Calcination conditions and stability of supported Ni₄La oxide for catalytic decomposition of N₂O

Abstract

A series of novel supported Ni₄La oxide catalysts (S-Ni₄La for short) with the same mass loading amount of 10%, using pretreated cordierite ceramics as carrier, was prepared by an impregnation method and tested for catalytic decomposition of N₂O at low temperature. The effects of calcination temperature and atmosphere on the catalytic performance were mainly studied, and the stability of the S-Ni₄La in reaction was evaluated. Meanwhile, the solid-phase structure, micro-structure morphology, redox properties, valence and content of ions were characterized by the techniques of XRD, SEM, H₂-TPR, N₂O-TPD and XPS, respectively. Moreover, the catalytic mechanism for N₂O decomposition over the S-Ni₄La was discussed. The results showed that the S-Ni₄La calcined at 400 °C in a nitrogen atmosphere completely decomposed N₂O at 375 °C, which successfully breaks the technical bottleneck that low-cost supported metal oxides were not able to completely decompose N₂O at below 400 °C. La₂O₃ and LaNiO₃ were not active phases for catalytic decomposition of N₂O, while NiO was a major active phase in reaction. The reducing atmosphere decreased crystallization and refined the grain size, so as to increase the effective specific surface area, thereby improving the catalytic performance. Furthermore, the La_n+1Ni_nO_3n+1+σ formed possessed a perfect migration performance of oxygen species, particularly for the catalyst calcined in a nitrogen atmosphere, and consequently the S-Ni₄La calcined in a nitrogen atmosphere revealed a much better catalytic performance.

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1. Introduction

N₂O elimination has become an urgent task in the building of a low carbon society.^1–3 Among all the N₂O emission control technologies, direct catalytic decomposition of N₂O has become a current mainstream technology and a development direction for N₂O elimination due to its high removal efficiency, without requiring a reducing agent, and its decomposition products being nitrogen and oxygen.^4,5 As the technological core of the direct catalytic decomposition method, catalyst development has been attracting wide attention. Metal oxides are regarded as the most promising catalysts because of their high catalytic activities at low temperatures, strong anti-poisoning abilities and low costs, particularly for complex oxides.⁶ Supported metal oxides might be particularly suited for industrial applications due to requiring less metal oxide for their production, their higher dispersion and lower cost.⁷ However, the active temperature of supported metal oxides, at which N₂O is completely decomposed, is much higher than those of the metal oxides before loading, and until now supported metal oxides have completely decomposed N₂O merely at above 400 °C,^7,8 which has been a technical bottleneck that directly hinders development of low-cost supported metal oxides for the application of low-temperature catalytic decomposition of N₂O. Therefore, improving the catalytic activity of the supported metal oxides at temperatures below 400 °C has been an important scientific problem.

According to semiconductor catalytic theory, p-type semiconductors are applicable for catalytically decomposing N₂O, because the p-type semiconductor has a low Fermi level.⁹ The active sites of the metal oxides are metal ions and oxygen vacancies, so their bonding state, coordinated state and electronic state will directly affect catalytic activity.¹⁰ To the best of our knowledge, NiO is a typical non-stoichiometric compound with positive ion defects, belonging to the p-type semiconductors, thus NiO may be used to catalytically decompose N₂O. Pasha et al.² reported that pure bulk NiO showed a 100% N₂O conversion at 350 °C in the catalytic decomposition of N₂O, whereas the most active Cs-promoted NiO (Cs/Ni = 0.1) catalyst exhibited a 100% N₂O conversion at 250 °C. Zhou et al.¹¹ reported that a pure Ni₉₀Ce₁₀ mixed oxide synthesized by a citrate acid method could completely decompose N₂O at 400 °C in the presence of oxygen. Lan et al.⁸ reported that a supported catalyst of NiO/mullite fully decomposed N₂O at 480 °C. As stated above, Ni-based complex oxides possess much lower full decomposition temperatures in reaction than that of pure NiO, while the full decomposition temperature of supported NiO is much higher than that of pure NiO.

In our previous work,⁵ a novel supported metal oxide catalyst system of supported Ni–La–O_x (S-Ni_xLa for short, where x is the molar ratio of Ni/La), calcined at 550 °C in an air atmosphere, was studied for the direct decomposition of N₂O; it was revealed that the supported Ni₄La complex oxide possessed the best catalytic performance and completely decomposed N₂O at 435 °C. However, the full decomposition temperature was still higher than 400 °C. In this work, the effects of calcination temperature and atmosphere on the catalytic performance of the S-Ni₄La for catalytic decomposition of N₂O were mainly studied. The catalytic stability of the S-Ni₄La in reaction was also evaluated so as to obtain the optimized preparation parameters for the catalyst, and break the technical bottleneck that, until the present, supported metal oxides have not been able to completely decompose N₂O at temperatures below 400 °C. In addition, the catalytic mechanism for catalytic decomposition of N₂O over the S-Ni₄La was discussed.

2. Experimental

2.1 Catalyst preparation

A series of supported Ni₄La complex oxides (S-Ni₄La for short) with a mass loading amount of 10% were prepared by an impregnation method. Some pretreated cordierite ceramic blocks, as carriers, were immersed into the Ni₄La precursor solution for 1 h, followed by drying at 60 °C for 3 h and calcination in air and nitrogen atmospheres at temperatures between 350 and 600 °C, in steps of 50 °C. The active Ni₄La complex oxides used for characterization were prepared by thermal decomposition of the Ni₄La precursor solution with the same firing system. The Ni₄La precursor solution with the Ni/La molar ratio of 4 was made from Ni(NO₃)₂ (Xilong Chemical Co., Ltd, AR 98%) and La(NO₃)₃ (Sinopharm Chemical Reagent Co., Ltd, AR 99.8) under vigorous stirring at room temperature for 3 h. Some cordierite honeycomb ceramics without catalytic activity were crushed into particle sizes of 6–10 mesh, and then the ceramic blocks were pretreated by an acid pickling technique which entailed washing with boiling 10 wt% HNO₃ solutions at 100 °C for 30 minutes. After that, the ceramic blocks were washed with distilled water until its pH was neutral, followed by drying at 80 °C for 24 h.

2.2 Activity measurement

The direct decomposition of N₂O was carried out in a fixed-bed continuous flow reactor using 5 ml of catalyst at a gas hourly space velocity (GHSV for short) of 2400 h⁻¹. The inner diameter of the quartz tube reactor was 10 mm. The S-Ni₄La samples were placed into the quartz reactor for catalytic decomposition of 5000 ppmv N₂O at temperatures between 250 and 450 °C, in steps of 50 °C. The concentrations of N₂O at the reactor outlet were analyzed online by a gas chromatograph (Shimadzu GC 2014) equipped with a thermal conductivity detector (TCD) and a Porapac Q column after 20 min reaction at each temperature. The catalytic activities of the S-Ni₄La samples were evaluated in terms of the N₂O conversion (X) according to the following equation:

2.3 Catalyst characterization

The specific surface areas of the selected Ni₄La samples were evaluated from liquid N₂ adsorption–desorption isotherms obtained at 77 K over the whole range of relative pressures, using a Micromeritics ASAP 2020 automatic analyzer on samples previously outgassed at 473 K for 2 h. The Brunauer–Emmett–Teller (BET) method was performed to estimate the specific surface areas of the Ni₄La samples.

X-ray powder diffraction (XRD) patterns were recorded in the two-theta range from 10° to 80° (0.6° min⁻¹) using an X-ray diffractometer (Rigaku DMAX-RB) with Cu Kα radiation (λ = 1.5406 Å) using 40 kV and 30 mA. The crystal phases were confirmed according to JCPDS files.

Scanning electron microscopy (SEM) was carried out using a JEOL (JSM-5900, Japan) instrument, using a 10 kV acceleration voltage, to determine the morphology and particle size of the Ni₄La sample.

The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a RBD upgraded PHI-5000C ESCA system (Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV) or Al Kα radiation (hν = 1486.6 eV). In general, the X-ray anode was run at 250 W and the high voltage was kept at 14.0 kV with a detection angle at 54°. The pass energy was fixed at 23.5, 46.95 or 93.90 eV to ensure sufficient resolution and sensitivity. The base pressure of the analyzer chamber was about 5 × 10⁻⁸ Pa. The sample was directly pressed into a self-supported disk (10 × 10 mm) and mounted on a sample holder, then transferred into the analyzer chamber. The binding energies of Ni 2p, La 3d and O 1s core levels were determined, referencing to a binding energy of adventitious C 1s signal at 284.6 eV, which gave an accuracy of ±0.1 eV. The data analysis was carried out using the RBD AugerScan 3.21 software provided by RBD Enterprises and XPSPeak 4.1 provided by Raymund W. M. Kwok (The Chinese University of Hong Kong, China). A standard Shirley background and Gaussian (Y%)–Lorentzian (X%) were used for each component.

Hydrogen temperature-programmed reduction (H₂-TPR) profiles were obtained using a semiautomatic Micromeritics TPD/TPR 2900 analyzer. The samples were pre-treated, before the reduction measurement, at 150 °C for 0.5 h in a helium flow, and then cooled to 50 °C. Reduction profiles were obtained by passing a 10% H₂/Ar flow at a flow rate of 50 ml min⁻¹ through around 30 mg of sample. The temperature was increased from 50 to 900 °C with a rate of 5 °C min⁻¹. The hydrogen consumption was monitored using a thermal conductivity detector.

The Ni₄La samples were analyzed by the temperature-programmed desorption of N₂O (N₂O-TPD). The samples consisted of solid particles in a 25–40 mesh size range. Prior to N₂O-TPD, samples of around 100 mg were exposed to 30 ml min⁻¹ of helium held at 550 °C (10 °C min⁻¹) for 1 h, then switched to a N₂O flow (30 ml min⁻¹) and isothermal treatment at this temperature for 0.5 h, followed by cooling down to 50 °C in a N₂O flow. Afterwards, helium was fed to the reactor at a 10 ml min⁻¹ flow and kept flowing for 0.5 h in order to remove any excess oxygen species. The sample was then heated up to 600 °C (10 °C min⁻¹) under a helium flow (10 ml min⁻¹); O₂ desorbed during the heating was recorded with the change of temperature.

3. Results and discussion

3.1 Effects of the calcination temperature on catalytic performance

The calcination temperature not only affects the catalyst’s solid-phase structure, but also affects the interactions of active components in the catalyst. The properties of a catalyst depend on its structure and composition. Herein, the effects of calcination temperature on the catalytic performance of the S-Ni₄La, calcined in an air atmosphere, for catalytic decomposition of N₂O were studied. Fig. 1 shows the profiles of N₂O conversion versus reaction temperature over the S-Ni₄La catalysts calcined at temperatures between 350 and 600 °C, in steps of 50 °C. The general trend of the N₂O conversion was a tendency to increase as reaction temperature increased. The adsorption and desorption rates of the gas molecules on the surface of the S-Ni₄La increase with rising temperature; correspondingly, the quantity of activated molecules increases, the effective collision frequency increases,¹² and consequently the reaction rate of N₂O decomposition increases. However, the detailed catalytic activities of the catalysts exhibited differences between each other: the catalytic activity increased as the calcination temperature increased from 350 °C to 400 °C, while the catalytic activity decreased with further increase in calcination temperature from 400 °C to 600 °C. As shown in Table 1, although the specific surface area of the Ni₄La calcined at 400 °C is smaller than that of the sample calcined at 350 °C, the S-Ni₄La calcined at 400 °C demonstrated the highest catalytic activity, completely decomposing N₂O at 400 °C. Moreover, the average pore diameter of the Ni₄La calcined at 400 °C is 2.1 nm, the same as that of the sample calcined at 350 °C; this indicates that the increase in calcination temperature from 350 °C to 400 °C does not affect the appropriate pore size for the mass transfer channel in the catalytic decomposition of N₂O, thus maintaining the effective specific surface area for the reaction. However, use of the excessively high calcination temperatures from 400 °C to 600 °C will cause crystallization enrichment of the active components, resulting in a decrease of the specific surface area, thus reducing catalytic activity.

Fig. 1 Catalytic performances of the S-Ni₄La catalysts calcined in air at different temperatures.

Table 1 Texture parameters of Ni₄La calcined at different temperatures

Calcination temperature, T/°C	Specific surface area, SBET/(m^2 g^−1)	Total pore volume, Vtotal/(cm^3 g^−1)	Average pore diameter, Dpore/nm
350	38.8	0.020	2.1
400	25.7	0.014	2.1
500	15.1	0.008	2.3
600	10.4	0.006	2.1

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As we know, if a catalyst has not been fully sintered, its structure will change with further increase of the heat treatment temperature. This will affect catalytic performance. Herein, since the reaction temperature ranges up to 400 °C, excessively high reaction temperatures of more than 350 °C may affect the catalytic performance of S-Ni₄La sintered at 350 °C. Moreover, the catalytic activity increased with an increase in calcination temperature from 350 °C to 400 °C. This indicates that a further increase in the reaction temperature from 350 °C to 400 °C will improve the catalytic activity of S-Ni₄La calcined at 350 °C, because the solid-phase structure of S-Ni₄La calcined at 400 °C is more active and stable in reaction compared with the sample calcined at 350 °C.

3.2 Effects of the calcination atmosphere on catalytic performance

To the best of our knowledge, oxygen vacancies and the valence states of metal ions in a catalyst, and their adsorption–desorption behavior, can directly affect catalytic performance. Furthermore, the surface ions of the S-Ni₄La catalysts separately calcined in oxidizing and reducing atmospheres may possess different valence distributions and different abilities to gain or lose electrons. On the basis of the established optimum calcination temperature, how does the calcination atmosphere affect catalytic performance of the S-Ni₄La calcined at 400 °C? Fig. 2 shows the effects of nitrogen and air atmospheres on the catalytic performance of the S-Ni₄La calcined at 400 °C. It is obvious that the S-Ni₄La calcined in a nitrogen atmosphere has a higher catalytic activity at the same reaction temperature compared with the sample calcined in an air atmosphere. The S-Ni₄La calcined in nitrogen completely decomposed N₂O at 375 °C. The concrete reason why the catalyst calcined in a nitrogen atmosphere exhibits better catalytic performance will be analyzed in combination with the following characterization.

Fig. 2 Catalytic performances of the S-Ni₄La catalysts calcined at 400 °C in nitrogen and air.

3.3 Stability of the S-Ni₄La calcined in nitrogen in reaction

The stability of the S-Ni₄La catalyst calcined at 400 °C in a nitrogen atmosphere was studied by testing the activity during the cooling step after the maximum temperature (400 °C) was reached. Fig. 3 plots the heating and cooling activity branches for the S-Ni₄La catalyst calcined at 400 °C in a nitrogen atmosphere. The S-Ni₄La shows an improvement of activity during the first cooling step: the S-Ni₄La was able to completely decompose N₂O at 360 °C. This improvement may be related to N₂O mass transfer from outside to inside in the initial cycle. Moreover, the catalyst showed no loss of activity in the second cycle of heating up and cooling down, and the catalytic activity curves completely overlap, indicating that the S-Ni₄La calcined at 400 °C in nitrogen was very stable with respect to low- and high-temperature treatments under reaction conditions. Moreover, the heating and cooling activity branches in the initial cycle reveal a hysteresis loop, which is similar to the typical IV isotherm shape with the adsorption characteristics of an unrestricted single molecular layer and a multi-molecular layer. In our previous work,⁵ we already confirmed that the Ni₄La calcined at 550 °C in air displayed an average pore diameter of about 17.4 nm, which was the appropriate pore size for the mass transfer channel in N₂O decomposition. Herein, the S-Ni₄La calcined at 400 °C in nitrogen may also possess an appropriate mass transfer channel in reaction, which also results in perfect stability of the catalytic activity.

Fig. 3 Stability test of the S-Ni₄La (400 °C, nitrogen), by a cooling down step, in N₂O decomposition.

3.4 XRD analysis

Fig. 4 shows the X-ray diffractograms of the Ni₄La powders calcined in air at different temperatures. All the diffractograms of the Ni₄La samples include a typical diffraction pattern for cubic NiO (PDF 47-1049, marked as ●). As the calcination temperature increased, typical diffraction patterns corresponding to La₂O₃ (PDF 73-2141, marked as ◆) and perovskite-type LaNiO₃ (PDF 34-1181, marked as ★) became able to be found, indicating that the rise in calcination temperature accelerates La₂O₃ grain growth, and increases formation of the solid-phase reaction product LaNiO₃. Accordingly, the catalytic activity decreased when the calcination temperature was increased from 400 to 600 °C, as, comparatively, the solid-phases La₂O₃ and LaNiO₃ may be not catalytically-active phases for the catalytic decomposition of N₂O. This may have resulted from the decrease in the effective specific surface area of the Ni₄La catalyst caused by the overly high calcination temperature.

Fig. 4 XRD patterns of the Ni₄La powders calcined at different temperatures.

Fig. 5 shows the X-ray diffractograms of the Ni₄La powders separately calcined at 400 °C in air and nitrogen atmospheres. The diffractograms of the two Ni₄La samples also include a typical diffraction pattern for cubic NiO (PDF 47-1049, marked as ●). However, the crystallization degree of the Ni₄La calcined in nitrogen is very low, and the diffraction peak intensities of the cubic NiO in the Ni₄La calcined in nitrogen are much lower compared with those for the sample calcined in air. Obviously the grain growth of the NiO is largely inhibited by the nitrogen atmosphere. In addition, the catalytic activity of the S-Ni₄La calcined in a nitrogen atmosphere is higher than that of the sample calcined in air. This further implies that the reduced state metal ions and oxygen vacancies are more active for the catalytic decomposition of N₂O.

Fig. 5 XRD patterns of the Ni₄La samples calcined in different atmospheres.

3.5 Micro-morphology

Fig. 6 shows the surface micro-topography of the Ni₄La samples calcined at 400 °C in air and nitrogen atmospheres. As shown in Fig. 6(a), the surface microstructure of the Ni₄La calcined in air looks very loose, with a coral-like structure. Fig. 6(b) shows the micro-structure of the sample calcined in nitrogen, where comparatively there are a large number of irregular interstitial holes existing in staggered accumulations of microscopic particles. The particle size is obviously smaller than that of the sample calcined in air; it can be estimated by SEM that the equivalent diameter of the microscopic particles is about 150–200 nm. A small particle size can contribute a large specific surface, and the irregular interstitial holes provide the channels for reaction gas diffusion and mass transfer inside the catalyst.

Fig. 6 SEM photographs of the Ni₄La calcined at 400 °C in an air atmosphere (a) and in a nitrogen atmosphere (b).

3.6 XPS

XPS measurements were carried out to examine the surface electronic states of the Ni₄La sample calcined in nitrogen. Fig. 7 and 8 present spectra of the Ni 2p, La 3d and O 1s core-levels, respectively. The binding energies of Ni 2p_1/2, 2p_3/2, La 3d_3/2, 3d_5/2 and O 1s determined by XPS are summarized in Table 2.

Fig. 7 Ni 2p and La 3d core level spectra of the Ni₄La sample calcined at 400 °C in nitrogen.

Fig. 8 O 1s core level spectrum of the Ni₄La sample calcined at 400 °C in nitrogen.

Table 2 Fitting results of the Ni 2p, La 3d and O 1s photoelectron spectra of the Ni₄La sample calcined at 400 °C

Peak parameters		Peak identification
Fresh Ni4La catalyst		Species	Line	Assignment
BE (eV)	FWHM (eV)
872.3	4.13	NiO	2p1/2	Core
854.8	3.57	NiO	2p3/2	Core
861.0	6.57			Satellite
878.7	8.07			Satellite
852.2	3.52	Lan+1NinO3n+1+σ	3d3/2	Core
855.5	3.69			Shake up
835.4	3.89	Lan+1NinO3n+1+σ	3d5/2	Core
838.7	2.78			Shake up
531.6	3.29	Ni^II–O	1s	Core
529.3	1.9	La–O	1s	Core

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Fig. 7 shows the peak fitting results of the Ni 2p and La 3d binding energies, which are determined by peak fitting through XPSPeak 4.1. As we know, the La 3d_3/2 peak overlaps the Ni 2p_3/2 peak,¹³ and the La 3d core level splits into 3d_5/2 and 3d_3/2 components due to a spin–orbit interaction. Additionally, each line splits into a main line (3d⁰4f⁰ final state configuration) and a satellite line (3d⁰4f¹L final state configuration).^14,15 The binding energy of La 3d_5/2 is 835.4 eV, which is slightly higher than that of the sample calcined at 550 °C in air.⁵ However, the peak of La 3d_5/2 appearing at 835.4 eV doesn’t belong to La₂O₃ or LaNiO₃, because the XRD already confirmed that no solid-phases of La₂O₃ and LaNiO₃ could be found in the Ni₄La sample calcined at 400 °C in nitrogen. Bassat reported that this binding energy belonged to the layered composite oxide of La_n+1Ni_nO_3n+1+σ.¹⁶ The binding energies of 852.2 eV and 855.5 eV correspond to the La 3d_3/2 and its splitting peak, respectively. In addition, the binding energies of 872.3 eV and 854.8 eV separately correspond to Ni 2p_1/2 and Ni 2p_3/2, accompanied by two satellite peaks.

The core levels of the O 1s spectra for the Ni₄La calcined at 400 °C in nitrogen shown in Fig. 8. The peak appearing at 531.6 eV is ascribed to the lattice O²⁻ anion in NiO, while the peak appearing at 529.3 eV is ascribed to the lattice O²⁻ anion in lanthanum oxides.¹⁷ The layered structure of the La_n+1Ni_nO_3n+1+σ is formed by separation of the LaO layer; the oxygen ions in its structure are of good electron properties because they are anisotropic. Its oxygen diffusion mechanism is that the oxygen atoms in the alternating layers of LaO jump in the structure through the interstitial mechanism. Since charge transfer and vacancies increase in the p-type semiconductor, correspondingly the concentration of Ni–O–Ni bonds which are responsible for electron conduction increases, and thereby the charge and oxygen species migration performance are increased. So this phase has an excellent migration performance of oxygen species and a higher diffusion coefficient.¹⁸ In any case, oxygen species migration and desorption performances can affect the catalytic activity in the catalytic decomposition of N₂O. Therefore, the existence of La_n+1Ni_nO_3n+1+σ may result in quicker migration of oxygen species, and thereby improve the catalytic performance.

3.7 H₂-TPR

The H₂-TPR measurements were performed over the Ni₄La samples calcined at 400 °C in air and nitrogen atmospheres in order to determine the reducibility and its correlation with catalytic performance. The H₂-TPR profiles of the samples are shown in Fig. 9. The narrow peaks appearing at low temperature around 250–300 °C are named α peaks, which are attributed to the reduction of surface absorbed O species and dispersed NiO. The formation of LaNiO₃ results in generation of oxygen vacancies, which adsorb oxygen species easily. Therefore, very active oxygen species are formed, which are reduced by H₂ at low temperature. The wide β peaks appearing at around 300–450 °C correspond to the reduction of NiO to metallic nickel. Zhou et al.¹¹ reported that the reduction process of NiO usually followed NiO → Ni^δ+ → Ni⁰, due to the large grain size of NiO. However, the stepwise reduction of NiO to metallic nickel is not observed in the Ni₄La samples. This illustrates that La additives minimize the grain size of NiO, consequently promoting oxygen species migration and leading to only one main peak being observed. Moreover, the β peak area of the sample calcined in a nitrogen atmosphere is larger than that of the sample calcined in air, implying that the grain size of the NiO is much smaller in the Ni₄La calcined in nitrogen, because the reducing atmosphere may inhibit crystallization enrichment of the NiO. These results are consistent with the results of XRD analysis. The weak γ peaks appearing at around 500–600 °C correspond to the reduction of La_n+1Ni_nO_3n+1+σ, such as expressed in the equation: La_n+1Ni_nO_3n+1+σ + H₂ → Ni⁰ + La₂O₃ + H₂O.¹⁹ The XPS also confirmed that the γ peak should be attributed to reduction of Ni²⁺, in La_n+1Ni_nO_3n+1+σ. However, the XRD did not detect solid-phase La_n+1Ni_nO_3n+1+σ, therefore it may be that the La_n+1Ni_nO_3n+1+σ content is very low.

Fig. 9 H₂-TPR profiles of the Ni₄La samples calcined at 400 °C in air and in nitrogen atmospheres. Conditions: 10% H₂/Ar, 50 ml min⁻¹ and a ramping rate of 5 °C min⁻¹.

3.8 N₂O-TPD

In the case of the Ni₄La samples, Ni²⁺ serves as the active sites in the reaction. In any case, the removal of adsorbed oxygen is the rate-determining step of N₂O decomposition. Therefore, N₂O temperature-programmed desorption (N₂O-TPD) experiments were performed in order to investigate the relationship between N₂O catalytic activity and O₂ desorption. Fig. 10 shows the N₂O-TPD profiles of the Ni₄La samples which were calcined at 400 °C in air and nitrogen atmospheres. Several peaks can be seen in the profiles in the temperature range from 50 to 600 °C. The weak peak appearing from 250 to 400 °C is named the α peak, and was not found in the N₂O-TPD profile of the sample calcined at 550 °C in air;⁵ it corresponds to desorption of oxygen species in the La_n+1Ni_nO_3n+1+σ. The La_n+1Ni_nO_3n+1+σ possesses perfect migration performance of oxygen species, so the samples calcined at 400 °C reveal good catalytic performances, especially the sample calcined in a nitrogen atmosphere. In addition, the β peak appearing at around 500 °C can be attributed to desorption of subsurface oxygen, and the γ peak appearing at over 600 °C is attributed to desorption of lattice oxygen.

Fig. 10 N₂O-TPD profiles of the Ni₄La samples calcined at 400 °C in air and nitrogen.

3.9 Catalytic mechanism discussion

Generally, calcined NiO crystals always possess Ni²⁺ cation vacancies; simultaneously, the crystal must contain some O⁻ or Ni³⁺ species in order to maintain electrical neutrality.²⁰ In the NiO crystal, the Fermi level, marked as E_F, is close to the maximum energy level of the valence band, as shown in Fig. 11(a). Some electrons in the valence band jump to the empty band, and form the conduction band. Accordingly, holes are left in the valence band. NiO relies on the electron holes in the valence band and the electrons in the conduction band to be able to conduct electricity, so NiO is a p-type semiconductor. In the catalytic decomposition of N₂O, only the Fermi level energy is lower than the ionization potential of adsorbed O⁻; electrons can transfer to the surface of the catalyst, so high catalytic activity can be obtained. Therefore, a larger number of electron holes are produced in the valence band, and its electrical conductivity δ and electron work function φ are increased; the electrons of the adsorbed O⁻ will be easier to transfer to the surface of the catalyst, thereby the catalytic activity is improved. In the Ni₄La calcined at 400 °C, if a La³⁺ ion replaces a Ni²⁺ ion, then the crystal should lose a redundant positive charge. Since the NiO is a p-type semiconductor, a vacancy must exist. According to the principle of charge conservation, the Ni³⁺ related to hole conduction must become Ni²⁺ to achieve the purpose of the loss of a positive charge, such as expressed in eqn (2). The charge of the entire material is zero, the conductive hole must be decreased, then the Fermi level becomes higher, so that the ability of accepting electrons on the surface of the catalyst is reduced, and thus the catalytic performance of the catalyst becomes worse. However, the S-Ni₄La exhibits better catalytic performance. If a La³⁺ ion replaces a Ni²⁺ ion, accordingly positive ion vacancies will form. The lack of charge will be compensated by the La³⁺ ion to keep charge conservation, thereby increasing the hole concentration and lowering the Fermi level, so that it is easier for the catalyst surface to accept electrons, and the catalyst possesses a better catalytic performance in the catalytic decomposition of N₂O, as shown in Fig. 11(b). When the Ni₄La catalyst was calcined at 400 °C in a nitrogen atmosphere, the hole concentration was further improved, thereby lowering the Fermi level, so that the catalyst revealed a much better catalytic performance, as shown in Fig. 11(c).

Fig. 11 Energy band structure of NiO in different samples (a) without electron holes; (b) with one electron hole; (c) with more electron holes.

As stated above, as long as the catalyst possesses a lower Fermi level, a larger number of vacancies and a better migration performance of oxygen species, it will exhibit better catalytic activity in the process of N₂O decomposition.

4. Conclusions

The S-Ni₄La calcined at 400 °C in an air atmosphere completely decomposed N₂O at 400 °C. In contrast, S-Ni₄La calcined at 400 °C in a nitrogen atmosphere completely decomposed N₂O at 375 °C and the catalyst exhibited perfect stability in reaction, which successfully breaks the technical bottleneck that a low-cost supported metal oxide catalyst could not until now completely decompose N₂O below 400 °C. Overly high calcination temperatures accelerate the formation of La₂O₃ and LaNiO₃, phases which are not active for the catalytic decomposition of N₂O, while the NiO is a major active phase in the reaction. Furthermore, a reducing atmosphere decreases the crystallization of the Ni₄La complex oxide and refines microscopic particles including grains, so as to increase the effective specific surface area of the catalyst, thereby improving the catalytic performance. In addition, the La_n+1Ni_nO_3n+1+σ formed possesses a perfect migration performance of oxygen species, particularly for the catalyst calcined in nitrogen atmosphere, so that the S-Ni₄La calcined in nitrogen atmosphere reveals a much better catalytic performance. In any case, as long as the catalyst possesses a lower Fermi level, a larger number of vacancies and a better migration performance of oxygen species, it will demonstrate a better catalytic activity in the process of N₂O decomposition.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 21106071, 51272105), the New Teachers’ Fund for Doctor Stations Sponsored by the Ministry of Education of China (no. 20113221120004), the Research Subject of Environmental Protection Department of Jiangsu Province of China (no. 2012016), the Jiangsu Provincial Science and Technology Supporting Program (no. BE2013718), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1146), and Nanjing Tech University Talent Cultivation Project.

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