Nurbosyn U. Zhanpeisov

The effect of the framework structure on the chemical properties of the vanadium oxide species incorporated within zeolites

Abstract V-silicalite catalysts (VS-1 and VS-2) prepared by hydrothermal synthesis have been studied by ESR, XAFS (XANES and EXAFS) and photoluminescence spectroscopy. The in situ characterization of these V-silicalites shows that vanadium is present within the zeolitic framework as a highly dispersed tetrahedrally coordinated V-oxides, VO4 unit, having a short VO bond length. Photoluminescence spectroscopy in static and dynamic mode, as well as XAFS studies allow to detect in the V-silicalites different V species than that present in V-HMS or V/SiO2, in terms of VO bond length, vibrational energy, bond angle and lifetime of the excited triplet state. It is suggested that the combined contribution of the neighboring SiOH group attached to the VO4 unit and the zeolitic rigid framework structure of V-silicalites cause a more significant and pronounced effect on the chemical properties of the VO4 unit than the flexible structure of V-HMS or V/SiO2. Moreover, the dynamic quenching of the phosphorescence by the addition of reactant molecules such as NO or propane indicates that the V species in the excited triplet state can be expected to be the active sites for the photocatalytic reactions.

Quantum Chemical Calculations on the Structure and Adsorption Properties of NO and N2O on Ag+ and Cu+ Ion-Exchanged Zeolites

Ab initio cluster quantum chemical calculations at the Hartree–Fock (HF/Lanl2dz) and correlated second-order Moller–Plesset perturbation theory (MP2/Lanl2dz) levels were performed for NO and N2O interactions with Ag+ and Cu+ ion-exchanged zeolites. The interaction energies were estimated in a conventional way and also corrected for basis set superposition errors. It was shown that the highly dispersed Ag+ counterions establish twofold coordination to the lattice oxygens on the zeolite surface, similar to the case of Cu+ ions. However, both NO and N2O bind relatively strongly to the Cu active sites of Cu+ ion-exchanged zeolites than those of the Ag+ site of the Ag+ ion-exchanged zeolites. Based on the results of these calculations, the two different forms of adsorption for these molecules on the catalyst surface, the nature of their binding and characteristics of the adsorption properties have been discussed. Finally, some comparisons with the results obtained by a variety of density functional theory calculations on target systems have been presented.

A density functional theory study of the oxidation of methanol to formaldehyde over vanadia supported on silica, titania and zirconia

The mechanism of the selective oxidation of methanol to formaldehyde over vanadia supported on silica, titania and zirconia, suggested recently by Khaliullin and Bell, has been critically reconsidered at the same density functional theory (B3LYP/6-31G*) level. It was shown that an improper use of cluster models mimicking an intrinsic support structure may result in the failure to explain the observed experimental findings like those found in the above paper, i.e. when considering the activation energies and TOF between those three different supports, as well as the next-nearestneighbor V environment geometry for vanadia supported on titania catalyst.

Intrinsic band gap in semiconductor oxides and Ti-silicalite: ab initio and DFT study

Here we present the results of ab initio and DFT calculations mimicking the effect of transition metal ions on electronic and structural properties of Ti-silicalite, as well as of Na and N on rutile structures. The main discussion focuses on the excitation shifts into the longer wavelength region and their relation to the frontier orbitals that are expected to be responsible for the observed band gap changes. The results obtained allow us to give some insight in understanding the target phenomenon, as well as to point out on the importance of boundary frontier orbitals. These and other related findings are presented.

Local structure of highly dispersed lead containing zeolite. An ab initio and density functional theory study

Abstract Ab initio quantum chemical studies at the HF/Lanl2dz, B3PW91/Lanl2dz and B3LYP/Lanl2dz levels were carried out to investigate the local structures of the Pb 2+ active site of lead containing zeolites. It was shown that the coordination number which equals three is found to be the most favorable one for the Pb 2+ site within the zeolite lattice. The latter arises from the ground electronic state for the Pb 2+ within the zeolite lattice that involves lone pair of electrons at the 6s-AO. Thus, the Pb 2+ site would be in a strongly distorted tetrahedral environment. Due to the presence of the above lone pair of electrons, the adsorbate molecules would experience the repulsion from these electrons. The latter may explain some differences observed for the decomposition of pollutant molecules over the Cu + as well as Ag + ion-exchanged zeolites and the Pb 2+ ion-exchanged ones.

Density-functional theory of potassium atoms in zeolite

Abstract We have constructed a model of a zeolite which we have optimized using density-functional theory (DFT) in local-density approximation. We have calculated the energy of the zeolite model with and without one potassium atom. From these calculations the adsorption energy of the K atom is found to be −1.057 eV. We find that the bonds in the zeolite ring show an expansion upon addition of a K atom. We have also calculated the spin-dependent bond energy for two K atoms in a zeolite, from which we predict that ferromagnetic order occurs.

Theoretical DFT Study on the Interaction of NO and Br2 with the Pt(111) Surface.

Density functional calculations were performed at the B3LYP level using combined basis sets for the NO and bromine interactions with the Pt(111) surface mimicked by the two-layer Pt10 cluster model. It explains well an attractive bonding interaction not only for bromine and Pt(111) but also for all three adsorption modes of NO on the Pt(111) surface. In accordance with the experimental observations, the calculations predict that the first peak in the IR spectra appears at around 1515 cm(-)(1) at the initial stage of low NO coverage, while it would shift to 1707 cm(-)(1) at high NO coverage. The bonding of NO on the 3-fold hollow fcc and hcp sites of Pt(111) proceeds via predominant back-donation interactions, while for the on-top adsorption, both the donation and back-donation interactions become equally important. Energetic criteria show also that the STM tip (made from Pt and Ir alloys) immersed into a bromine solution may contain only dissociated bromine atoms that bind strongly with the surface Pt atoms. As a result, the νBrBr stretching vibration mode for the bromine molecule may not be seen in the IR spectra because of its dissociation into adsorbed atoms. This leads to an appearance of a blue shifted band centered at ca. 202 cm(-)(1).

The nature of the active sites of titanium oxide photocatalysts stabilized on an active carbon surface. A theoretical ab initio study

Abstract Ab initio quantum chemical studies at the HF/Lanl2dz level of theory were performed for two different cluster models representing a graphite surface and their interactions with dihydrogen, water, and four-coordinated titanium oxide. Both associative and dissociative channels of their interaction with the specific active sites on graphite were considered. Based on the results of these calculations, it was suggested that active sites for the titanium oxides on active carbon can be represented by the OTi(OH) 3 fragment formed through the dehydroxylation of the surface OH-groups by the four-coordinated titanium oxide species.

Structure and chemical activity of transition metal and metal oxide catalysts: An insight from theoretical DFT studies

The present theoretical DFT study discusses the structure and chemical activity of transition metal and metal oxide catalysts within the well-known cluster approach. Selective oxidation of carbon monoxide on gold supported on titania (Au/TiO2 (110)) as well as some key points in understanding the effect of non-metal doping on TiO2 with the aim to increase its photocatalytic functionality have been briefly discussed. It was shown that Au (with formal oxidation state equal to plus one) stabilized on water-assisted and vacancy containing TiO2 (110) can explain selective oxidation of CO. Here binding of O2 with the vacancy site is energetically preferable than its adsorption on an Au site. Conversely, CO adsorbs on an Au center of Au/TiO2 (110) which is energetically much more profitable than its interaction with the oxygen vacancy site. Also, carbon and nitrogen doping on TiO2 (110) leads to two different structures. Energetically most profitable is that carbon occupies an interstitial position in deep bulk while nitrogen replaces the protruded oxygen atom and forms a surface N-H group.

Local structure of highly dispersed lead species incorporated within zeolite: experimental and theoretical studies

XAFS (both XANES and FT-EXAFS) measurements revealed that the Pb2+ /ZSM-5 catalyst prepared from precursor H-ZSM-5 by a conventional ion-exchange method includes a highly dispersed 3-fold coordinated Pb2+ ion species within the zeolite framework. UV-irradiation of Pb2+ /ZSM-5 led to effective decomposition of NO and N2O producing N2. The photocatalytic decomposition of NO is found to be slightly preferable than that of N2O. The isolated Pb2+ ions play a significant role in the decomposition of pollutant NOx. Ab initio and DFT quantum chemical studies at the HF/Lanl2dz and B3PW91/Lanl2dz levels further shed light on local structures of the Pb2+ active site of lead-containing zeolites, as well as on their interactions with pollutant NO and N2O molecules. In agreement with experiments, 3-fold coordination was found to be the most favorable state for the Pb2+ site within the zeolite framework.