion of aggregated anatase nanoparticles (in the micrometer size range). In order to tune the particle surfaces, to ensure a critical degree of hydrophobicity (so that they disperse in water and generate foam), the particles were subjected to low-temperature plasma treatment in the presence of a vapor-phase silane coupling agents. From ESCA it was shown that hydrophobization only occurred at a small number of surface sites. Foamability (foam generation) experiments were carried out under well-defined conditions at a range of gas flow rates using the Bikermann Foaming Column.1 The volume of the steady-state foams was determined under constant gas flow conditions, but on removing the gas flow, transient foams with short decay times (<5 s) were observed. The foamability of the steady-state foams was found to be dependent on (a) the time of plasma treatment of the particles (surface hydrophobicity), (b) the particle concentration in the suspension, and (c) the state of dispersion of the particles. High foamability was promoted in the neutral pH regions where the charged particles were highly dispersed. In the low and high pH regions where the particles were coagulated, the foamability was considerably reduced. This behavior was explained by the fact that the large coagula were less easily captured by the bubbles and more easily detached from the interface (during the turbulent foaming conditions) than individual dispersed particles.     

A density functional theory study on pelargonidin

A complete conformational analysis on the isolated and polarizable continuum model (PCM) modeled aqueous solution cation, quinonoidal, and anion forms of pelargonidin, comprising the diverse tautomers of the latter forms, was carried out at the B3LYP/6-31++G(d,p) level. The results indicate that the most stable conformer of cationic and quinonoidal forms of pelargonidin are completely planar in the gas phase, whereas that of the anionic form is not planar. In contrast, PCM calculations show that the plane of the B ring is slightly rotated with regard to the AC bicycle in the most stable conformer of the cation and quinonoidal form. The most stable conformers of the cation, both in gas phase and aqueous solution, display anti and syn orientations for, respectively, C2-C3-O-H and C6-C5-O-H dihedral angles, whereas syn and anti orientation of hydroxyls at 7 and 4' positions are nearly isoenergetic. The most stable tautomer of quinonoidal pelargonidin is obtained by deprotonating hydroxyl at C5 in gas phase but at C7 according to PCM. Also, the most stable tautomer of the anion is different in gas phase (hydrogens are abstracted from hydroxyls at C5 and C4') and PCM simulation (C3 and C5). Tautomeric equilibria affect substantially the geometries of the AC-B backbone providing bond length variations that basically agree with the predictions of the resonance model. Most of the conformers obtained display an intramolecular hydrogen bond between O3 and H6'. Nevertheless, this interaction is not present in the most stable anions. Ionization potentials and O-H bond dissociation energies computed for the most stable conformers of cation, quinonoidal, and anion forms are consistent with an important antioxidant activity.