Selective catalytic oxidation of ammonia to nitrogen over MnO2 prepared by urea-assisted hydrothermal method

Selective catalytic oxidation of ammonia to nitrogen over MnO2 prepared by urea-assisted hydrothermal method

Applied Surface Science 351 (2015) 573–579 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 351 (2015) 573–579

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Selective catalytic oxidation of ammonia to nitrogen over MnO2 prepared by urea-assisted hydrothermal method Zhenping Qu ∗ , Rui Fan, Zhong Wang, Hui Wang, Lei Miao Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Linggong Road 2, Dalian 116024, China

a r t i c l e

i n f o

Article history: Received 25 March 2015 Received in revised form 21 May 2015 Accepted 24 May 2015 Available online 1 June 2015 Keywords: Urea-assisted hydrothermal method MnO2 Microcrystal NH3 -SCO

a b s t r a c t Selective catalytic oxidation of ammonia was studied over MnO2 catalysts synthesized by urea-assisted and tranditional hydrothermal method. A contrastive research was also made to a commercial MnO2 catalyst. The catalytic performance of MnO2 for NH3 -SCO was found to be connected with the properties of the catalysts which were obviously determined by the preparation methods. The best catalytic activity was achieved at 170 ◦ C over the MnO2 synthesized by urea-assisted hydrothermal method. The properties of catalysts were characterized by BET, XRD, SEM, XPS, H2 -TPR and NH3 -TPD techniques. And it was found that MnO2 prepared by urea-assisted synthetic method showed larger surface area and lower reduction temperature. Abundant NH3 adsorption sites and oxygen vacancies were formed on its surface compared to the other catalysts, which evidently enhanced the catalytic performance for NH3 selective oxidation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The abatement of ammonia from waste streams was becoming a severe issue due to the increasing environmental concerns [1,2]. Selective catalytic oxidation (SCO) of ammonia to nitrogen and water was known as a potentially ideal technology to reduce ammonia because of its environmentally benign product and had attracted numerous attention in recent years [3]. The critical matter of such a technology was the availability of high-performance catalysts. Noble-metal-oxide, such as Ag2 O [4], RuO2 [5] were the most active catalysts with T100 at 180 ◦ C and 240 ◦ C, but the nitrogen selectivity was low (38% and 78%). Moreover the exorbitant price of noble metals also limited their widespread application. Relatively cheap transition-metal-oxide catalysts, including CeO2 [6,7] and Co3 O4 [8], also showed appropriate activities for NH3 -SCO, but the operation temperature remained unsatisfactory (300–450 ◦ C). Therefore, it is essential to develop the efficient transition-metaloxide catalysts for the removal of NH3 at low temperatures. MnO2 exhibited good catalytic performance in many fields because of its multivalent and polycrystalline type [9–11]. For example, Shi et al. pointed out that ␣-MnO2 and ␧-MnO2 showed good catalytic activities in the combustion of toluene [11]. Liang et al. found that ␣-MnO2 and ␦-MnO2 were active for CO oxidation

∗ Corresponding author. Tel.: +86 411 84708083; fax: +86 411 84708083. E-mail address: [email protected] (Z. Qu). http://dx.doi.org/10.1016/j.apsusc.2015.05.154 0169-4332/© 2015 Elsevier B.V. All rights reserved.

[9]. However, as far as we know, rare researches were carried out on the catalytic application of MnO2 with various morphologies and different crystal structures for NH3 oxidation. At the same time, various preparation methods could cause the change on the surface properties, crystal structures and morphology of MnO2 and hence on the activity. In this paper, the urea-assisted and tranditional hydrothermal method were adopted to prepare MnO2 catalysts. The catalytic properties of the synthesized MnO2 in selective oxidation of ammonia were also compared to that of the commercial one in order to elucidate the effect of both synthetic method and urea addition on the structure of MnO2 and its catalytic activity.

2. Experimental methods 2.1. Preparation of catalysts In our experiment, the hydrothermal method was employed to prepare catalysts, as shown in Fig. 1. In a typical synthesis, 50% Mn(NO3 )2 solution and a certain amount of KMnO4 were dissolved in 150 ml deionized water at room temperature and magnetically stirred to form a homogeneous solution A. Then some amount of urea was added into solution A with vigorously stirring to obtain solution B. The mixed solution B was transferred into two 100 ml-capacity Teflon-lined stainless steel autoclaves after stirring for 12 h at room temperature. Then the autoclaves were sealed and hydrothermally

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Fig. 1. Synthetic process of the catalysts.

2.2. Characterization techniques The N2 adsorption/desorption measurements were carried out on Quantachrom quadrasorb S1. Before analysis, each catalyst was heated for 4 h at 300 ◦ C under vacuum. The surface area was calculated using the Brunauer–Emmett–Teller (BET) model. The pore size distributions were determined by the Barrett–Joyner–Halenda (BJH) method. The crystallinity of the catalysts were measured by X-ray diffraction (Rigaku D/max-␥b X-ray diffractometer) with a Cu-K␣ radiation in the 10◦ ≤ 2Â ≤ 90◦ range at RT. Scanning electron microscope (SEM) experiments were performed on a JSM-5600LV microscope. The H2 -TPR was performed on a Quantachrom Automated Chemisorption Analyzer with 30 mg catalyst. Before the reduction, the catalyst was purged with helium at 200 ◦ C for 30 min and then cooled down to room temperature. The 10% H2 /Ar passed through the reactor and the temperature was heated to 600 ◦ C at a ramping rate of 10 ◦ C/min. The H2 consumption was monitored by a thermal conduction detector (TCD). XPS was measured using an X-ray photoelectron spectrometer (AMICUS, Shimadzu) with a monochromatic X-ray source of Al K␣ under ultra-high vacuum. The binding energies were calibrated internally by the carbon deposit C1s binding energy (BE) at 284.8 eV. The temperature programmed desorption of ammonia was performed in a fixed-bed flow reactor with a computer-interfaced quadruple mass spectrometer (MS) as detector. Before the experiment, 100 mg of catalyst was pretreated in He at 200 ◦ C for 30 min. After cooling to room temperature, NH3 was adsorbed at room temperature until the MS showed the stable intensity (m/e = 17). Then the reactor was purged with He, and heated to 700 ◦ C at a ramping rate of 10 ◦ C/min in a flow of He. 2.3. Catalytic activity tests

concentrations and N2 selectivity were measured by the NH3 analyzer (GXH-1050, Beijing) and Gas Chromatograph using a 5A column with a TCD detector, respectively. 3. Results and discussion 3.1. Catalytic activity for ammonia SCO The catalytic activity of the three catalysts for NH3 oxidation is shown in Fig. 2. For all the catalysts, the NH3 conversion increased with the reaction temperature. The MnO2 (UH) catalyst showed the best catalytic activity for the oxidation of NH3 during the whole detected temperature range and achieved 90% of NH3 conversion at 140 ◦ C. While only 68% and 28% ammonia transformed at this temperature for the MnO2 (H) and commercial MnO2 , respectively. With further increase of the temperature, 100% NH3 conversion was achieved on the MnO2 (UH) catalyst at 170 ◦ C, which is significantly lower than that on the MnO2 (H) (200 ◦ C) and commercial MnO2 (260 ◦ C). Furthermore, such low complete conversion temperature (170 ◦ C) is even lower than the pure Ag2 O [2], RuO2 [3]. Therefore, the MnO2 (UH) catalyst with the T100 at 170 ◦ C is probably an ideal material for the low-temperature NH3 oxidation reaction. The further research for the N2 selectivity over the MnO2 (UH) catalyst is shown in Fig. 3. The N2 selectivity changed in the range of 65–49% (100–170 ◦ C), which was dissatisfactory for the environmentally friendly technique. The reason for the poor selectivity maybe the generation of by-products, such as N2 O, NO, NO2 , which will be discussed in the latter part. 3.2. Physical properties, phase structures and morphology The physical properties of the manganese dioxide catalysts were listed in Table 1. As could be seen from Table 1, the average pore

100

NH3 Conversion (%)

treated in an oven at 120 ◦ C for 12 h. After the natural cooling of the autoclaves down to room temperature, the resulting precipitates were collected by centrifuging and rinsing several times with deionized water and absolute alcohol to remove excess ions, and then dried at 80 ◦ C in air overnight. Finally, MnO2 (UH) was obtained by calcining the precipitates in air at 400 ◦ C for 6 h. For the preparation of MnO2 (H), solution A was directly shifted into Teflon-lined stainless steel autoclave without the addition of urea. The following steps remained the same as described above. The commercial MnO2 sample was purchased from Tianjin Kemiou Chemical Reagent Company, and used after calcining in a muffle furnace at 400 ◦ C for 6 h.

80

MnO2 (UH) MnO2 (H)

60

Commercial

40

20

The catalytic activity test of the catalysts for the NH3 oxidation were carried out in a fixed-bed flow reactor (8 mm in interior diameter) under atmospheric pressure. Typically, 0.2 g catalyst was loaded in a quartz tube reactor for activity test. The catalyst was calcined at 200 ◦ C for 30 min in the flow of He before reaction to remove the impurities adsorbed on the surface. The typical reactant gas composition was as follows: 1000 ppm NH3 , 10 vol.% O2 , and balance He. The total flow rate was 100 ml/min. The NH3

0 60

80

100

120

140

160

180

200

220

240

260

Temperture(oC) Fig. 2. NH3 conversion over the manganese dioxide catalysts.

280

300

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575

Fig. 3. The N2 selectivity of the MnO2 (UH) catalyst.

sizes of the three catalysts were similar, and all were within the range of 10.37–12.72 nm. While the BET surface areas together with the corresponding pore volumes of these catalysts showed great difference. Compared with the commercial manganese dioxide catalyst, the catalysts prepared by hydrothermal methods showed the larger BET surface area and pore volume, especially the MnO2 (UH) catalyst with the BET surface area of 187.5 m2 /g and the pore volume of 0.4877 m3 /g, which provided more exposed active sites [12]. It revealed that the addition of urea in the synthesis procedure resulted in a significant increase of the BET surface area and pore volume. Meanwhile, associating with the activity test results, it could be concluded that the large BET surface area and pore volume had something to do with the activity of NH3 -SCO, and promoted the catalytic performance of catalyst to same extent. Furthermore, the reaction rate for NH3 oxidation at 100 ◦ C over the three catalysts were also calculated and listed in Table 1, and it was found that the MnO2 (UH) catalyst exhibited the highest reaction rate (9.0 ␮mol g−1 s−1 ) among the three catalysts. Fig. 4 showed the XRD patterns of the manganese dioxide catalysts for the investigation of the crystallographic nature. Compared with the standard XRD patterns of the ␣-MnO2 (JCPDS PDF 440141) and the ␤-MnO2 (JCPDS PDF 24-0735), the XRD results revealed that the commercial MnO2 showed the characteristic peaks of ␤-MnO2 at 2Â = 28.62◦ , 37.32◦ , 42.8◦ , 56.63◦ , 64.78◦ , 72.37◦ . However all the diffraction peaks of the as-prepared catalysts could be well indexed to a tetragonal ␣-MnO2 structure but with a distinct difference in intensity. When urea was imported as precursor, the intensities and widths of the diffraction peaks of the ␣-MnO2 were greatly lowered and widened without change in its crystal phase. Therefore, it was revealed that the moderate amount of urea in the reaction mixture could lead to smaller crystallite sizes, and MnO2 (UH) was composed of microcrystalline structured ␣-MnO2 . In order to further investigate the catalyst morphology and the influence of the synthesis conditions on the structural features, the panoramic morphologies of the manganese dioxide catalysts were executed. It could be apparently seen from the Fig. 5, the

Fig. 4. X-ray diffraction patterns of the manganese dioxide catalysts: (a) MnO2 (H), (b) MnO2 (UH), (c) commercial MnO2 .

as-obtained catalysts by hydrothermal methods showed smaller sizes than the commercial MnO2 . Particularly, the urea-assisted synthesized MnO2 showed highly uniform and monodisperse, and the microcrystal particles (0.5–0.8 ␮m) were composed of nanoplates in high quantity and high porosity (Fig. 5d). Specially, compared with the high-magnification between MnO2 (UH) and MnO2 (H) catalyst (Fig. 5b and d), it could obtained that urea prompted the formation of the nanoplates rather than nanorods, and made the nanoplates to aggregate the flower-like polyporous structure. Therefore, the addition of appropriate amount of urea was a vital procedure in this paper. These results were also in accordance with the BET surface area and XRD results (Table 1), the microcrystal ␣-MnO2 sample showed the large surface area which would be helpful to the adsorption performance. 3.3. Reducibility study H2 -TPR experiments were carried out to investigate the reducibility of the manganese dioxide catalysts (Fig. 6). Among all the MnO2 catalysts, the MnO2 (UH) catalyst exhibited the lowest reduction temperature (378 ◦ C and 427 ◦ C), corresponding to a total H2 consumptions of 6.84 mmol/g (Table 2). The MnO2 (H) sample only showed one broad reduction peak centered at 421 ◦ C and the corresponding H2 consumptions was 8.28 mmol/g (Table 2). And a main reduction peak at 399 ◦ C with a shoulder at 428 ◦ C was observed for the commercial MnO2 catalyst. According to the results reported previously [13], the reduction process of MnO2 could be reasonably divided into two steps: (1) Mn4+ → Mn3+ and (2) Mn3+ → Mn2+ . Theoretically, the H2 consumptions for the reduction of MnO2 to MnO were 11.5 mmol/g. Obviously, the experimental H2 consumptions were lower than the theoretical value. The XRD patterns (Fig. 7) of the three samples after H2 -TPR further confirmed that the products after H2 -TPR were

Table 1 Chemical analysis of the manganese dioxide catalysts and the reaction rate (R) for NH3 oxidation at 100 ◦ C. Catalysts

Surface area (m2 /g)

Pore volume (m3 /g)

Pore size (nm)

R ( ␮mol g−1 s−1 )a

MnO2 (H) MnO2 (UH) Commercial MnO2

47.94 187.50 26.10

0.1530 0.4877 0.0682

12.72 10.37 11.26

9.0 4.0 1.5

a

R = (F/W) × x; where F was the flow of the gas in cm3 /s, W was the weight of the catalyst in g and x was the fractional conversion.

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Fig. 5. SEM images of the manganese dioxide catalysts: (a), (c) and (e) were the low magnification for the MnO2 (H), MnO2 (UH) and commercial MnO2 ; (b) and (d) were the high-magnification for the MnO2 (H)and MnO2 (UH) catalyst.

Table 2 H2 consumption, AOS, Mn3+ /Mn4+ and Oads /Olatt of the catalyst. Catalysts

H2 consumption (mmol/g)

AOS of the catalyst

Mn3+ /Mn4+

Oads /Olatt

MnO2 (H) MnO2 (UH) Commercial MnO2

8.28 6.84 –

3.4 3.2 –

0.42 0.46 0.33

0.67 1.24 0.58

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577

Fig. 6. H2 -TPR profiles of the manganese dioxide catalysts.

pure MnO (JCPDS PDF 65-0640). Therefore, one could see that 59% and 72% Mn4+ and Mn3+ were reduced to MnO for the MnO2 (UH) catalyst and MnO2 (H) catalyst, and AOS (average oxidation state) of manganese were calculated to be 3.2 and 3.4 for the MnO2 (UH) catalyst and MnO2 (H) catalyst (Table 2). It has been known that only MnO2 phase and no other phase was detected in XRD detection, therefore one could conclude that Mn3+ was highly dispersed on the surface of the as-prepared catalysts, and there existed more Mn3+ species dispersed on the MnO2 (UH) catalyst. 3.4. XPS The oxidation states of the elements and the relative composition on the surface of all the three samples were further characterized by XPS. Typical Mn 2p3/2 and O 1s spectra for the manganese dioxide catalysts were illustrated in Fig. 8. Fig. 8A showed that the Mn 2p3/2 XPS spectrum could be deconvoluted to two components at 641.8 eV and 642.8 eV, which were attributed to the surface Mn3+ and Mn4+ species [14], respectively. Meanwhile,

Fig. 7. X-ray diffraction patterns of the manganese dioxide catalysts after H2 TPR.

Fig. 8. XPS spectra for the manganese dioxide catalysts: (A) the Mn 2p spectra and (B) the O 1s spectra.

the surface Mn3+ /Mn4+ molar ratios of the catalysts were summarized in Table 2. It was found that the MnO2 (UH) catalyst possessed the highest surface Mn3+ /Mn4+ molar ratio (0.46) and the commercial MnO2 showed the lowest one (0.33). Apparently, the synthesis conditions exhibited a significant effect on the surface Mn3+ /Mn4+ molar ratios, and the hydrothermal method strongly improved the surface Mn3+ /Mn4+ molar ratios contrasting to the commercial MnO2 . The assist of urea could further enhance the ratios. Based on the principle of electroneutrality, the couple of Mn3+ /Mn4+ would contribute to the generation of the oxygen vacancies in the prepared samples. Usually, the oxygen molecules were adsorbed on the oxygen vacancies of the catalysts, so one could deduce that the oxygen adspecies on the MnO2 (UH) catalyst would be the highest. Such a deduction was demonstrated by the illustration of the O 1s. As shown in Fig. 8B, two kinds of surface oxygen species could be distinguished in the O 1s XPS. The lower binding energy at 529.8 eV was attributed to the lattice oxygen (Olatt ) species [15], whereas the higher at 531.0 eV corresponding to the surface chemisorbed adsorbed oxygen (Oads ) species [16]. The ratio of Oads /Olatt on the surface of the catalysts was listed in Table 2. It was clear that the ratio of Oads /Olatt decreased in the sequence of MnO2 (UH)

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Fig. 9. TPD profiles of ammonia over the manganese dioxide catalysts: (a) MnO2 (UH), (b) MnO2 (H), (c) commercial MnO2 .

(1.24) > MnO2 (H) (0.67) > commercial MnO2 (0.58), and the result was in accordance with the order of the Mn3+ /Mn4+ molar ratio on the surface of the catalysts. Moreover, the result was consistent with the order of SCO activity. It had been known that Oads was more beneficial than Olatt for oxidation reactions because of its higher mobility [17]. Thus, the significantly improved performance of the MnO2 (UH) catalyst for NH3 SCO reaction, as compared with MnO2 (H) catalyst and commercial MnO2 (Fig. 2), could be ascribed to the observably increased amount of surface chemisorbed oxygen (Oads ) caused by the urea in the synthetic process. 3.5. NH3 -TPD In order to further investigate the surface adsorption capacity of the catalysts, NH3 -TPD experiment was measured and showed in Fig. 9. It could be seen that ammonia desorption occurred over a broad temperature range (70–600 ◦ C) with two maximums centered at low temperature region (LT, 100–170 ◦ C) and high temperature region (HT, 250–400 ◦ C), indicating that there were two major NH3 chemical adsorption sites differing in thermal stability on the surface of the catalysts. The quantitative analysis indicated that the amount of chemisorbed NH3 decreased in the relative order of MnO2 (UH) > MnO2 (H) > commercial MnO2 . This was completely in accordance with their surface area (Table 1). Furthermore, the adsorption capacities at LT were higher than that at HT on the MnO2 (UH) catalyst, and the similar phenomenon was also found on the other catalysts. In addition, the desorption temperature (LT and HT) of the MnO2 (UH) catalyst was lower than the MnO2 (H) catalyst and the commercial MnO2 . NH3 species adsorbed on the surface of MnO2 (UH) catalyst was more easily to be activated. Therefore, combining with the activity test, it could be concluded that the introduction of urea could prominently improve the amount of NH3 adsorption, facilitate the activation of the adsorbed NH3 and enhance the low-temperature activity eventually. Through integrating into account the results of BET, H2 -TPR, XPS and NH3 -TPD, it could be known that the urea-assisted hydrothermal method significantly improved the surface area of MnO2 , modificated its surface structure, and further generated more NH3 adsorption sites and oxygen vacancies on its surface. The ammonia adsorbed on the sites was revitalited and further oxidized by the oxygen. In parallel with the NH3 desorption, the products profiles during NH3 -TPD on the MnO2 (UH) catalyst were also detected, as shown in

Fig. 10. The products profiles during NH3 -TPD on the MnO2 (UH) catalyst.

Fig. 10. The N2 profile showed a maximum at 150 ◦ C, and meanwhile a weak peak for N2 O and NO at 230 ◦ C was also observed. No trace of NO2 was observed over the entire examined temperature range. Therefore, it was reasonable to deduce that the lattice oxygen could react with NH3 , and produce N2 , N2 O and NO. Combining with the selectivity of the MnO2 (UH) catalyst, it could be deduced that N2 O was the main by-product that led to the lower selectivity. 3.6. Stability test for the MnO2 (UH) catalyst The reaction stability of MnO2 (UH) catalyst was tested for NH3 oxidation at 170 ◦ C for 48 h (Fig. 11). It was found that the catalyst displayed a relatively stable activity and the NH3 conversion remained >98% during 48 h. Furthermore, we also evaluated the reusability of MnO2 (UH) catalyst for NH3 oxidation after two and three consecutive runs (Fig. 11 (inset)). It could be seen that the performance of the catalyst after two operations exhibited appropriate decrease by less than 8% relative to the first run, while the third cycle test was slightly decreased only 4% with respect to the conversion at second circulation. The NH3 conversion still remained above 91% conversion at 140 ◦ C after three time runs. In conclusion, the MnO2 (UH) catalyst was believed to be relatively stable.

Fig. 11. NH3 conversion as a function of stream time over the MnO2 (UH) catalyst and the resuse of the MnO2 (UH) catalyst (inset).

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4. Conclusions MnO2 catalysts were prepared by the hydrothermal method and studied for ammonia selective catalytic oxidation. Compared to the commercial catalyst, the as-prepared catalysts, especially MnO2 (UH), exhibited higher catalytic activity. Associating the characterization results and reaction activity, it could be concluded that the addition of urea effectively reformed the surface properties of samples. And MnO2 (UH) catalyst possessed a larger specific surface area, more adsorption sites for NH3 and abundant oxygen vacancies on its surface. All these features were closely related to the catalytic activity in NH3 -SCO, and contributed to the improved performance of MnO2 (UH) catalyst with the complete ammonia conversion temperature of 170 ◦ C. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21377016), the Natural Science Foundation of Liaoning Province (2014020011) and Program for Changjiang Scholars and Innovative Research Team in University (IRT 13R05). References [1] J. Gong, R.A. Ojifinni, T.S. Kim, J.M. White, C.B. Mullins, Selective catalytic oxidation of ammonia to nitrogen on atomic oxygen precovered Au(111), J. Am. Chem. Soc. 128 (2006) 9012–9013. [2] R.Q. Long, R.T. Yang, Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen, Chem. Commun. 17 (2000) 1651–1652. [3] Z. Qu, H. Wang, S. Wang, H. Cheng, Y. Qin, Z. Wang, Role of the support on the behavior of Ag-based catalysts for NH3 selective catalytic oxidation (NH3 -SCO), Appl. Surf. Sci. 316 (2014) 373–379. [4] L. Gang, B.G. Anderson, J. van Grondelle, R.A. van Santen, Low temperature selective oxidation of ammonia to nitrogen on silver-based catalysts, Appl. Catal. B: Environ. 40 (2003) 101–110.

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