Quantum-Chemical Investigations

CLUSTER MODEL DFT STUDY OF CO ADSORPTION TO GALLIUM IONS IN Ga/HZSM-5

G.M. Zhidomirov, A.A. Shubin, M.A. Milov, V.B. Kazansky*, R.A. van Santen**, E.J.M. Hensen**
(*Zelinsky Institute of Organic Chemistry, Moscow, Russia; **Eindhoven University of Technology, Eindhoven, The Netherlands)

J. Phys. Chem. C, 112(9) (2008) pp. 3321-3326.

Cluster model DFT calculations of CO adsorption on various possible forms of gallium in Ga/HZSM-5 zeolites have been performed. CO was found to only weakly interact with Ga⁺, (GaO)⁺, and (Ga(OH)_2-nH_n)⁺ (n = 1, 2) cationic clusters. The resulting shifts of the CO stretching frequency (Δν_CO) are only very small. On the other hand, CO coordination to small mononuclear and binuclear Ga³⁺ hydride/hydroxide/oxide species results in positive shifts in the stretching frequency in the range Δν_CO-1. Larger shifts (Δν_CO = 70-90 cm^-1) are associated with CO coordination to Ga ions at the corners of small three-dimensional Ga-oxide clusters. The experimentally observed changes in the infrared spectrum of adsorbed CO over Ga/HZSM-5 zeolites upon reductive and oxidative treatments are interpreted with these insights. Possibilities for the formation of such polynuclear oxide species in the zeolite micropore space are discussed. On the basis of recent literature insights, it is suggested that large shifts derive from CO coordination to oligomeric Ga cationic complexes stabilized by the negative zeolite charge.

NON-LOCALIZED CHARGE COMPENSATION IN ZEOLITES: A PERIODIC DFT STUDY OF CATIONIC GALLIUM-OXIDE CLUSTERS IN MORDENITE

E.A. Pidko*, E.J.M. Hensen*, G.M. Zhidomirov, R.A. van Santen*
(*Eindhoven University of Technology, Eindhoven, The Netherlands)

J. Catal., 255(2) (2008) pp. 139-143.

Periodic DFT calculations show that stability of binuclear cationic gallium-oxo clusters in high-silica zeolites is mainly controlled by the favorable geometrical environment of the Ga³⁺ ions, whereas the effect of the direct interaction with the charge-compensating framework anionic sites is less important. Extraframework cyclic Ga₂O²⁺₂ cations are shown to be active for light alkane dehydrogenation.

ADSORPTION PROPERTIES OF OXIDIZED GALLIUM-MODIFIED ZEOLITE ZSM-5 FROM DIFFUSE-REFLECTANCE IR-SPECTROSCOPIC AND QUANTUM-CHEMICAL DATA: II. INTERACTION WITH CARBON MONOXIDE AND WATER

I.R. Subbotina*, N.A. Sokolova*, I.V. Kuza'min*, A.A. Shubin, G.M. Zhidomirov, V.B. Kazanskii*
(*Zelinsky Institute of Organic Chemistry, Moscow, Russia)

Kinet. Catal., 49(1) (2008) pp. 149-155.

Diffuse-reflectance IR spectroscopy was used to study the adsorption and subsequent high-temperature transformations of water and carbon monoxide molecules on the oxidation-treated gallium-modified zeolite Ga/HZSM-5. The results were correlated with the corresponding quantum-chemical calculation data. Usually, it is thought that the oxo ions [Ga=O]⁺ are formed in the oxidation of Ga/HZSM-5. Based on the experimental and calculated data, the possible reactions of the gallium oxo ions with the above molecules are considered. The oxo ions were found highly reactive, and it is likely that polynuclear gallium oxide nanoclusters were formed in the oxidation of the gallium-substituted zeolite Ga/HZSM-5. The Ga⁺ ions, which appeared in the course of Ga/HZSM-5 reduction, were partially oxidized by water at 573 K; in turn, this could initiate the formation of polynuclear nanoclusters. It was found that ~25% of the Ga⁺ ions were oxidized in the interaction with water to liberate molecular hydrogen. The thermal reduction of a nitrous oxideaˆ“preoxidized Ga/HZSM-5 sample with carbon monoxide was studied, and a conclusion on dissimilar states of oxygen bound to gallium was drawn.

A THEORETICAL INVESTIGATION OF THE ADSORPTION SURFACE SITES OF THE ACTIVATED MgCl₂

D.A. Trubitsyn, V.A. Zakharov, I.I. Zakharov

J. Mol. Catal. A: Chem., 270(1-2) (2007) pp. 164-170.

The adsorption of carbon monoxide on activated MgCl₂ has been investigated within DFT using different models of the MgCl₂ surface. All the models were Mg6Cl10 clusters with two saturating OH groups. It has been found that the adsorption sites within the models based on the geometry of the ideal MgCl₂ crystal are stronger than they are in the experiment. It has also been found that relaxed clusters based on the geometry of the relaxed MgCl₂ surface present more accurate models of the MgCl₂ surface and account the relaxation effects properly. IR spectra of carbon monoxide bounded to three different adsorption sites calculated within relaxed clusters approximation is in excellent agreement with the experimental data. Such an agreement allows to conclude that adsorption sites of the activated MgCl₂ surface in general are 3-, 4-and 5-fold Mg atoms and the structure of these sites follows the structure of corresponded relaxed MgCl₂ crystallographic faces.

SIZE-DEPENDENCE OF ADSORPTION PROPERTIES OF METAL NANOPARTICLES: A DENSITY FUNCTIONAL STUDY ON Pd NANOCLUSTERS

I.V. Yudanov, M. Metzner*, A. Genest*, N. Rösch*
(*Technische Universität München, Garching, Germany)

J. Phys. Chem. C, 112(51) (2008) pp. 20269aˆ“20275.

Interatomic distances in metal nanoparticles are reduced from their values in the bulk. It was studied computationally how this size-dependent geometry change (from the bulk) relates to the size-dependence of other properties of large metal clusters, including their reactivity. For this purpose, using an all-electron scalar-relativistic density-functional approach, structures and binding energies for the example of CO adsorption on 3-fold hollow sites at the center of (111) facets of cuboctahedral nanoscale clusters Pd_n (n = 55-260) were calculated. The average nearest-neighbor Pd-Pd distance of optimized structures is 4-7 pm (2-3%) shorter than the extrapolated limit of the lateral distance within an infinite (111) surface. In consequence, the energy of CO adsorption on a cluster of aˆ“100 atoms is aˆ“15 kJ mol^-1 smaller than the extrapolated limit. On the basis of these results, a strategy was suggested for modeling particles of larger size, e.g. of 1000 atoms and more, with the help of smaller model particles of up to aˆ“300 atoms where one keeps the core of a model cluster fixed at the bulk structure and restricts the structure optimization to the outermost shell of cluster atoms.

HOW THE C-O BOND BREAKS DURING METHANOL DECOMPOSITION ON NANOCRYSTALLITES OF PALLADIUM CATALYSTS

I.V. Yudanov, A.V. Matveev, K.M. Neyman*, N. Rösch**
(*Universitat de Barcelona, Barcelona, Spain; **Technische Universität München, Garching, Germany)

J. Amer. Chem. Soc., 130(29) (2008) pp. 9342-9352.

Experimental findings imply that edge sites (and other defects) on Pd nanocrystallites exposing mainly (111) facets in supported model catalysts are crucial for catalyst modification via deposition of CH_x (x=0-3) byproducts of methanol decomposition. To explore this problem computationally, the authors applied recently developed approach to model realistically metal catalyst particles as moderately large three-dimensional crystallites. The first results of this advanced approach are presented here where the authors comprehensively quantify the reactivity of a metal catalyst in an important chemical process. In particular, to unravel the mechanism of how CHx species are formed, density functional calculations of C-O bond scission in methanol and various dehydrogenated intermediates (CH₃O, CH₂OH, CH₂O, CHO, CO), deposited on the cuboctahedron model particle Pd₇₉, were carried out. The lowest activation barriers, ~130 kJ mol^-1, of C-O bond breaking and the most favorable thermodynamics for the adsorbed species CH₃O and CH₂OH which feature a C-O single bond were calculated. In contrast, dissociation of adsorbed CO was characterized as negligibly slow. From the computational result that the decomposition products CH₃ and CH₂ preferentially adsorb at edge sites of nanoparticles, the authors rationalize experimental data on catalyst poisoning.

HYDROGEN ACTIVATION ON SILVER: A COMPUTATIONAL STUDY ON SURFACE AND SUBSURFACE OXYGEN SPECIES

A.B. Mohammad*, I.V. Yudanov, K.H. Lim**, K.M. Neyman***, N. Rösch*
(*Technische Universität München, Garching, Germany; **Nanyang Technological University, Singapore; ***Universitat de Barcelona, Barcelona, Spain) J. Phys.

Chem. C, 112(5) (2008) pp. 1628-1635.

Clean silver is known to be inert toward H₂ dissociation. Nevertheless, silver catalysts recently have been found to exhibit a noteworthy selectivity in the hydrogenation of unsaturated aldehydes to unsaturated alcohols. Experimental studies indicate that pretreatment in oxygen atmosphere activates the catalyst. To examine the role of oxygen in activation of hydrogenation catalysts, a density functional study on periodic slab models of H₂ dissociation at various oxygen species on silver surfaces, including subsurface oxygen, were carried out. According to calculations all oxygen forms under scrutiny promote dissociation of molecular hydrogen. With hydrogenation reactions in mind, the authors discuss a mechanism according to which an oxygen species, before it desorbs as a water molecule, produces one or two active hydrogen atoms on a metal terrace.

SPIN STATES OF IRON-NITROSYL ADSORPTION COMPLEXES FORMED IN Fe-ZSM5 ZEOLITES

S.E. Malykhin, A.M. Volodin, G.M. Zhidomirov
Appl. Magnet. Resonance, 33(1-2) (2008) pp. 153-166.

The capability of extra-framework monoiron sites to adsorb up to three NO molecules has been proven by quantum chemical calculations. Spin states of iron-nitrosyl adsorption complexes formed in Fe-ZSM5 zeolites have been found. For an initial iron site with S = 2, successive adsorption of one, two and three NO molecules changes S to 3/2, 1 and 1/2, respectively. Thus, the electron paramagnetic resonance (EPR) S = 1/2 signal observed after NO adsorption on the Fe-ZSM5 catalyst may be assigned to the Fe²⁺(NO)₃ species. Some peculiarities of the S = 1/2 EPR spectra obtained for iron-nitrosyl species in zeolites are discussed.

ESEEM MEASUREMENTS OF LOCAL WATER CONCENTRATION IN D₂O-CONTAINING SPIN-LABELED SYSTEMS

A.D. Milov*, R.I. Samoilova*, A.A. Shubin, Yu.A. Grishin*, S.A. Dzuba*
(*Institute of Chemical Kinetics and Combustion, Novosibirsk, Russia)

Appl. Magn. Reson., 35 (2008) pp. 73-94.

To calibrate electron spin echo envelope modulation (ESEEM) amplitudes with the respect to the deuterium water content in spin-labeled biological systems, ESEEM of nitroxide TEMPO has been studied in frozen glassy D₂O-dimethylsulfoxide mixtures of different composition. The interaction between the unpaired electron of nitroxide and the deuterium nuclei manifests itself in a cosine Fourier transform spectrum as the sum of a narrow line with the doublet quadrupole splitting and of a broad one. The narrow line arises from interaction with distant deuterium nuclei, the broad one arises from interaction with nearby nuclei belonging to nitroxide-water molecule complexes. The dependence on water concentration was found to be nonlinear for the intensity of the narrow line and close to linear for the intensity of the quadrupole doublet. Therefore, the intensity of the quadrupole doublet is suggested as a measure of concentration of free water around a spin label in biological objects. Fourier transform line shape was theoretically simulated for different model distributions of water molecules around the spin label. Simulations confirm the linear dependence of the quadrupole doublet intensity on water concentration seen in the experiment. The suggested approach was applied to analyze data for spin-labeled dipalmitoylphosphatidylcholine (DPPC) and DPPC-cholesterol D₂O-hydrated model membranes. The concentration of free water near the spin-labeled fourth carbon atom along the lipid chain was estimated as 5.2 and 7.2 M for DPPC and DPPC-cholesterol membranes, respectively.

ELECTRONIC STRUCTURE AND CHARGE TRANSPORT PROPERTIES OF AMORPHOUS Ta₂O₅ FILMS

V.A. Shvets*, V.Sh. Aliev*, D.V. Gritsenko*, S.S. Shaimeev*, E.V. Fedosenko*, S.V. Rykhlitski*, V.V. Atuchin*, V.A. Gritsenko*, V.M. Tapilin, H. Wong**
(*Institute of Semiconductor Physics, Novosibirsk, Russia; **City University of Hong Kong, Kowloon, Hong Kong)

J. Non-Cryst. Solids, 354(26) (2008) pp. 3025-3033.

Amorphous Ta₂O₅ films were deposited by sputtering Ta onto silicon substrates with reactive ion beam. Electron energy loss spectroscopy measurements on the film found that the plasma oscillation energy is 23.1 eV. The refractive index and the extinction coefficient were measured with spectroscopic ellipsometry over the spectral range of 1.9-4.9 eV. The optical band gap is found to be 4.2A±0.05 eV. The valence band consists of three bands separated by ionic gaps. The values of electron effective masses were estimated with ab initio quantum chemical calculation. Experiments on injection of minority carriers from silicon into oxide were also conducted and the authors found that the electron component of conduction current governed by the electron current in the amorphous Ta₂O₅.

ON THE MECHANISM OF MECHANOCHEMICAL DIMERIZATION OF ANTHRACENE. QUANTUM-CHEMICAL CALCULATION OF THE ELECTRONIC STRUCTURE OF ANTHRACENE AND ITS DIMER

V.M. Tapilin, N.N. Bulgakov, A.P. Chupakhin*, A.A. Politov*^,**
(*Novosibirsk State University, Novosibirsk, Russia; **Institute of Solid State Chemistry and Mechanochemistry, Novosibirsk, Russia)

J. Struct. Chem., 49(4) (2008) pp. 581-586.

The electronic structure of anthracene, its dimer, and intermediate structures composed of two anthracene molecules were calculated in the density functional theory. The calculated potential barrier to anthracene dimerization is ~55 kcal/mol; the dissociation barrier is ~45 kcal/mol. The pressure required for the reaction to reach the transition state and acting on the anthracene crystal is ~60 kbar. Lower pressures, ~10 kbar, are required for molecules to approach each other to distances of ~3 A…, at which tunnel dimerization is possible for photoexcited molecules.

NEW APPROACH TO THE CORRELATION PROBLEM: ELECTRON INTERACTION POTENTIAL AS A VARIABLE IN SOLVING THE MANY-PARTICLE SCHRÖDINGER EQUATION

V.M. Tapilin
J. Struct. Chem., 49(3) (2008) pp. 387-394.

The many-electron wave function is represented as the product of the wave function of the independent particles and the function that depends only on the value of the interelectron interaction potential. The function defines the electron correlation effects; a standard linear differential equation was derived to define the function. The equation depends on the functions of independent particles; a generalization of the Hartree-Fock equations including electron correlation was obtained for these functions. The total energy calculation of two-electron ions shows that even solving an ordinary differential equation for the function of independent particles represented by the functions of noninteracting electrons leads to higher accuracy than the one achieved in the Hartree-Fock theory.

Страницы 1 - 1 из 14
Начало | Пред. | 1 2 3 4 5 | След. | Конец | Все