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J. Phys. Chem. C 2010, 114, 15042–15048
Insight into Fundamental, Overtone, and Combination IR Bands of Surface and Bulk Ba(NO3)2 by Ab Initio Molecular Dynamics Holger Hesske,† Atsushi Urakawa,*,†,‡ Joost VandeVondele,§ and Alfons Baiker*,† Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Ho¨nggerberg, HCI, CH-8093 Zurich, Switzerland, Institute of Chemical Research of Catalonia, ICIQ, AV. Paı¨sos Catalans 16, E-43007 Tarragona, Spain, and Institute of Physical Chemistry, UniVersity of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland ReceiVed: June 14, 2010; ReVised Manuscript ReceiVed: August 10, 2010
Vibrational characteristics of Ba(NO3)2, one of the key components in an important automotive catalytic technology, NOx storage and reduction (NSR), were investigated by ab initio molecular dynamics. In particular, the fundamental, overtone, and combination bands of surface and bulk Ba(NO3)2 were calculated and compared with experimental infrared (IR) spectra measured by internal and diffuse reflection sampling configurations. Using the densities of characteristic internal vibrational modes, the origins of the experimental IR bands in the regions of fundamental as well as overtone and combination vibrations were clarified. Furthermore, the molecular dynamics based vibrational analysis showed that the bands in the overtone and combination band region (1600-3000 cm-1), typically neglected in NSR studies, contain chemically rich information and can assist in the firm identification of surface nitrates and their adsorption configurations. Introduction In situ vibrational spectroscopic methods, commonly in situ IR and Raman spectroscopy, are valuable tools to gain specific information about molecular species involved in a chemical reaction. Based on observed bands, particular functional groups, chemical species, or intermediates present during the reactions can be identified. In reality, band assignments are occasionally not straightforward because of the strong dependence of band characteristics, such as position (vibrational frequency), intensity, and width on the chemical environment (e.g., in vacuum, solvated, or coordinating to a surface). As a particular example, the surrounding environment of chemical species is often diverse and ill-defined when they reside at or close to a surface. In such a case, band assignments are rather empirical, consequently leading to unjustified band assignments and thus ambiguous interpretations of underlying chemistry. Ba(NO3)2 has been subject to recent extensive studies because of its important function in the NOx storage and reduction (NSR) catalysis, where Ba(NO3)2 is formed after the storage of gaseous NOx under oxygen-rich conditions over Ba species existing as BaO, BaO2, Ba(OH)2, and BaCO3.1-3 A number of vibrational spectroscopic studies have been reported to elucidate NOx storage mechanisms by clarifying the structure of barium nitrates and surface nitrate species formed on metal oxide support materials during the storage process.4-12 In spite of considerable insight gained by recent in situ/operando vibrational spectroscopic studies, the reported range of vibrational frequencies for barium nitrates, particularly those formed during NSR, is broad, scattered, and often contradicting, which is partially explained by the different local sensitivities of the applied sampling configurations, for example, transmission or diffuse reflection IR method, giving rise to distinct spectral features.13 Clearly * To whom correspondence should be addressed. Fax: +41 44 632 11 63. E-mail:
[email protected];
[email protected]. † ETH Zurich. ‡ Institute of Chemical Research of Catalonia. § University of Zurich.
firm band assignments of bulk and surface Ba(NO3)2 are required to identify the nature of bands observed during NSR. Over the past 15 years, the positions of IR-active bands of Ba(NO3)2 have been discussed controversially in several studies. In the first IR study on Ba(NO3)2 reported by Brooker et al.14 the observed bands up to 1800 cm-1 were carefully assigned and discussed. In 1994 Taha and Toson15 assigned observed overtones and combination bands at different temperatures. The usefulness of this overtone assignment for the identification of NOx surface species was discussed by Hadjiivanov.16 Vibrational frequencies and spectra of bulk Ba(NO3)2 have been reported using different sampling configurations in the mid-IR range, transmission far-IR, Raman, and theoretical approaches.5,6,9-13,17-24 Most of the experimental studies employed materials containing components (e.g., Ba/Al2O3) typically contained in the formulation of NSR catalysts (e.g., Pt-Ba/Al2O3), investigated NO/O2 or NO2 adsorption on these materials, and proposed various possible configurations of surface adsorbed NOx. With the help of density functional theory (DFT) calculations, assignments of observed bands to different adsorbed surface species have been attempted.5,8,24-29 To the best of our knowledge, calculated IR band positions and intensities originating from nitrates residing at the surface of Ba(NO3)2 crystal have not been reported so far, although a firm assignment of these bands is a necessary prerequisite for interpreting nitrate bands of more complex systems. Therefore, this study aims at identifying and differentiating vibrational characteristics arising from the bulk and the surface of Ba(NO3)2 by state-of-the-art ab initio molecular dynamics (MD) simulations. Moreover, strong focus is given to the assignment of overtone and combination bands of surface and bulk nitrates using ab initio MD in order to explore the potential of these bands to gain further chemical information of surface nitrate species and determine configurations allowing more unambiguous interpretation of experimental vibrational bands.
10.1021/jp105435h 2010 American Chemical Society Published on Web 08/19/2010
IR Study of Ba(NO3)2 by Ab Initio MD
Figure 1. Investigated slab system and the respective surface unit cell of Ba(NO3)2. Surface nitrates showing the same configuration are accounted into layers and accordingly colored (layer 1, cyan, chelating bidentate; layer 2, orange, bridging monodentate; layer 3, yellow, flat). Barium atoms are shown in black, while bulk nitrates are depicted in silver/gray.
Methods Computational Details. A unit cell of Ba(NO3)2 was constructed in such a way that the xy-plane represents the {111}facet, because the surface of natural and synthetic Ba(NO3)2 contains almost exclusively {111}-facets.30 The smallest unit cell that fulfills this requirement is of hexagonal symmetry (a ) b ) 11.756 Å, c ) 14.332 Å, R ) β ) 90°, γ ) 60°) and contains 108 atoms. In Figure 1, the slab system used for our MD-based IR spectra calculation is shown composed of 1 × 1 × 4 unit cells and additional 17 Å of vacuum to separate the periodic images from each other. The surface nitrate ions were categorized into three types (layers 1-3) to investigate different vibrational contributions from the respective surface layers. Recently, we have reported the fundamental vibrational frequencies and modes of bulk Ba(NO3)2 obtained by DFT calculation using plane wave basis sets and vibrational analysis based on the finite difference method (PW-FD).24 The study showed an excellent agreement with experiments and clarified some controversy in the band assignments. However, a direct use of this approach to surface vibrational spectrum calculation is not suited because of a considerable increase of computational costs by the use of a slab system containing surface and a vacuum space to ensure the absence of interaction between surfaces of periodic images. Therefore, in this study a computationally efficient Gaussian and plane wave (GPW) basis set was used to describe the Kohn-Sham orbitals as implemented in the CP2K code.31-33 In this approach, the interaction of atomic cores and valence electrons is described by pseudopotentials. The barium pseudopotential, containing 10 valence electrons, was created and tested for bulk BaO and BaO2 structures, showing a very good agreement with experimental lattice parameters (
