Catalytic performance and SO2 tolerance of tetragonal-zirconia-based catalysts for low-temperature selective catalytic reduction

Rong Liu; Lingchen Ji; Yifan Xu; Fei Ye; Feng Jia

doi:10.1557/jmr.2016.283

Catalytic performance and SO2 tolerance of tetragonal-zirconia-based catalysts for low-temperature selective catalytic reduction

Published online by Cambridge University Press: 15 August 2016

Rong Liu ,

Lingchen Ji ,

Yifan Xu ,

Fei Ye and

Feng Jia

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Rong Liu*: Affiliation:
Jangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing Normal University, Nanjing, China; School of Environment, Nanjing Normal University, Nanjing, China; and Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Controlling, Nanjing Normal University, Nanjing, China
Lingchen Ji: Affiliation:
Jangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing Normal University, Nanjing, China; School of Environment, Nanjing Normal University, Nanjing, China; and Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Controlling, Nanjing Normal University, Nanjing, China
Yifan Xu: Affiliation:
Jangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing Normal University, Nanjing, China; School of Environment, Nanjing Normal University, Nanjing, China; and Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Controlling, Nanjing Normal University, Nanjing, China
Fei Ye: Affiliation:
Jangsu Center for Collaborative Innovation in Geographical Information Resource Development and Application, Nanjing Normal University, Nanjing, China; School of Environment, Nanjing Normal University, Nanjing, China; and Jiangsu Provincial Key Laboratory of Materials Cycling and Pollution Controlling, Nanjing Normal University, Nanjing, China
Feng Jia: Affiliation:
School of Environment, Nanjing Normal University, Nanjing, China; and Nanoparticle and Air Quality Laboratory, Institute of Environmental Engineering, National Chiao Tung University, Taiwan, China
*: a) Address all correspondence to this author. e-mail: liurongle@163.com

Article contents

Abstract
INTRODUCTION
EXPERIMENTAL METHODS
RESULTS AND DISCUSSION
CONCLUSIONS
References

Abstract

MnOx –CeO2/t-ZrO2 catalyst was prepared by impregnation of nanotetragonal zirconia. The NO conversion of 5 wt% MnOx –CeO2/t-ZrO2 catalyst was 68.1% at 100 °C while that of 30 wt% MnOx –CeO2/t-ZrO2 catalyst was 97.4%. The x-ray diffraction, Brunner–Emmet–Teller measurements (BET), and H2-TPR showed surface properties of the prepared catalysts were good for selective catalytic reduction reactions. X-ray photoelectron spectroscopy analysis indicated that Mn4+ and Ce4+ oxidation states were predominant on the surface of the catalyst and so was lattice oxygen which was conducive to Lewis acid sites. NH3-TPD test results demonstrated that Lewis acid sites are predominant on the surface of catalyst. The presence of SO2 reduced the catalyst activity. The realized conversion dramatically decreased to 47% from nearly 100% after 8 h. Characterization of fresh and spent catalysts indicated the deterioration of active component and deposition of NH4HSO4 or (NH4)2SO4 contribute to SO2 poisoning.

Keywords

tetragonal zirconia MnOx –CeO2/t-ZrO2 catalyst catalytic activity SO2 poisoning mechanism low-temperature SCR

Information

Type: Articles
Information: Journal of Materials Research , Volume 31 , Issue 17 , 14 September 2016 , pp. 2590 - 2597

DOI: https://doi.org/10.1557/jmr.2016.283 [Opens in a new window]
Copyright: Copyright © Materials Research Society 2016

I. INTRODUCTION

NO_x is potentially harmful to humans as a kind of primary pollutant. And NO_x are the main cause of many environment problems, such as acid rain, surface ozone pollution, and particulate matter 2.5.^{Reference Tang, Hao, Xu and Li1} The emission of NO_x was 2337.8 tons in China in 2011 and 2275.4 tons in 2012.² Emission standard of air pollutants for thermal power plants which came into effect on January 1, 2012 requires the emission concentration of NO_x under 100 mg/m³.

Selective catalytic reduction (SCR) of NO_x with NH₃ is the most promising method to remove NO_x. Catalysts with high activity play a decisive part in low-temperature SCR technology. Many researches about metal oxide as SCR catalyst support have been reported recently, such as TiO₂,^{Reference Qi and Yang3} Al₂O₃,^{Reference Sullivan and Doherty4} activated carbon^{Reference Zhu, Liu, Niu, Liu, Hu, L and Xie5,Reference Yoshikawa, Yasutake and Mochida6} and molecular sieve.^{Reference Delahay, Valade, Guzmfin-Vargas and Cop7} Zirconium oxide has attracted considerable attention recently as a catalyst support because of its special characteristics. Cao et al.^{Reference Fan, Xiang, Su, Wang, Sun, Hu and Lei8} found that incorporating the Zr⁴⁺ ions into the CeO₂ crystal could form solid solution, and increase the BET surface area and poresit, comparing with Mn–Ce catalyst. Yan et al.^{Reference Yan, Hu and Xu9} reported that the existence of ZrO₂ in catalysts can enhance the dispersion of CeO₂ on catalysts which results in high activity of the catalysts. Takahashi et al.^{Reference Takahashi, Suda and Hachisuka10} investigated the influence of various compositions of TiO₂ and ZrO₂ on NO_x removal over a sulfur-treated NO_x storage and reduction (NSR) catalyst and found that TiO₂–ZrO₂-supported metal oxide catalysts have the best catalytic activity. Reddy et al.^{Reference Reddy, Khan, Yamada, Kobayashi, Loridant and Volta11} investigated structural characteristics of nano-sized ceria–silica, ceria–titania, and ceria–zirconia mixed oxide catalysts and found that these mixed oxides exhibit better redox properties than pure CeO₂. Song et al.^{Reference Song, Ping, Zhang, Li, Zhang, Wang, Liu and Huang12} found that CeO₂–ZrO₂–WO₃ catalyst exhibits high performance and superior hydrothermal stability at high temperature, it can achieve over 90% of NO_x conversion at 250–525 °C. The experimental and kinetic model by Shen et al.^{Reference Shen, Wang, Wang and Liu13} indicated that MnO_x(0.6)/Ce_0.5Zr_0.5O₂ catalyst has good resistance to H₂O and SO₂ and has high N₂ selectivity.

It is well known that ZrO₂ exists mainly in three polymorphs with monoclinic (m-ZrO₂), tetragonal (t-ZrO₂), and cubic (c-ZrO₂) structures.^{Reference Zhen and Chen14} ZrO₂ polymorphs have different amphoteric characters connected to surface hydroxyl groups. The crystalline phase of ZrO₂ has a great effect on the structure, activity, and selectivity of catalysts. Therefore, it is valuable to investigate the effects of nanocrystalline zirconia polymorphs on catalytic properties of MnO_x–CeO₂/ZrO₂ catalysts which few researchers have concerned about. In this study, we investigated the catalytic activity and resistance to SO₂ poisoning of SCR catalysts with manganese oxide and cerium oxide supported on pure t-ZrO₂.

II. EXPERIMENTAL METHODS

A. Catalyst preparation

Pure nanocrystalline tetragonal zirconia (t-ZrO₂) was prepared by coprecipitation based on our previous researches.^{Reference Guan, Gong, Liu and Yang15} A series of manganese oxide and cerium oxide supported t-ZrO₂ catalysts with varying active component content (MnO_x–CeO₂) from 5 to 30 wt% were prepared by wet impregnation method. The molar ratio of Mn/Ce was 2.^{Reference Liu and Yang16} MnO_x–CeO₂/t-ZrO₂ catalyst samples were finally prepared through calcining at 500 °C for 4 h after being heated at 120 °C for 12 h.

B. Catalytic activity tests

SCR activity measurements were carried out in a fixed-bed reactor at 30–240 °C containing 3 mL catalyst with a gas velocity (GHSV) of 32,000 h⁻¹. Particle size of catalysts was 40–60 mesh. The typical reactant gas consisted of 0.05 vol% NO, 0.05 vol% NH₃, 7–8 vol% O₂, 0.025 vol% SO₂ (when needed) and the balance was N₂. SO₂ was introduced into the flue gas to investigate the resistance to SO₂ poisoning when 100% of NO conversion was reached at 130 °C. The reactants were all sent out from gas cylinder and preheated in a gas mixer, and then the mixed gas was sent to the reactor. The concentrations of NO and N₂O in the inlet and outlet streams of the reactor were monitored using an on-line MGA5 flue gas analyzer.

The conversion of NO was calculated by

$${\left[ {{\rm{Conv}}.} \right]_{{\rm{NO}}}} = {{{{\left[ {{\rm{NO}}} \right]}_{{\rm{in}}}} - {{\left[ {{\rm{NO}}} \right]}_{{\rm{out}}}}} \over {{{\left[ {{\rm{NO}}} \right]}_{{\rm{in}}}}}} \times 100\% \quad ,$$

where [Conv.]_NO is NO conversion efficiency and [NO]_in and [NO]_out refer to the NO concentration at the reactor inlet and outlet, respectively.

C. Catalyst characterization

X-ray diffraction (XRD) studies were carried out on a DX2500 diffractometer (Rigaku Corporation, Tokyo, Japan). BET tests were carried out on a NOVA 1000e surface area measurement (Quantachrome Instruments, Shanghai, China). X-ray photoelectron spectroscopy (XPS) tests were carried out on an escalab 250Xi x-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, Massachusetts). Transmission electron microscopy (TEM) tests were carried on a H7650 transmission electron microscope (Hitachi Corp., Tokyo, Japan). Temperature-programmed reduction of H₂ and also temperature-programmed desorption of NH₃ were carried out on TP-5000 III Multi-purpose adsorption instrument (XianQuan Company, Tianjin, China). Fourier transform infrared spectroscopy (FT-IR) tests were carried on an iS5 Fourier transform infrared spectrometer (Thermo Nicolet Corporation, Madison, Wisconsin).

III. RESULTS AND DISCUSSION

A. Catalytic activity

1. Effect of active component on catalytic activity

The activity of the prepared catalyst with the variation of active component is presented in Fig. 1. It can be seen from Fig. 1 that pure tetragonal zirconia has almost no catalytic activity for the low-temperature SCR of NO with NH₃ and the activity increased with the increasing active component. The NO conversion of 5 wt% MnO_x–CeO₂/t-ZrO₂ was 68.1% at 100 °C while that of 30 wt% MnO_x–CeO₂/t-ZrO₂ was 97.4%.

FIG. 1.

Variations of NO conversion efficiency with active component for prepared MnO_x–CeO₂/t-ZrO₂ catalysts at 100 °C.

2. Effect of temperature on catalytic activity

The activity of the prepared catalyst with the variation of temperature from RT to 130 °C is presented in Fig. 2. It can be seen that the NO conversion of all samples increased with the increasing temperature. The MnO_x–CeO₂/t-ZrO₂ catalyst showed high catalytic activity for the low-temperature SCR of NO with NH₃. The NO conversion of 5 wt% MnO_x–CeO₂/t-ZrO₂ was 51.3% at 90 °C while that of 10 wt% MnO_x–CeO₂/t-ZrO₂, 20 wt% MnO_x–CeO₂/t-ZrO₂ and 30 wt% MnO_x–CeO₂/t-ZrO₂ were 67.5%, 75.4, and 81%, respectively. 100% NO conversion of 5 wt% MnO_x–CeO₂/t-ZrO₂, 10 wt% MnO_x–CeO₂/t-ZrO₂, 20 wt% MnO_x–CeO₂/t-ZrO₂, and 30 wt% MnO_x–CeO₂/t-ZrO₂ were obtained at 130, 118, 107, and 104 °C, respectively.

FIG. 2.

Variations of NO conversion efficiency with temperature for prepared MnO_x–CeO₂/t-ZrO₂ catalysts with different MnO_x–CeO₂ dopant contents. Conditions: 0.05 vol% NO, 0.05 vol% NH₃, 7–8 vol% O₂, and balance N₂; GHSV = 32,000 h⁻¹.

B. Characterization

1. X-ray diffraction

The XRD patterns of various MnO_x–CeO₂/t-ZrO₂ catalysts are shown in Fig. 3. It can be seen that minor diffraction peaks corresponding to monoclinic zirconia were observed for samples with 5, 10, and 20 wt% MnO_x–CeO₂/t-ZrO₂ catalysts. According to quantitative analysis, the ratios of m-ZrO₂ are 2.48, 3.41, and 8.74% for 5, 10 and 20 wt% MnO_x–CeO₂/t-ZrO₂ catalysts, respectively. At MnO_x–CeO₂ noticing of 30 wt%, peaks corresponding to CeO₂ were detected and monoclinic zirconia was not observed. This could be because cerium oxide precursors stabilized the tetragonal zirconia polymorph.

FIG. 3.

XRD patterns of various MnO_x–CeO₂/t-ZrO₂ catalysts with different MnO_x–CeO₂ dopant contents.

For MnO_x–CeO₂ ≤20 wt%, the amount of new generated m-ZrO₂ increased with increasing active component. This can be explained by the diffusion of precursors into nanocrystalline ZrO₂ resulting in the transformation of ZrO₂ polymorphs. For all samples, no peaks corresponding to manganese oxide were identified. This indicated that manganese and cerium oxides were well dispersed on the tetragonal zirconia carrier, which was good for SCR reactions.^{Reference Sager, Kondarides and Verykios17}

2. BET surface area and pore size distribution

The specific surface areas, pore volumes, and average pore diameters of the MnO_x–CeO₂/t-ZrO₂ catalysts prepared with different amounts of active component are summarized in Table I. Small decreases in surface areas and increases in average pore sizes were observed after MnO_x–CeO₂ deposited on the pure tetragonal zirconia and calcined. This could be caused by the agglomeration of metallic oxide at the surface of MnO_x–CeO₂/t-ZrO₂ catalysts in calcine process. The pore volume of all samples was in the mesopore size range (d 2–50 nm) which is good for NH₃ adsorption.

TABLE I.

Physical properties of the catalysts prepared with different amounts of active component.

For further research on catalytic properties of MnO_x–CeO₂/t-ZrO₂ catalysts, 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst was investigated because 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst showed a relatively higher SCR activity.

3. X-ray photoelectron spectroscopy

The 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst samples were analyzed using XPS to identify the surface atomic concentrations of Ce, Mn, and O as detected with the photoelectron spectra of Mn 2p, Ce 3d, and O 1s (in Fig. 4).

FIG. 4.

XPS spectra of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst, (a) Mn 2p; (b) Ce 3d; and (c) O 1s.

The Mn 2p spectra are presented in Fig. 4(a). The Mn 2p region consisted of a spin–orbit doublet with Mn 2p _1/2 with a BE of about 653.3 eV and Mn 2p _3/2 with a BE of 641.7 eV,^{Reference Zhang, Ji, Zhang and Yang18,Reference Peña, Uphade and Smirniotis19} which are characteristic of a mixed-valence manganese system (Mn⁴⁺ and Mn³⁺). The Mn 2p _3/2 spectra can be split into two peaks. Mn⁴⁺ oxidation state is predominant. The Ce 3d spectra are presented in Fig. 4(b), which contain two multiple peaks. The peaks are attributed to Ce⁴⁺ or Ce³⁺ by split, and Ce⁴⁺ oxidation state is predominant.

It can be seen in Fig. 4(c), all the O 1s peaks could be fitted into two peaks referred to the lattice oxygen and the chemisorbed oxygen.^{Reference Xie, Fang, He, Chen, Fu and Chen20} Lattice oxygen is predominant. Surface hydroxide radical has been reported to play an important role in the oxidation of NO to NO₂.^{Reference Wu, Liu and Wang21}

4. Temperature-programmed reduction of H₂

TPR profile of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst is shown in Fig. 5. It can be seen that the prominent peak is in the temperature region of 200–400 °C. The peaks correspond to the reduction of MnO₂ to Mn₂O₃ → Mn₃O₄ → MnO.^{Reference Thompson and Pembeaon22} This indicates that the reducibility of catalysts is good for the reduction of NO to NO₂.

FIG. 5.

H₂-TPR patterns of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst.

5. Temperature-programmed desorption of NH₃

In the present investigation, the acidity measurements have been carried out by NH₃-TPD method. The results are shown in Fig. 6. As can be seen from the curve, a weak and moderate peak at 150 °C corresponding to desorption of NH₃ adsorbed at weak acid sites was observed. The sharp peaks at temperature region from 450 to 650 °C correspond to desorption of NH₃ adsorbed at Lewis acid sites. Lewis acid sites are predominant on the surface of 10 wt% MnO_x–CeO₂/t-ZrO₂.

FIG. 6.

NH₃-TPD pattern of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst.

C. Resistance to SO₂ poisoning

1. Effect of SO₂ on catalytic activity

Figure 7 shows the resistance of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst to SO₂ poisoning during SCR of NO with NH₃. As can be seen from the curve, once SO₂ was introduced into the treated flue gas, the NO conversion enabled by the catalysts exhibited a rapid decrease, demonstrating that the presence of SO₂ in the gas certainly reduced the catalyst activity. Its realized NO conversion could still reach 85% in the tested 2.5 h. But in the next tested 5 h, the conversion dramatically decreased to 47%.

FIG. 7.

Effect of SO₂ on catalytic activity of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst at 130 °C.

2. TEM analysis

Figure 8 presents the TEM images for fresh and spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst. The TEM image in Fig. 8(a) indicates that active component (MnO_x–CeO₂) evenly coats the surface of t-ZrO₂, catalyst particles are almost same size and the average size is in the range of 20–40 nm, some catalyst particles aggregate together. Figure 8(b) shows that there is considerable amount of many white protuberant depositions on the surface after being poisoned by SO₂.

FIG. 8.

TEM images for 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst, (a) fresh sample, (b) spent sample.

3. FT-IR analysis

Figure 9 shows the FT-IR spectra of tested fresh and spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst. The spent catalyst was from tests in flue gas for 4 h containing SO₂. Both fresh and spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst samples exhibited vibration absorptions centering at wave numbers of 3420 cm⁻¹ and 1630 cm⁻¹. These absorptions featured H₂O. In comparison with the fresh 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst, there were absorptions at 1115 cm⁻¹ for the spent catalyst, showing the existence of SO₄ ²⁺. Meanwhile, the spent 10% MnO_x–CeO₂/t-ZrO₂ catalyst had new absorptions at 1400 cm⁻¹ to indicate the presence of NH₄ ⁺ species, such as (NH₄)₂SO₄, that were chemisorbed on the Brönsted acid sites.^{Reference Takagi, Kawai and Soma23} These results of FT-IR analysis are consistent with Yu et al.^{Reference Yu, Guo, Wang, Zhu, Liu, Su, Gao and Xu24} and demonstrated that the inhibitory effect of SO₂ was surely due to the formation of ammonium sulfate which deposited on the active sites and blocked the active channels of catalyst. Compared to the fresh 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst, the absorptions at 2925 cm⁻¹ and 2850 cm⁻¹ disappeared for the spent catalyst, which featured NO₃ ⁻.

FIG. 9.

FT-IR spectra of fresh and spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst.

4. XPS characterization and SO₂ poisoning mechanism

The surface atomic concentrations of Ce, Mn, S are summarized in Table II. It can be seen from Table II that sulfur was observed on the surface of spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst samples, which is in good agreement with the results of FT-IR analysis (Fig. 9). In comparison with the fresh samples, the surface Mn and Ce concentrations of spent samples was lower.

TABLE II.

Surface atomic concentrations of Ce, Mn, S of fresh and used 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst.

The XPS spectra of fresh and spent 10 wt% MnO_x–CeO₂/t-ZrO₂ catalyst shown in Fig. 10 can be analyzed to identify further the species causing SO₂ poisoning. Compared to the spectra of fresh catalyst samples, the spectra intensity of Mn 2p and Ce 3d became lower after reactions in SO₂ containing gas to indicate a decrease of the Mn and Ce surface atomic concentration. The binding energy of Mn 2p and Ce 3d became larger, indicating that MnSO₄ existed on the surface. For spent catalyst samples, S 2p peaks were detected and they could be split into three peaks at 168.5, 169.3, and 169.9 eV, respectively. The peak at a bending energy of 168.5 eV corresponds to SO₃ ²⁻ and the peaks at bending energy of 169.3 and 169.9 eV correspond to SO₄ ²⁻.

FIG. 10.

XPS spectra of fresh and spent 10% MnO_x–CeO₂/t-ZrO₂ catalyst, (a) Mn 2p, (b) Ce 3d, (c) S 2p.

These results are consistent with the reported literature.^{Reference Wei, Li, Zhang, Yan, Lv, Chen and Gong25–Reference Li, Chen, Ke, Luo and Hao29} The inhibitory effect of SO₂ was surely due to the formation of metal sulfate causing decrease of catalyst activity because the active component deteriorated. Furthermore, the deposition of NH₄HSO₄ or (NH₄)₂SO₄ on the active sites decreased the catalytic activity by blocking the active channels of catalysts.

IV. CONCLUSIONS

(1) The NO conversion increases with the increased amounts of MnO_x–CeO₂. The NO conversion of 5 wt% MnO_x–CeO₂/t-ZrO₂ catalyst was 68.1% at 100 °C while that of 30 wt% MnO_x–CeO₂/t-ZrO₂ catalyst was 97.4%.
(2) The MnO_x–CeO₂/t-ZrO₂ catalyst shows high catalytic activity for low-temperature SCR of NO with NH₃. 100% of NO conversion was obtained at 130, 118, 107, and 104 °C for 5, 10, 20, and 30 wt% MnO_x–CeO₂/t-ZrO₂, respectively.
(3) The BET surface areas of MnO_x–CeO₂/t-ZrO₂ catalysts decreased marginally and average pore sizes increased after MnO_x–CeO₂ deposited on the pure tetragonal zirconia. The pore volume of all samples was in the mesopore size range (d 2–50 nm).
(4) Mn⁴⁺ and Ce⁴⁺ oxidation states are predominant on the surface of 10 wt% MnO_x–CeO₂/t-ZrO₂ catalysts. The catalysts have excellent redox ability on the surface and Lewis acid sites are predominant.
(5) The catalytic activity decreased rapidly once SO₂ is introduced into the flue gas. The deterioration of active component and deposition of NH₄HSO₄ or (NH₄)₂SO₄ contribute to SO₂ poisoning.

ACKNOWLEDGMENTS

This research was supported by the Jiangsu Provincial Environmental Protection Office Project (2013032) and a sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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FIG. 1. Variations of NO conversion efficiency with active component for prepared MnOx–CeO2/t-ZrO2 catalysts at 100 °C.

FIG. 2. Variations of NO conversion efficiency with temperature for prepared MnOx–CeO2/t-ZrO2 catalysts with different MnOx–CeO2 dopant contents. Conditions: 0.05 vol% NO, 0.05 vol% NH3, 7–8 vol% O2, and balance N2; GHSV = 32,000 h−1.