Article pubs.acs.org/JPCC
Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides Satoshi Hinokuma,*,†,‡ Yusuke Kawabata,† Shun Matsuki,† 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 local structures and catalytic NH3 combustion properties of copper oxides (CuOx) and silver (Ag) catalysts supported on aluminum oxides (Al2O3) were studied. In order to achieve high catalytic NH3 combustion activity and high N2 (low N2O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In comparison with the single CuOx/Al2O3 and Ag/Al2O3, binary CuOx−Ag supported on Al2O3 showed high performance for catalytic NH3 combustion. Among the binary catalysts, sequentially impregnated CuOx/Ag/Al2O3 exhibited the highest activity and N2 selectivity. Because the combustion activity is closely associated with the Ag−Ag coordination number estimated from Ag K-edge XAFS, highly dispersed Ag nanoparticles supported on Al2O3 are considered to play a key role in the lowtemperature light-off of NH3. CuOx/Ag/Al2O3 also showed higher N2 (lower NO) selectivity for temperatures at which NH3 conversion reached approximately 100%, indicating that the N2 is directly produced from the NH3 combustion reaction over CuOx/Ag/Al2O3. Based on several analyses, a reaction mechanism for catalytic NH3 combustion over CuOx/Ag/Al2O3 was finally suggested.
1. INTRODUCTION
Previously, we demonstrated that the NH3 combustion activity of metal oxides increases with a decrease in their metal−oxygen bond energy5 and that copper oxides (CuOx) supported on aluminum oxide borates exhibit higher N2 selectivity and thermal stability than the other CuOx supported on various materials.6 To achieve high performance for catalytic NH3 oxidation to N2, CuOx-based catalysts have been widely studied, and they were recently reviewed by Jablonska and Palkovits.7 Moreover, Chmielarz and Jablonska also reviewed the effective catalysts for the selective oxidation of NH3 emission on automotive and energy production sectors.8 For the selective catalytic reduction of NO by NH3, novel CuOxbased catalysts were also recently reported.9−12 However, binary systems comprising CuOx and other materials, e.g., CuOx−Ag,13−15 −Pt,16−20 −Au,21 and −CeO2 (−Li2O3),22 supported on Al2O3 catalysts have been previously reported to exhibit high performances for catalytic NH3 oxidation to N2. Among the Al2O3-supprted binary CuOx−metal and/or −oxides systems, CuOx−Ag/Al2O3 exhibits particularly high NH3 oxidation activity and N2 selectivity. Gang et al. first
Recently, NH3 has become regarded as a renewable and carbon-free energy source due to its high energy density (3160 Wh·L−1) and negligible thermal NOx emission.1,2 Furthermore, an NH3-fueled micro gas turbine was demonstrated to show potential as an NH3-fired power plant at Fukushima Renewable Energy Institute in Japan.2 However, in comparison with fossil fuels, NH3 has the following problems: (1) high ignition temperature, (2) low combustion rate, and (3) N2O/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 regarded as a promising technique for decreasing emissions from hydrocarbon-based fuels and was thus actively studied for use in gas turbines, boilers, and jet engines in the 1980s.3,4 This system has many advantages over conventional noncatalytic combustion, as NOx emission is greatly diminished by the 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 that enables low ignition temperatures as well as negligible N2O/NOx emission to realize environmentally friendly catalytic combustors for NH3 fuel. © XXXX American Chemical Society
Received: December 28, 2016 Revised: January 24, 2017
A
DOI: 10.1021/acs.jpcc.6b13024 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
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 and/or 1% NO/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 and/or NO in the effluent gas were analyzed using an online quadrupole mass spectrometer (Belmass, Bel Japan, Inc.). XPS spectra were obtained using a K-Alpha spectrometer and vacuum transfer module (Thermo Scientific) under Al Kα radiation (12 kV). The C 1s signal at 285.0 eV, derived from adventitious carbon, was used as a reference to correct the effect of surface charge. Cu and Ag K-edge XAFS were performed on the BL9A and NW10A 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 Cu K-edge XAFS 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. The Ag K-edge XAFS spectra were recorded in transmission mode using an ionization chamber filled with Ar, another chamber filled with Kr, and a Si(311) monochromator. Reference samples (Cu2O, CuO, CuAl2O4, Ag2O, and AgAlO2) 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 Tests. Catalytic NH3 combustion (NH3−O2 reaction) was performed in a flow reactor at atmospheric pressure. Catalysts (10−20 mesh, 50 mg) were 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 O2-excess ratio of NH3 combustion was expressed as λ = (pO2/ pNH3)exp/(pO2/pNH3)stoichiom. To confirm NH3−NO reactivity, NH3−NO−O2 reaction test (0.8% NH3, 0.2% NO, 1.4% O2, He balance) was also performed. 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), NDIR (VA-3011, Horiba), and chemiluminescence (NOA-7000, Shimadzu). The calculation formulas for the concentration ratios are shown in the Supporting Information. In situ Fourier transform (FT)-infrared (IR) spectroscopy was performed with 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 200 °C, the catalyst cell subsequently purged with He and then filled with a gas mixture of 0.3% NH3/He, and the spectra finally collected while the catalysts maintained under a stream of NH3/He.
reported the optimum Ag/Cu weight ratio (1−3), structures, and NH3 oxidation properties of coimpregnated CuOx−Ag/ Al2O3.13 Additionally, they revealed the roles of Ag (oxidizing NH3 to NO) and CuOx (reducing NO by NH3), the bifunctional mechanism of which was induced by the close physical proximity and/or intimate chemical interaction between CuOx and Ag nanoparticles. A similar coimpregnated CuOx−Ag/Al2O3 was also prepared by Yang et al., who studied the production profiles and adsorption species of the catalyst for NH3 oxidation.14 More recently, Lee et al. also reported that the catalytic NH3 oxidation properties of CuOx−Ag/Al2O3.15 However, these published studies regarded NH3 as air pollution; therefore, their NH3 oxidation tests were evaluated at low reaction temperatures of less than approximately 400 °C. In this study, we focused on Al2O3-supported binary CuOx− Ag catalysts and studied their catalytic NH3 (as an energy source) combustion properties at high reaction temperatures (≤900 °C). To achieve high catalytic NH3 combustion activity and high N2 (low N2O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In addition, because sequentially impregnated CuOx/Ag/Al2O3 exhibited the highest performance for catalytic NH3 combustion among the binary catalysts, its thermal stability and local structures were studied using high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX) mapping, X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and gas adsorption techniques. Finally, we discussed the relationship between the local structures and catalytic NH3 combustion mechanism of CuOx/Ag/Al2O3.
2. MATERIALS AND METHODS 2.1. Catalyst Preparation. γ-Al2O3 (JRC-ALO-8, supplied by Catalysis Society of Japan, SBET = 173 m2·g−1) was used as the support material. CuOx (6 wt % loading as CuO) and Ag (10 wt % loading as Ag) supported on γ-Al2O3 were prepared by coimpregnation of an aqueous solution of Cu(NO3)2 (Wako Pure Chemicals) and AgNO3 (Wako Pure Chemicals), followed by drying and calcination at 600 °C for 3 h in air (CuOx−Ag/Al2O3). The sequentially impregnated binary catalysts (i.e., Ag and subsequent CuOx (CuOx/Ag/Al2O3) and CuOx and subsequent Ag (Ag/CuOx/Al2O3)) were also prepared in a similar fashion. Ag/CuO, impregnated single catalysts (CuOx/Al2O3, Ag/Al2O3), and the physically mixed catalysts (CuO + Ag/Al2O3, CuAl2O4 + Ag/Al2O3, CuOx/ Al2O3 + Ag/Al2O3) with the same composition ratios were also prepared. To evaluate their thermal stability and catalytic properties, the as-prepared catalysts were thermally aged at 900 °C for 100 h in air. Because as-prepared CuOx/Ag/Al2O3 showed high performance for catalytic NH3 combustion, the catalyst was also thermally aged at 700 or 800 °C for 100 h in air to study its thermal stability in detail. 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.). The NH3 and/or NO adsorbability of the catalysts were also studied through temperature-programmed desorption (TPD) (Bel-cat, B
DOI: 10.1021/acs.jpcc.6b13024 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
3. RESULTS AND DISCUSSION 3.1. Local Structures of CuOx/Ag/Al2O3. Figure 1 shows the XRD patterns of the Al2O3-supported single and binary catalysts before and after thermal aging at 700−900 °C for 100 h in air. For all as-prepared catalysts, the diffraction peaks for γAl2O3, Ag, and AgAlO2 can be observed; however, those of the Cu species could not be obtained, probably because of the high dispersion of the CuOx. Similar behavior is confirmed in CuOx/ Ag/Al2O3 aged at 700 °C and/or 800 °C. In contrast, after thermal aging at 900 °C, the diffraction peaks of all the catalysts can be assigned to Al2O3, Ag, and CuAl2O4. Because the Al2O3 phase for the binary catalysts is transformed from γ to α and the diffraction peaks for Ag are sharpened by thermal aging at 900 °C, it is suggested that particle growth and sintering of the respective materials were induced. To study the local structure around the Cu and Ag of the catalysts before and after thermal aging, Cu and Ag K-edge XAFS data were recorded. Figure 2 shows the Cu K-edge EXAFS oscillations in k-space and normalized XANES spectra of the catalysts together with those of three references (Cu2O, CuO, and CuAl2O4). The oscillations of binary catalysts before and after thermal aging are similar to that of CuAl2O4. However, as-prepared CuOx/Ag/Al2O3 shows an oscillation slightly similar to that of CuO (at approximately 8 Å−1), probably because the formation of CuAl2O4 is prevented by the presence of Ag particles supported on the Al2O3. For the XANES spectra, because the pre-edge (at approximately 8985 eV assigned to Cu2+ 1s → 4p + ligand, and Cu2+ 1s → Cu2+ charge-transfer excitation)23,24 for CuO is not observed in the spectra of the other catalysts, it is also suggested that the local structure around Cu of these binary catalysts can be assigned to CuAl2O4. Figure 3 also shows Ag K-edge XAFS profiles of the catalysts together with those of three references (Ag foil, Ag2O, and AgAlO2). The EXAFS oscillations in k-space and normalized XANES spectra of the all catalysts are similar those of Ag foil, and the amplitudes of the oscillations are Figure 2. (a) Cu K-edge EXAFS oscillations and (b) normalized XANES spectra of catalysts before and after thermal aging together with three references (Cu2O, CuO, and CuAl2O4).
increased by thermal aging due to the increase of Ag−Ag coordination number induced by the probable sintering of Ag particles. These XAFS analyses are consistent with the XRD patterns of the catalysts. The catalysts before and after thermal aging at 900 °C were characterized using HAADF-STEM and EDX mapping (Figure 4 and Supporting Information). Figure 4 shows the HAADFSTEM image (a), EDX mapping (b), and enlarged HAADFSTEM image (c) for as-prepared CuOx/Ag/Al2O3. In the HAADF-STEM observation (Figure 4a), highly dispersed nanoparticles with a bright contrast supported on Al2O3 particles with the size of approximately 20 nm can be observed, and the EDX mapping (Figure 4b) shows the presence of Ag (yellow in appearance). The histogram analysis of the Ag nanoparticles suggests a narrow size distribution ranging from 1 nm−6 nm and an average particle size of 3.2 ± 1.0 nm, which is similar to that of the single Ag/Al2O3 (2.9 ± 0.9 nm) (Supporting Information). However, the size distribution and average particle size of Ag broadens and increases to 18 ± 5.7 nm, respectively, upon thermal aging at 900 °C (Supporting Information). For the CuOx species of as-prepared CuOx/Ag/
Figure 1. XRD patterns of supports and catalysts before and after thermal aging. C
DOI: 10.1021/acs.jpcc.6b13024 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
Figure 4. (a) HAADF-STEM image, (b) EDX mapping analysis, and (c) high-magnification HAADF-STEM image of as-prepared CuOx/ Ag/Al2O3. The blue, green, and yellow points in panel b correspond to the Al−K, Cu−L, and Ag−L fluorescence lines.
3.2. Catalytic NH3 Combustion Properties of CuOx/Ag/ Al2O3. Table 1 summarizes the catalytic properties of the catalysts. The activity is expressed in terms of the light-off temperature at which 10% conversion of NH3 is reached (T10), and the product selectivities are evaluated at the reaction temperature where the NH3 conversion is 90% (T90). The asprepared impregnated binary catalysts show similarly high catalytic activities and high N2 selectivities to those of single Ag/Al2O3 and CuOx/Al2O3, respectively, whereas binary catalysts after thermal aging at 900 °C show low surface areas and sintering of Ag nanoparticles and therefore low activities and N2 selectivities. In contrast, the physically mixed catalysts and Ag/CuO also exhibit performances lower than those of asprepared impregnated binary catalysts, probably because of the lower dispersion CuOx and Ag nanoparticles in the proximity, as shown in the Supporting Information. Therefore, to improve the performance (high activity and high N2 selectivity) for catalytic NH3 combustion, it is suggested that highly dispersed as well as proximate CuOx and Ag nanoparticles on Al2O3 are required. Because the combustion activity is closely associated with the Ag−Ag coordination number estimated from Ag Kedge XAFS (plot shown in the Supporting Information), highly dispersed Ag nanoparticles supported on Al2O3 are considered to play a key role in the low-temperature light-off of NH3. The oxidation states of Cu and Ag were estimated using Cu 2p and Ag 3d XPS spectra, respectively (Table 1 and Supporting Information). The intensity of each Cu 2p and Ag 3d spectrum was normalized by Al 2p. All binary catalysts show fractions of Cu2+ and Ag0 slightly lower than those of the single CuOx/Al2O3 and Ag/Al2O3, respectively. Considering the dispersion and proximity for CuOx and Ag nanoparticles obtained in Figure 4, it is probable that the surface of Ag nanoparticles is partly oxidized by the lattice oxygen of proximate CuOx. Moreover, the Ag 3d XPS depth profiles for as-prepared CuOx/Ag/Al2O3 and Ag/Al2O3 show that the fraction of Ag0 decreases with increasing etch step, which implies that the Ag nanoparticles at the Ag−Al2O3 interface form the AgAlO2 observed by XRD (Figure 1). Therefore, it is suggested that highly dispersed Ag nanoparticles are anchored by the formation of AgAlO2 as well as by Ag−O−Al
Figure 3. (a) Ag K-edge EXAFS oscillations and (b) normalized XANES spectra of catalysts before and after thermal aging together with three references (Ag foil, Ag2O, and AgAlO2).
Al2O3, the EDX mapping (Figure 4b) also shows the presence of highly dispersed CuOx (green in appearance) on the Al2O3. As is evident from our previous study,6 a similar dispersion of CuOx is obtained from the HAADF-STEM image and EDX mapping of the single CuOx/Al2O3 (Supporting Information). Moreover, it should be noted that several fluorescence lines of Ag−L and Cu−L overlap on the same place, as shown by the solid arrows in Figure 4a,b. From the side-view HAADF-STEM observation (Figure 4c), the presence of CuOx dispersed around Ag nanoparticles shown by dashed arrows is obtained. Similar dispersions were also slightly found in as-prepared CuOx−Ag/Al2O3 and Ag/CuOx/Al2O3 (Supporting Information). However, it is probably expected that these nanophases may change during NH3 combustion reaction, because Brosius et al. reported that Ag particles in Ag/Al2O3 formed the nitrate species during selective catalytic reduction of NOx with hydrocarbons, which was monitored by in situ UV−vis spectroscopy.25 By contrast, such close physical proximity between CuOx and Ag nanoparticles is not found in CuOx/Ag/ Al2O3(900 °C) and the physically mixed catalysts (CuOx/Al2O3 + Ag/Al2O3) (Supporting Information). D
DOI: 10.1021/acs.jpcc.6b13024 J. Phys. Chem. C XXXX, XXX, XXX−XXX
E
Ag/CuAl2O4/γ-Al2O3 Ag/CuAl2O4/α-Al2O3
536 303 295 199 185 247 268 212 221 234 289 279 253 290 322 206 271 232
γ-Al2O3 CuAl2O4/γ-Al2O3 CuAl2O4/α, γ-Al2O3 Ag/γ-Al2O3 Ag/γ-Al2O3 CuAl2O4−Ag/γ-Al2O3 CuAl2O4−Ag/α-Al2O3 CuAl2O4/Ag/γ-Al2O3 CuAl2O4/Ag/γ-Al2O3 CuAl2O4/Ag/α, γ-Al2O3 CuAl2O4/Ag/α-Al2O3 818 476 450 327 326 426 634 370 346 389 423 576 365 450 525 324 346 354
T90 (°C)a 78 92 91 87 76 97 85 98 95 97 78 96 99 82 96 88 87 88
N2
