Supported binary CuO_x–Pt catalysts with high activity and thermal stability for the combustion of NH₃ as a carbon-free energy source

Abstract

Recently, NH₃ has been thought to be a renewable and carbon-free energy source. The use of NH₃ fuel, however, is hindered by its high ignition temperature and N₂O/NO production. To overcome these issues, in this study, the combustion of NH₃ over copper oxide (CuO_x) and platinum (Pt) catalysts supported on aluminium silicates (3Al₂O₃·2SiO₂), aluminium oxides (Al₂O₃), and silicon oxides (SiO₂) were compared. To achieve high catalytic activity for the combustion of NH₃ and high selectivity for N₂ (or low selectively for N₂O/NO), conditions for the preparation of impregnated binary catalysts were optimised. With respect to the binary catalysts, sequentially impregnated CuO_x/Pt/Al₂O₃ exhibited relatively higher activity, N₂ selectivity, and thermal stability. From XRD and XAFS analyses, CuO_x and Pt in CuO_x/Pt/Al₂O₃ were present as CuAl₂O₄ and metallic Pt, respectively. Given that the combustion activity was closely associated with the Pt nanoparticle size, which was estimated from the Scherrer equation and the pulsed CO technique, highly dispersed Pt nanoparticles were crucial for the low-temperature light-off of NH₃. For single and binary catalysts, although NH (imide) deformation modes as a key species for N₂O production were detected by in situ FTIR spectral analysis, the band intensity of CuO_x/Pt/Al₂O₃ was less than those of CuO_x/Al₂O₃ and Pt/Al₂O₃. Therefore, CuO_x/Pt/Al₂O₃ exhibits high selectivity for N₂ in NH₃ combustion.

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Introduction

Recent studies have reportedly considered NH₃ to serve not only as a H₂ carrier material but also as a renewable, carbon-free energy source because of its high energy density (3160 W h L⁻¹) and negligible thermal NO_x emission.¹ Currently, NH₃ is industrially produced at a high pressure using N₂ (produced from air) and H₂ (produced from natural gas) by employing the Haber–Bosch process, which consumes a large amount of energy. However, the energy and costs associated with H₂ production and natural gas is 70–90% of that required for NH₃ production.² Therefore, the low energy and costs that are associated with H₂ production along with the fact that it is a renewable energy source should be emphasised to ensure that the society utilises H₂ along with NH₃. As a proof-of-concept for using NH₃ as a fuel, an NH₃-fuelled micro-gas turbine has been employed to demonstrate the potential of NH₃-fired power plants at the Fukushima Renewable Energy Institute in Japan, as well as an NH₃-fuelled industrial furnace.³ Compared to fossil fuels, however, NH₃ exhibits disadvantages of a high ignition temperature, low combustion rate, and N₂O/NO_x production. Therefore, to overcome these issues, it is imperative to develop a novel NH₃ combustion system. First, a catalytic NH₃ combustion system, which decreases the ignition temperature and N₂O/NO_x emissions, was developed. Previously, our group has successfully developed a catalytic NH₃ combustion system and novel catalysts exhibiting high activity, N₂ selectivity, and thermal stability.⁴ For example, copper oxides (CuO_x) supported on Al₂O₃-based composite oxide materials (such as 10Al₂O₃·2B₂O₃ and 3Al₂O₃·2SiO₂) exhibit high N₂ selectivity and thermal stability, whereas binary CuO_x and silver (Ag) supported on Al₂O₃ exhibit high activity and N₂ selectivity.⁴ However, the former catalyst exhibits a low activity (ignition temperature at ∼300 °C), while the latter catalyst exhibits low thermal stability (deactivation temperature at ∼900 °C likely because of the low melting point of Ag).⁴ Therefore, it is imperative to develop catalysts to exhibit high activities and thermal stabilities for NH₃ combustion.

On the other hand, binary systems comprising supported CuO_x–Ag,⁵ CuO_x–Pt,⁶ CuO_x–Au,⁷ CuO_x–CeO₂⁸ and –Li₂O₃⁸ catalysts exhibiting a high performance for the selective catalytic oxidation of NH₃ (NH₃-SCO) have been previously reported. In particular, CuO_x–Pt systems are expected to exhibit high thermal stability because of the high melting point of Pt (∼1770 °C). Recently, Sun et al. have synthesised impregnated CuO_x–Pt/ZSM-5 catalysts and reported that CuO_x on Pt species can lead to higher NH₃-SCO performance compared to single catalysts.⁶ On the other hand, Jabłońska has synthesised Al₂O₃-, TiO₂-, and ZrO₂-supported CuO_x–Pt, –Pd and –Rh catalysts⁶ and found that the supported CuO_x–Pt catalyst exhibits high activity and N₂ selectivity for NH₃-SCO. However, in these studies, NH₃ is considered to cause air pollution; therefore, the thermal stability is investigated at temperatures of less than 900 °C.

In this study, supported binary CuO_x–Pt catalysts were synthesised, and their catalytic properties for the combustion of NH₃ (as an energy source) at high reaction temperatures were examined. To achieve high catalytic activity for the combustion of NH₃, N₂ (low N₂O/NO) selectivity and thermal stability, preparation conditions for impregnated binary catalysts were optimised. Moreover, the relationship between the local structure and catalytic properties for the combustion of NH₃ over supported CuO_x–Pt catalysts was discussed.

Experimental

Catalyst preparation

3Al₂O₃·2SiO₂ (3A2S) of the catalyst support material was prepared by using an alkoxide method. According to previous studies,^4,9 Si(OC₂H₅)₄ (TEOS, Wako Pure Chemicals) was first dissolved in C₂H₅OH (Wako Pure Chemicals) at a concentration of 1 M at room temperature (RT). Second, after the addition of H₂O and HCl (Wako Pure Chemicals), the solution was stirred at 70 °C for 5 h, and a Si solution was prepared. Third, the alcohol was dehydrated. The H₂O and HCl concentrations were H₂O/TEOS = 2 (mol mol⁻¹) and HCl/TEOS = 0.1 (mol mol⁻¹), respectively. Next, Al[OCH(CH₃)₂]₃ (AIP, Sigma-Aldrich) was subjected to reflux conditions and dissolved in (CH₃)₂CHCH₂OH (Nacalai Tesque) to prepare an Al solution. The precursor solution was prepared by mixing the Al and Si solutions in a volume ratio of 3 : 2 at RT. H₂O was added, and the solution was stirred for 1 h to prepare the precursor solution. The precursor sols were dried at 110 °C for 48 h, affording xerogel precursor powders. These powders were pulverised and calcined at 600 °C for 3 h and finally at 1200 °C for 5 h in air. In addition, α-Al₂O₃ (Wako Pure Chemicals), γ-Al₂O₃ (JRC-ALO-8, Catalysis Society of Japan), SiO₂ (JRC-SIO-10, Catalysis Society of Japan), and CuO (Wako Pure Chemicals) were used as support materials.

Single and binary supported CuO_x (CuO loading of 6 wt%) and/or Pt (Pt loading of 2 wt%) catalysts were prepared by the co-impregnation (CuO_x–Pt) of an aqueous solution of Cu(NO₃)₂ (Wako Pure Chemicals) and [Pt(NH₃)₂(NO₃)₂] (Tanaka Kikinzoku Kogyo), followed by drying and calcination at 600 °C for 3 h in air. In addition, the sequentially impregnated binary catalysts, i.e. Pt and subsequent CuO_x (CuO_x/Pt) and/or CuO_x and subsequent Pt (Pt/CuO_x), were synthesised in a similar fashion. As CuO_x/Pt/Al₂O₃ exhibited a high catalytic activity for the combustion of NH₃, physically mixed catalysts (CuO + Pt/Al₂O_3, CuAl₂O₄ + Pt/Al₂O₃, and CuO_x/Al₂O₃ + Pt/Al₂O₃) with the same composition ratios were also prepared. To evaluate their thermal stabilities and catalytic properties, the as-prepared catalysts were subjected to thermal aging at 900 °C for 100 h in air and/or at 1000 °C for 5 h in air.

Characterisation

Powder X-ray diffraction (XRD) patterns were recorded using monochromatic Cu Kα radiation (30 kV, 20 mA, Multiflex, Rigaku). The average size of the Pt particles was calculated by the XRD line broadening method by using the Scherrer equation.¹⁰ The chemical composition was determined by X-ray fluorescence (XRF; EDXL-300, Rigaku) measurements. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping images were recorded on a JEM-ARM200CF (JEOL) system. Brunauer–Emmett–Teller (BET) surface area (S_BET) values were calculated from the N₂ adsorption isotherms, which were recorded at −196 °C (Belsorp, Bel Japan, Inc.). Pt dispersion and particle size were determined by the pulsed CO technique (BELCAT-B, BEL Japan, Inc.) at 50 °C after O₂ oxidation and subsequent H₂ reduction at 400 °C according to previous studies.¹¹ Pt dispersion and particle size were calculated by assuming hemispherical particles with the stoichiometric adsorption of CO on Pt at a CO : Pt ratio of 1 : 1. Cu K-edge and Pt L₃-edge X-ray absorption fine structure (XAFS) spectral analyses were carried out at BL9A of PF and BL01B1 of SPring-8. XAFS spectra were recorded in the transmission mode at RT using an ionisation chamber filled with N₂ for the incident beam, another chamber filled with 75% N₂/Ar for the Cu K-edge and 50% N₂/Ar for Pt L₃-edge for the transmitted beam, and a Si(111) double-crystal monochromator. Reference samples (e.g. Cu₂O, CuO, CuAl₂O₄, and PtO₂) 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. XAFS data were processed using the IFEFFIT software package (Athena and Artemis).

Catalytic NH₃ combustion tests

The catalytic combustion of NH₃ was performed in a flow reactor at atmospheric pressure. Catalysts (10–20 mesh, <0.3 mm thickness, 50 mg) were fixed in a quartz tube (OD: 6 mm, ID: 4 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 RT to 900 °C at a constant rate of 10 °C min⁻¹ while a gas mixture containing 1.0% NH₃, 1.5% O₂ and He balance at 100 cm³ min⁻¹ (W/F = 5.0 × 10⁻⁴ g min cm⁻³) was supplied. The O₂ excess ratio for NH₃ combustion was expressed as λ = (pO₂/pNH₃)_exp./(pO₂/pNH₃)_stoichiom., where p denotes pressure. The NH₃/N₂O/NO, N₂ and NO₂ gas concentrations were analysed using a nondispersive infrared gas analyser (VA-3011, Horiba), chromatography (GC-8A, Shimadzu) and chemiluminescence (NOA-7000, Shimadzu) measurements. The ESI† provides the calculation formulae used to determine the concentration ratios.

In situ Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 spectrometer using a diffuse-reflectance reaction cell with a BaF₂ window connected to a gas supply and a heating system to enable measurements at atmospheric pressure. First, the catalysts were preheated in situ in flowing He at 400 °C for 30 min prior to each experiment. After pre-treatment, the temperature of the catalyst was decreased to 200 °C, followed by the subsequent purging of the cell with He and then filling with 0.3% NH₃/He mixed gas. Finally, FTIR spectra were recorded while the catalysts were maintained under a stream of NH₃/He.

Computational details

Spin-restricted density functional theory (DFT) computations were carried out for NH₃/Pt_x (x = 13 and/or 20) at the M06 level using the def-SV(P) basis sets, with the 60-electron relativistic effective core potential for Pt, as implemented in TURBOMOLE under the resolution of the identity approximation.¹² The model clusters of Pt₁₃ with I_h symmetry and Pt₂₀ with T_d symmetry having four (111) faces were locally optimised until imaginary frequencies were removed. One atop geometry for NH₃–Pt₁₃ and three initial geometries for NH₃–Pt₂₀ were considered, i.e. atop the (111) face; bridge-to-edge site; and atop the vertex site, respectively. These geometries were optimised until no imaginary frequencies were observed. The adsorption energy of NH₃ on Pt_x was estimated as E_ads = E(NH₃) + E(Pt_x) − E(NH₃–Pt_x), where E(A) denotes the electronic energy of species A.

Results & discussion

Local structures of supported CuO_x and Pt catalysts

Fig. 1 shows the XRD patterns of sequentially impregnated binary (CuO_x/Pt) catalysts, because CuO_x/Pt/Al₂O₃ exhibited higher activity, N₂ (lower N₂O/NO) selectivity, and thermal stability for the combustion of NH₃ compared to all binary catalysts before and after thermal ageing at 900 °C (Table 1). ESI† shows the XRD patterns of the other catalysts. The XRD patterns of as-prepared CuO_x/Pt/3A2S and CuO_x/Pt/SiO₂ before and after aging revealed peaks corresponding to CuO, whereas peaks corresponding to CuAl₂O₄ were observed for CuO_x/Pt/3A2S and CuO_x/Pt/Al₂O₃ (900 °C), possibly related to the solid-state reaction between γ-Al₂O₃ and CuO_x. Diffraction peaks for Cu species were not observed for as-prepared CuO_x/Pt/Al₂O₃, possibly related to the high dispersion of CuO_x. The Al₂O₃ phase for the binary catalysts was transformed from γ to α, and the diffraction peaks for Pt became sharper after thermal aging at temperatures of higher than 900 °C. Therefore, the growth of particles and sintering of Al₂O₃ and Pt are induced by thermal aging. Indeed, the Pt particle sizes calculated by the Scherrer equation increased with the aging temperature (Table 1). On the other hand, as peaks corresponding to CuO were observed for the binary catalysts prepared using α-Al₂O₃ (CuO_x/Pt/α-Al₂O₃) before and after aging, it was possibly difficult to form CuAl₂O₄ from the solid-state reaction between α-Al₂O₃ and CuO_x (ESI†).

Fig. 1 XRD patterns of sequentially impregnated catalysts before and after thermal aging 900 °C for 100 h in air.

Table 1 Catalytic properties of supported catalysts before and after thermal aging at 900 °C for 100 h in air

Catalyst	Phase	T10 ^a/°C	T90 ^a/°C	Selectivity at T90 ^a/%			SBET /m^2 g^−1	Pt particle size/nm		Desorbed gas ^d/μmol m^−2
				N2	N2O	NO		^b XRD	^c Pulsed CO	NH3	NO
a Temperature at which NH3 conversion reached 10% and 90%.b Calculated from XRD line broadening method.c Calculated from pulsed CO chemisorption.d Estimated by NH3- and NO-TPD ranging from 50 °C to 500 °C.e The amount of CO chemisorption was not detected (n. d.).
CuOx/Pt/3A2S	CuO/Pt/3A2S	210	356	90	8	2	32		18
CuOx/Pt/3A2S (900 °C)	CuAl2O4/Pt/3A2S	203	344	89	10	1	27		50
CuOx/Al2O3	CuAl2O4/γ-Al2O3	303	476	92	6	2	149			1.3	0.050
CuOx/Al2O3 (900 °C)	CuAl2O4/α, γ-Al2O3	295	450	91	8	1	102			0.4	0.063
Pt/Al2O3	Pt/γ-Al2O3	188	289	81	19	<1	156	11	11	0.6	0.051
Pt/Al2O3 (900 °C)	Pt/γ, θ-Al2O3	213	266	87	12	<1	99	31	^e n. d.	2.0	0.140
CuOx/Pt/Al2O3	CuAl2O4/Pt/γ-Al2O3	189	278	86	13	<1	139	15	5	1.6	0.064
CuOx/Pt/Al2O3 (900 °C)	CuAl2O4/Pt/α-Al2O3	220	339	86	13	1	14	33	176	1.5	0.444
CuOx/Pt/SiO2	CuO/Pt/SiO2	234	316	93	7	<1	182	13	^e n. d.
CuOx/Pt/SiO2 (900 °C)	CuO/Pt/SiO2	301	456	96	<1	3	81	24	^e n. d.

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Fig. 2 shows the normalised Cu K-edge X-ray absorption near-edge structure (XANES) spectra and k-space extended X-ray absorption fine structure (EXAFS) oscillations of the catalysts and the three reference compounds (Cu₂O, CuO and CuAl₂O₄, respectively). Cu K-edge XANES spectra and k-space EXAFS oscillations of CuO_x/Pt/3A2S (900 °C) and CuO_x/Pt/Al₂O₃ before and after thermal aging at 900 °C were similar to those of CuAl₂O₄, whereas those of CuO_x/Pt/3A2S and CuO_x/Pt/SiO₂ before and after thermal aging at 900 °C were similar to those of CuO. In the XANES spectra of CuO_x/Pt/3A2S and CuO_x/Pt/SiO₂ before and after aging, the pre-edge (at ∼8985 eV, corresponding to Cu²⁺ 1s → 4p + ligand and Cu²⁺ 1s → Cu²⁺ charge-transfer excitation)¹³ for CuO was observed. XAFS analyses were consistent with the XRD patterns of the catalysts. The Pt L₃-edge XAFS profiles of the catalysts and two references (i.e. Pt foil and PtO₂, respectively) are also depicted in Fig. 3. For as-prepared CuO_x/Pt/Al₂O₃, the XANES profile was slightly similar to that observed for PtO₂. According to the calculated thermodynamic Pt–PtO_x phase equilibrium (ESI†), PtO₂ and metallic Pt exhibited thermodynamic stability at RT and 600 °C, respectively. Therefore, Pt nanoparticles (especially, their surfaces) in CuO_x/Pt/Al₂O₃ are thought to be oxidised during the cooling process of calcination. By contrast, the XANES profiles of the other catalysts were similar to that of Pt foil (metallic Pt) as against the calculated phase equilibrium. This disagreement is supposedly caused by the calcination and/or thermal aging-induced sintering of metallic Pt nanoparticles in these catalysts; hence, the bulk metallic Pt particles are stable at RT.

Fig. 2 Cu K-edge (upper) normalised XANES spectra and (lower) EXAFS oscillations of sequentially impregnated catalysts before and after thermal aging and three references (i.e. Cu₂O, CuO, and CuAl₂O₄, respectively).

Fig. 3 Pt L₃-edge (upper) normalised XANES spectra and (lower) EXAFS oscillations of sequentially impregnated catalysts before and after thermal aging and two references (i.e. Pt foil and PtO₂, respectively).

Next, HAADF-STEM and EDS mapping images of sequentially impregnated CuO_x/Pt/Al₂O₃ (900 °C) were recorded (Fig. 4). From the HAADF-STEM image, dispersed nanoparticles with a bright contrast supported on Al₂O₃ particles (blue for Al) were observed, and the EDS mapping images revealed the presence of Pt (red). On the other hand, for the CuO_x species of CuO_x/Pt/Al₂O₃ (900 °C), the EDS mapping images revealed the presence of CuO_x aggregates (green). However, because the overlap of the fluorescence lines of Pt-L and Cu-K, as denoted by the solid arrows in the overlaid image, a physical proximity between Pt and CuO_x nanoparticles in CuO_x/Pt/Al₂O₃ was supposed after aging at 900 °C. On the other hand, Pt particles with sizes larger than 200 nm were clearly observed for CuO_x/Pt/SiO₂ (900 °C) (ESI†).

Fig. 4 HAADF-STEM image and EDS mapping analysis of CuO_x/Pt/Al₂O₃ (900 °C). Red, green, and blue points denote the Pt-L, Cu-K, and Al-K fluorescence lines, respectively.

Combustion properties of supported CuO_x and Pt catalysts

Fig. 5 shows the comparison of the temperature dependence on the selectivities of products obtained over supported CuO_x/Pt, CuO_x/Al₂O₃ and Pt/Al₂O₃ before and after thermal aging at 900 °C under O₂ excess conditions (λ = 2). ESI† shows those of the other catalysts. With respect to the supported CuO_x/Pt catalysts before and after aging, CuO_x/Pt/Al₂O₃ exhibited a comparatively higher catalytic NH₃ combustion activity and N₂ (lower N₂O/NO) selectivity. For CuO_x/Pt/Al₂O₃ and Pt/Al₂O₃ before and after aging, the light-off curves for NH₃ were obtained at ∼200 °C, whereas CuO_x/Al₂O₃ exhibited lower catalytic activity (T₁₀: ∼300 °C). On the other hand, before and after aging, CuO_x/Pt/Al₂O₃ and CuO_x/Al₂O₃ exhibited lower NO selectivity compared to Pt/Al₂O₃ although NO selectivities for all catalysts were observed after the NH₃ conversion reached ∼90%. In contrast, the selectivity of N₂O over the catalysts before and after aging increased in the order of CuO_x/Al₂O₃ < CuO_x/Pt/Al₂O₃ < Pt/Al₂O₃. Finally, CuO_x/Pt/Al₂O₃ exhibited higher N₂ selectivity. Therefore, binary CuO_x/Pt/Al₂O₃ renders synergistic effects of Pt (enhancing the NH₃–O₂ reaction) and CuO_x (increasing N₂ selectivity). To investigate the low NO selectivity of CuO_x/Pt/Al₂O₃ and CuO_x/Al₂O₃, an NH₃–NO–O₂ reaction test (0.8% NH₃, 0.2% NO, 1.4% O₂, He balance) was also performed (shown in ESI†). For the NH₃–NO reaction, CuO_x/Pt/Al₂O₃ also exhibited higher activity compared to CuO_x/Al₂O₃ and Pt/Al₂O₃. NH₃ was supposedly consumed not only by O₂ but also by NO, which was formed as an intermediate during the light-off of NH₃. However, the selectivity of N₂O for the NH₃–NO–O₂ reaction over CuO_x/Pt/Al₂O₃ was higher than that for the combustion of NH₃ (Fig. 5), indicating that the N₂O is produced from the NH₃–NO reaction.

Fig. 5 Product selectivities for the catalytic combustion of NH₃ (NH₃–O₂) over catalysts before and after thermal aging in air at 900 °C for 100 h. Reaction conditions: 1.0% NH₃, 1.5% O₂, λ = 2, He balance, W/F = 5.0 × 10⁻⁴ g min cm⁻³.

For N₂O production, on the other hand, according to the kinetic model for NH₃ oxidation, the elementary reaction of NH (imide) + NO → N₂O + H exhibited high sensitivity for N₂O production.¹⁴ Moreover, for catalytic NH₃ oxidation and NH₃–NO reactions, NH was also regarded as a key species for N₂O production.¹⁵ Therefore, to verify the adsorption of NH on the catalysts, in situ FTIR spectra of NH₃ adsorbed on the catalysts before and after aging were recorded at 200 °C (Fig. 6); this temperature is the approximate initiation temperature for NH₃ combustion (Fig. 5). As has been reported previously,¹⁵ FTIR bands were observed at 1250 and 1625 cm⁻¹, corresponding to the deformation modes of NH₃ adsorbed on Lewis acid sites, for all catalysts. Moreover, for Pt/Al₂O₃ and CuO_x/Pt/Al₂O₃, a set of bands were slightly observed at 1395 and 1695 cm⁻¹, corresponding to the asymmetric and symmetric bending vibrations of NH₃ species on the Brønsted acid sites. Although the single band at 1458 cm⁻¹ corresponding to the NH (considered as a species for N₂O production) deformation modes was observed for all catalysts, the band intensity for CuO_x/Pt/Al₂O₃ was less than that for Pt/Al₂O₃. As CuO_x/Pt/Al₂O₃ prevented the production of NO from the combustion of NH₃ and the dissociative adsorption of NH₃, the catalyst finally achieved high N₂ selectivity.

Fig. 6 In situ FTIR spectra of NH₃ adsorbed on supports and catalysts before and after thermal aging in air at 900 °C for 100 h. The spectra were recorded at 200 °C in gas feeds of 0.3% NH₃ with He balance.

Catalytic properties of supported CuO_x and/or Pt

Table 1 summarises the catalytic properties of supported CuO_x/Pt, CuO_x/Al₂O₃ and Pt/Al₂O₃ before and after thermal aging at 900 °C, and ESI† shows those of the other catalysts. Catalytic activity was expressed in terms of the light-off temperature at which 10% conversion of NH₃ was achieved (T₁₀), and the product selectivities were evaluated at the reaction temperature at which 90% NH₃ conversion was achieved (T₉₀). ESI† shows the temperature dependence of the product selectivities for the combustion of NH₃ over the catalysts. For single catalysts supported on each material (i.e. 3A2S, Al₂O₃ and SiO₂), supported CuO_x exhibited low activity and N₂O/NO selectivities, whereas supported Pt exhibited high activity and N₂O/NO selectivity (ESI†). On the other hand, binary supported catalysts exhibited the same high activity as the single supported Pt, albeit a slightly lower N₂O/NO selectivity. As the combustion activity was closely associated with the Pt particle size, which was calculated by the Scherrer equation (plot shown in the ESI†), highly dispersed Pt nanoparticles supposedly played a key role in the low-temperature light-off of NH₃. In this study, for pulsed CO chemisorption, it is expected that CO is adsorbed not only on Pt but also on CuO_x in case of binary system catalysts.¹⁶ Therefore, the correlation between the NH₃ combustion activity (T₁₀) and the Pt particle size can be estimated using the XRD line broadening method by employing the Scherrer equation.

To elucidate the correlation between the activity and size of Pt nanoparticles, the adsorption energy (E_ads) between NH₃ and Pt₁₃ and/or Pt₂₀ clusters (Pt–N) were estimated by DFT computations (Fig. 7). Compared with the Pt–N E_ads of the vertex for the Pt₂₀ (larger) cluster, that of the vertex for the Pt₁₃ (smaller) cluster was higher, indicating that NH₃ preferentially adsorbs on highly dispersed Pt nanoparticles as well as coordinatively unsaturated (cus) Pt atoms. Indeed, for the Pt₂₀ cluster, the Pt–N E_ads of three initial geometries (see inset in Fig. 7) increased in the order of atop < edge < vertex. Moreover, the bond distance of N–H in NH₃ adsorbed on Pt₁₃ was longer than that on Pt₂₀, suggesting that cus Pt exhibits high activity for the dissociation of the N–H bond in adsorbed NH₃. Compared with the activity of binary CuO_x–Pt catalysts prepared by co-impregnation, those of the sequentially impregnated CuO_x/Pt catalysts were higher probably because Pt nanoparticles can be highly dispersed onto each support material without inhibition from the presence of CuO_x.

Fig. 7 Adsorption energy (E_ads) between NH₃ and Pt₁₃ and/or Pt₂₀ clusters (Pt–N) and bond distances of N–H in NH₃ adsorbed on Pt₁₃ and/or Pt₂₀ obtained by the DFT computations.

Among the CuO_x/Pt catalysts supported on various support materials, CuO_x/Pt/Al₂O₃ exhibited a relatively higher activity, N₂ selectivity, and thermal stability (Table 1). However, a significant decrease in S_BET was observed for CuO_x/Pt/Al₂O₃ (900 °C) (139 → 14 m² g⁻¹) in comparison with that observed for CuO_x/Al₂O₃ (900 °C) and Pt/Al₂O₃ (900 °C). For this reason, first, it is considered that the significant decrease in specific surface area can be attributed to the phase transition of (γ → α) Al₂O₃ as well as thermal aging at 900 °C. However, in case of only γ-Al₂O₃, the phase transformation to α-Al₂O₃ was observed at approximately 1200 °C.¹⁷ Next, however, according to a previous report related to the effects of divalent cation additives on the phase transition of (γ → α) Al₂O₃, the additive Cu²⁺ species that coexist with γ-Al₂O₃ accelerates its phase transition to α-Al₂O₃ at a low temperature of 1200 °C, which can be attributed to the fact that CuAl₂O₄ is formed at a temperature of lower than 1200 °C.¹⁸ Indeed, in this study, α-Al₂O₃ was observed in CuO_x/Al₂O₃ after thermal aging at 900 °C (Table 1). Finally, it is probable that the additive and co-existing Pt also accelerates the formation of CuAl₂O₄ as well as the phase transition to α-Al₂O₃. Although no concrete data are available, a similar significant decrease of the specific surface area could be observed in both Cu–Al–O and Cu–Pt–Al–O system catalysts after thermal aging occurred at 1000 °C.¹⁹

Notably, the high performance for CuO_x/Pt/Al₂O₃ was maintained after thermal aging at 1000 °C despite the phase transition (γ → α) and decrease in the surface area. On the other hand, CuO_x/Pt supported on α-Al₂O₃ exhibited slightly higher N₂O/NO selectivities compared to CuO_x/Pt/Al₂O₃ (using γ-Al₂O₃), the results of which can be explained from the difference in the CuO_x phase of CuO in CuO_x/Pt/α-Al₂O₃ and CuAl₂O₄ in CuO_x/Pt/γ-Al₂O₃. Previously, our group has reported that supported CuAl₂O₄ exhibits a higher NH₃–NO–O₂ reaction activity to N₂ compared to supported CuO because supported CuAl₂O₄ exhibits a higher fraction of the Cu²⁺ active species for the reaction.⁴ Therefore, the trend for N₂O/NO selectivities over CuO_x/Pt/α-Al₂O₃ and CuO_x/Pt/Al₂O₃ can be explained in a similar manner. Moreover, physically mixed catalysts (CuO + Pt/Al₂O₃, CuAl₂O₄ + Pt/Al₂O₃ and CuO_x/Al₂O₃ + Pt/Al₂O₃) exhibited lower N₂ selectivity at high reaction temperatures (ca. > 600 °C) compared to binary catalysts (ESI†), indicating that highly dispersed Pt and proximate CuO_x particles are required to achieve high N₂ selectivity for catalytic NH₃ combustion. On the other hand, Pt/CuO exhibited high N₂ selectivity, albeit low catalytic activity (ESI†), probably because of low Pt dispersion. To examine the acid and base properties of these catalysts, the amount of desorbed gas per surface area of the catalyst was estimated using NH₃- and NO-temperature-programmed desorption (TPD) over a range of 50–500 °C (ESI†). Table 1 summarises this data. For all catalysts after aging, the amount of desorbed NO increased noticeably, likely because of decreased surface area. However, before and after aging, CuO_x/Pt/Al₂O₃ exhibited relatively higher NH₃ and NO adsorption properties compared to CuO/Al₂O₃ and Pt/Al₂O₃. These results indicated that CuO_x/Pt/Al₂O₃ exhibits higher acidity and basicity compared to single supported catalysts; therefore, the reactivity of NH₃–NO to N₂ is enhanced on the CuO_x/Pt/Al₂O₃ surface.

Finally, the effects of CuO, Cu₂O and metallic Cu on the catalytic NH₃ combustion properties are discussed. ESI† denotes the comparison of the temperature dependence on the selectivities of the products that were obtained when compared to those of the commercially available CuO, Cu₂O and metallic Cu. The metallic Cu was prepared from CuO that was reduced at 400 °C for 30 min in 5% H₂/He because CuO can be reduced to metallic Cu at an approximate temperature of 300 °C, as demonstrated from the H₂-TPR measurement.⁴ For observing the NH₃ combustion activity, the light-off curve for CuO was obtained at approximately 300 °C; further, Cu₂O and metallic Cu exhibited lower activity when compared to that exhibited by CuO. Therefore, it can be considered that CuO (Cu²⁺ oxidation state) acts as an active site during the reaction. However, while considering the selectivities of the products, Cu₂O and metallic Cu exhibited low N₂O/NO selectivities because their activities (NH₃ conversions) were observed to be low. In the O₂-excess condition for NH₃ combustion (λ = 2.0), it can be considered that the combustion over CuO_x catalysts proceeds because of redox cycling between CuO and Cu₂O.⁴ Further, the XRD patterns of CuO and Cu₂O before and after the reaction (λ = 2.0) also indicate redox cycling because the patterns of Cu₂O after the reaction revealed diffraction peaks corresponding to those of CuO (ESI†). In addition, based on our previous studies related to operando XAFS, similar redox behaviours could be observed in the supported CuO catalysts (CuO_x/SiO₂).⁴ Therefore, it can be assumed that the effect of metallic Cu on combustion is negligible.

Conclusions

In this study, binary CuO_x and Pt catalysts supported on 3A2S, Al₂O₃ and SiO₂ suppressed the production of N₂O/NO_x via the catalytic combustion of NH₃ as a carbon-free energy source. Among the binary supported CuO_x and Pt catalysts, sequentially impregnated CuO_x/Pt catalysts tended to exhibit higher activity because Pt nanoparticles were highly dispersed onto each support material without inhibition due to the presence of CuO_x. Compared to sequentially impregnated CuO_x/Pt/3A2S and CuO_x/Pt/SiO₂, CuO_x/Pt/Al₂O₃ exhibited high NH₃ combustion activity, N₂ selectivity, and thermal stability. Before and after aging, CuO_x/Pt/Al₂O₃ was characterised as proximate CuAl₂O₄ and dispersed metallic Pt particles. As the combustion activity was closely associated with the Pt particle size, highly dispersed Pt nanoparticles were thought to play a key role in the low-temperature light-off of NH₃. For selectivity, the proximate CuAl₂O₄ in CuO_x/Pt/Al₂O₃ enhanced the NH₃–NO to N₂ reaction, and the catalyst prevented the dissociative adsorption of NH₃ to NH, which was regarded as an intermediate species obtained during the N₂O production. Therefore, CuO_x/Pt/Al₂O₃ exhibits high N₂ selectivity.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by JST, PRESTO (JPMJPR1344) and JSPS (18K14326). This work was partly supported by JST-Mirai Program and the Advanced Characterization Platform of the Nanotechnology Platform Japan sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The XAFS experiments were performed at BL9A with the approval of PF, KEK (Proposal No. 2016G527), and at the BL01B1 of SPring-8 with the approval of JASRI (Proposal No. 2017A1050).

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Footnote

† Electronic supplementary information (ESI) available: Calculation formulae, calculated thermodynamic Pt–PtO_x phase equilibrium, XRD patterns, HAADF-STEM/EDX mapping images, product selectivities for the NH₃–O₂ and NH₃–NO–O₂ reactions, relationship between NH₃ combustion activity and Pt particle size, NH₃- and NO-TPD profiles. See DOI: 10.1039/c8ra07969b

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