Quantum chemical modeling (DFT) of active species on the VWO catalyst surface in various redox conditions☆
Introduction
Vanadia or vanadia–tungsta catalysts are usually used to reduce NOX by ammonia in off-gases of the stationary sources (Bosh and Janssen, 1988, Bond and Tahir, 1991). Nitrogen oxides NOX (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 NOX 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 NOx 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 VVO and VWO 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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2011, Journal of Molecular Catalysis A: ChemicalCitation Excerpt :Onal et al. [26] report that the adsorption of small molecules, such as H2O and NH3, is a relatively local phenomenon and a small metal oxide cluster can sufficiently represent larger cluster surfaces. Hence, a V2O9H8 cluster was selected as a working model of the vanadium-like surface species in the present work [27]. In this cluster, two structurally inequivalent oxygen sites, i.e., terminal vanadyl oxygen and bridging oxygen, were represented, and all of the peripheral oxygen atoms were saturated by hydrogen atoms (Fig. 1a).
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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.