MnO x /Ce 0.6 Zr 0.4 O 2 Catalysts for Low-Temperature Selective Catalytic Reduction of NO x with NH 3

Abstract

Innovative catalysts of manganese oxides supported on Ce_0.6Zr_0.4O₂ prepared by different methods (e.g., citric acid method, coprecipitation method, and combustion method) were studied for low-temperature selective catalytic reduction of NO_x with NH₃ at 80°C–260°C. Compared with MnO_x/Ce_0.6Zr_0.4O₂ (coprecipitation method) and MnO_x/Ce_0.6Zr_0.4O₂ (combustion method), MnO_x/Ce_0.6Zr_0.4O₂ (citric acid method) exhibited the best performance on selective catalytic reduction activity even in the presence of H₂O and SO₂. X-ray diffraction, N₂ adsorption, X-ray photoelectron spectroscopy, and temperature-programmed experiments (H₂-TPR and NH₃-TPD) were used to characterize properties of the catalysts. Large surface area, small crystalline size, high surface chemisorbed oxygen, and high dispersion of manganese oxides might be the main reasons for the excellent performance of the catalysts.

Introduction

The most efficient technology to remove NO_x from stationary sources is the selective catalytic reduction (SCR) of NO_x with NH₃. V₂O₅-WO₃/TiO₂ has been widely accepted as industrial catalysts for SCR of NO_x with NH₃. However, this vanadium-based catalyst is highly active only within a narrow temperature range of 300°C–400°C, which makes it necessary to locate the SCR unit upstream of the desulfurizer and/or particulate control device to avoid reheating the flue gas. The life of the catalysts is shortened because of high concentrations of SO₂ and ash in the flue gas. In addition, retrofitting SCR devices into existing systems for flue gas cleaning is costly because space and access in many power plants are extremely limited. Thus, there is great interest in the development of low-temperature SCR catalysts. This catalyst can be placed downstream of the desulfurizer and/or particulate control device, where most of sulfur dioxide and ash are removed and the deactivation is weakened.

Manganese (MnO_x)-based catalysts (Qi and Yang, 2003) have been reported to be highly active for low-temperature SCR of NO_x with NH₃ at a similar space velocity to that of V₂O₅-WO₃/TiO₂ catalysts. Therefore, it could be expected that high NO_x conversion was achieved over the similar volume of low-temperature SCR catalysts compared with V₂O₅-WO₃/TiO₂ catalysts. Unsupported MnO_x prepared by a precipitation method using sodium carbonate exhibited excellent activity for SCR of NO_x with NH₃ in the temperature range of 75°C–200°C (Kang et al., 2006). Supporting MnO_x catalysts, such as MnO_x/TiO₂ (Ettireddy et al., 2007; Wu et al., 2008a; Jiang et al., 2009), MnO_x/Al₂O₃ (Singoredjo et al., 1992; Kijlstra et al., 1997), and MnO_x/ACF (Yoshikawa et al., 1998; Marbán and Fuertes, 2001), were also active for low-temperature SCR of NO_x with NH₃. The nature of the support was found to be a very important factor in SCR of NO_x with NH₃ (Smirniotis et al., 2006).

Because of high redox property, thermal stability, and catalytic activity, Ce_xZr_1−xO₂–mixed oxide has attracted much attention as a candidate for noble metal (Pd, Pt, or Rh) support in automotive three-way catalysts (Anderson et al., 2006; Guo et al., 2007, 2008). Ce_xZr_1−xO₂–supported other transition metals, such as CuO/Ce_0.8Zr_0.2O₂ (Ma et al., 2003), CuO-CoO_x/Ce_0.67Zr_0.33O₂ (Liu et al., 2009b), and CuO/WO₃/Ce_0.5Zr_0.5O₂ (Li et al., 2005), were also investigated for abatement of NO and CO from automobile exhausts. Recently, Ce_xZr_1−xO₂–based catalysts were studied for SCR of NO with NH₃; for example, WO₃/CeO₂-ZrO₂ (Li et al., 2008) and V₂O₅/CeO₂-ZrO₂ (Putluru et al., 2009) have shown to be promising catalysts for SCR of NO with NH₃ in the medium temperature range (200°C–450°C). However, there were few reports on the application of Ce_xZr_1−xO₂–based catalysts in low-temperature SCR of NO_x with NH₃ (80°C–260°C).

In this work, the innovative catalysts of MnO_x supported on Ce_0.6Zr_0.4O₂ prepared by different methods (citric acid method, precipitation method, and combustion method) were investigated for low-temperature SCR of NO_x with NH₃ under similar reaction conditions to that of MnO_x-based catalysts (Qi and Yang, 2003). The H₂O and SO₂ tolerance of the catalysts under low-temperature SCR conditions was also considered.

Experimental

Preparation of Ce_0.6Zr_0.4O₂

Three different preparation methods for Ce_0.6Zr_0.4O₂ were introduced, namely citric acid method, coprecipitation method, and combustion method. The preparation methods are described below.

Citric acid method

A certain amount of Ce(NO₃)₃·6H₂O and ZrO(NO₃)₂·2H₂O were dissolved in water at 80°C, and the solution of citric acid was slowly added into the mixture solution under vigorous stirring. The molar ratio of Ce:Zr:citric acid was 0.6:0.4:1. The mixture was stirred at room temperature for 1 h. Then the solution was dried at 80°C for 12 h, and some white powders were obtained. These powders were calcined at 200°C for 6 h to produce the oxide and then calcined at 550°C for 6 h under static air in muffle furnace. The obtained sample was denoted as Ce_0.6Zr_0.4O₂ (CA).

Coprecipitation method

After dissolving Ce(NO₃)₃·6H₂O and ZrO(NO₃)₂ · 2H₂O (Ce:Zr = 0.6:0.4) in water at 80°C, the ammonium hydroxide solution (25% ammonia) was slowly dropped into the mixture under vigorous stirring until pH 10. The blend was stirred at room temperature for 3 h and then aged for 1 h. The precipitation was washed with deionized water, filtered in vacuo, and dried at 80°C for 12 h. The obtained solid was calcined at 550°C for 6 h under static air in muffle furnace. The obtained sample was denoted as Ce_0.6Zr_0.4O₂ (CP).

Combustion method

The solution (20 mL) of Ce(NO₃)₃·6H₂O, ZrO(NO₃)₂ · 2H₂O, and CO(NH₂)₂ (urea) with a molar ratio of Ce:Zr:CO(NH₂)₂ of 0.6:0.4:2.5 was directly calcined in a muffle furnace preheated to 550°C. The combustion mixture boiled and frothed, followed by the appearance of flame, yielding voluminous yellow solid materials within 5 min. The obtained sample was denoted as Ce_0.6Zr_0.4O₂ (CB).

Preparation of MnO_x/Ce_0.6Zr_0.4O₂

MnO_x/Ce_0.6Zr_0.4O₂ was prepared by incipient wetness impregnation of Ce_0.6Zr_0.4O₂ with an aqueous solution of Mn(NO₃)₂. The prepared samples were dried in a water bath at 80°C for 12 h and then calcined at 550°C for 6 h under static air in a muffle furnace. The obtained catalysts were denoted as MnO_x/Ce_0.6Zr_0.4O₂ (CA), MnO_x/Ce_0.6Zr_0.4O₂ (CP), and MnO_x/Ce_0.6Zr_0.4O₂ (CB), respectively. In all the catalysts, the molar ratio of Mn:Ce:Zr was 0.4:0.6:0.4. P25-TiO₂ and γ-Al₂O₃, common SCR supports, were also used to prepare MnO_x/P25-TiO₂ and MnO_x/γ-Al₂O₃ catalysts by the same procedure. The molar ratio of Mn:Ti (or Al) was designated as 0.4:1.

Catalytic activity test

The SCR activity of the catalysts for NO_x removal with NH₃ was carried out in a fixed-bed flow reactor. Six gas streams, 0.06 vol% NO, 0.06 vol% NH₃, 3 vol% O₂, 3 vol% H₂O (when used), 0.01 vol% SO₂ (when used), and pure N₂ in balance, were used to simulate the flue gas. In all the runs, the total gas flow rate was maintained at 300 mL/min over 0.75 g of catalyst (40–60 mesh). The feed gases were mixed and preheated in a chamber before entering the reactor. The water vapor was generated by passing N₂ through a heated gas-wash bottle containing deionized water (80°C). During the measurements, the concentrations of NO and NO₂ at the inlet and outlet of the reactor were monitored by a Flue Gas Analyzer (KM940; Kane International Ltd.) equipped with NO, NO₂, and SO₂ sensors. The activities of the catalysts were measured by the conversion of NO_x calculated as follows: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland,xspace}\usepackage{amsmath,amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} {\rm NO}_x \ {\rm conversion} = \frac {[{\rm NO}_x]_{\rm in}-[{\rm NO}_x]_{\rm out}}{[{\rm NO}_x]_{\rm in}}\times 100 \% \end{align*} \end{document}

with [NO_x] = [NO] + [NO₂].

Characterization of the catalysts

The surface areas (Brunauer-Emmett-Teller [BET] method) were determined by N₂ isotherms at −196°C using a NOVA 2000 automated gas sorption system. The powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2500 system using Cu Kα radiation (40 kV and 100 mA). The X-ray photoelectron spectroscopy (XPS) was used to analyze the surface atomic state of the catalysts using a Kratos Axis Ultra DLD spectrometer equipped with a monochromatic Al Kα radiation (1,486.6 eV).

The temperature-programmed reduction (H₂-TPR) and temperature-programmed desorption (NH₃-TPD) experiments were performed on a tp-5080 automated chemisorption analyzer using 0.1 g catalysts. Prior to H₂-TPR experiments, the samples were first pretreated in pure N₂ at 500°C for 1 h and then cooled down to room temperature. TPR runs were carried out in a flow of H₂ (5%) in N₂ (30 mL/min) from room temperature to 900°C, with a heating rate of 10°C/min. For NH₃-TPD experiments, after a pretreatment in pure N₂ at 500°C for 1 h, the catalyst was cooled down to room temperature in pure N₂ and then saturated for 30 min with a stream of pure NH₃ (total flow rate = 1 mL/min [STP]). After saturation, the catalysts were flushed in a pure N₂ flow for 30 min at 100°C. Finally, the TPD of NH₃ was carried out in pure N₂ at a heating rate of 10°C/min from 100°C to 500°C. The NH₃ desorption (or H₂ uptake) was measured quantitatively by a thermal conductivity detector.

Results

Structural and textural properties

The XRD patterns of Ce_0.6Zr_0.4O₂ and MnO_x/Ce_0.6Zr_0.4O₂ for each method are shown in Fig. 1. For Ce_0.6Zr_0.4O₂ in Fig. 1a, all profiles are attributed to the cubic, fluorite structure of CeO₂ [(111), (200), (220), (311), (222), (400), and (311)]. The diffraction peaks for CeO₂ phase became sharper in the order of Ce_0.6Zr_0.4O₂ (CA) < Ce_0.6Zr_0.4O₂ (CP) < Ce_0.6Zr_0.4O₂ (CB). Figure 1b exhibits the XRD patterns of all three MnO_x/Ce_0.6Zr_0.4O₂ samples. The diffraction peaks for CeO₂ phase could also be seen on all three MnO_x/Ce_0.6Zr_0.4O₂ XRD curves. Besides the CeO₂ phase, several weak peaks assigned to Mn₂O₃ phase [(211), (400), (332), (440), and (622)] are observed as the secondary phase for MnO_x/Ce_0.6Zr_0.4O₂ (CB) and MnO_x/Ce_0.6Zr_0.4O₂ (CP). However, no Mn₂O₃ phase appeared for MnO_x/Ce_0.6Zr_0.4O₂ (CA).

FIG. 1.

X-ray diffraction patterns of fresh samples: (a) Ce_0.6Zr_0.4O₂ and (b) MnO_x/Ce_0.6Zr_0.4O₂.

BET surface areas, crystallite sizes, and lattice parameters of all three MnO_x/Ce_0.6Zr_0.4O₂ samples are presented in Table 1 and compared with Ce_0.6Zr_0.4O₂ samples. MnO_x/Ce_0.6Zr_0.4O₂ (CA) exhibited the largest surface area among all three MnO_x/Ce_0.6Zr_0.4O₂ samples. The crystallite sizes and lattice parameters of all samples were calculated according to Bragg's law and Debye–Scherrer equation. MnO_x/Ce_0.6Zr_0.4O₂ (CA) had small crystallite size compared with MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB). The lattice parameter of MnO_x/Ce_0.6Zr_0.4O₂ (CA) was smaller than that of Ce_0.6Zr_0.4O₂ (CA), whereas no variation was observed for Ce_0.6Zr_0.4O₂ (CP) and Ce_0.6Zr_0.4O₂ (CB) after the loading of manganese oxides.

Table 1.

Structural and Textural Properties of Ce_0.6Zr_0.4O₂ and MnO_x/Ce_0.6Zr_0.4O₂ Samples

Samples	BET surface area (m²/g)	Crystallite size (nm)	Lattice parameter (Å)
Ce_0.6Zr_0.4O₂ (CA)	58.66	11.29	5.325
Ce_0.6Zr_0.4O₂ (CP)	50.57	13.17	5.390
Ce_0.6Zr_0.4O₂ (CB)	5.90	22.52	5.332
MnO_x/Ce_0.6Zr_0.4O₂ (CA)	55.38	11.37	5.307
MnO_x/Ce_0.6Zr_0.4O₂ (CP)	34.66	15.12	5.390
MnO_x/Ce_0.6Zr_0.4O₂ CB)	3.66	21.22	5.332

Crystallite size and lattice parameters were both derived from (111) planes.

XPS characterization

Table 2 shows the surface elemental concentrations and atomic ratio of MnO_x/Ce_0.6Zr_0.4O₂ determined by XPS. The surface Mn concentrations and Mn/(Ce+Zr) ratio for MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB) increased significantly compared with MnO_x/Ce_0.6Zr_0.4O₂ (CA). Especially for MnO_x/Ce_0.6Zr_0.4O₂ (CB), the surface Mn concentrations and Mn/(Ce+Zr) ratio reached 70.18% and 2.35%, respectively. According to above XRD results, crystalline Mn₂O₃ phase was formed on the surface of MnO_x/Ce_0.6Zr_0.4O₂ (CB), which might result in a high Mn/(Ce+Zr) ratio for MnO_x/Ce_0.6Zr_0.4O₂ (CB).

Table 2.

Surface Elemental Concentrations (%) and Atomic Ratio of MnO_x/Ce_0.6Zr_0.4O₂ Determined by X-Ray Photoelectron Spectroscopy

	Surface elemental concentrations (%)
	Ce				O
Samples	Ce⁴⁺	Ce³⁺	Zr	Mn	O_α	O_β	Mn/(Ce + Zr)
MnO_x/Ce_0.6Zr_0.4O₂ (CA)	7.88	1.23	4.10	10.46	36.50	39.83	0.79
MnO_x/Ce_0.6Zr_0.4O₂ (CP)	7.36	2.07	4.25	13.98	33.75	38.59	1.02
MnO_x/Ce_0.6Zr_0.4O₂ (CB)	6.64	1.82	3.22	27.49	29.58	31.25	2.35

The oxidation states of Ce, Zr, Mn, and O were investigated by analyzing the photoelectron spectra of Ce 3d, Zr 3d, Mn 2p, and O1s levels obtained from XPS measurements. As shown in Fig. 2a, the curves of Ce 3d spectra were fitted with eight peaks in accordance with the fitting and labeling method reported elsewhere (Wu et al., 2008b). The relative percentages of the cerium species obtained by the area ratios of the Ce⁴⁺ (Ce⁴⁺ = U′′′ + U′′ + U + V′′′ + V′′ + V)/Ce³⁺ (Ce³⁺ = U′ + V′) are given in Table 2. From the results, it could be seen that the cerium species were mainly in Ce⁴⁺ oxidation state in all three MnO_x/Ce_0.6Zr_0.4O₂ samples. The typical examples of observed Zr 3d spectra are presented in Fig. 2b. The binding energy of Zr 3d_5/2 between 182.10 and 182.35 eV indicated that Zr was present only in Zr⁴⁺ oxidation state (Alifanti et al., 2003; Sun et al., 2003).

FIG. 2.

(a) Ce 3d, (b) Zr 3d, (c) Mn 2p, and (d) O 1s X-ray photoelectron spectroscopy spectra of all three MnO_x/Ce_0.6Zr_0.4O₂ samples [1, MnO_x/Ce_0.6Zr_0.4O₂ (CA); 2, MnO_x/Ce_0.6Zr_0.4O₂ (CP); 3, MnO_x/Ce_0.6Zr_0.4O₂ (CB)].

Figure 2c shows the Mn 2p photoelectron peaks of all three MnO_x/Ce_0.6Zr_0.4O₂ samples. For MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB), the binding energies of Mn 2p_3/2 fell in a very narrow range (641.3–641.5 eV). This value was in the range of Mn₂O₃ (641.3–641.7 eV) (Morales et al., 2006). For MnO_x/Ce_0.6Zr_0.4O₂ (CA), the binding energy of Mn 2p_3/2 was 642.0 eV, which had been reported to be characteristic of a mixed-valence manganese system (Mn³⁺ and Mn⁴⁺) (Ettireddy et al., 2007).

The XPS spectra for O1s showed that the O1s spectra gave two distinct peaks (Fig. 2d). The peak at 529.6–530.0 eV was assigned to lattice oxygen (denoted as O_α) from metal oxides, whereas the one at 531.3–531.7 eV was assigned to chemisorbed oxygen (denoted as O_β), which had been reported to have a positive effect on SCR reaction (Wu et al., 2008b). As shown in Table 2 and Fig. 2d, the chemisorbed oxygen on the surface of all three MnO_x/Ce_0.6Zr_0.4O₂ samples increased in the order of MnO_x/Ce_0.6Zr_0.4O₂ (CB) < MnO_x/Ce_0.6Zr_0.4O₂ (CP) < MnO_x/Ce_0.6Zr_0.4O₂ (CA).

H₂-TPR and NH₃-TPD

The H₂-TPR for Ce_0.6Zr_0.4O₂ and MnO_x/Ce_0.6Zr_0.4O₂ samples are shown in Fig. 3a. Figure 3a shows Ce_0.6Zr_0.4O₂ (CA), Ce_0.6Zr_0.4O₂ (CP), and Ce_0.6Zr_0.4O₂ (CB) have reduction peaks centered at 570°C, 534°C, and 591°C, respectively, which might be attributed to the reduction of Ce⁴⁺ in Ce_0.6Zr_0.4O₂ samples (Wei et al., 2008). However, a shoulder peak at 827°C was observed in the TPR profiles of Ce_0.6Zr_0.4O₂ (CP). As pointed out by recent studies, the shoulder peak at high temperature was assigned to the bulk reduction of cerium phase (Guo et al., 2007).

FIG. 3.

(a) H₂-TPR and (b) NH₃-TPD spectra of the samples.

Compared with Ce_0.6Zr_0.4O₂ for each method, the reduction peaks of corresponding MnO_x/Ce_0.6Zr_0.4O₂ shifted to low temperature. A similar behavior was also observed for Mn- and Cu-doped CeO₂-ZrO₂ solid solutions (Terribile et al., 1999). For MnO_x/Ce_0.6Zr_0.4O₂ (CB) and MnO_x/Ce_0.6Zr_0.4O₂ (CP), the reduction peaks emerged at 367°C and 440°C. A three-step reduction profile was observed for MnO_x/Ce_0.6Zr_0.4O₂ (CA). An additional shoulder peak appeared at 267°C except the other two peaks at 367°C and 440°C. The H₂ consumption values of all three MnO_x/Ce_0.6Zr_0.4O₂ samples are given in Table 3. MnO_x/Ce_0.6Zr_0.4O₂ (CA) gave almost same H₂ consumption for MnO_x/Ce_0.6Zr_0.4O₂ (CB) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) samples.

Table 3.

H₂-TPR and NH₃-TPD Results of Three MnO_x/Ce_0.6Zr_0.4O₂ Samples

Samples	H₂ consumption (μmol/g)	Acidity (μmol/g)
MnO_x/Ce_0.6Zr_0.4O₂ (CA)	1,099.3	37.1
MnO_x/Ce_0.6Zr_0.4O₂ (CP)	1,076.0	36.6
MnO_x/Ce_0.6Zr_0.4O₂ CB)	1,084.7	15.8

The NH₃-TPD was carried out to investigate the catalyst acidity for all three MnO_x/Ce_0.6Zr_0.4O₂ samples. The results of NH₃-TPD runs performed over all three samples are shown in Fig. 3b. With increasing desorption temperature at 100°C–500°C, MnO_x/Ce_0.6Zr_0.4O₂ (CB) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) exhibited similar NH₃-TPD curves with one strong peak centered at 106°C and 141°C, respectively. The NH₃-TPD curve for MnO_x/Ce_0.6Zr_0.4O₂ (CA) showed different features, with two desorption peaks centered at 119°C and 269°C, respectively. The peaks centered at 106°C, 119°C, and 141°C were assigned to the NH₃ molecules adsorbed by weak acid sites, whereas the one centered at 269°C was assigned to the NH₃ molecules adsorbed by moderate acid sites. This indicated that the chemisorbed NH₃ molecules adsorbed by moderate acid sites might play a more important role in the adsorption of NH₃ on MnO_x/Ce_0.6Zr_0.4O₂ (CA) than on the other two catalysts. According to the total acidity of all three MnO_x/Ce_0.6Zr_0.4O₂ samples as shown in Table 3, MnO_x/Ce_0.6Zr_0.4O₂ (CA) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) had similar chemisorbed NH₃ amount, which was comparatively higher than that of MnO_x/Ce_0.6Zr_0.4O₂ (CB).

Activity test

The catalytic activities for NO_x removal with NH₃ over all three MnO_x/Ce_0.6Zr_0.4O₂ samples were investigated at a temperature of 80°C–260°C. As shown in Fig. 4a, MnO_x/Ce_0.6Zr_0.4O₂ (CA) performed most efficiently, and nearly 100% NO_x conversion was obtained at 160°C–220°C. MnO_x/Ce_0.6Zr_0.4O₂ (CP) showed nearly 90% NO_x conversion at 160°C–220°C. For MnO_x/Ce_0.6Zr_0.4O₂ (CB), only 85% NO_x conversion was obtained when the temperature reached 220°C. It could also been seen from Fig. 4a that low NO_x-SCR activities were observed over all three Ce_0.6Zr_0.4O₂ samples at a temperature of 80°C–180°C. However, the NO_x-SCR activities were improved significantly when the temperature was higher than 180°C. Figure 4b shows the comparison of NO_x conversion as a function of temperature over MnO_x/Ce_0.6Zr_0.4O₂ (CA), MnO_x/P25-TiO₂, and MnO_x/γ-Al₂O₃. The results indicated that MnO_x/Ce_0.6Zr_0.4O₂ (CA) could provide higher NO_x conversion than MnO_x/P25-TiO₂ and MnO_x/γ-Al₂O₃, especially at a temperature of 80°C–180°C.

FIG. 4.

(a) NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ and Ce_0.6Zr_0.4O₂ samples at different temperatures. (b) Comparison of NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ (CA), MnO_x/Al₂O₃, and MnO_x/TiO₂. Reaction conditions: 0.06 vol% NO, 0.06 vol% NH₃, 3 vol% O₂, balance N₂, W/F = 2.5 × 10⁻³ g/(mL min).

The NO oxidation activities of all three MnO_x/Ce_0.6Zr_0.4O₂ samples with the variation of temperature were also investigated (Fig. 5). For all three MnO_x/Ce_0.6Zr_0.4O₂ samples, the NO oxidation activity increased with the increasing temperature, especially at 180°C–260°C. It could also be seen from Fig. 5 that the NO oxidation activities of all three MnO_x/Ce_0.6Zr_0.4O₂ samples in the whole temperature range (80°C–260°C) increased in the following sequence: MnO_x/Ce_0.6Zr_0.4O₂ (CB) < MnO_x/Ce_0.6Zr_0.4O₂ (CP) < MnO_x/Ce_0.6Zr_0.4O₂ (CA).

FIG. 5.

Oxidation activity of NO to NO₂ by O₂ over MnO_x/Ce_0.6Zr_0.4O₂ samples. Reaction conditions: 0.06 vol% NO, 3 vol% O₂, balance N₂, W/F = 2.5 × 10⁻³ g/(mL min).

Figure 6 shows the effect of H₂O and SO₂ on activities of all three samples for NO_x removal with NH₃ at 180°C. MnO_x/Ce_0.6Zr_0.4O₂ (CA) exhibited the best resistance to H₂O and SO₂ in the presence of 3 vol% H₂O and 100 ppm SO₂. A barely detectable decrease in NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ (CA) was observed in 6 h in the presence of 3 vol% H₂O and 100 ppm SO₂. The NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ (CP) decreased slowly to 79.1% and then stabilized under low-temperature SCR conditions including 3 vol% H₂O and 100 ppm SO₂. The effects of SO₂ concentration on NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ (CA) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) had also been studied and shown in the inset graph of Fig. 6. It could be seen that the effect of 3 vol% H₂O and 200 ppm SO₂ on NO_x conversion over the catalysts was a little stronger than that of 3 vol% H₂O and 100 ppm SO₂. However, MnO_x/Ce_0.6Zr_0.4O₂ (CA) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) exhibited excellent resistance to low concentrations of H₂O and SO₂ at 180°C anyway. For MnO_x/Ce_0.6Zr_0.4O₂ (CB), the catalytic activity decreased sharply upon 3 vol% H₂O and 100 ppm SO₂ addition into the flue gases, and the NO_x conversion decreased from initial 65.6% to 13% in the first 6 h. The original SCR activity was not restored after cutting off of H₂O and SO₂ supply.

FIG. 6.

Effect of H₂O and SO₂ on NO_x conversion over MnO_x/Ce_0.6Zr_0.4O₂ samples. Reaction conditions: 0.06 vol% NO, 0.06 vol% NH₃, 3 vol% O₂, balance N₂, reaction temperature = 180°C, W/F = 2.5 × 10⁻³ g/(mL min).

Discussion

The XRD patterns of Ce_0.6Zr_0.4O₂ samples displayed only diffraction peaks corresponding to fluorite structure of CeO₂, and no diffraction peaks for ZrO₂ phase were detected. As shown in Table 1, the lattice parameters of all three Ce_0.6Zr_0.4O₂ samples were smaller than that of CeO₂ (5.410 Å, JCPDS 78-0694). Because of the smaller ionic radius for Zr⁴⁺ (0.084 nm) compared with that of Ce⁴⁺ ions (0.097 nm), the incorporation of Zr⁴⁺ into CeO₂ phase might decrease the lattice parameters of all Ce_0.6Zr_0.4O₂ samples.

In the preparation of Ce_0.6Zr_0.4O₂ (CA), citric acid was used to chelate various cations (Ce⁴⁺ and Zr⁴⁺) to form homogeneous metal chelates of citric acid, which might favor the small crystallite size and high surface area. Ce_0.6Zr_0.4O₂ (CA) with large surface area and small crystallite size was favorable to be a catalyst support in the SCR of NO_x with NH₃. For Ce_0.6Zr_0.4O₂ (CB), the small surface area (5.90m²/g) and large crystallite size (22.52 nm) indicated the agglomeration by combustion method.

According to XRD results, no intense peaks attributable to manganese oxides appeared for MnO_x/Ce_0.6Zr_0.4O₂ (CA), which indicated the high dispersion and/or low crystalline nature of manganese oxides in this sample. Crystalline Mn₂O₃ was formed using MnO_x/Ce_0.6Zr_0.4O₂ (CB) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) catalysts. Therefore, manganese oxides dispersed well in MnO_x/Ce_0.6Zr_0.4O₂ (CA) compared with MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB). It has been reported that amorphous manganese oxide performed well in SCR reaction, whereas the crystalline one contributed little to the activity (Wu et al., 2008a; Jiang et al., 2009).

From the H₂-TPR of all three MnO_x/Ce_0.6Zr_0.4O₂ samples, the low-temperature reduction features attributable to the reduction of manganese oxides could be observed. According to XPS results, manganese oxides in MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB) are exclusively in Mn³⁺ state, whereas Mn³⁺ and Mn⁴⁺ coexist in MnO_x/Ce_0.6Zr_0.4O₂ (CA). This might be the reason why MnO_x/Ce_0.6Zr_0.4O₂ (CA) presented a three-step reduction profile different from that of MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB). The first reduction peak at 267°C might be attributed to the reduction of Mn⁴⁺ to Mn³⁺, which has also been observed for MnO_x/TiO₂ catalysts at 261°C because of the interaction between titania and manganese oxide (Ettireddy et al., 2007). The other two peaks (at 367°C and 440°C) could be speculated to be assigned to the reduction of Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO. Because of the exclusive presence of Mn₂O₃ in Ce_0.6Zr_0.4O₂ (CB) and Ce_0.6Zr_0.4O₂ (CP), the reduction peaks emerged only at 367.0°C and 440°C, which were due to the reduction of Mn₂O₃ to Mn₃O₄, and Mn₃O₄ to MnO, respectively. However, the reduction of Ce_0.6Zr_0.4O₂ phase could not be distinguished clearly in H₂-TPR curves, which indicated the complicated interaction between manganese oxide and Ce_0.6Zr_0.4O₂ phase in all three catalysts.

XPS results indicated that there were different amounts of chemisorbed oxygen on the surface of all three samples. The chemisorbed oxygen has been reported to be helpful to the oxidation of NO to NO₂, which could lead to enhanced SCR activities because of the occurrence of the “fast SCR”: NO + NO₂ + 2NH₃ → 2N₂ + 3H₂O (Liu et al., 2009a). It could be seen from Fig. 5 that the oxidation activities of all three samples are in good agreement with the variation in surface chemisorbed oxygen shown in Table 2. Therefore, it could be expected that MnO_x/Ce_0.6Zr_0.4O₂ (CA) with the highest chemisorbed oxygen in all three samples performed well in the low-temperature SCR reaction.

Because the SCR catalyst is usually deactivated mainly by water vapor (H₂O) and residue SO₂ in the flue gases, the resistance of the catalysts to H₂O and SO₂ has also been investigated at 180°C, as shown in Fig. 6. MnO_x/Ce_0.6Zr_0.4O₂ (CA) and MnO_x/Ce_0.6Zr_0.4O₂ (CP) performed well in the presence of 3 vol% H₂O and 100 ppm SO₂ in the flue gas. However, the deactivation of MnO_x/Ce_0.6Zr_0.4O₂ (CB) by 3 vol% H₂O and 100 ppm SO₂ was observed as some other SCR catalysts in previous literatures (Wu et al., 2009; Shen et al., 2010). At a low temperature, SO₂ could be adsorbed by metal oxides in the catalysts, producing metal sulfates. On the other hand, some ammonium sulfates [such as NH₄HSO₄ and (NH₄)₂SO₄] would be formed by the reaction between SO₂ and the reactants (NH₃ and O₂). Because of small surface area, the active sites of MnO_x/Ce_0.6Zr_0.4O₂ (CB) would be occupied and deactivated more easily by the formed metal sulfates and ammonium sulfates than that of MnO_x/Ce_0.6Zr_0.4O₂ (CA) and MnO_x/Ce_0.6Zr_0.4O₂ (CP).

The lattice parameters of Ce_0.6Zr_0.4O₂ (CP) and Ce_0.6Zr_0.4O₂ (CB) did not change with the loadings of manganese oxides. However, the lattice parameter of MnO_x/Ce_0.6Zr_0.4O₂ (CA) was found to be smaller than that of Ce_0.6Zr_0.4O₂ (CA). Some manganese ions with small ion radius (e.g., 0.067 nm for Mn³⁺ and 0.05 nm for Mn⁴⁺) might incorporate into the CeO₂ phase, which led to the shrinkage of the lattice parameters for MnO_x/Ce_0.6Zr_0.4O₂ (CA). However, there were no more evidence to support the presumption for the behaviors of manganese oxides in MnO_x/Ce_0.6Zr_0.4O₂ (CA). Large surface area, small crystallite size, high dispersion of manganese oxide, and high surface chemisorbed oxygen observed for MnO_x/Ce_0.6Zr_0.4O₂ (CA) might be related to its high activity and good resistance to SO₂ and H₂O.

Conclusions

Among all three MnO_x/Ce_0.6Zr_0.4O₂ samples, MnO_x/Ce_0.6Zr_0.4O₂ (CA) exhibited the highest activity for SCR of NO_x with NH₃, and nearly 100% NO_x conversion was obtained at 160°C–220°C. Compared with MnO_x/Ce_0.6Zr_0.4O₂ (CP) and MnO_x/Ce_0.6Zr_0.4O₂ (CB), MnO_x/Ce_0.6Zr_0.4O₂ (CA) showed excellent resistance to H₂O and SO₂ under low-temperature SCR conditions. According to the results of BET, XRD, and XPS, large surface area, small crystalline size, high dispersion of manganese oxides, and high surface chemisorbed oxygen were observed for MnO_x/Ce_0.6Zr_0.4O₂ (CA), which might be the main reasons for the excellent performance of MnO_x/Ce_0.6Zr_0.4O₂ (CA) catalyst.

Footnotes

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (NSFC 90610018 and 50976050) and the Key Special Technologies R&D Program of Tianjin (09ZCKFSH01900).

Author Disclosure Statement

The authors declare that no conflicting financial interests exist.

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MnO x /Ce 0.6 Zr 0.4 O 2 Catalysts for Low-Temperature Selective Catalytic Reduction of NO x with NH 3

Abstract

Abstract

Introduction

Experimental

Preparation of Ce0.6Zr0.4O2

Citric acid method

Coprecipitation method

Combustion method

Preparation of MnOx/Ce0.6Zr0.4O2

Catalytic activity test

Characterization of the catalysts

Results

Structural and textural properties

XPS characterization

H2-TPR and NH3-TPD

Activity test

Discussion

Conclusions

Footnotes

Acknowledgments

Author Disclosure Statement

References

Preparation of Ce_0.6Zr_0.4O₂

Preparation of MnO_x/Ce_0.6Zr_0.4O₂

H₂-TPR and NH₃-TPD