NO and NO2 Adsorption on Al2O3 and Ga Modified Al2O3 Surfaces: A

Feb 19, 2010 - Both the NO and NO2 adsorption and activation are promoted on the Ga modified Al2O3 (110) surface. Moreover, the activation energy barr...
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J. Phys. Chem. C 2010, 114, 4445–4450

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NO and NO2 Adsorption on Al2O3 and Ga Modified Al2O3 Surfaces: A Density Functional Theory Study Zhiming Liu,*,† Lingling Ma,*,‡ and Abu S. M. Junaid§ State Key Laboratory of Chemical Resource Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, Key Laboratory of Nuclear Analytical Techniques, Multidisciplinary InitiatiVe Center, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, T6G 2G6, Canada ReceiVed: August 17, 2009; ReVised Manuscript ReceiVed: January 25, 2010

Al2O3 and its supported metal catalysts are widely used in deNOx catalysis, but the true nature of the catalytic sites and the structure-activity relationships are still unclear. By a set of systematic and comparative calculations, this study investigates the adsorption of NO and NO2, and nitrate formation via the oxidation of NO on Al2O3 and Ga modified Al2O3 surfaces using density functional theory. It is found that NOx gases (NO and NO2) preferentially adsorb on (110) planes, and are oriented in different configurations. While NO bonds with the (110) surfaces through an N-down orientation, the most stable mode of adsorption of NO2 on the (110) surfaces is a bidentate configuration, causing much higher net charge transfer from the surface and noticeable elongation of the N-O bond. Both the NO and NO2 adsorption and activation are promoted on the Ga modified Al2O3 (110) surface. Moreover, the activation energy barrier for nitrate formation via NO oxidation, a process crucial for the selective catalytic reduction of NOx, is about 35% less on the Ga modified Al2O3 (110) surface compared to the pristine Al2O3 (110) surface. This is one of the reasons for the high activity of Ga2O3-Al2O3 catalyst for the selective catalytic reduction of NOx. 1. Introduction NOx removal from lean exhaust streams remains one of the major challenges in environmental catalysis. Selective catalytic reduction of NOx by hydrocarbons (HC-SCR) in the presence of oxygen, a potential method to remove NOx from lean-burn and diesel exhausts,1,2 commonly employs Al2O3 because of the high catalytic activities of its supported metal (Ag, Sn, Ga, In, etc.) oxides for the SCR reaction.3-6 Al2O3 also serves as a typical NOx storage and reduction (NSR) catalyst support. According to Westerberg et al.,7 Al2O3 plays an important role as a storage site. Adsorption and activation processes of NOx on the catalytic sites, and the role of NO2 are crucial for SCR reactions.1 Therefore, fundamental understanding of the mechanism of NOx adsorption over Al2O3 is extremely important for the design and development of active deNOx catalysts. Although some reports are available on the adsorption of NOx on TiO2 and BaO,8-10 no theoretical investigation has been conducted on Al2O3 surfaces. Hence the nature of adsorption sites, and the structural dependence of NO and NO2 adsorption over Al2O3 supported deNOx catalysts remain unclear to date. Among Al2O3 supported metal oxide catalysts, Ga2O3-Al2O3 stands out as a promising NOx reduction catalyst based on its unique activity.6,11 Different from other supported metal (Ag, Sn, In etc.) oxide catalysts, whose active metal is highly dispersed on the catalyst surface and does not substitute the Al atom of Al2O3,3-5 [GaxAl(1-x)]2O3 was formed for the Ga2O3-Al2O3 catalyst and the high activity was attributed to the formation of this solid solution.11 However, * Corresponding author. Tel: +86-10-64421077. E-mail: liuzm@ mail.buct.edu.cn (Z.M.L.); [email protected] (L.L.M.). † Beijing University of Chemical Technology. ‡ Chinese Academy of Sciences. § University of Alberta.

the relationship between the activity and the substitution of Al by Ga on the Al2O3 surface remains an open question. This study investigates the nature and role of NOx adsorption on Al2O3 (100) and (110) surfaces using density functional theory (DFT) calculations to illustrate the effect of surface structure on the adsorption and activation of NOx in the SCR reaction. Additionally, the adsorption phenomenon of NOx on Al2O3 and Ga modified Al2O3 surfaces has been compared to elucidate the promoting effect of Ga on Al2O3 for the SCR reaction. Such investigation will benefit future mechanistic studies on NOx removal by Al2O3-based catalysts. 2. Computational Details All calculations are based on DFT, and are performed using Materials Studio (MS) Modeling DMol3 from Accelrys (version 4.3).12 The double-numerical plus polarization (DNP) functions and Becke exchange13 plus Perdew-Wang approximation14 nonlocal functionals (GGA-PW91) are used in all calculations. The real space cutoff radius is maintained as 4.2 Å. On the basis of the convergence test for k-point sampling, the k-point set of (4 × 2 × 1) is used for Al2O3 (100) calculations, and (2 × 2 × 1) is used for Al2O3 (110) calculations. To determine the activation energy for a specific reaction path, a transition state connecting two stable structures through a minimum energy path is identified by complete synchronous transit (LST) and quadratic synchronous transit (QST) search methods, followed by transition-state confirmation through the nudged elastic band (NEB) method.15-17 The flow of charges is estimated by Mulliken population analysis, which has been shown to be a useful tool.8,11 Spin polarization is applied to all calculations. The crystallographic data for γ-Al2O3 bulk structure used in this work is taken from Digne et al.’s model.18 The structure is subsequently geometrically optimized (energy minimization) for

10.1021/jp907925w  2010 American Chemical Society Published on Web 02/19/2010

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Figure 2. NO adsorption on (A) Al2O3 (110), (B) GaAlIV (110), and (C) GaAlIII (110) surfaces (O atoms, red spheres; Al atoms, pink spheres; Ga atoms, gray spheres; N atoms, blue spheres; bulk alumina, light lines). Figure 1. Periodic model of (A) Al2O3 (100) and (B) Al2O3 (110) surfaces (O atoms, red spheres; Al atoms, pink spheres).

further refinement of the model, and the calculated lattice parameters of bulk γ-Al2O3 are a ) 5.59 Å, b ) 8.41 Å, c ) 8.07 Å, and β ) 90.59°, which are in good agreement with previous calculations.19 The choice of crystallographic surfaces is based on the morphology of γ-alumina nanoparticles.20 The (110) surface corresponds to a rectangular oxygen atom sublattice in the spinel description and exhibits the predominant area of 70-83%.21,22 The (100) surface, corresponding to a square oxygen atom sublattice, is less abundant and accounts for 17% of the area.21 The (111) surface can also be exposed but is subjected to the synthesis conditions.22 Hence, this study will focus on the (100) and (110) surfaces. Models with four to seven atomic layers were tested for NOx adsorption on Al2O3 and Ga-modified Al2O3 surfaces, and it was found that the energy variation is less than 0.03 eV as the layers increased from six to seven. Thus in the present study models consisting of six layers of Al and O atoms, along with a 15 Å vacuum layer in the z-direction were used to simulate the surfaces (see Figure 1). On the Al2O3 (110) surface, two types of Al atoms are present: the first type (AlIV) is inherited from bulk phase octahedral aluminum atoms, and the second type (AlIII) from bulk phase tetrahedral aluminum atoms. It has been reported that the Ga2O3-Al2O3 catalyst is active for the SCR of NOx within the loading range 30-50%.6 Therefore, the Ga modified Al2O3 (denoted by GaAl from now onward) surface is modeled in the current study by replacing an Al atom by a Ga atom, which corresponds to ∼38% Ga2O3 loading. For the Ga modified Al2O3 (110) surface, GaAlIV and GaAlIII represent AlIV and AlIII replaced by Ga, respectively. Adsorption energies of NO or NO2 on the Al2O3 and GaAl surfaces are calculated using the following equations:

∆ENO ) E(NO adsorbed surface) - E(clean surface) E(free NO) (1) ∆ENO2 ) E(NO2adsorbed surface) - E(clean surface) E(free NO2)

(2)

3. Results and Discussion 3.1. NO Adsorption. Different initial configurations for NO adsorption on both Al2O3 (100) and GaAl (100) surfaces, each with a fractional coverage of 0.25 monolayer, are considered to find the optimized structures with minimum energy. It was found that NO does not interact significantly with these surfaces

TABLE 1: Adsorption of NO on Al2O3 (110) and GaAl (110) Surfaces

NO(g) NO on Al2O3 (110) NO on GaAlIV (110) NO on GaAlIII (110)

adsorption energy (eV)

NO charge (electrons)

bond length (Å)

0.00 -1.08 -1.12 -1.04

0.00 -0.07 -0.09 -0.07

1.163 1.181 1.184 1.184

and only undergoes physisorption. A similar phenomenon was reported in the literature for NO adsorption on the MgO (100) surface.23 Geometry optimization shows that NO prefers to adsorb on the AlIII site on the Al2O3 (110) surface through an N-down orientation, tilting away from the AIII-O bond on the surface by an O-AIII-N angle of 106°. The adsorption energy calculated using eq 1 is -1.08 eV/NO. When the N atom is on the top of an AlIV atom or between two AlIV atoms, NO interacts with the surface in the bridging position, forming two bonds with the two AlIV atoms via a N atom. In this case the adsorption energy is -0.76 eV/NO, which is lower than that of the former adsorption mode. If the O atom in an NO molecule is oriented toward the Al2O3 (110) surface, weak interaction and physisorption takes place. Figure 2A represents the most stable configuration for NO adsorption on the Al2O3 (110) surface. Table 1 shows that a flow of charge (0.07 electrons) occurs from the Al2O3 (110) surface to the adsorbed NO molecule. NO has one electron localized in the antibonding 2π* state,24 and electron transfer from the Al2O3 surface to the antibonding LUMO (the lowest unoccupied molecular orbital) of NO weakens the N-O bond and increases the bond length from 1.163 to 1.181 Å. Al substitution by Ga generally appears to promote adsorption and activation of NO on the Al2O3 (110) surface. When an AlIV atom on the surface is replaced by a Ga atom, a NO molecule preferentially interacts with the AlIII atom in a manner similar to that on a nonreplaced Al2O3 surface (Figure 2B), with slightly increased adsorption energy (-1.12 eV/NO) and N-O bond length (1.184 Å). NO-TPD studies by Haneda et al.11 also found that the amount of NO adsorbed on the Ga2O3-Al2O3 catalyst is somewhat higher than that on Al2O3. In the case of an AlIII atom replaced by a Ga atom, NO forms a bond with the Ga atom, with the N-O bond length is elongated to 1.184 Å (Figure 2C). Irrespective of the type of Al atom available for substitution, the presence of Ga appears to promote the activation of NO. Adsorption of NO on the GaAlIII (110) surface, with an adsorption energy of -1.04 eV/NO, is less favorable. 3.2. NO2 Adsorption. Compared to NO, NO2 interacts with the Al2O3 (100) surface more strongly. If NO2 is placed in the bridging position between two Al atoms via two O atoms on

NO and NO2 Adsorption on Al2O3 Surfaces

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Figure 3. NO2 adsorption on (A) Al2O3 (100) and (B) GaAl (100) surfaces (O atoms, red spheres; Al atoms, pink spheres; Ga atoms, gray spheres; N atoms, blue spheres; bulk alumina, light lines).

TABLE 2: Adsorption of NO2 on Al2O3 (100) and GaAl (100) Surfaces adsorption energy (eV) NO2 charge (electrons) NO2 on Al2O3 (100) NO2 on GaAl (100)

-0.56 -0.58

-0.17 -0.21

the Al2O3 (100) surface, it does not interact. But when the molecule is placed on the top of the O atom on the surface in an N-down approach, producing NO3-like species, NO2 migrates and bonds with Al via O, forming the most stable structure. The optimized configuration is shown in Figure 3A. The adsorption energy for this configuration is -0.56 eV/NO2. Population analysis indicates that a charge transfer of 0.17 electrons occurs from the surface. Adsorption also causes elongation of the surface-bound N-O bond to 1.268 Å as compared to the bond length of 1.209 Å of the free unperturbed N-O. Ga substitution of Al results in an even stronger interaction between NO2 and the (100) surface. The most stable mode of adsorption is similar to that on an unsubstituted surface (see Figure 3B). The adsorption energy, amount of charge transfer, and N-O bond length, however, become larger (-0.58 eV/ NO2, 0.21 electrons, and 1.272 Å respectively), as presented in Table 2 and Figure 3B. NO2 has several stable adsorption modes on (110) surfaces. Unlike its behavior on the Al2O3 (100) surface, NO2 forms a bond with the two AlIV atoms via two O atoms on the Al2O3 (110) surface (Figure 4A). The adsorption energy for this configuration is -1.81 eV/NO2. The molecule can also form a bond with one AlIV atom (Figure 4B) or AlIII atom (Figure 4C) via one O atom. Comparing the two configurations, it can be seen that in the case of NO2 forming one bond with the AlIII atom, the adsorption energy becomes higher (-2.03 eV/NO2) and the surface-bound N-O bond becomes longer (1.370 Å), indicating that the AlIII atom is more active for the NO2 adsorption and activation. The higher unsaturation level of the AlIII atom could be responsible for its higher activity.25 However, in the most stable configuration for adsorption on the Al2O3 (110) surface, NO2 bonds with one AlIII atom and one AlIV atom via two O atoms, as shown in Figure 4D. The adsorption energy for this configuration is -2.30 eV/NO2. This bidentate mode of adsorption was also observed on the CeO2 (110) surface.26 NO2 also adsorbs on the TiO2 (110) surface, having the strongest interactions with the two Ti atoms.8 However, NO2 can bond with the TiO2 (110) surface O via N to produce NO3-like species on the surface, which is not possible on the Al2O3 (110) surface.

Figure 4. Configurations for the adsorption of NO2 on the Al2O3 (110) surface (O atoms, red spheres; Al atoms, pink spheres; N atoms, blue spheres; bulk alumina, light lines).

TABLE 3: Adsorption of NO2 on Al2O3 (110) and GaAl (110) Surfaces

NO2 on Al2O3 (110)

NO2 on GaAlIV (110)

NO2 on GaAlIII (110)

configurationa

adsorption energy (eV)

NO2 charge (electrons)

A B C D A B C D E A B C D

-1.81 -1.81 -2.03 -2.30 -1.82 -1.73 -1.57 -2.08 -2.34 -1.75 -1.16 -1.90 -2.14

-0.47 -0.46 -0.39 -0.47 -0.44 -0.42 -0.48 -0.40 -0.49 -0.48 -0.37 -0.38 -0.42

a A, B, C, D, and E are favorable modes of adsorption presented in Figures 4-6, respectively.

Higher net charge transfer and significant distortion of N-O bonds indicate that NO2 is more easily adsorbed and activated on the Al2O3 (110) surface than on the Al2O3 (100) surface, or compared to NO adsorption on the Al2O3 (110) surface. Table 3 shows that the net charge on the most stable configuration for adsorbed NO2 on Al2O3 (110) (Figure 4D) is -0.47 eV/ NO2, which is much higher than that on the Al2O3 (100) surface. The N-O bond of NO2 can be lengthened by 0.092 Å, whereas on the Al2O3 (100) surface the bond is lengthened by 0.059 Å. This significant distortion of adsorbed NO2 from free molecular NO2 and the degree of charge transfer are even more pronounced than those occurring on the perfect TiO2 (110) surface and are similar to those on the TiO2 (110) surface with defects.8 With higher adsorption energy and larger net charge transfer, NO2 also interacts with the Al2O3 (110) surface more strongly than NO. A similar phenomenon was also observed on the TiO2 (110) surface.8 Gas phase NO and NO2 have low-lying half-occupied antibonding orbitals (2π* in NO and 6a1 in NO2). The relative energy positions of the NO (2π*) and NO2 (6a1) orbitals indicate

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Figure 6. Configuration for the adsorption of NO2 on the GaAlIII (110) surface (O atoms, red spheres; Al atoms, pink spheres; Ga atoms, gray spheres; N atoms, blue spheres; bulk alumina, light ines).

TABLE 4: Activation and Reaction Energy of NO Oxidation to NO3-

Figure 5. Configurations for the adsorption of NO2 on the GaAlIV (110) surface (O atoms, red spheres; Al atoms, pink spheres; Ga atoms, gray spheres; N atoms, blue spheres; bulk alumina, light lines).

that the dioxide should be the better electron acceptor and thus the more reactive species on Al2O3.27 NO2 strongly adsorbed on Al2O3 surfaces, blocking sites for NO adsorption. Therefore, some Al2O3-based catalysts,28,29 such as Co/Al2O3 and Ag/Al2O3 catalysts, exhibited low activity for the NO oxidation to NO2 in the absence of reducing agent. Replacement of the AlIV atoms by Ga facilitates adsorption and activation of NO2 on the (110) surface. The most stable configuration for NO2 adsorption on GaAlIV (110) shows that NO2 bonds with one AlIII atom and one AlIV atom via two O atoms with an adsorption energy of -2.34 eV/NO2, which is higher than that for the nonreplaced Al2O3 (110) surface (-2.30 eV/NO2) or GaAlIII (110) surface (-2.14 eV/NO2). The higher adsorption energy indicates that the replacement of AlIV atom by Ga contributes to the adsorption and activation of NO2, which adsorbs more easily on this surface. The promoted adsorption and activation of NOx on the GaAl (110) surface is expected to benefit the conversion of NOx. The configurations of NO2 adsorption on GaAlIV (110) and GaAlIII (110) surfaces are shown in Figures 5 and 6, respectively. According to the calculated adsorption energies for some configurations for NO2 adsorption on the GaAl surface (Table 3), NO2 preferentially adsorbs on the Ga atom over the Al atom, which is very much contrary to the nature of NO adsorption on the GaAl (110) surface, on which NO always prefers the Al to the Ga atom for adsorption. Ga substitution of Al is found to promote the nitrate formation on the surface, which plays an important role in the reduction

of NOx over Al2O3-based catalysts.29-31 IR studies showed that the formed nitrates via NO oxidation by O2 are highly reactive toward hydrocarbons to form N2 over the Ga2O3-Al2O3 catalyst.32 Table 4 shows that the oxidation of NO by O2 over Al2O3 requires an activation energy of 0.31 eV. Ga substitution of Al on the Al2O3 surface reduces the activation energy to 0.20 eV, which demonstrates that the formation of nitrate on the Ga2O3-Al2O3 catalyst is much easier. This is in accordance with the experimental studies of Haneda et al.,33 which found that the rate of formation of nitrate species was about 1.6 times faster on Ga2O3-Al2O3 than on Al2O3. Thus the effect of Ga is to promote the formation of nitrate. In the HC-SCR process the formed nitrate, not NO, is crucial for reacting with the oxygenated hydrocarbon to form N2.34 3.3. Electronic Structure. The electronic structures of the Al2O3 (100), GaAl (100), Al2O3 (110), and Ga AlIV (110)

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Figure 7. Density of state (DOS) of (A) the Al2O3 (100) surface and (B) O atoms in the surface.

Figure 9. Density of state (DOS) of (A) the Al2O3 (110) surface and (B) O atoms in the surface.

Figure 8. Density of state (DOS) of conduction bands in the (A) Al2O3 (100) and (B) GaAl (100) surfaces.

surfaces are analyzed in terms of density of states (DOS) to explain the results described above. DOS is a powerful tool to analyze the energetic levels of slabs. The region between the top of the valence band (the HOMO in molecules) and the conduction band (the LUMO in molecules) is related to the reactivity: the system with a small band gap (HOMO-LUMO) is more reactive than that with a high value.35 The DOS projected on the outermost layer of the Al2O3 (100) surface is shown in Figure 7. From the comparison of panels A and B of Figure 7, we can see that the upper valence band is mainly composed of the oxygen 2p orbital, while the lower valence band is composed of the oxygen 2s orbital. The Al orbital contributes to the conduction band. This phenomenon is in accordance with the previous report by Bandura et al.36 that oxygen 2p states are the main contributors to the valence band DOS of most metal oxides. As illustrated in Figure 8A, the band gap for the Al2O3 (100) surface is 3.5 eV. While for the GaAl (100) surface (shown in Figure 8B), the value is decreased to 3.2 eV, indicating that the band gap becomes narrower due to the substitution of Al with Ga. Thus the GaAl (100) surface is more reactive than Al2O3 (100) for NO2 adsorption. Figure 9 shows the DOS projected on the outermost layer of the Al2O3 (110) surface. Compared with Figure 7, it is evident that the band gap of the Al2O3 (110) surface (1.4 eV) is significantly narrower than that of Al2O3 (100). The higher reactivity of the Al2O3 (110) surface than Al2O3 (100) could be attributed to its smaller band gap. For the GaAlIV (110) surface, the band gap is decreased to a even lower value (0.9 eV) (Figure 10). This suggests that the replacement of AlIV by Ga induces the narrowing of the band gap. Therefore, the GaAlIV (110) surface becomes more reactive than Al2O3 (110) for NOx adsorption and activation.

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Liu et al. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant No. 20907003), Beijing University of Chemical Technology Startup Program for Attracting High-level Talents (2008), and the State Key Laboratory of Chemical Resource Engineering Foundation (2009). References and Notes

Figure 10. Density of state (DOS) of conduction bands in the (A) Al2O3 (110) and (B) GaAlIV (110) surfaces.

4. Conclusions Understanding NOx adsorption on Al2O3 surfaces is vital for the development of the role of Al2O3 and Al2O3 supported metal catalysts for deNOx catalysis. The current DFT study investigated the possible mechanisms of NO and NO2 adsorption on the Al2O3 and Ga modified Al2O3 surfaces, as well as the effect of Ga substitution on the Al2O3 surfaces. On the basis of our calculations we found that while NO only undergoes physisorption on the Al2O3 (100) surface, it can adsorb on the AlIII site of the Al2O3 (110) surface through an N-down orientation. On the other hand, NO2 can bond with the Al2O3 (100) surface through one of its O atoms and have four possible modes of adsorption on the Al2O3 (110) surface. The most stable configuration of NO2 bonds with one AlIII atom and one AlIV atom via two O atoms. The high charge transfer and the elongation of the N-O bond suggest that the Al2O3 (110) surface is active for the adsorption and activation of NO and NO2. Substitution of Al by Ga facilitates the adsorption of NO and NO2 on Al2O3 (110) surface. The lower activation energy required for the formation of nitrate via NO oxidation by O2 on Ga modified Al2O3 (110) surface, as compared to the Al2O3 (110) surface, indicates that the NOx reduction proceeds more easily on Ga modified Al2O3 catalyst.

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