Direct N₂O decomposition over La₂NiO₄-based perovskite-type oxides

Abstract

Direct decomposition of N₂O by perovskite-structure catalysts including La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ was investigated. The catalysts were prepared by the Pechini method and characterized by x-ray diffraction (XRD), BET, scanning electron microscopy (SEM), and O₂-TPD. Experimental results indicate that the properties of La₂NiO₄ are significantly improved by partially substituting La with Sr and Ce. N₂O decomposition efficiencies achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ are 44 and 36%, respectively, at 400ºC. As the temperature was increased to 600ºC, N₂O decomposition efficiency achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ reached 100% at an inlet N₂O concentration of 1,000 ppm, while the space velocity was fixed at 8,000 hr⁻¹. In addition, effects of various parameters including oxygen, water vapor, and space velocity were also explored. The results indicate that N₂O decomposition efficiencies achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ are not significantly affected as space velocity is increased from 8,000 to 20,000 hr⁻¹, while La_0.7Ce_0.3SrNiO₄ shows better tolerance for O₂ and H₂O_(g). On the other hand, N₂ yield with LaSrNiO₄ as catalyst can be significantly improved by doping Ce. At a gas hour space velocity of 8,000 hr^−1, and a temperature of 600ºC, high N₂O decomposition efficiency and N₂ yield were maintained throughout the durability test of 60 hr, indicating the long-term stability of La_0.7Ce_0.3SrNiO₄ for N₂O decomposition.

Implications:

Nitrous oxide (N₂O) not only has a high global warming potential (GWP₁₀₀ = 310), but also potentially destroys ozone in the stratosphere. Pervoskite-type catalysts including La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ are applied for direct N₂O decomposition. The results show that N₂O decomposition can be enhanced as Sr and Ce are doped into La₂NiO₄. At 600ºC, N₂O decomposition efficiencies achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ reach 100%, demonstrating high activity and good potential for direct N₂O decomposition. Effects of O₂ and H₂O_(g) contents on catalytic activities are also evaluated and discussed.

Introduction

Nitrous oxide (N₂O) has been recognized as one of the major air pollutants with increasing atmospheric concentration year by year. It not only destroys the ozone layer in the stratosphere, but also is a strong greenhouse gas (Rodhe, 1990; Wójtowicz et al., 1993; Kramlich and Linak, 1994; Russo et al., 2007). Previous studies indicate that N₂O can be converted into nitrogen oxide (NO) when it migrates into the stratosphere, furthermore, triggering ozone-destroying reactions (Jacob, 1999). In addition, N₂O has a long lifetime in atmosphere (150 years) and a high global warming potential (GWP₁₀₀ = 310), which aggravates the greenhouse effect (Russo et al., 2007). N₂O emission may originate from natural and anthropogenic sources. Annual global N₂O emission is approximately 4.7–7 million tons, with 30–40% coming from natural sources (Wójtowicz et al., 1993; Jacob, 1999). Among anthropogenic sources, adipic acid production, nitric acid manufacturing, and fossil fuel combustion are considered as significant. Therefore, developing effective control technology to reduce N₂O emissions has become an emerging issue.

Generally speaking, methods for N₂O removal include thermal decomposition, selective catalytic reduction with hydrocarbons (HC-SCR), adsorption, and direct catalytic decomposition into O₂ and N₂ (Kumar et al., 2012). Among these methods, direct catalytic decomposition of N₂O into N₂ and O₂ is considered a relatively simple and economical option to reduce N₂O emission. Previous studies indicate that catalysts available for direct N₂O catalytic decomposition include noble metals (Hinshelwood and Prichard, 1925; Lintz and Riekert, 1984; Siva Sankar Reddy et al., 2007), metal oxide catalysts (Winter, 1974; Perez-Alonso et al., 2006; Granger et al., 2006), and zeolite-based catalysts (Pannov et al., 1990; Li and Armor, 1992; Sobolev et al., 1993). However, most zeolite-based catalysts are unpromising for practical use due to low hydrothermal stability and poisoning by sulfur-containing gases or water vapor (Li and Armor, 1992; Van den Brink et al., 2001). Noble metal catalysts are expensive although they show excellent performance; for example, the price of platinum (Pt) is about U$ 1,470 per ounce in 2014, while 1 ounce of rhodium (Rh) costs U$ 1,110 (NASDAQ, 2014). On the other hand, perovskite-type catalysts are interesting alternatives as they are low-cost materials and stable even at a temperature above 1,000ºC (Voorhoeve et al., 1972).

Perovskite-type catalysts are with a general formula of ABO₃ or A₂BO₄, where the A site is either an alkaline metal or rare earth element and a relatively large ion—typically La is applied—while the B site is generally a 3d transition metal like Cu, Fe, Co, Ni, or Mn. The B site is mainly responsible for catalytic activity, because it is located next to the oxide anion and triggers desorption, while the A site can change the electronic state of the B site and influence catalysis indirectly. Perovskite-type catalysts with different physical properties can be synthesized by substituting different elements at the A and B sites and by varying the chemical compositions. The mechanisms of N₂O decomposition with perovskite-type catalysts have been proposed in previous study as the reactions 1–3 (Kapteijn et al., 1996; Russo et al., 2007):

1

2

3

where an asterisk represents an oxygen vacant site, which is considered as an active site for N₂O decomposition. First N₂O can be adsorbed on an active site of catalyst, and then the active site offers charge into antibonding orbitals of N₂O to destabilize the N–O bond. Finally, N₂ can be desorbed from the active site of the catalyst, and the adsorbed O atom and another nearby O atom of an oxygen vacant site combine to form an O₂ molecule and desorb (Kapteijn et al., 1996).

ABO₃-type catalysts including LaMO₃ (M = Cr, Co, Ni, Cr, Mn, and Fe) (Wang et al., 1995; Dacquin et al., 2009), and A₂BO₄-type catalysts including La₂MO₄ (M = Co, Ni, Cu) (Wang et al., 1995), M₂NiO₄ (M = La, Pr, Nd) (Ramanujachary et al., 1982), and M₂CuO₄ (M = La, Pr, Nd, Sm, Gd) (Ramanujachary et al., 1984) have been studied for N₂O decomposition. Furthermore, modified perovskite-type catalysts were prepared by partial substitution at the A site and/or B site, including LaB_1−x B′ _xO₃ (B = Mn, Fe and B′ = Cu, Ni) (Alini et al., 2007) and La_1−xSr_xM_1−yM_yO₃ (M = Co, Ni, Cu) (Tofan et al., 2002a) for the improvement of catalyst activity. Overall, the highest N₂O decomposition efficiencies achieved with perovskite-type catalysts could reach 100%, but they have to be operated at high temperatures (≥700°C) for high N₂O decomposition activities. Additionally, oxygen and water vapor usually exist in industrial flue gases, so tolerances of oxygen and water vapor should be considered for good performance of catalyst.

Catalytic performance of perovskite-type catalysts can be enhanced by substituting different elements at the A and/or B sites. For example, strontium (Sr) can be applied to partially substitute La at the A site to improve the activity of the catalyst. Previous study indicates that nonstoichiometric perovskites (La_1−xSr_xM_1−yM_yO_3−δ) show several advantages, such as fast electron transfer, high oxygen mobility, and easy oxygen desorption (Tabata and Misono, 1990; Kirchnerova and Klvana, 1994). As a low-valence cation (Sr²⁺) occupies the frame at the La³⁺ site, the oxide anion vacancies can be produced by charge compensation (Zhao et al., 1996). Recently, cerium (Ce) received much attention in preparing a catalyst because it has excellent performance for oxygen storing and scavenging. Catalytic activity can be enhanced when Ce is partially substituted for the A site due to its redox property. Kirchnerova et al. (2002) found that the activity of LaCoO₃ was significantly improved when LaCoO₃ was doped with CeO₂. Belessi et al. (2001) indicate that the NO decomposition rate could be accelerated by increasing CeO₂ content in La-Sr-Ce-Fe-O catalyst. Nitadori and Misono (1985) applied Sr, Ce, and Th to substitute LaMO₃ (M = Mn, Fe, Co) and indicated that Ce-substituted samples improved the property of oxygen mobility. This redox effect appears prominently on the A₂BO₄-type perovskite catalysts, because the lattice of the A₂BO₄-type catalyst has larger cavity space compared with the ABO₃ type. A site of A₂BO₄ type can accommodate more relatively large ions of Ce, and previous study indicates that La_1−xCe_xSrNiO₄ lattice can store up to 10 times more cerium ions than can La_1−xCe_xNiO₃ (Imanaka and Masui, 1996). Another study also indicates that oxides with A₂BO₄ structure are more stable than that of ABO₃ structure (Zhao et al., 1996).

In this study, the A₂BO₄-type perovskite catalyst (La₂NiO₄) is prepared and evaluated for effectiveness in direct N₂O decomposition. Previous study has shown that nickel-based perovskites are excellent catalysts for a variety of oxygen-involving reactions (Kirchnerova and Klvana, 1994; Klvana et al., 1994). Furthermore, as the A site of La₂NiO₄ is partially substituted by doping Sr and Ce to prepare LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ catalysts, it is expected that properties and activities of the modified catalysts can be improved. In addition, the influences of space velocity, oxygen, and water vapor on N₂O decomposition are also explored with a lab-scale experimental system.

Experimental

Catalyst preparation

The provskite-type catalysts tested are of A₂BO₄ structure, including La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄, which were prepared by the Pechini method (Pechini, 1967). The preparation method includes several steps; first, appropriate metal nitrates were dissolved in deionized water (DI water) as precursors for preparing the aqueous solution with appropriate stoichiometry, and this aqueous solution was continuously stirred at room temperature. Then citric acid and ethylene glycol were added into the mixed aqueous solution of metal nitrates. The mixed aqueous solution was stirred for 1 hr at 90ºC as citric acid was added into the mixed aqueous solution; subsequently, ethylene glycol was added and then stirred at 85ºC to evaporate the water. Citric acid was added into the mixed aqueous solution of metal nitrates with designated proportion so that the molar ratio of citric acid to the total metal cations was 4 to 1. Ethylene glycol was then added into the mixed aqueous solution to control the molar ratio of citric acid to the ethylene glycol as 1 to 1. Second, the mixture just described was dried at 110ºC for 24 hr, followed by heat treatment at 600ºC for 1 hr in air, and calcination in air at 950ºC for 6 hr. Finally, the samples were manually milled to powders by using a porcelain mortar and pestle; subsequently, the synthesized particles were then pulverized to the final 200 mesh size and used as such.

Characterization of catalysts

The specific surface areas of the catalysts were measured by N₂ adsorption at –196ºC with a Micromeritics ASAP 2010 (ASAP2010, Micromeritics, USA), while measurements of the surface morphology and composition of catalyst were conducted using scanning electron microscopy (SEM) with associated energy-dispersive x-ray spectroscopy (EDS) (S80 JEOL, Japan). In addition, an x-ray diffractometer (D8AXRD, BRUKER, Germany) was utilized to identify the structure of catalyst, operating at 40 kV and 10 mA, by using Cu-Kα radiation. Diffraction patterns were obtained within a 2θ range of 5–80º at a scanning rate of 6º min⁻¹. Additionally, O₂-TPD was carried out with apparatus (AutoChem2920, Micromeritics, USA) equipped with a thermal conductivity detector (TCD). The samples (0.2 g) were first treated in O₂ at 1,000ºC for 1 hr and cooled to room temperature in the same atmosphere, then swept with helium at a rate of 11.8 mL/min. Finally, the sample was heated at a rate of 20ºC/min in helium and the TPD profile was recorded.

Catalytic activity measurement

Experimental tests were carried out with a lab-scale experimental system, and catalytic activity of direct N₂O decomposition was evaluated with a fixed-bed quartz reactor (ID 20 mm) containing 6 g catalyst. The feeding gas stream contained 1,000 ppm N₂O with helium (He) as carrier gas. The total gas flow rate was fixed at ˜628 ccm (cm³/min), corresponding to a gas hourly space velocity (GHSV) of 8,000 hr⁻¹. In addition, experimental tests were also conducted with the space velocity of 15,000 hr⁻¹ and 20,000 hr⁻¹, respectively, to evaluate the space velocity effect. In order to investigate the effects of O₂ and H₂O_(g) contents on the activities of catalysts for direct N₂O decomposition, 1–5% O₂ and 5–10% H₂O_(g) were introduced into the feeding gas, respectively. Sources of N₂O and O₂ are from cylinders, and designated N₂O and O₂ concentrations can be obtained by adjusting the flow rate of individual gas stream. In addition, H₂O is introduced into gas stream by a peristaltic pump. Experimental parameters are selected to simulate real conditions; that is, N₂O concentration is in the range of 0–1,000 ppm, while O₂ and H₂O_(g) concentrations are about 2–10% and 10% (Kapteijn et al., 1996), respectively. However, the space velocity applied in this study is smaller if compared with the field due to the limitation of the lab-scale experimental system.

Mass flow controllers (MFC) were utilized to adjust and control the gas flow rates, and all gases were mixed before reaching the reactor. Finally, operating temperature was varied from 300 to 700ºC to evaluate the temperature effect. The N₂O concentration and products were analyzed before and after the reaction by using a Fourier-transform infrared spectrophotometer (Nicolet 6700, Thermo Scientific, USA) and NO_x detector (Testo350, Testo, Germany), respectively, while analysis of N₂ was conducted using gas chromatography (Technologies6890N, Agilent, USA) with a TCD and columns packed with 5A molecular sieve (capillary: 30.0 m × 530 μm × 25 μm nominal). All experimental tests were conducted three times to check the stability and repeatability, and the experimental data were taken when the catalytic reactions practically reached steady state. Decomposition efficiency of N₂O and yields of N₂ were calculated by eqs 4 and 5, respectively:

4

5

where [ ]_in and [ ]_out are the concentrations of species measured before and after the reactor, respectively.

Results and Discussion

Characterization of the catalysts

Figure 1 shows XRD patterns of three catalysts tested, indicating that all the catalysts are of perovskite-like structure (A₂BO₄-type). XRD patterns of La_0.7Ce_0.3SrNiO₄, LaSrNiO₄, and La₂NiO₄ are presented in Figures 1a, 1b, and 1c, respectively, while triangles and squares represent La₂NiO₄ and LaSrNiO₄ crystalline phases. For Figure 1 c, there exist strong main diffraction peaks, which are assigned to those from La₂NiO₄. As Sr is partially substituted for the A site of La₂NiO₄, its main diffraction peaks are assigned to LaSrNiO₄. For Ce-substituted catalyst, the diffraction peaks are mainly assigned to LaSrNiO₄, but there exists one abrupt peak at 2θ= ∼28^ο, generally, which is attributed to CeO₂. However, 2θ = ∼28^ο might imply the diffraction peak of La_1−xCe_xSrNiO₄. Previous study suggests that the peak of 2θ = ∼28^οmay not be attributed to CeO₂ when Ce is added into LaSrNiO₄; that is, Ce may occupy the La-site without destroying the matrix structure (Zhu et al., 2005). In order to clarify whether this peak (2θ = ∼28^ο) belong to CeO₂, more information is needed for understanding the diffraction peaks of CeO₂. According to Hernández-Enríquez et al. (2012), the diffraction peaks of CeO₂ are mainly distributed at 2θ = 28.5°, 33.0°, 47.6°, 56.4°, 59.1°, and 69.5°, respectively, which only belongs to the CeO₂ phase. However, the Ce-substituted catalyst does not generate peaks of CeO₂ at 2θ = 33°, 47.6°, 56.4°, 59.1°, and 69.5° in this study; hence, it is clear that 2θ = ∼28^ο is ascribed to La_1−xCe_xSrNiO₄ but not to CeO₂. These results demonstrate that Ce substitutes for lattice position, and this agrees with the literature (Zhu et al., 2005).

Figure 1. X-ray diffraction patterns of catalysts (a) La_0.7Ce_0.3SrNiO₄, (b) LaSrNiO₄, and (c) La₂NiO₄ (Δ:La₂NiO₄, □: LaSrNiO₄) .

Table 1 summarizes BET surface areas of the catalysts tested. The results indicate that the BET surface areas of La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ are 3.7, 5.0, and 5.2 m²/g, respectively, indicating that all three catalysts have small surface area. Generally speaking, BET surface areas of perovskite-type catalysts are between 5 and 50 m²/g. Figure 2 displays the SEM patterns of La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄, respectively, and all three catalysts present as pellet with slight agglomeration, corresponding to the low BET surface area possibly due to high-temperature calcination. EDX results are shown in Table 2, indicating that the elemental composition of the catalysts deviates a little from the designated formula of catalysts.

Table 1. BET surface areas of the catalysts prepared

Catalyst	BET (m^2/g)
La2NiO4	3.7
LaSrNiO4	5.0
La0.7Ce0.3SrNiO4	5.2

Figure 2. SEM patterns of catalysts (a) La₂NiO₄, (b) LaSrNiO₄, and (c) La_0.7Ce_0.3SrNiO₄.

Table 2. Elemental composition of the catalysts prepared

	EDX (mass %)					Designated formula (mass %)
Catalyst	La	Ce	Sr	Ni	O	La	Ce	Sr	Ni	O
La2NiO4	39.8	—	—	20.6	39.7	40	—	—	20	40
LaSrNiO4	21.3	—	23.5	23.1	31.9	20	—	20	20	40
La0.7Ce0.3SrNiO4	13.9	5.9	19.8	22.1	38.3	14	6	20	20	40

O₂ Temperature-programmed desorption (O₂-TPD)

O₂ temperature-programmed desorption (TPD) is conducted for La₂NiO_4, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ individually for better understanding the behavior of oxygen desorption, since it has important implications regarding the catalytic N₂O decomposition. Typical perovskite-type catalysts have two kinds of oxygen desorption peaks for the O₂-TPD test. The peak appearing at a relatively low temperature is denoted by α oxygen, which is relatively weak, while the peak appearing at higher temperatures is denoted as β oxygen. Generally, the temperatures from 40 to 500ºC are defined as the low temperature range, while those higher than 500ºC are defined as the high temperature range. However, peaks of α oxygen and β oxygen are variant with different perovskite-type catalysts, implying that their peaks may appear at lower or higher temperatures due to different properties of catalysts. The α oxygen is chemically desorbed from the surface, while β oxygen is desorbed from the oxygen of vacancy and lattice. Hence, the performance of the perovskite-type catalysts mainly depends on the intensity of β oxygen peak for N₂O decomposition, namely, the β oxygen is released from the B site of perovskite-type catalysts.

The O₂ desorption profiles of three selected catalysts are shown in Figure 3. For La₂NiO₄, there exists one abrupt desorption peak of oxygen, which is β oxygen corresponding to T > 790ºC, while α and β oxygen of LaSrNiO₄ are located in the regions of 220–450ºC and 520–925ºC, respectively. For Ce-improved catalyst, its desorption peak of β oxygen is between 510 and 805ºC. As presented in Figure 3, it is clear that α oxygen does not appear as significant desorption peaks for La₂NiO₄ and La_0.7Ce_0.3SrNiO₄; this result is different from previous studies (Zhao et al., 1996; Zhu et al., 2005). It is speculated that different preparing methods and conditions may change properties of catalysts. However, α oxygen is not mainly responsible for catalytic activity.

Figure 3. O₂-TPD profiles for La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄, respectively.

O₂-TPD results indicate that adsorption of oxygen for La₂NiO₄ is strong and not easy to desorb at temperatures lower than 790ºC. If compared with the modified catalysts, thisd clearly shows that the β oxygen peak of La₂NiO₄ is shifted to lower temperature with partial substitution at the A site. Obviously, physicochemical properties of La₂NiO₄ are improved due to Sr substitution. In addition, we can see that the desorption area of β oxygen decreases when the A site of LaSrNiO₄ is partially substituted with Ce, implying that the number of oxygen vacancies decreases due to Ce doping. Previous study suggests that existence of Ce in the frame of LaSrNiO₄ may be of the Ce⁴⁺ form. On the contrary, the β oxygen desorption area does not decrease due to the same oxidative state of La³⁺ and Ce³⁺ (Zhu et al., 2005).

However, the highest β oxygen peak of LaSrNiO₄ is shifted from 830 to 780ºC when Ce partially substitutes for the A site of LaSrNiO₄; that is, oxygen desorption from the Ce-substituted catalyst is easier than that from LaSrNiO₄. Therefore, the increasing order of the mobility of lattice oxygen in three perovskite-type catalysts tested is listed as La_0.7Ce_0.3SrNiO₄ > LaSrNiO₄ > La₂NiO₄. Possible enhancement of oxygen mobilization and desorption is discussed later for modified catalysts.

Activities of catalysts for N₂O decomposition

The catalytic N₂O decomposition activities of three perovskite-type catalysts prepared were first evaluated as a function of reaction temperature with He as the carrier gas. The reaction temperature was raised gradually from 300 to 700ºC. N₂O decomposition efficiencies achieved with La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄, respectively, are presented in Figure 4. The results indicate that the activities of three catalysts for N₂O decomposition increase monotonously with increasing temperature. At initial temperature of 300ºC, N₂O decomposition efficiencies achieved with La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ are only 6.8%, 3.4%, and 6.6%, respectively. As temperature is increased to 400ºC, activity of La₂NiO₄ is significantly higher than that of LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄, and their N₂O decomposition efficiencies are 88.7%, 44.3%, and 36.5%, respectively. At 600ºC, the N₂O decomposition efficiencies achieved with all three catalysts reached 100%, implying that the gap of N₂O decomposition efficiencies becomes smaller with increasing temperature. In addition, the efficiency curves are frequently characterized by two parameters, that is, T₅₀ and T₉₀. T₅₀ is defined as the temperature required to remove 50% of the pollutant, while T₉₀ is the temperature needed to achieve 90% removal. As shown in Table 3, T₅₀ for N₂O decomposition with La₂NiO₄ is 350ºC, while T₅₀ of LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ is 410ºC and 430ºC, respectively. The results indicate that La₂NiO₄ has the lowest T₅₀. Likewise, T₉₀ with La₂NiO₄ is also the lowest if compared with two other catalysts. The N₂O decomposition efficiencies achieved with three perovskite-type catalysts rank in the order of La₂NiO₄ > LaSrNiO₄ > La_0.7Ce_0.3SrNiO₄ for the same operating condition.

Figure 4. Activities of La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄ for N₂O decomposition at various temperatures ( = 0.1%, GHSV = 8,000 hr⁻¹, He as carrier gas).

Table 3. Temperature for 50% and 90% N₂O conversion

Catalyst	T50 (ºC)	T90 (ºC)
La2NiO4	350	410
LaSrNiO4	410	470
La0.7Ce0.3SrNiO4	430	480

O₂-TPD results show that desorption peak of β oxygen of La₂NiO₄ mainly appeared at temperatures higher than 790ºC. As previously mentioned, oxygen adsorption of La₂NiO₄ is strong and not easy to desorb at temperature lower than 790ºC. Inhibition of N₂O decomposition by oxygen can be explained by the reversible dissociative adsorption of oxygen, that is, the back reaction of eq 2, to block active sites of catalyst, or via molecular adsorption of oxygen, as proposed in reaction 6 (Kapteijn et al., 1996):

6

Therefore, La₂NiO₄ may have a shorter lifetime due to the poisoning with oxygen, although it shows excellent performance for N₂O removal, because active sites of La₂NiO₄ cannot regenerate if oxygen adsorbed does not desorb from catalyst lattice. In addition, as presented in Figure 3, it is clear that β oxygen peaks of LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ are located at lower temperature if compared with La₂NiO₄, implying that LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ have better mobility of oxygen and it is easy to adsorb oxygen from the lattice.

On the other hand, activity of LaSrNiO₄ is slightly higher than that of La_0.7Ce_0.3SrNiO₄ at 400–500ºC for N₂O decomposition. The areas of β oxygen of both catalysts indicate that the number of oxygen vacancies may decrease when Ce partially substitutes for the A site of LaSrNiO₄, as shown in Figure 4. However, N₂O decomposition efficiencies achieved with three catalysts selected are close to 100% at 500ºC. Alini et al. (2007) reported that N₂O decomposition efficiencies achieved with LaMn_0.7Cu_0.3O₃ and LaFe_0.7Cu_0.3O₃ are less than 10% and 38%, respectively, at 500ºC with the GHSV of 22,100 hr⁻¹. Dacquin et al. (2009) applied LaCoO₃ to decompose N₂O at 600ºC with a space velocity of 10,000 hr⁻¹, and indicate that N₂O decomposition efficiency is 50%. Kumar et al. (2012) studied the catalytic activity of Pr_0.8Ba_0.2O₃ for N₂O decomposition and reported that 92% of N₂O decomposition efficiency could be achieved at 500ºC, with a GHSV of 7,500 hr⁻¹.

Previous studies indicate that the perovskite-type catalysts have to be operated at a temperature higher than 700ºC for high N₂O decomposition (Kapteijn et al., 1996), Overall, three catalysts selected (La₂NiO₄, LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄) indeed show better performance for N₂O decomposition at 500–600ºC compared with the catalysts tested in literature.

Effect of Sr and Ce on activity and properties of La₂NiO₄

The information on ion valence of catalyst is important for better understanding the N₂O decomposition. As reported earlier, the La cation generally is trivalent (La³⁺) at the A site for perovskite-type catalysts, while Ni ion mainly presents as Ni²⁺ in La₂NiO₄. As La³⁺ at the A site is partially replaced by a lower valency cation (Sr²⁺), according to the principle of electroneutrality, the positive charge reduced could be balanced either via formation of higher oxidation state ion at the B site, that is, Ni²⁺ → Ni³⁺, or via formation of an oxygen vacancy (V_o). Thus, the mixed oxidation state of Ni and the presence of oxygen vacancy in LaSrNiO₄ could be explained by reactions 7 and 8 (Zhao et al., 1996):

7

8

where V_o indicates oxygen vacancies, asterisks are cation holes, apostrophes are anion electrons, and λ is an oxygen defect. Thus, as a low valence cation (Sr²⁺) occupied the frame at the La³⁺ site, the oxidative state of Ni as well as the number of oxygen vacancy would increase to satisfy the electroneutrality.

On the other hand, valence of Ni cannot be measured for La_0.7Ce_0.3SrNiO₄, because valence of Ce is alterable (trivalent and tetravalent). As mentioned earlier, valence of Ni for La₂NiO₄ is mainly Ni²⁺, while it presents as Ni³⁺ in LaSrNiO₄. From the formula of LaSrNiO₄, La_1−xCe_xSrNiO₄, and La₂NiO₄ (according to the valence of Sr²⁺ and Ce⁴⁺), the amount of Ni³⁺ for La_1−xCe_xSrNiO₄ is between those of LaSrNiO₄ and La₂NiO₄. Hence, it is clear that Ni³⁺ amount of LaSrNiO₄ decreases as the high-valence cation (Ce⁴⁺) substitutes with the frame of La³⁺ of LaSrNiO₄. Ladavos and Pornonis (1991) plotted the reaction rate per unit area as a function of the content of Ni²⁺ to Ni³⁺, and indicate that the maximum activity appears in two cases, with one appearing at a low ratio of Ni²⁺ and Ni³⁺, implying that Ni exists in the Ni³⁺ state. In this case, the electronic configuration of the Ni ion (Ni³⁺, d⁷) should favor reaction. The other one is when the ratio of Ni²⁺ and Ni³⁺ is close to 1, and in such conditions, the catalyst possesses a higher percentage of “Ni²⁺-[ ]-Ni³⁺” pairs, which are active sites. This favors the occurrence of the surface reaction, resulting in higher activity (Ladavos and Pornonis, 1991).

In this study, Ce in the frame of LaSrNiO₄ is of the Ce⁴⁺ form. Therefore, it decreases the content of Ni³⁺; furthermore, the ratio of Ni²⁺ and Ni³⁺ may be close to 1 by changing Ce⁴⁺. Generally, the active sites of perovskite-type catalysts modified are composed of oxygen vacancy for N₂O decomposition, described as “Ni²⁺-[ ]-Ni³⁺”. However, for the La_0.7Ce_0.3SrNiO₄ case, there may be a new complex active site designated as Ce³⁺-(O)-(Ni²⁺-[ ]-Ni³⁺) with the addition of Ce, of which the valence is alterable (Ce⁴⁺ ⇔ Ce³⁺). Hence, higher activity can be expected as the La site is replaced by the higher valency cation (Ce). Thus, the Ce in the La site would take part in the reaction and is one part of the active sites. In this study, the participation of Ce in N₂O decomposition reaction may be described as (Zhu et al., 2005):

9

10

To summarize, LaSrNiO₄ may give rise to two actions simultaneously as Ce substitutes for the La site. First, it can decrease the number of oxygen vacancies, and second it can improve the catalytic activity for N₂O decomposition. This seems contradictory. However, Ce⁴⁺ has higher Pauling electronegativity, so it has stronger force to draw the electron from “Ni³⁺-O-Ni³⁺” (Zhu et al., 2005); hence, it can easily desorb oxygen to form “Ni³⁺-[ ]-Ni²⁺” and enhance the regeneration of active sites.

The results just described indicate that properties of La₂NiO₄ are significantly improved by doping with Sr and Ce; hence, the modified catalysts (LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄) are mainly applied for the follow-up experimental tests.

Influence of space velocity on N₂O decomposition

Figure 5 shows the effects of contact time on N₂O decomposition achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄, respectively. The results indicate that decomposition efficiency of N₂O decreased slightly with increasing space velocity at 300–500ºC for both catalysts. Since the BET surface areas of LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ are only 5.0 and 5.2 m²/g, respectively, N₂O decomposition reaction with a larger space velocity might be reduced due to diffusion limitations of reactant to the active site. However, N₂O decomposition efficiencies achieved with both catalysts reached 100% at 600ºC, implying that activities of both catalysts are relatively high at T ≥ 600ºC. According to the reaction kinetics, the reaction rate increases with increasing temperature, because kinetic energy of pollutant molecule is enhanced as temperature is increased; furthermore, this can promote the chance of pollutant collision with catalyst.

Figure 5. Influences of gas residence time on activities of LaSrNiO₄, and La_0.7Ce_0.3SrNiO₄, respectively, at various temperatures ( = 0.1%, He as carrier gas).

Overall, N₂O decomposition efficiency achieved with LaSrNiO₄ is affected slightly by decreasing residence time at temperatures less than 600ºC compared with La_0.7Ce_0.3SrNiO₄.

Influence of O₂ and water vapor on N₂O decomposition activity

Generally, oxygen and water vapor exist in most flue gases, and their influences should be considered for field application. A catalyst is successfully evolved if it can effectively tolerate oxygen and water vapor. However, the suppression of N₂O decomposition by oxygen remains one of the important obstacles to overcome in the effective N₂O decomposition. Inhibition of catalytic activity by oxygen has been described in the previous section. However, as the oxygen removal from the catalyst surface is slow, oxygen inhibition may be significant (Hussain et al., 2014). The effects of oxygen content on the N₂O decomposition over LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄, respectively, are presented in Figure 6. With 1–5% oxygen in the gas stream, the efficiencies of N₂O decomposition decrease slightly for both catalysts if compared with the case without oxygen at low temperatures (300–500ºC). For LaSrNiO₄, N₂O decomposition efficiency decreases from 44% to 27% at 400ºC when 1% oxygen is introduced into the gas stream, while activity of La_0.7Ce_0.3SrNiO₄ decreases from 37% to 27%. As oxygen content is further increased to 5%, N₂O decomposition efficiencies achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ decrease to 22 and 20%, respectively. However, influence of O₂ content gradually decreases with increasing temperature for both catalysts, and the N₂O decomposition efficiencies achieved with both catalysts even reached 100% at 600ºC. The same trends also appear in the space velocity test, implying that both catalysts show excellent performance for N₂O decomposition when operated at a temperature higher than 600ºC. On the other hand, activities of both catalysts for N₂O decomposition would be substantially affected by oxygen at 400ºC.

Figure 6. Effects of oxygen contents on N₂O decomposition at various temperatures ( = 0.1%, = 1–5%, GHSV = 8,000 hr⁻¹, He as carrier gas).

In order to investigate the effect of water vapor on N₂O decomposition, 5 and 10% H₂O_(g) are introduced into system, respectively. Figure 7 shows the effect of 5 and 10% H₂O_(g) on N₂O decomposition achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO_4, respectively, at 600ºC. The results indicate that N₂O decomposition efficiencies decrease from 100% to 94 and 95% as 5% H₂O_(g) is incorporated for LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄, respectively. As H₂O_(g) is further increased to 10%, the N₂O decomposition efficiencies achieved with LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄ decrease to 86% and 88%, respectively. It is clear that activities of both catalysts suffer from H₂O_(g). Tofan et al. (2002b) suggested that H₂O_(g) is easily adsorbed on all metal oxide catalysts, including perovskite-type catalysts used in this study, and H₂O_(g) adsorbed on catalyst surface causes a reduction of active sites for N₂O decomposition. Another possibility is that the existence of H₂O_(g) may induce hydroxylation on catalyst surface and reduce active site density. However, the N₂O decomposition efficiency achieved with LaSrNiO₄ still maintains at 86%, while La_0.7Ce_0.3SrNiO₄ maintains close to 90% at 10% H₂O_(g). Overall, both catalysts show good tolerance for H₂O_(g) when operated at 600ºC.

Figure 7. Effects of H₂O_(g) contents on N₂O decomposition at 600ºC ( = 0.1%, = 5–10%, GHSV = 8,000 hr⁻¹, He as carrier gas).

Product analysis

Table 4 summarizes N₂, NO, and NO₂ yields for three selected catalysts. The results indicate that NO and NO₂ are not produced during N₂O decomposition, and it demonstrates that N₂O is purely decomposed into N₂ and O₂. N₂ yield clearly depends on the temperature. Among the three catalysts, La₂NiO₄ shows the best N₂O decomposition efficiency (88%) and the highest N₂ yield (72%) at 400ºC. As temperature is increased to 700ºC, N₂ yield achieved with La₂NiO₄ reached 90%. However, as shown in Figure 3, the oxygen adsorbed by La₂NiO₄ is not easy to desorb; therefore, it may have a short lifetime, and its activity can also suffer from O₂ seriously. On the other hand, N₂ yield with LaSrNiO₄ increases from 28% to 92% as temperature is increased from 400 to 700ºC, while N₂ yield is increased from 30% to 100% as Ce is added into LaSrNiO₄. Indeed, the activity of LaSrNiO₄ at high temperatures can be improved by doping with Ce. Since La_0.7Ce_0.3SrNiO₄ possesses better mobility of lattice oxygen, it promotes desorption of O₂ and regeneration of the active sites.

Table 4. Products analysis and efficiencies of N₂O decomposition achieved with three catalysts at various temperatures

Catalyst	Reaction temperature (°C)	N2O decomposition (%)	Yield (%)			with 5 % O2	with 5% H2O(g)
			N2	NO	NO2	N2	N2
La2NiO4	400	88	72	0	0	—	—
	500	100	80	0	0	—	—
	600	100	84	0	0	—	—
	700	100	90	0	0	—	—
LaSrNiO4	400	44	28	0	0	18	—
	500	98	69	0	0	68	—
	600	100	82	0	0	75	80
	700	100	92	0	0	79	—
La0.7Ce0.3SrNiO4	400	36	30	0	0	28	—
	500	97	80	0	0	79	—
	600	100	93	0	0	90	89
	700	100	100	0	0	99	—

Notes: —: not available

N₂O: 0.1%; He: balance; GHSV = 8,000 h⁻¹

Furthermore, with 5% oxygen in the gas stream, N₂ yields are 28–99% at 400–700ºC for Ce improved catalyst, implying that it is not substantially affected by oxygen content when operated at 700ºC. In addition, N₂ yields for both catalysts (LaSrNiO₄ and La_0.7Ce_0.3SrNiO₄) are only slightly reduced (82% to 80% and 93% to 89%, respectively) as 5% H₂O_(g) is introduced into the gas streams.

Durability test

The durability of catalysts is important in determining their practical usefulness. Figure 8 indicates that N₂O decomposition efficiency achieved with La_0.7Ce_0.3SrNiO₄ maintains 100% at 600ºC, and no significant deactivation is observed for continuous operation for 60 hr, while N₂ yield is between 90 and 94% for the durability test.

Figure 8. Durability test on the La_0.7Ce_0.3SrNiO₄ catalyst for N₂O decomposition ( = 0.1%, GHSV = 8,000 hr⁻¹, reaction temperature = 600ºC, He as carrier gas).

Conclusion

Development of an effective N₂O control technology is essential to reduce the environmental impact associated with N₂O. In this study, we not only evaluate the effects of various parameters on N₂O decomposition, but also explain oxygen mobility in bulk by doping various cations. Additionally, yields of N₂ with different catalysts are also investigated.

The N₂O decomposition efficiencies achieved with three perovskite-type catalysts rank in the order La₂NiO₄ > LaSrNiO₄ > La_0.7Ce_0.3SrNiO₄ for lower temperature operating conditions. However, La₂NiO₄ may have a shorter lifetime due to poisoning by oxygen. Indeed, physicochemical properties of La₂NiO₄ for N₂O decomposition are significantly improved by partially substituting La with Sr. The mobilities of lattice oxygen with three perovskite-type catalysts are in the order La_0.7Ce_0.3SrNiO₄> LaSrNiO₄ >La₂NiO₄. Additionally, activity of LaSrNiO₄ can be improved by doping Ce for N₂ yield. Compared to previous studies using other compounds to dope La, reasonable N₂O decomposition efficiencies (97–100%) are measured at lower temperatures (500-600ºC) for these three catalysts.

Moreover, contents of oxygen and water vapor are introduced into the reaction system to evaluate the effects, and La_0.7Ce_0.3SrNiO₄ shows better tolerance for oxygen and water vapor, implying that the catalyst has a new highly active site Ce³⁺-(O)-(Ni²⁺-[ ]-Ni³⁺) due to the addition of Ce, which may facilitate oxygen mobilization and desorption at low temperature (˜300ºC). However, more studies are needed to further enhance the activity of these catalysts and to improve their tolerance to the complex composition for industrial application.

Additional information

Notes on contributors

Kuan Lun Pan

Kuan Lun Pan is a Ph.D. student and Moo Been Chang is a distinguished professor at the Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan.

Sheng Jen Yu

Sheng Jen Yu and Shaw Yi Yan are researchers at the Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan.

Shaw Yi Yan

Sheng Jen Yu and Shaw Yi Yan are researchers at the Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan.

Moo Been Chang

Kuan Lun Pan is a Ph.D. student and Moo Been Chang is a distinguished professor at the Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan.