Journal of Molecular Catalysis A: Chemical 363–364 (2012) 380–386 Contents lists available at SciVerse ScienceDirect Journal of Molecular Catalysis A: Chemical jou rn al h om epa ge: www.elsevier.com/locate/molcata Nitrogen oxides and SO 2 adsorption on Au/MOR catalyst: Adsorption sites, thermodynamic and vibrational frequencies. ONIOM study Anibal Sierraalta ∗ , Lenin Diaz, Rafael A˜ nez Laboratorio de Química Computacional, Centro de Química, Instituto Venezolano de Investigaciones Científicas, Apartado 21827, Caracas 1020-A, Venezuela a r t i c l e i n f o Article history: Received 13 February 2012 Received in revised form 6 July 2012 Accepted 12 July 2012 Available online 21 July 2012 Keywords: ONIOM DFT DeNOx Gold Mordenite Au a b s t r a c t Quantum chemical calculations were carried out to determine geometries, vibrational frequencies and adsorption energies of NO, NO 2 , N 2 O, and SO 2 molecules on a gold-exchanged mordenite catalyst (Au/MOR). The results show that the adsorption energies are greater on [Au] + than on [Au 3 ] + species. In general, the charge transferred from Au to the adsorbed molecules is higher for [Au 3 ] + than for [Au] + . It is shown for first time, that the SO 2 adsorption is thermodynamically favored on the [Au] + species, but not on [Au 3 ] + species. Therefore at low Au content, the Au/MOR catalyst could be able to adsorb SO 2 . © 2012 Elsevier B.V. All rights reserved. 1. Introduction Nitrogen dioxide (NO 2 ) and nitric oxide (NO) are extremely harmful contaminants for human health, living species (animal and vegetable ecosystems) and the environment [1,2]. These nitrogen oxides or NO x along with N 2 O plays an important role in atmo- spheric chemistry and in the climate system. NO x and SO 2 are among others responsible for the photochemical smog, greenhouse effect, acid rain and many respiratory diseases [3–7]. Some of the important sources of NO x and SO 2 are the anthropogenic activities including lean-burn automobiles, oil refineries, biomass burning, etc. [6,7]. The NO x and SO 2 removal from gases is possible employ- ing catalysts. Depending on the nature of the processes, various kinds for DeNO x and DeSO x catalysts have been developed [4,6,8] among them, transition metal-exchanged zeolite catalysts. These catalysts have shown potential applications in the direct decom- position of NO x and N 2 O to N 2 and O 2 [8]. Unfortunately, transition metal-exchanged zeolite catalysts are susceptible to be poisoned by H 2 O or SO 2 [9–11] Since 1980s, intensive researches have been performed to produce efficient DeNO x catalysts to eliminate NO x through reduc- tive mechanisms [12–14]. To reach this goal, several studies have been carried out with different systems such as Cu/ZSM- 5 [14–17], Fe/ZSM-5, [18,19] Co/ZSM-5 [20–22] and Au/ZSM-5 ∗ Corresponding author. Tel.: +58 0212 5041774; fax: +58 0212 5041350. E-mail addresses: asierral@ivic.gob.ve, asierral@gmail.com (A. Sierraalta). [23,24]. Ichikawa and coworkers [24] showed that there is a sig- nificant enhancement of the NO reduction by H 2 over Au/NaY and Au/ZSM-5 catalysts. Additionally when oxygen is present; Au based catalysts have been used in CO oxidation [25,26] and in green chem- istry [27]. Different oxidation states and structures for the active site have been proposed in the literature to explain the catalytic activity of Au in zeolites. Pestryakov and coworkers [28] proposed that for Au catalyst prepared by ionic exchange using mordenite (Au/MOR), the active sites correspond to Au + , Au 3+ ions and charged clusters Au n d+ . Ichikawa and coworkers [29] studied the Au/NaY, Au/MOR, and Au/NaZSM-5 catalysts and concluded that Au + is the dominant active site on which the reactions take place. Sachtler and coworkers [30] using FTIR, XRD, HRAEM (high-resolution analyti- cal electron microscopy) and CO-TPR analyzed the Au/MFI system and concluded that Au is present mainly as Au + and Au 3+ . Besides the DeNO x activity, it has been reported in the literature that the Au catalysts are able to adsorb and dissociate SO 2 [4,5,31] which in principle could be an indication of potential DeSO x activity. Although Au/MOR is a good catalyst for CO oxidation [32–34] and in general, the Au/zeolite catalysts are able to reduce NO x [18,23,35], the mechanism or the active species are not well known. In spite of the experimental works done to characterize the active sites [28,29,33,34], some questions remain unclear. Is the NO adsorption thermodynamically more favored than the correspond- ing to NO 2 ? If N 2 O and NO 2 are intermediates in the NO reduction, are the N 2 O and NO 2 adsorptions more favored on isolated Au cation or on aggregates? Are there bond activations in the adsorp- tion process? or charge transfer? Details of the manner in which 1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2012.07.014