J. Phys. Chem. C 2009, 113, 7779–7789
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First-Principles Analysis of NOx Adsorption on Anhydrous γ-Al2O3 Surfaces Donghai Mei,*,† Qingfeng Ge,‡ Janos Szanyi,† and Charles H. F. Peden† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99352, Department of Chemistry and Biochemistry, Southern Illinois UniVersity, Carbondale, Illinois 62901 ReceiVed: NoVember 25, 2008; ReVised Manuscript ReceiVed: March 16, 2009
The interaction of nitrogen oxides NOx (x ) 1-3) with γ-Al2O3 has been investigated using first-principles density functional theory calculations. NO and NO2 weakly physisorb on the clean, dehydrated (100) and (110) surfaces of γ-Al2O3, whereas the adsorption of the NO3 radical is rather strong. Only the basic-like O-down adsorption configurations were found to be stable. The interaction between NOx and γ-Al2O3 can be described as a surface-mediated electron transfer process. For single NOx adsorption, greater electron transfer from the surface to the adsorbate (negatively charged) yields stronger interaction between NOx and the surface. The adsorption of four combinations of NOx + NOy (x ) 1-3, y ) 2, 3) pairs on the (100) and the (110) facets of γ-Al2O3 were investigated. Except for the NO2 + NO2 pair, a strong cooperative effect that substantially enhances the stability of NOx on both γ-Al2O3 surfaces was found. This cooperative effect consists of surface-mediated electron transfer processes resulting in a favorable electrostatic interaction between two adsorbed NOx species. The NO+δNO3-δ pair was found to be the thermodynamically most stable state among the coadsorbed NOx + NOy pairs on both γ-Al2O3 surfaces. The results are used to analyze the experimentally observed NOx evolution during temperature programmed desorption from NO2-saturated γ-Al2O3 substrates. 1. Introduction The strong demand to improve fuel efficiency has led to increasing the use of engine technologies (e.g., diesel and leanburn gasoline engines) that operate under oxygen rich (lean) conditions.1-4 However, under lean-burn conditions, traditional three-way catalysts cannot effectively reduce NOx from the exhaust emissions.1-4 Consequently, several lean-burn NOx emission control technologies are being developed to meet this challenge, and one of the most promising is NOx storagereduction (NSR).2 In the NSR technology, NO from the emissions is first oxidized to NO2 over the noble metal sites during lean-burn engine operation. The NO2 is then stored as nitrites and nitrates in BaO nanoparticles well dispersed on the γ-Al2O3 substrate. During a short fuel-rich excursion, the stored NOx will be released and reduced to N2 over the noble metal sites.2 In this catalytic oxidation/storage/release/reduction cycle, the NOx storage and release capabilities of the catalyst are essential to effective NOx removal and overall efficiency with the BaO component playing the primary role as the NOx storage material.2,5-7 However, the γ-Al2O3 substrate may also contribute to the total NOx storage capacity.8 To gain more insights into the overall storage and release processes of NOx over BaO/ γ-Al2O3-based NSR catalysts, a fundamental understanding of how NOx interacts with the γ-Al2O3 substrate at the atomic level is pursued in this study. The adsorption of NOx on the γ-Al2O3 substrate has been experimentally studied using Fourier transform infrared (FTIR) spectroscopy and temperature programmed desorption (TPD) techniques.8-14 Most of these experimental studies have focused on identifying the adsorbed NOx species by vibrational spectroscopy. The large number of possible surface species with different oxidation states and adsorption structures, coupled with * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ Southern Illinois University.
the complex surface structure of the γ-Al2O3 substrate itself, has made assignment of the observed infrared (IR) vibrational features to specific surface NOx species difficult.15 The presence of surface hydroxyl groups on γ-Al2O3 under experimental conditions further complicates the situation. In a recent study, Szanyi et al. reported that both nitrites and nitrates were formed on the anhydrous γ-Al2O3 surface upon low NO2 exposure at room temperature.8 They collected a series of IR spectra as a function of NO2 pressure and observed IR features centered at 1230 and 1320 cm-1 were assigned to bridging bidentate nitrites with a band at 1463 cm-1 associated with linear surface nitrites. By increasing the NO2 exposure, only nitrate species were observed.8 On the basis of these IR spectra and prior literature assignments, three different types of nitrates on the surface were proposed: bridging bidentate (1247, 1618, and 1654 cm-1), chelating bidentate (1296 and 1592 cm-1), and monodentate nitrates (1296 and 1570 cm-1). Similar spectral features of nitrates on the γ-Al2O3 particles have been recently reported by Baltrusaitis et al.16 In the former study, an additional broad IR peak developed between 1940 and 1980 cm-1 at high NO2 exposures that was assigned to adsorbed N2O3, formed from a reaction between NO and NO2 in the gas phase that is then weakly adsorbed.8 The presence of adsorbed N2O3 species on γ-Al2O3 was first proposed by Venkov et al.13 Westerberg and Fridell have investigated the NOx storage properties of γ-Al2O3 by exposures to both NO + O2 and NO2 + O2 mixtures in the temperature range of 373∼673 K14 and proposed a model to fit about 21 observed IR bands with different adsorption structures of both surface nitrites and nitrates. They found that the peak position of surface nitrates shifted to higher wavenumbers as the coverage increased and suggested that the stability of surface nitrates was in the order of bridging bidentate > chelating bidentate > monodentate, in accord with the strength of covalent bonding and the number of bonds made between nitrate and the γ-Al2O3 surface.14 Of
10.1021/jp8103563 CCC: $40.75 2009 American Chemical Society Published on Web 04/09/2009
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additional interest is the observation of Goodman et al. that the formation of surface nitrates was first order in NO2 pressure.10 The interaction of NOx species with transition metal17-19 and alkaline earth oxide (AEO)20-26 surfaces have been studied using first-principles density functional theory (DFT) calculations. In particular, Schneider systematically investigated the adsorption of NOx on a series of AEO(001) surfaces.25 Different adsorption structures of nitrite and nitrate anions (acidic, basic, and ionic) were identified, and NOx adsorption was found to involve both electron transfer and acid-base interactions with the oxide surfaces.25 The increasing adsorption strengths of NOx in the order of BaO > SrO > CaO > MgO was attributed to an increasing oxidizing ability along the AEO family. Furthermore, Schneider described a cooperative effect arising from an electron transfer process between coadsorbed NOx that species dramatically enhanced the stability of NOx adsorption on the AEO surfaces.25 Broqvist et al. have studied the dynamics of NOx species on the BaO(100) surface using ab initio molecular dynamics simulations,27 where a large number of configurations of nitrite and nitrate species were predicted. These authors found that the nature of the interaction between NOx and the BaO surface was primarily electrostatic. They also suggested that the experimentally measured broad vibrational signatures could be attributed to the continuous interconversion between different adsorption structures of highly mobile nitrite and nitrate species at elevated temperatures.27 Theoretical studies of NOx adsorption on metal oxide surfaces other than those of AEOs are rare. To model adsorbed NOx structures on alumina surfaces, Baltrusaitis et al. calculated the structural parameters and the vibrational frequencies of nitrate ion species adsorbed on a binuclear Al2(OH)5(µ-OH) cluster using quantum chemical theory at the B3LYP/6-31+G(d) level.16 The calculated low and high frequency components of degenerate V3 vibrational modes for three types of nitrate configurations (monodentate, chelating bidentate, and bridging bidentate) were in good agreement with experimental frequencies. To the best of our knowledge, no theoretical investigation of NOx adsorption on γ-Al2O3 surfaces has been reported to date. In the present work, we investigated the adsorption structures and energetics of single NOx (x ) 1-3) and NOx + NOy (y ) 2, 3) pairs on the two most commonly occurring stable surface facets, that is, (100) and (110) of the anhydrous γ-Al2O3 substrate using first-principles DFT calculations. The bonding interactions between NOx and the surface were analyzed by Bader charge calculations for all adsorption configurations studied. The NOx adsorption mechanism on γ-Al2O3 will also be discussed using a thermodynamic stability analysis. 2. Computational Method All calculations were performed within the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional28 for exchange-correlation implemented in the Vienna ab initio simulation package (VASP).29 The core and valence electrons were represented by the projector augmented wave (PAW) method30,31 and plane wave functions were expanded up to a kinetic cutoff energy of 400 eV. The groundstate atomic geometries of the bulk and surface were obtained by minimizing the forces on each atom to below 0.05 eV/Å. The convergence of energy differences with respect to different k-points sampling schemes and system sizes were tested as discussed in our previous work.32 In this work, spin-polarization was applied to all calculations. The (100) and the (110) structures of γ-Al2O3 were studied here because they likely account for at least 90% of the exposed
Mei et al.
Figure 1. Optimized structures of the fully dehydrated γ-Al2O3 surfaces. The top view (a) and side view (b) of (100) surface. The top view (c) and side view (d) of (110) surface. The Al atoms are in magenta, and the O atoms are in red. The numbers shown in the figure are the averaged Bader charges (in electrons) of Al and O atoms.
surface for its nonspinel bulk structure.33 Each surface structure was modeled using a periodic slab of eight atomic layers, and a height of 15 Å in the z direction was used to separate the surface slab and its images. As shown in Figure 1a, the optimized γ-Al2O3(100)-(2 × 1) surface includes four Al2O3 surface units with all 8 surface Al atoms being penta-coordinated (AlV) and all 12 surface O atoms tricoordinated (OIII). The optimized γ-Al2O3(110)-(1 × 1) surface structure, consisting of two surface Al2O3 units, is shown in Figure 1c where three tetra-coordinated Al atoms (AlIV) and one tricoordinated Al atom (AlIII) are present. The surface O atoms are either tricoordinated (OIII) or dicoordinated (OII) in a 1:1 ratio. Different k-point grid samplings ranging from (1 × 1 × 1) to (4 × 4 × 1) were tested for both bare (100) and (110) surface slabs. We found that the (2 × 2 × 1) k-point sampling scheme was sufficiently accurate (
