Enhancement of nitric oxide decomposition efficiency achieved with lanthanum-based perovskite-type catalyst

ABSTRACT

Direct decompositions of nitric oxide (NO) by La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are experimentally investigated, and the catalysts are tested with different operating parameters to evaluate their activities. Experimental results indicate that the physical and chemical properties of La_0.7Ce_0.3SrNiO₄ are significantly improved by doping with Ba and partial substitution with Pr. NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are 32% and 68%, respectively, at 400 °C with He as carrier gas. As the temperature is increased to 600 °C, NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, reach 100% with the inlet NO concentration of 1000 ppm while the space velocity is fixed at 8000 hr⁻¹. Effects of O₂, H₂O_(g), and CO₂ contents and space velocity on NO decomposition are also explored. The results indicate that NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, are slightly reduced as space velocity is increased from 8000 to 20,000 hr⁻¹ at 500 °C. In addition, the activities of both catalysts (La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄) for NO decomposition are slightly reduced in the presence of 5% O₂, 5% CO₂, or 5% H₂O_(g). For durability test, with the space velocity of 8000 hr⁻¹ and operating temperature of 600 °C, high N₂ yield is maintained throughout the durability test of 60 hr, revealing the long-term stability of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ for NO decomposition. Overall, Pr_0.4Ba_0.4Ce_0.2SrNiO₄ shows good catalytic activity for NO decomposition.

Implications: Nitrous oxide (NO) not only causes adverse environmental effects such as acid rain, photochemical smog, and deterioration of visibility and water quality, but also harms human lungs and respiratory system. Pervoskite-type catalysts, including La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, are applied for direct NO decomposition. The results show that NO decomposition can be enhanced as La_0.7Ce_0.3SrNiO₄ is substituted with Ba and/or Pr. At 600 °C, NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ reach 100%, demonstrating high activity and good potential for direct NO decomposition. Effects of O₂, H₂O_(g), and CO₂ contents on catalytic activities are also evaluated and discussed.

Introduction

Oxides of nitrogen (NO_x) have been regarded as one of the most serious air pollutants, and their emission has received much public concern. They not only cause adverse environmental effects such as acid rain, photochemical smog, and deterioration of visibility and water quality, but also harm human lungs and respiratory system (Bowker, 2008). Hence, how to effectively reduce NO_x emission at a reasonable cost has become an emerging issue. NO_x originate mainly from combustion processes, including mobile and stationary sources. Traditional technologies for effective removal of NO_x can be generally divided into precombustion treatment, combustion modification, and postcombustion treatment (Wood, 1994). As a postcombustion treatment, selective catalytic reduction (SCR) has the highest nitric oxide (NO) removal efficiency, which is the most effective technique for removing NO from gas streams. However, SCR must rely on the injection of large amount of reducing agent such as ammonia (NH₃), which is costly and may cause secondary pollution. Alternatively, direct NO_x decomposition with catalysts is of high interest, since no additional reactant is required and the products are exclusively composed of N₂ and O₂.

Some catalysts, including noble metals (Sugisawa et al., 2001; Pietraszek et al., 2007; Wu et al., 2013), metal oxide catalysts (Teraoka et al., 1998; Silveira et al., 2008; Hong et al., 2011), and zeolite-based catalysts (Iwamoto and Hamada, 1991; Iwamoto, 1996; Lisi et al., 2009), have been investigated for direct NO decomposition. However, several disadvantages associated with these catalysts need to be overcome. For example, most zeolite-based catalysts are poisoned to some extent by water vapor (H₂O_(g)) or sulfur-containing gases; also, they are of low hydrothermal stability (Van den Brink et al., 2001). Therefore, they are difficult to apply in industry, although they may have high activities at low temperatures, whereas NO_x decomposition efficiency and yield of N₂ achieved with metal oxide catalysts are typically low (Hong et al., 2011). Noble metal catalysts, including Pt, Pd, Rh, and Au, are of high activity for NO_x decomposition; however, they are expensive (National Association of Securities Dealers Automated Quotations System [NASDAQ], 2014). On the other hand, previous studies indicate that perovskite-type catalysts have the potential to replace noble metal catalysts for NO_x removal (Voorhoeve et al., 1972; Khan and Frey, 1993) due to their high activities in decomposing NO_x into N₂ and O₂ (Ishihara et al., 2003; Zhang et al., 2006; Russo et al., 2007; Zhu et al 2009; Zhao et al., 2009; Ivanov et al., 2011; Wang et al., 2012).

Generally, the mechanism of NO decomposition over perovskite-type catalysts can be described by eqs 1–3 (Teraoka et al., 1998). First, NO molecules are adsorbed on oxygen vacancies (active sites of catalyst) at the catalyst surface, and two adjacent N–O bonds can be dissociated to produce a N₂, which is subsequently released into the gas stream from the catalyst surface (Teraoka et al., 1998). Previous study indicates that adsorption of the second NO molecule on the pair site (eq 2) is the rate-determining step (Imanaka and Masui, 2012); hence, NO decomposition would depend on NO adsorption potential and the number of vacancies available.

(1)

(2)

(3)

where O is an oxygen atom and V_o is a surface oxygen vacant site.

Perovskite-type catalyst is of the general form ABO₃ or A₂BO₄, where A can be either an alkaline metal or rare earth element, typically La is applied. On the other hand, 3d transition metal with a relatively small ion such as Cu, Fe, Co, Ni, or Mn (Pérez-Ramírez et al., 2003) is generally applied at the B site. The number of vacancies available depends on the B site, and it directly influences catalyst activity. Also, catalytic performance can be enhanced by partial substitution of the A site, because it may lead to formation of lattice vacancy. Further, lattice vacancy increases mobility of oxygen. Therefore, physical and chemical properties of perovskite-type catalysts may vary by substituting different elements at the A and/or B sites.

ABO₃-type and A₂BO₄-type catalysts such as LaMO₃ (M = Fe, Mn, and Ni) (Wu et al., 2013; Zhang et al., 2006; Wang et al., 2012) and La₂MO₄ (M = Ni, Cu) (Zhao et al., 1996; Gao et al., 2008) have been studied for NO decomposition. Catalysts with rather complex composition are synthesized by partial substitution at A-site and/or B-site to improve the activity. For example, La_1-xA_xM_1-yB_yO₃ (A = Ba, Th; M = Cu, Cr, Fe, Mn; and B = Ga, In) (Ishihara et al., 2003; Zhu et al., 2006) and La_1−xSr_xNiO₄ (Zhao et al., 1996) have been evaluated for the effectiveness in direct NO decomposition. In another study, La_1−xCe_xSrNiO₄ is supported on MgO to enhance NO decomposition (Zhu et al., 2006).

As high as 100% NO decomposition efficiency could be achieved with perovskite-type catalysts; however, they need to be operated at high temperatures (>700 °C) for a high NO decomposition efficiency. Additionally, NO decomposition is strongly inhibited by the presence of oxygen (O₂) and carbon dioxide (CO₂). For example, the N₂ yield achieved with La_0.2Ba_0.8Mn_0.8Mg_0.2O₃ reaches 85% in the absence of CO₂ at 850 °C, but it decreases dramatically to below 20% in the presence of 5% CO₂ (Iwakuni, 2008). Furthermore, activity of perovskite-type catalysts may be influenced by the presence of H₂O_(g) (Tofan et al., 2002). Since CO₂, O₂, and H₂O_(g) commonly exist in industrial flue gases, tolerances of CO₂, O₂, and H₂O_(g) should be considered for field application.

With respect to the enhancement of perovskite-type catalyst activity, several methods have been proposed. For instance, La at the A site was partially substituted by Sr to improve the activity because the oxide vacancies can be produced by charge compensation as a low-valence cation (Sr²⁺) occupies the frame at the La³⁺ site (Zhao et al., 1996; Zhu et al., 2005). Previous study suggests that nonstoichiometric perovskites (La_1−xSr_xM_1−yM_yO_3−δ) own several advantages, such as fast electron transfer, high oxygen mobility, and easy oxygen desorption (Tabata and Misono, 1990). Also, the activity of perovskite-type catalysts can be enhanced when A is partially substituted with Ce. Ce possesses good properties such as oxygen storing and scavenging, resulting in easy regeneration of active sites. Kirchnerova et al. (2002) show that the activity of LaCoO₃ is significantly improved if doped with CeO₂ (Kirchnerova et al., 2002). Belessi et al. (2001) show that the NO decomposition rate achieved with La_1−x−ySr_xCe_yFeO₃ can be improved by increasing CeO₂ content (Belessi et al., 2001). Nitadori and Misono (1985) applied Sr, Ce, and Th to substitute LaMO₃ (M = Mn, Fe, Co) and show that Ce-substituted catalysts increase the oxygen mobility (Nitadori and Misono, 1985). Iwakuni et al. (2007) showed that the lattice of A₂BO₄-type catalyst could accommodate more cerium ions due to larger cavity space; therefore, redox effect of A₂BO₄-type perovskite catalysts is more prominent than that of ABO₃-type (Iwakuni et al., 2007).

Ba possesses a high basicity, and it has been applied to partially substitute the A site of perovskite-type catalyst for enhancing NO decomposition (Iwakuni, 2008; Iwakuni et al., 2007). LaMnO₃ doped with Ba also exhibits a higher activity for direct NO decomposition at over 800 °C (Khan and Frey, 1993). Praseodymium (Pr) is one of important rare earth elements for preparing perovskite-type catalyst, with its electron distribution and structural characteristics being similar to light rare earth elements such as lanthanum (La) and cerium (Ce). Recently, Pr-based perovskites have been prepared and investigated for their catalytic properties. Ran et al. (2005) applied Pr_0.8Ce_0.2MnO₃ to decompose NO, and the results indicate that NO decomposition efficiency achieved with Pr_0.8Ce_0.2MnO₃ reaches 90% at 650 °C (Ran et al., 2005). However, application of Pr-based perovskites-type catalyst is limited, especially for NO decomposition.

In this study, A₂BO₄-type perovskite catalyst (La_0.7Ce_0.3SrNiO₄) is prepared and evaluated for the effectiveness in direct NO decomposition. Nickel (Ni) is frequently applied due to a variety of oxygen-involving reactions (Kirchnerova and Klvana, 1994; Klvana et al., 1994). The A site of La_0.7Ce_0.3SrNiO₄ is partially substituted by doping with Ba to prepare La_0.4Ba_0.4Ce_0.2SrNiO₄ catalyst, and then the La site is further substituted by Pr to prepare Pr_0.4Ba_0.4Ce_0.2SrNiO₄. It is expected that the properties and activities of the catalysts can be improved to further reduce the reaction temperature needed for NO decomposition. Influences of space velocity, O₂, H₂O_(g), and CO₂ on NO decomposition are also evaluated with a laboratory-scale experimental system.

Experimental

Catalyst preparation

A₂BO₄-type perovskite catalysts, including La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, were prepared by the Pechini method (Pechini, 1967). The corresponding metal nitrates were used as starting material for preparing aqueous solution with appropriate stoichiometry. Citric acid was added into the mixed aqueous solution of metal nitrates in a designated proportion so that the molar ratio of citric acid to the total metal cations was 4:1. Ethylene glycol was then added into the mixed aqueous solution to control the molar ratio of citric acid to the ethylene glycol as 1:1. The mixed aqueous solution was then heated and stirred to evaporate the water. Thereafter, the residual solid was dried at 140 °C for 24 hr, followed by heat treatment at 600 °C for 1 hr in air, and finally calcination in air at 950 °C for 6 hr, and then manually milled to powders by using an agate mortar and pestle. Subsequently, the synthesized particles were pulverized to the final 100 mesh size and used as such.

Characterization of the catalysts prepared

Brunauer-Emmett-Teller (BET) surface areas of the catalysts prepared were measured by using N₂ adsorption at −196 °C with a Micromeritics ASAP 2010 analyzer (Micromeritics, Norcross, GA, USA). Scanning electron microscopy (SEM) was utilized to identify the morphology and structure of catalysts, and powder X-ray diffraction (XRD) data were obtained using an X-ray diffractometer (model D8AXRD; Bruker, Germany) that was operated at 40 kV and 10 mA by using Cu-Kα radiation combined with a nickel filter. Diffraction patterns were obtained within a 2θ rang of 10°–80° at a scanning rate of 6° min⁻¹. Additionally, oxygen mobility of the catalysts prepared was evaluated via temperature-programmed desorption of oxygen (O₂-TPD), which was carried out by using thermal conductivity detector (TCD), in which the sample (0.2 g) was treated at 1000 °C in the presence of O₂ for 1 hr. Subsequently, it was cooled down to the room temperature and swept with helium (He) at a rate of 11.8 mL/min. Finally, the sample was heated with a rate of 20 °C/min in He and the data profile was recorded.

Catalytic activity measurement

Direct NO decomposition test was performed with a fixed-bed quartz reactor (inner diameter [i.d.] 20 mm) containing 4 g of catalyst. The gas stream containing 1000 ppm NO with He as carrier was introduced into the reactor continuously. The total gas flow rate was fixed at 628 mL/min, corresponding to a gas hourly space velocity (GHSV) of 8000 hr⁻¹. Also, two space velocities (15,000 and 20,000 hr⁻¹) were tested to evaluate the space velocity effect. As mentioned previously, O₂, H₂O_(g), and CO₂ commonly exist in real flue gas; therefore, effects of O₂, H₂O_(g), and CO₂ on the activities of catalysts for direct NO decomposition are also investigated. Contents of O₂, H₂O_(g), and CO₂ of the feeding gas stream were controlled within the ranges of 1–5%, 5–10%, and 5%, respectively. Sources of NO, O₂, and CO₂ were from gas cylinders, and designated NO, O₂, and CO₂ concentrations could be obtained by adjusting the flow rate of individual gas stream. In addition, a peristaltic pump was used to inject H₂O_(l) into the gas stream to control the H₂O_(g) content. Mass-flow controllers (MFCs) were utilized to adjust and control the gas flow rates, and all gases were well mixed before reaching the reactor. The operating temperature was varied from 300 to 900 °C to evaluate the temperature effect. NO concentrations were analyzed before and after the reaction by using Fourier transform infrared spectrophotometer (Nicolet 6700; Thermo Scientific, USA) and NO_x detector (Testo 350; Testo, Germany), whereas analysis of N₂ was conducted using gas chromatography (model 6890N; Agilent Technologies, USA) with a TCD and a column packed with 5A molecular sieve (capillary: 30.0 m × 530 μm × 25 μm nominal). All experimental data were taken after ca. 3–4 hr on stream at each temperature, while the catalytic reaction practically reached a steady state. Decomposition efficiency of NO and yield of N₂ were calculated by eqs 4 and 5, respectively:

(4)

(5)

where and are NO concentrations measured before and after the reactor, respectively, and is the N₂ concentration measured at reactor outlet.

Results and discussion

Characterization of the catalysts

The XRD patterns of the three catalysts prepared are presented in Figure 1, indicating that all catalysts prepared are of perovskite-like structure (A₂BO₄-type). As shown in Figure 1, La_0.7Ce_0.3SrNiO₄ diffraction peaks are attributed to crystalline of LaSrNiO₄, but there exists one abrupt peak at 2θ ~ 28°, which could be attributed to CeO₂ according to XRD database. However, the characteristic peaks of CeO₂ should include 2θ = 28.5°, 33.0°, 47.6°, 56.4°, 58°, and 69.5°, respectively. In this study, characteristic peaks of CeO₂ barely appear at 2θ = 28°, 47°, 56°, 58°, whereas other diffraction peaks are mainly assigned to LaSrNiO₄. Previous study indicates that diffraction peak at 2θ ~ 28° might imply La_1−xCe_xSrNiO₄ (Zhu et al., 2005). This result demonstrates that Ce occupies the La site without destroying the matrix structure. In addition, La_0.4Ba_0.4Ce_0.2SrNiO₄ shows strong main diffraction peaks that are attributed to those from LaSrNiO₄. Some peaks are also observed in the XRD pattern of La_0.4Ba_0.4Ce_0.2SrNiO₄, and they belong to La₂NiO₄ and BaO, respectively, Crystalline of BaO formed due to excess addition of Ba element, although the lattice of A₂BO₄-type catalyst has larger cavity space to accommodate other ions. On the other hand, characteristic peaks of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are mainly assigned to Pr₂NiO₄. Clearly, crystalline of Pr_1-xM_xNiO₄-like is not detected. Possibly, Sr is incorporated into the lattice of Pr-based catalyst to form a good solid solution, and additional peaks of Pr₂O₃ and BaO are observed in the XRD pattern of the Pr-based catalyst. In addition, characteristic peaks of CeO₂ are not observed for La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ due to addition of low content of Ce.

Figure 1. X-ray diffraction patterns of the catalysts (Δ:Pr₂NiO₄, La₂NiO₄, ○: LaSrNiO_4, ●:CeO₂, ♦:Pr₂O₃, ◊:BaO).

The BET surface areas of La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are 5.2, 25.5, and 29.9 m²/g (see Table 1), respectively, indicating that La_0.7Ce_0.3SrNiO₄ has the smallest BET surface area. Generally, BET surface areas of perovskite-type catalysts are in the range of 5–50 m²/g. However, the BET surface area is significantly increased by the addition of Ba and substitution of Pr. Figure 2 displays the SEM patterns of all three catalysts, including three catalysts present as pellet with slight agglomeration. Obviously, the morphology of Pr-based catalysts is more compact and uniform when compared with La-based catalysts.

Table 1. BET surface areas of the catalysts prepared.

Catalyst	BET (m^2/g)
La0.7Ce0.3SrNiO4	5.2
La0.4Ba0.4Ce0.2SrNiO4	25.5
Pr0.4Ba0.4Ce0.2SrNiO4	29.9

Figure 2. SEM patterns of the catalysts (a) La_0.7Ce_0.3SrNiO₄, (b) La_0.4Ba_0.4Ce_0.2SrNiO₄, and (c) Pr_0.4Ba_0.4Ce_0.2SrNiO₄.

O₂ temperature-programmed desorption (O₂-TPD)

The role of perovskite-type oxides for NO decomposition can be explored by O₂-TPD analysis because it provides the information on oxygen nonstoichiometry of substituted perovskite (Kumar et al., 2012). Typically, two kinds of oxygen desorption peaks are observed in the O₂-TPD process and can be denoted as α oxygen and β oxygen, respectively. The peak of α oxygen appears at a relatively lower temperature and generally it is weaker when compared with the peak of β oxygen.

Peaks of α oxygen and β oxygen vary with different perovskite-type catalysts, implying that their peaks may appear at lower or higher temperatures, sometimes peaks of α oxygen are not even observed in the O₂-TPD profile. As previously reported, α oxygen is chemically desorbed from the surface whereas β oxygen is desorbed from the oxygen of vacancy and lattice. Hence, the intensity of β oxygen can be an indicator of catalytic performance (Ferri and Forni, 1998; Belessi et al., 2000). The O₂ desorption profiles of the three catalysts prepared are shown in Figure 3. For La_0.7Ce_0.3SrNiO₄, there exists one abrupt desorption peak of oxygen, which is β oxygen corresponding to the temperature of 590–810 °C, whereas β oxygen of La_0.4Ba_0.4Ce_0.2SrNiO₄ is located in the region of 550–820 °C. For Pr-based catalyst, the desorption peak of β oxygen is found at 450–780 °C. As presented in Figure 3, α oxygen does not appear as significant desorption peaks for the three catalysts prepared.

Figure 3. O₂-TPD profiles for La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Cec_0.2SrNiO₄, respectively.

The O₂-TPD results indicate that La_0.7Ce_0.3SrNiO₄ has a smaller area of β oxygen desorption when compared with the modified catalysts. Previous study indicates that area of β oxygen desorption is related to the number of oxygen vacancies. In other words, a perovskite-type catalyst would have excellent performance for NO decomposition if it has a wide area of β oxygen desorption (Zhu et al., 2005). As shown in Figure 3, the highest peak for La_0.7Ce_0.3SrNiO₄ is located at 790 °C, whereas those of La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are located at 670 and 590 °C, respectively. The O₂-TPD results show that β oxygen peaks are shifted to lower temperature with partial substitution at the A site. Presumably, physical and chemical properties of La_0.7Ce_0.3SrNiO₄ are improved due to Ba substitution. In addition, the highest β oxygen peak of La_0.4Ba_0.4Ce_0.2SrNiO₄ is shifted from 670 to 590 °C when Pr is substituted for the A site of La_0.4Ba_0.4Ce_0.2SrNiO₄. The order of the mobility of lattice oxygen in the three perovskite-type catalysts tested is Pr_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.7Ce_0.3SrNiO₄.

Activities of catalysts for NO decomposition

The catalytic activities of the three perovskite-type catalysts prepared were tested at various temperatures (300–900 °C) to evaluate the effect on NO decomposition. NO decomposition efficiencies achieved with La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, are presented in Figure 4. The results indicate that the activities of the three catalysts for NO decomposition all increase with increasing temperature. At initial temperature of 300 °C, NO decomposition efficiencies achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and La_0.7Ce_0.3SrNiO₄ are 24.5%, 17.2%, and 2.0%, respectively. The activity of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ is significantly higher than that of La_0.7Ce_0.3SrNiO₄ and La_0.4Ba_0.4Ce_0.2SrNiO₄. NO decomposition efficiencies achieved with the three catalysts at 400 °C are 68.3%, 32.6%, and 10.2%, respectively. As the temperature is further increased to 600 °C, the NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, reach 100%. On the other hand, La_0.7Ce_0.3SrNiO₄ has to be operated at 700 °C to achieve 100% NO decomposition efficiency. The reaction rates per surface area for the three catalysts operated at 600 °C are presented in Table 2. The result indicates that Pr_0.4Ba_0.4Ce_0.2SrNiO₄ has the highest value (6.58 × 10⁻⁵ mg/L·min·m²) when compared with La_0.4Ba_0.4Ce_0.2SrNiO₄ and La_0.7Ce_0.3SrNiO₄ (5.33 × 10⁻⁵ and 2.02 × 10⁻⁵ mg/L·min·m²).

Table 2. Catalytic activities (the reaction rate) per surface area for the three catalysts (T = 500 °C).

Catalyst	Reaction Rate/Surface Area (mg/L·min·m^2)
Pr0.4Ba0.4Ce0.2SrNiO4	6.58 × 10^−5
La0.4Ba0.4Ce0.2SrNiO4	5.33 × 10^−5
La0.7Ce0.3SrNiO4	2.02 × 10^−5

Figure 4. Activities of La_0.7Ce_0.3SrNiO₄, La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ for NO decomposition at different temperatures with He as the carrier gas (C_NO = 0.1%, GHSV = 8000 hr⁻¹).

In addition, the efficiency curves are frequently characterized by two parameters, i.e., T₅₀ and T₉₀. T₅₀ is defined as the temperature required to convert 50% of the pollutant, whereas T₉₀ is the temperature needed to achieve 90% removal. As shown in Table 3, T₅₀ for NO decomposition with La_0.7Ce_0.3SrNiO₄ is 480 °C, whereas T₅₀ values of La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are 450 and 350 °C, respectively. The results indicate that Pr_0.4Ba_0.4Ce_0.2SrNiO₄ has the lowest T₅₀. Likewise, T₉₀ with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ is also the lowest when compared with the other two catalysts, indicating that the performance of the three perovskite-type catalysts for NO decomposition can be ranked in the order of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.7Ce_0.3SrNiO₄ at same operating conditions. Accordingly, the modified perovskite-type catalysts have more active sites for NO decomposition.

Table 3. Temperatures for 50% and 90% NO decomposition.

Catalyst	T50 (°C)	T90 (°C)
La0.7Ce0.3SrNiO4	480	625
La0.4Ba0.4Ce0.2SrNiO4	435	545
Pr0.4Ba0.4Ce0.2SrNiO4	350	475

Previous study indicates that activity of La_0.7Ce_0.3SrNiO₄ is mainly from a complex active site described as Ce³⁺-(O)-(Ni²⁺-[]-Ni³⁺), in which valence of Ce is alterable as Ce⁴⁺ ↔ Ce³⁺ during reaction (Zhu et al., 2005). This redox reaction promotes regeneration of active sites. Additionally, basicity of catalyst surface is an important factor affecting the chemical reactivity of NO, and the catalyst with a high basicity generally has excellent performance for NO decomposition (Zhang et al., 2006). Due to its high basicity, Ba is frequently applied to prepare NO reduction catalysts, such as TWCs (three-way catalysts) and NSR (NO_x storage and reduction) catalysts. Since NO is acidic, adsorption of NO can be significantly enhanced by increasing the basicity of catalyst. Indeed, activity of La_0.7Ce_0.3SrNiO₄ is improved as the A site is partly substituted with Ba. In addition, addition of Ba may also produce oxygen vacancies by charge compensation (generally, Ba is bivalent). Ce promotes the generation of active sites, without destroying the structure of the lattice, and forms a complementary system. Previous study indicates that the activity of Pr-based catalyst can be improved by partial substituting with Sr and Ce (Ran et al., 2005) to induce structural default, further increasing the number of oxygen vacancies. Indeed, Pr_0.4Ba_0.4Ce_0.2SrNiO₄ possesses a broad β oxygen desorption peak, as shown in Figure 3.

Overall, the modified catalysts (La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄) show better performance in decomposing NO at a temperature range of 500–900 °C compared with that reported in literature (Russo et al., 2007; Pérez-Ramírez et al., 2003; Gao, et al., 2008). Experimental results indicate that properties of La_0.7Ce_0.3SrNiO₄ are significantly improved by doping with Ba and partial substitution with Pr at the La site; hence, the modified catalysts (La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄) are mainly applied for the follow-up experimental tests.

Influences of space velocity on NO decomposition

Figure 5 shows the effects of space velocity on NO decomposition achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively. The results indicate that NO decomposition efficiency decreases with increasing space velocity at a temperature range of 300–600 °C for both catalysts. Obviously, NO decomposition efficiency might suffer from a larger space velocity due to diffusion limitation. However, NO decomposition efficiency achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ reaches 100% at 600 °C, implying that activity of Pr-based catalyst is relatively high at T ≥ 600 °C. Higher activity observed at a higher operating temperature may be attributed to higher mobility of lattice oxygen. O₂-TPD results indicate that La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ have significant desorption peak of lattice oxygen at about 600 °C; it is believed that this is one of the reasons for high activity. Overall, NO decomposition efficiency achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ is affected more significantly by decreasing residence time at a temperature lower than 600 °C compared with Pr_0.4Ba_0.4Ce_0.2SrNiO₄. However, influence of space velocity can be improved by loading the active phase on the support with a large surface area.

Figure 5. Influences of space velocity on activities of La_0.4Ba_0.4Ce_0.2SrNiO₄, and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, for NO decomposition (C_NO = 0.1%; He as carrier gas).

Influences of O₂ and H₂O_(g) on NO decomposition

O₂ and H₂O_(g) exist in typical flue gases and they may cause poisoning of catalyst. Therefore, their influences on catalyst active cannot be ignored. The lifetime of catalyst is successfully increased if it can effectively tolerate O₂ and H₂O_(g). The suppression of NO decomposition by oxygen remains one of the important obstacles to overcome in effective NO decomposition. Inhibition of NO decomposition by oxygen can be explained by the reversible dissociative adsorption of oxygen or via molecular adsorption of oxygen, as shown in eq 5:

(8)

Effects of oxygen content on the NO decomposition over La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, are presented in Figure 6. With 1–3% oxygen in the gas stream, the NO decomposition efficiencies achieved with both catalysts decrease slightly when compared with the case without oxygen at low temperatures (300–400 °C). NO decomposition efficiency achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ decreases from 32.0% to 24.3% at 400 °C as 1% oxygen is introduced into the gas stream, whereas that achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ decreases from 68.0% to 63.5%. As oxygen content is further increased to 5%, NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO decrease to 20.4% and 42.3%, respectively, at 400 °C. However, influence of O₂ content decreases gradually with increasing temperature for both catalysts. The NO decomposition efficiency achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ reaches 95.6% at 600 °C with 1% of O₂, whereas that achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ is 94.3%. With 5% O₂, NO decomposition efficiencies achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ and La_0.4Ba_0.4Ce_0.2SrNiO₄ decrease to 90.3% and 86.3%, respectively, at 600 °C. The same trend also appears in the space velocity test, and both catalysts show excellent performance for NO decomposition when operated at a temperature higher than 600 °C. However, activities of both catalysts for NO decomposition would be slightly affected at a high oxygen content.

Figure 6. Effects of oxygen contents on NO decomposition at various temperatures (C_NO = 0.1%, C_O2 = 1–5%, GHSV = 8000 hr⁻¹; He as carrier gas).

Figure 7 shows the effects of 5% and 10% H₂O_(g) on NO decomposition achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, at 600 °C. The results indicate that NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ decrease from 100% to 86.7% and 92.4%, respectively, as 5% H₂O_(g) is incorporated into the gas stream. With 10% H₂O_(g), the NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ further decrease to 78.9% and 86.3%, respectively. Tofan et al. (2002) suggest that influence of H₂O_(g) is mainly from adsorption effect because H₂O_(g) is easily adsorbed on the catalyst surface, resulting in the reduction of active sites (Tofan et al., 2002). Nevertheless, the NO decomposition efficiency achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ maintains at 86% with 10% H₂O_(g) in the gas stream. The results indicate that of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ has good tolerance for H₂O_(g) when operated at 600 °C.

Figure 7. Effects of H₂O_(g) contents on NO decomposition achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, at 600 °C (C_NO = 0.1%, = 5–10%, GHSV = 8000 hr⁻¹; He as carrier gas).

Influences of CO₂ on NO decomposition

Figure 8 shows the effect of CO₂ in the gas stream on NO decomposition achieved with the two catalysts at 600 °C. The results indicate that NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ decrease from 100% to 80.6% and 87.3%, respectively, as 5% CO₂ is introduced into the system. As temperature is controlled at 400 °C, NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄ are greatly reduced to 20.8% and 42.3%, respectively. Obviously, the activities of both catalysts are severely affected by CO₂ at low temperatures. Previous study indicates that high basicity of catalyst facilitates adsorption of CO₂ in addition to NO (Zhang et al., 2006). Therefore, catalyst poisoning by CO₂ adsorption can be severe. Pr_0.4Ba_0.4Ce_0.2SrNiO₄ shows higher tolerance to O₂, CO₂, and H₂O_(g) when compared with La_0.4Ba_0.4Ce_0.2SrNiO₄. Characterization of the catalysts indicates that Pr_0.4Ba_0.4Ce_0.2SrNiO₄ may have higher oxygen mobility to reduce poison. In addition, previous study also indicates that Pr-based catalyst can induce structural defaults by partial substituting with Sr and Ce, further increasing the number of oxygen vacancies (Ran et al., 2005).

Figure 8. Effects of CO₂ on NO decomposition achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, at 600°C (C_NO = 0.1%, CO₂= 5%, GHSV = 8000 hr⁻¹; He as carrier gas).

Catalytic tests in the presence of O₂, CO₂, and H₂O_(g)

Testing of catalytic activity with real gas is very important in determining the catalysts’ practical usefulness. Therefore, 5% O₂, 5% CO₂, and 5% H₂O_(g) are simultaneously introduced into the catalysis system to investigate the effect on activity of Pr_0.4Ba_0.4Ce_0.2SrNiO₄. As shown in Figure 9, NO decomposition efficiency could maintain 100% at initial period of the reaction, i.e., without O₂, CO₂, and H₂O_(g) condition. As 5% O₂, 5% CO₂, and 5% H₂O_(g) are simultaneously introduced into the gas streams, NO decomposition efficiency gradually decreases from 100% to 82%. Obviously, activity of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ can be affected by simultaneous presence of O₂, CO₂, and H₂O_(g). However, the NO decomposition efficiency returns to 100% as the supplies of O₂, CO₂, and H₂O_(g) are terminated.

Figure 9. Catalytic test of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ for NO removal in the absence or presence of O₂, CO₂, and H₂ (C_NO = 0.1%, GHSV= 8000 hr⁻¹, T = 600 °C; He as carrier gas).

Product analysis and durability test

Table 4 summarizes the N₂ yields from NO decomposition achieved with the three selected catalysts. The results indicate that N₂ yield clearly depends on the temperature. Among the three catalysts, Pr_0.4Ba_0.4Ce_0.2SrNiO₄ shows the best NO decomposition efficiency (68%) and the highest N₂ yield (54%) at 400 °C. As the temperature is increased to 600 °C, N₂ yield achieved with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ reached 92%. For La_0.4Ba_0.4Ce_0.2SrNiO₄, N₂ yield increases from 15% to 88% as the temperature is increased from 400 to 600 °C, and in the meantime, N₂ yield increases from 3% to 63% for La_0.7Ce_0.3SrNiO₄. Results indicate that Pr_0.4Ba_0.4Ce_0.2SrNiO₄ possesses the best activity in decomposing NO due to its excellent oxygen mobility.

Table 4. Products analysis and efficiencies of NO decomposition achieved with the three catalysts at various temperatures.

			N2 Selectivity (%)
Catalyst	Reaction Temperature (°C)	NO Decomposition (%)	Yield (%)	With 5% O2	With 5% H2O(g)	With 5% CO2
LaCe0.7Sr0.3NiO4	400	10	3	—	—	—
	500	65	30	—	—	—
	600	85	63	—	—	—
La0.4Ba0.4Ce0.2SrNiO4	400	32	15	—	—	—
	500	84	81	—	—	—
	600	100	88	78	81	74
Pr0.4Ba0.4Ce0.2SrNiO4	400	68	54	—	—	—
	500	100	90	—	—	—
	600	100	92	89	90	82

Notes: — = not tested. NO: 0.1%; He: balance; GHSV = 8000 hr⁻¹.

Also, effects of 5% O₂ and 5% H₂O_(g) on N₂ yield are evaluated, respectively. The results indicated that N₂ yields achieved with Pr-based catalyst is 89% at 600 °C in the presence of 5% O₂, whereas N₂ yields is 90% in the presence of 5% H₂O_(g). Furthermore, with 5% CO₂ in the gas stream, N₂ yield is 82% at 600 °C for Pr-based catalyst.

The durability of catalysts is important in determining their practical application, and Figure 10 shows the results of durability test with Pr_0.4Ba_0.4Ce_0.2SrNiO₄ as the catalyst. The results indicate that N₂ yield maintains 92% at 600 °C and no significant deactivation is observed during the continuous operation of 60 h.

Figure 10. N₂ yield during the durability test of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ catalyst for NO decomposition (C_NO = 0.1%, GHSV = 8000 hr⁻¹, T = 600 °C; He as carrier gas).

Conclusions

Development of NO decomposition technology is essential for reducing the environmental impact associated with NO_x. This study discusses the activities of perovskite-type catalysts for NO decomposition and evaluates the influences of O₂, H₂O_(g), and CO₂. The results indicate that catalytic activities of La-based catalysts and their tolerance to the complex composition of typical flue gases can be improved by partial substitution with Pr at the La site. Important conclusions can be drawn from this study as follows:

1. NO decomposition efficiencies achieved with three perovskite-type oxide catalysts prepared rank in the order of Pr_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.7Ce_0.3SrNiO₄ at the same operating condition.

2. O₂-TPD results show that the order of the mobility of lattice oxygen in the three perovskite-type catalysts tested is Pr_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.4Ba_0.4Ce_0.2SrNiO₄ > La_0.7Ce_0.3SrNiO₄. Physical and chemical properties of La-based for NO decomposition are improved by substituting with Pr.

3. NO decomposition efficiencies achieved with La_0.4Ba_0.4Ce_0.2SrNiO₄ and Pr_0.4Ba_0.4Ce_0.2SrNiO₄, respectively, decrease with the addition of 5% O₂ or 5% CO₂ because high basicity of the catalysts facilitates adsorption of O₂ and CO₂, reducing the active sites for NO decomposition.

4. NO decomposition efficiencies achieved with catalysts modified slightly decrease as 10% H₂O_(g) is introduced the gas stream. Possibly, H₂O_(g) can be adsorbed on the catalyst surface, resulting in the reduction of active sites.

5. Development of the catalysts that can tolerate relatively high O₂, H₂O_(g), and CO₂ contents of the typical flue gases is essential for successful NO decomposition application.

Acknowledgment

The authors thank Dr. G.T. Pan for valuable discussion.

Funding

The authors would like to thank the Industrial Technology Research Institute for financial support.

Additional information

Funding

The authors would like to thank the Industrial Technology Research Institute for financial support.

Notes on contributors

Kuan Lun Pan

Kuan Lun Pan is a Ph.D. student at the Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan, Republic of China.

Mei Chung Chen

Mei Chung Chen is a master’s degree student at the Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan, Republic of China.

Sheng Jen Yu

Sheng Jen Yu is a researcher at the Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan, Republic of China.

Shaw Yi Yan

Shaw Yi Yan is a researcher at the Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan, Republic of China.

Moo Been Chang

Moo Been Chang is Distinguished Professor at the Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan, Republic of China.