Understanding the catalytic properties for synthesizing NO_x derivative NH₃ by an alternate gas-switching process

Abstract

Reactive nitrogen compounds like NO_x are not environment-friendly but have potential as some of the chemical sources for the low-energy synthesis of NH₃ and value-added chemicals. Our recent initiatives have revealed catalytic conversion to NH₃via a gas-switching NO_x storage and reduction process. This concept was demonstrated absolutely by using nanocomposite catalysts based on mesoporous metal oxides with noble metal catalysts and NO_x storage components like BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂. Here, we investigate and compare the properties of another BaO/Pt@mCeO₂-type nanocomposite catalyst. After stabilization by zirconium doping, BaO/Pt@mCe_0.8Zr_0.2O₂ exhibited acceptable NO_x storage properties at temperatures higher than 250 °C with extra NO_x storage by the CeO₂-based support in addition to the major capacity of BaO. However, Pt nanoparticles were made to be very fine by a strong metal–support interaction to CeO₂ and possibly worked very well, which was not preferable for the formation of NH₃. From this viewpoint, the hydrogenation property was suppressed by lowering the reaction temperature, leading to an improvement in the NH₃ selectivity from less than 10% to more than 70% even in the low NO_x storage state.

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1. Introduction

Various recycling processes of nitrogen compounds, such as nitrogen oxides (NO_x) and nitrate (NO₃⁻), as ammonia (NH₃) have been investigated for overcoming issues with the environment, energy and resources.^1–3 This approach is quite promising for the design of a nitrogen circular economy, where NH₃ is used to obtain value-added organonitrogen compounds like amines.^4–7 Accordingly, the reaction converting harmful NO_x to valuable NH₃ (named as the NTA reaction) is worth investigating through the rational design of heterogeneous catalysts showing high NH₃ selectivity. As one of the interesting approaches, the NO_x storage reduction, NSR, process has been applied by using nanocomposite catalysts, e.g., BaO/Pt/γ-Al₂O₃, for the selective synthesis of NH₃.^8–10 The chemical composition of the catalysts is similar to that developed for the purification to nitrogen (N₂) by NSR.^11–13 In the NTA reaction by modified NSR, the use of hydrogen (H₂) is the standard process for the reduction of NO_x to NH₃.^14,15 From this viewpoint, the chemical composition of the catalysts, such as metal oxide supports and novel metal catalysts, should be investigated further for understanding the catalytic properties. The oxidation of NO, the storage of NO₂, the activation of H₂ and the hydrogenation of the stored NO₃⁻ proceeded sequentially as the major reactions.

Fundamental study progressed through the use of well-designed nanocomposite catalysts (e.g., BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂) for understanding the properties arising from typical components.^16,17 Metal oxide supports such as alumina (Al₂O₃) and titania (TiO₂) were surveyed by combining barium oxide (BaO) and calcium oxide (CaO) as the NO_x storage components and platinum (Pt) as the metal catalyst for activating oxygen (O₂) and H₂. After learning about common NSR catalysts for the purification of NO_x to N₂ in the lean-burn gasoline engine system of automobiles, we proposed a gas-switching NSR process for the synthesis of NH₃ as a game changing technology.^14–17 By modifying the NSR process as a completely different process for the recycling of NO_x, a selectivity towards NH₃ of up to 90% or more was reached in a flow of 5% H₂ by using BaO/Pt@mAl₂O₃ at around 250 °C and CaO/Pt@mAl₂O₃ at around 300 °C.¹⁶ Due to the surface property of the metal oxide support, which supply dissociated hydrogen species, the selectivity of NH₃ was almost maintained in a flow of 1% H₂ by using BaO/Pt@mTiO₂ even at a low temperature (e.g., 175 °C).¹⁷ In this study, for further understanding the significance of the metal oxide support for our gas-switching NSR process for the selective synthesis of NH₃, an additional nanocomposite catalyst (e.g., BaO/Pt@mCeO₂) was prepared in the same way using mesoporous ceria (mCeO₂) and compared to BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂.

2. Experimental

2.1. Chemicals

Cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O), zirconyl nitrate dihydrate (ZrO(NO₃)₂·2H₂O) and citric acid (C(OH)(CH₂COOH)₂COOH) were obtained from FUJIFILM Wako Pure Chemical Co. Hydrogen hexachloroplatinate(IV) hexahydrate (H₂PtCl₆·6H₂O) aqueous solution was purchased from Tanaka Precious Metals. Pluronic P123 (EO₂₀PO₇₀PO₂₀), as one of the commercial poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (EO_nPO_mEO_n) triblock copolymers, was purchased from Sigma-Aldrich. Barium acetate (Ba(CH₃COO)₂) was also obtained from FUJIFILM Wako Pure Chemical Co.

2.2. Synthesis of BaO/Pt@mCe_0.8Zr_0.2O₂

The Pluronic P123-templated mesoporous structure of pure CeO₂ was not stable and was improved by mixing an appropriate amount of zirconia (ZrO₂). Ce(NO₃)₃·6H₂O (4 mmol, 17.4 g), ZrO(NO₃)₂·2H₂O (1 mmol, 2.7 g) and citric acid (10 mmol, 19.2 g) were dissolved in an aqueous solution of Pluronic P123 (5 g in 120 mL H₂O) and stirred for 90 min. After combining with another aqueous solution of H₂PtCl₆·6H₂O (405.5 μL, 200 g L⁻¹ Pt) and stirring for another 30 min, the resultant precursor solution was solidified by using a spray dryer (Yamato Scientific Co., Ltd., ADL311S-A). The inlet temperature and the blower rate were 170 °C and 1.0 m³ min⁻¹, respectively. After calcination at 400 °C (heating rate; 2 °C min⁻¹) for 3 h in a flow of dry air, Pt@mCe_0.8Zr_0.2O₂ (1.01 g) was dispersed in H₂O (20 mL), followed by the dropwise addition of an aqueous solution of Ba(CH₃COO)₂ (5 mL containing 0.1 g of Ba) under stirring for 1 h and recovered by using a rotary evaporator at 60 °C under vacuum. The weight ratio of Ba : Pt : Ce_0.8Zr_0.2O₂ was 10 : 1 : 100. The resultant powder sample was finally calcined at 500 °C (10 °C min⁻¹) for 3 h in a flow of dry air.

2.3. Characterization of BaO/Pt@mCe_0.8Zr_0.2O₂

Low- and wide-angle X-ray diffraction (XRD) patterns were recorded by using a Rigaku RINT 2100 with monochromated Fe Kα radiation and a Rigaku RINT 2000 with monochromated Cu Kα radiation, respectively. N₂ adsorption–desorption isotherms were measured by using an Anton Paar Autosorb-iQ at 77 K after the sample was degassed at 110 °C for 6 h under vacuum. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. The total pore volume was estimated by using the volume of N₂ at P/P₀ = 0.99. The pore size distribution curve was drawn by the Barrett–Joyner–Halenda (BJH) method using the desorption branch. Field-emission scanning electron microscopic (FE-SEM) images were taken by using a HITACHI SU9000, operated at 1.0 kV. Transmission electron microscopy (TEM) images were taken by using an FEI Tecnai Osiris, operated at 200 kV, in high-angle annular dark field scanning TEM (HAADF-STEM) mode to enhance the contrast of Pt nanoparticles (NPs) for obtaining clear elemental mapping by energy dispersive X-ray spectroscopy (EDS).

2.4. Catalytic test using BaO/Pt@mCe_0.8Zr_0.2O₂

According to our previous study,^16,17 the catalytic properties were investigated through our typical gas-switching process between the storage of NO_x and the reduction of stored NO_x (typically NO₃⁻) species. The inlet gas was alternated between 1000 ppm NO with 10% O₂ for 60 min and 1% H₂ for 60 min at temperatures ranging from 300 °C down to 175 °C using a fixed-bed flow reactor. All the flow rates were 100 mL min⁻¹ balanced with N₂. During the gas-switching operation, the reactor was purged with N₂ for 5 min after every process. Initially, BaO/Pt@mCe_0.8Zr_0.2O₂ (approximately 100 mg) was placed on quartz glass wool in a quartz glass tube reactor and pre-treated at 300 °C for 60 min in a flow of 1% H₂. The amount of stored NO_x was evaluated on the basis of total NO_x in the gas phase. The formation of nitrogen compounds (e.g., NO, NO₂, N₂O and NH₃) arising from the hydrogenation of the NO₃⁻ species was monitored using a Fourier-transform infrared (FT-IR) spectrometer (Nicolet iS20, Thermo Fisher Scientific) equipped with a deuterated triglycine sulfate (TGS) detector and a 2.4 m gas cell (Pike Technologies).

Temperature-programmed reduction (TPR) profiles in a flow of H₂ were measured by using a BELCAT II (Microtrac BEL) equipped with a thermal conductivity detector (TCD). A powder sample (approx. 50 mg) was placed inside a triple-walled quartz tube and pre-treated in a flow of O₂ at 300 °C to oxidize adequately, and then cooled down to 40 °C in a flow of He. TPR measurement was conducted at temperatures ranging from 40 °C to 900 °C with a heating rate of 10 °C min⁻¹ in a flow of 4% H₂ (Ar balanced, 30 mL min⁻¹) where the resultant water was trapped during the measurement by using a molecular sieve before introduction into the detector.

3. Results and discussion

High-surface-area metal oxides are very important for designing high-performance heterogeneous catalysts. A huge number of papers have so far demonstrated the synthesis of mesoporous metal oxides by applying a drying process of clear precursor solutions containing soluble metal oxide species and an amphiphilic organic compound. However, it is still not easy to reproduce ordered mesoporous metal oxides in powder forms with the self-assembly of amphiphilic organic molecules. We have therefore developed several aerosol-assisted synthetic methods for obtaining powder samples of mesoporous metal oxides, such as Al₂O₃ and TiO₂.^18–22 However, a wet-impregnation method cannot be applied for preparing Pt-supported catalysts based on EO_nPO_mEO_n (e.g., Pluronic P123)-templated mesoporous metal oxides owing to their lower stability in a highly acidic solution of H₂PtCl₆.^23,24 Accordingly, a series of nanocomposite catalysts have been carefully designed by using such mesoporous metal oxides as the catalyst support, especially those prepared using EO_nPO_mEO_n. Alkali earth metal species (e.g., BaO) were impregnated after the direct introduction of the metal catalysts (e.g., Pt) during the spray-drying process. The growth of Pt nanoparticles (NPs) progressed during calcination to remove organic moieties with the crystallization of the metal oxide frameworks. Here, another BaO/Pt@mCe_0.8Zr_0.2O₂-type nanocomposite catalyst was prepared by the same synthetic process. A Pluronic P123-templated and stabilized mesoporous CeO₂ (mCeO₂)containing Pt species was obtained in one-pot with the preferable addition of ZrO₂ (Pt@mCe_0.8Zr_0.2O₂). Pt NPs were mainly grown during calcination at 400 °C for 3 h in dry air with the crystallization of CeO₂-based frameworks.

3.1. Structural characterization of Pt@mCe_0.8Zr_0.2O₂ and BaO/Pt@mCe_0.8Zr_0.2O₂

Low- and wide-angle XRD patterns, N₂ adsorption–desorption isotherms and SEM images of Pt@mCe_0.8Zr_0.2O₂ and BaO/Pt@mCe_0.8Zr_0.2O₂ are shown in Fig. 1. A single dominant peak was observed at 2θ = 1.4° (d = 7.9 nm) in the low-angle XRD pattern of Pt@mCe_0.8Zr_0.2O₂ (Fig. 1a), indicating the presence of a mesostructural ordering derived from the self-assembly of EO₂₀PO₇₀EO₂₀ molecules. The wide-angle XRD pattern clearly showed several peaks that can be assigned to the cubic structure of Ce_xZr_1−xO₂ (Fig. 1b). The peaks were slightly shifted to higher diffraction angles compared to those observed for pure CeO₂ (Table S1, ESI†). For example, the (111) peak was shifted from 2θ = 28.5° to 28.8° without any peak splitting, suggesting the isomorphous substitution of large Ce⁴⁺ (ionic radius; 0.97 Å) with small Zr⁴⁺ (0.84 Å).²⁵ The N₂ adsorption–desorption isotherm was type IV with a hysteresis loop (Fig. 1c), indicating the presence of mesopores. The BET surface area and the total pore volume were 97 m² g⁻¹ and 0.10 cm³ g⁻¹, respectively. The average pore diameter was calculated to be 3.6 nm by the BJH method. A representative SEM image showed partially crushed spherical particles with mesopores (Fig. 1d).

Fig. 1 (a) and (e) Low- and (b), (f) wide-angle XRD patterns, (c) and (g) N₂ adsorption–desorption isotherms and (d), (h) SEM images of (a)–(d) Pt@mCe_0.8Zr_0.2O₂ calcined at 400 °C for 3 h in dry air and (e)–(h) BaO/Pt@mCe_0.8Zr_0.2O₂ calcined further at 500 °C for 3 h in dry air.

After the impregnation of Ba(CH₃COO)₂ followed by calcination at 500 °C for 3 h in a flow of dry air, a modest peak was still observed at almost the same 2θ position in the low-angle XRD pattern (Fig. 1e). This result revealed that the original mesoporous structure of Pt@mCe_0.8Zr_0.2O₂ was maintained and accommodated the resultant barium species like BaO. As in the case of our previous BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂ catalysts,^16,17,23,24 the formation of crystalline BaO species was not confirmed by XRD, but additional peaks were detected at 2θ = 23.9° and 42.0° due to the presence of witherite-type BaCO₃ (Fig. 1f).²³ This arises from the thermal decomposition of Ba(CH₃COO)₂ to BaO and subsequent reaction with atmospheric CO₂. The N₂ adsorption–desorption isotherm showed a type IV behavior with a hysteresis loop (Fig. 1g). The BET surface area (52 m² g⁻¹) and the total pore volume (0.07 cm³ g⁻¹) decreased with the impregnation of Ba species, which is related to the presence of Ba species inside the mesopores (3.6 nm). A partially crushed spherical porous morphology was also found through SEM observation even after the impregnation of Ba species (Fig. 1h).

Here, spherical mesopores (pore size; 3.6 nm) were designed through the formation of Ce_0.8Zr_0.2O₂-type frameworks with about 1 mass% Pt using a general-purpose spray dryer where the specific surface area and the total pore volume were 97 m² g⁻¹ and 0.10 cm³ g⁻¹, respectively. These values are comparable to those reported in several successful literature studies.^26,27 2D hexagonal mesoporous Ce_0.5Zr_0.5O₂ (specific surface area; 132 m² g⁻¹, pore volume; 0.19 cm³ g⁻¹ and pore size; 3.7 nm) was prepared by drying a precursor solution containing Pluronic P123, followed by calcination at 400 °C (heating rate; 1 °C min⁻¹).²⁶ An intense peak was observed at the d-spacing of 9.0 nm in the low diffraction angle region after calcination at 400 °C, which deformed with the increase in calcination temperature and almost disappeared after calcination at 600 °C. In another case of Ce_0.8Zr_0.2O₂, the intensity of the corresponding XRD peaks was weakened after calcination at 400 °C where the specific surface area, pore volume and pore size were 107 m² g⁻¹, 0.20 cm³ g⁻¹ and 3.7 nm, respectively. Such 2D hexagonal mesoporous Ce_0.5Zr_0.5O₂ (specific surface area; 98 m² g⁻¹, pore volume; 0.08 cm³ g⁻¹ and pore size; 4.0 nm after calcination at 400 °C) was reproduced using Pluronic P123.²⁷ The mesoporous structure can be confirmed by TEM but seemed to deviate (specific surface area; 56 m² g⁻¹, pore volume; 0.01 cm³ g⁻¹ and pore size; 7.8 nm).

XRD peaks due to the formation of Pt NPs were not detected in the wide-angle XRD patterns of Pt@mCe_0.8Zr_0.2O₂ after calcination at 400 °C in a flow of dry air (see Fig. 1b) and Ba/Pt@mCe_0.8Zr_0.2O₂ even after calcination at 500 °C (see Fig. 1f). The presence of metallic Pt species is typically confirmed by the presence of the (111) reflection at around 2θ = 40°. The results possibly suggest that the resultant Pt species are extremely fine, being quite different from those observed in our previous case of BaO/Pt@mTiO₂ calcined by the same process.¹⁷ The presence of Pt NPs was then surveyed by HAADF-STEM combined with EDS analysis. Typical bright-field TEM observations of Pt@mCe_0.8Zr_0.2O₂ and BaO/Pt@mCe_0.8Zr_0.2O₂ were not successful (see Fig. S1, ESI†) because Ce and Zr are both high atomic number atoms. However, HAADF-STEM, detecting scattered electrons, was quite helpful for illustrating the internal structure at an identical acceleration voltage, revealing a partially hollow morphology of the spray-dried spherical particles (Fig. 2a and e). The presence of Pt NPs was not observed directly by HAADF-STEM, but the EDS spectra suggested the presence of reasonable Pt species (see Fig. S2, ∼0.8 mass%, ESI†). The elemental mappings of Ce, Zr, Pt and/or Ba can be illustrated by using the data (Fig. 2b–d and f–i). The resultant size of the Pt NPs is possibly smaller than ∼1 nm, under the detection limit by HAADF-STEM. However, the presence of Pt NPs with an average size of around 1 nm was confirmed after the catalytic test (see Fig. S3, ESI†). In addition, a small amount (0.1–0.3 wt%) of chlorine (Cl) was detected for Pt@Ce_0.8Zr_0.2O₂ and Ba/Pt@Ce_0.8Zr_0.2O₂, which arose from the utilization of H₂PtCl₆·6H₂O as the Pt source.

Fig. 2 (a) and (e) HAADF-STEM images and (b)–(d) and (f)—(i) EDS mappings overlapped with Ce, Zr, Pt and/or Ba of (a)–(d) Pt@mCe_0.8Zr_0.2O₂ and (e)–(i) BaO/Pt@mCe_0.8Zr_0.2O₂.

3.2. Catalytic properties of BaO/Pt@mCe_0.8Zr_0.2O₂ during the gas-switching NSR process

Catalytic reduction of stored NO₃⁻ using BaO/Pt@mCe_0.8Zr_0.2O₂ was investigated by an alternate switching of the inlet gas every 60 min between 1000 ppm NO with 10% O₂ and 1% H₂ at temperatures ranging from 300 °C down to 175 °C. As in the case of our recent study,^16,17 Pt NPs were enveloped inside the resultant particles of mCe_0.8Zr_0.2O₂ through the spray-drying of a precursor solution with Pt species followed by calcination at 400 °C. The BaO species were then impregnated using stepwise design to obtain a BaO/Pt@mCeO₂ type nanocomposite catalyst for the storage of NO_x and the subsequent hydrogenation to NH₃ and/or N₂. The typical raw data that can be collected under our standard reaction conditions are shown in Fig. 3. The formation of NH₃ was favorable at lower temperatures (e.g., 200 °C and lower). Before starting the reaction process, the electric state of CeO₂ would be initially changed in part from Ce⁴⁺ to reduced Ce³⁺ in addition to the reduction of Pt NPs to metallic Pt(0) by pretreatment at 300 °C for 60 min in a flow of H₂. In the gas-switching process, it is rational to consider that Ce³⁺ was oxidized to Ce⁴⁺ during the storage process in a flow of 1000 ppm NO with 10% O₂ and reduced again to Ce³⁺ during the reduction process in 1% H₂.²⁸ This kind of NSR process is mainly divided into 4 catalytic reactions such as (1) the oxidation of NO by Pt NPs, (2) the storage of NO₂ with BaO, (3) the activation of H₂ over Pt NPs and (4) the hydrogenation of stored NO₃⁻. The reaction steps utilizing BaO/Pt@mCe_0.8Zr_0.2O₂ were assessed one-by-one to understand the role of the CeO₂ support and compared to those reported for our BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂-type catalysts.

Fig. 3 Time-course plots of nitrogen compounds (e.g., NO, NO₂, N₂O and NH₃) during the alternate gas-switching operation of the inlet gas between 1000 ppm NO with 10% O₂ and 1% H₂ at (a) 300 °C, (b) 250 °C and (c) 200 °C, and down to (d) 175 °C.

The NO oxidation activity over BaO/Pt@mCe_0.8Zr_0.2O₂ in a flow of 1000 ppm NO with 10% O₂ is summarized in Fig. 4, showing the gradual increase with an increase in the reaction temperature. In a previous study on a similar NSR process using common Pt/BaO/CeO₂ (500 ppm NO with 5% O₂ ↔ 5% H₂), the steady-state conversion of NO to NO₂ was increased up to 300 °C and saturated at the equilibrium (e.g., 400 °C).²⁹ However, the NO oxidation property of BaO/Pt@mCe_0.8Zr_0.2O₂ was not sufficient even at higher temperature (e.g., 250 °C and 300 °C). Additionally, the amount of NO_x, mainly stored as NO₃⁻ with BaO, is quantified in Fig. 5 at each temperature. This reaction step can be progressed even in a continuous flow of NO₂ as the minor component after a little oxidation of NO by O₂. The storage of NO_x reached up to 0.58 mmol g⁻¹ for 60 min after the 3rd cycle at 300 °C, being superior to those observed for our BaO/Pt@mAl₂O₃ (∼0.5 mmol g⁻¹) and BaO/Pt@mTiO₂ (∼0.2 mmol g⁻¹ at 300 °C and 0.26 mmol g⁻¹ at 250 °C at the maximum).^16,17 This is very reasonable because CeO₂ can work as an additional NO_x storage component in addition to BaO.^30,31 The amount of stored NO_x was also abundant even at 250 °C (e.g., 0.43 mmol g⁻¹) but drastically decreased at around 200 °C (0.20 mmol g⁻¹) or lower (0.20 mmol g⁻¹ at 175 °C). This tendency for the storage of NO_x was not noticeable in our cases using BaO/Pt@mAl₂O₃ and BaO/Pt@mTiO₂-type catalysts.

Fig. 4 Temperature-dependence of the NO oxidation property, calculated as the ratio of NO₂/(NO + NO₂) at the end of the 3rd NO_x storage process.

Fig. 5 The amount of stored NO_x in a flow of 1000 ppm NO with 10% O₂ and subsequent formation of NH₃ in a flow of 1% H₂ by using BaO/Pt@mCe_0.8Zr_0.2O₂ at (a) 300 °C, (b) 250 °C, (c) 200 °C and (d) 175 °C.

The amount of resultant NH₃ during the reduction process in a flow of 1% H₂ is also displayed in Fig. 5. The slight formation of N₂O was confirmed at the beginning of the reduction process, being similar to the case using BaO/Pt@mTiO₂.¹⁷ After the activation of H₂ and subsequent hydrogenation of the stored NO_x, the recovery of NH₃ was very low at temperatures higher than 250 °C (∼0.05 mmol g⁻¹), where the formation rate of NH₃ was below 10%. This reaction step was promoted at lower temperatures (0.13 mmol g⁻¹ at 200 °C and 0.12 mmol g⁻¹ at 175 °C) with ∼70% selectivity of NH₃. This behavior is mainly caused by the presence of fine Pt NPs smaller than those grown in mAl₂O₃ (∼21 nm as the average) and mTiO₂ (∼11 nm as the average).^16,17 As observed by HAADF-STEM of BaO/Pt@mCe_0.8Zr_0.2O₂ (see Fig. 2), the hydrogenation of the stored NO_x was accelerated by the presence of extremely fine Pt NPs (smaller than 1 nm). In the case using CeO₂, further oxidation of the resultant NH₃ to N₂ is an extra issue at around 300 °C in the presence of oxidated CeO₂ and thus suppressed below 200 °C.^32,33

The average size of the resultant Pt NPs is changed with the synthetic process, typically the calcination temperature. Metallic Pt species are dispersed over the surfaces of metal oxides depending on the relationship between the strength of interaction and the amount of interactive sites. For example, Pt interacts strongly with unsaturated penta-coordinated Al³⁺ species (Al_penta³⁺) and is then anchored over the γ-Al₂O₃ surface.³⁴ Electronic defects are designed at the surfaces of reducible metal oxides (e.g., CeO₂ and TiO₂), leading to strong metal–support interaction (abbreviated as SMSI).^35,36 From this viewpoint, the resultant size of the Pt NPs in Pt@mCeO₂ (less than 1 nm, calcined at 400 °C) was very reasonable, being smaller than those observed for Pt@mTiO₂ (ave. 11 nm, calcined at 400 °C) and much smaller than those observed for Pt@mAl₂O₃ (ave. 21 nm, calcined at 850 °C). The hydrogenation was completed very quickly in the presence of highly active smaller Pt NPs preferable for the formation of N₂. Accordingly, lower temperature is very suitable for the selective synthesis of NH₃ (∼70% at 200 °C, smaller than the ∼90% observed for BaO/Pt@mTiO₂ at 175 °C).¹⁷

This consideration was supported by TPR measurements in a flow of H₂. As shown in Fig. 6, a major peak was observed for Pt@mCe_0.8Zr_0.2O₂ at around 250 °C (starting at around 200 °C) and shifted to higher temperature in the presence of Ba species (around 300 °C, starting at around 250 °C). This peak is assignable to the reduction of PtO to Pt, which is promoted with the surface reduction of CeO₂.²⁸ This variation is due to the decrease of accessible Pt surfaces with the coverage through the impregnation of the Ba species. Additionally, the reduction temperature (around 250 °C, starting at around 200 °C) is higher than the case using common Pt/Ce_0.25Zr_0.75O₂ that was prepared by impregnating the same 1.0 wt% Pt and comparable to that observed for Ce_0.25Zr_0.75O₂ having very low Pt (0.001 wt%).³⁷ It would be better to consider that some of the Pt NPs smaller than 1 nm are possibly embedded inside the framework of Ce_0.8Zr_0.2O₂ with a decrease in the NO oxidation property. As a specific feature of the CeO₂-based catalyst support, a possible formation route of NH₃ by using BaO/Pt@mCe_0.8Zr_0.2O₂ is illustrated in Fig. 7. During the gas-switching NTA process by the storage of NO_x through the oxidation of NO and subsequent reduction by H₂, lower temperature (e.g., 200 °C and lower) is eventually suitable for the high recovery of NH₃ (improved up to ∼70% NH₃ selectivity).

Fig. 6 TPR profiles of Pt@mCe_0.8Zr_0.2O₂ and BaO/Pt@mCe_0.8Zr_0.2O₂.

Fig. 7 Possible routes for the formation of NH₃ and N₂ through the reduction of stored NO_x by H₂ utilizing BaO/Pt@mCe_0.8Zr_0.2O₂.

4. Conclusions

The general properties of a BaO/Pt@mCeO₂-type nanocomposite catalyst, specifically those of BaO/Pt@mCe_0.8Zr_0.2O₂, were investigated for the synthesis of NH₃ through an alternate switching of the inlet gas between 1000 ppm NO with 10% O₂ and 1% H₂ at temperatures below 300 °C. Considering the major components for the catalytic conversion of NO_x in high-temperature combustion gas, we have designed several nanocomposite catalysts, including BaO/Pt@mCe_0.8Zr_0.2O₂. The resultant size of the Pt NPs was predominantly determined by the synthetic conditions, mainly the calcination temperature. Furthermore, SMSI was very significant for the formation of fine Pt NPs. As a result, the resultant fine Pt NPs (smaller than 1 nm for fresh BaO/Pt@mCe_0.8Zr_0.2O₂), were highly active as a catalyst, but not fully accessible. Accordingly, high temperature is suitable for the oxidation of NO and subsequent storage as NO₃⁻ by extra CeO₂ in addition to BaO. For the synthesis of NH₃ by the hydrogenation of stored NO_x species after the activation of H₂, rapid hydrogenation is not suitable for the formation of NH₃.^16,17 This is the main reason that lower reaction temperature is preferable for the high recovery of NH₃. In addition, due to the specific features of CeO₂, like the oxygen storage properties, the oxidation of NH₃ as well as the consumption of H₂ should be suppressed by lowering the reaction temperature, which allows the oxygen storage properties of the CeO₂-based frameworks to be shown (∼250 °C for BaO/Pt@mCe_0.8Zr_0.2O₂).

Data availability

Date will be made available on request. Extra data supporting this article are included as a part of the ESI.†

Conflicts of interest

The author declares no competing financial interest.

Acknowledgements

This work has been mainly supported as one of the research projects (JPNP18016, commissioned by the New Energy and Industrial Technology Development Organization, NEDO) in the “Moonshot R&D program (Moonshot Goal 4: Realization of sustainable resource circulation to recover the global environment by 2050).”

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj05459h

‡ These authors equally contributed to this research.

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