ORIGINAL PAPER Selective catalytic oxidation of ammonia into

Chemical Papers DOI: 10.1515/chempap-2015-0120

ORIGINAL PAPER

Selective catalytic oxidation of ammonia into nitrogen and water vapour over transition metals modified Al2 O3 , TiO2 and ZrO2 Magdalena Jablo´ nska* Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Kraków, Poland Received 4 December 2014; Revised 2 February 2015; Accepted 26 February 2015

Copper or iron supported on commercially available oxides, such as γ-Al2 O3 , TiO2 (anatase) and monoclinic tetragonal ZrO2 (mt-ZrO2 ) were tested as catalysts for selective catalytic oxidation of ammonia into nitrogen and water vapour (NH3 -SCO) in the low temperature range. Different commercial oxides were used in this study to determine the influence of the specific surface area, acidic nature of the support and crystalline phases as well as of the type of species and aggregation state of transition metals on the catalytic performance in selective ammonia oxidation. Copper modified oxide supports were found to be more active and selective to nitrogen than catalysts impregnated with iron. Activities of both transition metal modified samples decreased in the following order: mt-ZrO2 , TiO2 (anatase), γ-Al2 O3 . Quantitative total ammonia conversion was achieved with the Cu/ZrO2 catalytic system at 400 ◦C. Characterisation techniques, e.g. H2 -temperature programmed reduction, UV-VIS-diffuse reflectance spectroscopy, suggest that easily reducible copper oxide species are important in achieving high catalytic performances at low temperatures. c 2015 Institute of Chemistry, Slovak Academy of Sciences Keywords: oxide supports, selective oxidation of ammonia, NH3 -SCO, copper, iron

Introduction Low temperature selective catalytic oxidation of ammonia into nitrogen and water vapour (NH3 -SCO) is considered as a potentially efficient, stable and simple technique for ammonia removal from oxygencontaining waste gases (Il’chenko, 1976; Li & Armor, 1997). Different types of materials have been reported to be active in selective ammonia oxidation; these materials can be divided into three main groups: (i) metallic catalysts, (ii) mixed oxide catalysts and (iii) ion-exchanged zeolites (Il’chenko, 1976; Gang et al., 2002; Jablo´ nska, 2014; Jablo´ nska et al., 2014). Among them especially copper and/or iron catalysts have been reported to be some of the most interesting systems. Therefore, this paper gives an overview of selective ammonia oxidation over copper or iron supported on commercially available oxides, such as γ-Al2 O3 , TiO2 (anatase) and mt-ZrO2. Some results of selective ammonia oxidation over other supported copper

or iron oxides found in scientific literature are listed in Table 1, where the highest ammonia conversion is presented. As it can be clearly seen, the activity and selectivity to nitrogen depend mainly on the chemistry of the starting support. γ-Al2 O3 , TiO2 (anatase) and mt-ZrO2 are characterised by different properties, such as specific surface area, surface acidity and reducibility. Some of the particular physicochemical properties of these oxides are listed below while a more detailed and comprehensive review can be found in literature (Matsuda & Kato, 1983; Trueba & Trasatti, 2005; Kelly & Denry, 2008). γ-Al2 O3 is a transition alumina stable between 350– 1000 ◦C (Trueba & Trasatti, 2005). γ-Al2 O3 has Lewis acid sites that have a significant effect on the distribution and state of the active components as well as on the catalytic activity in numerous reactions (Morterra & Magnacca, 1996). Additionally, a defective spinel structure (Al8 [Al13(1/3) 2(2/3) ]O32 ]) facilitates the interaction of the active component with the support

*Corresponding author, e-mail: [email protected]

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M. Jablo´ nska/Chemical Papers

Table 1. Literature data on modified copper or iron catalysts tested for the low temperature selective ammonia oxidation; W/F = catalyst mass/feed flow rate; GHSV = gas hourly space velocity Catalysts mass composition (metal oxide precursor)/%

Composition of testing gas/vol. %

Conditions

Temperature/ ◦C

Conversion of NH3 /%

Selectivity to N2 /%

Reference

Cu, 3.4/Al2 O3 (NO− 3 )

NH3 , 0.54; O2 , 8

W/F = 0.075 g s cm−3 , catalysts: 25 mg

325

82

89

Lenihan and Curtin (2009)

Cu, 10.0/TiO2 (c)

NH3 , 0.04: O2 , 10

GHSV = 50000 h−1 , catalysts: 200 mg

250

100

95

He et al. (2007)

CuO, 4.3/Al2 O3

NH3 , 0.54; O2 , 8 W/F = 0.03 g s cm−3 , catalysts: 25 mg

325

63

95

Curtin et al. (2000)

350

75

96

100

90

100

93

(NO− 3 ) Cu, 5.0/Al2 O3 (NO− 3 ) Cu, 10.0/Al2 O3

NH3 , 1.14; O2 , 8.21


NH3 , 0.1; O2 , 10


350

NH3 , 0.04; O2 , 10


NH3 , 1.14; O2 , 8.21


(NO− 3 ) Cu, 10.0/Al2 O3 (C2 H6 O2− 2 ) Cu, 10.0/Al2 O3

Gang et al. (1999)

Liang et al. (2012)

97

100

400

100

95

He et al. (2007)

350

100

94

Gang et al. (2000)

NH3 , 0.04; O2 , 18.2; N2 , 69.6; H2 , 2.9; CO, 4.1; CO2 , 3.9; CH4 , 1.3

500

100

–

NH3 , 0.04; O2 , 1.2; N2 , 87.1; H2 , 2.8; CO, 4.0; CO2 , 3.7; GHSV = 100000 h−1 , CH4 , 1.2 catalysts: 15–20 mass %, loading of NH3 , 0.04; .O2 , washcoat 18.2; N2 , 69.6; H2 ,

500

79

–

(NO− 3 ) Cu, 10.0/Al2 O3 (NO− 3 ) Cu, 15.0/Al2 O3 (NO− 3 )

Cu 10.0/Al2 O3 (NO− 3 )

Fe, 5.0/Al2 O3 (NO− 3 )

380

98

–

500

24

–

500

100

94

2.9; CO, 4.1; CO2 , 3.9 CH4 , 1.3 NH3 , 0.04; O2 , 1.2; N2 , 87.1; H2 , 2.8; CO, 4.0; CO2 , 3.7; CH4 , 1.2

Fe, 5.0/Al2 O3 (NO− 3 )

Kušar et al. (2005)

NH3 , 0.1; O2 , 18


(Ivanova et al., 2010). TiO2 (anatase) – one of the three crystalline modifications of titanium oxide, is stable at low temperatures. Sintering of the material at above 700 ◦C is accompanied by the transformation of anatase to rutile (Matsuda & Kato, 1983). TiO2

Amblard et al. (1999)

(anatase) suffers from low surface area (Reddy et al., 1999), however, it exhibits high SO2 durability (Nakajima & Hamada, 1996). Additionally, TiO2 has been proved to play a role both as a support and as a promoter of activity (Matsuda & Kato, 1983). ZrO2 exists



in three different forms: monoclinic (m-ZrO2 ), tetragonal (t-ZrO2 ) and cubic (c-ZrO2 ) (Iglesia et al., 1996; Chang & Doong, 2005). m-ZrO2 is stable from room temperature to 1170 ◦C, while the tetragonal form is stable between 1170–2370 ◦C (Basu et al., 2004). The study of c-ZrO2 is seldom addressed, mainly due to the lack of stability of this form at ambient conditions (Chang & Doong, 2005). Among all phases of zirconia, t-ZrO2 exhibits the most favourable features such as chemical resistance, high fracture toughness and hardness (Chary et al., 2007). Thus, the mixture of monoclinic and tetragonal phase is most frequently used in catalytic applications (Chary et al., 2007; Sato et al., 2012). The presence of both acidic and basic properties makes zirconia a very attractive carrier for numerous catalytic applications (Yamaguchi, 1994). As stated above, physicochemical properties of commercially available oxides are an interesting starting point in the preparation of efficient catalysts for selective ammonia oxidation. The current paper presents results of the NH3 -SCO process over copper or iron modified γ-Al2 O3 , TiO2 (anatase) and mt-ZrO2 and their structure (XRD, UV-VIS-DRS), specific surface area (SBET ), redox properties (H2 -TPR) and chemical surface composition (XPS) are discussed. This approach enabled the comparison of the role of active components deposited on the surface of the oxide supports in the NH3 -SCO process. It seems that there are no studies on selective ammonia oxidation over transition metals modified zirconia in scientific literature. The objective of this paper is to fill this gap.

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Various commercially available oxides such as γAl2 O3 (Merck, Germany), TiO2 (anatase) (Sigma– Aldrich) and mt-ZrO2 (Sigma–Aldrich, Poland) were used in this study. Cu- or Fe-doped catalysts with 1.0 mass % of transition metal were prepared by the incipient wetness impregnation method using aqueous solutions of the following metal nitrates: Fe(NO3 )3 · 9H2 O (POCH) and Cu(NO3 )2 · 3H2 O (Merck). The prepared catalytic materials were calcined at 600 ◦C for 12 h. The catalyst obtained grade grain was in the range of 1.60–3.15 mm.

tometer (Brucker) with CuKα radiation (λ = 1.54060 ˚ A, 30 kV, 10 mA). Temperature programmed ammonia desorption (NH3 -TPD) of the samples was performed in a flow microreactor system equipped with a QMS detector (VG QUARTZ). Prior to the ammonia sorption, the sample (100 mg) was outgassed in a flow of pure helium at 500 ◦C for 1 h. Subsequently, the microreactor was cooled down to 70 ◦C and the sample was saturated in a flow (20 cm3 min−1 ) of a gas mixture containing 1.0 vol. % of NH3 diluted in helium for about 2 h. Then, the sample was purged in a flow of pure helium until a constant baseline level was attained (about 1.5 h). In the next step, the temperature of the reactor was increased in the range of 70–500 ◦C at the linear heating rate of 10 ◦C min−1 in a flow of pure helium. The total flow rate was 20 cm3 min−1 . H2 -TPR of the samples was carried out between room temperature and 1100 ◦C using a linear heating rate of 5 ◦C min−1 . The measurements were performed in a fixed-bed flow microreactor. The TPR runs were carried out in a flow of 5.0 vol. % of H2 diluted in argon, the flow rate was 6 cm3 min−1 . Evolving water was removed from the effluent gas by means of a cold trap. The evolution of hydrogen was detected by a micro volume TCD (Valco). UV-VIS-DRS spectra of the samples were recorded using an Evolution 600 (Thermo) spectrophotometer. The measurements were performed in the range of 200–900 nm with the resolution of 1 nm. The spectra were recorded in air at room temperature and the data transformed according to the Kubelka–Munk equation. X-ray photoelectron spectra (XPS) were measured on a VSW spectrometer equipped with a hemispherical analyser. The photoelectron spectra were measured using a magnesium MgKα source (E = 1253.6 eV). The base pressure in the analysis chamber during the measurements was 3 × 10−6 Pa and the spectra were calibrated on a main carbon C 1s peak at 284.6 eV. The composition and chemical surrounding of the sample surface were investigated based on the areas and binding energies of Cu 2p, Zr 3d, C 1s and O 1s photoelectron peaks. Mathematical analyses of the XPS spectra were carried out using the XPSpeak 4.1 computer software (RWM. Kwok, The Chinese University of Hong Kong).

Catalysts characterisation

Catalytic tests

Specific surface areas of the calcined catalysts were determined by the BET method. The measurements were performed using a Quantasorb Junior sorptometer (Ankersmit). Prior to the nitrogen adsorption at –196 ◦C, the samples were outgassed under nitrogen atmosphere at 250 ◦C for 2 h. X-ray diffraction (XRD) patterns of the catalytic materials were recorded using a D2 Phaser diffrac-

The catalytic experiments were performed in a fixed-bed flow microreactor (ID = 7 mm, l = 240 mm). The analysis of the reaction products was performed using a QMS detector (PREVAC). Prior to the activity tests, the catalyst sample (100 mg) was outgassed at 500 ◦C for 1 h in a flow of pure helium (20 cm3 min−1 ). Catalytic tests were performed for a mixture containing: 0.5 vol. % NH3 , 2.5 vol. % O2 ,

Experimental Catalysts preparation


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Table 2. Specific surface areas and chemical compositions of catalytic materials Sample code Al2 O3 Al2 O3 -600 Al2 O3 -600-Cu Al2 O3 -600-Fe

SBET /(m2 g−1 ) 135 124 98 129

TiO2 TiO2 -600 TiO2 -600-Cu TiO2 -600-Fe

9 8 7 8

ZrO2 ZrO2 -600 ZrO2 -600-Cu ZrO2 -600-Fe

5 4 4 4

97.0 vol. % He. The total flow rate of the reaction mixture was 40 cm3 min−1 . The studies were performed in the temperature range of 100–500 ◦C at the linear heating rate of 10 ◦C min−1 . Intensities of the mass lines corresponding to all reactants and possible products were measured at a given temperature at least 30 min after the reaction had reached a steady-state. The signal of the helium line served as the internal standard to compensate small fluctuations of the operating pressure. Sensitivity factors of the analysed lines were calibrated using commercial mixtures of gases. Possible changes in the molar flow caused by the NH3 conversion were negligible in the diluted reaction mixtures. Differences between the reactor inlet and outlet molar flows of the reactants were used to determine their conversion. Additionally, the catalysts were tested in the process of selective catalytic reduction of NO with ammonia (NH3 -SCR). The experiments were performed in a fixed-bed flow microreactor (ID = 7 mm, l = 240 mm) and the reactant concentrations were continuously measured using a quadruple mass spectrometer (PREVAC) connected directly to the reactor outlet. Prior to the reaction, each catalyst sample (100 mg) was outgassed in a flow of pure helium at 500 ◦C for 1 h. The following composition of the gas mixture was used: 0.25 vol. % NO, 0.25 vol. % NH3 , 2.5 vol. % O2 . Helium was used as the balancing gas at the total flow rate of 40 cm3 min−1 , while the space velocity was about 15,400 h−1 .

Fig. 1. X-ray diffraction patterns for commercial oxides: Al2 O3 (A), TiO2 (B), ZrO2 (C); G – γ-Al2 O3 , A – TiO2 anatase, M – ZrO2 monoclinic, T – ZrO2 tetragonal.

Results and discussion Specific surface areas of commercial oxides such as γ-Al2 O3 , TiO2 (anatase), mt-ZrO2 and their modifications with transition metals are presented in Table 2. Alumina exhibited the highest specific surface area, 135 m2 g−1 . Titania and zirconia have very low val-

ues of the specific surface area, 9 m2 g−1 and 5 m2 g−1 , respectively. Decrease in the surface area after calcination at 600 ◦C was prominent in case of all support oxides. A further decrease in the specific surface area of the supports after the deposition of transition met-



Fig. 2. NH3 -TPD profile of oxides; experimental conditions: mass of catalyst = 100 mg, sorption: NH3 , 1.0 vol. %; He, 99.0 vol. %; desorption: He, 100.0 vol. %; flow rate = 20 cm3 min−1 ; linear heating rate of 10 ◦C min−1 ; A – γ-Al2 O3 , B – TiO2 (anatase), C – mt-ZrO2 .

als – copper or iron, is a normal phenomenon in the impregnation process as the surface is covered by low surface area clusters of active components. Powder XRD diffraction patterns obtained for the commercial oxides are presented in Fig. 1. Characteristic XRD diffraction peaks corresponding to the reflections of γ-Al2 O3 support were observed at 2θ = 19.9◦ , 32.6◦, 37.5◦ , 39.6◦ , 45.9◦ , 61.1◦ and 67.0◦ (Fig. 1A) (Wan et al., 2010). Characteristic diffraction peaks for TiO2 (anatase) were found at 2θ = 25.4◦ , 37.8◦ , 48.1◦ , 54.0◦ and 55.1◦ (Fig. 1B) (Zhang et al., 2009). ZrO2 exists in three crystallographic polymorphs: monoclinic, tetragonal and cubic (Iglesia et al., 1996). Zirconia used in the present study comprises mixed phases of monoclinic and tetragonal polymorphs (Fig. 1C). XRD peaks of ZrO2 were observed at 2θ = 17.4◦ , 24.1◦ , 28.2◦ , 31.5◦, 34.2◦ and 49.3◦ for the monoclinic phase of zirconia and at 2θ = 50.4◦ and 59.6◦ which correspond to tetragonal zirconia (Sato et al., 2012). No changes in the structure of oxides were reported either after their calcination in air at 600 ◦C for 12 h or after impregnation with copper or iron (results not shown), which indicates that deposited oxides, i.e. CuOx , Fex Oy , were dispersed homogenously on the supports in amorphous or poorly crystalline state. Fig. 2. shows the NH3 -TPD spectra employed to measure the total acidity of the commercial oxides and their relative acid strength. The total ammonia desorption obtained for γ-Al2 O3 were 169.6 mol g−1 ; the acid sites were distributed in two temperature regions, which indicates the presence of two types of adsorbed ammonia species with different thermal stabilities (Smirniotis et al., 2006). The amount of ammonia desorbed in case of TiO2 (specific surface area, SBET , 9 m2 g−1 ) and ZrO2 (SBET , 5 m2 g−1 ) was quite small and reached only 12.1 mol g−1 and 10.6 mol g−1 ,

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respectively. Deposition of transition metals on oxide supports resulted in only slight increase in their surface acidity (results not shown). H2 -TPR profiles of commercial oxides and supported copper or iron oxide catalysts are shown in Fig. 3. The TPR profile for γ-Al2 O3 did not show any H2 uptake (Fig. 3A). Reduction temperature of TiO2 in the presence of H2 is reported to be 1300 ◦C (Dewan et al., 2009). Under the presented experimental conditions, TiO2 was reduced at lower temperatures in two stages. The broad peak located at 370 ◦C can be assigned to partial reduction of titania to a mixture of Ti4 O7 and Ti3 O5 that was further converted into Ti3 O5 and Ti2 O3 at 1030 ◦C. Further transformation to TiO was not observed under the studied experimental conditions (ambient temperature–1100 ◦C) due to insufficient temperature. As it was reported before (Dewan et al., 2009), TiO starts to appear at temperatures above 1324 ◦C. The reduction to a mixture of Ti3 O5 and Ti2 O3 in pure TiO2 was promoted by the presence of dispersed metal crystallites at lower temperatures in case of Cu- or Fe-containing titania (1024 ◦C and 926 ◦C, respectively). Iron doping facilitated even stronger reduction, indicating robust interaction between metal and the support. A similar effect was described previously over gold–iron supported on titania (Parida et al., 2010). The broad peak located at around 540 ◦C present on pure ZrO2 can be assigned to the reaction of hydrogen with oxygen atoms present on the surface of zirconia. According to literature data (van den Berg et al., 1985), maximum temperature of this peak can vary depending on the preparation procedure of zirconia and thus on the crystalline phase composition. Generally, the spectra obtained for the Cu-containing samples were fitted by bands presented by two sharp low and high temperature reduction peaks. The low temperature peak is related to the reduction of highly dispersed copper oxide species which include isolated copper ions and small two- and threedimensional clusters. The high temperature peak can be attributed to the reduction of the bulk-like copper oxide phases (Dow et al., 2000; Chary et al., 2008). For Al2 O3 -600-Cu sample, reduction peaks located at around 300 ◦C, 670 ◦C and 894 ◦C were found. The first peak is due to the partial reduction of Cu2+ present in highly dispersed copper oxide species generating Cu1+ and Cu◦ (Mozer & Passos, 2011), while the peaks located in the higher temperature range of 557– 825 ◦C (maximum at about 670 ◦C) and 894 ◦C are related to the reduction of copper in bulky CuO clusters and bulk CuAl2 O4 phase, respectively. Bulk CuAl2 O4 phase was not detected by XRD, which is consistent with previous studies (e.g. Luo et al., 2005). In particular, it was reported that at low loadings (lower than 4 mass % of CuO for alumina with the surface area of 100 m2 g−1 ) and low calcination temperature (around 500–600 ◦C), defective spinel-type surface


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in which about 60 % of the copper ions have tetragonal coordination and 40 % have octahedral coordination (Strohmeier et al., 1985). In the TiO2 -600-Cu sample, reduction peaks were located at 205 ◦C and 261 ◦C, which are related to the interaction of highly dispersed CuO species with TiO2 , and the reduction of bulk CuO, respectively (Larsson & Anderson, 1998). Similarly, the TPR profile of the ZrO2 -600-Cu catalyst exhibited two reduction peaks centred at about 203 ◦C and 269 ◦C. The low temperature TPR peak is attributed to the reduction of highly dispersed surface CuO species, indicating CuO–ZrO2 interactions. The high-temperature TPR peak can be assigned to the reduction of CuO due to weak interaction with the ZrO2 surface or bulk CuO (Sato et al., 2012). The H2 -TPR spectra obtained for the profiles of Fe-containing samples are presented mainly by three reduction peaks which correspond to the following sequence (1) (Hayashi et al., 2002): 3 Fe2 O3 (hematite) → 2 Fe3 O4 (magnetite) → 6 FeO (w¨ ustite) → 6 Fe

Fig. 3. H2 -TPR profiles of oxides and their modified forms with transition metals γ-Al2 O3 (A), TiO2 (anatase) (B), mt-ZrO2 (C); experimental conditions: mass of the catalyst = 30 mg; H2 , 5.0 vol. %; Ar, 95.0 vol. %; flow rate = 6 cm3 min−1 ; liner heating rate of 5 ◦C min−1 .

species (CuAl2 O4 ) are formed with most Cu2+ ions in a distorted octahedral geometry, unlike bulk CuAl2 O4

(1)

For the Al2 O3 -600-Fe catalyst, reduction peaks located at about 233–781 ◦C (maximum at about 418 ◦C) and a broad peak located at about 982 ◦C were observed. The first TPR peak indicates that the isolated Fe2 O3 was reduced to Fe2+ and then to Fe◦ , while the second peak corresponds to the reduction of FeAl2 O4 to Fe2+ and further also to metallic iron (de Morais Batista et al., 2010). In the TiO2 -600-Fe sample, the maximum of hydrogen consumption located at 448 ◦C can be attributed to the first stage of Fe2 O3 reduction to Fe3 O4 , and the next broad maximum at 528 ◦C to the subsequent reduction to FeO, which was in the next step reduced to metallic iron at 796 ◦C. The same reduction peaks can be observed in the H2 -TPR spectra of the ZrO2 -600-Fe sample; however, the peaks were located at lower temperatures (376 ◦C, 482 ◦C and 630 ◦C, respectively), which indicates that iron species were more easily reduced than in the case of iron supported on titania. UV-VIS-DRS were employed to prove the H2 -TPR results related to the oxidation state and the chemical environment of transition metals present in the catalytic materials. Fig. 4. shows the UV-VIS-DR spectra of the oxide supports impregnated with copper or iron. The diffuse reflectance UV-VIS-DR spectrum of γ-Al2 O3 (results not shown) revealed its optical transparency. Strong absorption at 200–340 nm is characteristic of TiO2 (Parida et al., 2010), while TiO2 (anatase) usually shows an absorption threshold at above 400 nm. A band centred at 315 nm in this case can be ascribed to the O2− → Ti4+ charge-transfer transition (Liu et al., 2010). The UV-VIS spectrum of pure ZrO2 , which is actually a mixture of the monoclinic and tetragonal phases, exhibited a characteris-



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Fig. 4. UV-VIS-DR spectra of oxides and their modified forms with transition metals: γ-Al2 O3 (A), TiO2 (anatase) (B), mt-ZrO2 (C). Table 3. Literature data related to charge–transfer and d–d transfer transitions of copper and iron reference compounds Species CuO [Cu—O—Cu]n [FeO4 ] [FeO6 ] (FeO)n Fe2 O3

Mn+

λmax /nm

Assignment

Cu2+

234 700 350

O2− → Cu2+ 2E → 2T g 2g O2− → Cu2+

222, 252 275 390 533 540, 570

O2− → Fe3+ O2− → Fe3+ O2− → Fe3+ 6 6A 4 4 1g → T1g A1g → T2g Fe3+ + Fe3+ → Fe4+ + Fe2+ or 2T 2 2 2g → A2g , T1g

Fe3+

tic band at around 232 nm due to the O2− → Zr4+ 2 charge–transfer transition (Morterra et al., 1998). Spectra obtained for the Cu- or Fe-containing samples were fitted by bands presented in Table 3. The investigation provided information on the copper or iron oxide state. It can be concluded that two different forms of copper: isolated Cu2+ cations and bulky clusters of CuO, were introduced onto γ-Al2 O3 . These forms are represented by a main peak centred at about 235 nm with a broad shoulder at above 400– 500 nm. Absorption spectra of CuAl2 O4 were not recorded (Salavati-Niasari et al., 2009). The assignment of bands in case of both Cu-containing TiO2 (anatase) and mt-ZrO2 was very difficult or even impossible due to overlapping bands of CuOx species and oxide supports. Therefore, discussion of possible CuOx species in case of Cu-containing TiO2 and ZrO2 samples is very speculative. The same was found for TiO2 -600-Fe sample. Iron species deposited on γ-Al2 O3 existed mainly

Reference Wan et al. (2010) Praliaud et al. (1998) Mendes and Schmal (1997) Timofeeva et al. (2007), Pérez-Ramírez et al. (2004) Dossi et al. (1999) Schwidder et al. (2005), Pirngruber et al. (2006) Neat¸u et al. (2009), Wu et al. (2012)

in the form of isolated Fe3+ cations and oligonuclear (FeO)n species. The bands centred at 230 nm and 310 nm are related to monomeric Fe3+ species in tetrahedral and octahedral coordination, respectively (Pérez-Ramírez et al., 2004; Timofeeva et al., 2007). The band located at around 350 nm can be assigned to the Fe3+ ions in oligonuclear (FeO)n species, while the shoulder at about 480 nm can be assigned to the aggregated iron oxide clusters (Ohishi et al., 2005). Spectra of the ZrO2 -600-Fe sample showed an effective increase in the visible light absorption capacity compared to pure oxide; however, some isolated Fe3+ , oligonuclear (FeO)n and Fe2 O3 clusters with bands located at 230 nm and 305 nm, and at 365 nm and 500 nm, respectively, can be distinguished (Ohishi et al., 2005). Fig. 5. shows the Cu 2p and O 1s peaks of the XPS spectra of ZrO2 impregnated with copper, while Table 4. summarizes the values of the Cu 2p, O 1s and Zr 3d BEs spectra. The low binding energy in the range


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Fig. 5. X-ray photoelectron spectra of zirconia modified with copper: Cu 2p (A) and O 1s (B).

Table 4. XPS results of zirconia modified with copper Positiona

FWHMb

Peak eV 932.4 (main peak) 942.6 (satellite peak) 934.3

2.8 3.9 3.1

O 1s

529.3 530.3 531.9

2.0 1.9 2.2

Zr 3d

181.7 182.8

1.9 2.2

Cu 2p

a) Position of binding energy; b) full width at half-maximum.

of 932.2–933.1 eV is characteristic of the Cu1+ ion (Chary et al., 2007). The results indicate that copper species on the surface of ZrO2 -600-Cu were present as Cu1+ ions characterised by the maximum of the main photoelectron peak at 932.4 eV. Additionally, the shoulder peak at BE of 934.3 eV for the Cu 2p3/2 peak can be attributed to the Cu2+ species interacting strongly with ZrO2 (Wu et al., 2009). O 1s XPS spectra of ZrO2 -600-Cu were also investigated. According to the position of different O species (Wu et al., 2009), the O 1s peak was separated into three peaks at about 529.3 eV, 530.0 eV and 531.9 eV which correspond to the O atom of CuO, ZrO2 or Cu2 O and of —OH, respectively. All obtained samples were tested as catalysts for selective oxidation of ammonia into nitrogen and water vapour (NH3 -SCO). Nitrogen was the main product of ammonia oxidation; however, also a significant amount of NO and N2 O in the reaction products was observed. Fig. 6. presents the results of activity exper-

Fig. 6. Results of catalytic test for the NH3 -SCO process performed in an empty reactor (without a catalyst): NH3 conversion ( ), N2 selectivity ( ), NO selectivity ( ), N2 O selectivity (); reaction conditions: NH3 , 0.5 vol. %; O2 , 2.5 vol. %; He, 97.0 vol. %; total flow rate = 40 cm3 min−1 ; liner heating rate of 10 ◦C min−1 .

•

iment performed in the absence of catalyst, i.e. empty reactor. Ammonia conversion started at temperatures as high as 375 ◦C and at 500 ◦C it did not exceed 20 %. Results of the catalytic tests performed for commercial oxides and their derivatives with deposited transition metals are shown in Fig. 7. Among the oxide supports studied, zirconia best enhanced the oxidation activity and the oxidation selectivity. Ammonia conversion for pure mt-ZrO2 started at 300 ◦C and at 500 ◦C, NH3 conversion of about 60 % was achieved. Nitrogen was the main product of ammonia oxidation; however, also a significant amount of NOx especially at higher temperatures was found. The NH3 -SCO performance of other




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x


Fig. 7. Results of catalytic tests for the NH3 -SCO process performed for oxides and their modifications with transition metals: Al2 O3 (A); Al2 O3 -600-Cu (B); Al2 O3 -600-Fe (C); TiO2 (D); TiO2 -600-Cu (E); TiO2 -600-Fe (F); ZrO2 (G); ZrO2 -600-Cu (H); ZrO2 -600-Fe (I). NH3 conversion ( ), N2 selectivity ( ), NO selectivity (), N2 O selectivity (); reaction conditions: mass of catalyst = 100 mg; NH3 , 0.5 vol. %; O2 , 2.5 vol. %; He, 97.0 vol. %; total flow rate = 40 cm3 min−1 ; linear heating rate of 10 ◦C min−1 .

•

oxides was lower compared to that of mt-ZrO2 and decreased in the following order: mt-ZrO2 , γ-Al2 O3 , TiO2 (anatase). Deposition of transition metals on oxide supports resulted in their catalytic activation. Among the catalysts studied, copper deposited on both ZrO2 and TiO2 showed superior catalytic performance over other samples. Complete NH3 oxidation with the N2 selectivity of 77 % and 54 % was achieved at 400 ◦C and 425 ◦C over ZrO2 -600-Cu and TiO2 -600-Cu, respectively. Good performance of these two supports was not expected because of their extremely low surface areas (ZrO2 -600-Cu, SBET , 4 m2 g−1 ; TiO2 -600Cu, SBET , 7 m2 g−1 ). On the other hand, the activity of Cu-containing mt-ZrO2 and TiO2 (anatase) catalysts can be attributed to the interaction between the highly dispersed easily reducible CuO species and the

oxide support. Catalytic performance of the studied Cu-containing samples was determined mainly by the redox properties of the copper oxide species. The catalysts containing easily reduced copper oxide species presented superior catalytic activity at lower temperatures but also a significant decrease in the selectivity to N2 at higher temperatures. These results are consistent with previous findings obtained over hydrotalcite originated mixed metal oxides containing copper (Jablo´ nska et al., 2013a, 2013b). On the other side, more aggregated CuO species with lower reducibility are responsible for the higher selectivity to N2 (Jablo´ nska et al., 2013a, 2013b). For γ-Al2 O3 impregnated with copper, the obtained selectivity to N2 in the whole temperature range was more stable and higher in comparison to the other samples. These results can be ascribed to the more aggregated copper


xi


oxides species present in form of CuO and CuAl2 O4 as it was shown by the H2 -TPR studies. Quantitatively, for the Al2 O3 -600-Cu catalyst, ammonia oxidation started at 275 ◦C with a nearly 100 % selectivity to N2 and its complete removal from the reaction mixture was achieved at 500 ◦C with a 96 % selectivity to N2 . A series of Fe-containing samples were operated at significantly higher temperatures than coppermodified samples. However, also in this case, higher activity was obtained compared to pure support oxides. An exception was the TiO2 -600-Fe sample with the catalytic activity quite similar to that of pure material, which indicates that the deposition of iron influences the catalytic activity of the oxide support only slightly. Total ammonia conversion in the studied temperature range was achieved only with ZrO2 -600-Fe at 500 ◦C with an 87 % selectivity to N2 . As it can be seen from H2 -TPR or UV-VIS-DRS, these results can be explained by the presence of relatively easily reducible iron oxide species. In conclusion, the obtained results show that both kinds of oxide supports and transition metal oxides strongly influence the activity and selectivity of the NH3 -SCO process. The activity of the supported copper catalysts decrease in the following order: mt-ZrO2 , TiO2 (anatase), γ-Al2 O3 . A similar order, except for the TiO2 -600-Fe sample, was found for catalysts containing iron. A comparison of obtained data with results reported in scientific literature (cf. Table 1.) showed lower catalyst performance of the tested materials compared to other materials studied (Amblard et al., 1999; Gang et al., 1999, 2000; Kušar et al., 2005;

Liang et al., 2012). However, it is well known that copper loading of around 10.0 mass % supported on alumina, titania or incorporated into their structure, has been considered as the most efficient system until now by the majority of researchers (Gang et al., 1999, 2000; He et al., 2007). On the other hand, there are no reports concerning copper supported on zirconia. Therefore, the objective of this paper was a comparative study of the selective ammonia oxidation into N2 over pure support oxides and their modified forms with transition metals – copper or iron, as the starting point for the preparation of highly active and selective catalytic systems. Another aim of this research was to investigate the reaction mechanism of these materials. From the three mechanisms presented in literature, such as hydrazine (N2 H4 ) mechanism (Ramis et al., 1996), imide (NH) mechanism (Zawadzki, 1950) and internal selective ammonia oxidation mechanism (iSCR) (Jablo´ nska et al., 2013a, 2013b), the last one is accepted by the majority of the researchers. The i-SCR mechanism consists in two steps (Jablo´ nska, 2014; Jablo´ nska et al., 2014). In the first step, a part of ammonia is oxidised to NO, according to Eq. (2), while in the second step, NO is reduced by ammonia unreacted in the first step according to the NH3 SCR reaction (Eq. (3)). The main products are N2 and H2 O, whereas the formation of N2 O is also possible considering Eq. (4): 4NH3 + 5O2 → 4NO + 6H2 O

(2)

4NO + 4NH3 + O2 → 4N2 + 6H2 O

(3)

4NO + 4NH3 + 3O2 → 4N2 O + 6H2 O

(4)

Fig. 8. Results of activity tests for the NH3 -SCR process performed for oxide supports modified with copper, selectivity to N2 (A) and NO conversion (B): Al2 O3 -600-Cu ( ), TiO2 -600-Cu (), ZrO2 -600-Cu ( ). Conditions: mass of catalyst = 100 mg; NH3 , 0.25 vol. %; NO, 0.25 vol. %; O2 , = 2.5 vol. %; He, 97.0 vol. %; total flow rate = 40 cm3 min−1 , linear heating rate of 10 ◦C min−1 .

•


xii


Fig. 9. Results of catalytic tests for the NH3 -SCO (diamonds) and NH3 -SCR (squares) processes performed for zirconia modified with copper, ZrO2 -600-Cu; reaction conditions: mass of catalyst = 100 mg; NH3 , 0.5 vol. % (in NH3 -SCR: NH3 , 0.25 vol. %; NO, 0.25 vol. %); O2 , 2.5 vol. %; He, 97.0 vol. %; total flow rate = 40 cm3 min−1 , liner heating rate of 10 ◦C min−1 .

process can be considered as the first indication of the i-SCR mechanism. In order to determine the reaction mechanism in detail, additional studies, e.g. FT-IR of adsorbed ammonia proposed by Chen et al. (2014) or Zhang and He (2009), should be carried out. However, such experiments have to be investigated separately. A comparison of the results of both catalytic tests, i.e. NH3 -SCO and NH3 -SCR led to the conclusion that the second reaction is faster than ammonia oxidation (Fig. 9). It was observed that ammonia oxidation in the NH3 -SCO process over the ZrO2 -600-Cu catalyst started at about 200 ◦C, whereas the conversion of NO in the NH3 -SCR process was observed at temperatures lower by about 25 ◦C. Therefore, it can be concluded that oxidation of NH3 to NO is the rate determining step in the low temperature range. Consequently, the activity of the catalyst can be improved by its modification with small amounts of noble metals which are responsible for selective ammonia oxidation to NO. Such studies over bi-functional systems are underway.

Conclusions Among various catalytic systems, this mechanism was proposed also for Cu- or Fe-containing catalysts, such as Cu/Al2 O3 (Curtin et al., 2000), CuMgAlOx (Jablo´ nska et al., 2013a, 2013b), Fe2 O3 (Pérez-Ramírez & Kondratenko, 2007), Fe2 O3 –Al2 O3 , Fe2 O3 –TiO2 , Fe2 O3 –ZrO2, Fe2 O3 –SiO2 (Long & Yang, 2002) or Fe–ZSM–5 (Akah et al., 2005). It seems also to be valid in case of the tested materials. If the process of ammonia oxidation proceeds according to the i-SCR mechanism, the studied catalysts should also be active in the selective catalytic reduction of NO with ammonia (NH3 -SCR, DeNOx ). Results of the catalytic studies of the NH3 -SCR process for copper-modified γ-Al2 O3 , TiO2 (anatase) and mtZrO2 samples are presented in Fig. 8. The experiments performed using these materials for the catalytic reduction of NO by ammonia proved their catalytic activity in the studied temperature range. N2 and N2 O were the only detected in the N-containing reaction products. At low temperatures, the NH3 -SCR reaction dominated and NO conversion increased with the increasing temperature; increasing temperature led to the predominant role of the ammonia oxidation. Similarly, to the results obtained for the NH3 -SCO reaction, also in this case, copper deposited on mt-ZrO2 showed superior catalytic performance than over other catalysts. However, the selectivity to N2 obtained over this catalyst was the lowest among all catalysts studied, but still above 75 % in the studied temperature range. Selectivity to N2 of above 93 % was achieved over the Al2 O3 -600-Cu sample with the maximum conversion for this catalyst of about 60 % at 400 ◦C. At the higher temperatures, the NO conversion decreased due to the side process of ammonia oxidation. Presented activities of the samples in the NH3 -SCR

Preparation, physicochemical characterisation and catalytic performance in the selective ammonia oxidation in the presence of copper or iron containing γ-Al2 O3 , TiO2 (anatase) and mt-ZrO2 were studied. Among the tested catalysts, copper deposited on mtZrO2 showed the highest catalytic performance with complete ammonia conversion and a 77 % N2 selectivity at 400 ◦C. The same catalyst revealed the highest activity however with the lowest selectivity to N2 over all studied Cu-containing catalysts in the NH3 SCR process. Redox properties of catalytically active species are an important parameter influencing the activity and selectivity of the catalysts. Highly dispersed easily reducible transition oxide species were responsible for the higher NH3 -SCO and NH3 -SCR activity at relatively low temperatures, but also for the lower selectivity to N2 at higher temperatures. A comparison of the results of NH3 -SCO and NH3 -SCR tests over the same catalysts showed that ammonia oxidation to NO is the rate determining step in the low temperature range. Acknowledgements. The author appreciates the possibility to measure X-ray photoelectron spectra of the Cu-ZrO2 sample at the laboratory of Prof. Marek Nocu´ n (AGH University of Science and Technology, Kraków). Part of the research was done with equipment purchased in the frame of the European Regional Development Fund (Polish Innovation Economy Operational Program – contract no. POIG.02.01.00-12-023/08).

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