MnO₂ catalysts uniformly decorated on polyphenylene sulfide filter felt by a polypyrrole-assisted method for use in the selective catalytic reduction of NO with NH₃

Abstract

A manganese dioxide (MnO₂)/polypyrrole (PPy) nanocoating was uniformly decorated on the surface of polyphenylene sulfide (PPS) filter felt via an in situ synthesis method to fabricate a catalytic filter material. The pyrrole functioned as a dispersant for the MnO₂ catalysts and the PPy generated acted as a binder to adhere the MnO₂ catalysts and filter felt together. The catalytic filter material obtained, had a high adhesive strength between that of the MnO₂/PPy nanocoating and the PPS filter felt, and was used for the selective catalytic reduction of nitric oxide (NO) with ammonia under model conditions without any sulfur dioxide or water vapor in the gas. More than 70% conversion of NO was achieved at 160–180 °C at a high space velocity of 38 000 h⁻¹.

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1. Introduction

Nitrogen oxides (NO_x) and fly ash originating from both mobile and stationary sources are major air pollutants and can give rise to a variety of environmental and health-related issues.^1–4 A two-stage gas cleaning technology using a particulate filter integrated with selective catalytic reduction of NO_x by ammonia (NH₃-SCR) is widely applied in power plants to remove these pollutants.⁵ A new catalytic filter has previously been proposed for the simultaneous removal of fly ash and NO_x from flue gases.^5–7 The focus of this earlier study was the preparation of ceramic foam catalysts, which were then enclosed in a polymer filter bag which was resistant to high temperatures.^6,8,9 However, this filter bag takes up a substantial amount of space and is uneconomical. If the catalysts for removing NO_x could be directly incorporated onto the filter bag, the space requirements and costs would be reduced.¹⁰

Polymer filter bags, usually constructed from non-woven felt made from polyphenylene sulfide (PPS) fibers,¹¹ are widely used in industrial particle separators. They are usually located downstream of the desulfurizer and electrostatic precipitator, thus avoiding the high concentrations of sulfur dioxide and ash present in the flue gas. The temperature at this location is always about 170 °C. A suitable catalyst for the removal of nitric oxide (NO) should not only achieve a high catalytic activity in the operating temperature range, but should also require only a low catalyst loading for use in the high efficiency catalytic filter felt. Manganese oxides are well known for their high activity in low temperature selective catalytic reduction reactions^12–17 and, among the pure manganese oxides, manganese oxide (MnO₂) shows the optimum activity for NO abatement.^14,18 MnO₂ can easily be obtained from the reduction of potassium permanganate (KMnO₄).^19–21

The adhesive strength between the catalyst and the polymer filter felt is a consideration in the preparation of high-performance catalytic filter felt. The inert chemical surface of PPS fibers typically makes it very difficult for it to adhere to inorganic catalysts. In the research reported here, we decorated MnO₂ catalysts on the PPS filter felt using a polypyrrole (PPy)-assisted method for NH₃-SCR (Fig. 1). As a result of the π–π conjugation effect, the pyrrole monomers were uniformly adsorbed on the surface of the PPS filter fibers. PPy was then formed via in situ polymerization in acidic KMnO₄ solution. At the same time, the KMnO₄ was reduced to MnO₂, which was embedded into the PPy matrix at the nano-scale.^22,23 The catalytic filter felt obtained showed a high adhesive strength between the MnO₂/PPy nanocoating and the PPS filter felt. The NH₃-SCR activity of this catalytic filter felt was investigated and >70% conversion of NO was achieved at 160–180 °C at a high space velocity of 38 000 h⁻¹. These results suggest that this method is an effective strategy for preparing catalytic filter materials for the removal of NO_x from flue gases.

Fig. 1 Schematic diagram of the process for the synthesis of the catalytic filter felt.

2. Experimental

2.1. Synthesis of materials

The PPS filter felts (500 g m⁻²) with a thickness of 1.8 mm were obtained from Xiamen Savings Environmental Company. Pyrrole was purified by distillation under reduced pressure prior to use. In a typical synthesis procedure, a piece of circular PPS filter felt (id 40 mm) without acid treatment, was directly immersed in 0.3 M pyrrole acetone solution for 12 h. After drying naturally for 30 min, the PPS filter felt with the absorbed pyrrole monomers was dipped into an acidic 0.05 M KMnO₄ solution containing 1 M sulfuric acid (H₂SO₄) and mixed ultrasonically for 30 min. The resulting product was rinsed several times with deionized water and ethanol to remove the residual reactants and was finally dried at 110 °C for 12 h. For comparison, catalytic filter felts were also synthesized using different concentrations of KMnO₄ and H₂SO₄.

2.2. Catalyst characterization

The chemical and elemental compositions in the near-surface region of the catalytic filter felt were investigated using X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific ESCALAB 250 spectrometer equipped with a dual Al/Mg anode with 0.6 eV resolution at the C 1s peak. The morphology and structure of the catalytic filter felt were studied using transmission electron spectroscopy (TEM; FEI Tecnai G2) and field emission scanning electron microscopy (FESEM; FEI Nova NanoSEM 230 and Carl Zeiss SUPRA 55 Sapphire). An X-ray probe was used to measure the energy-dispersive X-ray (EXD) spectra with 10 keV primary electron energy and about 127 eV energy resolution in the Mn (Kα) peak. To prepare the TEM test sample, the as-obtained catalytic filter felt was first cut into short fibers. The short fibers were then ground into single fibers and dispersed in ethanol and were then analysed using TEM. Ultrasonic treatment in ethanol for 1 h was used to investigate the adhesive strength between the MnO₂/PPy nanocoating and the PPS filter felt.

2.3. Catalytic activity tests

The SCR activity of the catalytic filter felt was measured in a custom-made tubular flow reactor (id 28 mm) under model conditions without SO₂ or water vapor in the gas. The gas composition was 500 ppm NO, 500 ppm NH₃ and 5% oxygen (O₂) balanced by nitrogen (N₂). The total flow-rate was 700 mL min⁻¹, which corresponded to a gas hourly space velocity (GHSV) of about 38 000 h⁻¹. The gas composition (NO, nitrogen dioxide and O₂) was monitored using a KM940 flue gas analyzer (Kane International Limited). All the data were recorded after 30 min when the SCR reaction reached a steady state.

3. Results and discussion

The surface chemical composition of the catalytic filter felt was investigated using XPS. The XPS full spectrum (Fig. 2) only shows the signals from elemental C, Mn and O; the magnified spectrum of the Mn 2p region in the inset with a spin energy separation of 11.7 eV between Mn 2p^3/2 (642.2 eV) and Mn 2p^1/2 (653.9 eV) suggests that MnO₂ was formed.²⁴ Previous studies have indicated that the binding energies of Mn 2p^3/2 for MnO₂, Mn₂O₃ and Mn₃O₄ are 641.1–642.4, 641.2–641.9 and 641.3–641.4 eV, respectively.^25–28 It is clear from these studies that only the binding energy of Mn 2p^3/2 for MnO₂ is >642 eV, which is in good agreement with our XPS results and suggests the formation of MnO₂. The surface atomic concentrations of C, Mn and O were 45.46%, 13.59%, and 38.23%, respectively. It should be noted that the elemental ratio of O/Mn (2.81) was higher than the theoretical elemental ratio of 0.58; this was attributed to the chemical oxygen species bound to Mn, which are helpful in the NH₃-SCR reaction.^3,4,16,29 The absence of signals from S and N in the XPS full spectrum was a result of their low surface content and indicates that most of the surface of the catalytic filter felt was completely covered by the MnO₂ catalysts.

Fig. 2 XPS full spectrum results for the catalytic filter felt; the inset is the magnified spectrum of the Mn 2p region.

Fig. 3 shows the corresponding morphologies of the raw PPS filter felt and the catalytic filter felt characterized by FESEM. The raw PPS filter felt had a three-dimensional structure and a smooth surface (Fig. 3a). After deposition of the MnO₂/PPy nanocoating, the three-dimensional porous structure in the catalytic filter felt was still preserved without apparent agglomeration and the surface of the catalytic filter felt was decorated by the MnO₂/PPy nanocoating (Fig. 3b). Elemental mapping analysis was used to determine the uniformity of the MnO₂ catalysts deposited on the surface of the catalytic filter fiber (Fig. 4). The C, Mn, O, and S signals in the full spectrum suggest the existence of MnO₂ and PPS. The missing signal for N may be ascribed to its low surface content. In addition, a low intensity Mn signal compared to that of sulfur was observed in the EDX pattern. The reason for this phenomenon was the low thickness of the coating and the large penetration depth of the EDX probe, which meant sulfur contributed more to the signal. The mapping shape for Mn showed that the MnO₂ catalysts were uniformly dispersed on the fiber surface, which supports the NH₃-SCR reaction. Wang et al.²² proposed a synthetic strategy to uniformly coat the MnO₂/PPy nanocoating on the carbon nanofiber substrate and found that acid treatment of the carbon nanofiber fabric could introduce oxygen containing functional groups (–OH, –COOH). These negatively charged functional groups attracted pyrrole monomers via electrostatic interactions and provided nucleation centers for the subsequent in situ polymerization of pyrrole and the formation of MnO₂ catalysts. Without this acid treatment, a thick MnO₂/PPy coating was deposited on the surface of the carbon nanofiber substrate. In this work, the PPS filter felt was not treated with acid, although a uniform nanocoating was generated on the inert chemical surface of the PPS fiber felt. It was therefore concluded that the π–π conjugation effect between PPS and pyrrole may help to disperse the MnO₂ catalysts on the surface of the PPS fibers.^3,29–31

Fig. 3 FESEM images of (a) the raw PPS filter felt and (b) the catalytic filter felt.

Fig. 4 (a and b) FESEM images of the surface of a single catalytic filter fibre and element mapping of (c) Mn, (d) S, (e) O, (f) C and (g) the EDX pattern of the selected region in (b).

Fig. 5 shows the TEM image of the catalytic filter felt. An MnO₂/PPy nanocoating with a thickness <50 nm was coated on to the surface of the PPS fibers. The catalytic filter felt was mixed ultrasonically in ethanol for 1 h without any apparent exfoliation or weight loss, suggesting that the MnO₂/PPy nanocoating could effectively adhere to the surface of PPS filter felt and that this preparation method was feasible.

Fig. 5 TEM image of the catalytic filter felt.

The catalytic activity of the catalytic filter felt was measured in a custom made stainless steel tubular flow reactor. Fig. 6 shows the NO conversion as a function of temperature from 90 °C to 180 °C with a high GHSV of 38 000 h⁻¹. The raw PPS filter felt showed 8–15% NO conversion in the temperature range tested. The NO conversion for the catalytic filter felt was low below 140 °C, but 44% NO conversion was obtained at 140 °C, which was higher than for the raw PPS filter felt. The NO conversion of the catalytic filter felt was significantly higher at higher temperatures and was >70% between 160 °C and 180 °C, the operating temperature required for the PPS filter felt. As seen in the FESEM images of the filter felt (Fig. 3), the hollow inner cavities among the fibers were much larger than the aggregated pores of commonly used solid catalysts and this means that less gas could be retained, which influences the SCR reaction. However, the three-dimensional structure of the filter felt offers more opportunities for contact between the reactant gases and the fiber surface. Therefore, it was feasible to produce the composite catalytic filter felt with complete abatement of the SCR unit via the direct deposition of the catalysts on the surface of the polymer filter felt.

Fig. 6 Effect of reaction temperature on NO conversion of the raw PPS filter felt and the catalytic filter felt prepared with the 0.05 M acidic KMnO₄ solution (containing 1 M H₂SO₄). Reaction conditions: [NO] = [NH₃] = 500 ppm, [O₂] = 5%, N₂ as balance gas, GHSV = 38 000 h⁻¹.

For comparison, catalytic filter felts were prepared using different concentrations of KMnO₄ and H₂SO₄. Fig. 7 shows the NO conversion at 160 °C and catalyst loading of the catalytic filter felts prepared via the PPy-assisted method with a constant H₂SO₄ concentration (1 M), but different KMnO₄ concentrations. The catalyst loading was encouraged with increasing concentrations of the KMnO₄ solution. NO conversion reached a peak value when the KMnO₄ concentration was 0.05 M. Fig. 8 shows that low concentrations of KMnO₄ (0.01 M) result in a low catalyst loading and the MnO₂/PPy nanocoating did not completely cover the surface of the PPS filter felt. However, although a high concentration of KMnO₄ (0.1 M) enhanced the catalyst loading, it also induced severe aggregation of the MnO₂/PPy nanocoating. In summary, both these two examples resulted in poor conditions for the selective catalytic reduction of NO with NH₃. In contrast, the catalytic filter felt showed remarkable catalytic activity when the concentration of KMnO₄ was 0.05 M. It was also found that the concentration of H₂SO₄ played an important part in the morphology of the catalytic filter felt and the catalytic activity. Fig. 9 shows that when only a neutral KMnO₄ solution was used to prepare the catalytic filter felt, the NO conversion at 160 °C was almost the same as for the raw PPS filter felt (15%) as a result of the low catalyst loading. After 0.1 M H₂SO₄ had been added to the KMnO₄ solution, the catalyst loading and NO conversion were significantly encouraged and reached 6.2 wt% and 64%, respectively. Further increasing the H₂SO₄ concentration gradually improved the NO conversion while the catalyst loading sustained a nearly constant value. For example, the NO conversion at 160 °C for the catalytic filter felt fabricated using an acidic 2 M H₂SO₄/KMnO₄ solution was 88%. For an acidic 3 M H₂SO₄/KMnO₄ solution, the NO conversion reached 94% at the same temperature. The higher NO conversions for this catalytic filter felt might be attributed to the different morphologies of the MnO₂/PPy nanocoating (Fig. 10).

Fig. 7 NO conversion (left) and catalyst loading (right) as a function of KMnO₄ concentration (containing 1 M H₂SO₄) for the catalytic filter felts. Reaction conditions: 160 °C, [NO] = [NH₃] = 500 ppm, [O₂] = 5%, N₂ as balance gas, GHSV = 38 000 h⁻¹.

Fig. 8 FESEM images of the catalytic filter felt prepared using (a) low (0.01 M) or (b) high (0.1 M) KMnO₄ concentrations.

Fig. 9 NO conversion (left) and catalyst loading (right) as a function of H₂SO₄ concentration (containing 0.05 M KMnO₄) for the catalytic filter felts. Reaction conditions: 160 °C, [NO] = [NH₃] = 500 ppm, [O₂] = 5%, N₂ as balance gas, GHSV = 38 000 h⁻¹.

Fig. 10 FESEM images of the catalytic filter felts prepared by different concentrations of acidic KMnO₄ solution: (a) 2 M H₂SO₄ and (b) 3 M H₂SO₄.

The adhesive strength between the MnO₂/PPy nanocoating and the PPS filter felt are important in the preparation of the catalytic filter felt. Catalytic filter felts obtained via ultrasonic treatment in ethanol for 1 h were used to investigate the adhesive strength. Fig. 11 shows that the catalytic filter felt prepared using a 1 M H₂SO₄ solution did not show an obvious weight loss after the ultrasonic treatment, although the catalytic filter felt prepared using a higher concentration of H₂SO₄ solution (2 M or 3 M) showed an apparent weight loss during the process, indicating that the adhesive strength between the MnO₂/PPy nanocoating and the PPS filter felt was weak. Thus, an appropriate acidity of the KMnO₄ solution should be used in the preparation of these practical catalytic filter felts.

Fig. 11 Histograms of catalyst loading before and after ultrasonic treatment of the catalytic filter felts prepared via different H₂SO₄ concentrations (containing 0.05 M KMnO₄ solution).

4. Conclusion

MnO₂ catalysts were uniformly decorated on the surface of PPS filter felt by a PPy-assisted method to fabricate catalytic filter felt for the removal of NO_x from flue gases. The high adhesive strength between the MnO₂/PPy nanocoating and the PPS filter felt contributed to the greater than 70% NO conversion obtained when using this catalytic filter felt. A too high or too low a concentration of KMnO₄ was unsuitable for the preparation of a highly efficient catalytic filter felt. The catalytic filter felt prepared using a highly acidic KMnO₄ solution showed only weak adhesion between the MnO₂/PPy nanocoating and the PPS filter fiber. Therefore, the focus of further studies will not only be in improving the NO conversion of the catalytic filter felt, but also in increasing the adhesive strength between the MnO₂/PPy nanocoating and the PPS filter fibers.

Acknowledgements

This work was supported by Scientific and Technological Innovation Project of Fujian Province (Grant no. 2012H6008), and the Scientific and Technological Innovation Project of Fuzhou City (Grant no. 2013-G-92).

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