Novel MnOx-CeO2 nanosphere catalyst for low-temperature NH3-SCR

Accepted Manuscript Novel MnOx-CeO2 nanosphere catalyst for low-temperature NH3-SCR

Lulu Li, Bowen Sun, Jingfang Sun, Shuohan Yu, Chengyan Ge, Changjin Tang, Lin Dong PII: DOI: Reference:

S1566-7367(17)30250-9 doi: 10.1016/j.catcom.2017.06.019 CATCOM 5085

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

9 February 2017 15 June 2017 15 June 2017

Please cite this article as: Lulu Li, Bowen Sun, Jingfang Sun, Shuohan Yu, Chengyan Ge, Changjin Tang, Lin Dong , Novel MnOx-CeO2 nanosphere catalyst for low-temperature NH3-SCR. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Catcom(2017), doi: 10.1016/j.catcom.2017.06.019

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ACCEPTED MANUSCRIPT Novel MnOx -CeO2 nanosphere catalyst for low-temperature NH3 -SCR Lulu Lia, Bowen Suna, Jingfang Suna,b , Shuohan Yua, Chengyan Gec, Changjin Tanga* , Lin Donga,b*

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Key Laboratory of M esoscopic Chemistry of M OE, School of Chemistry and Chemical Engineering, Nanjing

b

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University, Nanjing 210093, PR China Jiangsu Key Laboratory of Vehicle Emissions Control, Center of M odern Analysis, Nanjing University, Nanjing

School of Chemistry & Chemical Engineering, Jiangsu YanCheng insititue of technology, Yancheng 224051, P.R.

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c

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210093, PR China

China

Corresponding author:

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*

Fax: +86 25 83317761; Phone: +86 25 83592290

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Address: Hankou Road 22#, Nanjing, Jiangsu province, P. R. China, 210093

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E-mail address: [email protected] (L. Dong); [email protected] (C. J. Tang)

ACCEPTED MANUSCRIPT Abstract A novel MnOx-CeO2 nanosphere catalyst with assembled structure from tiny particles was prepared and tested towards low-temperature NH3 -SCR. A superior deNOx performance compared to that obtained over the catalyst composed of MnOx supported on CeO2 was obtained. A good resistance to high space velocity and SO2 present in the feed and an excellent stability (no o

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deactivation was observed during 100-h of testing at 150 C) were obtained. The characterization results suggested that the higher specific surface area, better redox behavior and larger

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concentration of surface active oxygen species were responsible for the above mentioned excellent performance. The results of the present study suggest that control of catalyst morphology can be

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an effective strategy to upgrade low-temperature NH3 -SCR performance.

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Keywords: Selective catalytic reduction, low-temperature SCR activity, SO2 resistance and stability，MnOx-CeO2 nanosphere.

ACCEPTED MANUSCRIPT 1. Introduction o

Low-temperature NH3 -SCR (<200 C), which can be placed downstream of the electrostatic precipitator and even the desulfurizer, where most of SO2 and dusts are removed, has received great attention in the past decades[1]. Among the various reported catalysts, Mn-based catalysts have been confirmed to demonstrate excellent low-temperature SCR activity, such as Mn/USY[2], Mn/ACF[3] and Mn/TiO2 [4]. As a typical rare-earth oxide, ceria (CeO2 ) has been widely

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investigated as catalyst and catalyst support in many important catalytic reactions owing to its excellent redox properties, high oxygen storage ability and low cost characteristics[5, 6]. Recently,

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CeO2 -promoted MnOx catalysts were found to exhibit superior activity compared to other

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Mn-based SCR catalysts due to their facile activation of adsorbed NOx related species and the synergistic interactions between Mn and Ce [7, 8]. In 2003, R. T. Yang et al. first reported that

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Mn-Ce mixed oxide catalyst showed over 80% NO conversion at 80

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C for the NH3 -SCR

reaction[9]. Our group also fabricated a MnOx-CeO2 SCR catalyst with hollow structure and

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which displayed outstanding low-temperature activity[5]. As a key component in Mn-Ce catalysts, the morphology, crystal size, pore structure and other properties of ceria often strongly influenced

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its catalytic performance. For example, Han et al. [10]designed Fe/CeO2 nanorods and Fe/CeO2 nanopolyhedra catalysts for NH3 -SCR and found that Fe/CeO2 nanorods displayed higher catalytic

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activity because of its population of a larger concentration of adsorbed surface oxygen, oxygen defects and Fe. Xu et al. [11]prepared a Au/CeO2 nanosphere catalyst for the reduction of 4-nitrophenol with enhanced activity compared to commercial CeO2 powder supported solid. The

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former behavior was due to the unique porosity structure of the support and the uniform distribution of metallic Au particles. This suggests that nanosphere structures can effectively enhance spatial metal dispersion, the specific surface area and promote more suitable pore structures.

Inspired by the above-mentioned facts, it is reasonable to believe that ceria nanosphere would have a positive effect on the NH3 -SCR catalytic performance. However, to the best of our knowledge, MnOx-CeO2 catalysts with nanosphere shape have not yet been applied towards the NH3 -SCR reaction. In this study, MnOx-CeO2 nanosphere catalyst with unique assembled structure was successfully prepared and tested in the NH3 -SCR. It was found that this novel MnOx-CeO2 nanosphere catalyst with morphology and structure exhibits high low-temperature activity,

ACCEPTED MANUSCRIPT stability and excellent resistance to high space velocities and the presence of SO2 in the feed stream. 2. Experimental Section 2.1 Catalyst preparation The CeO2 nanosphere was synthesized using a glycothermal approach. 1.0 g of

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Ce(NO3 )3 .6H2 O was dissolved in 1 ml of deionized water. Afterwards, 30 ml of ethylene glycol and 1 ml of propionic acid were added under stirring. The solution was then transferred into a o

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Teflon-lined stainless steel autoclave and heated at 160 C for 200 min. After cooling to room

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temperature, 30 ml of acetone were added and the resulting mixture was washed several times with ethanol. The resulting powder was dried overnight and calcined at 400 o C for 4 h in air. The

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obtained nanospheric-CeO2 is marked as CeO2 -NS. A normal CeO2 was obtained by the direct thermal decomposition of cerium nitrate hydrate. The MnO2 -CeO2 (denoted as MC) and

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MnO2 -CeO2 nanosphere (denoted as MCN) catalysts were synthesized by the wet impregnation .

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method using Mn(CH3 COO)2 4H2 O (the molar ratio of Mn/(Mn+Ce)=0.4) and calcined at 400 C

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for 4 h. Finally, all catalysts samples were crushed and sieved to 40-60 mesh for testing. 2.2 Catalyst characterization

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X-ray diffraction (XRD), BET surface area measurements, H2 -temperatureprogrammed reduction (H2 -TPR), X-ray photoelectron spectroscopy (XPS), Laser Raman spectroscopy (Raman) and Transmission electron microscopy (TEM) were performed on an XRD-6000 X-ray

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diffractometer (Shimadzu), an ASAP2020 physical adsorption instrument (Micromeritics), a PHI 5000 VersaProbe spectrophotometer and a chemisorption analyzer, Renishaw invia Laser Raman spectrometer and a JEM-2100 at an acceleration voltage of 200 kV, respectively. NH3 -temperature programmed desorption (NH3 -TPD) studies were performed using a multifunction chemisorption analyzer, whereas NOx-TPD experiments were carried out on a Nicolet 5700 FT-IR spectrometer . In situ DRIFTS spectra were collected in the range of 650 - 4000 cm-1 at a spectral resolution of 4 -1

cm (number of scans, 32) on a Nicolet 5700 FT-IR spectrometer.

2.3 Catalytic performance The catalytic performance of the various Mn-Ce-O solids for the selective catalytic reduction

ACCEPTED MANUSCRIPT of NO by NH3 was determined under steady state reaction conditions: 500 ppm NO, 500 ppm NH3 , 5% O2 , 100 ppm SO2 (when used), and N2 as balance gas. The catalyst sample was placed in a quartz tube and pre-treated in a highly purified N2 gas stream at 200 o C for 1 h. The reaction was o

carried out at different temperatures in the 50-250 C range and the concentrations of NO, NH3 , NO2 and N2 O at the effluent gas stream from the reactor were measured by a Thermofisher IS10 o

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FTIR spectrometer equipped with a 2 m path-length gas cell (250 ml volume) held at 150 C.

3. Results and discussion

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Fig. 1 shows the NH3 -SCR activity of MC and MCN catalysts in terms of NO conversion using

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a GHSV of 60000 ml g-1 h-1 . It is seen that the NO conversion of MCN achieves a conversion of about 80% at 75 o C which is maintained almost 100% in the whole 125-250 o C range. On the

in the range of 175-250

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other hand, the NO conversion of MC is only 39% at 75 o C and could only reach the value of 93% C. Clearly, the MCN catalyst exhibits an excellent low-temperature

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NH3 -SCR activity as opposed to the MC catalyst. Moreover, the N2 -selectivity of MC and MCN catalysts is shown in Fig. S1(B), which is found to decline

gradually with

increasing

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temperature. This might be due to the strong redox behavior of Mn-based catalysts, which could be linked to the oxidation of NH3 by gaseous O2 (present in the feed) towards the formation of the

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by-product NO2 and/or N2 O[5]. Fig. S1(C) also reports the influence of SO2 in the feed stream on the NO conversion over the MC and MCN catalysts, where the latter displays better ability for sulfur tolerance. Furthermore, the stability of MCN catalyst was also tested under the same GHSV

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(ca. 60000 ml g-1 h-1) and results are presented in Fig. S1(D). The obtained results indicate that there is no deactivation during the successive testing performed over the period of 100 h at 150 o C. Thus, the MCN catalyst appears to be highly stable for NO elimination at low temperatures at least for the examined period of operation. According to the above results, it can be concluded that MCN catalyst exhibits superior catalytic performance compared to the MC catalyst in the low-temperature NH3 -SCR. In order to explore some fundamental reasons of the distinct improvement of NH3 -SCR performance over the MCN catalyst, the physochemical properties of the obtained catalysts were investigated. Firstly, the morphologies of MC and MCN samples were investigated by TEM. As shown in Fig. S2a, MC solid catalyst appears in the form of irregular particles without any defined

ACCEPTED MANUSCRIPT morphology, whereas MCN catalyst exhibits uniform nanosphere shapes with an average size ~80-100 nm composed of ultrafine CeO2 nanoparticles (Fig. S2b and c). As shown in Fig. S3 depicting the XRD patterns obtained, no phases of manganese oxides could be detected, indicating that these manganese oxides formed primary crystals of less than about 4 nm. This result could be supported by the likely developed interactions between MnOx and ceria at the moderate

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calcination temperature used. Meanwhile, the diffraction peaks of CeO2 -NS and MCN solid samples were broadened as evidenced by the smaller grain size of MCN (Table 1). Fig. S4 and

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Table 1 presents results from the textural characterization of the solids. Obviously, the specific surface area and pore volume of MCN are higher than those of MC because of its unique

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nanostructure and smaller primary particle sizes. Moreover, it is worth mentioning that the MCN catalyst exhibits bimodal distribution with a pore diameter around ~ 3 nm and ~ 16 nm, which is

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in good agreement with the findings in the case of CuO supported on CeO2 nanosphere[12]. Based on the significant difference in the specific surface area obtained between MC and MCN

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solids, the NO conversion per unit area (%/m2 ) of MC and MCN was calculated and corresponding results are presented in Fig. S5(B). It can be clearly seen that the value of MCN is

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still higher than that of MC, strongly suggesting that there must be other intrinsic kinetic factors that influence the rate of NO conversion over these catalysts. Raman spectra were obtained to

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better understand differences in the structural features and defects of the MC, MCN and the corresponding support materials. As depicted in Fig. 2A, the two supports exhibit the characteristic band of the CeO2 phase centered at ~ 460 cm -1 and one weak band at ~ 600 cm -1,

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which is assigned to the Raman-active F2g mode of fluorite cubic structured of CeO 2 and defect-induced mode, respectively[13]. Fig. 2B shows the relative intensity ratio of I600 /I460 which is related to the extent of oxygen defect sites in the catalysts[14]. Evidently, the surface oxygen defect concentration in CeO2 -NS is higher than that in the ordinary CeO2 and the band width of F2g vibration mode is broadened due to its smaller particle size. After the deposition of MnOx, the Raman spectra for MC and MCN appear similar to those of supports, but a new Raman band is -1

observed at about 655 cm , which is related with the presence of manganese oxide. In addition, the intensity ratio of I 600 /I460 increases compared the bare supports, which suggests the existence of some kind of synergistic interactions between MnOx and CeOx. The surface oxygen defect concentration in the four samples follows the sequence: MCN > MC > CeO2 -NS > CeO2 . This

ACCEPTED MANUSCRIPT observation may also suggest that the interaction between manganese oxide and CeO2 -NS is stronger than that with ordinary CeO2 . The generation of oxygen vacancies is directly related to the conversion of Ce 4+ to Ce3+and 3+

therefore MCN contains more Ce

ions compared with the bare MC and CeO2 supports.

Previous studies revealed that the high concentration of surface chemisorbed oxygen promotes 3+

as well as higher oxidation states of

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activation of reactant molecules in NH3 -SCR and that Ce

Mn favor redox properties of MCN catalysts[15]. To investigate this issue, XPS data are

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reported in Table 1, where MCN appears to have more accessible surface chemisorbed oxygen (Oα), Mn4+ and Ce3+ species.

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In order to study differences in the adsorption behavior of reactants among the present solids, NH3 -TPD and NOx-TPD experiments were performed and the obtained results are depicted in

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Fig. S7, Fig. S8 and Table S1. It is illustrated that CeO2 -NS and MCN samples have more acid sites and accommodate larger concentration of adsorbed NOx species compared to ordinary

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CeO2 and MC solids. This means that more acid sites to activate NH3 molecules and more NOx species are formed that could potentially participate in the NH3 -SCR reaction [16]. Based on the

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above results, it becomes evident that smaller grain s izes, higher concentrations of surface Ce3+, Mn4+ and oxygen species, as well as more adsorbed NH3 and NOx species are the factors that

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make MCN catalyst to exhibit higher de-NOx efficiency in the NH3 -SCR. The redox properties usually play an important role in the NH3 -SCR performance over metal oxides[17]. As shown in Fig. 3, the H2 -TPR traces of MC and MCN catalysts exhibit three obvious o

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reduction peaks in the low-temperature range of 100-450 C, which could be reasonably ascribed to the stepwise reduction of MnOx (MnO2  Mn2 O3  Mn3 O4  MnO)[14]. The high-temperature reduction peak at ~ 760 o C is associated with the reduction of bulk CeO 2 [18]. Compared to MC, Mn species over the MCN catalyst are reduced at comparatively lower temperatures. The reason might be that the interaction between Mn species and CeO2 nanosphere is stronger than that between Mn species and normal CeO2 nanoparticles. Furthermore, the relative area of the first reduction peak over MCN is larger than that of MC due to the larger surface concentration of reducible Mn4+ ions, result that is supported by the present XPS results (Table 1). Apparently, the redox properties of MCN appear better than of MC, which becomes beneficial for the improvement of low-temperature NH3 -SCR activity. Further evidence for this is provided by

ACCEPTED MANUSCRIPT the NO oxidation studies performed (Fig. 4). It is shown that the NO conversion over MCN is higher than that over MC. Many researchers have found that the conversion of NO to NO 2 could improve the NH3 -SCR low-temperature activity by facilitating the ‘‘fast SCR process’’ [17, 19]. Thus, it is reasonable that MCN exhibits remarkable NO conversion than MC in the lowtemperature range investigated in the present work.

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In situ DRIFTS studies were employed to explore some essential aspects of the reaction mechanism over the MCN catalyst. First, NH3 adsorption experiments were carried out and the

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obtained spectra are presented in Fig. S9(A). After NH3 adsorption and N2 purge, strong bands were recorded at 1654, 1337 and 1096 cm -1 and which are attributed to coordinated NH3 species to

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Lewis acid sites over the MC sample. Besides, the amide species (-NH2 ) was also observed (1557 cm-1 ), which is regarded as a reactive intermediate in the NH3 -SCR reaction. For the MCN

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catalyst, the band intensities associated to adsorbed NH3 on Lewis acid sites (1654, 1303 and 1165 cm-1 ) are greatly enhanced compared to MC. In addition, a new band at 1601 cm-1 attributed to

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coordinated NH3 on Lewis acid sites was detected, which is due to the presence of enriched x+

-1

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surface Mn species[20]. Moreover, the IR band at 1434 cm attributed to ionic NH4 specie

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bound to Brønsted acid sites is observed due to the larger concentration of Ce3+ appeared on the surface of MCN compared to MC. Previous works revealed that the presence of Ce3+ species could

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create a charge imbalance and unsaturated chemical bonds on the catalyst surface, which would lead to an increase in oxide defects or hydroxyl-like groups. Thus, NH3 adsorption linked to Brønsted acid sites could be enhanced due to the production of such adsorption sites. In situ

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DRIFT NO+O2 adsorption spectra were also recorded over these catalysts. As shown in Fig. S9(B), several bands were detected in the range of 1000-1700 cm-1 , which could be ascribed to bridging nitrate (1621 and 1233 cm-1), monodentate nitrate (1510 and 1293 cm-1 ) and bidentate nitrate species (1563 cm-1 ) [21]. Furthermore, the intensities of these adsorbed nitrate species over the MCN catalyst surface were stronger than those over the MC catalyst surface. A previous study reported that the reactivity of bridging nitrate was much higher than other nitrates in the NH3 -SCR reaction, which contributed to the enhanced low -temperature SCR activity [22]. In order to follow the reaction course over the MCN catalyst, the reaction between NO+O2 and pre-adsorbed NH3 at 100 o C was recorded as a function of time as depicted in Fig. S9(C). After NH3 pre-adsorption, the IR bands assigned to adsorbed NH3 species on Brønsted and Lewis acid

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sites (1432, 1185, 1298 and 1601 cm ) were all present. When the NO+O2 flow was turned on, the intensities of all bands ascribed to adsorbed NH3 species rapidly disappeared and then replaced by several kinds of nitrates and molecularly adsorbed NO species (bidentate nitrate (1578 cm-1 ), -1

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bridging nitrate (1628, 1602 and 1214 cm ) and monodentate nitrate (1278 cm ), which is similar to the above-mentioned in situ DFIFTS NO+O2 results. In addition, the IR band recorded at 1747 -1

cm and which is assigned to molecularly adsorbed NO was also detected. This result indicates

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that both ionic NH4 + and coordinated NH3 over the MCN surface could act as active species for

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the reduction of NOx and the NH3 -SCR reaction can proceed through an E-R mechanism [16]. Meanwhile, the reaction between pre-adsorbed NOx species and NH3 over the MCN catalyst was

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also studied and corresponding results are provided in Fig. S9(D). It can be clearly seen that after the introduction of NH3 , the intensity of bridging nitrate (1614 and 1215 cm-1 ) decreases with

a

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concomitant increase in the intensity of monodentate nitrate (1304 and 1496 cm-1 ) and bidentate nitrate (1538 cm-1 ) species. It is therefore reasonable to suggest that the bridging bidentate nitrates

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are likely active species in the pathway of NH3 -SCR reaction and a L-H mechanism seems to apply for the NH3 -SCR on MCN. Furthermore, similar experiments also performed on the MC

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catalyst (Fig. S10) indicate that NH3 -SCR over the MC catalyst follows the L-H mechanism only.

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4. Conclusions

A novel MnOx-CeO2 nanosphere catalyst was prepared and tested towards the NH3 -SCR. An excellent low-temperature activity and ability for SO2 tolerance, which was much superior to

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the counterpart MnOx-CeO2 catalyst without any defined structural morphology. Moreover, the stability of MnOx-CeO2 nanosphere NH3 -SCR catalyst at 150 o C for long time on stream was also outstanding. The better redox properties, smaller grain size, increased surface concentration of Ce3+, Mn4+ and oxygen species, as well as of adsorbed NH3 and NOx species were found to be the main factors that explain very well the higher de-NOx efficiency compared to the non structured MnOx-CeO2 catalyst. In situ DRIFTS studies suggested that the NH3 -SCR reaction over the MnOx-CeO2 nanosphere catalyst could proceed through the E-R and L-H mechanisms.

ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National High-tech Research and Development (863) Program of China (2015AA03A401), Science and Technology Support Program of Jiangsu Province (Industrial, BE2014130), Scientific Research Project of Environmental Protection Department of Jiangsu Province (2016048) and the National Natural Science Foundation of China (21670182,

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21607122). Fundamental Research Funds for the Central Universities (020514380124)

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Communications, 84 (2016) 75-79.

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Fig.1 The NH3 -SCR performance of the MC and MCN.

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Fig. 2 The Raman spectra of CeO 2 , CeO2 -NS, MC and MCN and the intensity ratio of I 600 /I 400 (B).

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Fig.3 H2 -TPR profiles of MC and MCN.

NO conversion in NO oxidation over MC and MCN.

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Fig.4

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ACCEPTED MANUSCRIPT Table.1 BET surface area (the values in brackets were the surface area of the corresponding supports), pore volume, CeO2 crystallite size, average pore diameter, atomic concentration and atomic ratio of MC and MCN catalysts Catalysts

S BET 2

（m /g）

Pore

CeO2

Average

volume

sizes

pore

3

(nm)

Diameter

(cm /g)

Atomic

Atomic ratio/%

concentration/mol.% Ce

Mn

Mn4+/Mn Ce3+/Ce

O

Oα/O（α+β ）

（Å） 74（84）

0.19

8.5

91

13.42

7.41 79.17

20.26

11.04

27.12

MCN

118（155）

0.26

4.3

150

14.40

8.49 77.11

21.67

15.97

34.36

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MC

ACCEPTED MANUSCRIPT Highlights MnOx/CeO2 nanosphere catalyst showed over 80% NO conversion at 75-250 o C. MnOx/CeO2 nanosphere possessed high amounts of acid sites adsorbed NOx species. Excellent sulfur-tolerance ability and stability.

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Reaction pathway for NH 3 -SCR over MnOx/CeO2 nanosphere catalyst was illustrated.

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