The effects of BaO on the catalytic activity of La1.6Ba0.4NiO4 in direct decomposition of NO

Journal of Molecular Catalysis A: Chemical 423 (2016) 277–284

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The effects of BaO on the catalytic activity of La1.6 Ba0.4 NiO4 in direct decomposition of NO Liqiang Chen a,b , Xiaoyu Niu a , Zhibin Li a , Yongli Dong a , Dong Wang a , Fulong Yuan a,∗ , Yujun Zhu a,∗ a Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin 150080, PR China b School of Science, Heihe University, Heihe 164300, PR China

a r t i c l e

i n f o

Article history: Received 3 February 2016 Received in revised form 7 July 2016 Accepted 10 July 2016 Available online 11 July 2016 Keywords: NO decomposition Lattice oxygen BaO Oxygen vacancy NOx spillover

a b s t r a c t The La1.6 Ba0.4 NiO4 -x%BaO (x = 0, 5, 10, 15, 20, 25, 30) catalysts were prepared by heating the mixture of Ba(NO3 )2 and La1.6 Ba0.4 NiO4 in situ, and their catalytic performances were evaluated for NO direct decomposition. The results showed that the activities of the La1.6 Ba0.4 NiO4 -x%BaO catalysts had been improved with the increasing of BaO amount. Among these catalysts, the La1.6 Ba0.4 NiO4 -20%BaO exhibited the best catalytic performance for NO direct decomposition, and the yield of N2 kept 57% during 500 h at 923 K in the absence of O2 , even the concentration of O2 was 0.2% in the feed, the N2 yield still up to 57% at 923 K. So much higher activity for the perovskite(-like) oxides catalysts at such reaction temperature was first observed. In order to understand the role of BaO, a serial of experiments and characterizations were carried out on the La1.6 Ba0.4 NiO4 -x%BaO catalysts. The results revealed that the number of chemical adsorption oxygen adsorbed on the oxygen vacancies increased and the mobility of the lattice oxygen could be improved due to the BaO addition. Moreover, BaO may play an important role in NOx transportation and storage, which is favorable for the regeneration of the active sites. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen oxides (NOx ), emitted from coal and fossil combustion, are extremely toxic to human health and also harmful to the environment, which can result in acid rain, photochemical smog, ozone depletion and atmospheric visibility degradation [1]. In order to minimize the NOx emission, several techniques have been developed in the past years, such as three-way catalyst (TWC) [2], NOx storage reduction catalysts (NSR) [3] and selective catalytic reduction (SCR) by NH3 [4]. TWC and SCR have been applied in the automobile engine exhaust and stationary source emission, respectively. However, the direct decomposition of nitric oxide (NO) into harmless N2 and O2 is regarded as one of the most attractive methods for NO removal based on its negative Gibbs free energy and thermodynamically favorable [5–7]. Moreover, this reaction is simple, economical and does not require any other reductants like NH3 , H2 , CO, or hydrocarbons.

∗ Corresponding authors. E-mail addresses: [email protected] (F. Yuan), [email protected], [email protected] (Y. Zhu). http://dx.doi.org/10.1016/j.molcata.2016.07.022 1381-1169/© 2016 Elsevier B.V. All rights reserved.

Since the early work carried out by Winter [8], a number of catalysts have been examined for the direct decomposition of NO, such as precious metals [9,10], metal oxides [8,11], ion exchanged zeolites [12,13] and perovskites [14,15]. However, activities of the proposed catalysts up to now are not high enough for the use in a practical application and it is also known that these catalysts are easily deactivated by adsorption of oxygen formed by direct decomposition of NO. Perovskite(-like) oxides, ABO3 and A2 BO4 structures, have high structural stability, and the A- and/or B-site cations can be substituted by a foreign cation having different oxidation states or radius [16,17]. Thus, the oxidation state of B-site cation and the content of oxygen vacancy can be controlled when desired foreign cation is used, offering a convenient and feasible way of correlating physicochemical properties with catalytic performance of the materials. Many perovskite-like oxide catalysts have been investigated on NO direct decomposition based on their structural properties. Teraoka et al. reported that La0.8 Sr0.2 CoO3 was highly active in NO decomposition, the yield of N2 and O2 reached to 72% and 40% at 1073 K, respectively [15]. They found that the yield of N2 was much higher than that of O2 , which suggested that the surface of the catalyst was covered by strongly adsorbed oxygen, and the removal of surface oxygen followed by adsorption of NO might be the most important

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steps for the NO decomposition reaction. La0.7 Ba0.3 Mn0.8 In0.2 O3 perovskite oxides exhibited much higher activity to NO direct decomposition over 1073 K, which was reported by Ishihara et al. [18]. Zhao et al. also found that LaSrNiO4 showed far much higher activity than La2 NiO4 in the reaction, with NO conversion of 94% and 20%, respectively. The reason was that LaSrNiO4 with an oxygen vacancy could show good NO adsorption capacity even at room temperature, but La2 NiO4 with excess oxygen showed no adsorption capacity for NO at low temperature [19]. Zhu et al. thought the perovskite oxides with an oxygen defect were beneficial to NO removal. On the one hand, oxygen vacancy could provide space for NO adsorption, and on the other hand, electrons could be generated in the structure to activate NO, in the form of B4+ -(NO− )-B3+ , as a result of the oscillation between B3+ -B3+ , and B4+ -(e− )-B3+ [16]. Many other works also have shown that the presence of oxygen vacancy and the redox ability of B-site cation were two major factors influencing NO adsorption and activation [7,15,17,18,20–24]. But, the reaction temperature for the NO direct decomposition is high over the perovskite(-like) oxides (>1023 K), and how to improve the regeneration ability of active sites to obtain the high activity of NO decomposition at low temperature is still a challenge over the perovskite(-like) oxide catalysts. In our previous study [20], NO direct decomposition over the La2-x Bax NiO4 (x ≤ 1.2) catalysts had been investigated. It was found that the highest N2 yield was achieved with La1.2 Ba0.8 NiO4 composed of perovskite-like structure phase and BaCO3 in the absence/presence of oxygen. It is suggested that the BaCO3 phase, NOx storage component, contributes to the increase in activity, and plays an important role in quickening up the run of catalytic NO decomposition recycle. Therefore, in this study, the effect of BaO on the NO decomposition over the La1.6 Ba0.4 NiO4 was studied in details in the absence/presence of oxygen because the alkaline of BaO is stronger than BaCO3 . The La1.6 Ba0.4 NiO4 -20%BaO exhibits an excellent catalytic performance for the NO decomposition at 923 K, BaO plays a key role in NOx transportation and storage, which is favorable for the regeneration of the active sites and the promotion of the catalytic activity of La1.6 Ba0.4 NiO4 −20% BaO sample. The possible reaction routes are brought out for the NO decomposition over the La1.6 Ba0.4 NiO4 -20%BaO catalyst.

2. Experimental 2.1. Preparation of catalysts La1.6 Ba0.4 NiO4 was prepared by citrate combustion method as described elsewhere [6,20]. 16 mmol of La(NO3 )3 ·6H2 O, 10 mmol Ni(NO3 )2 ·6H2 O and 4 mmol of Ba(NO3 )2 were dissolved in 100 mL of deionized water under stirring for 30 min. At the same time, 45 mmol C6 H8 O7 was dissolved in 225 mL of deionized water under stirring for 30 min. Subsequently, the above citric acid solution was dripped into the nitrate solution dropwisely and stirred for 30 min. The resulting solution was evaporated to dryness, and then the precursor was calcined at 573 and 873 K for 1 h, respectively. Finally, the precursors were palletized and calcined at 1123 K in air for 6 h, and then the synthesized pellets were pulverized to 40–60 mesh size. Because BaO reacts easily with CO2 and generates BaCO3 , La1.6 Ba0.4 NiO4 -x%BaO (x = 5, 10, 15, 20, 25, 30) was prepared by mixing La1.6 Ba0.4 NiO4 with Ba(NO3 )2 and heated them to obtain in situ. A certain amount of the mixture (40–60 meshes) without dilution was set into the quartz reactor (containing La1.6 Ba0.4 NiO4 0.500 ± 0.001 g), which was firstly treated by 1% O2 from 293 K to 1023 K at 10 K/min, and kept at 1023 K for 1 h, then cooled to 423 K in 1% O2 , finally switched to 1% NO.

2.2. Characterizations X-ray diffraction patterns (XRD) were obtained with a BrukerD8 Advance from 30 to 900 ◦ C, using Cu K␣ radiation combined with Ni-filter, wavelength of 0.15406 nm, voltage of 40 kV, current at 40 mA, scan range 2 of 10–65◦ , scan speed of 10◦ /min, and sampling interval of 0.02◦ . Temperature programmed reduction with hydrogen (H2 -TPR) was carried out in a conventional self-made apparatus equipped with TCD as a detector. 30 mg of catalyst was pretreated by 1% O2 with a total flow rate of 25 mL/min from 323 K to 1023 K at 10 K/min, and kept at 1023 K for 1.5 h, then cooled to 323 K in the same atmosphere, then switched to 5% (volume) H2 /N2 for 2 h in order to remove the residual O2 . Finally, the sample was reduced with a 5% (volume) H2 /N2 mixture (25 mL/min) by heating from 323 K to 1023 K at a rate of 10 K/min Temperature programmed desorption of O2 (O2 -TPD) was performed on a conventional self-made apparatus equipped with mass spectrometry (MS Hiden-Qic-20) as a detector. 100 mg of sample was fist treated in 10% O2 (50 mL/min) by heating from 323 K to 1023 K at a rate of 10 K/min and hold for 1 h at 1023 K. After cooled to 323 K in the same atmosphere, then, the sample was swept with Ar at a rate of 50 mL/min for 30 min in order to remove the residual O2 . Finally, the sample was heated to 1023 K at a rate of 10 K/min in Ar to record the TPD spectra with MS. Temperature programmed desorption of NO (NO-TPD) was performed on a conventional self-made apparatus equipped with mass spectrometry (MS Hiden-Qic-20) as a detector. 100 mg of sample was fist treated in 2000 ppm NO (50 mL/min) by heating from 323 K to 1023 K at a rate of 10 K/min and for 1 h at 1023 K. After cooled to 323 K in the same atmosphere, then, the sample was swept with Ar at a rate of 50 mL/min for 30 min in order to remove the residual O2 . Finally, the sample was heated to 1023 K at a rate of 10 K/min in Ar to record the TPD spectra with MS.

2.3. Catalytic activity tests Direct decomposition of NO was performed in a fixed-bed quartz reactor with an inner diameter of 6 mm under 1% NO diluted with He. A certain of La1.6 Ba0.4 NiO4 -x%BaO (x = 0, 5, 10, 15, 20, 25, 30) (containing La1.6 Ba0.4 NiO4 0.500 ± 0.001 g) prepared in situ without dilution was always set in the reactor by using quartz wool. The feed rate of the reactant was fixed at W/F = 1.2 gcat s mL−1 , where W and F were the catalyst weight (La1.6 Ba0.4 NiO4 ) and the gas flow rate. Produced N2 , O2 , and the fed NO were analyzed by gas chromatography online, which had a molecular sieve 5 A column and a thermal conductivity detector (TCD). In the coexistence of O2 experiment, a certain of La1.6 Ba0.4 NiO4 -20%BaO (containing La1.6 Ba0.4 NiO4 0.500 ± 0.001 g) was adopted and the gas reactant of the 1% NO, balance He and the concentration of O2 varied from 0.2% to 1.0% at constant W/F of 1.2 gcat s mL−1 . In any particular run, the data was recorded after temperature change every time to ensure that the equilibrium of catalytic reaction was reached. The activity of NO decomposition was evaluated by the following equations: N2 yield% = 2[N2 ]out /[NO]in × 100%

(1)

O2 yield% = 2[O2 ]out /[NO]in × 100%

(2)

NOconversion% = ([NO]in − [NO]out )/[NO]in × 100%

(3)

Where [NO]in was the concentration of NO measured before the reaction, [NO]out , [N2 ]out and [O2 ]out was the concentration of NO, N2 and O2 measured after the reaction, respectively.

L. Chen et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 277–284

90

160

80

N2 yield / %

Conversion and Yield / %

180

100

140

70

120

60

100

50 40

873K 923K 973K 1023K 1073K 1123K

30 20 10 0

279

0

5%

10%

15%

20%

25%

30%

60

b

40 20 0

Content of x%BaO

a

80

c 0

Fig. 1. Yield of N2 over La1.6 Ba0.4 NiO4 -x%BaO as a function of x (x = 0, 5, 10, 15, 20, 25, 30).

5

10

15

20 -3

Time on stream ×10 /min

25

30

Fig. 2. Stability tests of La1.6 Ba0.4 NiO4 -20%BaO at 923 K (a. conversion of NO, b. N2 yield, c. O2 yield).

3. Results 900 → 30 C o

3.1. Effects of BaO contents on activity

o

900 C o

800 C o

Intensity (a.u.)

The results of effect of different BaO contents on the catalytic activities of NO direct decomposition over the La1.6 Ba0.4 NiO4 catalyst are shown in Fig. 1. The La1.6 Ba0.4 NiO4 catalyst performed low activity below 1023 K. The N2 yield attained only a value of 4.7%, 9.7%, 19.8% and 34.1% at 873, 923, 973 and 1023 K, respectively. When the temperature increased to above 1073 K, the N2 yields were only a value of 53%, 76.5% at 1073 and 1123 K, respectively. The positive effect in terms of activity for deNOx could clearly be observed by in situ mixing different BaO contents with La1.6 Ba0.4 NiO4 . The yield of N2 increased from 9.7% to 53% at 923 K over the La1.6 Ba0.4 NiO4 -5%BaO catalyst. When the reaction temperature was 1023 K, the yield of N2 got to 70.8%. Moreover, the yield of N2 for each reaction temperature continuously increased with the amount of BaO increasing, which was 20% higher than that of the La1.6 Ba0.4 NiO4 in range of 923–1123 K. The yields of N2 over the La1.6 Ba0.4 NiO4 -20%BaO attained to 65% at 923 K and 85% at 1023 K. Increasing further in the content of BaO from 20% to 30%, the yield of N2 increased less. La1.6 Ba0.4 NiO4 -20%BaO has shown improved activity in the decomposition of NO reaction than that of catalysts reported in the literatures [18,25–28]. The promoting action of BaO was obviously observed between 873 and 1073 K with the increase in the BaO amount. BaO showed especial promotion action for NO decomposition over the La1.6 Ba0.4 NiO4 below 1073 K, and the enhanced value of N2 yield was about 50% for the La1.6 Ba0.4 NiO4 -20%BaO catalyst. However, the promoting effect of BaO on the yield of N2 became smaller and smaller with the increase in temperature. The results indicate that the existence of BaO can promote the activity of La1.6 Ba0.4 NiO4 for the NO direct decomposition into N2 and O2 . The conversion of NO and yields of N2 and O2 with the reaction time over the La1.6 Ba0.4 NiO4 -20%BaO catalyst at 923 K are shown in Fig. 2. As far as we know, this is almost higher than those of mixed oxide catalysts reported at this reaction temperature (923 K) [5,25,26,29]. It is interesting that the initial yields of N2 and O2 increased rapidly and more than 100%. With the increase in reaction time, the yields of N2 and O2 decreased, the value of N2 yield attained stable at about 65% during 360–1560 min, however, the value decreased from 65% to 60% during 1560–2880 min and last up to 7620 min, the subsequent value of the yield of N2 kept at 57% until to 30,000 min (500 h). It is clear that the yield of N2 is always much higher than that of O2 , and the conversion of NO is also always

750 C o

700 C o 600 C o

500 C o

400 C o 300 C o

200 C o

100 C o

30 C

20

30

40

2θ (degree)

50

60

Fig. 3. XRD patterns of the La1.6 Ba0.4 NiO4 -20%BaO at different temperatures ( BaO and  La2 NiO4 ).

higher than that of the yield of N2 from 300 min. La1.6 Ba0.4 NiO4 20%BaO exhibits much longer stable activity period at 923 K for the NO direct decomposition. In previous study, it was generally considered that the catalytic activity of La2 NiO4 for NO decomposition should be much lower at 923 K, but this study reveals that BaO has a beneficial effect for the NO decomposition over the La1.6 Ba0.4 NiO4 20%BaO catalyst. 3.2. Effects of BaO on the properties of La1.6 Ba0.4 NiO4 Fig. 3 illustrates the in situ XRD patterns of the mixture of La1.6 Ba0.4 NiO4 and Ba(NO3 )2 (calculated to 20% BaO) at different temperatures from 30 ◦ C to 900 ◦ C. Because BaO reacts easily with CO2 and generates BaCO3 , La1.6 Ba0.4 NiO4 -20%BaO was prepared by mixing La1.6 Ba0.4 NiO4 with Ba(NO3 )2 and heated them to obtain in situ. The diffraction peaks at 2 18.79, 21.80, 27.03, 36.44, 38.10, 48.70, 50.15, 59.20◦ are attributed to the phase of Ba(NO3 )2 at 30 ◦ C. The intensities of diffraction peaks got weaker and weaker with the increase in the temperature range from 30 ◦ C to 750 ◦ C, and the diffraction peaks of Ba(NO3 )2 totally disappeared at 750 ◦ C. It is noticed that the new diffraction peaks appeared at 32.03 and

L. Chen et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 277–284

Sample

T (◦ C)

1 (2)

2 (2)

3 (2)

4 (2)

La1.6 Ba0.4 NiO4 La1.6 Ba0.4 NiO4 La1.6 Ba0.4 NiO4 -20%BaO La1.6 Ba0.4 NiO4 -20%BaO La1.6 Ba0.4 NiO4 -20%BaO La1.6 Ba0.4 NiO4 -20%BaO D-value−900a D-value−900 ↔ 30b

30 900 750 800 900 900 → 30

32.84 32.70 32.76 32.66 32.52 32.67 0.18 0.15

42.28 41.92 41.90 41.88 41.86 41.88 0.06 0.02

47.20 46.84 46.90 46.88 46.74 46.78 0.10 0.04

57.72 57.28 57.33 57.26 57.22 57.26 0.06 0.04

a The different values of diffraction peaks at 900 ◦ C respectively on La1.6 Ba0.4 NiO4 20% BaO and La1.6 Ba0.4 NiO4 . b The different values of diffraction peaks on La1.6 Ba0.4 NiO4- 20%BaO respectively at 900 ◦ C and cool from 900 to 30 ◦ C.

45.94◦ which could be ascribed to the BaO phase at 750 ◦ C. The results suggest that Ba(NO3 )2 completely decomposed into BaO at 750 ◦ C. Diffraction peak positions (2 angle) of La1.6 Ba0.4 NiO4 and La1.6 Ba0.4 NiO4 -20%BaO at different temperatures were also listed in Table 1 in order to understand the structural changes from XRD patterns. In the Table 1, the 2 angle value of the diffraction peaks attributed to the La1.6 Ba0.4 NiO4 phase shifted to low angle when the measured temperature rose from 30 ◦ C to 900 ◦ C. The reason is that the lattice expansion of La1.6 Ba0.4 NiO4 occurs at high temperature, resulting in the diffraction peaks appear at lower angle. The corresponding diffraction peaks of the La1.6 Ba0.4 NiO4 phase in the La1.6 Ba0.4 NiO4 -20%BaO also moved gradually to lower 2 angle value when the tested temperature increased from 750 ◦ C to 800 ◦ C (900 ◦ C). The shifts of the diffraction peaks to the low angle have two reasons: the first is the expansion of lattice at high temperature; the second is that a small amount of Ba may incorporate into the La1.6 Ba0.4 NiO4 lattice structure leading to enlarge the lattice parameter. In addition, the intensity of the diffraction peaks attributed to the La2 NiO4 perovskite-like phase in the La1.6 Ba0.4 NiO4 -20%BaO become weak at 900 ◦ C, the reason is that the existence of BaO causes the crystal distortion. Ba(NO3 )2 melts before 600 ◦ C, which may cause BaO well dispersion in the lattice gap of the La1.6 Ba0.4 NiO4 . Thus, we suggest that partial Ba may incorporate into the lattice of the La1.6 Ba0.4 NiO4 phase except the existence as BaO for the La1.6 Ba0.4 NiO4 -20%BaO sample. The XRD pattern of (900 → 30 ◦ C) was also obtained when the La1.6 Ba0.4 NiO4 -20%BaO sample was cooled to 30 ◦ C from 900 ◦ C (donated as La1.6 Ba0.4 NiO4 -20%BaO (900 → 30 ◦ C)). Compared the 2 value of the diffraction peaks between the La1.6 Ba0.4 NiO4 and La1.6 Ba0.4 NiO4 -20%BaO (900 → 30 ◦ C) at 30 ◦ C, the 2 value for the La1.6 Ba0.4 NiO4 -20%BaO (900 → 30 ◦ C) is smaller than that of the La1.6 Ba0.4 NiO4 catalyst, which also indicates that the crystal lattice expansion results from the partial Ba incorporates into the lattice of La1.6 Ba0.4 NiO4 . In order to understand the role of BaO in the La1.6 Ba0.4 NiO4 20%BaO catalyst, a serial of experiments were carried out over the La1.6 Ba0.4 NiO4 -z%BaCO3 and La1.6 Ba0.4 NiO4 -24.32% BaCO3 catalysts (Fig. 4). The La1.6 Ba0.4 NiO4 -z%BaCO3 was obtained by making the La1.6 Ba0.4 NiO4 -20%BaO exposed in an atmosphere of air. It is well known that BaO would react with CO2 to transfer BaCO3 completely when it was adequately exposed to the air. So, the La1.6 Ba0.4 NiO4 -20%BaO catalyst could be changed into the La1.6 Ba0.4 NiO4 -z%BaCO3 that the z should be 24.32 according to the calculation of reaction equation in theory. The catalytic activity of the La1.6 Ba0.4 NiO4 -z%BaCO3 shows that the yield of N2 increases from 22% to 89% with temperature changes in range of 873–1123 K, which is clearly lower than that of the La1.6 Ba0.4 NiO4 20%BaO catalyst (51.7%-98.8% at 873–1123 K) (Fig. 4). However, the catalytic activity of the La1.6 Ba0.4 NiO4 -z%BaCO3 obtained from the La1.6 Ba0.4 NiO4 -20%BaO is different from that of the

100

75

50

25 La1.6Ba0.4NiO4-20% BaO La1.6Ba0.4NiO4-24.32% BaCO3 La1.6Ba0.4NiO4-z% BaCO3 La1.6Ba0.4NiO4

0 850

900

950

1000

1050

1100

T/K Fig. 4. The effects of BaO and BaCO3 for La1.6 Ba0.4 NiO4 catalyst on NO decomposition activity (1% NO 25 mL/min; catalyst containing La1.6 Ba0.4 NiO4 : 0.500 ± 0.001 g).

e d c b a

TCD signal / a.u.

Table 1 Diffraction peaks of La1.6 Ba0.4 NiO4 -x%BaO (x = 0, 20) at different temperatures.

N2 yield / %

280

300

400

500

600

700

800

900

1000

T/K Fig. 5. H2 -TPR profiles of La1.6 Ba0.4 NiO4 -x%BaO(x = 0–20) catalysts ((a) BaO, (b) La1.6 Ba0.4 NiO4 -0%BaO, (c) La1.6 Ba0.4 NiO4 -10%BaO, (d) La1.6 Ba0.4 NiO4 -15%BaO and (e) La1.6 Ba0.4 NiO4 -20%BaO).

La1.6 Ba0.4 NiO4 -24.32%BaCO3 prepared by mixing La1.6 Ba0.4 NiO4 and 24.32%BaCO3 . The La1.6 Ba0.4 NiO4 -z%BaCO3 exhibits much higher N2 yield than the La1.6 Ba0.4 NiO4 -24.32%BaCO3 . The order of the catalytic activity is La1.6 Ba0.4 NiO4 -20%BaO > La1.6 Ba0.4 NiO4 The z%BaCO3 > La1.6 Ba0.4 NiO4 -24.32%BaCO3 > La1.6 Ba0.4 NiO4 . La1.6 Ba0.4 NiO4 -z%BaCO3 and La1.6 Ba0.4 NiO4 -24.32%BaCO3 catalysts should present the similar catalytic activities because they have the same amount of BaCO3 in theory. Nevertheless, the N2 yield with temperature is different according to the above results. It suggests that BaO in the La1.6 Ba0.4 NiO4 -20%BaO have not been changed completely to BaCO3 in fact. The effect of BaO and BaCO3 on NO decomposition activity over La1.6 Ba0.4 NiO4 was distinctively different. The H2 -TPR was performed to evaluate the reduction properties of the catalysts. The TPR profiles of La1.6 Ba0.4 NiO4 -x%BaO (x = 0–20) catalysts are demonstrated in Fig. 5. All of H2 -TPR profiles show two reduction peaks, in which the first reduction peak (T1 ) at low temperature, which is mainly ascribed to the reduction of Ni3+ → Ni2+ accompanying the reduction of chemical adsorption oxygen adsorbed on the oxygen vacancies [1,8,13]. The second peak (T2 ) at high temperature is ascribed to Ni2+ → Ni0 , accompanying the reduction of lattice oxygen at high temperature region. The data of reduction peak sites and areas are listed in Table 2. The first reduction peak shifts from 641 K to 620 K and the corresponding area increases from 143 to 176 with the increase in the BaO content from 0% to 20%. The result indicates that the formation of more

L. Chen et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 277–284

281

Table 2 Data obtained from H2 -TPR of La1.6 Ba0.4 NiO4 -x%BaO (x = 0-20). Ni3+ → Ni2+ (T1 /K)

Ni3+ → Ni2+ peak area

Ni2+ → Ni0 (T2 /K)

Ni2+ → Ni0 peak area

T1 –T2 /K

La1.6 Ba0.4 NiO4 -0%BaO La1.6 Ba0.4 NiO4 -10%BaO La1.6 Ba0.4 NiO4 -15%BaO La1.6 Ba0.4 NiO4 -20%BaO

641 631 623 620

143 158 162 176

823 814 768 727

1134 1146 1157 1166

182 183 145 109

Ms Signal / a.u.

Sample

b a 300

450

600

750

900

1050

T/K Fig. 6. O2 -TPD profiles of (a) La1.6 Ba0.4 NiO4 , (b) La1.6 Ba0.4 NiO4 -20% BaO.

chemical adsorption oxygen species adsorbed on the oxygen vacancies for La1.6 Ba0.4 NiO4 -20%BaO due to partial Ba incorporates into the lattice of La1.6 Ba0.4 NiO4 derived from XRD results (Fig. 3). So, it suggests that the increase in amount of chemical adsorption oxygen should relate to the increase in oxygen vacancies with BaO content [19,20]. It is noticed that the H2 -consumption amount of the second peak is very similar for all La1.6 Ba0.4 NiO4 -x%BaO (x = 0–20) catalysts, however, the peak shifts obviously to low temperature from 823 K to 727 K with the increase in amount of BaO from 0% to 20%. The results suggest that the reduction process of the second peak becomes easier with the increase in the BaO amount for the La1.6 Ba0.4 NiO4 -x%BaO (x = 0–20) catalysts. Thus, the downward shift of T2 reveals the improvement of the reducibility of the lattice oxygen. The enhanced activities of the oxygen vacancies and lattice oxygen are favorable for the direct catalytic decomposition of NO. Based on the special activity and good reducibility of the La1.6 Ba0.4 NiO4 -20%BaO catalyst, a serial of further experiments and characterizations were carried out to investigate the essential factor of BaO on the catalytic activity. The O2 -TPD measurements (Fig. 6) were conducted to study the adsorption behavior of O2 and active sites of the La1.6 Ba0.4 NiO4 and La1.6 Ba0.4 NiO4 -20%BaO. The La1.6 Ba0.4 NiO4 catalyst showed only a weak oxygen desorption peak in range of 457–651 K, which was attributed to the desorption of the oxygen chemically adsorbed on oxygen vacancies. Obviously, two O2 desorption peaks at 468–602 K and 711–946 K were observed for the La1.6 Ba0.4 NiO4 -20%BaO catalyst. The low temperature peak is attributed to the desorption of the chemical adsorbed oxygen on oxygen vacancies, namely, the oxygen is released by reduction of Ni3+ according to the following reaction: 2Ni3+ -O2− → 2Ni2+ + Vo + 1/2O2 . The peak appeared in the range of 711–946 K, is assigned to the desorption of lattice oxygen. It is noteworthy that the desorption area of oxygen adsorbing on oxygen vacancies over the La1.6 Ba0.4 NiO4 -20%BaO is bigger than that of the La1.6 Ba0.4 NiO4 , which indicates that the existence of BaO is able to increase the amount of oxygen vacancies for the La1.6 Ba0.4 NiO4 -20%BaO. Furthermore, a desorption peak of lattice oxygen is presented significantly at 875 K, and its area is much larger than that of the chemical adsorption oxygen adsorbed on oxygen vacancies. However, only a very small O2 desorption was able to obtained for the BaO sample when the desorption

temperature was up to 1023 K (the curve is not given). Therefore, the desorption O2 at high temperature is not caused by the decomposition of BaO. It demonstrates that the BaO promotes the redox capability of the La1.6 Ba0.4 NiO4 catalyst, increases of the number oxygen vacancies and improves the mobility of lattice oxygen, which possibly leads to good catalytic performance. It partly accounts for the fact that the La1.6 Ba0.4 NiO4 -20%BaO performs excellent catalytic performance. Since the adsorption and activation of NO are important steps for NO decomposition, the NO-TPD was studied over the La1.6 Ba0.4 NiO4 -20%BaO and La1.6 Ba0.4 NiO4 catalysts. The NO desorption profiles are exhibited in Fig. 7. As shown in NO-TPD curves of the La1.6 Ba0.4 NiO4 (Fig. 7a and c), only small amount of NO desorption in company without O2 desorption was observed in the low temperature range of 420 K to 603 K. Accordingly, the ion current intensity of O2 decreased slightly with the increase in temperature. When the desorption temperature increased further, a large NO desorption peak accompanying with O2 desorption peak was observed at 733 K. It indicated that partial adsorbed NO reacted with the surface oxygen to form NOx species that adsorbed on the surface of the catalyst at low temperature. Then, the NOx species was decomposed to NO and O2 at 733 K. It is noticed that a NO2 desorption peak emerged at 733 K (Fig. 7e) that confirms the above speculation. It is obviously seen that the NO-TPD results are different for the existence of BaO for the La1.6 Ba0.4 NiO4 -20%BaO sample (Fig. 7b, d, f). Only a NO desorption peak over the La1.6 Ba0.4 NiO4 -20%BaO at 918 K companying with O2 desorption was observed in the NO-TPD curves. Moreover, the ion current intensity of O2 drifted seriously down without NO desorption peak at low temperature region (<800 K), which implies that the adsorbed NO act with O2 to form the NO2 and NO2 -derived species (nitrate and/or nitrite) on the surface of the La1.6 Ba0.4 NiO4 -20%BaO sample. However, a NO2 desorption peak appeared at 733 K compared with the La1.6 Ba0.4 NiO4 sample, meanwhile, a NO2 desorprtion peak was observed at much higher temperature (918 K) for the La1.6 Ba0.4 NiO4 -20%BaO sample (Fig. 7f). It is obvious that the existence of BaO is an advantage to the adsorption and reaction of NO over the La1.6 Ba0.4 NiO4 -20%BaO sample. We consider that the adsorbed NO and O2 species on the surface of La1.6 Ba0.4 NiO4 sample transfer to the BaO surface by spill-over action forming barium nitrate and/or nitrite. Due to the much stronger basicity of BaO, the barium nitrate and/or nitrite species can decompose into NO and O2 only at much higher temperature (918 K). In addition, a desorption peak of O2 is observed at 827 K companying without NO desorption in NO-TPD curves over the La1.6 Ba0.4 NiO4 -20%BaO sample, which can be attributed to the desorption of the lattice oxygen and confirmed at the same temperature in the O2 -TPD of the La1.6 Ba0.4 NiO4 -20% BaO sample. These results mean that the existence of BaO can improve the mobility of the lattice oxygen of the La1.6 Ba0.4 NiO4 , which is in agreement with the results of H2 -TPR and O2 -TPD of the La1.6 Ba0.4 NiO4 -20%BaO sample. In the experiments, it is observed that the yield of N2 is lower than the conversion of NO, however, it is higher than the yield of O2 in whole activity evaluation for the La1.6 Ba0.4 NiO4 -20%BaO catalyst. Similarly, others have also noticed the same phenomenon, which has been confirmed that NO2 and NO2 -derived species are impor-

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b

a c

300

400

500

600

700

800

900

1000

e

MS Signal / a.u.

MS Signal / a.u.

d

f 300

400

500

T/K

600

700

800

900

1000

T/K

Fig. 7. NO-TPD profiles of La1.6 Ba0.4 NiO4 -20%BaO and La1.6 Ba0.4 NiO4 (O2 signal of (a) La1.6 Ba0.4 NiO4 and (b) La1.6 Ba0.4 NiO4 -20%BaO; NO signal of (c) La1.6 Ba0.4 NiO4 and (d) La1.6 Ba0.4 NiO4 -20%BaO; NO2 signal of (e) La1.6 Ba0.4 NiO4 and (f) La1.6 Ba0.4 NiO4 -20%BaO).

b c d

N2 yield / %

80 60 40

a

60

N2 Yield / %

a

100

O2 ON

45

O2 OFF

b c

30

d

e

15

20

0 900

950

1000

T/K

1050

1100

Fig. 8. N2 yields over La1.6 Ba0.4 NiO4 -20%BaO with different contents of O2 (Reaction conditions: 1% NO, balance He, W/F = 1.2 gcat s mL−1 , a: 0.0% O2 , b: 0.2% O2 , c: 0.4% O2 , d: 0.6% O2 ).

tant intermediates during NO decomposition process [25,29,30]. Hence, it is assumed that the oxygen dissociated by NO also partly reacts with NO to form intermediates NO2 species, besides the oxygen adsorbed on active site of catalysts. Many researchers have reported that the formed oxygen adsorbed on active site of catalysts than desorption, resulting in the yield of O2 is lower than N2 [15,20]. Noticeably, NO2 desorption does not occurred in the NO-TPD curves of the La1.6 Ba0.4 NiO4 -20%BaO (Fig. 7). It indicates that NO2 species should be stored on the surface of the catalyst. Thus, based on the composition of the La1.6 Ba0.4 NiO4 -20%BaO catalyst, BaO phase plays a key role in NO species storage in the above process. 3.3. Effects of O2 contents on La1.6 Ba0.4 NiO4 -20% BaO for NO decomposition The effect of co-feeding of O2 on the activity was also investigated over the La1.6 Ba0.4 NiO4 -20%BaO catalyst. The N2 yield as a function of the additional concentration of O2 at different temperatures was shown in Fig. 8. It is noticed that the N2 yield decreased sharply when the concentration of O2 (1% NO balance He, W/F = 1.2 gcat s mL−1 ) varied from 0.2% to 0.6%, and the yield of N2 got to merely 42% and 50% at 1073 and 1123 K under the O2 concentration of 0.6% over the La1.6 Ba0.4 NiO4 -20%BaO, respectively. Therefore, the catalytic activity of La1.6 Ba0.4 NiO4 -20%BaO for NO decomposition is inhibited when the O2 is added in the feed, and the more amount of O2 , the lower N2 yield. Interestingly, negative effects of coexistence of O2 was less at 923 K than at other temperatures, and reasonably higher N2 yield was observed under the O2 concentration of 0.2% coexistence condition. So, the

250

300

350

400

450

Time on stream ×10 /min

500

Fig. 9. N2 yields as a function of the O2 content over La1.6 Ba0.4 NiO4 -20%BaO at 923 K(1% NO, balance He, W/F = 1.2 gcat s mL−1 , a: 0.2% O2 , b: 0.4% O2 , c: 0.6% O2 , d: 0.8% O2 , e: 1% O2 ).

La1.6 Ba0.4 NiO4 -20%BaO catalyst shows high anti-passivation of O2 at 923 K. Maybe, 923 K is a special temperature point for the storage and decomposition of NOx , therefore, the effect of O2 on the NO decomposition over the La1.6 Ba0.4 NiO4 -20%BaO at 923 K would be further researched. In conventional study, co-feeding of O2 significantly lowered the activity for the NO decomposition and only few catalysts could exhibit much higher NO conversion under coexistence of O2 [8,14,30–34]. The effect of different concentration of O2 on the NO direct decomposition over the La1.6 Ba0.4 NiO4 -20%BaO at 923 K is shown in Fig. 9. The yield of N2 decreases with the increase of the concentration of O2 in the reaction gas. The yield of N2 was about 66% at 923 K without O2 in the feed. It is noted that the N2 yield reduces sharply with the concentration of O2 increasing from 0.2% to 0.6%, which decreases from 57% to 38% at 923 K. The possible reason is that coexisting O2 strongly adsorbs on the active sites of the La1.6 Ba0.4 NiO4 -20%BaO under O2 -rich conditions. However, the N2 yield is much higher under 0.2% O2 coexistence, and the N2 yield is sustained at 57%. Only a limited number of catalysts showed stable NO decomposition activity under the coexistence of O2 in the literatures [8]. Teraoka et al. reported that Sr0.6 La0.4 Mn0.8 Ni0.2 O3 could decompose NO into N2 under coexistence of O2 up to 10% [8]. When O2 amounted to 0.4%, N2 formation rate decreased from 67% to about 48%, the yield of N2 reduced with the concentration of O2 increasing. It is interesting that when O2 is stopped to be added into the feed over the La1.6 Ba0.4 NiO4 -20%BaO, the yield of N2 immediately returns to the normal level. It is noted that the inhibition of O2 is reversible, and the presence of less than 1% O2 can strongly adsorb on the active sites of the La1.6 Ba0.4 NiO4 -20%BaO,

L. Chen et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 277–284

283

a

80 70 60 50

NO N2 O2

40 30 1

2

3

4

5

6

7

3 -1

GHSV (× 10 h )

Conversion and Yield (%)

Conversion and Yield (%)

90

b

90 75 60 45

NO N2 O2

30 2

3

4

5

6

7

3 -1

GHSV (× 10 h )

Fig. 10. Conversions of NO and the yields of N2 and O2 with different GHSV over the La1.6 Ba0.4 NiO4 -20%BaO at different temperatures (a: 923 K, b: 973 K).

which reduces the regeneration of oxygen vacancy and leads to the inhibition for the NO decomposition.

route 1

route 2

recycle 1

3.4. Effects of different space velocities for NO decomposition The effect of the different space velocities on the activity of NO decomposition was evaluated over the La1.6 Ba0.4 NiO4 -20%BaO at 923 and 973 K. The results are shown in Fig. 10. Under the lower space velocity, NO conversion, N2 and O2 yields were much higher. 87% NO could be converted to N2 at 1200 h−1 , and 71% NO to N2 at 7200 h−1 at 923 K (Fig. 10a). The NO conversion could also maintain high level under high space velocity at 973 K (Fig. 10b). Meanwhile, 96% and 80% of NO conversions could be attained at 973 K under 1800 and 7200 h−1 , respectively. With the increase in space velocity, N2 and O2 yields became quickly smaller and smaller. However, the conversion of NO maintained still high value at large space velocity. Therefore, the La1.6 Ba0.4 NiO4 -20%BaO displays much higher catalytic activity with the altering of the space velocities. 4. Discussion The above results (Figs. 1, 2 and 4) indicate that BaO can promote the catalytic activity for NO decomposition for the La1.6 Ba0.4 NiO4 . In order to understand the role of BaO in the La1.6 Ba0.4 NiO4 -20%BaO catalyst, the catalysts were carefully examined by XRD, H2 -TPR, O2 TPD and NO-TPD measurements. The results of XRD show that the diffraction peaks of the La1.6 Ba0.4 NiO4 -20%BaO sample shift to low angle compared with the La1.6 Ba0.4 NiO4 sample, which indicates that the partial Ba incorporate into the lattice of the La1.6 Ba0.4 NiO4 phase except the existence as BaO for the La1.6 Ba0.4 NiO4 -20%BaO sample. The results of H2 -TPR suggest that the presence of BaO is favorable for the reduction of Ni3+ → Ni2+ and the reduction of chemical adsorption oxygen adsorbed on the oxygen vacancies, in the meanwhile, the numbers of the oxygen vacancies increase with increasing the Ba amount. The O2 -TPD results also confirm the above results reported in Fig. 6. So we think that BaO leads to the formation of more chemical adsorption oxygen for La1.6 Ba0.4 NiO4 x%BaO (x = 5–20) catalysts. In addition, the results of H2 -TPR also suggest that the presence of BaO is propitious to the reduction of Ni2+ → Ni0 , as well as the reduction of lattice oxygen at high temperature region, however, the reduction temperature shift to low temperature region with the increase in the amount of BaO. The O2 -TPD profiles of the La1.6 Ba0.4 NiO4 -20%BaO shows a large peak in range of 711–946 K compared with the La1.6 Ba0.4 NiO4 , which also reveals that the existence of BaO can increase the mobility of lattice oxygen. The NO-TPD curves of La1.6 Ba0.4 NiO4 -20%BaO shows a large lattice O2 desorption peak in range of 711–865 K without NO desorption peak, which further indicates that mobility of the

2Ni3+-Vo-Ni2++2NO

1/2N2(g)

2Ni3+-NO--Ni3+

1/2O2(g)

Ni3+-O--Ni3+ + Ni3+-NO--Ni3+ recycle 2 1/2O 2(g) Ni -NO2--Ni3+ 3+

1/2O2

+ BaO

recycle 3

barium-nitro phase/ barium-nitrate phase

route 3

BaO

Scheme 1. Reaction routes of NO decomposition over La1.6 Ba0.4 NiO4 -x% BaO.

lattice oxygen is improved by BaO. Furthermore, all the evidences indicate that the adsorbed NO and O2 species on the surface of La1.6 Ba0.4 NiO4 sample transfers to the BaO surface by spill-over action forming barium nitrate and/or nitrite. So we consider that BaO plays a key role on transferring and storing of NOx species in the process of NO decomposition over the La1.6 Ba0.4 NiO4 -20%BaO. Thus, the reasons that BaO can promote the catalytic activity for the NO decomposition over the La1.6 Ba0.4 NiO4 -20%BaO can be summarized as: First, increasing the number of oxygen vacancies. XRD, H2 -TPR and O2 -TPD results can confirm the formation of more oxygen vacancies, when the BaO is added. Second, the mobility of the lattice oxygen is improved, which is also proved by the H2 -TPR, O2 -TPD and NO-TPD results. Third, BaO plays an important role in NOx transportation and storage, which is demonstrated by NO-TPD. The possible reaction route of the NO direct decomposition over the La1.6 Ba0.4 NiO4 -x%BaO is shown in Scheme 1. Direct decomposition of NO over perovskites-like oxides depends mainly on the numbers of oxygen vacancies and the redox-capability of B-site cations. Ni3+ -Vo-Ni2+ is regarded as the active sites to dissociate NO over the perovskites(-like) oxide catalysts [6,19,35–37]. The gaseous NO adsorbs first on the active site like reaction (1), and dissociates into N2 and atomic oxygen (reaction (2)). The atomic oxygen on oxygen vacancy (Ni3+ -O− -Ni3+ ) is desorbed to form O2 (O2(g) ) with another one. Thus, the active site can be regenerated (route 1). But this process is slow and only occurs at much higher temperature. Iglesia et al. and Yang et al. suggested that the recycle of NO2 (its formation and dissociation) played an important role in the reaction over perovskite-like oxides [6,38]. The gaseous NO (NO(g)) or adsorbed NO (NO(a) ) could reacts with the surface oxygen to form adsorbed NO2 (NO2(a) ) species on the active sites. The adsorbed NO2 (NO2(a) ) species can decompose into O2 and NO under some reaction conditions (route 2). In addition, the desorption of NO2(a) species can form gaseous NO2 (NO2(g) ). Obviously, two NO2 desorption peaks at 650–820 K and >920 K were observed for the La1.6 Ba0.4 NiO4 sample, and a small NO2 des-

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orption peak was detected at 825–990 K in our experiments as shown in Fig. 7. According to the above discussion (Fig. 6), the basic BaO could adsorb intermediate NO2 species resulting in a new catalytic route. It is suggested that the gaseous NO or adsorbed NO (Ni3+ -NO− -Ni3+ ) could react with surface oxygen (Ni3+ -O− -Ni3+ ) to form NO2 -BaO transition state species (nitrates or nitrites, denoted as: (NO2 -BaO)*) resulting in regeneration of Vo (Ni3+ -Vo-Ni2+ ) in the function of BaO described as reaction 3 and 4. In high reaction temperature, the (NO2 -BaO)* species is strongly unstable, and consequently decompose rapidly to NO and O2 (reaction (5)). Thus, BaO, as an assistant, takes the function of accelerating the regeneration of Vo and speeding up the run of catalytic NO decomposition recycle (route 3 in Scheme 1). It should be pointed out that the number of Vo and the property of B site cations are essential factors to NO decomposition [6,7,19,20]. This is maybe why La0.4 Ba1.6 NiO4 performed poor activity below 1023 K, but La1.6 Ba0.4 NiO4 -20%BaO showed high activity for NO direct decomposition. The possible key reaction steps of NO decomposition in presence of BaO are as follows: Ni3+ -Vo-Ni2+ + NO(g) → Ni3+ -NO− -Ni3+

(4)

2Ni3+ -NO− -Ni3+ → N2(g) + 2Ni3+ -O− -Ni3+ 3+

Ni

Ni

−

3+

-NO -Ni

3+

−

+ Ni

3+

-O -Ni

3+

−

-O -Ni

3+

+ BaO → 2Ni

+ NO(g) + BaO → Ni

3+

3+

(5) 2+

-Vo-Ni

-Vo-Ni

(NO2 -BaO)∗ → BaO + NO(g) + 1/2O2(g)

+ (NO2 -BaO)∗

2+

+ (NO2 -BaO)∗

(6) (7) (8)

5. Conclusion In summary, La1.6 Ba0.4 NiO4 -x%BaO (x = 5–20) catalysts are successfully prepared in situ as high improved performance deNOx catalysts. La1.6 Ba0.4 NiO4 -20%BaO displays stable activity for NO direct decomposition, and the yiled of N2 keeps 57% during 500 h at 923 K in the absence of O2 . Even the concentration of O2 was 0.2% in the feed, the N2 yield still up to 57% at 923 K. It reveals that La1.6 Ba0.4 NiO4 -20%BaO exhibits much higher activity in direct NO decomposition even in presence of O2 , which is attributed to three aspects: (1) A small amount of Ba entering into the lattice of La1.6 Ba0.4 NiO4 when BaO forms in suit, resulting in the increase in the number of oxygen vacancies; (2) improving the mobility of the lattice oxygen and strengthening the redox capability oxidation of La1.6 Ba0.4 NiO4 due to the interaction between BaO and La1.6 Ba0.4 NiO4 , which is also beneficial to NO decomposition; (3) BaO plays an important role in NOx transportation and storage, which is favorable for the regeneration of the active sites and the promotion of the catalytic activity. Therefore, the La1.6 Ba0.4 NiO4 20%BaO catalyst with better catalytic performance could open up great opportunities for the application of de-NOx catalysts. Acknowledgements This work is supported by the the Natural Science Foundation of Heilongjiang Province (B2015009), Innovation Research

Team of Heilongjiang University (Hdtd2010-10), Foundation of Educational committee in Heilongjiang province (11531286), Postdoctoral Scientific Research Project of Heilongjiang Province (LBH-Q12022), and Innovative Talents Program of Heilongjiang University (Q20130202). References [1] H. Bosch, F. Janssen, Preface Catal. Today, vol. 2, Elsevier, Amsterdam, 1988, pp. 369–521. [2] J. Kaˇspar, P. Fornasiero, N. Hickey, Catal. Today 77 (2003) 419–449. [3] S. Elbouazzaoui, E.C. Corbos, X. Courtois, P. Marecot, D. Duprez, Appl. Catal. B: Environ. 61 (2005) 236–243. [4] G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B: Environ. 18 (1998) 1–36. [5] W.J. Hong, M. Uedaa, M. Inoue, Appl. Catal. B: Environ. 106 (2011) 142–148. [6] J.J. Zhu, D.H. Xiao, J. Li, X.F. Xie, X.G. Yang, Y. Wu, J. Mol. Catal. A: Chem. 233 (2005) 29–34. [7] J.J. Zhu, X.Y. Yang, X.L. Xu, W. Kemei, J. Phys. Chem. C 111 (2007) 1487–1490. [8] E.R.S. Winter, J. Catal. 22 (1971) 158–170. [9] S.B.C. Pergher, R.M. Dallago, R.C. Veses, C.E. Gigola, I.M. Baibich, J. Mol. Catal. A: Chem. 209 (2004) 107–115. [10] R.J. Wu, T.Y. Chou, C.T. Yeh, Appl. Catal. B: Environ. 6 (1995) 105–116. [11] E.R.S. Winter, J. Catal. 34 (1974) 440–444. [12] M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett. 18 (1989) 213–216. [13] S. Morpurgo, G. Moretti, M. Bossa, J. Mol. Catal. A: Chem. 358 (2012) 134–144. [14] Y. Teraoka, H. Fukuda, S. Kagawa, Chem. Lett. 19 (1990) 1–4. [15] Y. Teraoka, T. Harada, S. Kagawa, J. Chem. Soc. Faraday Trans. 94 (1998) 1887–1891. [16] J.J. Zhu, H.L. Li, L.Y. Zhong, P. Xiao, X.L. Xu, X.G. Yang, Z. Zhao, J.L. Li, ACS Catal. 234 (2014) 2917–2940. [17] S. Shin, H. Arakawa, Y. Hatakeyama, K. Ogawa, K. Shimomura, Mater. Res. Bull. 14 (1979) 633–639. [18] T. Ishihara, A. Ando, K. Takiishi, K. Yamada, H. Nishiguchi, Y. Takita, J. Catal. 220 (2003) 104–114. [19] Z. Zhao, X.G. Yang, Y. Wu, Appl. Catal. B: Environ. 8 (1996) 281–297. [20] Y.J. Zhu, D. Wang, F.L. Yuan, G. Zhang, H.G. Fu, Appl. Catal. B: Environ. 82 (2008) 255–263. [21] J. Deng, L. Zhang, Y. Xia, H. Dai, H. He, J. Environ. Sci. 22 (2010) 448–453. [22] C. Tofan, D. Klvana, J. Kirchnerova, Appl. Catal. A 223 (2002) 275–286. [23] J. Zhu, X. Yang, X. Xu, K. Wei, Sci. China Ser. B 50 (2007) 41–46. [24] H. Yasuda, N. Mizuno, M. Misono, J. Chem. Soc. Chem. Commun. 16 (1990) 1094–1096. [25] K. Goto, H. Matsumoto, Top. Catal. 52 (2009) 1776–1780. [26] W.J. Hong, S.J. Iwamoto, M. Inoue, Catal. Lett. 135 (2010) 190–196. [27] J.H. Kwak, D.H. Kim, J. Szanyi, S.J. Cho, H.F. Peden, Top. Catal. 55 (2012) 70–77. [28] W.J. Hong, M. Ueda, S. Iwamoto, S. Hosokawa, K.J. Wada, M. Inoue, Catal. Lett. 142 (2012) 32–41. [29] T. Ishihara, K. Goto, Catal. Today 164 (2011) 484–488. [30] H. Iwakuni, Y. Shinmyou, H. Yano, H. Matsumoto, T. Ishihara, Appl. Catal. B: Environ. 74 (2007) 299–306. [31] S. Xie, M.P. Rosynek, J.H. Lunsford, J. Catal. 188 (1999) 24–31. [32] Y.F. Chang, J.G. McCarty, J. Catal. 178 (1998) 408–413. [33] M.A. Vannice, A.B. Walters, X.J. Zhang, J. Catal. 159 (1996) 119–126. [34] M. Iwamoto, H. Yahiro, K. Tanda, N. Mizuno, Y. Mine, S. Kagawa, J. Phys. Chem. 95 (1991) 3727–3730. [35] J.J. Zhu, Z. Zhao, D.H. Xiao, J. Li, X.G. Yang, Y. Wu, J. Mol. Catal. A: Chem. 238 (2005) 35–40. ˜ J.L.G. Fierro, Chem. Rev. 101 (2001) 1981–2017. [36] M.A. Pena, [37] J.J. Zhu, D.H. Xiao, J. Li, X.G. Yang, Y. Wu, J. Mol. Catal. A: Chem. 234 (2005) 99–105. [38] B. Modén, P.D. Costa, B. Fonfé, D.K. Lee, E. Iglesia, J. Catal. 209 (2002) 75–86.