Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Direct Molten Polymerization Synthesis of Highly Active Samarium Manganese Perovskites with Different Morphologies for VOC Removal Lizhong Liu, Hongbo Zhang, Jinping Jia, Tonghua Sun,* and Mengmeng Sun School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P.R. China
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ABSTRACT: A morphology-controlled molten polymerization route was developed to synthesize SmMnO3 (SMO) perovskite catalysts with netlike (SMO-N), granular-like (SMO-G), and bulk (SMO-B) structures. The SMO perovskites were formed directly by a molten polymerization method, and their morphologies were controlled by using the derivative polymers as templates. Among all catalysts, the porous SMO-N exhibited the highest activity, over which the toluene, benzene, and o-xylene were completely oxidized to CO2 at 240, 270, and 300 °C, respectively, which was comparable to that of typical noble-metal catalysts. The apparent activation energies of toluene over SMO-N (56.4 kJ·mol−1) was much lower than that of SMO-G (70.8 kJ·mol−1) and SMO-B (90.1 kJ·mol−1). Based on the results of scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and H2 temperature-programmed reduction characterization, we deduce that the excellent removal efficiency of volatile organic compounds (VOCs) over SMO-N catalyst was attributable to the special structure, high surface Mn4+/Mn3+ and Olatt/Oads molar ratios, and strong reducibility. Due to the high activity, low cost, and simple preparation strategy, the SMO catalyst is a promising catalyst for VOC removal.
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INTRODUCTION Volatile organic compounds (VOCs), as the byproducts of the chemical and manufacturing industries, are endangering the atmosphere and human health. Many countries and organizations have constituted stringent legislations and regulations to curb the emission of VOCs.1−3 Among the present technologies for removal of VOCs, catalytic oxidation is the most promising approach as it could convert VOCs into CO2, H2O, and other less hazardous compounds at low temperature.4−7 Until now, researchers have developed a large number of catalysts, such as noble-metal materials8,9 and single10,11 or hybrid transition metal oxides12,13 for catalytic oxidation of VOCs. Although noble-metal catalysts exhibit great catalytic properties at low temperature, there are many problems, including low thermal stability, high cost, easy sintering, and susceptibility to poisoning, limiting their further applications. By contrast, transition metal oxides have been considered as the candidate catalysts for VOC oxidation due to their low cost and great thermal stability.12−15 Therefore, it is very important to develop advanced transition metal oxide catalysts with excellent performance for catalytic oxidation of VOCs. As a potential substitute of noble-metal catalyst and one of the common transition metal oxides, the ABO3 perovskite-type oxide has received extensive attention in the catalytic oxidation of VOCs.16−20 Si et al.21 and Liu et al.22 prepared a threedimensional structure of 3D LaMnO3 perovskites by using © XXXX American Chemical Society
poly(methyl methacrylate) microspheres as templates and applied them to the study of catalytic oxidation of toluene or methanol. Suárez-Vázquez et al.23 reported the synthesis of SrTi1−xBxO3 (B = Cu and Mn) perovskites with dendritic morphology via a hydrothermal route, presenting its catalytic activity with complete conversion of toluene to CO2 at 350 °C. However, those methods either are complex or require harsh conditions. It is known that large quantities of catalysts are required to cast honeycomb or catalytic filter modules for the construction of industrial catalysts.24 This means that it is necessary to develop simple fabrication processes for scale-up production of catalysts with high catalytic activity. The textural properties of catalysts, such as the crystal plane, crystallinity, and morphology, is related to their catalytic behavior.25 The morphology control has been a novel approach for adjusting the catalytic performance of perovskites, and the synthesis strategy is a key factor to control their morphologies.26,27 To tackle the aforementioned challenges, we herein present an industrially scalable method to prepare SmMnO3 (SMO) perovskites with netlike, granular-like, and bulk structures. During the process, the SmMnO3 perovskite was synthesized by a molten polymerization strategy, and the morphology control of SMO perovskites was realized by citric acid polymerization, wherein the molten metal salts served as a Received: April 24, 2018
A
DOI: 10.1021/acs.inorgchem.8b01125 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry solvent, and the polymerized derivatives were used as templates to control their morphologies. The preparation is not only simple and efficient in enhancing the nature of the catalysts but also effective in advancing catalytic activity on VOC (toluene, o-xylene, and benzene) oxidation. Utilizing various catalyst characterizations, the relationship between physicochemical performance and catalytic behavior as well as mechanisms of VOC oxidation was also explored. More importantly, the catalytic activity of SMO-N is also much higher than that of 3D-LaMnO319 and La0.6Sr0.4Fe0.8Bi0.2O3−δ.28
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EXPERIMENTAL SECTION Figure 1. XRD patterns of SMO-N, SMO-G, and SMO-B.
Sample Preparation. The SmMnO3 perovskites with different morphologies were synthesized via a one-step molten polymerization strategy. The procedure of SMO-N was as follows: 1.23 g of manganese acetate tetrahydrate, 2.22 g of samarium(III) nitrate hexahydrate, and 3.15 g of citric acid (CA) were mixed. Next, the mixture was heated to 750 °C at a heating rate of 2 °C·min−1 and then held for 2 h in a electrical resistance furnace (YFX7/120-60) under air atmosphere. The amount of CA was adjusted to 2.10 and 1.05 g to obtain SMO-G and SMO-B, repectively, and no CA was added to prepare the sample designated NCA. Catalytic Evaluation. The catalytic activities of the samples for the catalytic oxidation of VOCs were evaluated in a continuous flow fixed bed quartz microreactor (inner diameter = 6.0 mm). Then, 0.15 g of the sample was used to measure the activity (height of catalyst bed = 160 mm). The VOC source was produced via an air-blowing strategy, passing airflow through a bottle containing pure VOC chilled in an ice−water isothermal bath. The feed gas contained 1000 ppm VOC, air (21% O2 and 79% N2), and saturated vapor (RH = 100%). The total flow rate was 80 mL·min−1, giving a weight hourly space velocity (WHSV) of 32000 mL·g−1·h−1. In addition, when the effect of WHSV on the catalytic activity was evaluated, the amount of catalyst was adjusted to 0.2 g (WHSV = 24000 mL·g−1·h−1) or 0.1 g (WHSV = 48000 mL·g−1·h−1). To investigate the influence of water vapor, the moisture of the catalytic system was also regulated (RH = 0, 50, and 100%). The concentration of VOCs was analyzed by a gas chromatograph (Shimadzu, GC-1020) equipped with a flame ionization detector (FID). The CO2 was measured using a gas chromatograph (HXSP, GC-950) equipped with a FID detector and a methane conversion oven. Additionally, the VOC conversion, αvoc, and the CO2 yield, YCO2, were calculated by the following equations: αvoc =
YCO2 =
[Cx Hy]in − [Cx Hy]out [Cx Hy]in
structures. Compared with those of SMO, NCA had some noticeable impurities (e.g., Sm2O3 and MnOx), which could be confirmed by the XRD result of NCA (Figure S1). It is inferred that when there is no CA, the distribution of the metal salts was nonuniform, which restricted the precursors from contacting each other adequately. The morphologies of the samples could be clearly revealed by scanning electron microscopy (SEM) and and transmission electron microscopy (TEM) (Figure 2a−h). As shown in Figure 2a,e, the SMO-B sample exhibited a compact surface, suggesting that SMO-B had experienced serious agglomeration, which could cause low surface area and small amounts of active sites.21 In contrast, it could be evidently observed from the SEM image in Figure 2b,c that the obtained SMO-N sample exhibited a three-dimensional interconnected netlike structure, and its arrangement became relatively orderly. Meanwhile, the SMO-N sample manifested small sizes in comparison with those of SMO-B, indicating that the agglomeration of the sample was reduced to some extent. Moreover, it could also be seen that a large number of nanopores and cavities with different diameters were present on the SMO-N sample (Figure 2f,g), which might be attributed to the release of gas from the decomposition of carbon materials. The special structure may be advantageous for the toluene molecule to approach the catalyst interior and enhance the area of toluene contact with the catalyst, thus favoring the occurrence of a catalytic reaction. Moreover, the SMO-G sample, as shown in the SEM and TEM images (Figure 2d,h), displayed disordered granular-like nanoparticles, revealing that the precursors of the catalyst were overdispersed. Those results showed that by adjusting the polymerization of the CA monomer, the morphology of the catalyst can be effectively modified. A simultaneous differential scanning calorimetry/thermogravimetric analysis (DSC-TGA) trace of the mixed system is provided in Figure 3a,b. The curve in Figure 3a shows significant weight loss at about 80 °C, which should result from the loss of water in the system; the first endothermic peak at ca. 70 °C in the DSC curve (Figure 3b) was caused by the metal salts melting. The adjacent endothermic peak at 95 °C may be attributed to the dissolution of citric acid in molten salt. The weight loss peak at 118 °C was associated with the dehydration due to the polymerization. A strong exothermic peak at 342 °C in the DSC curve and a weight loss peak at 336 °C in the DTG curve indicated that the CA derivatives were gradually self-combusted and decarbonized. The synthetic mechanism of SMO with different morphologies is illustrated in Figure 4. As is known, the melting points of manganese acetate and samarium trinitrate are much lower
× 100%
[CO2 ]out − [CO2 ]in × 100% x[Cx Hy]in
where [CxHy]in, [CxHy]out, [CO2]in, and [CO2]out denote the inlet and outlet concentrations of VOC and CO2, respectively.
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RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) patterns of the asprepared materials under different preparation conditions. It can be found that the XRD pattern of three samples depicted diffraction peaks in the 2θ range of 15−70°, which well corresponded to the XRD pattern of the standard SmMnO3 (JCPDS PDF# 25-0747), indicating that the SmMnO3 perovskite structure was formed after the calcination process.29 In addition, it should be noted that the diffraction peaks of the catalysts became gradually broader in order SMO-B < SMO-N < SMO-G, showing the decrease in the crystalline degree of the SmMnO3 catalyst was due to the enhanced amount of CA, which indicated that there may be differences in their B
DOI: 10.1021/acs.inorgchem.8b01125 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. SEM images of SMO-B (a), SMO-N (b,c), and SMO-G (d); TEM images of SMO-B (e), SMO-N (f,g), and SMO-G (h) samples.
Figure 3. (a) TGA-DTG and (b) DSC traces of the mixed system.
N2 adsorption−desorption isotherms and pore-size distributions of SMO-N, SMO-G, and SMO-B perovskites are displayed in Figure 5a,b, and the Brunauer−Emmett−Teller
Figure 5. (a) N2 adsorption−desorption isotherms and (b) pore-size distributions of the three samples. Figure 4. Schematic illustration of SMO synthesis.
than that of CA. Therefore, under heat, the manganese acetate and samarium trinitrate were first melted into a liquid, and the citric acid was dissolved quickly into the liquid to form a homogeneous mixture. Afterward, citric acid underwent polymerization to generate CA-derived polymers with complex structures,30 wherein the samarium−manganese metal salts doped in CA-derived polymers would be homogeneously dispersed in the polymers (Figure S2). Namely, the asobtained polymer was used as a template for the growth of perovskite precursors. Therefore, the change in the amount of CA could lead to different morphologies of catalyst precursors. As the temperature continued to increase, the derived polymers were decomposed and gradually decarbonized. Eventually, the different morphologies of SMO catalysts were generated at high temperature.31
(BET) surface areas are summarized in Table 1. As shown in Figure 5a, the isotherms of each sample showed a typical IUPAC type II pattern with slit-type pores (H3-type) of the BET hysteresis loop in the P/P0 range of 0.55−1.0, which are characteristic of the combination of mesoporous and macroporous structures.32,33 Figure 5b shows the pore-size distribution scattering from 4 to 110 nm of each sample, which is consistent with the result of the isotherms. In addition, the BET surface area of SMO-G, SMO-N, and SMOB was 27.7, 25.6, and 13.2 m2·g−1, respectively, which was much higher than that of LaMnO 3 (7.3 m 2 ·g −1 ), 32 La0.6Sr0.4Fe0.8Bi0.2O3−δ (4.5 m2·g−1),34 La0.6Sr0.4MnO3 (2.6 m2·g−1),35 and CeO2/LaCoO3 (17.6 m2·g−1).36 The relatively high specific surface area could contribute to the improvement of catalytic activity for VOC oxidation.37 C
DOI: 10.1021/acs.inorgchem.8b01125 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Surface Element Compositions, BET Surface Areas, and H2 Consumption of Samples XPS (molar ratio)
H2 consumption (mmol/g)
samples
Sm/Mn
Olatt/Oads
Mn4+/Mn3+
SBET (m2·g−1)
T < 310 °C
310 < T < 578 °C
T > 578 °C
total
SMO-N SMO-G SMO-B
1.02 1.11 1.25
1.27 0.87 0.95
1.02 0.79 0.53
25.6 27.7 13.2
0.28 0.15 0.09
1.10 0.84 0.65
1.60 1.72 1.91
2.98 2.71 2.65
The information on surface element compositions, metal oxidation states, and adsorbed species of SMO perovskites with different morphologies can be obtained by an X-ray photoelectron spectroscopy (XPS) technique. The Mn 2p3/2 and O 1s XPS spectra of the SMO-N, SMO-G, and SMO-B are presented in Figure 6a,b, and the quantitative analysis results
Figure 7. H2-TPR profiles of SMO-N, SMO-G, and SMO-B.
the peak at 427 °C was assigned to the single-electron reduction of Mn3+ located in a coordination-unsaturated microenvironment, and the reduction peak about 726 °C was from the reduction of Mn3+ to Mn2+.41−43 The temperatures of all reduction peaks of SMO-N were much lower in comparison to those of SMO-G and SMO-B, suggesting that the three-dimensional network with porous structure was beneficial for the reduction of the sample. In order to facilitate the comparison of the hydrogen consumption of samples, the reduction of each sample was performed with three regions (e.g., less than 310, 310−578, and greater than 578 °C, respectively). With regard to the removal of the surface oxygen adspecies, the SMO-N displayed the best consumption of H2 (0.28 mmol·g−1) below 310 °C by comparison to that of SMO-G (0.15 mmol·g−1) and SMO-B (0.09 mmol·g−1) (Table 1). In the range of 310−578 °C, all three samples showed a greater amount of hydrogen consumption, and the H2 consumption of SMO-N, SMO-G, and SMO-B was 1.10, 0.84, and 0.65 mmol·g−1, respectively; in the high-temperature region (above 578 °C), the amount of hydrogen consumed was 1.60, 1.72, and 1.91 mmol·g−1 for samples SMO-N, SMO-G, and SMO-B. Catalytic Performance. Before testing the catalytic activity, we performed the blank test under the condition of no catalyst, and no conversion of VOCs was observed below 350 °C. The result showed that there was no occurrence of homogeneous reactions under the adopted reaction conditions. The catalytic activities of toluene, benzene, and o-xylene oxidation over as-prepared SMO catalysts are displayed in Figure 8a−d, where the measured conditions were as follows: WHSV = 32000 mL·g−1·h−1, RH = 100%, VOC concentration = 1000 ppm. Figure 8a shows the toluene conversion over the three samples, and the SMO-N presented the best catalytic activity. The T10%, T50%, and T90% (the reaction temperatures vs toluene conversions of 10, 50, and 90%, respectively) were utilized to evaluate the activities of the samples for catalytic oxidation of toluene. The values of T10%, T50%, and T90% were 170, 190, and 215 °C for SMO-N, 175, 206, and 228 °C for SMO-G, and 188, 223, and 258 °C for SMO-B, respectively. The result indicated that SMO-N held a lower light-off temperature.
Figure 6. (a) Mn 2p2/3 XPS spectra; (b) O 1s XPS spectra of SMON, SMO-G, and SMO-B.
are summarized in Table 1. The asymmetrical XPS peaks can be decomposed by curve fitting. As shown in Figure 6a, the three components divided from the Mn 2p2/3 XPS spectrum at 642.9, 641.7, and 640.7 eV were attributed to the surface Mn2+, Mn3+, and Mn4+, respectively.38 As known, the oxidation reaction occurs on the surface of a catalyst, and the catalytic activity could be associated with the extent of the Mn4+ ⇔ Mn3+ redox process; the higher molar ratio of surface Mn4+/ Mn3+ was conducive to improving the redox cycles of manganese ions, thus enhancing its catalytic capacity.20 For the as-prepared catalyst, the molar ratios of surface Mn4+/Mn3+ were 1.05 (SMO-N), 0.79 (SMO-G), and 0.53 (SMO-B), indicating that the porous SMO-N may hold higher catalytic activity for VOC oxidation. Additionally, it can also be found from O 1s XPS spectra (Figure 6b) of the samples that the adsorbed oxygen species (O2−, O22−, or O−), surface lattice oxygen (Olatt), and carbonate (CO32−) or hydroxide (OH−), located at 528.9, 530.6, and 532.8 eV, respectively, were copresent on the surface of the catalysts.39,40 The surface Olatt/ Oads increases in the order SMO-B < SMO-G < SMO-N, indicating the increase in Olatt concentration, which could give rise to improved catalytic activity of SMO-N. From Table 1, furthermore, the surface Sm/Mn molar ratios for samples SMO-N, SMO-G, and SMO-B were 1.03, 1.15, and 1.21, respectively, indicating the surface of SMO-N catalyst held more abundant Mn elements. The reducibility is an important parameter that affects the catalytic activity of the catalysts. The reducibility of SMO-N, SMO-G, and SMO-B was acquired by a H2 temperatureprogrammed reduction (TPR) experiment, as shown in Figure 7a,b, and their corresponding H2 consumption is listed in Table 1. For the SMO-N, the reduction peak at 293 °C was attributed to the surface oxygen adspecies of the catalyst, and the peak at 349 °C was due to the reduction of Mn4+ to Mn3+; D
DOI: 10.1021/acs.inorgchem.8b01125 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
that toluene oxidation over SMO perovskites should adhere to a Mars−van Krevelen mechanism,22,44 wherein toluene molecules are attracted by high valent manganese ions and react with the surface Olatt over catalysts to convert into H2O and CO2, creating oxygen vacancies; afterward, gas-phase oxygen molecules are activated by oxygen vacancies to generate active oxygen, supplementing the consumption of the Olatt due to toluene oxidation. Compared with SMO-G and SMO-B, the SMO-N took on higher low-temperature reducibility and higher molar ratios of Mn4+/Mn3+ and Olatt/ Oads, promoting the conversion of Mn4+ ⇔ Mn3+ and O2 → Olatt, thereby improving the catalytic activity of the catalysts for oxidation of toluene. The obvious porous and cavity structures could also give the toluene molecules easy access into the inside of SMO-N, further accelerating the catalytic reaction process. Furthermore, it can be found that SMO-N also has good catalytic activities for benzene and o-xylene oxidation (Figure 8d). Over the SMO-N catalyst, toluene, benzene, and o-xylene can be completely oxidized to CO2 at 240, 270, and 300 °C, respectively, which was lower than that over Pd/ZrO245 (280 °C based on toluene oxidation), Au/CeTi46 (350 °C based on toluene oxidation), Au/Al2O347 (290 °C based on benzene oxidation), Ag/α-MnO248 (400 °C based on benzene oxidation), or Pt/zeolite49 (350 °C based on o-xylene oxidation). The difference in reactivity among VOCs over SMO-N may be attributed to the degree of substrate adsorption and structural effects.50 Moreover, by comparing the toluene conversion and the yield of CO2 (Figure 8a,d), it can also be observed that there is a difference in the numerical values at the low-temperature reaction stage, indicating that there may be some intermediates not completely mineralized in the exhaust gas.51 In addition, the catalytic stability of SMO-N was investigated (Figure 9). The tests were performed under feed gas
Figure 8. (a) Toluene conversion versus reaction temperature over SMO-N, SMO-G, and SMO-B; (b) Arrhenius plots of the three samples for toluene oxidation; (c) effect of WHSV on toluene oxidation over SMO-N; (d) CO2 yield from toluene, benzene, and oxylene oxidation versus reaction temperature over SMO-N.
Moreover, in the case of excess oxygen, the reaction of toluene oxidation obeys a first-order kinetic mechanism with respect to toluene concentration (c, mol·g−1): r = −kc = (−A exp(−Ea/RT))c, where r, k, A, and Ea are the reaction rate (mol·g−1·s−1), rate constant (s−1), pre-exponential factor, and apparent activation energy (kJ·mol−1), respectively. The Arrhenius plots for toluene conversion over the three samples at the toluene conversion of
