Enhanced Toluene Combustion over Highly Homogeneous Iron

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Enhanced Toluene Combustion over Highly Homogeneous Iron Manganese Oxide Nanocatalysts Yu Wang, Lingxia Zhang, and Limin Guo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00258 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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ACS Applied Nano Materials

Enhanced Toluene Combustion over Highly Homogeneous Iron Manganese Oxide Nanocatalysts Yu Wang a, Lingxia Zhang b, Limin Guo a*

a

School of Environmental Science and Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China b

State Key Laboratory of High Performance Ceramic and Superfine Microstructure,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China

*

Corresponding author Tel.: +86 02787792101.

E-mail address: [email protected] (L. M. Guo). 1 ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Employing “redox-precipitation” reactions among MnO4+, Fe2+ and Mn2+, a facile synthetic route for highly homogeneous FeMn oxide (Fe1Mn2-RP) has been developed, which can achieve a superior catalytic performance in the toluene combustion with T50 and T90 at 186 oC and 197 oC, respectively (500 ppm toluene/humid air, GHSV = 50,000 h-1). XRD, N2 sorption isotherms and TEM characterization demonstrated the formation of a homogeneous Fe-Mn-O solid solution with higher surface area for the Fe1Mn2-RP catalyst. The highly homogeneous Fe-Mn-O structure can result in better low-temperature reducibility and higher amount of oxygen electrophilic species evidenced by H2-TPR and XPS, thus significantly increasing the catalytic activity. At the same time, the Fe1Mn2-RP catalyst showed the excellent water/methanol-resistance and stability, demonstrating its great potential for practical application.

Keywords: Redox-precipitation, FeMn oxides, Heterogeneous catalysts, Catalytic combustion, VOCs

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1 Introduction Volatile organic compounds (VOCs) are extensively applied in industrial applications1. As a results, VOCs as one of the major air pollutants contribute to a number of environmental problems such as ground-level ozone, photochemical smog, PM2.5, toxic air emissions and to undesirable human health effects2. As an effective method, low-temperature catalytic combustion has been gaining the increasing attention. Its low operating cost and reaction temperature, as well as high destructive efficiency can well meet the environmental requirement1. Non-noble transition-metal oxides such as Fe2O3, Co3O4, MnO2, CuO and rare earth such as CeO2, LaMnO3, and LaFeO33-16 have been tested as catalysts for combustion of VOCs. Among the metal oxide based catalysts for VOCs combustion, FeMn oxide catalyst has attracted much research attention due to its noticeable performance in VOCs elimination17-18, adsorption of heavy metal ions19 and Fischer-Tropsch synthesis20-21. The promotion of catalytic performance was attributed to a favorable synergetic effect of manganese and iron oxide22. Duran et al.17 studied the Fe/Mn atomic ratio of Mn2O3-Fe2O3 mixtures and found these FeMn oxides were efficient for the catalytic ethanol, ethyl acetate and toluene oxidation at low temperatures. A synergetic effect between Fe and Mn oxide was proposed, which led to the formation of a Mn-Fe-O solid solution. Navrotsky et al.23 have found that the redox reactions between Fe and Mn atom should play a more important role in stabilizing the spinels compared with the binary oxides. Cadus et al.24 obtained MnFe mixed oxide catalysts by co-precipitation and the MnFe catalyst consisted of an oxide mixture (Fe2O3, Mn2O3 and Mn5O8), which have an excellent performance in ethanol combustion. 3 ACS Paragon Plus Environment


It is commonly accepted that the electrophilic oxygen species are important to catalytic oxidation reaction25-27. Normally, different type and concentration of electrophilic oxygen species could significantly affect the catalytic performance28. However, it is challenging to find a facile strategy, which can increase the surface oxygen species concentration of FeMn oxide catalyst. Generally, the synthesis method plays a key role to improve the reactivity of the FeMn oxide catalyst. Recently, a novel “redox-precipitation” method has been developed to synthesize well-dispersed Mn-based binary oxides by conducting two or three parallel reactions simultaneously. And the Sn-Mn29, Fe-Mn18, Cu-Mn30 and Ce-Mn31 catalysts synthesized by “redox-precipitation” method possessed great textural and redox properties in comparison with the counterparts synthesized by co-precipitated method. Importantly, the strong metal-MnOx interaction along with higher average oxidation number (AON) of both Mn and metal (Sn, Fe, Cu and Ce) ions have been assigned to be responsible for the great promotion of the availability of reactive surface oxygen and the catalytic activity32-33. This paper reported the highly homogeneous FeMn oxide preparation driven by redox-precipitation process and its performance for catalytic toluene combustion (toluene was chosen as a typical VOC) have been evaluated. Experimental data demonstrated that the redox-precipitation process led to the formation of highly homogeneous Fe-Mn-O solid solution with higher surface area and narrow pore size distribution, which greatly promoted the electrophilic oxygen species quantity and resulted in better low-temperature reducibility. Finally, the enhanced catalytic toluene combustion based on as-prepared FeMn catalysts has been achieved.

2 Experimental 4 ACS Paragon Plus Environment

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2.1. Catalyst preparation (a) Redox-precipitation method: FeMn oxide catalysts with different Fe/Mn ratio (Fe/Mn = 3/1-1/2) were prepared via the “redox-precipitation” route according to the following procedure13, 31-33. The proper amount of the KMnO4 aqueous solution (10% stoichiometric excess) was titrated at 80 oC with a solution of FeSO4·7H2O and Mn(NO3)2 precursors under vigorous stirring. Here, the pH value of the solution was kept around 8.5 by the addition of 0.2 M KOH solution, according to the following reaction scheme:

MnO 4− + 3e− + 2H 2 O  → MnO2 ( ↓ ) + 4HO −

(1)

Mn 2+ + 2H 2O  → MnO2 ( ↓ ) + 2e − + 4H +

(2)

2Fe 2+ + 3H 2 O  → Fe2 O3 ( ↓ ) + 2e − + 6H +

(3)

Thereafter, the turbid solution was stirred for 2 hours at 80 oC and then filtered, washed and dried at 100 oC for 12 hours. The as-obtained powder without further calcination was denoted as Fe3Mn1-RP, Fe1Mn1-RP and Fe1Mn2-RP, which represented Fe/Mn = 3/1, 1/1 and 1/2, respectively. (b) Co-precipitation method: The FeMn oxide catalyst with Fe/Mn = 1/2 was also prepared by co-precipitation method as the reference. 2M NaOH aqueous solution was added dropwise to Mn(NO3)2 and Fe(NO3)3·9H2O to reach a pH value of 8.5 under vigorous agitation. After an ageing period of 3 h, the solid was filtered out, washed, dried at 80 oC overnight and calcined at 350 oC for 5 h. The as-obtained powder was denoted as Fe1Mn2-CP. The Fe2O3, MnOx and β-MnO2 34 samples were also prepared by the similar procedure.

2.2. Catalyst characterization



The catalysts were characterized by XRD, N2 adsorption-desorption, ICP, TEM, SEM, XPS and H2-TPR. The detailed procedures were enclosed in the Supporting Information.

2.3. Catalytic activity measurement The toluene combustion activity measurements were carried out in a fixed-bed quartz reactor, and the detailed procedures were enclosed in the Supporting Information.

3 Results and discussion 3.1 Catalysts characterization Normally, the co-precipitated method is difficult to control the formation of homogeneous mutual-dispersion in mixed metal oxides. As depicted in Scheme 1, the co-precipitate of mixed Fe and Mn hydroxides is formed in the existence of NaOH solution, However, the huge difference in solubility product constant (Ksp values of Fe(OH)3 and Mn(OH)2 are 1.6 × 10-39 and 1.6 × 10-13, respectively) causes Fe3+ more easily precipitated than Mn2+ ions. This results in the preferred self-aggregation of Fe(OH)3 before well-dispersion in MnOx precipitate and formation of Fe-rich oxide cores surrounded by Mn oxides after calcination. In order to maximize the homogeneity of Fe and Mn ions, the preparation of FeMn oxide driven by a redox reaction can be proposed as shown in Scheme 1. The simultaneous redox reactions among MnO4-, Fe2+ and Mn2+ ions into MnO2 and Fe2O3 were used to prepare Fe-Mn-O nanocomposites with highly homogeneous Fe and Mn oxides. Herein, around 10 g/L KMnO4 precursor was titrated by a Fe2+ and Mn2+ solution at pH = 8.5 ± 0.2, and the above reactions (Equation 1-3) happened. The XRD patterns of the FeMn oxide, Fe2O3 and MnOx samples were shown in Figure 1. The diffraction peaks of Fe2O3 could be ascribed to α-Fe2O3 (PDF#33-0664) and γ-Fe2O3 6 ACS Paragon Plus Environment

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(PDF#25-1402)35-36. XRD peaks for the MnOx samples can be assigned to α-MnO2 (PDF#44-0141) and Mn5O8 (PDF#39-1218). For the Fe3Mn1-RP and Fe1Mn1-RP samples synthesized via the “redox-precipitation” route, only diffraction peaks related to the α-Fe2O3 could be observed, and the intensity of diffraction peaks of α-Fe2O3 decreased with the Fe/Mn ratio decreasing. For Fe/Mn = 1/2, the diffraction peaks corresponding to Mn3O4 (PDF#24-0734) were observed over the conventional CP sample17-18, which was with the well crystalline structure. While the XRD pattern of the RP sample showed the much poor cystalline, which was ascribed to rutile-like β-MnO2 (PDF#30-0820)37-38 as convinced in

Figure S1. And the diffraction patterns of “redox-precipitated” Fe1Mn2-RP sample implied the homogeneity at a (quasi)molecular level13, 31-33. It was observed that there was a slight negative-shift for the strongest diffraction peak around 2θ = 37.06o for pure β-MnO2 to 2θ = 36.24o for Fe1Mn2-RP (Figure S1). Besides, it was also notable that there was a slight positive-shift for the strongest diffraction peak from 2θ = 35.53o for pure α-Fe2O3 to 2θ = 35.74o for Fe3Mn1-RP, but the diffraction peak for Fe1Mn1-RP (2θ = 35.54o) was almost consistent with regard to the pure α-Fe2O3 (Figure 1). These could be ascribed to the variation of metal-oxygen bond length due to the formation of Fe-Mn-O solid solution24. In addition, it was observed that α-Fe2O3 could achieve ca. 30% solubility in Mn2O3, and Mn2O3 ca. 10% in α-Fe2O3 vice versa39. Therefore, it may form the solid solution between MnOx and α-Fe2O3 in the current FeMn catalysts synthesized by the RP method with Fe/Mn ratio at 3/1 and 1/2. Then, the “redox-precipitation” route in the current work can form homogenous FeMn solid solution by incorporating Fe into the MnO2 lattice (i.e., Fe1Mn2-RP) or by incorporating Mn into the α-Fe2O3 lattice (i.e., Fe3Mn1-RP). 7 ACS Paragon Plus Environment


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It was also found that manganese in the lattice of spinel can strongly affect the redox properties of the iron and then embedded influence on their catalytic activity and stability17. The N2 sorption isotherms were used to understand the overall microstructural characteristics of as-prepared samples (Figure 2). The N2 sorption isotherms of RP samples showed the type-IV isotherms with the capillary condensation at the relative pressure between about 0.7 and 0.9. As the reference, the isotherm of Fe1Mn2-CP showed the capillary condensation at relative pressure between about 0.4 and 0.8. Interestingly, the Fe1Mn2-CP, Fe2O3 and MnOx have been prepared by similar procedure (co-precipitation). However, the quite difference isotherms were observed. This result can be ascribed to the different aggregated structure of the corresponding metal oxides. For the Fe-Mn oxide sample, some mesostructure was formed via the strong interaction of Fe and Mn precursor, which showed the hysteresis loop in the P/P0 range of 0.4-1.0 and relative high N2 adsorption volume. Whereas the pore structure of Fe2O3 and MnOx originated from the irregular compiling of Fe2O3 and MnOx nanoparticles. Then the isotherms showed the relative low N2 adsorption volume and the hysteresis loop was in relative higher P/P0 value. The microstructural data of as-prepared samples were summarized in Table 1. The SBET (114-178 m2/g) and Pv (0.227-0.352 cm3/g) values of the FexMny-RP samples increased with the Mn/Fe ratio increasing. And the phenomena were well consistent with the decrease of diffraction intensity for hematite phases from XRD results (Figure 1). The SBET and Pv values of Fe1Mn2-CP were 133 m2/g and 0.185 cm3/g, respectively, which was much lower than that of Fe1Mn2-RP.

Actually,

the

microstructure

of

the

sample

synthesized

by

the

“redox-precipitation” method was independent on the Fe/Mn ratio and the phenomena were 8 ACS Paragon Plus Environment

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different from other synthesis methods31. The pore size distribution (PSD) of Fe1Mn2-RP (Figure S2-A) distributed in the range 1-100 nm and had a maximum at ca. 15 nm. The porous network was generated by the aggregation of FeMn nanoparticles. According to the TG-DTA result (Figure S3), there was around 20% weight loss up to 800 oC, which can be ascribed to the dehydroxylation of the as-synthesized sample. This can be convinced by the XRD results that the crystalline of Fe1Mn2-RP sample did not change after 350 oC calcination (Figure S1). For Fe1Mn2-CP sample (Figure S2-B), the distribution was in the range of 1-50 nm and had a sharp maximum at ca. 5 nm. The SEM images of the Fe1Mn2-CP and RP samples (Figure S4) showed the particles over Fe1Mn2-RP were more homogeneous than those over Fe1Mn2-CP sample. Further insights into the structural feature of Fe1Mn2-RP was checked by TEM observation and the representative images were shown in Figure 3. The co-precipitated Fe1Mn2-CP consisted of irregular particles (Figure 3A). And the interplanar distances of d = 3.09, 4.92, and 2.88 Å can be attributed to (112), (101) and (200) planes of the Mn3O4 phase at high magnification (Figure 3B), respectively, which was consistent with XRD estimation. And the isolated FeOx and MnOx can been observed from the elements mapping for Fe1Mn2-CP sample (Figure

3C), which was mainly owing to solubility product constant of Fe(OH)3 and Mn(OH)2. Figure 3D showed that the Fe1Mn2-RP possessed porous architectures, which were results of the tiny irregular nanoparticles assembly. It can be observed at high magnification (Figure 3E) that the phases can be identified according to interplanar distance (d = 2.45 Å), which was in the range between 2.42 Å ((1 0 0) plane of β-MnO2) and 2.52 Å ((1 1 0) plane 9 ACS Paragon Plus Environment


of α-Fe2O3). The element mapping demonstrated that Mn and Fe were homogeneously dispersed according to the fact that the completely overlapping of color images for Mn and Fe element (Figure 3F). TEM results well proved the introduction of Fe ions into the β-MnO2 lattice to form the solid solution, suppress the crystallization of β-MnO2 and finally, the amorphous dominated structure formed, which favored the occurrence of the lattice defects and the exposure of inner atoms, and enlarged the surface area of materials17, 24, 40. Importantly, for the TEM image of Fe1Mn2-RP, some crystalline can be observed and the XRD pattern gave the overall amorphous state of the sample. In the current work, it was found that the improved performance of Fe-Mn redox catalysts could arise from higher amorphous state, which could lead to higher surface area, higher concentration of lattice defects and oxygen vacancies, and better low-temperature reducibility. The crystals may also show promotion in the catalytic oxidation of toluene, but the activity should be much lower considering its lower surface oxygen species. The XPS measurements have been used to study the surface composition and dispersion of the as-synthesized catalysts. The XPS O 1s spectra were shown in Figure 4A. By deconvolution, the estimated the distribution of oxygen species were summarized in Table 2. Two kinds of oxygen species were observed, the one with binding energy (BE) at around 532.0 eV was assigned to surface adsorbed oxygen (Oads)41, while that BE at around 529.9 eV was ascribed to surface lattice oxygen (Olatt)42. The results revealed that FexMny-RP samples (Oads/Olatt = 0.75, 0.83, and 0.90, respectively) were greatly enriched with surface adsorbed oxygen compared with Fe1Mn2-CP (Oads/Olatt = 0.48). The surface adsorbed oxygen was important for physical property of the materials as well as for their catalytic applications43, 10 ACS Paragon Plus Environment

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which was thought to be beneficial to Mars-van-Krevelen mechanism for VOCs oxidation.

Figure 4B and C illustrated the Fe 2p and Mn 2p spectra of the samples. The Fe 2p region were composed of two main spin-orbital lines, the Fe2+ valence state was characterized by an asymmetric 2p3/2 peak (BE = 710.6 eV), whereas the Fe 2p3/2 signal at BE = 713.1 eV together with the shake-up satellites (S1) at BE = 718.4 eV was indicative of surface Fe3+ presence44-45. The Mn 2p peaks were composed of two main spin-orbital lines, the Mn 2p3/2 at BE = 642.0 eV was indicative of surface Mn3+ presence, whereas the Mn 2p3/2 signal at BE = 643.4 eV was indicative of surface Mn4+ presence45-46. By curve-fitting the Fe 2p and Mn 2p peaks (Table 2), the Fe3+/Fe2+ ratio was noted to increase from 0.29 to 0.47 and the Mn4+/Mn3+ ratio increase from 0.95 to 1.31 with decrease Fe/Mn ratio in the RP samples, respectively. Based on the XRD, BET, TEM and XPS results, a strong “redox-interaction” between Fe and Mn ions during the “redox-precipitation” process was proposed, which could be explained on the basis of an electron transfer between Fe2+, Mn2+ and Mn7+ via the reaction of Fe2++Mn2++Mn7+→Fe3++ 2Mn4+. The substitution of Fe2+ ions in the crystal lattice of β-MnO2 by Fe3+ ion suggested that the Mn4+/Mn3+ ratio must increase to maintain a charge balance47. However, both the surface Fe3+ and Mn4+ species concentration increased with decreasing Fe/Mn ratio, which indicated the redox reaction occurred more readily to form maximum dispersion of Mn and Fe oxides at manganese-rich compositions. For Fe1Mn2-CP, the Fe3+/Fe2+ ratio (0.27) and Mn4+/Mn3+ atomic ratio (1.12) were much lower than those of Fe1Mn2-RP (0.47 and 1.31, respectively). In fact, the superior catalytic performance of the Mn-based materials had been associated with their higher AON31. The “well-dispersed” manganese ions could play a key role in absorb oxygen and be active in 11 ACS Paragon Plus Environment


an oxidizing atmosphere, and hence result in the formation of highly active electrophilic oxygen species14, 48. Furthermore, the surface Fe/Mn atomic ratio based on the XPS data of the RP and CP catalysts (e.g., Fe 2p/Mn 2p) were compared with the bulk Fe/Mn atomic ratio (Table 1) in

Figure S5. An accurate linear relation was observed for RP catalysts, which was consistent with the well dispersion of the phase. In contrast, with a Fe/Mn surface value lower than the counterpart Fe1Mn2-RP sample, XPS analysis provided evidence that a Fe-rich oxide cores surrounded by Mn oxides in the Fe1Mn2-CP sample. Notably, the surface Fe and Mn elemental ratio of the RP samples were also lower than the designed value, although they were much close to the designed value as compared with the co-precipitated samples. Although the RP method can maximize the homogeneity of Fe and Mn ions, the Fe3+ was still easily precipitated, which could result in the preferred self-aggregation of Fe(OH)3 to some extent before MnOx precipitate. H2-TPR results (Figure 5A) showed the redox properties of the FeMn oxide catalysts. For better understand the TPR profiles of the binary FeMn oxide samples, α-Fe2O3 and MnOx samples were also measured as references (inset of Figure 5A). For the α-Fe2O3 sample, three reduction peaks were present at 345, 404 and 533 oC. The peak at 345 oC can be attributed to the reduction of surface oxygen and hydroxyl species, the two peaks at 404 and 533 oC corresponded to the stepwise reduction of Fe2O3→Fe3O4→FeO49. For MnOx, the H2-TPR result showed a successive reduction process. The sample MnOx (contained both MnO2 and Mn5O8 phases) had a mixed valence nature, which corresponded to the 2D octahedral sheets of [Mn3O8]4− consisting of Mn4+ ions in the bc plane separated by Mn2+ 12 ACS Paragon Plus Environment

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layers. Therefore the H2 consumption was due to the reduction of both Mn5O8 and MnO250-51. The peaks were due to the stepwise reductions of Mn5O8 and MnO2 owing to: Mn4+→Mn3+→ Mn2+. For all the FeMn oxide samples shown in Figure 5A, the first reduction peaks (240-278 o

C) was due to the reduction of Oads species generated from the presence of oxygen vacancies.

These peaks between ca. 284 and 490 oC were due to the successive reduction of bulk oxygen species of FeMn oxides, involving the transformation of Fe2O3→Fe3O4→FeO and MnO2→Mn3O4→MnO, respectively. Notably, the reduction of bulk oxygen species was much easier than those in pure α-Fe2O3 and MnOx, indicating that both CP and RP methods would not lead to a simple oxide mixture. The reduction peaks of bulk oxygen species were shifted to lower temperature with decrease Fe/Mn ratio, which was advantageous to enhance the catalytic oxidation reaction. The H2 consumption and extent of reduction were calculated and depicted in Table 2. The H2 consumption was much higher over Fe1Mn2-RP (3.76 mmol/g) than over Fe1Mn2-CP (3.01 mmol/g). For the FexMny-RP catalysts, maximum H2 consumption was observed over the Fe1Mn2-RP catalyst as well, suggesting that among the catalysts, the amount of oxygen available on Fe1Mn2-RP was the largest. These results were related to the different textural and structural properties of the as-prepared FeMn oxides. Actually, the “well-dispersed” manganese and iron oxide promoted the huge amount of active Oads species formation (i.e., O2-, O22-, O-, etc.)52-56, which remarkably enhanced the oxidizing ability of the catalysts in a temperature range suitable for VOCs catalytic combustion14-15. Herein the initial H2 consumption rate (where reduction was less than 20%) was also calculated to evaluate the 13 ACS Paragon Plus Environment


low-temperature reducibility of the FeMn oxide catalysts as shown in Figure 5B. It was obviously seen that the initial H2 consumption rates of the FeMn oxides decreased in the order of Fe1Mn2-RP > Fe1Mn1-RP > Fe3Mn1-RP > Fe1Mn2-CP, which were in good agreement with the sequence of catalytic performance shown below.

3.3 Catalytic activity The catalytic performance of the FeMn oxide catalysts (Figure 6A) was evaluated using the temperatures of 50% (T50) and 90% (T90) toluene conversion at a GHSV of 50,000 h-1. With the reaction temperature increasing, the conversion of toluene increased over the catalysts. And the RP catalysts showed much better performance than that of co-precipitated catalyst. As summarized in Table 3, the order of activity was Fe1Mn2-RP > Fe1Mn1-RP > Fe3Mn1-RP > Fe1Mn2-CP > MnOx > Fe2O3, which was consistent with the order of reducibility of the catalysts. When the manganese doping amount was down to a Fe/Mn ratio of 3/1, there was a decline of toluene conversion. The Fe1Mn2-RP catalyst was the most active among all the catalysts and the T50 and T90 were at 186 and 197 oC, respectively. This much enhanced catalytic behavior was closely related to the formation of solid solution between β-MnO2 and α-Fe2O3, its higher surface area (SBET) and pore volume (Pv), higher AON of Mn ions, better reducibility and the rich active oxygen species. The Fe1Mn2-CP catalyst exhibited relatively low activity, the lower AON of Mn ions, poor reducibility of the catalyst at lower temperature and the decreased value of pore volume may be the reasons for its poor performance. In fact, the combustion of toluene has been studied over various catalysts in recent years. The activity characterization of the catalysts can be based on the rate per unit of surface area 14 ACS Paragon Plus Environment

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or the TOFs (µmol/(m2 h) or mmol/(gcat h)) value, which was defined as the ratio of the reaction rate to the active site density of catalysts (the detailed calculative procedures were described in the Supporting Information). As listed in Table 4 (the reaction rate was calculated at 200 oC). Among the non-noble metal oxide catalysts, the Fe1Mn2-R catalyst showed the highest toluene consumption rate per gram, and the reaction rate at 200 oC on Fe1Mn2-R was about 0.89 mmol/(g h) in this work, which was clearly greater than a variety of manganese oxide such as flower-like ε-MnO2 (0.16 mmol/(g h))57, rod-like α-MnO2 (0.30 mmol/(g h))12 and 3D mesoporous MnO2 (0.36 mmol/(g h))58. The effect of GHSV on the catalytic performance of Fe1Mn2-RP catalyst for toluene oxidation was shown in Figure 6B. As expected, conversion of toluene dropped with a rise of GHSV from 50,000 to 400,000 h-1. Nevertheless, the Fe1Mn2-RP catalyst still exhibited a considerable toluene conversion and the T50 and T90 were at 280 and 326 oC (Table S1), respectively, even at an extremely high GHSV of 400,000 h-1. In other words, the “redox-precipitated” catalyst exhibited much better tolerance to high GHSV and consequently had higher toluene conversion, which was a key factor for practical use. To compare the catalytic stability depending on the synthesis method used, the 50 h on-stream reaction experiment was conducted over the Fe1Mn2-RP and CP catalysts and the result was shown in Figure S6. It can be seen that the toluene conversion to CO2 gradually decreased from 74% to 68% over Fe1Mn2-CP. In contrast, toluene conversion to CO2 remained at 70% during the entire test over Fe1Mn2-RP, suggesting that Fe1Mn2-RP exhibited better catalytic stability.

Figure 7A showed the toluene conversion variation with reaction temperature over 15 ACS Paragon Plus Environment


Fe1Mn2-RP catalyst at four humidity levels. Impressively, the presence of water vapor from 0 to 3.7 wt.% had only slight effect on toluene conversion over the Fe1Mn2-RP catalyst. In the worst case, the conversion decreased by about 20% at 200 oC in the level of 6.3 wt% H2O. The inhibiting effect of 6.3 wt% H2O on catalytic activity was reversible after shutdown the H2O (results not shown for brevity), and similar results can also been found in the literatures16, 59-60. The results for the simulated exhaust with 500 ppm toluene and 1000 ppm methanol (in 10 vol.% O2/N2, GHSV = 50,000 h-1) over Fe1Mn2-RP catalyst were shown in Figure 7B. And the two VOCs conversions were plotted as functions of reaction temperature and the results for the simulated exhaust with only 500 ppm of toluene or 1000 ppm of methanol were also put into the figure as references. The complete combustion of simulated exhaust with toluene was 200 oC and the complete combustion of simulated exhaust with methanol was 120 oC. For the simulated exhaust with a mixture of toluene and methanol, methanol combustion was suppressed (Figure 7B). In the case of toluene conversion, no significant drop from that of simulated exhaust with toluene alone was observed, which may be attributed to the strong adsorption of toluene on over Fe1Mn2-RP catalyst compared with methanol and the inhibition effect of adsorbed toluene on methanol combustion59. After switching off the toluene atmosphere, the methanol conversion remained unchanged (results not shown), which implied that the inhibition effect of adsorbed toluene on methanol combustion was irreversible.

3.4 Kinetic parameters Assuming that the toluene combustion in excessive O2 would obey a first-order reaction 16 ACS Paragon Plus Environment

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mechanism with respect to toluene concentration14-15, the Arrhenius plots for the rates of toluene combustion was showed in Figure 8. As summarized in Table 3, it could be observed that the apparent activation energy (Ea) value for toluene combustion decreased in a sequence of Fe1Mn2-CP > α-Fe2O3 > Fe3Mn1-RP > Fe1Mn1-RP > MnOx ≈ Fe1Mn2-RP. Such a result suggests that toluene combustion proceeded more readily over the “redox-precipitated” samples. Actually, the pre-exponential factor reflects the active site density. It was observed that the pre-exponential factor (A) decreased in a tendency same as the Ea value. This finding suggested that the catalytic efficiency of the active site enhanced although the active site density dropped. It was of interest to note that the apparent activation energy did not vary appreciably for the MnOx and Fe1Mn2-RP catalysts investigated, but the pre-exponential factor of MnOx (3.6×1012 s-1) was much lower than that of Fe1Mn2-RP (1.6×1014 s-1). The striking difference in pre-exponential factor can be possibly referred to the difference in active sites number, which was directly related to the extent of lattice defects and the exposure of inner atoms61. It can be clearly seen that the Ea of Fe1Mn2-RP catalyst was 132-177 kJ/mol at four humidity levels, and when the water vapor was at 1.6 wt.%, the catalyst exhibited the lowest

Ea value (132 kJ/mol). Appropriate amount of water may provide another reaction pathway with lower Ea values for toluene combustion. The proper amount of moisture may generate hydroxyl groups on the Fe1Mn2-RP catalyst surface, then the moisture can facilitate the activation of O2 molecules. And the above-proposed mechanism had been also reported in previous reports 9, 62-63. However, excessive water would play an inhibiting effect on catalytic performance, which can be due to competitive adsorption of water with toluene for surface 17 ACS Paragon Plus Environment


active sites. The phenomena was also found by some previous studies16, 59.

4 Conclusions In this paper, different preparation method of FeMn oxide catalysts have been addressed. The catalytic oxidation properties of the redox-precipitation system have been documented. The redox-precipitation method was an appropriate approach to obtain homogeneous FeMn oxide catalysts. In particular, the well dispersion of Fe and Mn ions enhanced the reducibility and promoted the surface oxygen mobility. The close correlations of Oads species concentration and low-temperature reducibility with catalytic toluene combustion activity of the FeMn oxides were observed. The Fe1Mn2-RP catalyst showed the best catalytic activity, and the apparent activation energies over Fe1Mn2-RP were lowest in all the as-prepared samples. The excellent catalytic performance and water/methanol-resistance of Fe1Mn2-RP sample demonstrated its high potential for the practical applications.

Acknowledgments The work was financially supported by the Huazhong University of Science and Technology, the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure (No. SKL201707SIC) and China Postdoctoral Science Foundation (No. 2016M602303). The authors thank the Analysis and Testing Center of Huazhong University of Science and Technology for analytical support.

Supporting Information Available: The detailed procedures for characterizations and catalytic activity measurement, the catalytic activities (Table S1), XRD patterns (Figure S1), pore size distribution curves (Figure S2), TG-DTA curves (Figure S3), SEM images (Figure S4), Fe/Mn atomic ratio (surface vs bulk) (Figure S5) and the catalytic stability (Figure S6) of 18 ACS Paragon Plus Environment

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as-prepared catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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2618-2623.


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Figure captions Scheme 1. An illustration of two pathways for FeMn oxide synthesis: (i) co-precipitation method and (ii) redox-precipitation method.

Fig. 1. XRD patterns of the FeMn oxide, Fe2O3 and MnOx catalysts. Fig. 2. N2 sorption isotherms of the FeMn oxide, Fe2O3 and MnOx catalysts. Fig. 3. TEM images of (A-C) Fe1Mn2-CP catalyst and (D-F) Fe1Mn2-RP catalyst. Fig. 4. (A) O 1s, (B) Fe 2p, and (C) Mn 2p XPS spectra of the FeMn oxide catalysts. Fig. 5. (A) H2-TPR profiles and (B) initial H2 consumption rate as a function of inverse temperature of the FeMn oxide, Fe2O3 and MnOx catalysts.

Fig. 6. (A) Conversion of toluene to CO2 over the FeMn oxide, Fe2O3 and MnOx catalysts (500 ppm toluene/1.6 wt.% H2O/10 vol.% O2/N2, GHSV = 50,000 h-1); (B) Effect of GHSV on conversion of toluene to CO2 over the Fe1Mn2-RP catalyst (500 ppm toluene/1.6 wt.% H2O/10 vol.% O2/N2, GHSV = 50,000-400,000 h-1).

Fig. 7. (A) Effect of H2O content on conversion of toluene to CO2 over the Fe1Mn2-RP catalyst (500 ppm toluene/0-6.3 wt.% H2O/10 vol.% O2/N2, GHSV = 50,000 h-1); (B) Comparison of toluene and methanol conversion alone or in mixture over the Fe1Mn2-RP catalyst (Toluene alone: 500 ppm toluene/10 vol.% O2/N2; methanol alone: 1000 ppm methanol/10 vol.% O2/N2; mixture of the two VOCs: 500 ppm toluene/1000 ppm methanol/10 vol.% O2/N2, GHSV = 50,000 h-1).

Fig. 8. Arrhenius plots for catalytic combustion of toluene over the FeMn oxide, Fe2O3 and MnOx catalysts.


ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

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Tables Table 1. Preparation condition, BET surface area, pore volume and average pore size of the FeMn oxide, Fe2O3 and MnOx catalysts. Catalyst

Preparation

Precursor

method Fe3Mn1-RP Fe1Mn1-RP Fe1Mn2-RP Fe1Mn2-CP Fe2O3 MnOx a

red.-prec. red.-prec. red.-prec. co-prec. prec. prec.

(Atomic ratio) 2+

-

Fe : MnO4 = 3:1

condition

2+

-

2+

2+

-

3+

2+

Fe : Mn : MnO4 = 5:2:3 Fe : Mn : MnO4 = 1:1:1 Fe : Mn = 1:2

APD d (nm)

Experimental

3.0

2.9

114

0.227

8.6

o

1.0

1.1

128

0.270

8.4

o

0.5

0.5

178

0.352

8.0

o

0.5

0.5

133

0.185

5.6

o

-

-

31

0.092

30.6

o

-

-

14

0.031

40.6

100 C 12 h 100 C 12 h 350 C 5 h

-

PV c (cm3/g)

Design

350 C 5 h

-

SBET b (m2/g)

o

100 C 12 h

2+

Fe/Mn a

Calcination

350 C 5 h b

Atomic ratio from design and ICP measurements. BET specific surface areas determined from the linear part of the BET equation (P/P0 = 0. 05-0.25). c The total

adsorption pore volume at P/P0 = 0.995. d Average pore diameter.

30

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Table 2. Surface element composition, and H2 consumption of the FeMn oxide, Fe2O3 and MnOx catalysts. Catalysts

XPS resolving Oads

H2 consumption Olatt

Oads/Olatt

Fe3+/Fe2+

Actual H2

Theoretical H2 a

consumption (mmol/g)

extent c (%)

1.12

3.01

12.9

23

0.29

0.95

3.16

12.2

26

0.83

0.38

1.01

3.45

12.0

29

529.9

0.90

0.47

1.31

3.76

11.8

32

-

-

-

-

-

1.44

12.5

12

-

-

-

-

-

6.80

11.5

59

(eV)

Fe1Mn2-CP

531.7

530.1

0.48

0.27

Fe3Mn1-RP

531.2

529.7

0.75

Fe1Mn1-RP

531.2

529.8

Fe1Mn2-RP

531.3

Fe2O3 MnOx

b

b

Reduction

consumption (mmol/g)

(eV)

a

Mn4+/Mn3+

The data were estimated by quantitatively analyzing the H2-TPR profiles. Assuming that iron and manganese in the catalysts was initially present as Fe2O3 or

MnO2 (Mn3O4 for CP method). c Equal to actual H2 consumption/Theoretical H2 consumption×100.

31



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Table 3. Toluene combustion activities of the FeMn oxide Fe2O3 and MnOx catalysts. Catalysts Fe1Mn2-CP Fe3Mn1-RP Fe1Mn1-RP Fe1Mn2-RP

H2O conc.

o

o

Reaction rate a

A -1

( C)

( C)

(s )

1.6

212

236 219

1.6 1.6 0 3.7 6.3

MnOx

T90

(wt.%)

1.6

Fe2O3

T50

1.6 1.6

198 189 184 186 185 189 312 232

(kJ/mol)

1.2×1026

0.09

0.69

249

16

0.48

4.21

154

14

0.75

5.86

140

15

0.84

4.74

148

14

0.89

5.02

132

15

0.85

4.77

146

19

0.67

3.76

177

20

0

0

235

12

0.01

0.57

9.1×10

197

1.6×10

198

4.6×10

212

1.1×10

338

3.5×10

253

Ea

(µmol/(m h))

8.4×10

197

2

(mmol/(g h))

1.7×10

207

Reaction rate

3.6×10

133 -1

a

The catalytic activities were measured under the conditions of 500 ppm toluene/0-6.3 wt.% H2O/10 vol.% O2/N2, GHSV=50,000 h . Toluene combustion rate was measured at 200 oC.

32


Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Table 4. Summary of previous research papers on toluene combustion over iron and manganese oxides related catalysts. Catalysts

Surface area 2

Conc.

T50/T90

GHSV -1

o

Reaction rate at 200 oC

Reaction rate at 200 oC

Ref.

2

(m /g)

(ppm)

(mL/g h or h )

( C)

(mmol/(g h))

(µmol/(m h))

Flower-like ε-MnO2

30

1000

20000

221/229

0.16

5.30

57

CoMn0.5

249

1200

60000

271/311

0.13

0.52

64

Mn3O4

18

1000

15000

245/270

-

-

4

MnOx/3DOM LaMnO3

25

1000

20000

193/215

0.59

21.00

65

Au/LaMnO3

33

1000

20000

201/226

0.41

12.59

61

MnO2/LaMnO3

145

2000

120000

263/279

0.54

3.70

11

La0.6Sr0.4Co0.9Fe0.1O3

21

1000

20000

220/239

0.18

8.49

66

Rod-like α-MnO2

83

1000

20000

210/225

0.30

3.65

12

3DOM LaMnO3

40

1000

20000

222/243

0.18

4.53

15

Manganese oxide

86

1020

60000

215/235

0.64

7.41

16

3D MnO2

184

1000

20000

213/220

0.36

1.94

58

Fe1Mn2-RP

178

500

50000

186/197

0.89

5.02

This work

33



Scheme. 1

34


Page 34 of 43

Page 35 of 43

Fig. 1

♣: α-Fe2O3 ♦: α-MnO2 ♥: Mn5O8 Φ: γ-Fe2O3 ∇: β-MnO2 ♠: Mn3O4 ♠

Intensity /cps

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


♠ ♠

♠ ♠

♠

∇ ♣♣

♣

♦

Fe1Mn2-CP

Fe1Mn2-RP ∇

∇

Fe1Mn1-RP

♣

♣ ♣ ♣

♣♣

♣

♣

♣

♣♣

♦ ♥ ♥♦♥ ♥ ♥

♥

♠

♣♣

♣ ♦

♠♠

♠

♣

Fe3Mn1-RP

MnOx ♥♥ ♦ ♥ ♥♥ ♦

♥ ♥

♣Φ ♣ ♣

10

20

Φ

30

♣♣

♣

♣

♣

♣♣

40 50 60 2θ /degree


Fe O Φ 2 3

70

80


Fig. 2

250 Volume Adsorbed (cm3/g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 43

Fe1Mn2-CP Fe3Mn1-RP Fe1Mn1-RP Fe1Mn2-RP Fe2O3

200

MnOx

150

100

50

0 0.0

0.2

0.4

0.6

Relative Pressure (P/P0)


0.8

1.0

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Fig. 3

37



Fig. 4

A

B

C

2+

Fe

Lattice O

Fe2p1/2

3+

Fe

Mn3+

Fe2p3/2

Mn2p3/2

Mn2p1/2

Fe1Mn2-CP

Fe1Mn2-CP

Adsorbed O

Mn4+

Fe1Mn2-CP

Fe3Mn1-RP

Fe1Mn1-RP

Intensity (a.u.)

Intensity (a.u.)

S1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 38 of 43

Fe3Mn1-RP

Fe1Mn1-RP

Fe3Mn1-RP

Fe1Mn1-RP

Fe1Mn2-RP

Fe1Mn2-RP

Fe1Mn2-RP

536

534

532 BE (eV)

530

528

735

730

725

720

715

710

BE (eV)

38


660

655

650

BE (eV)

645

640

Page 39 of 43

Fig. 5

Rate of H2 consumption (a.u.)

288

A

418

MnOx Fe2O3

404 345 533

100 200 300 400 500 600 Temperature (oC)

431 396

278 Fe1Mn2-CP

Fe1Mn2-RP

284 368 240 400 290 245

410 447

Fe1Mn1-RP Fe3Mn1-RP

250 327

410 490

100 200 300 400 500 600 Temperature (oC)

Initial H2 consumption rate (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe1Mn2-CP Fe1Mn2-RP Fe1Mn1-RP Fe3Mn1-RP

B

2.2

2.3

2.4

1000/T (K-1) 39 ACS Paragon Plus Environment

2.5

2.6


Toluene Co nver sion to CO2 / %

Fig. 6

100 80 60

Fe1Mn2-RP Fe1Mn1-RP Fe3Mn1-RP Fe1Mn2-CP MnOx Fe2 O3

40 20

A

0 80

120

160

200

240 280 320

360

o

T emperature / C

Toluene Conversion to CO2 / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

100 80 60

GHSV (h-1) 50,000 100,000 200,000 400,000

40 20

B 0 80

120

160

200

240

280 o

Temperature / C


320

360

Page 41 of 43

Toluene Conversion to CO2 / %

Fig. 7

100

A

80 60

H2O (wt.%)

40

0 1.6 3.7 6.3

20 0 80

120

160

200

240

o

Temperature / C

VOCs Conversion to CO2 / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

B

80 60 Methanol alone in mixture

40 20

Toluene alone in mixture

0 0

40

80

120

160

200 o

Temperature / C


240

280


Fig. 8

-2 -4

ln k

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 43

-6 -8

-10

Fe1Mn2-CP Fe3Mn1-RP Fe1Mn1-RP Fe1Mn2-RP(dry gas) Fe1Mn2-RP(1.6% H2O) Fe1Mn2-RP(3.7% H2O)

-12

Fe1Mn2-RP(6.3% H2O) Fe2O3 MnO2

1.8

2.0

2.2

1000/T (K

2.4 -1

)


2.6

Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


221x80mm (150 x 150 DPI)


Enhanced Toluene Combustion over Highly Homogeneous Iron

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