Quantum chemical modeling (DFT) of active species on the VWO catalyst surface in various redox conditions

doi:10.1016/S0097-8485(99)00090-X

Computers & Chemistry

Volume 24, Issues 3–4, May 2000, Pages 405-410

https://doi.org/10.1016/S0097-8485(99)00090-X Get rights and content

Abstract

This study concerns quantum chemical modeling of water behaviour on VWO systems. It was undertaken in order to validate the hypothesis that the presence of W atoms on (001) surface of the crystalline vanadia-like species, facilitates low temperature water dissociation. Quantum chemical calculations were done with the use of modern electronic structure methodology, based on the density functional theory (DFT). The program package DMol of Molecular Simulations, was applied for the calculations. The calculations were performed for small clusters representing two adjacent metal sites, in pentacoordinated oxygen environment, analogous to bipiramidal clusters, introduced in description of the (001) layers of vanadium pentoxide.

Introduction

Vanadia or vanadia–tungsta catalysts are usually used to reduce NO_X by ammonia in off-gases of the stationary sources (Bosh and Janssen, 1988, Bond and Tahir, 1991). Nitrogen oxides NO_X (x=1 or 2) belong to the most dangerous air pollutants. They demonstrate a wide spectrum of influence on animals and plants. The toxic impact of NO₂ and the influence of NO_X on acid rain and smog result in the waste of millions dollars (Hamill and Toon, 1991).

Vanadia–tungsta catalysts are known to have a wider temperature window for SCR (selective catalytic reduction) activity than vanadia alone (Chen and Yang, 1992, Alemany et al., 1995, Forzatti and Lietti, 1996, ). They become active at temperatures of about 100° lower than vanadia catalysts do. The lowering of the onset temperature for the SCR of NO_x is usually ascribed to the higher concentration of the Broensted acid sites on the surface of the mixed vanadia–tungsta species and the higher proton acidity of these sites (Chen and Yang, 1992).

DFT methodology and small local clusters of finite size, were already shown to perform very well for transition metal oxides, as well diatomic molecules as models of bulk oxides where the atoms couple via local covalent bonding with some admixture of ionic bonding (Broclawik and Salahub, 1993, Broclawik, 1995, Michalak and Witko, 1997).

This study presents an example of DFT methodology capacity, for performing modeling of the metal oxide catalysts properties.

Section snippets

Computational techniques

Quantum chemical calculations were done with the use of modern electronic structure methodology, based on the density functional theory (DFT). For the calculations the program package DMol of Molecular Simulations (DMol, 1995) was applied.

In this work DMol ab initio quantum chemistry software package was used, to perform calculation of:the equlibratium geometry of a VVO and VWO clusters and the clusters with water adsorbed on metal atoms in various redox conditions;harmonic vibrational

Cluster modeling

The calculations were performed for small clusters (Fig. 1, Fig. 2) representing two adjacent metal sites in pentacoordinated oxygen environment, analogous to bipiramidal clusters introduced in the description of the (001) (Bachman et al., 1961) surface of vanadium pentoxide. Similar modeling was already performed for pure vanadium pentoxide surface by Michalak and Witko (Michalak and Witko, 1997). In the present paper, bipiramidal cluster models were constructed to mimic surface vanadia-like

Results and discussion

Fig. 3 shows final geometries of a water molecule positioned on vanadium site in successively reduced vanadia cluster. The analysis of the calculation results, shows a few clear trends in water behaviour. The first one is correlated with redox conditions of modeled surface. The results presented in Fig. 3. show that water adsorption strongly depends on total number of electrons in modeling vanadia cluster. Its reduction (simulated by successive addition of electrons to the system) leads to

Acknowledgements

The subject was sponsored by Polish Committee of Scientific Research (KBN) (PB 0820/T 09/96/11).

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^☆: Workshop on New Trends in Computational Methods for Large Molecular Systems, Szklarska Poreba, Poland, 1–6 July 1999, Edited by W. Andrzej Sokalski and Morris Krauss.

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Quantum chemical modeling (DFT) of active species on the VWO catalyst surface in various redox conditions☆

Abstract

Introduction

Section snippets

Computational techniques

Cluster modeling

Results and discussion

Acknowledgements

Appl. Catal.

J. Mol. Catal.

Appl. Catal. A.

Surf Sci.

Solid State Ion.

J. Catal.

Z. Kristallogr.