Copper Oxides Supported on Aluminum Oxide Borates for Catalytic

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Copper Oxides Supported on Aluminum Oxide Borates for Catalytic Ammonia Combustion Satoshi Hinokuma,*,†,‡ Shun Matsuki,† Yusuke Kawabata,† Hiroki Shimanoe,† Saaya Kiritoshi,† and Masato Machida† †

Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡ Japan Science and Technology Agency, Precursory Research for Embryonic Science and Technology, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan S Supporting Information *

ABSTRACT: The catalytic NH3 combustion properties and local structures of copper oxides (CuOx) supported on aluminum oxide borates (Al20B4O36, 10Al2O3·2B2O3: 10A2B) were studied by means of high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray mapping, X-ray absorption fine structure, X-ray photoelectron spectroscopy, gas adsorption techniques, etc. Among the CuOx supported on various metal oxide materials, CuOx/ 10A2B exhibited high catalytic NH3 combustion activity, highest N2 (lowest N2O·NO) selectivity, and high thermal stability. Because the combustion activity is closely associated with the reducibility and dispersion of CuOx, highly dispersed CuOx nanoparticles on supports are considered to play a key role in the low temperature light-off of NH3. For NO and N2O selectivities, the oxidation state of CuOx and the dissociative species of adsorbed NH3 are suggested to be important catalytic combustion properties, respectively. On the basis of these discussions, the reaction mechanism of catalytic NH3 combustion over CuOx/10A2B is described. stoichiometric NH3 combustion (the NH3−O2 reaction).5 For selective catalytic NH3 oxidation, Il’chenko previously also reported that the activity of metal oxides was associated with their metal−oxygen bond energies and that CuOx showed high N2 selectivity.6 Moreover, to achieve high performance for the catalytic oxidation of NH3 to N2, CuOx-based catalysts have also been widely studied and were recently reviewed by Jablonska and Palkovits.7 The review particularly summarizes selective catalytic NH3 oxidation over CuOx, supported CuOx, and copper-exchanged zeolites required for various technical applications.7 However, to the best of our knowledge, there are no published studies regarding CuOx-based catalysts for the combustion of NH3 as a carbon-free energy source. In this study, therefore, we focused on CuOx catalysts supported on various materials to suppress N 2 O·NO x production under lean-burn conditions for the combustion of NH3 as energy source. In addition, because the highly thermal stable CuOx/10A2B catalyst exhibited high activity and N2 selectivity in comparison with other supported CuOx species, local structural characterization of this compound was performed using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), energy

1. INTRODUCTION Recently, NH3 has been considered as a renewable and carbonfree energy source due to its high energy density (3160 Wh· L−1) and negligible thermal NOx emission;1,2 furthermore, a micro gas turbine firing NH3 fuel has been demonstrated at the Fukushima Renewable Energy Institute in Japan.2 However, in comparison with fossil fuels, the use of NH3 as a fuel poses the following problems: (1) high ignition temperature, (2) low combustion rate, and (3) N2O and fuel NOx production. In order to solve these problems, the development of new NH3 combustion systems is required. One possible candidate is catalytic combustion, which is considered to be a promising technique for decreasing emissions from hydrocarbon-based fuels and has been actively studied for use in gas turbines, boilers, and jet engines in the 1980s.3,4 This system has multiple advantages over conventional noncatalytic combustion, as NOx emission is greatly diminished by low operating temperatures and high efficiency can be achieved through stable combustion. Therefore, it is desirable to develop a novel catalyst with high thermal stability, which enables low ignition temperature as well as negligible emission of N2O·NOx to realize environmentally friendly catalytic combustors for NH3 fuel. Previously, we have demonstrated that the NH 3 combustion activity of metal oxides increases with decreasing metal−oxygen bond energy; copper oxide (CuOx) catalysts exhibited higher N2 selectivity than other studied catalysts in © XXXX American Chemical Society

Received: July 17, 2016 Revised: October 12, 2016

A

DOI: 10.1021/acs.jpcc.6b07157 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Catalytic Properties of As-Prepared CuOx Supported on Various Metal Oxide Materials selectivity at T90 (%)a support

T10 (°C)

MgO Al2O3 10A2B AlPO4 SiO2 TiO2 ZrO2 La2O3 CeO2

360 303 307 351 334 340 306 290 301

a

T90 (°C) 487 476 474 501 490 481 438 440 476

a

N2

N2O

NO

SBET (m2·g−1)

reduction temperature (°C)b

Cu dispersion (%)c

Cu particle size (nm)c

83 92 97 95 97 73 84 57 88

1 6 2 3 1 4 2 6 1

16 2 1 2 2 23 14 37 11

33 149 77 64 177 9 11 13 137

212 185 210 222 220 220 210 162 172

29 43 27 12 11 19 25 30 42

3.6 2.4 3.9 8.6 9.4 5.3 4.1 3.4 2.5

Temperature at which NH3 conversion reached 10% and 90%. bTemperature of the first H2 consumption peak determined by H2-TPR. cCalculated from H2-TPR after H2-reduction and subsequent N2O-reoxidation. a

dispersive X-ray (EDX) mapping, X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), gas adsorption techniques, etc. Finally, we discuss the relation between the local structure of CuOx/10A2B and its catalytic NH3 combustion properties.

particle size were calculated from H2-TPR after H2-reduction and subsequent N2O-reoxidation technique (Bel-cat, Bel Japan, Inc.) according to Sagar et al.10 Before H2-TPR, the catalysts were reduced (bulk: CuOx → metallic Cu) by flowing 5% H2/ Ar at 400 °C for 30 min, subsequently purged with He and cooled to 60 °C, and reoxidized (surface: metallic Cu → Cu2O) by flowing 1% N2O/He at the same temperature for 1 h, and the catalyst cell was purged with Ar and cooled to 50 °C. Finally, H2-TPR of the surface Cu2O was performed, and the H2 consumption was used to calculate the Cu dispersion and particle size. XPS spectra were obtained using a K-Alpha spectrometer and vacuum transfer module (Thermo Scientific) under Al Kα radiation (12 kV). To study the oxidation states of the catalysts under NH3 combustion at different reaction temperature, the spent catalysts were transferred under vacuum to XPS chamber without exposure to air. The C 1s signal at 285.0 eV, derived from adventitious carbon, was used as a reference to correct the effect of surface charge. XAFS for Cu K-edge was obtained on the BL9A station of the Photon Factory (PF), High Energy Accelerator Research Organization (KEK), and the BL01B1 station of SPring-8, Japan Synchrotron Radiation Research Institute (JASRI). The spectra were recorded at ambient temperature in transmission mode using an ionization chamber filled with N2 for the incident beam, another chamber filled with 75% N2 + 25% Ar for the transmitted beam, and a Si (111) double-crystal monochromator. Reference samples (CuO and CuAl2O4) were mixed with boron nitride (BN) powder to achieve an appropriate absorbance at the edge energy, whereas the catalysts were used without mixing with BN. The XAFS data were processed using the IFEFFIT software package (Athena and Artemis). 2.3. Catalytic NH3 Combustion Test. Catalytic NH3 combustion was performed in a flow reactor at atmospheric pressure. Supported CuOx (10−20 mesh, 50 mg) was fixed in a quartz tube (O.D., ϕ6 mm; I.D., ϕ4 mm) with quartz wool at both ends of the catalyst bed. The temperature dependence of the catalytic activity was evaluated by heating the catalyst bed from room temperature to 900 °C at a constant rate of 10 °C· min−1 while a gas mixture containing NH3 (0.6−1.0%), O2 (1.5−3.75%), and He (balance) at 100 cm3·min−1 (W/F = 5.0 × 10−4 g·min·cm−3) was supplied. The dependence of the NH3 combustion activity and product selectivities on the O2-excess ratio was also studied. The O2-excess ratio of NH3 combustion was expressed as λ = (pO2/pNH3)exp/(pO2/pNH3)stoichiom. The concentrations of NH3, N2, N2O·NO, and NO2 gas were analyzed using a nondispersive infrared gas analyzer (NDIR, EIA-51d, Horiba), gas chromatography (GC-8A, Shimadzu),

2. MATERIALS AND METHODS 2.1. Catalyst Preparation. A wide variety of commercially available metal oxides (Table 1) were used as support materials for CuOx. As a support with high performance for NH3 combustion, aluminum oxide borate (Al20B4O36, 10Al2O3· 2B2O3: 10A2B) was prepared by a reverse coprecipitation method.8,9 An aqueous solution containing Al(NO3)3 and H3BO3 (Wako Pure Chemicals) with a molar ratio of Al/B = 10:2 was added dropwise to a 3 mol·L−1 solution of (NH4)2CO3 (Wako Pure Chemicals) under vigorous stirring until the pH reached 7.0. The precipitate thus formed was calcined at 500 °C for 3 h and finally at 1000 °C for 5 h in air. Supported CuOx (6 wt % loading as CuO) was prepared by impregnation of an aqueous solution of Cu(NO3)2, followed by drying and calcination at 600 °C for 3 h in air. To evaluate their thermal stability and catalytic properties, the as-prepared catalysts were thermally aged at 900 °C for 100 h in air. Catalyst preparation methods for the other catalysts are shown in the Supporting Information. 2.2. Characterization. Powder X-ray diffraction (XRD) measurements were performed using monochromated Cu Kα radiation (30 kV, 20 mA, Multiflex, Rigaku). The chemical composition was determined by X-ray fluorescence measurements (XRF, EDXL-300, Rigaku). HAADF-STEM and EDX mapping were performed using a JEM-ARM200CF (Jeol). Brunauer−Emmett−Teller (BET) surface area (SBET) calculations were performed using N2 adsorption isotherms, which were obtained at −196 °C (Belsorp, Bel Japan, Inc.). Temperature-programmed reduction by H2 (H2-TPR) was performed in a flow system (5% H2/Ar) at a constant rate of 10 °C·min−1. The NH3, NO, and/or CO2 adsorbability of the catalysts were also studied through temperature-programmed desorption (TPD) (Bel-cat, Bel Japan, Inc.). Prior to measurement, the catalysts were treated at 500 °C for 1 h under He flow and subsequent cooling at 100 °C for 30 min in 5% NH3/He, 1% NO/He, and/or 1% CO2/He (50 cm3· min−1). After pretreatment, the catalysts were heated to 500 °C under He flow at a constant rate of 10 °C·min−1. The concentrations of the desorbed NH3, NO, and/or CO2 in the effluent gas were analyzed using an online quadrupole mass spectrometer (Belmass, Bel Japan, Inc.). The Cu dispersion and B

DOI: 10.1021/acs.jpcc.6b07157 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C NDIR (VA-3011, Horiba), and chemiluminescence (NOA7000, Shimadzu). The calculation formulas of the concentration ratios are shown in the Supporting Information. In situ Fourier transform (FT)-infrared (IR) spectroscopy was performed by a Nicolet 6700 spectrometer using a temperature-controllable diffuse reflectance reaction cell with a BaF2 window connected to a gas supply system to enable measurements under controlled gas environments at atmospheric pressure. The catalysts were first preheated in situ in flowing He at 400 °C for 30 min prior to each experiment. Thereafter, the temperature of the catalyst was decreased to 300 °C, the catalyst cell was subsequently purged with He and then filled with a gas mixture of 0.3% NH3/He, and the spectra were finally collected, while the catalysts were maintained under a stream of NH3/He.

3. RESULTS AND DISCUSSION 3.1. CuO x Supported on Various Metal Oxide Materials. The XRD patterns (Supporting Information) of as-prepared CuOx supported on 10A2B, AlPO4, SiO2, TiO2, and ZrO2 showed peaks ascribable to CuO, whereas these diffraction peaks could not be observed for the other support materials (MgO, Al2O3, La2O3, and CeO2) probably because of the high dispersion of the Cu species. Table 1 summarizes the catalytic properties of the catalysts. The activity was expressed in terms of the light-off temperature at which 10% conversion of NH3 was reached (T10), and the product selectivities were evaluated at reaction temperature where the NH3 conversion was 90% (T90). The temperature dependence of the product selectivities for NH3 combustion over the catalysts is shown in the Supporting Information. The activity decreased in the following order: La2O3 > CeO2 > Al2O3 > ZrO2 ≈ 10A2B > SiO2 > TiO2 > AlPO4 > MgO. This is almost consistent with the order of reduction temperatures observed in the H2-TPR experiments (Supporting Information), suggesting that the reduction of CuOx is closely associated with the combustion activity. The reduction temperature of CuOx should be influenced by its interaction with the support materials. In addition, the supported CuOx catalysts with higher Cu dispersion, i.e., with smaller particle sizes, tended to exhibit lower reduction temperatures (Table 1). The product selectivity for N2 was also dependent on the support materials. Although the catalysts supported on basic oxides such as La2O3 and CeO2 showed lower T10 values, they yielded a larger amount of unfavorable N2O·NO. In contrast, the N2 selectivity at T90 decreased in the following order: 10A2B ≈ SiO2 > AlPO4 > Al2O3 > CeO2 > ZrO2 > MgO > TiO2 > La2O3. CuOx supported on 10A2B achieved 97% of N2 selectivity. Although CuOx/SiO2 also exhibited high N2 selectivity, it was deactivated by thermal aging at 900 °C in air. 3.2. Local Structures of CuOx/10A2B. Because the CuOx/10A2B catalyst exhibited high NH3 combustion activity and N2 selectivity, its local structure and catalytic NH3 combustion properties were studied in greater detail. 10A2B has potential as a support material, exhibiting high specific surface area (SBET = 78 m2·g−1) and thermal stability (m.p.: 1950 °C); its crystal structure is described as mullite analogues comprising AlO6 octahedra, irregular AlO5 polyhedra, AlO4 tetrahedra, and planar BO3 triangles (Supporting Information).11−15 The catalytic application of this material was first reported for catalytic CH4 combustion over Pd/10A2B,15 and we have studied this for automotive three-way catalysts.8,9 Figure 1 shows the XRD patterns of CuOx/10A2B and CuOx/

Figure 1. XRD patterns of supports, CuOx/10A2B and CuOx/Al2O3, before and after thermal aging at 900 °C for 100 h in air and references (ICSD Collection Code).

Al2O3 (Al2O3: JRC-ALO-8, supplied by Catalysis Society of Japan) as a reference before and after thermal aging at 900 °C for 100 h in air. For as-prepared catalysts, the diffraction peaks of CuOx/10A2B could be assigned to 10A2B and CuO; however, those of CuOx/Al2O3 could be assigned to only γAl2O3. In contrast, after thermal aging, the diffraction peaks of both catalysts could be assigned to the respective catalyst support and CuAl2O4. The Al2O3 phase was partly transformed from γ to α by thermal aging. Raman spectra of the catalysts were also obtained; they showed similar structural assignments (Supporting Information). To study the local structure around Cu of the catalysts before and after thermal aging, Cu K-edge XAFS was measured. Figure 2 shows EXAFS oscillations in k-space and normalized XANES spectra of the catalysts together with two references (CuO and CuAl2O4). In comparison with the two references, the oscillation of as-prepared CuOx/10A2B was similar to that of CuO, and the oscillations of as-prepared CuOx/Al2O3 and both aged catalysts were similar to that of CuAl2O4. This result is consistent with the XRD patterns of the catalysts. For the XANES spectra, as-prepared CuOx/10A2B slightly showed the pre-edge (approximately 8985 eV) assigned to Cu2+ 1s → 4p + ligand and Cu2+ 1s → Cu2+ charge-transfer excitation,16,17 which was observed in the XANES spectrum of CuO. However, because the pre-edge was not observed in the spectra of the other catalysts, it is also suggested that the local structure around Cu of these catalysts can be assigned to CuAl2O4. After thermal aging, CuOx/10A2B was next characterized by using HAADF-STEM and EDX mapping (Figure 3). From the bright-field STEM observation (a), the aged catalyst comprised 10A2B particles with sizes of approximately 50 nm. From the EDX mapping (b), CuOx particles (green in appearance) having sizes larger than 20 nm could be observed. Moreover, it should be noted that an enlarged STEM image (d) of the square region in (a) exhibited the presence of highly dispersed C

DOI: 10.1021/acs.jpcc.6b07157 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

selectivity, the NH3−NO−O2 reaction test (0.8% NH3, 0.2% NO, 1.4% O2, He balance, λ = 2) was performed (Supporting Information). CuOx/10A2B exhibited higher activity for the NH3−NO−O2 reaction and higher selectivity to N2 than CuOx/Al2O3. It is considered that NH3 is consumed not only by O2 but also by NO, which is formed as an intermediate during the light-off of NH3. Because the NH3−NO reaction is particularly promoted by CuOx/10A2B, NH3 combustion with extremely high N2 selectivity is possible over this catalyst. CuOx/10A2B(aged) also exhibited lower N2O selectivity (3%) at T90 compared to CuOx/Al2O3(aged) (8%) (Table 2). According to previous reports for selective catalytic NO reduction by NH3 (NH3−SCR) over the supported Cu species,23−26 the oxidation state of Cu2+ acts as an active site for the NH3−NO reaction. After the NH3−NO reaction, the active Cu2+ is reduced to Cu+, which is not readily reoxidized to Cu2+. Therefore, to study the oxidation states of CuOx for the catalysts under NH3 combustion at each reaction temperature, the spent catalysts were transferred under vacuum to XPS chamber without exposure to air. Figure 5 shows the Cu 2p XPS spectra of CuOx/10A2B(aged) and CuOx/Al2O3(aged) after NH3 combustion (λ = 2) at reaction temperatures of 200, 400, and 600 °C. The intensity of each Cu 2p spectrum was normalized by Al 2p. The deconvolution of the observed peaks revealed two sets of spin−orbital doublets assigned to Cu+ and Cu2+. Two satellite peaks (940.0 and 944.0 eV), attributed to Cu2+ with the 3d9 electronic state, were also found. CuOx/ 10A2B(aged) (Cu+, 34%; Cu2+, 66%) contained more Cu2+ than CuOx/Al2O3(aged) (Cu+, 63%; Cu2+, 37%). Unlike the thermodynamically stable CuO (Cu2+) in atmosphere, Cu+ was observed in both aged catalysts, which is indicative of highly dispersed CuOx nanoparticle (Figure 3 and Supporting Information) having high fraction of surface CuOx species with lower oxygen coordination numbers than bulk CuOx (Cu2+); therefore, such a highly dispersed CuOx nanoparticle contains coordinatively unsaturated (cus) Cu and probably shows lower oxidation state (Cu+). Although the Cu2+ fractions of both catalysts after combustion at 400 and 600 °C decreased slightly probably due to the reduction of CuOx nanoparticles by the NH3 combustion and/or NH3−NO reaction, CuOx/ 10A2B(aged) after NH3 combustion at each reaction temperature also showed a higher fraction of Cu2+ in comparison with CuOx/Al2O3(aged). Al 2p, O 1s, and/or B 1s XPS spectra of these catalysts were also measured; however, no noticeable difference was observed. Similar trends of the abundance of Cu2+ in CuOx/10A2B were also observed for the as-prepared catalysts after NH3 combustion (Supporting Information). Therefore, it is suggested that CuOx/10A2B with an abundance of active Cu2+ exhibited high NH3−NO reactivity and thus low NO selectivity for the NH3 combustion reaction. For N2O selectivity, according to the kinetic model of NH3 oxidation, the elementary reaction NH(imide) + NO → N2O + H shows high sensitivity for N2O production.2,27 In the catalytic NH3 oxidation and NH3−NO reaction, NH produced from the dissociative adsorption of NH3 is also considered as a key species for N2O production.28−34 Liu et al. reported that, in NH3−NO reaction, NH2 (amine) species also produced from the dissociative adsorption of NH3 react with NO to N2, whereas NH species react with NO to N2O.34 Although Darvell et al. suggested that the intermediate HNO species produced from the reaction of NH−O2 react with NO to N2O,30 it is also regarded that NH adsorbed on the catalysts is key species for N2O production. Therefore, to confirm the presence of NH

Figure 2. (top) Cu K-edge EXAFS oscillations and (bottom) normalized XANES spectra of (a) CuOx/10A2B, (b) CuOx/ 10A2B(aged), (c) CuOx/Al2O3, and (d) CuOx/Al2O3(aged) together with two references of CuO and CuAl2O4.

nanoparticles with a slight bright contrast, and the EDX mapping showed the presence of Cu. Similar highly dispersed CuOx nanoparticles could be observed in different fields of view in CuOx/10A2B(aged) and CuOx/Al2O3(aged) (Supporting Information). The dispersion of the observed CuOx nanoparticles is further confirmed by measuring the Cu particle size calculated from H2-TPR (Table 2) and previous reports for CuOx/Al2O3.10,18−22 Jensen et al. reported that Cu nanoparticles on Al2O3(0001) substrate showed high thermal stability because of the formation of Cu−O−Al bonds, which prevent the sintering of Cu nanoparticles. This was revealed by density functional theory calculations and high-resolution dynamic scanning force microscopy observations.21 Therefore, considering the XRD and XAFS results, it is probable that similar Cu−O−Al interactions occurred in these catalysts during the generation of CuAl2O4. Thus, the thermally stable CuOx nanoparticles were highly dispersed onto their supports. 3.3. Catalytic NH3 Combustion Properties. Figure 4 compares the temperature dependence of the product selectivities for CuOx/10A2B and CuOx/Al2O3 before and after thermal aging under O2-excess conditions (λ = 2). For all catalysts, the light-off curves of NH3 were obtained at approximately 300 °C, and the selectivities of NO was observed after the NH3 conversion reached approximately 90%. However, the NO selectivity at T90 for CuOx/10A2B before and after aging was lower than that for CuOx/Al2O3 (Table 1 and Table 2). The same trend of lower N2O and NO selectivity for CuOx/10A2B was observed under O2-excess conditions (λ = 3 to 5) (Supporting Information). To elucidate the lower NO D

DOI: 10.1021/acs.jpcc.6b07157 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. (a) Bright-field STEM image, (b) EDX mapping analysis, and (c,d) HAADF-STEM images of CuOx/10A2B(aged). The blue and green points in (b) correspond to the Al−K and Cu−L fluorescence lines, respectively; (d) was taken from the area shown by a black square in (a).

Table 2. Catalytic Properties of Supports, CuOx/10A2B and CuOx/Al2O3 before and after Thermal Aging at 900 °C for 100 h in Air desorbed gas (μmol·m−2)e

selec. at T90 (%)a catalyst

phase

10A2B CuOx/10A2B CuOx/ 10A2B(aged) Al2O3 CuOx/Al2O3

10A2B CuO/10A2B CuAl2O4/ 10A2B γ-Al2O3 CuAl2O4/γAl2O3 CuAl2O4/α,γAl2O3

CuOx/ Al2O3(aged)

Cu2+/Cu (%)d

NH3

NO

CO2

3.9 9.7

55 66

1.5 2.1 1.3

0.044 0.025 0.015

0.10 0.07 0.09

43

2.4

34

1.1 1.3

0.054 0.050

0.38 0.27

56

1.8

37

0.4

0.063

0.11

T10 (°C)a

T90 (°C)a

N2

N2O

NO

SBET (m2·g−1)

575 307 325

819 474 536

91 97 96