Low-Temperature Selective Catalytic Reduction of NOx

Article pubs.acs.org/IECR

Low-Temperature Selective Catalytic Reduction of NOx with NH3 over Novel Mn−Zr Mixed Oxide Catalysts Jianliang Zuo,† Zhihang Chen,‡ Furong Wang,† Yinghao Yu,† Lefu Wang,† and Xuehui Li*,† †

School of Chemistry and Chemical Engineering, Pulp & Paper Engineering State Key Laboratory of China, South China University of Technology, Guangzhou 510640, China ‡ South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510655, China ABSTRACT: Novel Mn−Zr mixed oxide catalysts have been prepared by the citric acid method for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. They have been characterized by a series of techniques, specifically N2 adsorption−desorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). It was found that an Mn(0.5)− ZrOx-450 (Mn/(Mn + Zr) mole ratio of 0.5) catalyst showed the highest activity, giving 100% NOx conversion at 100 °C with a space velocity of 30 000 h−1. XRD results suggested that an Mn−Zr solid solution was formed in the Mn(0.5)−ZrOx-450 catalyst, with highly dispersed MnOx. TPR data indicated a strong interaction between the zirconium oxide and manganese oxide, which improved the reduction ability of the MnOx. The TPD results indicated that an appropriate NH3 adsorption ability was beneficial for the low-temperature SCR. The catalyst showed a certain level of sulfur tolerance and water resistance. The effect of H2O could be quickly eliminated after its removal, whereas deactivation by SO2 proved to be irreversible. 100% (reaction conditions: 50 °C, NO = 500 ppm, gas hourly space velocity (GHSV) = 30 000 h−1), but NOx conversion rapidly decreased to 60% in the presence of SO2 (100 ppm). They also found that mixed oxides with the spinel structure such as CuMn2O4 showed enhanced low-temperature SCR activity. Long et al.18 found that Fe−Mn mixed oxide catalysts gave 100% NO conversion at 100−180 °C in the SCR of NO with NH3 at a relatively low space velocity of 15 000 h−1. Qi et al.19 reported that NO x conversions over MnO x−CeO2 catalysts prepared by both the citric acid method and the coprecipitation method exceeded 95% below 150 °C (GHSV = 42 000 h−1) and that their activities decreased a little in the presence of water or SO2. In our previous studies, various Mnbased mixed oxides, such as Cr−MnOx, Fe−MnOx, and Zr− MnOx, were found to be potential low-temperature SCR catalysts after the screening of 120 kinds of mixed oxide catalysts prepared by the solid-phase method through the combination of 16 potential metals, viz., Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Mo, Ba, W, or Bi in pairs. A novel Cr− MnOx catalyst with CrMn1.5O4 as the active phase and a Fe− MnOx catalyst with Fe3Mn3O8 as the active phase were thereby identified.20,21 However, the deactivation caused by water and SO2 remains an unresolved issue, as for other catalysts reported in the literature,4 and there is as yet no commercial lowtemperature SCR process. Therefore, the development of new catalyst systems with good water and sulfur tolerance is still the principal goal in this field. Mn−Zr mixed oxides have been reported to be suitable catalysts for oxidation and combustion, as well as good

1. INTRODUCTION It is well-known that nitrogen oxides (NO, NO2, and N2O) contribute to a lot of environmental problems, such as acid rain, photochemical smog, ozone depletion, and the greenhouse effect. Selective catalytic reduction (SCR) of NOx (x = 1, 2) by ammonia is the major technology for reducing nitrogen oxides emitted from stationary sources such as power stations, industrial heaters, and cogeneration and has been successfully commercialized.1,2 The general reaction of this SCR process is as follows: 4NO + 4NH3 + O2 → 4N2 + 6H 2O

Vanadium-based catalysts such as V2O5/TiO2 (anatase) mixed with WO3 or MoO3 are typical commercial catalysts for this process; however, they are only active within a narrow temperature window of 300−400 °C and are susceptible to deactivation by dust deposition or SO2 poisoning.3 Thus, there has been a very strong incentive to develop highly efficient deNOx catalysts for low-temperature SCR processes in which these catalysts would be placed downstream of the desulfurizer and electrostatic precipitator in the power generation system,4 and much effort has recently been directed toward this. From surveying the literature, transition metal oxides and supported oxides, such as MnOx,5−7 Co3O4,8,9 MnO/Al2O3,10,11 CuO/ Al2O3,12 CuO/TiO2,13 Fe2O3/TiO2,14 and V2O5/activated carbon (AC),15,16 as well as mixed transition metal oxides with Mg, Ni, Cu, Co, Mn, Ce, Cr, or other elements, have been shown to be active in low- and/or medium-temperature SCR of NO with NH3 in the presence of excess oxygen. Among all of the screened catalysts, Mn-based mixed catalysts have exhibited higher activity for the conversion of NOx. Kang et al.17 studied the low-temperature SCR activity of Cu−Mn mixed oxides and found that a Cu0.01Mn0.25 catalyst showed the best lowtemperature SCR activity, with an NOx conversion close to © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2647

December 13, 2013 January 28, 2014 January 28, 2014 January 28, 2014 dx.doi.org/10.1021/ie404224y | Ind. Eng. Chem. Res. 2014, 53, 2647−2655

Industrial & Engineering Chemistry Research

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pretreated in a flow of He at 300 °C for 1.0 h. After cooling to 80 °C, the sample was exposed to a gas mixture of 2.5% NH3/ He with a total flow rate of 30 mL·min−1 (NTP) at 80 °C for 1.0 h. It was then purged with He at 80 °C for 1.0 h to remove NH3 physically adsorbed on the surface. Finally, TPD was carried out from 80 to 450 °C in a flow of He (30 mL·min−1, NTP) with a temperature ramp of 10 °C·min−1. X-ray photoelectron spectroscopy (XPS) analyses were performed with a Quantum-2000 scanning ESCA microprobe (Physical Electronics) with a hemispherical detector operating at constant pass energy (PE = 46.95 eV). An X-ray source at 210 W (I = 15 mA, U = 14 kV) and Al Kα radiation (1486.6 eV) were used. All binding energies were corrected using the C 1s line at 284.6 eV, and the values were measured with a precision of ±0.1 eV. 2.3. Catalytic Activity Measurement. SCR activities were measured using a fixed-bed quartz reactor (i.d. = 8 mm). A typical reactant gas composition was as follows: 1000 ppm NO, 1000 ppm NH3, 3% O2, 5−10% water vapor, and 50−100 ppm SO2 (when used), with a balance of N2. A 1.2 mL (about 2.0 g) sample of catalyst (60−100 mesh) was used in each run. The total flow rate was 600 mL·min−1 (under ambient conditions). Thus, a high gas hourly space velocity (GHSV) with respect to the powder was obtained (30 000 h−1). The compositions of the feed gases and the effluent streams were monitored continuously using online sensors with emission monitors, namely, an SWG-300 gas analyzer (MRU, Germany) for NO, O2, NO2, and SO2. All data were collected between 10 and 30 min (steady state) at the chosen temperature. From the concentration of the gases at the steady state, the conversion was calculated according to the following equation:

materials for NOx trapping and storage.22−25 However, to the best of our knowledge, no attention has been paid to catalysis by Mn−Zr mixed oxides for low-temperature NH3-SCR. Therefore, a systematic investigation of the physicochemical properties of Mn−Zr mixed oxide catalysts prepared by the citric acid method has been carried out based on our previous combinatorial screening results. The effects of Mn/Zr ratio and calcination temperature on the SCR activity have been considered, and the catalysts have been characterized by various methods, namely, N2 adsorption−desorption, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS). The relationship between structure and activity and the interaction between zirconium and manganese ions are also discussed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The catalysts were prepared by the citric acid method as used in our previous work.20 Manganese acetate and zirconium nitrate were mixed in designated ratios and added to aqueous citric acid solution (1.0 mol/L), and the mixtures were stirred at room temperature until complete dissolution of the salts. The molar ratio of citric acid to the total number of moles of zirconium and manganese was set at 1.0. The solution was then dried at 110 °C for 12 h resulting in a porous, foamlike solid. This precursor was calcined at the desired temperature for 4.0 h in air in a temperature programmed muffle furnace. Finally, the samples were crushed and sieved to 60−100 mesh prior to use. Pure zirconium oxide (ZrO2) and manganese oxide (MnOx) were also prepared following the same procedure. The mixed oxides are denoted as Mn(y)−ZrOx-z, where y is the molar ratio of Mn/(Mn + Zr) and z is the calcination temperature in degrees Celsius. 2.2. Catalyst Characterization. A Micromeritics ASAP 2020 micropore size analyzer was used to measure the N2 adsorption isotherms of the samples at liquid N2 temperature (−196 °C). The specific surface area was determined from the linear portion of the Brunauer−Emmett−Teller (BET) plot. The pore-size distribution was calculated from the desorption branch of the N2 adsorption isotherm using the Barrett− Joyner−Halenda (BJH) formula. Prior to the surface area and pore-size distribution measurements, the samples were degassed in vacuum at 300 °C for 24 h. X-ray diffraction (XRD) patterns were obtained using a Rigaku D/MAX-3A Auto X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Intensity data were collected over a 2θ range of 10−80° with a 0.05° step size and a counting time of 1 s per point. The XRD phases were identified by comparison with reference data from International Center for Diffraction Data (ICDD) files. Temperature programmed reduction (TPR) experiments were carried out on a Micromeritics AutoChem 2920 chemisorption analyzer. A 30 mL·min−1 (NTP) mixed gas flow of 10% H2/N2 was passed over samples of about 50 mg in a quartz reactor through a cold trap to the detector. The reduction temperature was increased linearly at 10 °C·min−1 from 30 to 700 °C. Prior to the analysis, all samples were pretreated at 300 °C for 1.0 h in a pure N2 (30 mL·min−1, NTP) stream. Temperature programmed desorption (TPD) experiments on NH3 were also carried out on a Micromeritics AutoChem 2920 chemisorption analyzer. A sample of about 100 mg was

NOx conversion (%) =

[NOx ]in − [NOx ]out [NOx ]in

where [NOx] = [NO] + [NO2], and the subscripts “in” and “out” indicate the inlet and outlet concentrations at the steady state, respectively. Because the reaction was carried out at low temperature, it was important to ensure that the decrease in NO concentration was not caused by its adsorption on the catalyst material. Hence, the catalyst material was purged until the NO concentration reached the expected value in each run.

3. RESULTS AND DISCUSSION 3.1. SCR Activity of Mn−ZrOx Catalysts. The effect of temperature on NO conversions over Mn−ZrOx mixed oxide catalysts calcined at 450 °C with different molar ratios of Mn/ (Mn + Zr) was first investigated, and the results are shown in Figure 1 and Table 1. Pure ZrO2 showed almost no activity and MnOx showed relatively high activity, as reported previously.20 It is interesting to note that almost 100% NO conversion was obtained at a space velocity of 30 000 h−1 over most of the Mn−ZrOx mixed oxides at 140−200 °C, except for Mn(0.2)− ZrOx-450. Figure 1 also shows the effect of the Mn/(Mn + Zr) ratio on the SCR activity. The activity increased with increasing Mn/(Mn + Zr) molar ratio and peaked at a value of 0.5−0.6. Evidently, a higher Mn/(Mn + Zr) ratio is not beneficial for this low-temperature SCR process. This observation indicated that Mn−ZrOx mixed oxides with an appropriate Mn/(Mn + Zr) ratio can facilitate high NOx conversions. The activities of Mn(0.5)−ZrOx mixed oxide catalysts calcined at different temperatures were investigated, and the 2648

dx.doi.org/10.1021/ie404224y | Ind. Eng. Chem. Res. 2014, 53, 2647−2655


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Figure 1. SCR activities of Mn−ZrOx mixed oxide catalysts with different molar ratios of Mn/(Mn + Zr). Reaction conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3%, GHSV = 30 000 h−1, and catalysts calcined at 450 °C.

Figure 2. SCR activities of Mn(0.5)−ZrOx catalysts calcined at different temperatures. Reaction conditions: [NH3] = [NO] = 1000 ppm, [O2] = 3%, and GHSV = 30 000 h−1.

results are shown in Figure 2 and Table 1. Mn−ZrOx mixed oxides calcined at different temperatures exhibited different SCR activities at lower temperatures, but all of them gave almost 100% NOx conversion at above 140 °C. However, Mn(0.5)−ZrOx catalysts calcined at below 650 °C showed notably better catalytic activities in the temperature range 80− 100 °C. Therefore, the lifetime of the Mn(0.5)−ZrOx-450 catalyst at 120 °C was further investigated (Figure 3). The experimental results showed that the initial SCR activity of Mn(0.5)−ZrOx-450 was very high (100% NOx conversion) and remained at 95% after 200 h of continuous operation. Moreover, its activity was completely restored after in situ regeneration (calcination in air at 450 °C for 2 h in the reactor). The results clearly indicated that the Mn(0.5)−ZrOx450 catalyst displayed satisfactory stability in our laboratoryscale work. The NH3-SCR activity of the Mn(0.5)−ZrOx-450 catalyst at different GHSVs was also investigated, and the results are shown in the inset in Figure 3. It can be seen that an increase in GHSV only resulted in a decrease of SCR activity at lower temperatures. Almost 100% NOx conversion was maintained at above 140 °C, even at a very high GHSV of 120 000 h−1, suggesting that this Mn(0.5)−ZrOx-450 catalyst is highly resistant to a high space velocity.

Figure 3. Stability of Mn(0.5)−ZrOx-450 catalyst and SCR activity of under different GHSVs (inset). Reaction conditions: [NH3] = [NO] = 1000 ppm, and [O2] = 3%.

3.2. Influences of H2O and SO2. It is well-known that water vapor is one of the main components in flue gases, and is especially relevant for the low-temperature SCR process operating downstream of the desulfurizer, since it can lead to

Table 1. Surface Areas, Pore Characterization, and deNOx Activities of Mn−ZrOx Catalysts NOx conversion (%) catalyst

surf. area (m /g)

pore vol (cm /g)

av pore size (nm)

80 °C

100 °C

120 °C

ZrO2-450 MnOx-450 Mn(0.2)−ZrOx-450 Mn(0.3)−ZrOx-450 Mn(0.4)−ZrOx-450 Mn(0.5)−ZrOx-450 Mn(0.6)−ZrOx-450 Mn(0.7)−ZrOx-450 Mn(0.5)−ZrOx-350 Mn(0.5)−ZrOx-550 Mn(0.5)−ZrOx-650 Mn(0.5)−ZrOx-750

2.6 11.3 102.1 126.1 165.3 138.7 117.2 84.5 160.6 95.8 51.4 14.9

0.006 0.170 0.021 0.049 0.062 0.080 0.080 0.086 0.073 0.074 0.063 0.038

39.4 42.6 6.3 2.8 3.0 3.1 3.7 4.3 4.2 3.9 4.8 7.5

0 4.8 32.1 52.4 74.8 87.0 86.2 82.4 80.7 83.7 53.3 24.6

0 12.8 52.4 79.3 99.6 100 100 100 100 100 88.8 58.7

0 25.2 72.9 96.4 100 100 100 100 100 100 99.3 92.3

2

3

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mixed oxides,18,19,31 and this property is very important for the industrial prospects of such catalysts. However, the NO conversion over Mn(0.5)−ZrOx-450 decreased to 47% in the following 2 h, indicating the progressive and accumulative nature of the deactivation, which is further certified by the progressive deactivation of the catalyst under a high GHSV of 200 000 h−1 as shown in the inset of Figure 4. When the SO2 supply was turned off, the catalytic activity did not recover but remained at a steady state of about 40%, showing the poisoning by SO2 to be irreversible. The Mn(0.5)−ZrOx-450 catalyst proved to be very stable over the first 4 h when the combined effects of both SO2 and water vapor were examined (Figure 4). It is also clear that the effect of SO2 and water vapor together was accumulative and combinatorial as the NO conversion decreased to 39% in the following 8 h with comparsion to the one above 47% achieved with SO2 alone (no water). Moreover, it was also observed that the activity could be only partly restored to about 60% NO conversion and remained at this steady level when the supply of SO2 and water vapor was turned off, showing the deactivation to be irreversible and pointing to a synergistic effect between H2O and SO2. It is also noteworthy that, after removal of all of the pollutants, the catalyst deactivated by the combination of water and SO2 still showed a higher activity than that exposed to SO2 alone, which further corroborated that the adsorption of H2O at the active sites was reversible. Further investigations of the significant deactivation process and mechanism are underway. 3.3. BET−BJH Analysis. The BET surface areas, pore volumes, and pore sizes of the studied Mn−ZrOx mixed oxide catalysts are summarized in Table 1. It can be seen that the surface areas of the pure manganese and zirconium oxides were far lower than those of the mixed oxides. The surface area apparently increased as the content of manganese increased and peaked at Mn(0.4)−ZrOx, and then decreased on further increase of the Mn content. However, the pore volume increased steadily with increasing Mn content, with pure MnOx showing the maximum value. The average pore diameters in these mixed oxides were 2.8−6.3 nm, and no regular changes could be discerned. It was found that the BET surface areas of the samples continuously decreased with increasing calcination temperature, while the pore volume increased a little as the calcination temperature was increased from 350 to 450 °C. Higher temperature resulted in a decrease in the pore volume and an increase in the average pore size. The sharp decrease in the activity of the catalysts calcined at 650 and 750 °C can be attributed to lower surface area of the crystalline material due to sintering. This indicated that the apparent properties of these catalysts were not the only parameters affecting activity as mixed oxides calcined at 450 °C showed the maximum activity (Figure 2). Thus, there must be a complex interplay between apparent properties and other factors such as phase composition and so on. 3.4. XRD Analysis. The XRD patterns of pure oxides and the Mn−ZrOx mixed oxide catalysts with various ratios of Mn/ (Mn + Zr) are presented in Figure 5A. The XRD pattern of pure MnOx featured intense and sharp peaks corresponding to mixed oxides of Mn2O3 (ICDD PDF No. 65-7467, 2θ = 23.2, 33.0, and 55.2°) and Mn5O8 (2Mn2O3·MnO2) (ICDD PDF No. 39-1218, 2θ = 18.1, 31.9, and 38.3°). This was quite distinct from the pattern of the catalyst calcined at above 650 °C as we reported previously,20 but was in good accordance with that of the MnOx phase structure obtained by calcining

deactivation of the catalyst. Hence, better water resistance is required for this SCR process. At the same time, any remaining traces of SO2 after the desulfurizer will undoubtedly have a negative effect on the deNOx activity. Hence, the effects of water and/or SO2 on the above Mn(0.5)−ZrOx-450 catalyst were further investigated, as it was found that good deNOx activity was maintained (Figures 2 and 3). Usually, two classic procedures are applied to test the influences of H2O and SO2 on SCR activities, performed either at a given temperature or over a wide temperature range.26 It is well-known that a catalyst is unable to rapidly attain the steady state in the lowtemperature SCR process. Hence, tests of H2O and SO2 resistance conducted at a given temperature are more likely to precisely reflect the real behavior of the catalyst. Because the temperature of flue gas downstream of the desulfurizer and electrostatic precipitator is about 150−160 °C,27 here, the SCR reaction was operated at 150 °C under a GHSV of 30 000 h−1 and allowed to stabilize (operate) for 1 h before the addition of H2O and/or SO2. It was clearly observed that 5% H2O in the flow had no effect on the deNOx activity (Figure 4), and it was

Figure 4. Effect of H2O and/or SO2 on SCR activity of Mn(0.5)− ZrOx-450. Reaction conditions: 150 °C, [NH3] = [NO] = 1000 ppm, and [O2] = 3%.

interesting to note that the NO conversion decreased to about 90% in 1 h and remained at a steady value even when 10% H2O was added. Furthermore, the activity was rapidly restored to 100% once more when the supply of water vapor was turned off. The results indicated that the Mn(0.5)−ZrOx-450 catalyst showed good water resistance and that the inhibitory effect of water was reversible. The reversible inhibition of H2O was also certified by the experiment under a very high GHSV of 200 000 h−1 as shown in the inset of Figure 4. It is believed that competing adsorption of H2O resulted in a partial decrease in the deNOx activity,28 as has also been observed with other Mnbased catalysts, for instance, Fe−Mn27 and Mn−Ce29 systems. On the other hand, the deNOx activity may also have been suppressed as a result of partial hydrolysis of surface acidic sites on ZrO2 under these conditions.30 When 100 ppm SO2 was added to the reactants, the NO conversion showed no decrease until 8 h later. This demonstrated that the combination of ZrO2 with MnOx had greatly improved sulfur tolerance compared to the Cr−MnOx mixed oxides that we reported previously20 as well as Mn-based 2650



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MnOx, which leads to improved deNOx activity and better water and sulfur tolerances. The XRD patterns of Mn(0.5)−ZrOx mixed oxide catalysts calcined at different temperatures are shown in Figure 6. A

Figure 5. XRD patterns of Mn−ZrOx catalysts with different Mn contents: (a) ZrO2-450, (b) Mn(0.2)−ZrOx-450, (c) Mn(0.3)−ZrOx450, (d) Mn(0.4)−ZrOx-450, (e) Mn(0.5)−ZrOx-450, (f) Mn(0.6)− ZrOx-450, (g) Mn(0.7)−ZrOx-450, (h) Mn(0.8)−ZrOx-450, and (i) MnOx-450.

Figure 6. XRD patterns of Mn(0.5)−ZrOx catalysts calcined at different temperatures (°C): (a) 350, (b) 450, (c) 550, (d) 650, and (e) 750.

manganese citrate in oxygen for 8 h at 427 °C, as reported by Kapteijn.5 The pure ZrO2 showed broad peaks corresponding to tetragonal ZrO2 (ICDD PDF No. 88-1007, 2θ = 30.3, 50.4, and 60.2°), indicating that it was poorly crystallized or highly dispersed with a small size. The diffraction patterns of all of the mixed oxides showed broad peaks corresponding to the tetragonal ZrO2 phase structure, which became wider with increasing Mn content. No distinct peaks corresponding to manganese oxides were observed, except for Mn(0.7)−ZrOx450, for which a very weak peak was visible (2θ = 33.0°). To further elucidate the effect of the Mn/(Mn + Zr) ratio on the structure of the Mn−ZrOx catalysts, Mn(0.8)−ZrOx-450 was prepared and characterized. It showed very distinctive diffraction peaks for the MnOx phases, corresponding to Mn2O3 (2θ = 23.2, 33.0, and 55.2°) and Mn5O8 (2θ = 18.1 and 38.3°), as seen for pure MnOx. The diffraction peaks for the ZrO2 phase shifted to higher angles as shown in Figure 5B, and the crystal lattice parameters of the tetragonal ZrO2 phase (listed in Table 2) gradually decreased with increasing Mn

lower calcination temperature, such as 350 °C, resulted in amorphous ZrO2, giving rise to a broad peak between 25 and 39°. Catalysts calcined at higher temperature showed the same characteristic XRD patterns attributable to ZrO2 with a tetragonal structure (ZrO2 ICDD PDF No. 88-1007, 2θ = 30.3, 50.4, and 60.2°). These characteristic diffraction peaks became stronger and narrower with increasing calcination temperature. On the other hand, the characteristic diffraction peaks of MnOx gradually emerged with increasing calcination temperature. For example, a weak peak at 33.0° corresponding to Mn2O3 was seen at a calcination temperature of 650 °C, and peaks corresponding to the Mn2O3 phase appeared at 23.2, 33.0, and 55.2° (ICDD PDF No. 65-7467) at an elevated temperature of 750 °C. This implied that the dispersed amorphous MnOx was transformed into the Mn2O3 crystal phase at higher temperature. It is interesting to note that all of the diffraction peaks of the ZrO2 phase became sharp and shifted toward lower angles, implying that both distinct phase separation and aggregation took place at above 650 °C. Evidently, this resulted in decreases in surface area and pore volume (Table 1). 3.5. H2-TPR Analysis. The H2-TPR patterns of the Mn− ZrOx catalysts with different Mn contents are shown in Figure 7. The broad, weak peak beginning at 320 °C and centered at 520 °C for pure ZrO2 can be attributed to reduction of the surface oxygen and some of the lattice oxygen of ZrO2,24,33 although ZrO2 is difficult to reduce. The MnOx showed two reduction steps, with peaks centered at 292 and 423 °C, respectively. The first stage can be ascribed to the reduction of both Mn2O3 and Mn5O8 to Mn3O4, and the second stage can be ascribed to reduction of Mn3O4 to MnO, according to the literature and our previous work.5,20 The reduction behavior of mixed oxides is quite different from those of pure MnOx and ZrO2. A broad peak in the lowtemperature range between 100 and 460 °C was observed for each of the Mn−ZrOx catalysts, which could be assigned to nonstoichiometrically dispersed MnOx phases, and the second peak in a higher temperature range between 480 and 710 °C could be assigned to the reduction of oxygen species of ZrO2 or

Table 2. XRD Analysis of Tetragonal ZrO2 Diffraction Peaks catalyst

2θ

d (Å)

pure ZrO2-450 Mn(0.2)−ZrOx-450 Mn(0.3)−ZrOx-450 Mn(0.4)−ZrOx-450 Mn(0.5)−ZrOx-450 Mn(0.6)−ZrOx-450 Mn(0.7)−ZrOx-450 Mn(0.8)−ZrOx-450

30.34 30.55 30.61 30.92 30.92 31.25 31.34 31.34

2.94 2.92 2.92 2.89 2.89 2.86 2.85 2.85

content, e.g., 2.94 Å for pure ZrO2 and 2.85 Å for Mn(0.7)− ZrOx. This means that Mnn+ with much a smaller ionic radius (0.53, 0.64, 0.83, and 0.84 Å for Mn4+, Mn3+, Mn2+, and Zr4+, respectively) can be incorporated into the ZrO2 crystal lattice to form a solid solution.32 Hence, it could be concluded that the formation of a solid solution results in highly dispersed 2651



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Figure 8. H2-TPR of Mn(0.5)−ZrOx catalysts calcined at different temperatures (°C): (a) 350, (b) 450, (c) 550, (d) 650, and (e) 750.

ZrO2, and the sintering of mixed oxide at high temperatures. The TPR results matched well with both the BET and XRD results. 3.6. NH3-TPD Analysis. The adsorption and activation of ammonia at surface active sites on a catalyst plays a crucial role in the NH3-SCR reaction according to both the Eley−Rideal and Langmuir−Hinshelwood mechanisms reported in the literature.4 Therefore, NH3-TPD was performed to investigate the surface acid properties of these mixed oxides (Figure 9).

Figure 7. H2-TPR of Mn−ZrOx catalysts with different Mn contents: (a) ZrO2-450, (b) Mn(0.2)−ZrOx-450, (c) Mn(0.3)−ZrOx-450, (d) Mn(0.4)−ZrOx-450, (e) Mn(0.5)−ZrOx-450, (f) Mn(0.6)−ZrOx-450, (g) Mn(0.7)−ZrOx-450, and (h) MnOx-450.

Mn−Zr solid solution.34,35 The area of the peak corresponding to the reduction of MnOx increased, and the reduction behavior of the mixed oxides became more similar to that of pure MnOx with increasing Mn content. For example, a very weak shoulder peak appeared at an Mn/(Mn + Zr) ratio of 0.6 (Figure 7f), and a further increase in Mn content resulted in splitting of this peak (Figure 7g). Hence, it is thought that aggregation of MnOx occurred at high Mn/(Mn + Zr) ratios. Highly dispersed MnOx is easier to reduce than bulk MnOx, which is in good accordance with the XRD results (Figure 5g) as a very weak peak corresponding to MnOx was seen for the Mn(0.7)−ZrOx450 catalyst. The reduction peak of Mn−Zr solid solution shifted to higher temperatures in all of the mixed oxides compared to pure ZrO2, and the peak area decreased with decreasing Zr content. According to the XRD results (Table 2), the doping of the ZrO2 lattice with the smaller Mnn+ resulted in lower crystal lattice parameters for the tetragonal ZrO2 phase in the mixed oxides in comparison to that for pure ZrO2, and this was undoubtedly responsible for the observed increase in the reduction temperature of the lattice oxygen of ZrO2. The H2-TPR patterns of Mn(0.5)−ZrOx catalysts calcined at different temperatures are shown in Figure 8. All of the catalysts showed two reduction peaks. The first broad peak within the temperature range 100−500 °C could be assigned to the reduction of MnOx, and the second peak in the temperature range 500−700 °C could be assigned to the reduction of lattice oxygen of ZrO2 or Mn−Zr solid solution as discussed above. The first reduction peak shifted to higher temperatures with increasing calcination temperature, and a shoulder peak appeared at elevated temperatures (Figure 8d,e), implying that high-temperature sintering leads to the formation of aggregated MnOx. It is also interesting to note that the second reduction peak area diminished following calcination at 750 °C. It is thought that the mobility of Mnn+ from the ZrO2 lattice and phase separation between MnOx and ZrO2 at much higher temperatures contribute to this, and also accounted for the surface area loss, the crystallization of amorphous MnOx and

Figure 9. NH3-TPD of Mn−ZrOx catalysts with different Mn contents: (a) ZrO2-450, (b) Mn(0.2)−ZrOx-450, (c) Mn(0.3)− ZrOx-450, (d) Mn(0.4)−ZrOx-450, (e) Mn(0.5)−ZrOx-450, (f) Mn(0.6)−ZrOx-450, (g) Mn(0.7)−ZrOx-450, and (h) MnOx-450.

Both ZrO2 and MnOx show very low acidity and had few acidic centers according to their lower surface areas and chemical nature. It is interesting to note that all of the mixed oxides showed weak but abundant acidic sites. According to the literature,26 the low-temperature peak centered in the range 150−200 °C can be attributed to NH3 desorbed from different weakly acidic sites. The high-temperature peak centered in the range 200−300 °C can be ascribed to the desorption of NH3 from Lewis acidic sites and/or the dissociation of NH4+ formed between NH3 and surface Brønsted acidic sites. The NH3 desorption peak became narrower and shifted to lower temperature with increasing Mn content, indicating that the surface acidic sites in the mixed oxides became weaker and much more homogeneous. It was noted that the overall acidity 2652



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Figure 10. XPS spectra for (A) Mn 2p, (B) Zr 3d, and (C) O 1s of Mn−ZrOx catalysts with different Mn contents: (a) Mn(0.2)−ZrOx-450, (b) Mn(0.3)−ZrOx-450, (c) Mn(0.4)−ZrOx-450, (d) Mn(0.5)−ZrOx-450, (e) Mn(0.6)−ZrOx-450, and (f) Mn(0.7)−ZrOx-450.

to all oxygen species (Oβ/(Oα + Oβ)) (about 36.7−46.8%) in the mixed oxide is thought to be beneficial for the NH3-SCR reaction. The surface atom contents calculated from the Mn 3p, Zr 3d, and O 1s peaks are listed in Table 3. The surface Mn contents

(calculated by integration of the desorption peak area; not shown) significantly increased with Mn incorporation and reached a maximum value for Mn(0.4)−ZrOx-450, which is in good accordance with results obtained by Gutiérrez-Ortiz when Mn−Zr mixed oxide catalysts were prepared by coprecipitation method and used for gas-phase oxidation of chlorocarbons.35 Hence, it is concluded that mixed oxides such as Mn(0.4)− ZrOx-450, with abundant and uniform weakly acidic centers, are beneficial for this low-temperature SCR process. 3.7. XPS Analysis. The surface chemical states of Mn 2p, Zr 3d, and O 1s in Mn−ZrOx catalysts with different Mn contents were investigated, and the results are shown in Figure 10. The Mn 2p region features a spin−orbit doublet with Mn 2p3/2 and Mn 2p1/2 having binding energies of about 642 and 653 eV, respectively (Figure 10A). The Mn 2p3/2 spectra can be separated into three characteristic peaks by deconvolution: 641.0−641.2, 642.2−642.5, and 644.3−644.9 eV, which correspond to the characteristic peaks of Mn2+, Mn3+, and Mn4+ respectively.36 The Mn 2p3/2 binding energies were higher than those of pure MnO, MnO2, and Mn2O3,37,38 which indicated a strong interaction between manganese and zirconium oxides. This is not only in good accordance with previous reports on Ce−Mn and Fe−Mn mixed oxides,21,36 but is also supportive of the above interpretations of the XRD and TPR results. At the same time, increasing peak intensity and area indicated increasing surface content of Mn. Figure 10B shows that Zr 3d5/2, with binding energy centered at 181.6 eV, appeared in all of the mixed oxides, which indicated the sole presence of Zr4+ ions. The binding energy values of Zr 3d5/2 were lower than that reported for pure ZrO2 (182.1 eV),39 which further confirmed the interaction between manganese and zirconium oxides. XPS results for O 1s (Figure 10 C) clearly showed that two kinds of surface oxygen species could be distinguished by deconvolution. The lower-binding-energy peak at 529.0−530.0 eV can be ascribed to the surface lattice oxygen (Oα), and the higher-energy peak at 531.0−532.0 eV can be assigned to chemisorbed oxygen and/or defect oxides (O β ).40 In the low-temperature SCR process, surface chemisorbed oxygen has been reported to be the most active type of oxygen and plays an important role in oxidation reactions.41 Hence, the relatively higher ratio of surface oxygen

Table 3. Surface Atom Contents in Mn−ZrOx Catalysts with Different Mn Contents catalyst

Mn (%)

Zr (%)

O (%)

Mn/(Mn + Zr)

Mn(0.2)−ZrOx-450 Mn(0.3)−ZrOx-450 Mn(0.4)−ZrOx-450 Mn(0.5)−ZrOx-450 Mn(0.6)−ZrOx-450 Mn(0.7)−ZrOx-450

4.7 8.4 12.1 15.8 18.6 20.3

21.9 19.8 17.1 13.6 10.7 10.2

73.4 71.8 70.8 70.6 70.8 69.5

0.18 0.30 0.41 0.54 0.63 0.67

were close to the stoichiometric compositions of the mixed oxides, and with increasing feed Mn content, the surface Mn content became a little higher than that in the bulk phase until the Mn/(Mn + Zr) ratio reached 0.7. Taking these results together with the XRD data, it is evident that the Mn is inserted into the ZrO2 crystal lattice to form a solid solution and that the excess Mn is highly dispersed on the surfaces of the mixed oxides. Highly dispersed Mn on the surface improved the NOx conversion activity. The dispersity of the Mn decreased on further increase of the Mn content of the mixed oxides, which was not beneficial for the deNOx activity.

4. CONCLUSION A series of novel Mn−Zr mixed oxides showing high activity in the low-temperature SCR of NOx with NH3 in the presence of oxygen has been synthesized. NOx conversion of 100% was obtained at 100 °C at a GHSV of 30 000 h−1 over an Mn(0.5)− ZrOx-450 catalyst prepared by the citric acid method. This Mn(0.5)−ZrOx-450 catalyst showed good water resistance and sulfur tolerance, although both SO2 and H2O in the feed gas could exert an adverse effect on the NOx conversion; the effect of H2O could be quickly eliminated after its removal, but the deactivation by SO2 proved to be irreversible. Detailed characterization and analysis has showed that the incorporation 2653



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of smaller Mnn+ ions into the ZrO2 crystal results in the formation of a solid solution with highly dispersed MnOx. The interaction between Mnn+ and Zr4+, highly dispersed Mn on the surface of the mixed oxides, higher concentrations of surface oxygen species, and a suitable amount of weakly bound NH3 on the surface are all beneficial for the low-temperature SCR properties.

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(14) Liu, F. D.; He, H.; Zhang, C. B. Novel iron titanate catalyst for the selective catalytic reduction of NO with NH3 in the medium temperature range. Chem. Commun. 2008, No. 17, 2043−2045. (15) Zhu, Z.; Liu, Z.; Niu, H.; Liu, S. Promoting Effect of SO2 on Activated Carbon-Supported Vanadia Catalyst for NO Reduction by NH3 at Low Temperatures. J. Catal. 1999, 187 (1), 245−248. (16) Liu, Z. Y.; Huang, Z. G.; Zhu, Z. P. Combined effect of H2O and SO2 on V2O5/AC catalysts for NO reduction with ammonia at lower temperatures. Appl. Catal., B 2002, 39 (4), 361−368. (17) Kang, M.; Park, E. D.; Kim, J. M.; Yie, J. E. Cu-Mn mixed oxides for low temperature NO reduction with NH3. Catal. Today 2006, 111 (3−4), 236−241. (18) Long, R. Q.; Yang, R. T.; Chang, R. Low temperature selective catalytic reduction (SCR) of NO with NH3 over Fe-Mn based catalysts. Chem. Commun. 2002, No. 5, 452−453. (19) Qi, G.; Yang, R. T. A superior catalyst for low-temperature NO reduction with NH3. Chem. Commun. 2003, No. 7, 848−849. (20) Chen, Z. H.; Yang, Q.; Li, H.; Li, X. H.; Wang, L. F.; Tsang, S. C. Cr-MnOx mixed-oxide catalysts for selective catalytic reduction of NOx with NH3 at low temperature. J. Catal. 2010, 276 (1), 56−65. (21) Chen, Z. H.; Wang, F. R.; Li, H.; Yang, Q.; Wang, L. F.; Li, X. H. Low-temperature selective catalytic reduction of NOx with NH3 over Fe-Mn mixed-oxide catalysts containing Fe3Mn3O8 phase. Ind. Eng. Chem. Res. 2012, 51 (1), 202−212. (22) Lopez, E. F.; Escribano, V. S.; Resini, C.; Gallardo-Amores, J. M.; Busca, G. A study of coprecipitated Mn-Zr oxides and their behaviour as oxidation catalysts. Appl. Catal., B 2001, 29 (4), 251− 261. (23) Zhao, Q.; Shih, W. Y.; Chang, H. L.; Shih, W. H. Redox activity and NO storage capacity of MnOx-ZrO2 with enhanced thermal stability at elevated temperatures. Ind. Eng. Chem. Res. 2010, 49 (4), 1725−1731. (24) Choudhary, V. R.; Uphade, B. S.; Pataskar, S. G. Low temperature complete combustion of dilute methane over Mn-doped ZrO2 catalysts: factors influencing the reactivity of lattice oxygen and methane combustion activity of the catalyst. Appl. Catal., A 2002, 227 (1−2), 29−41. (25) Matsukuma, I.; Kikuyama, S.; Kikuchi, R.; Sasaki, K.; Eguchi, K. Development of zirconia-based oxide sorbents for removal of NO and NO2. Appl. Catal., B 2002, 37 (2), 107−115. (26) Xu, H. D.; Zhang, Q. L.; Qiu, C. T.; Lin, T.; Gong, M. C.; Chen, Y. Q. Tungsten modified MnOx-CeO2/ZrO2 monolith catalysts for selective catalytic reduction of NOx with ammonia. Chem. Eng. Sci. 2012, 76, 120−128. (27) Qi, G. S.; Yang, R. T. Low-temperature selective catalytic reduction of NO with NH3 over iron and manganese oxides supported on titania. Appl. Catal., B 2003, 44 (3), 217−225. (28) Kijlstra, W. S.; Daamen, J. C. M. L.; van de Graaf, J. M.; van der Linden, B.; Poels, E. K.; Bliek, A. Inhibiting and deactivating effects of water on the selective catalytic reduction of nitric oxide with ammonia over MnOx/Al2O3. Appl. Catal., B 1996, 7 (3−4), 337−357. (29) Qi, G. S.; Yang, R. T. Performance and kinetics study for lowtemperature SCR of NO with NH3 over MnOx-CeO2 catalyst. J. Catal. 2003, 217 (2), 434−441. (30) Kourieh, R.; Retailleau, L.; Bennici, S.; Giroir-Fendler, A.; Auroux, A. Influence of the acidic properties of ZrO2 based mixed oxides catalysts in the selective reduction of NOx with n-decane. Catal. Lett. 2013, 143 (1), 74−83. (31) Yu, J.; Guo, F.; Wang, Y. L.; Zhu, J. H.; Liu, Y. Y.; Su, F. B.; Gao, S. Q.; Xu, G. W. Sulfur poisoning resistant mesoporous Mn-base catalyst for low-temperature SCR of NO with NH3. Appl. Catal., B 2010, 95 (1−2), 160−168. (32) Azalim, S.; Franco, M.; Brahmi, R.; Giraudon, J. M.; Lamonier, J. F. Removal of oxygenated volatile organic compounds by catalytic oxidation over Zr-Ce-Mn catalysts. J. Hazard. Mater. 2011, 188 (1−3), 422−427. (33) Esposito, S.; Turco, M.; Bagnasco, G.; Cammarano, C.; Pernice, P.; Aronne, A. Highly dispersed sol−gel synthesized Cu-ZrO2

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 0086 20 8711 4707. Fax: 0086 20 8711 4707. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Natural Science Foundation of China (Nos. 21276095, 21206048) and Sinopec Group (112022) are gratefully acknowledged for financial support of this work.

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Low-Temperature Selective Catalytic Reduction of NOx

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