The molecular adsorption of some probe molecules (CO, H2O, and H2S) on a 5-fold-coordinated Ti Lewis acid site of the TiO2 rutile (110) surface is studied within the density functional theory. The ...substrate is modeled with a small Ti7O9 cluster terminated with pseudo-hydrogens. This is found to describe the electronic and structural properties of the clean surface in good agreement with both experimental and periodic slab calculations. Adsorption energies (6.7, 19.3, and 7.0 kcal/mol for CO, H2O, and H2S, respectively) and adsorbate stretching frequencies compare favorably with available experimental data. The agreement is particularly good for the C−O stretching frequency shift (+56 cm-1), as was found in previous investigations carried out with the same theoretical approach on other oxides. In contrast, the HOH scissoring mode is poorly reproduced, suggesting that molecularly adsorbed H2O is actually involved in weak bonds with other adsorbed species. This agrees with predictions of very recent slab calculations by Lindan et al. (Phys. Rev. Lett. 1998, 80, 762). Differences with respect to calculations carried out by embedding the cluster in point-charge arrays are discussed.
MOCVD deposition of CoAl2O4 films CARTA, G; CASARIN, M; EL HABRA, N ...
Electrochimica acta,
08/2005, Letnik:
50, Številka:
23
Conference Proceeding, Journal Article
Recenzirano
Cobalt aluminate (CoAl204) thin films were grown in a low-pressure hot wall metal organic chemical vapour deposition (MOCVD) reactor on Si(100) and quartz substrates with a total pressure of 2 Torr ...using bis(eta5-cyclopentadienyl)Co(II) Co(eta5-C5H5)2 and aluminium dimethylisopropoxide AlMe2(O'130 as precursors at 500 and 900 DGC. Films showed a dark-brown and dark-green colouration, respectively, and after an overnight heat treatment in air at 1200 DGC, they turned blue. Film microstructure, composition and morphology were investigated in detail by X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS) analyses. Films were polycrystalline and the UVvis spectra showed three electronic transitions allowed by the spin (540-630 nm range) characteristic of Co(II) ions with 3d7 configuration in tetrahedral coordination. SEM micrographs of the heat-treated CoAl2O4 samples revealed the presence of agglomerated crystallites with a highly porous structure.
Thin films of HfO
2 were grown by metal-organic chemical vapour deposition on fused quartz substrates in the temperature range of 400–500 °C using some bis(cyclopentadienyl)bis(alkoxide)hafnium (IV) ...precursors, namely Cp
2Hf(O
i
Pr)
2, Cp
2Hf{OCH(CH
3)CH
2OCH
3}
2, Cp
2Hf{OC(CH
3)
2CH
2OCH
3}
2 and Cp
2Hf{OC(CH
2CH
3)
2CH
2OCH
3}
2. These complexes, analyzed by nuclear magnetic resonance and thermogravimetric measurements, resulted pure and very stable towards air and moisture. The obtained films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy and atomic force microscopy. The deposits contained hafnium and oxygen in the right stoichiometric ratio with a low carbon contamination and they consisted of monoclinic HfO
2 phase (baddeleyite) with a granular surface morphology.
In the present work, the control of a ceramic colloidal process is aimed to fabricate NiO-YSZ composite with specific microstructure and porosity for anode-supported solid oxide fuel cell ...applications. The optimization of dispersant and solid concentration in anode slurries provides a reduction in their viscosity, shear stress and yield stress; concurrently, solvent evaporation and hysteresis in flow behaviour are minimized, thus improving the suspension stability. The corresponding sintered compacts produced from the optimized slurry demonstrated uniform microstructure with specific porosity distribution. It is shown that the residual porosity can be tailored in the range of 12–23% exclusively by controlling the conditions of drying, burn-out and sintering process; the lower limit allows effective gas permeation during anode reduction owing to an interconnected pore structure obtained already at 12–14% porosity. This indicates a stable pore structure derived from the burn-out of pore former even for sintering temperature as high as 1400°C.
Density functional molecular cluster calculations have been used to study the adsorption of CO on the α-Al2O3(0001) surface. Substrate and adsorbate geometry modifications, adsorption enthalpies, and ...adsorbate vibrations are computed. Despite the rather small size of the employed cluster, relaxation phenomena evaluated for the clean surface agree well with experimental measurements and periodic slab calculations and mainly consist of an inward relaxation of the Lewis acid site ( ). Different adsorbate arrangements, perpendicular and parallel to the surface, have been considered. Among them, the most stable CO chemisorption geometry (ΔH ads ≈ −13 kcal/mol) is that corresponding to the adsorbate perpendicular to the surface, atop and C-down oriented. The C−O stretching frequency (νC - O) computed for such an arrangement is 2158 cm-1, i.e., blue shifted by 44 cm-1 with respect to the free adsorbate. The lack of experimental evidence pertaining to CO interacting with a well-defined α-Al2O3(0001) surface prevents the possibility of a direct check of the computed quantities. Nevertheless, low-temperature IR data for CO on alumina powders (Zecchina, A.; Escalona Platero, E.; Otero Areán, C. J. Catal. 1987, 107, 244) indicate for the chemisorbed species a Δν = 12 cm-1. The adsorbate−substrate interaction relieves some of the Ls a relaxation, even if the electronic structure is only slightly affected upon chemisorption.
The reactions of pyrazole (Hpz) with some copper(II) carboxylates in the presence of water yield trinuclear copper derivatives characterized by the triangular core Cu3(μ 3-OH)(μ-pz)3(RCOO)2 (R = H, ...C2H5, C3H7). Copper(II) formate gives Cu3(μ 3-OH)(μ-pz)3(HCOO)2(Hpz)2 (1), whereas copper propionate and butyrate afford Cu3(μ 3-OH)(μ-pz)3(C2H5COO)2(EtOH) (2) and Cu3(μ 3-OH)(μ-pz)3(C3H7COO)2(MeOH)(H2O) (3), respectively, both containing solvent molecules coordinated to copper atoms. Magnetic susceptibilities are consistent with a single unpaired electron for each trinuclear unit of 1−3, and EPR measurements indicate that higher spin states, generated by exchange coupling between copper atoms, may be populated at room temperature. Density-functional calculations provide the description of the electronic structures of 1−3, allowing, at the same time, the assignment of their UV−vis absorption spectra. X-ray molecular structure determinations show that triangular trinuclear units of 1 are connected to each other through single formate bridges, forming one-dimensional (1D) zigzag coordination polymers, whereas in 2 and 3, two oxygen atoms of two carboxylate ions doubly bridge two copper atoms of different triangles, thus generating hexanuclear units. Moreover, in 2, two other propionate ions link together two hexanuclear units yielding a 12-membered cycle and giving rise to 1D coordination polymers. The supramolecular assemblies of 1−3 are compared to that of the previously reported trinuclear triangular copper(II) derivative Cu3(μ 3-OH)(μ-pz)3(CH3COO)2(Hpz) (A), where a two-dimensional (2D) coordination polymer is present. The reactions of 3,5-dimethylpyrazole (Hpz*) with copper(II) carboxylates in the same conditions yield 1:2 Cu(RCOO)2/Hpz* adducts.
The almost quantitative formation of the triangular trinuclear copper derivative Cu3(μ3-OH)(μ-pz)3(MeCOO)2(Hpz) (1) (Hpz = pyrazole), has been simply achieved by adding Hpz to an ethanol solution of ...Cu(MeCOO)2·H2O. An X-ray molecular structure determination shows that 1 is completely unsymmetric and that trinuclear units result assembled in an extended bidimensional network formed through acetate bridges and hydrogen bonds. EPR and magnetic measurements are consistent with the presence of a single unpaired electron. Theoretical density functional calculations carried out for S = 1/2 provide a thorough description of the electronic structure of 1, allowing a detailed assignment of its UV−vis absorption spectrum. Compound 1 reacts with MeONa, yielding Cu3(μ3-OH)(μ-pz)3(MeCOO)(MeO)(Hpz) (2) and Cu3(μ3-OH)(μ-pz)3(MeO)2(Hpz) (3) through the substitution of one and two acetate ions, respectively, with MeO- ion(s). The spontaneous self-assembly of the triangular trinuclear Cu3 moiety seems to occur only with pyrazole as can be inferred by the results obtained in the reactions of copper(II) acetate with some substituted pyrazoles leading to the formation of mononuclear Cu(MeCOO)2(L)2 (4 − 8) and dinuclear Cu(MeCOO)2(L)2 (9 − 11) (L = substituted pyrazole) compounds. Also the presence of acetate ions seems to play a leading role in determining the formation of the trinuclear triangular arrangement, as indicated by the formation of a mononuclear derivative, Cu(CF3COO)2(Hpz)2 (compound 12), in the reaction of copper(II) trifluoroacetate with pyrazole. Compounds 1 − 3, as well as some other mono- and dinuclear copper(II)-substituted pyrazole complexes, have been tested as catalyst precursors in cyclopropanation reaction, observing the formation of products in a syn:anti ratio opposite that normally reported.
The spectral element method is used to discretize self-adjoint elliptic equations in three-dimensional domains. The domain is decomposed into hexahedral elements, and in each of the elements the ...discretization space is the set of polynomials of degree N in each variable. A conforming Galerkin formulation is used, the corresponding integrals are computed approximately with Gauss-Lobatto-Legendre (GLL) quadrature rules of order N, and a Lagrange interpolation basis associated with the GLL nodes is used. Fast methods are developed for solving the resulting linear system by the preconditioned conjugate gradient method. The conforming finite element space on the GLL mesh, consisting of piecewise Q1or P1functions, produces a stiffness matrix Khthat is known to be spectrally equivalent to the spectral element stiffness matrix KN. Khis replaced by a preconditioner K̃hwhich is well adapted to parallel computer architectures. The preconditioned operator is then K̃-1
hKN. Techniques for nonregular meshes are developed, which make it possible to estimate the condition number of K̃-1
hKN, where K̃his a standard finite element preconditioner of Kh, based on the GLL mesh. Two finite element-based preconditioners: the wirebasket method of Smith and the overlapping Schwarz algorithm for the spectral element method are given as examples of the use of these tools. Numerical experiments performed by Pahl are briefly discussed to illustrate the efficiency of these methods in two dimensions.
The selective reduction of NO with NH3 catalyzed by isolated VO x species grafted onto TiO2 (anatase) is studied by means of periodic density functional calculations. NH3 is adsorbed molecularly by ...the bare support both as a Lewis-bonded complex at (101) 5-fold coordinated Ti sites, and as a H-bonded complex at (001) Ti−OH sites. Analogous interactions are predicted for stable submonolayer VO x species, which provide V5+ Lewis acid sites and V−OH sites. Neither Ti−OH nor submonolayer V−OH groups act as Brønsted acids toward NH3. Reaction pathways where both Lewis-bonded and H-bonded NH3 complexes yield a NH2NO intermediate are found. In the former case, a (rate-determining) deprotonation step of NH3 is required, whereas, in the latter, NH2NO is formed directly through a concerted mechanism. This suggests that many channels may contribute to the NO reduction process.
