J. Phys. Chem. C 2007, 111, 1487-1490
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Active Site Structure of NO Decomposition on Perovskite(-like) Oxides: An Investigation from Experiment and Density Functional Theory Junjiang Zhu,†,* Xiangguang Yang,‡,* Xuelian Xu,§ and Kemei Wei† National Research Center of Chemical Fertilizer Catalysts, Fuzhou UniVersity, Fuzhou 350002, P.R. China, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P.R. China, and College of Chemistry and Chemical Engineering, Fuzhou UniVersity, Fuzhou 350002, P.R. China ReceiVed: September 22, 2006; In Final Form: October 26, 2006
Active site structure for NO decomposition carried out on perovskite-like oxides were discussed based on the N2 yield measured from LaSrNi1-xAlxO4 with different B-site cations and from La2-ySryCuO4 with different crystal phases. Results show that the active site contains two oxygen vacancies, two transition metals, and one lattice-oxygen, with the oxygen vacancy locating on the apex of MO6 octahedron, and the lattice oxygen locating between the two transition metals (i.e., M-O-M plane). Density functional theory (DFT) analysis to the structure shows that this new active site is the most active structure for NO adsorbing, and hence, for NO decomposition. The similar trend of the relative energies that are required for the formation of oxygen vacancies with f form (calculated from DFT), the amount of oxygen vacancies, and the activities (N2 yield) certifies this result further.
1. Introduction The growing concerns for the environment and increasingly stringent standards for NO emission have presented a major challenge to control NO emission from electric utility plants and automobiles.1 Catalytic NO decomposition is the most desirable way of removing NOx from exhaust gas streams since it does not involve the addition of a supplemental reductant and the products (N2 and O2) of the reaction are nontoxic.2,3 Therefore, to design a catalyst successfully for NO decomposition will greatly decrease the equipment and operation cost of NO control. However, this is still unsuccessful at present due to the lack of understanding of the mechanism and active site structure of NO decomposition. We have reported that NO decomposition on perovskite(-like) oxides might be carried out by a “recycle” mechanism;4 now we are interested in giving a discussion on the active site structure for NO decomposition carried out on perovskite(-like) oxides. Perovskite-type oxides can show high activity comparable to that of platinum catalyst for automotive exhaust,5,6 and thus, they have been investigated in many works7-10 regarding the removal of nitrogen oxides. In all the works, it was assumed that the active sites for NO decomposition are composed of two adjacent oxygen vacancies, as that suggested by Shin et al.10 However, to the best of our knowledge, no proof was presented to confirm the exact existent status of oxygen vacancies (which are used for NO decomposition) in all the literature. Therefore, the active site structure of NO decomposition needs the further deliberation, and we considered it worthwhile to shed some light on the active site structure to give an overall comprehension on the reaction. * To whom correspondence should be addressed. (J.Z.) Tel.: +86-59183731234-8410. Fax: +86-591-83738808. E-mail:
[email protected]. (XY) E-mail:
[email protected]. † National Research Center of Chemical Fertilizer Catalysts, Fuzhou University. ‡ Chinese Academy of Sciences. § College of Chemistry & Chemical Engineering, Fuzhou University.
Naturally, perovskite(-like) oxides have three existent forms, described as T′ (with B-O square), T (with B-O octahedron), and T* (with B-O pyramids) phases (see Figure 1), depending on the coordination number of oxygen for B cation.11 The contribution of different phases (T′, T, and T*) to NO decomposition is various,12 and it has been reported that the oxygen vacancy in T* phase plays important role on the mobility of lattice oxygen.13,14 On these bases, the active site structure for NO decomposition was discussed, as below. 2. Experimental Method LaSrNi1-xAlxO4 (0.0 e x e 1.0) and La2-ySryCuO4 (0.0 e y e 1.0) were prepared by the citrate method as described elsewhere.12 Briefly, to an aqueous solution of La3+, Sr2+, and Cu2+ nitrates (all are in AR grade purity) with appropriate stoichiometry, a solution of citric acid 100% in excess of cations was added. The resulting solution was evaporated to dryness, and then the precursors obtained were decomposed in air at 300 °C, calcined at 600 °C for 1 h, and finally pelletized and calcined at 900 °C in air for 6 h. The synthesized pellets were pulverized to ca. 40-80-mesh size to be used. Catalytic activity was carried out on a single-pass flow microreactor made of quartz with an internal diameter of 6 mm. The reactant gas (1.0 vol.% NO/He) was passed through 0.5 g of catalyst at a rate of 25 mL/min. The gas compositions were analyzed before and after the reaction by an online gas chromatograph, using a molecular sieve 5Å column for separating NO, N2, and O2. N2O was not detected because it is difficult to form between 500 °C and 850 °C as reported by Teraoka et al.3 Before the data were obtained, reactions were maintained for ∼2 h at each temperature to ensure steady-state conditions. The activity was evaluated according to the equation: N2 yield ) 2[N2]out/[NO]in, where [NO]in and [N2]out are the concentration of NO and N2 measured before and after the reaction, respectively. Powder X-ray diffraction (XRD) data were obtained from an X-ray diffractometer (type D/MAX-II B Rigaku) over the
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1488 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Zhu et al.
Figure 1. Crystal structure of A2BO4 compounds with different phases (T′, T, and T*); In the T* phase, the possible active site for NO decomposition proposed previously and presently are shown, described as routes ‘a’ and ‘b’, respectively.
Figure 2. Four possible existence forms (denoted as c, d, e, and f) of oxygen vacancies in perovskite-like oxides.
range 20° < 2θ < 80° at room temperature, operating at 40 kV and 10 mA, using Cu KR radiation combined with nickel filter. The nonstoichiometric oxygen (λ) was determined by iodometry titration after dissolving the sample in 6 M HCl solution containing an excess of KI, under flowing nitrogen gas. I- anions reduce Cu3+/Cu2+ into Cu+ (2Cu3+ + 6I- f 2CuI + I2, 2Cu2+ + 4I- f 2CuI + I2). The titration of the resulting I2 by a solution of Na2S2O3 sodium thiosulfate leads to determine the λ value. In this case, the valence of La, Sr, and O was assumed to be +3, +2, and -2, respectively.15,16 Considering that the active site of NO decomposition contains two oxygen vacancies,9,10 four routes for the active site (with two oxygen vacancies) formation were proposed, as shown in Figure 2. Density functional theory (B3LYP), a powerful tool for calculating the point defects in solids oxides,17,18 was employed to calculate the single point energy of these four forms of oxygen vacancies. The basis set at LANL2DZ level was used for all the systems consisting of lanthanum element. Perovskite(-like) oxides were modeled by a cluster as shown in Figure 2 (1/2 part in vertical). The oxygen vacancies were assumed to be formed by removing the corresponding oxygen atom, and the other atoms were all fixed at their crystallitic sites. All calculations were performed with the Gaussian 03 program.19 3. Results and Discussion XRD analysis shows the main phase of the sample is perovskite-like with A2BO4 structure (see Figure 3). For the LaSrNi1-xAlxO4 samples, A2O3 and LaAlO3 phases appeared at high Al content (x ) 0.8 and 1.0), indicating that Al cannot enter the frame of LaSrNi1-xAlxO4 entirely at high Al content. Therefore, in order to ensure that the catalyst system carried out in this work is single phase, only the samples with x ) 0.0 ∼ 0.6 were taken into account. (In fact, Al2O3 and LaAlO3 show no activity for NO decomposition under the present conditions.) For the La2-ySryCuO4 samples, only the T phase (denoted as
“+”) was observed at 0.0 e y < 0.5; while at 0.5 e y e 1.0, La(Sr)CuO4 with T* phase (denoted as “b”) appeared, and its peak intensity strengthened with the increase of Sr content.12 Table 1 shows the activity of LaSrNi1-xAlxO4 at different temperatures. The activity decreased with the increase of Al content, irrespective of the temperatures, indicating that the presence of transition metal with changeable valence (i.e., Ni) is essential for a high activity and the active site for NO decomposition contains dual transition metals, since the most important exchange mechanism (for B-site cations) is believed to be a longer-range superexchange interaction through two oxygens of the type B-O-B′-O-B.20 High Ni content increases the probability of the formation of active site (i.e., Ni-O-Ni, see Figure 1). Table 1 also shows the activity of La2-ySryCuO4 at 850 °C, together with the nonstoichiometric oxygen (λ). At 0.0 e y < 0.5, where only the T phase of La2-ySryCuO4 was present, although the oxygen vacancy increased substantially, the increase of activity was slight. At 0.5 e y e 1.0, where the T* phase emerged and its intensity increased with the increase of Sr content (see IT*/IT in Table 1), both the oxygen vacancy and the activity increased substantially. This suggests that NO decomposition does not relate much to the oxygen vacancy produced in the T phase of La2-ySryCuO4, but has a close correlation to that produced in the T* phase of La2-ySryCuO4. Besides, considering that two NO molecules are required in NO decomposition reaction and oxygen vacancy is the place for NO adsorbing,9,10 it is thus assumed that two oxygen vacancies were involved in the active site and they stand on the apex of MO6 octahedron, as that in the T* phase. On the basis of this result, we therefore considered that on perovskite(-like) oxides, the active site for NO decomposition contains two oxygen vacancies, two transition metals, and one lattice-oxygen, as shown in Figure 1 (route ‘b’). Density functional theory (DFT) was applied to illuminate the reasonableness of this new active site for NO decomposition. In this case, four possible existence forms of oxygen vacancies in the structure (see Figure 2) were assumed, and the energies for their formation were calculated, as listed in Table 2. The energy required for the formation of oxygen vacancies c f d f e f f increased continuously, indicating that the structure with c form is the most stable and that with f form is the most instable. That is to say, the structure with f form has the strongest ability to catch oxygen atoms on the oxygen vacancies. This means that the adsorption of NO on the oxygen vacancies with f form is most facilitated, since NO and O2 have similar molecular orbitals and, in both cases, it is the oxygen atom adsorbing on the oxygen vacancy.9 On the other, it was reported that NO adsorption on the oxygen vacancies is the rate determining step of NO decomposition on perovskite-type oxides at high temperatures (T > 600 °C);8 therefore, the
Active Site Structure of NO Decomposition
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1489
Figure 3. XRD patterns of (a) LaSrNi1-xAlxO4 (0.0 e x e 1.0) and (b) La2-ySryCuO4 (0.0 e y e 1.0; +: T phase, b: T* phase) calcined in air at 900 °C.
TABLE 1: NO Conversion and Physical Properties of LaSrNi1-xAlxO4+λ and La2-ySryCuO4+λ x in LaSrNi1-xAlxO4+λ
850 °C
N2 yield (%) 750 °C
650 °C
0.0 0.2 0.4 0.5 0.6 0.8a 1.0a
67.1 55.5 52.5 43.8 42.3 24.0 12.9
53.6 51.2 43.5 32.6 30.0 17.7 7.9
18.4 12.6 9.9 9.2 8.8 8.0 5.0
y in La2-ySryCuO4+λ
N2 yield at 850 °C (%)
l
IT*/IT
0.0 0.1 0.3 0.5 0.7 0.9 1.0
2.1 2.8 3.5 4.6 12.9 22.6 30.8
+0.02 +0.01 -0.07 -0.13 -0.22 -0.33 -0.38
0 0 0 0.3 0.8 1.3 1.6
a Although these two samples are not single phase, the activity of them has the same trend as that of the others.
TABLE 2: Energies Required for the Formation of Oxygen Vacancies with Different Forms
a
oxygen vacancies
computed results (Hartreea)
relative energies (kcal/mol)
form “c” form “d” form “e” form “f”
-3109.93077323 -3109.91628994 -3109.79504524 -3109.71397874
0.00 9.09 85.17 126.95
1 Hartree ) 627.51 kcal/mol.
presence of oxygen vacancies with the strongest ability for NO adsorption is the most important factor for NO decomposition carried out on perovskite-type oxides. This suggests that the two oxygen vacancies with f form are the active sites of the catalyst. In addition, by correlating the relative energies that are required for the formation of oxygen vacancies (calculated from DFT), the amount of oxygen vacancies, and the activities (N2 yield), as shown in Figure 4, it was found that the energies required for the formation of oxygen vacancies with f form have a similar trend as that of the amount of oxygen vacancies and/ or the activities (the slope (k) obtained from the Fit linear of the curves is similar, with the values of 48, 50, and 51, respectively). Whereas, when the energies required for the
Figure 4. Relationships between the energies that are required for the oxygen vacancy formation, the amount of oxygen vacancies, and the activities for La2-ySryCuO4 (0.5 e y e 1.0). Here, the data of samples 0.0 e y e 0.3 were not considered since no T* phase was observed and their activity is very low (
