Catalytic activity

The SCR activity tests were evaluated in a fixed-bed quartz reactor and gas composition included 440 ppm NH₃, 440 ppm NO, and 5% O₂, which are all balanced by N₂. The NO conversion versus temperature for PPS catalytic filter materials was displayed in Fig. 1. As shown in Fig. 1, the pristine PPS filter felt shows 8–15% NO conversion in the test temperature range. The 0.4, 0.6, and 0.8 Mn–CeO _x /PPS filter felt present better denitration activities in the test temperature range of 140–180 °C, and the denitration efficiencies could almost reach 100% at 160 °C at a high space velocity of 210,000 mL/g_cat/h. It should be noted that denitration activity of the 0.8 Mn–CeO _x /PPS filter is optimum in the test temperature range (80–180 °C), and could achieve 100% NO conversion at 140 °C. However, the catalytic activity of 1.0 Mn–CeO _x /PPS filters is lower in the test temperature range, which may be ascribed to the excessive loading of activity components.

Figure 1.

Effect of reaction temperature on NO conversion of the raw PPS filter felt and y Mn–CeO _x /PPS filter felt. Reaction conditions: [NO] = [NH₃] = 440 ppm, [O₂] = 5%, N₂ as balance gas, weight hourly space velocity (WHSV) = 210,000 mL/g_cat/h.

Field emission scanning electron microscopy (FESEM) images

In Fig. 2(a), it is evident that pristine PPS exhibits regular morphology. For Mn–CeO _x /PPS samples [Figs. 2(b)–2(e)], the nanospecies could be observed on PPS external surface after the introduction of activity components, revealing the formation of activity components on the surface of PPS. It can be also seen that the surface of the PPS filter where the catalyst gradually appeared crack and massive spalling with the growth of y. With the increase in the loading, compound PPS bag filter displays dehiscent external surface. The above results may be resulted from the aggregate activity of the components. Generally, the agglomerate components were not good for the catalytic activity, which was consistent with the conclusions of NO conversion.

Figure 2.

FESEM images of PPS filter felt (a), 1.0 Mn–CeO _x /PPS (b), 0.8 Mn–CeO _x /PPS (c), 0.6 Mn–CeO _x /PPS (d), and 0.4 Mn–CeO _x /PPS (e).

X-ray photoelectron spectroscopy (XPS) results

The XPS was used to analyze the concentration of atoms and the chemical states of elements in the near-surface region of 0.8 Mn–CeO _x /PPS catalytic filter material. According to the XPS total spectrum, the signals of Mn, Ce, C, and O could be detected, indicating the presence of Mn, Ce, C, and O elements. The binding energy of Mn 2p ^3/2 for MnO₂ is 641.1–642.2 eV, which is in line with the previous studies^[ Reference Wang, Zheng, Xu, Wang and Chen ^{12 ]}. Besides, the energy gap of 11.8 eV between Mn 2p ^3/2 (643 eV) and Mn 2p ^1/2 (654.8 eV) could be found in Fig. 3(b), further evidencing the formation of MnO₂.

Figure 3.

(a) XPS full spectrum and (b) Mn 2p spectrum of 0.6 Mn–CeO _x /PPS; (c) Ce 3d and (d) O 1s spectra for 0.6 Mn–CeO _x /PPS.

By the peak-fitting deconvolution, eight characteristic peaks originated from the Ce 3d spectra [Fig. 3(c)] of PPS catalytic filter felt could be obtained. The peaks denoted as U1 (883 eV), U3 (888.6 eV), U4 (898.6 eV), V1 (901.3 eV), V3 (907.3 eV), and V3 (916.9 eV) were indexed to Ce⁴⁺ species, whereas U2 (885.2 eV) and V2 (902.9 eV) were correlated to the Ce³⁺ species. In light of the above results, it could infer the coexistence of Ce³⁺ and Ce⁴⁺ species.^[ Reference Wu, Jin, Liu and Wang ^{13 ,} Reference Qiu, Meng, Pang, Zhang and OuYang ^{14 ]} The O 1s spectrum [Fig. 3(d)] of catalytic filter felt was fitted into three peaks. The peak located at 531.5 eV was corresponded to the lattice oxygen (O _β ), and the binding energies between 529.5 and 532.2 eV were ascribed to the surface oxygen (O _α ).^[ Reference Shen, Ma, He and Zhang ^{15 ]}

Transmission electron microscopy (TEM) images

The morphologies of PPS catalysts filter materials were listed in Fig. 4. Nanoflake-like species could be seen on the surface of acid-treated PPS after being supported by Mn–Ce mixed oxide catalysts [Fig. 4(a)]. Moreover, Fig. 4(b) revealed that the inapparent lattice fringe of manganese oxides or cerium oxides observed from high-resolution TEM (HRTEM) of 0.6 Mn–CeO _x /PPS, implying weakly crystalline Mn–Ce mixed oxide catalysts. Normally, MnO _x with weakly crystalline structure is in favor of the low-temperature catalytic activity.^[ Reference Jin, Liu, Wu, Wang and Gu ^{16 –} Reference Min, Park, Kom and Yie ^{19 ]} Furthermore, Fig. 4(c) corresponding to the energy dispersive X-ray spectroscopy (EDX) spectrum presents the signals of Mn, Ce, O, S, and C elements, demonstrating the formation of Mn–Ce mixed oxide catalysts on the surface of PPS.

Figure 4.

TEM and HRTEM of PPS-based catalysts: (a) and (b) 0.6 Mn–CeO _x /PPS; (c) EDX spectrum of 0.6 Mn–CeO _x /PPS from the annular region of Fig. 4(a).

X-ray diffraction (XRD) patterns

Figure 5 illustrates the XRD patterns of the acid-treated PPS filter, 0.4 Mn–CeO _x /PPS, 0.6 Mn–CeO _x /PPS, 0.8 Mn–CeO _x /PPS, and 1.0 Mn–CeO _x /PPS. It was observed from Fig. 5 that all samples presented two sharp and some weaker typical PPS peaks. After being loaded by Mn–Ce mixed oxide activity components, the diffraction peak intensity corresponding to PPS became weaker, which may be resulted from the interaction between active components and PPS. It should be pointed out that the typical peaks of MnO₂ could not be detected, suggesting its highly dispersed or weak crystalline characteristic, which was related to the HRTEM results.^[ Reference Guo, Liu, Pan, Zhen, Chen, Chen, Ding and Yang ^{20 ,} Reference Wang, Peng, Peng, Yu and Yang ^{21 ]}

Figure 5.

XRD patterns of (a) acid-treated PPS filter; (b) 0.4 Mn–CeO _x /PPS; (c) 0.6 Mn–CeO _x /PPS; (d) 0.8 Mn–CeO _x /PPS; (e) 1.0 Mn–CeO _x /PPS.