Catalytic activity and mechanism of selective catalytic oxidation of ammonia by Ag–CeO₂ under different preparation conditions

Abstract

Given the problem of the high-temperature window of CeO₂ catalyst activity, this study evaluated the catalytic properties of Ag/CeO₂ prepared by changing the preparation methods and loadings. Our experiments showed that Ag/CeO₂-IM catalysts prepared by the equal volume impregnation method could have better activity at lower temperatures. The Ag/CeO₂-IM catalyst achieves 90% NH₃ conversion at 200 °C, and the main reason is that the Ag/CeO₂-IM catalyst has more vital redox properties, and the NH₃ catalytic oxidation temperature is lower. However, its high-temperature N₂ selectivity still needs to be improved and may be related to the less acidic sites on the catalyst surface. On both catalyst surfaces, the i-SCR mechanism governs the NH₃-SCO reaction.

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1. Introduction

In recent years, ammonia has gradually been increasingly considered as a harmful gas. Excessive emission of ammonia will not only cause a series of impacts on the environment and the decline of biodiversity,^1,2 but also cause serious risks to human health. The lowest ammonia concentration that humans can perceive is 5.3 ppm,³ and the increased ammonia emissions will greatly irritate the eyes, respiratory tract, etc.⁴ Ammonia emissions in cities mainly come from diesel vehicle exhaust,⁵ as an unexpected result of introducing technology to reduce NO_x emissions.⁶ However, after implementing China's National VI Standard, ammonia emissions have strict requirements, and the treatment of excess ammonia gas becomes necessary.

Technologies to reduce ammonia emissions include adsorption, absorption, biodegradation, catalytic oxidation, thermal decomposition, direct decomposition, and membrane separation.^7–9 Ammonia selective catalytic oxidation technique (NH₃-SCO) is a highly effective and eco-friendly method of treating waste gases that contain ammonia. NH₃-SCO uses catalysts to oxidize NH₃ to N₂ and H₂O under aerobic conditions selectively. Its mechanism has been extensively studied, mainly including three pathways, namely the imide mechanism (–NH mechanism), hydrazine mechanism (N₂H₄ mechanism), and internal selective catalytic reduction mechanism (an i-SCR mechanism). In the imide mechanism, the adsorbed NH_3(ad) is dehydrogenated twice to form –NH, and –NH reacts with O to form the characteristic intermediate –HNO, after which –HNO combines with –NH to produce N₂ and/or N₂O;^10–12 in the hydrazine mechanism, NH_3(ad) removes H to become –NH₂ and the two combine to form the characteristic intermediate hydrazine (NH₂–NH₂), and then NH₂–NH₂ reacts with O to produce N₂ and H₂O;^10,13 in the i-SCR mechanism, NH_3(ad) first removes H to form –NH_x, which reacts with O₂ to form NO_x, and then is reduced to N₂ and H₂O by –NH_x.^14–16

Research on NH₃–SCO catalysts has focused on catalysts made of molecular sieves, noble metals, and transition metals. The surface of molecular sieve catalysts has acidic sites, and the active centers are equally distributed, but the hydrothermal stability is poor.^17–19 Noble metal catalysts are characterized by Ag, Pt, Au, Pd, and other precious elements as active components, and precious metals are usually loaded on different carriers. Generally, they have excellent low-temperature catalytic activity and can achieve a high NH₃ conversion at 300 °C. But its N₂ selectivity is poor, easy to sinter, and the cost is high.^20–22 Transition metal catalysts mainly refer to transition metals and their oxides, characterized by low price, abundant and easy availability, and have obtained extensive attention in catalytic oxidation. However, their low-temperature catalytic capacity needs to be improved.^23,24 Among them, CeO₂ has apparent advantages in catalytic oxidation due to its unique physicochemical properties, oxygen storage, and release capabilities.

In our previous NH₃-SCO experiments, although the CeO₂ catalyst exhibits excellent redox performance, its activity at cold temperatures needs further improvement. Therefore, we use the combination of Ag and CeO₂ to prepare noble metal-supported catalysts to attain high NH₃ conversion at cold temperatures and evaluate the performance of Ag/CeO₂ catalysts with various preparation methods and loadings. To explore the differences in NH₃-SCO properties of Ag/CeO₂ catalysts prepared in multiple ways, we adopted XRD, XPS, H₂-TPR, and NH₃-TPD characterization methods and in situ DRIFTS characterization was adopted to deduce the mechanism of the response.

2. Experimental

2.1 Catalyst synthesis

0.5 g of Ce (NO₃)₃·6H₂O (Tianjin Comio Chemical Co., Ltd) and 0.2 g of PVP (Tianjin Hienceoptech Co., Ltd) were dissolved in 15 mL ethylene glycol, and then the above solution was slowly added to 15 mL deionized water. After continuous stirring for 30 min, the clarified liquor was transferred to a 50 mL PTFE-lined autoclave and kept at 160 °C for 8 h. After the autoclave was cooled at indoor temperature, the samples were gathered by centrifugal washing. Then, the samples were dried overnight at 110 °C in an oven. Finally, the specimens were heat treated at 500 °C for 3 h in a muffle furnace to obtain a CeO₂ carrier.

2.1.1 Constant volume impregnation method. 0.0786 g of silver nitrate (Beijing Inokai Technology Co., Ltd) was weighed using an electronic balance, dissolved in deionized water, and prepared into 1 mg mL⁻¹ of silver nitrate solution. After that, a certain amount of carrier CeO₂ was weighed into the beaker, and a particular volume of silver nitrate solution was added to CeO₂ drop by drop. The mixture was continuously stirred while dropping. After dropping, the mixture was mixed evenly for 30 min. Then, the specimens were kept in an air-blast drying oven and dried at 120 °C for 12 h. Eventually, the dried sample was burned in a muffle furnace at 500 °C for 3 h to obtain the catalyst, x Ag/CeO₂-IM-T, where x represents the mass ratio of Ag to CeO₂, and T stands for different calcination temperatures (°C).

2.1.2 Deposition–precipitation method. An appropriate amount of CeO₂ was weighed using an electronic balance and then dissolved in deionized water. After stirring for 15 min, ammonium carbonate solution was added for proper alkalization. A certain amount of silver nitrate was dropped into the CeO₂ solution and stirred continuously. After the dripping, the solution was allowed to stand for 30 min. The combination was aged indoors for 1 h and then washed by centrifugation with deionized water. Consequently, the material was dried at 120 °C for an entire night. Finally, the sample was placed in a quartz boat, burned in a hydrogen tube furnace, and reduced by passing a mixture of 200 mL min⁻¹ (90% N₂ and 10% H₂) at 400 °C for 2 h to obtain x Ag/CeO₂-DP catalyst, where x represents the mass ratio of Ag to CeO₂.

2.2 Catalyst characterization

X-ray powder diffraction (XRD) characterization test was performed in a d8-Focus Type X-ray diffractometer produced by Germany Brock AXS Co., Ltd, using Cu target Kα ray (λ = 0.1540598 nm), and Jade 6.5 was used for XRD data analysis.

X-ray photoelectron spectroscopy (XPS) uses ESCALAB 250Xi X-ray photoelectron spectroscopy (USA), with a limit energy resolution of 0.43 eV, a ray source of Al Kα (1486.6 eV, power 150 W, 500 μm beam spots), and the resulting XPS spectrogram uses C 1s (284.8 eV) for charge correction for standard peaks and peak fitting using XPS Peak software.

NH₃-temperature-programmed desorption (NH₃-TPD) uses AutoChem II 2920 and Chembet TPDTPR-type chemosorbents. 100 mg of the sample was put in an N₂ atmosphere at a 10 °C min⁻¹ heating rate to 300 °C, thermostatic pretreatment was 1 h, and was cooled to 50 °C after completion. Then, 500 ppm NH₃ is passed for 1 h until adsorption saturation. After that, the flow of NH₃ is stopped and purged with N₂ for 30 min. After the signal is stabilized, it is heated from 50 °C to 900 °C at 10 °C min⁻¹ in an N₂ atmosphere while the NH₃ concentration is continuously recorded and output with a TCD signal.

H₂-temperature-programmed reduction (H₂-TPR) is performed on AutoChem II 2920 and Chemobelt TPDTPR-type chemosorption instruments. 100 mg of sample is placed in an N₂ atmosphere at a 10 °C min⁻¹ heating rate to 300 °C and pretreated at a constant temperature for 1 h. The samples were cooled to 50 °C after pretreatment. At the end of the cooling, a 10% H₂/Ar gas mixture is introduced, and the temperature rises to 900 °C at a rate of 10 °C min⁻¹. During this process, the H₂ consumption is recorded by the TCD detector.

In situ diffuse filter infrared spectroscopy (in situ DRIFTS) was performed using the German Bruker company Tensor II Fourier transform infrared spectrometer, supporting the MCT (Mercury Cadmium Telluride) detector, and the accessory is connected to the in situ diffuse reflection reaction cell. The adsorption, transient, and steady-state reaction experiments of the catalyst surface reaction species involved in this study were performed in this instrument, with a scanning range of 400–4000 cm⁻¹, a diaphragm of 6 mm, a resolution of 4 cm⁻¹, and several scans (64 times).

2.3 Activity testing

The NH₃-SCO activity analyses in this study were operated in the laboratory-made fixed-bed quartz reaction device for all catalysts. The composition and concentration of the inlet reaction gas were as follows: [O₂] = 10 vol, [NH₃] = 500 ppm, N₂ was the equilibrium gas, and 300 mL min⁻¹ was the overall gas flow rate. The reaction temperature range of this study was 90–400 °C, and every temperature was constant for 30 min, controlled by a tubular reactor thermostat and a thermocouple. The exhaust gas detection system uses the German Bruker Tensor II infrared spectrometer to detect the concentration of NH₃, N₂O, NO₂, and NO in import and export and uses computer supporting software for data analysis.

The evaluation indexes of NH₃-SCO catalyst performance are NH₃ conversion and N₂ selectivity, which are determined by applying eqn (1) and (2), respectively:

3. Findings and analysis

3.1 Catalytic effectiveness

Different preparation methods have a huge effect on NH₃-SCO activity, so we adopted the constant volume impregnation method and deposition–precipitation method to prepare Ag-based catalysts, and the NH₃-SCO activities for catalysts with different methods are shown in Fig. 1. Fig. 1a shows the NH₃ conversion results, and the T₅₀ of NH₃ conversion is 250 °C on the Ag/CeO₂-DP catalyst, which is 50 °C lower than the CeO₂ support. Still, the T₉₀ of NH₃ conversion is above 400 °C on the Ag/CeO₂-DP catalyst, higher than the CeO₂ support (T₉₀ = 400 °C). The NH₃ conversion of Ag/CeO₂-IM catalyst reaches 90% at 200 °C, and the T₉₀ is lower at 200 °C than the Ag/CeO₂-DP catalyst and has a more excellent ammonia oxidation activity. The results of N₂ selectivity are shown in Fig. 1b. The N₂ selectivity of the Ag/CeO₂-IM catalyst begins to decrease at 125 °C, and the N₂ selectivity is 70% at 200 °C, which then continues to decline with the increase in temperature. The Ag/CeO₂-DP catalyst's N₂ selectivity is marginally higher than that of the Ag/CeO₂-IM catalyst, and the N₂ selectivity starts to decrease at 150 °C, still higher than 90% at 200 °C, but the N₂ selectivity is less than 60% at 400 °C. The graphic shows that the CeO₂ has better N₂ selectivity than Ag/CeO₂-DP catalyst and Ag/CeO₂-IM catalyst from 100 °C to 300 °C, but the N₂ selectivity declined sharply from 300 °C to 30% at 400 °C. We have compared the catalytic performance of this work with other catalysts in recent years listed in Table 1, and it can be found that the Ag/CeO₂-IM catalyst in this study exhibits a high rate of ammonia oxidation over a wide temperature window. While other catalysts with good low-temperature activity have a narrow temperature window and catalysts with good high-temperature ammonia oxidation performance are less active in the low-temperature region, the Ag/CeO₂-IM catalyst maintains a more balanced ammonia oxidation performance with an ammonia conversion rate greater than 90% within a wider temperature window from low-temperature 200 °C to high-temperature 400 °C. Overall, catalysts prepared by equal volume impregnation with better NH₃ conversion were preferred for further exploration.

Fig. 1 (a) NH₃ conversion (b) N₂ selectivity of CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM catalysts (inlet reaction gas composition: [NH₃] = 500 ppm, [O₂] = 10 vol%, N₂ is equilibrium gas, GHSV = 60 000 h⁻¹).

Table 1 Literature review of catalytic performance results related to Ag or Ce based catalysts

Catalysis	Preparation method	Reaction conditions	Temperature window/°C	NH3 conversion/%	Ref.
Ag/CeO2-IM	Impregnation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 60 000 h^−1	200–400	>90	This work
Ag/CeO2-DP	Deposition–precipitation		400	87
1.75Ag/CeSnOx	Impregnation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 100 000 h^−1	250–400	100	14
Ag/Al2O3	Rotary evaporator	[NH3] = 1000 ppm, [O2] = 10%, GHSV = 35 000 h^−1	130–180	100	25
Ag/ZSM-5			110–140	100
Ag/meso-TiO2	Wet impregnation	[NH3] = 5000 ppm, [O2] = 2.5%	350–400	100	26
Ag/Al2O3–H2	Impregnation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 28 000 h^−1	120–180	100	27
Ag/nano-Al2O3	Impregnation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 28 000 h^−1	100–180	100	28
Ag/SiO2–TiO2	Impregnation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 28 000 h^−1	140–240	100	29
CuO/CeO2-NR	Wetness impregnation	[NH3] = 1000 ppm, [O2] = 10%, GHSV = 40 000 h^−1	240–250	100	30
CeSnOx	Coprecipitation	[NH3] = 500 ppm, [O2] = 10%, GHSV = 100 000 h^−1	300–400	90	31

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Fig. 2 depicts the impact of various Ag loading amounts on the catalytic activity of Ag/CeO₂-IM. As shown in Fig. 2a, with an increase in the amount of Ag loading from 0.2 wt% to 1 wt%, T₉₀ of NH₃ conversion increased from 400 °C to 200 °C on the Ag/CeO₂-IM catalysts. However, with the addition of Ag loading to 2 wt%, NH₃ conversion only increased a little at the low temperature. This is because the catalytic activity rises as the number of available surface-active sites increases.³²{Azadi, 2012 #48} The increased Ag loading dispersed more Ag particles on the CeO₂ carrier. However, with an increase in the amount of Ag packing from 1 wt% to 2 wt%, the dispersion of Ag nanoparticles formed may not be further changed. Therefore, the number of exposed active sites no longer increases, so the NH₃ conversion has not increased much. Fig. 2b shows the influence of Ag loading on the N₂ selectivity of the catalyst. As Ag has a more vital oxidation capacity,³³ NH₃ is prone to excessive oxidation at high temperatures. Therefore, appropriate Ag loading can significantly improve NH₃ conversion and maintain good N₂ selectivity. When the loading reaches 1 wt%, the NH₃ conversion reaches 90% at 200 °C, and the N₂ selectivity of the catalyst remains above 50% at 125–400 °C, which is the optimum loading capacity for the Ag/CeO₂-IM catalyst.

Fig. 2 (a) NH₃ conversion (b) N₂ selectivity of Ag/CeO₂-IM catalyst with different Ag loading amounts (inlet reaction gas composition: [NH₃] = 500 ppm, [O₂] = 10 vol%, N₂ is equilibrium gas, GHSV = 60 000 h⁻¹).

3.2 Catalyst phase analysis

The structure of the catalyst can be characterized by XRD. Fig. 3 displays the XRD results of Ag/CeO₂ catalysts formed by various preparation methods. It can be observed that the CeO₂ crystallinity increases after Ag loading, showing a typical CeO₂ cubic fluorite structure diffraction pattern. It also indicates that the Ag impregnation load will not significantly change the crystal structure of CeO₂ in the catalyst but also enhance the structural stability of CeO₂. Notably, a slight shift in the diffraction angle towards the higher angle was observed, indicating that Ag⁺ had merged into the CeO₂ lattice and changed the CeO₂ unit cell parameters.³⁴ In addition, the diffraction peak of Ag species was not observed, indicating that the Ag species were spread uniformly across the catalyst surface or did not reach the XRD detection limit.³⁵

Fig. 3 Results of XRD tests on the catalysts CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM.

3.3 Surface element valence and oxygen content

The XPS results for determining the state of valence and oxygen concentration of the components on the catalyst's exterior are displayed in Fig. 4 and Table 2. Fig. 4a displays the XPS Ag 3d spectra of CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM, where the Ag 3d_5/2 core energy level binding energy appears at 367.9 and 368.4 eV. According to reports in the literature, 367.9 eV corresponds to Ag oxide,³⁶ and 368.4 eV correlates to the metal Ag.³⁷ The peak areas in the graph indicate that Ag/CeO₂-IM catalysts have two types of Ag species on the surface: metallic Ag and Ag₂O. From the deconvolution results of the peaks in Table 2, we can see that Ag/CeO₂-IM has 56.02% of Ag⁰, which has more Ag⁰ acting as the active site for ammonia oxidation than Ag/CeO₂-DP, so Ag/CeO₂-IM has superior low-temperature ammonia oxidation performance. Fig. 4b exhibits the XPS Ce 3d spectra of Ag/CeO₂-DP, Ag/CeO₂-IM, and CeO₂ catalysts. The markers u, u′′, and u′′′ correspond to Ce⁴⁺ of Ce 3d_3/2, v, v′′ and v′′′ correspond to Ce⁴⁺ of Ce 3d_5/2, while u′ and v′ correspond to Ce³⁺ of Ce 3d_3/2 and Ce 3d_5/2, respectively.^38,39 The percentage Ce³⁺ content is shown in Table 2, with 18.30% Ce³⁺ on CeO₂, 13.96% Ce³⁺ in catalyst Ag/CeO₂-DP, and the least Ce³⁺ content of 10.8% in catalyst Ag/CeO₂-IM. This is due to the redox interaction between surface Ce³⁺ and Ag⁺, which in turn produces the Ag⁰–CeO₂ interface.⁴⁰ The Ce³⁺ sites are involved in Ag⁺ reduction to form more Ag⁰, which also corresponds to the metal Ag content outside the catalyst.

Fig. 4 Results of XPS tests on the catalysts CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM.

Table 2 XPS results

Catalyst	Actual Ag content (wt%)	Atomic ratio (%)
		Ag^0/(Ag^0 + Ag^+)	Ce^3+/(Ce^4+ + Ce^3+)	Oads/(Oads + Olatt)
CeO2	—	—	18.30	26.64
Ag/CeO2-DP	1.19	49.83	13.96	38.15
Ag/CeO2-IM	1.27	56.02	10.80	33.95

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Fig. 4c exhibits the XPS O1s spectra of CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM. The O1s spectra may be fitted to two peaks. The binding energy is about 531 eV for surface adsorption oxygen O_ads, and the surface lattice oxygen O_latt is at about 530 eV.^41–43 As seen in Table 2, the catalysts obtained by both methods have higher surface adsorption of oxygen. This indicates that species of Ag present affect the ability to activate oxygen by interacting with Ce. Similar phenomena are also found in the literature.⁴⁴ The Ag/CeO₂-IM catalyst's surface had somewhat less adsorption oxygen O_ads than the Ag/CeO₂-DP catalyst due to the formation of the Ag⁰–CeO₂ interface resulting in a slight increase in surface lattice oxygen.⁴⁰

3.4 Surface acidity

Fig. 5 depicts the results characterizing the quantity and potency of acid sites across the catalyst's exterior using NH₃-TPD. On the surface of the trigger, three different types of desorption peaks can be found: NH₃ physically adsorbed or species adsorbed on weak B-acid site is in charge of the temperature below 150 °C, species adsorbed on robust B-acid site is in charge of the peak between 150 °C and 250 °C, and species adsorbed on Lewis acid site is responsible for the peak beyond 250 °C.^38,45 Fig. 5 demonstrates that the total number of acid sites is CeO₂ > Ag/CeO₂-DP > Ag/CeO₂-IM in sequence. The Ag/CeO₂-DP catalyst retains a narrow peak below 150 °C, but the Ag/CeO₂-IM catalyst only has a weak peak attributed to the L-acid site. The reduction of stimulus NH₃ consumption after Ag support is mainly related to the drop in acid content brought on by the presence of surface silver species,²⁶ and Ag/CeO₂-IM has a very weak acidity due to the formation of the Ag⁰–CeO₂ interface covering more acidic sites. In addition, it has been reported that the N₂ selectivity of the ammonia oxidation catalyst will be impacted by its surface acidity,^46,47 which explains why the trigger with the lowest N₂ selectivity is the Ag/CeO₂-IM.

Fig. 5 Test results for the NH₃-TPD catalyst with CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM.

3.5 Redox performance

Fig. 6 displays the data of the H₂-TPR analysis of the catalyst's redox capability. There are three reduction crests in the H₂-TPR spectra of Ag/CeO₂-IM, Ag/CeO₂-DP, and CeO₂ catalysts. The reduction of Ce⁴⁺ to Ce³⁺ in bulk phase CeO₂ causes the peak above 750 °C. In contrast, the removal of CeO₂ on the exterior causes the peak between 350 and 650 °C,^48–50 and the graph shows that the catalyst's reducing peak at temperatures over 750 °C after supporting Ag is more prominent than CeO₂'s, and the peak between 350–650 °C disappears. The reduction of silver oxide species or the removal of oxygen on the surface of the Ag⁰–CeO₂ interface is responsible for the appearance of new prominent peaks at temperatures below 300 °C.^51,52 Moreover, Ag/CeO₂-IM catalyst has a lower reduction peak temperature, which is in line with the findings of the activity test and shows Ag/CeO₂-IM catalyst has a more extraordinary redox ability.

Fig. 6 Test results for the H₂-TPR catalyst with CeO₂, Ag/CeO₂-DP, and Ag/CeO₂-IM.

3.6 Surface reaction mechanism exploration

The DRIFTS findings of NH₃ + O₂ co-adsorption at various temperatures were recorded to understand the surface response of NH₃-SCO on multiple catalysts, as shown in Fig. 7. The infrared spectra of NH₃-SCO reactions on the Ag/CeO₂-IM catalyst are shown in Fig. 7a, where 965 cm⁻¹ correspond to physically adsorbed NH₃,⁵³ 1430 cm⁻¹ can be attributed to the species adsorbed at the Brønsted acid site,⁴⁷ 1320, 1591, and 1293 cm⁻¹ infrared peaks can be credited with NH₂ (ref. 47 and 54) and NH species,⁴⁶ 1030 cm⁻¹ corresponds to bidentate nitrate,⁵⁵ and 1222 and 1246 cm⁻¹ correspond to bridge nitrate species.⁴⁵ N₂O₂²⁻ species, the intermediate that gives rise to N₂O, is responsible for the peak at 1010 cm⁻¹.^56,57 It can be seen from the figure that NH₂ and NH species exist at 90 °C, and their peak strength is reduced to 150 °C and disappears, which indicates that some NH₃ species adsorbed at low temperatures can be dehydrogenated on the surface. species (1430 cm⁻¹) and NH₂ species (1591 and 1320 cm⁻¹) adsorbed at the Brønsted acid site appeared at 110 °C, and their peak strength first increased and disappeared after 200 °C, indicating that the Brønsted acid site was involved in the ammonia oxidation reaction. After 200 °C, bidentate nitrate (1030 cm⁻¹), bridge nitrate (1222 and 1246 cm⁻¹), and intermediates of N₂O (1010 cm⁻¹) appeared, and with rising temperature, their peak strength increased. The above detection results show that during the ammonia oxidation reaction of the Ag/CeO₂-IM catalyst, the catalyst surface-adsorbed NH₃ species underwent an ammonia oxidation process that was dehydrogenated to produce NH₂ and NH. Some NH_x species were over-oxidized to form NO_x. Subsequently, nitrate adsorbed on the catalyst's exterior reacts with the NH_x species to produce N₂ and a byproduct called N₂O; that is, the ammonia oxidation reaction of the Ag/CeO₂-IM catalyst followed the i-SCR mechanism.

Fig. 7 Ag/CeO₂-IM (a) and Ag/CeO₂-DP (b) catalysts acting as NH₃-SCO catalysts in situ DRIFTS reactions at various temperatures.

The infrared spectra of the NH₃-SCO reaction on the Ag/CeO₂-DP catalyst are shown in Fig. 7b. Brønsted acid sites' adsorbed species are responsible for the peaks at 1398 and 1467 cm⁻¹,⁴⁷ species adsorbed at Lewis acid sites are accountable for the peaks at 1145 and 1052 cm⁻¹⁴⁵, NH₂ and NH species are responsible for the peaks at 1355 and 1292 cm⁻¹,⁴⁷ and 1030, 1523, and 1554 cm⁻¹ correspond to bidentate nitrates,^55,58,59 and 1218 cm⁻¹ is classified as bridge nitrate.⁴⁵ The Ag/CeO₂-DP catalyst also follows the i-SCR reaction pathway, as shown in Fig. 7b. The double-toothed nitrate and bridge nitrate species were seen outside the Ag/CeO₂-DP catalyst at 300 °C. In contrast, these nitrate species were seen on the exterior of the Ag/CeO₂-IM catalyst at 200 °C, indicating that the Ag/CeO₂-IM catalyst had a lower NH₃ ignition temperature and better NH₃ oxidation capacity, which agreed with the outcomes of the two catalytic activity tests as well.

3.7 Hydrothermal stability

In practical applications, the diesel engine will undergo a stop-cooling-restart process, which will impact the catalyst's action, so we conducted a cyclic stability test on the motivation and simulated the actual situation to investigate the tolerance of the stimulus. Fig. 8 displays the Ag/CeO₂-IM catalyst's stability test results over numerous cycles. During the 5 processes of the cycling test, the NH₃ conversion results of the Ag/CeO₂-IM catalyst remained constant, T₉₀ was kept at 200 °C, and the N₂ selectivity at 400 °C remained relatively stable at about 30%, which indicates that the Ag/CeO₂-IM catalyst has good cycling stability and no major changes in the active sites during the cycling process.

Fig. 8 Comparison of T₅₀, T₉₀, and N₂ selectivity at 400 °C for 5 cycles of stability testing of Ag/CeO₂-IM catalysts (inlet reaction gas composition: [NH₃] = 500 ppm, [O₂] = 10 vol%, N₂ is equilibrium gas, GHSV = 100 000 h⁻¹).

In addition, diesel engines in practical applications usually have exhaust temperatures higher than 600 °C due to the regeneration of diesel particulate filters and contain about 10% H₂O.^60,61 Therefore, catalysts for diesel vehicles require high hydrothermal durability. Fig. 9 displays the Ag/CeO₂-IM catalyst's activity test results before and after hydrothermal ageing at 700 °C for 12 h. Ag/CeO₂-IM catalyst's hydrothermal ageing reduces NH₃ conversion at low temperatures, an increase of T₉₀ by 50 °C, and a decrease in N₂ selectivity to about 50% at 200 °C, which may be due to agglutination and sintering of Ag species at 700 °C. The hydrothermal stability of the Ag/CeO₂-IM catalyst needs to be improved.

Fig. 9 (a) NH₃ conversion (b) N₂ selectivity of Ag/CeO₂-IM catalyst and Ag/CeO₂-IM-700 catalyst after hydrothermal testing.

4. Conclusions

In this study, the deposition precipitation approach and equal volume impregnation method were used to create the Ag/CeO₂ catalyst, the impact of the preparation technique and load amount on ammonia oxidation performance was discussed, and the following conclusions were drawn:

(1) The Ag/CeO₂-IM catalyst prepared by the equal volume impregnation method has a better NH₃ oxidation capacity than the Ag/CeO₂-DP catalyst prepared by the deposition precipitation method. NH₃ conversion can reach 90% at 200 °C, and T₅₀ is below 150 °C. The Ag/CeO₂-IM catalyst has good cycle stability, but its hydrothermal stability at 700 °C still needs to be improved.

(2) The Ag/CeO₂-IM catalyst has a higher content of Ag⁰ and stronger redox, which achieves low temperature and high NH₃ conversion, but its surface acidity is less than that of Ag/CeO₂-DP, which results in N₂ selectivity decreasing slowly at 150–400 °C. According to the results of in situ DRIFTS, the Ag/CeO₂-IM catalyst had a lower NH₃ ignition temperature and better NH₃ oxidation capacity, which was the main reason for the higher activities of the Ag/CeO₂-IM catalyst.

(3) The mechanism of NH₃-SCO on the Ag/CeO₂-IM and Ag/CeO₂-DP catalysts both followed the i-SCR mechanism.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22076136), the Central Government Guides Local Science and Technology Development Fund Project (206Z3702G), and Hebei Province Major Scientific and Technological Achievement Transformation Fund Support Project (2021004012A and 22281401Z).

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