The Al promotional effect for Ce0.4Zr0.6O2 mixed oxides in selective catalytic oxidation of ammonia to nitrogen

The Al promotional effect for Ce0.4Zr0.6O2 mixed oxides in selective catalytic oxidation of ammonia to nitrogen

Separation and Purification Technology 147 (2015) 24–31 Contents lists available at ScienceDirect Separation and Purification Technology journal homep...

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Separation and Purification Technology 147 (2015) 24–31

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

The Al promotional effect for Ce0.4Zr0.6O2 mixed oxides in selective catalytic oxidation of ammonia to nitrogen Zhong Wang, Zhenping Qu ⇑, Rui Fan Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Sciences 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 7 July 2014 Received in revised form 3 April 2015 Accepted 6 April 2015 Available online 15 April 2015 Keywords: NH3 oxidation Ce0.4Zr0.6O2 catalyst Al modified Oxygen species Oxygen vacancy

a b s t r a c t A novel Al doped Ce0.4Zr0.6O2 catalyst prepared by surfactant-templated method was tested for NH3 selective oxidation, and it was found that the addition of an appropriate amount of Al to Ce0.4Zr0.6O2 catalysts improved the NH3 oxidation activity. Strikingly, the NH3 conversion obtained on 3% Al–Ce0.4Zr0.6O2 catalyst was, on average, 12.7% higher than that of undoped Ce0.4Zr0.6O2 catalyst. The XRD and H2-TPR results showed that the inclusion of Al components resulted in the formation of smaller sized CeO2 (4.4–5.8 nm), and improved the oxygen mobility and reducibility. The normalized BET and NH3-TPD analysis also confirmed that the adsorption centers for NH3 in Al–Ce0.4Zr0.6O2 catalysts was increased with the Al addition. More importantly, the oxygen vacancies, of which the amount was increased by modification of Al, were considered as the essential oxygen adsorption and activation sites. Consequently, more active oxygen species (O and O2) were formed, which facilitated the NH3 oxidation reaction. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Ammonia (NH3) was considered a highly hazardous material, which could cause severe irritation and borns and was suspected to have long-term effects such as bronchitis [1]. In order to control the ammonia slip, several reviews of different techniques used for the elimination of ammonia have been published, such as adsorption, absorption, chemical treatment, catalytic decomposition, and selective catalytic oxidation (SCO) [2]. The SCO of ammonia with O2 to nitrogen and water has been considered to be one of the most successful methods to eliminate NH3 pollution [2–4], and consequently it was of increasing interest in recent years. To date, various types of materials have been studied as catalysts in the process of NH3-SCO. Noble metals have been used as active catalysts for the ammonia oxidation at the low temperature region, such as Ag [5], Pt [3], Pd [6], Ru [7] and Au [8]. Unfortunately, the N2 selectivity on these catalysts was relatively poor (680%). Transition metal oxides catalysts with low cost were also developed for SCO of ammonia, such as CuO [9], Fe2O3 [10], MnO2 [11], Co3O4 [12], MoO3 [13], NiO [14]. These catalysts exhibited higher N2 selectivity, but the operation temperature was significantly higher (300–400 °C). In addition, a variety of mixed

⇑ Corresponding author. Fax: +86 411 8470 8083. E-mail address: [email protected] (Z. Qu). http://dx.doi.org/10.1016/j.seppur.2015.04.006 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

oxides (CeO2, Mg–Al–O) were suggested to be effective catalysts in NH3 oxidation [2,15]. Among these catalysts, Ce–Zr mixed oxides were well known for a high oxygen exchange capacity, which was related to the capacity of cerium to change oxidation states reversibly between Ce4+ and Ce3+ by receiving or giving up oxygen [2]. The Ce–Zr mixed oxides could possess the better catalytic properties as well as the thermal resistance. As our previous studies [2], it was found that the NH3 was completely oxidized at 360 °C over Ce0.4Zr0.6O2 mixed oxide, and the N2 selectivity was always above 95%. More importantly, it was found that the catalytic performance was strongly related with the surface oxygen vacancies and oxygen mobility of Ce0.4Zr0.6O2 catalyst, which gave a very promising pathway for promoting the NH3 oxidation property. Several research groups have reported that the doping of current catalysts with additional metal cations might modify their chemical and physical properties, and finally developed novel catalysts with the improved catalytic performance. Gao et al. proposed that the addition of Cu to Ce–Ti catalyst resulted in the production of a new oxygen species with high reducibility at low temperatures, which leaded to the activity promotion of the SCR reaction [16]. Li et al. reported the addition of Ce on V2O5–WO3/TiO2 could create a charge imbalance, the vacancies and unsaturated chemical bond, which increased the amount of the chemisorbed oxygen on the catalyst surface [17]. Li et al. also found that the modification

Z. Wang et al. / Separation and Purification Technology 147 (2015) 24–31

of the oxidation states in CoCr2O4 catalyst by substitution of Li+ for Co2+ could be accompanied by the formation of structural defects, thus producing more oxygen vacancies. The oxygen vacancies in Co1xLixCr2O4 catalyst would promote the adsorption and dissociation of oxygen molecules and facilitated the methane combustion [18]. However, little attention has been paid attention to the doping of metal component into Ce–Zr mixed oxide to change the physic-chemical properties of the original catalyst. Recently, some researchers found that the doping of Y, Ca or La into CeO2–ZrO2 solid solutions could well facilitate the diffusion of oxygen in the lattice and decrease the reduction temperature of the solid solutions [19]. In addition to these modifications, loading of noble metals in the Ce–Zr mixed oxides also improved the oxygen release property by a synergetic effect between the noble metals and the supports [20]. Aluminum compound was commonly accepted as catalytic support in various catalytic reactions for a period of time. Especially, it was lower cost and also played a key role in improving thermal stability, homogeneous mixing of components, oxygen-storage capability and reduction behavior [10,21]. Therefore, in this paper a novel and simple surfactant-templated method was proposed to introduce aluminum into Ce0.4Zr0.6O2 mixed oxide. In particular, the influence of Al on the structure, reducibility and oxygen vacancies of Ce0.4Zr0.6O2 catalyst were investigated in detail. In addition, we gave an insight into the influence mechanism of Al doping in the chemical and physical properties as well as the NH3 oxidation behavior. 2. Experimental 2.1. Catalyst preparation The aluminum-doped ceria–zirconia mixed oxides (Al– Ce0.4Zr0.6O2) catalysts were prepared via a surfactant-templated method. In the typical procedure, 2.3649 g of cetyltrimethyl ammonium bromide (CTAB) was dissolved in deionized water of 200 ml, followed by the addition 2.4318 g of Ce(NO3)36H2O, 3.6064 g of Zr(NO3)45H2O and 0.8336 g of Al(NO3)39H2O. The molar ratio of CTAB/([Ce] + [Zr] + [Al]) was 0.4. The mixture was stirred for 0.5 h and then aqueous ammonia (25%) was slowly added with stirring till the pH equaled 10. The above mixture was stirred for another 12 h and then the homogeneous slurry mixture was aged at 90 °C for 24 h. Subsequently, the precipitates were filtered, washed with deionized water and absolute ethanol. Finally, the obtained solids were dried at 100 °C overnight and then calcined at 550 °C for 3 h in air. The Al loading amount was 3 wt%. And the sample was referred to as 3-Al–CZ. Similarly, other Al–Ce0.4Zr0.6O2 samples with different Al loading content were referred to hereafter as X-Al–CZ, where X stood for weight percent of Al. 2.2. Catalyst characterization N2 adsorption/desorption isotherms at 196 °C were obtained using a Quantachrom NOVA-4200e. The powder X-ray diffraction (XRD) experiment was recorded on a Rigaku D/max-cb X-ray diffractometer with monochromatic detector equipped with Cu Ka radiation. X-ray photoelectron spectroscopy was measured using an X-ray photoelectron spectrometer (ESCALAB 250Xi, Thermo) with a monochromatic X-ray source of Al Ka under ultra-high vacuum (3–2  106 Pa). The binding energies were calibrated internally by the carbon deposit C 1s binding energy (BE) at 284.8 eV. Electron paramagnetic resonance (EPR) measurements were performed at room temperature using a Bruker (A2009.5/12) operating at the X band (9.8 GHz). H2 temperature-programmed reduction (H2-TPR) and the temperature programmed desorption of ammonia (NH3-TPD) were

25

performed on a Chem BET TPR/TPD Chemisorptions Analyzer. Typically, 100 mg of sample was pretreated in a flowing stream of He at 200 °C for 1 h. After that, H2–Ar mixture (10% H2 by volume) was introduced into the instrument and the temperature was ramped to 1000 °C at a heating rate of 10 °C/min. For the NH3-TPD experiments, following the pretreatment step, the catalyst was saturated by a flow of NH3–He mixture (10% NH3 by volume). The reactor was then purged with He for a further 2 h to remove weakly adsorbed NH3. Then He was passed through the reactor and the temperature was ramped from room temperature to 700 °C at a rate of 10 °C/min. 2.3. Catalytic activity tests The catalytic performance of Al–Ce0.4Zr0.6O2 catalysts in the SCO of ammonia was studied in a fixed-bed flow reactor (8 mm in interior diameter). The composition and amount of the inlet gas mixture was set by mass flow controllers. The typical reactant gas composition was as follows: 1000 ppm NH3, 10 vol% O2, 5 vol% H2O (when used), and balance He. Each test was carried out by loading 200 mg of catalyst. The total flow rate of the reaction mixture was 100 ml/ min, and the gas hourly space velocity (GHSV) was about 40,000 h1. The inlet and outlet gas were analyzed by Gas Chromatograph using a 5A column with a TCD detector for N2 and the NH3 analyzer (GXH-1050, Beijing) to monitor the concentration of ammonia. The signal of all reactants and possible products were measured step by step after stabilization of the signals at a given temperature. 3. Results 3.1. Catalytic performance in NH3 oxidation Fig. 1 reveals NH3 conversion and N2 selectivity curves of Al–CZ catalysts with different Al contents as a function of reaction temperature. It was worth pointing out that the NH3-SCO activity of CZ catalyst was improved with the addition of Al. T10%, T50% and T90% (temperature at which 10%, 50% or 90% NH3 conversion was reached) were used as indications of the relative reactivity of the examined catalysts, and the results are shown in Table 1. When the Al content increased up to 1 wt%, the NH3 conversion presented the monotonous increase, and the T90% was lowered to 340 °C compared with pure CZ catalyst (T90% = 355 °C). Strikingly, as 3 wt% Al was doped into CZ mixed oxide, it accomplished highly remarkable NH3 conversion, and T10%, T50% and T90% values were lowered by ca. 15 °C, 15 °C and 25 °C compared to that of undoped CZ catalyst, respectively. However, the further increase of the aluminum content to 5 wt% resulted in the decrease of NH3 conversion, and T90% value shifted ca. 15 °C to higher temperature (345 °C) than that of 3-Al–CZ catalyst. This indicated that proper amount of aluminum addition could be essential to obtain the relatively high SCO activity. Comparatively, the N2 selectivity of Al doped CZ catalyst was a little higher compared with pure CZ catalyst in the 280– 340 °C temperature range, and was above 95%. In addition, the other nitrogen-containing products were also detected by mass spectrometer (MS). Only negligible N2O and NO were formed during the reaction temperature range, suggesting that NH3 in SCO reaction over Al–CZ catalysts was exclusively converted to N2. Table 2 shows the comparison of catalytic performance of the NH3 oxidation over different catalysts. It could be seen that the catalytic activity of Al–CZ catalyst in NH3 oxidation was higher than other catalysts. Also it was noted that the catalytic performance of Al–CZ catalyst was comparable or even superior to that of noble metal catalysts. Therefore, the Al–CZ catalyst was a very potential catalyst for NH3 oxidation.

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o

0.4

o

300 C o 340 C

100

320 C o 360 C

90

3

80

5-Al-CZ

0.3

dV/dD (cm /g/nm)

NH3 conversion (%)

(A)

70 60 50

3-Al-CZ

0.2

1-Al-CZ

0.1

CZ

40 0.0

1

0

3

0

5

10

N2 selectivity (%)

(B)

90

80

CZ 1-Al-CZ 3-Al-CZ 5-Al-CZ

260

280

300

320

340

360

380

o

Fig. 1. NH3 conversion and N2 selectivity over Al–CZ catalysts.

Table 1 Specific surface areas, crystallite size and catalytic activities of Al–CZ catalysts.

b c

50

60

Table 2 Catalytic activity of various catalysts for NH3 oxidation reaction.

Temperature ( C)

a

40

Fig. 2. The pore size distribution curves of Al–CZ catalysts.

240

CZ 1-Al–CZ 3-Al–CZ 5-Al–CZ

30

100

70

Samples

20

Pore Diameter (nm)

Al loading content (%)

BET

Va

Pb

Catalytic activity (°C)

Crystallite sizec

(m2 g1)

(cm3 g1)

(nm)

T10%

T50%

T90%

(nm)

104.7 95.2 93.0 57.8

0.16 0.14 0.11 0.16

4.9 4.3 3.4 9.7

255 245 240 250

305 300 290 300

355 340 330 345

6.3 4.9 4.4 5.8

Total pore volume. Mean pore diameter. Calculated according to the [1 1 1] diffraction peak of CeO2.

3.2. BET surface area and XRD analysis The BET surface area, pore volume and pore diameter are summarized in Table 1. The isotherms of all Al–CZ samples with mesoporosity (pore size was between 4.3 and 9.7 nm) according to the IUPAC [25] were similar. Moreover, it was found that the aluminum doping significantly affected the surface area of the samples. The surface area of Al–CZ samples decreased with the increase of the Al doped amount. Compared with the initial CZ sample (104.7 m2/g), the surface area of 5-Al–CZ was only 57.8 m2/g. In addition, Fig. 2 exhibited that the pore size distribution curves of all samples apart from 5-Al–CZ catalyst were very narrow. Besides, it was of great interest to find out that the center of the pore size distribution curves moved from 4.9 to 3.4 nm with increasing aluminum content from 0 to 3 wt%. On the contrary, it was noticeable that the 5-Al–CZ catalyst exhibited

Catalysts

Temperature (°C)

NH3 conversion (%)

N2 selectivity (%)

References

3-Al–CZ 1-Al–CZ 5-Al–CZ Pd/Cu-Mg-Ala Rh/Fe–Mg–Ala Fe/Al2O3b Ni/Al2O3b Cu-ZSM-5c Co-ZSM-5c Cu–Ce (7:3)d Mn/Al2O3e

330 340 345 350 400 450 450 350 400 340 300

90 90 90 90 82 80 57 48 61 80 4

97 97 95 88 85 86 74 95 59 – 56

This study This study This study [22] [22] [14] [14] [23] [23] [15] [24]

a 5000 ppm NH3, 2.5% O2, He balance, total flow rate = 40 ml/min, GHSV = 15,400 h1. b 1000 ppm NH3, 18% O2, He balance, total flow rate = 300 ml/min, GHSV = 61,000 h1. c 0.1 g catalyst, 1000 ppm NH3, 2% O2, He balance, total flow rate = 500 ml/min, GHSV = 2.3  105 h1. d 1000 ppm NH3, 4% O2, RH = 12%, GHSV = 92,000 ml/h g. e 400 ppm NH3, 1.2% O2, 2.8% H2, 4% CO, 3.7% CO2, 1.2% CH4, 3.9% H2O, N2 balance, GHSV = 1.0  105 h1.

the broad pore size distribution (9.7 nm), which was presumably due to the interparticle pores after the addition of higher aluminum content [26]. The XRD patterns of Al–CZ catalysts are presented in Fig. 3. It was observed that all diffraction peaks of Al–CZ sample systematically shifted to higher diffraction angles and broadened than that of pure CeO2 catalyst, which was due to the isomorphous substitution of smaller Zr4+ (ionic radius 0.84 Å) with larger Ce4+ (ionic radius 0.98 Å), as our previous study [2]. This tendency suggested a cubic-tetragonal structural phase transition from pure CeO2 to Al– CZ catalyst [27]. In addition, it was asserted in the pioneer literatures that the formation of cubic structure was expected with ceria content up to 50 mol% and higher zirconia content helped to the formation of tetragonal type [28]. Thus, it was noticeable that the true nature of Al–CZ catalyst was a mixture of the two phases: cubic and tetragonal. For instance, the peak around 29.4° seems to contain the contribution of the cubic (1 1 1) and tetragonal (1 1 1) planes. Meanwhile, no peaks related to Al species in XRD appeared, suggesting that the partial Al species might incorporate into the CZ lattice to form the solid solution and/or Al species existed as the highly dispersed species [29].

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o

reported that the reduction peak at 397 °C over CeO2–ZrO2/TiO2 catalyst was due to the reduction of the most easily reducible surface oxygen of highly dispersed ceria species. Łamacz et al. [33] also proposed the reduction peak at 350 °C in Ce0.63Zr0.37O2 solid solution referred to surface reduction or nano-particle CeO2. Besides, the b and c reduction peaks shifted toward lower temperature region. Moreover, the a/(a + b + c) molar ratio decreased in the order of 3-Al–CZ > 1-Al–CZ > 5-Al–CZ catalysts, coinciding with the sequences of catalytic performance.

o

29.4

Intensity (a.u.)

28.5

o

o

48.9

34.0

o

58.2

5-Al-CZ o

47.2

3-Al-CZ o

56.3

1-Al-CZ

o

32.9

3.4. NH3-TPD

CZ CeO2

20

30

40

50

60

70

2θ (degree) Fig. 3. XRD patterns of Al–CZ catalysts.

γ α

o

530 C

o

371 C

5-Al-CZ

Intensity (a.u.)

o

638 C

β

o

648 C

o

530 C

o

374 C

3-Al-CZ

o

646 C o

555 C o

380 C

1-Al-CZ

o

556 C

o

662 C

CZ

o

o

o

560 C

900 C

650 C

CeO2

150

300

450

600

750

900

Temperature (°C) Fig. 4. H2-TPR profiles of Al–CZ catalysts.

3.3. H2-TPR analysis The H2-TPR profiles are illustrated in Fig. 4 to investigate the reducibility of Al–CZ catalysts. The TPR profile of CeO2 showed three reduction peaks at around 560 °C, 650 °C and 900 °C. The first two reduction peaks corresponded to the reduction of surface CeO2, and the third one could be attributed to the reduction process in the bulk [2,30]. For the CZ catalyst, the surface and bulk reduction of CeO2 phase would concurrently occur, and the clear distinction between them could not be expected [31]. The broad H2 consumption band for CZ sample was split in two bands at 556 °C (b) and 662 °C (c) (Table 3), respectively. Importantly, an unconspicuous H2 reduction peak at 371–380 °C (a) was also observed with the introduction of Al, which was attributed to the reduction of finely dispersed ceria species [2]. Reddy et al. [32]

The NH3-TPD profiles are shown in Fig. 5, and the area under the desorption peak was proportional to the total number of acidity in the catalyst [2,10]. Moreover, NH3-TPD result also determined NH3 adsorption and activation sites in NH3-SCO reaction. The appearance of the three desorption peaks of NH3 (130 °C, 350 °C and 460 °C) indicated that three major NH3 adsorbed species differing in thermal stability existed on catalyst surface. The first peak (130 °C) may be assigned to physically adsorbed NH3, but another two at higher temperature (350 °C and 460 °C) were related to the chemisorbed NH3 [34]. As evident from the results in Table 3, it was found that the ratio of desorption amount of chemisorbed NH3 to total desorption amount followed this relative order of 3-Al–CZ (0.76) > 1-Al–CZ (0.53) > 5-Al–CZ (0.47) > CZ (0.41), suggesting that the desorption amount of chemisorbed NH3 in Al–CZ catalysts was intensively greater than that of pure CZ catalyst. The result array was in consistent with the sequences of NH3 conversion (Fig. 1). Taking the NH3 oxidation activity into consideration, it was reasonable to conclude that the addition of Al provided much more NH3 adsorption and activation sites. In addition, in the chemisorbed NH3 curves, the moderate-temperature (350 °C) peak was ascribed to ammonia coordinated to Lewis acid sites, which was prone to be desorbed at temperatures above 200 °C [35]. The ammonia coordinated to Brønsted acid sites was more thermally stable, and was desorbed at high temperature (460 °C) [36]. The relative concentrations order of Lewis to Brønsted acid sites on all samples in Table 3 was 3-Al–CZ (0.49) > 1-Al–CZ (0.28) > 5-Al–CZ (0.21) > CZ (0.19), which was consistent with the result of NH3 oxidation activity. Therefore, It could be further suggested that the inclusion of Al components mainly resulted in the increase of Lewis acid sites, and Lewis acid sites for chemisorbed NH3 were responsible for ammonia activation. 3.5. EPR analysis Fig. 6A presents the room-temperature EPR spectra of Al–CZ catalysts. All Al–CZ catalysts spectra showed an axial signal with g\ = 1.963 and g// = 1.944, which was due to Ce3+ ions associated with oxygen vacancy [37]. More importantly, the intensity of Ce3+ signal increased with the increase of Al loadings, and the 5Al–CZ sample showed the highest intensity, as shown in Fig. 6B. The Al incorporation enhanced the concentration of Ce3+ in CZ

Table 3 The quantitative H2-TPR and NH3-TPD analyses of Al/CZ catalysts. Samples

CZ 1-Al–CZ 3-Al–CZ 5-Al–CZ a b c

Ratio of different strength acidity to total acidity

Reduction temperature (°C)

H2 consumption (lmol g1)

Weaka

Mediumb

Strongc

Peak a

Peak b

Peak c

Peak a

Peak b

Peak c

0.59 0.47 0.24 0.53

0.19 0.28 0.49 0.21

0.22 0.25 0.27 0.26

– 380 374 371

556 555 530 530

662 646 648 638

– 15.3 55.6 9.9

332.0 201.6 266.8 231.1

599.7 704.1 615.8 676.2

Calculated from the NH3 amount desorbed in the 80–230 °C. Calculated from the NH3 amount desorbed in the 230–400 °C (Lewis acid sites). Calculated from the NH3 amount desorbed in the 400–540 °C (Brønsted acid sites).

H2 consumption ratio peak [a/(a + b + c)] (%)

– 1.7 5.9 1.1

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induced significant lattice distortions and increasing of the oxygen mobility in the lattice.

o

130 C

o

o

350 C

460 C

3.6. XPS studies

Intensity (a.u.)

5-Al-CZ

The XPS analysis results of Al–CZ catalysts verifying the surface composition and elementary oxidation states are presented in Fig. 7, including the Ce3d, Zr 3d, Al 2p and O 1s bands. Fig. 7A illustrates the Al 2p spectrum of Al–CZ, where peaks at 73.7 eV were assigned to Al3+ species [40]. The values of binding energy were lower than that of pure Al2O3 (74.3 eV), which was an indication of the strong interaction of the Al species with Ce species due to the effect of electronic transfer [41]. This behavior could be interpreted as the formation of a Ce–Al–Zr solid solution [42,43]. And the detailed study of the surface composition obtained by XPS (Table 4) indicated a higher Al concentration on the surface compared with the theoretical content. Fig. 7B presents the XPS spectra of Ce 3d core levels for all samples. It was noticeable that the curves of Ce 3d spectra were composed of six peaks labeled as u (901.1 eV), u00 (907.6 eV), u000 (916.7 eV), v (882.7 eV), v00 (889.1 eV) and v 000 (898.4 eV) corresponding to three pairs of spin-orbit doublets, which could be identified as characteristic of Ce4+ 3d final state [2,44]. Compared with the bands of Ce4+ 3d, the character peaks marked as u0 (903.4 eV) and v0 (885.0 eV) were present for Ce3+ 3d final state [45,46]. The O1s spectra (Fig. 7C) clearly showed two surface oxygen species. The binding energy of 529.5 eV, denoted as Olatt, was characteristic of surface lattice oxygen [47,48], while the broad shoulder at a higher BE (531.8 eV) might be assigned to 2 surface adsorbed oxygen (Oads) (O, O 2 , O2 ) [49].

3-Al-CZ

1-Al-CZ

CZ

100

200

300

400

500

600

Temperature (°C) Fig. 5. NH3-TPD profiles of Al–CZ catalysts.

(A)

Intensity (a.u.)

1.963

5-Al-CZ

2.043

1.944

3-Al-CZ 1-Al-CZ CZ

2500

3000

3500

4000

4. Discussion 4500

(B)

4

EPR intensity (a.u.)

Magnetic Field (G)

3

1.0

3+

Ce (g⊥=1.963)

0.6

2 3+

Ce (g//=1.944)

1

0.4

EPR intensity (a.u.)

0.8

-

O2 (g=2.403)

0.2

0 0

1

2

3

4

5

Aluminum loading (%) Fig. 6. EPR spectra recorded at room temperature (A) and variation of Ce3+ (g// = 1.944 and g\ = 1.963) and O 2 (g = 2.403) signals intensity as the loading content of aluminum in Al–CZ catalysts (B) of Al–CZ catalysts.

sample. In addition to Ce3+ signal, a small signal characterized by g = 2.043 also appeared over Al–CZ catalysts, and the parameter of this signal was consistent with superoxide species (O 2 ) adsorbed on cerium centers [38]. From Fig. 6B, it was clear that the intensity of the O 2 signal strongly depended on the addition loadings of aluminum. The formation of sites suitable for the generation of O 2 species on the ceria component was enhanced by interaction with alumina [39]. So the more O 2 species could be specifically explained that the introduction of Al component to CZ catalyst

It was clear from Fig. 1 that Al–CZ catalyst provided the better activity compared with CZ catalyst. Notably, the change trend of surface area was not consistent with that of catalytic activity. That is to say, the surface area should not be a determinative factor influencing the catalytic performance of Al–CZ catalysts. After normalization by surface area, the sequence of the NH3-SCO catalytic activity was as follows (Fig. 8): 5-Al–CZ > 3-Al–CZ > 1-Al–CZ > CZ catalysts, and it was apparent that the addition of Al increased the normalized NH3 conversion compared with unmodified CZ catalyst, which confirmed that the Al addition could induce the formation of more active phases for NH3-SCO reaction. Moreover, the XRD results showed that the diffraction peaks of CeO2 on Al–CZ catalysts were obviously broadened due to the smaller crystalline size. As presented in Scheme 1, the addition of Al decreased the crystalline size of CeO2. It was notable that the 3-Al–CZ catalyst exhibited the smallest crystallite size of 4.4 nm and the further addition of 5 wt% Al resulted in the formation of the larger crystalline size (5.8 nm). As a whole, after Al addition, the interaction between CZ and Al suppressed the growth of crystal size of CeO2 on Al–CZ catalyst compared with pure CZ catalyst in calcination process. In addition, it was found from H2-TPR results that the reduction peaks in Al–CZ catalysts shifted to lower temperature and the reduction peak due to the finely dispersed ceria species also appeared after Al addition, which should correspond to the insertion of Al atoms in the CZ cubic matrix. The change of CZ structure resulted in the formation of lattice defect and the further decrease of the oxygen activation energy [25]. That is to say that the addition of Al enhanced the oxygen mobility and acted as the promoter in NH3 catalytic oxidation. Most notably, the relative amounts of Ce3+/Ce4+ and different oxygen species were confirmed by XPS analysis, as shown in Table 4. It could be found that the Al doped samples exhibited relatively significant concentration of Ce3+ than pure CZ sample, which

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73.7 eV

Al2p

(B)

Ce3d u'''

uv'''

d

529.5 eV

O1s

531.8 eV

v v' v''

Intensity (a.u.)

Intensity (a.u.)

u'' u'

(C)

d d

c

c

Intensity (a.u.)

(A)

c

b

b

b

a a

82 80 78 76 74 72 70 68 66 64

930 920 910 900 890 880 870

Binding Energy (eV)

Binding Energy (eV)

537

534

531

528

525

Binding Energy (eV)

Fig. 7. XPS spectra of Al 2p (A), Ce 3d (B) and O 1s (C) for Al–CZ catalysts: (a) CZ; (b) 1-Al–CZ; (c) 3-Al–CZ; (d) 5-Al–CZ.

Table 4 Surface elemental composition of various catalysts. Samples

2

Normalized NH3 conversion (%/m )

CZ 1-Al–CZ 3-Al–CZ 5–Al–CZ

Surface composition (wt%) Ce 3d

Zr 3d

Al 2p

O 1s

12.6 10.4 8.4 7.3

17.9 16.3 15.7 14.5

– 3.9 7.7 10.2

69.4 69.4 68.1 67.9

Oads/Olatt

Ce3+/Ce4+

0.56 0.62 1.04 0.58

0.21 0.23 0.24 0.40

9.0 7.5 6.0 4.5 3.0

CZ 1-Al-CZ 3-Al-CZ 5-Al-CZ

1.5 0.0 240

260

280

300

320

340

360

380

o

Temperature ( C)

Scheme 1. Schematic illustration of the effect mechanism of Al for CZ catalyst.

Fig. 8. NH3 conversion of Al–CZ catalysts normalized by BET surface area.

was consistent with the EPR result (Fig. 6B). The Ce3+ species should be in contact with Al species, which was taking in consideration the formation of Ce–Al–Zr solid solution [50]. In general, the presence of Ce3+ was assigned with the generation of oxygen vacancies according to the charge compensation [51]. Therefore, it was likely that the Al doping facilitated the reduction of Ce4+ to Ce3+, and the more oxygen vacancies were also easily generated on the surface of Al–CZ catalysts, as depicted in Scheme 1. It was also found that the presence of surface lattice oxygen and adsorbed oxygen species was strongly dependent on the Al content, and the doping of Al increased the amount of Oads species in Table 4. The surface Oads/Olatt molar ratio decreased in the order of 3-Al– CZ > 1-Al–CZ > 5-Al–CZ > CZ, coinciding with the catalytic activity. Thus it was reasonable to suggest that the oxygen adspecies should be a critical factor in determining the catalytic activity of Al–CZ samples.

Further study indicated the detailed information about oxygen adspecies in Al–CZ catalysts by EPR technology. The presence of  O 2 species in Al–CZ samples was clearly observed. The O2 species was formed by the following equation [52]:

Ce3þ  V o þ O2 ! Ce4þ  ðO  OÞ

ðV o : oxygen vacancyÞ

Such oxygen vacancy could promote the activation of adsorbed oxygen to form O 2 species. That is to say, the more the oxygen vacancies associated with Ce3+ species was, the higher the O 2 species intensity was. However, the integrated intensities of EPR signal (O 2 ) was increased in a trend of 3-Al–CZ > 1-Al–CZ > 5-Al–CZ > CZ sample (Fig. 6B). Obviously, although the 5-Al–CZ catalyst presented the most intense of oxygen vacancies (Fig. 6B), the amount of O 2 species formed on 5-Al–CZ catalyst was not the largest. The reason might be that the excess Al on the surface of 5-Al–CZ catalyst

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Z. Wang et al. / Separation and Purification Technology 147 (2015) 24–31

could occupy the oxygen vacancies and make them deactivate and block the formation of O 2 species. Interestingly, the sequence for the redox performance of Al–CZ was similar with that of the integrated O 2 signal intensities, which suggested that the excellent redox performance promoted the formation of O 2 species in CeO2, as reported by Machid et al. [53]. Meanwhile, combined with the crystallite sizes listed in Table 1, it was thought that the formation of O 2 species was related with not only oxygen vacancies but also the reduction properties and dispersion degree of ceria species. O 2 species was more easily formed on the CeO2 with smaller crystallite size [39]. The intensity sequence of O 2 signal observed in Fig. 6B was also in agreement with the catalytic performance (Fig. 1). So the formation of O 2 species was the ultimate reason for the enhanced catalytic activity when Al species was doped into the CZ sample, which was attributed to the general scheme [54]:  2 O2ads ! O2ads ! O2 2ads ! Oads ! Olatt 2 In fact, superoxide (O 2 ) and peroxide (O2 ) species could be considered as ‘‘virtual’’ intermediates formed during oxygen dissociation. The surface O species was thermally unstable and 2 converted into O2 2 species with the increase of the temperature, which subsequently dissociated to yield atomic oxygen (O) adsorbed on ceria surface [55]. So the more O 2 species could poten2 tially result in much more formation of O ads and O latt species, which would directly take part in the NH3 oxidation reaction. Our previous studies have evidenced that the surface lattice oxygen (O2 latt) was an important active oxygen species toward NH3 oxidation [2,34]. In addition, Che et al. reported that the O species readily reacted with 2 alkane compared with O2 on the low-coordinated sites [56]. 2 or O Dai et al. also suggested that a larger amount of O species might be  2 available through the conversion of O 2 and O2 to O species, giving rise to a great enhancement in catalytic activity of toluene [57]. Therefore, the O ads species might be another active oxygen species in NH3 oxidation besides surface lattice oxygen. Based on the above analysis, a new reaction path for NH3 oxidation on Al–CZ catalyst was proposed, as shown in Scheme 1. The addition of a suitable amount of Al increased the amount of the active sites for NH3 adsorption. Then the adsorbed NH3 species was activated and transformed into NHx and H+ species in the roles of strong electron state of Ce4+/Ce3+ over Al–CZ catalyst [2,34]. The formation of more oxygen vacancies over Al–CZ catalyst enhanced the ability of the catalyst to adsorb and activate O2 to form super oxide O 2 loaded in surface oxygen vacancies. The surface O2 species was the most thermally unstable and converted first into  peroxides O2 and O2 species. 2 , and then transformed into O Thus the O and O2 species further reacted with the activated NHx species to form N2 and H2O. Reversibly, the reoxidation of catalyst’s surface by O2 occurred, and the active oxygen species were generated again. So the better oxygen cycles was constituted among gaseous oxygen, oxygen vacancies and active oxygen species. Meanwhile, the formed H+ species during NH3 activation could also react with O2 to form H2O [58,59]:

Ce3þ þ Hþ þ 1=2O2 ! Ce4þ þ H2 O

5. Conclusions The effect of Al on the structure of CZ catalyst and the catalytic activity in NH3 oxidation has been investigated. The NH3 conversion was promoted with the addition of aluminum in Al–CZ catalysts. The 3-Al–CZ catalyst has been proved as the best NH3-SCO catalyst, and the NH3 conversion was, on average, 12.7% higher than that of pure CZ catalyst. The normalized BET and NH3-TPD results indicated that the Al addition improved the adsorption

and activation sites of NH3, significantly decreased the crystallite size of CeO2, and increased the oxygen mobility associated with finely dispersed CeO2 species. Moreover, the addition of Al resulted in the higher concentration of oxygen vacancies associated with Ce3+ on the surface of catalyst, which evidently promoted the activation of adsorbed oxygen to form superoxides (O 2 ) and surface lattice oxygen (O2). In addition, the adsorption and activation 2 ability for O2 to form O species was also strongly related 2 and O with the reduction properties and dispersion degree of ceria species. It could be suggested that the Al component contributed to the enhanced NH3 oxidation behavior, which was mainly due to more active oxygen species (O and O2).

Acknowledgements The work described above was supported by the National Nature Science Foundation of China (No. 21377016), the Natural Science Foundation of Liaoning Province in China (2014020011), Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05) and the ‘‘123’’ Project of Liaoning Environmental Research and Education (CEPF2012-123-1-10).

References [1] M. Amblard, R. Burch, B.W.L. Southward, A study of the mechanism of selective conversion of ammonia to nitrogen on Ni/c-Al2O3 under strongly oxidising conditions, Catal. Today 59 (2000) 365–371. [2] Z. Wang, Z.P. Qu, X. Quan, H. Wang, Selective catalytic oxidation of ammonia to nitrogen over ceria–zirconia mixed oxides, Appl. Catal. A: Gen. 411–412 (2012) 131–138. [3] G. Olofsson, L.R. Wallenberg, A. Andersson, Selective catalytic oxidation of ammonia to nitrogen at low temperature on Pt/CuO/Al2O3, J. Catal. 230 (2005) 1–13. [4] M.S. Kim, D.W. Lee, S.H. Chung, Y.K. Hong, S.H. Lee, S.H. Oh, H. Cho, K.Y. Lee, Oxidation of ammonia to nitrogen over Pt/Fe/ZSM5 catalyst: influence of catalyst support on the low temperature activity, J. Hazard. Mater. 237–238 (2012) 153–160. [5] L. Zhang, H. He, Mechanism of selective catalytic oxidation of ammonia to nitrogen over Ag/Al2O3, J. Catal. 268 (2009) 18–25. [6] R.Q. Long, R.T. Yang, Noble metal (Pt, Rh, Pd) promoted Fe-ZSM-5 for selective catalytic oxidation of ammonia to N2 at low temperatures, Catal. Lett. 78 (2002) 353–357. [7] X.Z. Cui, J. Zhou, Z.Q. Ye, H.R. Chen, L. Li, M.L. Ruan, J.L. Shi, Selective catalytic oxidation of ammonia to nitrogen over mesoporous CuO/RuO2 synthesized by co-nanocasting-replication method, J. Catal. 270 (2010) 310–317. [8] J.L. 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 (1 1 1), J. Am. Chem. Soc. 128 (2006) 9012–9013. [9] L. Chmielarz, P. Kustrowski, M. Drozdek, R. Dziembaj, P. Cool, E.F. Vansant, Selective catalytic oxidation of ammonia into nitrogen over PCH modified with copper and iron species, Catal. Today 114 (2006) 319–325. [10] A.C. Akah, G. Nkeng, A.A. Garforth, The role of Al and strong acidity in the selective catalytic oxidation of NH3 over Fe-ZSM-5, Appl. Catal. B: Environ. 74 (2007) 34–39. [11] R.Q. Long, R.T. Yang, Selective catalytic oxidation of ammonia to nitrogen over Fe2O3–TiO2 prepared with a sol–gel method, J. Catal. 207 (2002) 158–165. [12] W.K. Fung, M. Claeys, E. van Steen, Effective utilization of the catalytically active phase: NH3 oxidation over unsupported and supported Co3O4, Catal. Lett. 142 (2012) 445–451. [13] L. Lietti, G. Ramis, G. Busca, F. Bregani, P. Forzatti, Characterization and reactivity of MoO3/SiO2 catalysts in the selective catalytic oxidation of ammonia to N2, Catal. Today 61 (2000) 187–195. [14] M. Amblard, R. Burch, B.W.L. Southward, The selective conversion of ammonia to nitrogen on metal oxide catalysts under strongly oxidising conditions, Appl. Catal. B: Environ. 22 (1999) 159–166. [15] J.C. Lou, C.M. Hung, S.F. Yang, Selective catalytic oxidation of ammonia over copper-cerium composite catalyst, J. Air Waste Manage Assoc. 54 (2004) 68– 76. [16] X. Gao, X.S. Du, L.W. Cui, Y.C. Fu, Z.Y. Luo, K.F. Cen, A Ce–Cu–Ti oxide catalyst for the selective catalytic reduction of NO with NH3, Catal. Commun. 12 (2010) 255–258. [17] L. Chen, J.H. Li, M.F. Ge, Promotional effect of Ce-doped V2O5–WO3/TiO2 with low vanadium loadings for selective catalytic reduction of NOx by NH3, J. Phys. Chem. C 113 (2009) 21177–21184. [18] J.H. Chen, W.B. Shi, X.Y. Zhang, H. Arandiyan, D.F. Li, Junhua Li, Roles of Li+ and Zr4+ cations in the catalytic performances of Co1xMxCr2O4 (M = Li, Zr; x = 0– 0.2) for methane combustion, Environ. Sci. Technol. 45 (2011) 8491–8497.

Z. Wang et al. / Separation and Purification Technology 147 (2015) 24–31 [19] T. Ozaki, T. Masui, K. Machida, G. Adachi, T. Sakata, H. Mori, Redox behavior of surface-modified CeO2–ZrO2 catalysts by chemical filing process, Chem. Mater. 12 (2000) 643–649. [20] P. Fornasiero, R.D. Monte, G.R. Ranga, J. Kašpar, S. Meriani, A. Trovarelli, M. Graziani, Rh-loaded CeO2–ZrO2 solid-solutions as highly efficient oxygen exchangers: dependence of the reduction behavior and the oxygen storage capacity on the structural-properties, J. Catal. 151 (1995) 168–177. [21] S. Damyanov, C.A. Perez, J.M. Schmal, J.M.C. Bueno, Characterization of ceriacoated alumina carrier, Appl. Catal. A: Gen. 234 (2002) 271–282. [22] L. Chmielarz, P. Kuštrowski, A.R. Lasocha, R. Dziembaj, Selective oxidation of ammonia to nitrogen on transition metal containing mixed metal oxides, Appl. Catal. B: Environ. 58 (2005) 235–244. [23] R.Q. Long, R.T. Yang, Superior ion-exchanged ZSM-5 catalysts for selective catalytic oxidation of ammonia to nitrogen, Chem. Commun. (2000) 1651– 1652. [24] H.M.J. Kušar, A.G. Ersson, M. Vosecky´, S.G. Järås, Selective catalytic oxidation of NH3 to N2 for catalytic combustion of low heating value gas under lean/rich conditions, Appl. Catal. B: Environ. 58 (2005) 25–32. [25] M.H. Yao, R.J. Baird, F.W. Kunz, T.E. Hoost, An XRD and TEM investigation of the structure of alumina-supported ceria–zirconia, J. Catal. 166 (1997) 67–74. [26] X.D. Zhang, Z.P. Qu, J.X. Jia, Y. Wang, Ag nanoparticles supported on wormhole HMS material as catalysts for CO oxidation: effects of preparation methods, Powder Technol. 230 (2012) 212–218. [27] I. Atribak, B. Azambre, A.B. López, A.G. García, Effect of NOx adsorption/ desorption over ceria–zirconia catalysts on the catalytic combustion of model soot, Appl. Catal. B: Environ. 92 (2009) 126–137. [28] E. Aneggi, C. de Leitenburg, G. Dolcetti, A. Trovarelli, Promotional effect of rare earths and transition metals in the combustion of diesel soot over CeO2 and CeO2–ZrO2, Catal. Today 114 (2006) 40–47. [29] J. Wang, J. Wen, M.Q. Shen, Effect of interaction between Ce0.7Zr0.3O2 and Al2O3 on structural characteristics, thermal stability, and oxygen storage capacity, J. Phys. Chem. C 112 (2008) 5113–5122. [30] M. Haneda, K. Shinoda, A. Nagane, O. Houshito, H. Takagi, Y. Nakahara, K. Hiroe, T. Fujitani, H. Hamada, Catalytic performance of rhodium supported on ceria–zirconia mixed oxides for reduction of NO by propene, J. Catal. 259 (2008) 223–231. [31] Y. Guo, G.Z. Lu, Z.G. Zhang, S.H. Zhang, Y. Qi, Y. Liu, Preparation of CexZr1xO2 (x = 0.75, 0.62) solid solution and its application in Pd-only three-way catalysts, Catal. Today 126 (2007) 296–302. [32] B.M. Reddy, P. Bharali, P. Saikia, G. Thrimurthulu, Y. Yamada, T. Kobayashi, Thermal stability and dispersion behavior of nanostructured CexZr1xO2 mixed oxides over anatase-TiO2: a combined study of CO oxidation and characterization by XRD, XPS, TPR, HREM, and UV–Vis DRS, Ind. Eng. Chem. Res. 48 (2009) 453–462. [33] A. Łamacz, A. Krzton´, G.D. Mariadassou, Catalytic decomposition of nitrogen oxides from coal combustion flue gases on CeZrO2 supported Cu catalysts, Catal. Today 176 (2011) 126–130. [34] Z. Wang, Z.P. Qu, X. Quan, Z. Li, H. Wang, R. Fan, Selective catalytic oxidation of ammonia to nitrogen over CuO–CeO2 mixed oxides prepared by surfactanttemplated method, Appl. Catal. B: Environ. 134–135 (2013) 153–166. [35] Z.C. Si, D. Weng, X.D. Wu, J. Yang, B. Wang, Modifications of CeO2–ZrO2 solid solutions by nickel and sulfate as catalysts for NO reduction with ammonia in excess O2, Catal. Commun. 11 (2010) 1045–1048. [36] G.Y. Xie, Z.Y. Liu, Z.P. Zhu, Q.Y. Liu, J. Ge, Z.G. Huang, Simultaneous removal of SO2 and NOx from flue gas using a CuO/Al2O3 catalyst sorbent II. Promotion of SCR activity by SO2 at high temperatures, J. Catal. 224 (2004) 42–49. [37] E.A. Aad, A. Bennani, J.P. Bonnelle, Transition-metal ion dimers formed in CeO2: an EPR study, J. Chem. Soc., Faraday Trans. 91 (1995) 99–104. [38] A.M. Arias, R. Cataluña, J.C. Conesa, J. Soria, Effect of copper–ceria interactions on copper reduction in a Cu/CeO2/Al2O3 catalyst subjected to thermal treatments in CO, J. Phys. Chem. B 102 (1998) 809–817.

31

[39] J. Soria, J.M. Coronado, J.C. Conesa, Spectroscopic study of oxygen adsorption on CeO2/c-Al2O3 catalyst supports, J. Chem. Soc., Faraday Trans. 92 (1996) 1619–1626. [40] B.M. Reddy, B. Chowdhury, E.P. Reddy, A. Fernández, An XPS study of dispersion and chemical state of MoO3 on Al2O3–TiO2 binary oxide support, Appl. Catal. A: Gen. 213 (2001) 279–288. [41] A.F. Lucrédio, G.T. Filho, E.M. Assaf, Co/Mg/Al hydrotalcite-type precursor, promoted with La and Ce, studied by XPS and applied to methane steam reforming reactions, Appl. Surf. Sci. 255 (2009) 5851–5856. [42] M.J. Guittet, J.P. Crocombette, M.G. Soyer, Phy. Rev. B 63 (2001) 125117. [43] Tery L. Barr, Recent advances in X-ray photoelectron spectroscopy studies of oxides, J. Vac. Sci. Technol. A 9 (1991) 1793. [44] W.J. Shan, W.J. Shen, C. Li, Structural characteristics and redox behaviors of Ce1xCuxOy solid solutions, Chem. Mater. 15 (2003) 4761–4767. [45] V.M. Shinde, G. Madras, Synthesis of nanosized Ce0.85M0.1Ru0.05O2d (M = Si, Fe) solid solution exhibiting high CO oxidation and water gas shift activity, Appl. Catal. B: Environ. 138–139 (2013) 51–61. [46] W.P. Shan, F.D. Liu, H. He, X.Y. Shi, C.B. Zhang, A superior Ce–W–Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, Appl. Catal. B: Environ. 115–116 (2012) 100–106. [47] D.R. Sellick, A. Aranda, T. García, J.M. López, B. Solsona, A.M. Mastral, D.J. Morgan, A.F. Carley, S.H. Taylor, Influence of the preparation method on the activity of ceria zirconia mixed oxides for naphthalene total oxidation, Appl. Catal. B: Environ. 132–133 (2013) 98–106. [48] W.P. Shan, F.D. Liu, H. He, X.Y. Shi, C.B. Zhang, An environmentally-benign CeO2–TiO2 catalyst for the selective catalytic reduction of NOx with NH3 in simulated diesel exhaust, Catal. Today 184 (2012) 160–165. [49] Y.S. Xia, H.X. Dai, L. Zhang, J.G. Deng, H. He, C.T. Au, Ultrasound-assisted nanocasting fabrication and excellent catalytic performance of threedimensionally ordered mesoporous chromia for the combustion of formaldehyde, acetone, and methanol, Appl. Catal. B: Environ. 100 (2010) 229–237. [50] D.G. Cantrell, L.J. Gillie, A.F. Lee, K. Wilson, Structure–reactivity correlations in MgAl hydrotalcite catalysts for biodiesel synthesis, Appl. Catal. A: Gen. 287 (2005) 183–190. [51] J. Fan, X.D. Wu, X.D. Wu, Q. Liang, R. Ran, D. Weng, Thermal ageing of Pt on low-surface-area CeO2–ZrO2–La2O3mixed oxides: effect on the OSC performance, Appl. Catal. B: Environ. 81 (2008) 38–48. [52] C. Li, K. Domen, K. Maruya, T. Onishi, Dioxygen adsorption on well-outgassed and partially reduced cerium oxide studied by FT-IR, J. Am. Ceram. Soc. 111 (1989) 7683–7687. [53] M. Machid, Y. Murat, K. Kishikawa, D. Zhang, K. Ikeue, On the reasons for high activity of CeO2 catalyst for soot oxidation, Chem. Mater. 20 (2008) 4489– 4494. [54] C. Binet, M. Daturi, J.C. Lavalley, IR study of polycrystalline ceria properties in oxidised and reduced states, Catal. Today 50 (1999) 207–225. [55] K. Tabata, Y. Hirano, E.J. Suzuki, XPS studies on the oxygen species of LaMn1xCuxO3+k, Appl. Catal. A: Gen. 170 (1998) 245–254. [56] M. Che, A.J. Tench, Characterization and reactivity of molecular oxygen species, Adv. Catal. 32 (1983) 1–148. [57] F. Wang, H.X. Dai, J.G. Deng, G.M. Bai, K.M. Ji, Y.X. Liu, Manganese oxides with rod-, wire-, tube-, and flower-like morphologies: highly effective catalysts for the removal of toluene, Environ. Sci. Technol. 46 (2012) 4034–4041. [58] J.M.G. Amores, V.S. Escribano, G. Ramis, G. Busca, An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides, Appl. Catal. B: Environ. 13 (1997) 45–58. [59] L.I. Darvell, K. Heiskanen, J.M. Jones, A.B. Ross, P. Simell, A. Williams, An investigation of alumina-supported catalysts for the selective catalytic oxidation of ammonia in biomass gasification, Catal. Today 81 (2003) 681– 692.