Carefully design hollow MnxCo3-xO4 polyhedron derived by In-situ

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Carefully design hollow MnxCo3-xO4 polyhedron derived by In-situ pyrolysis of MOFs for the outstanding low temperature catalytic oxidation performance Jiuhu Zhao, Weiliang Han, Zhicheng Tang, and Jiyi Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00677 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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Crystal Growth & Design

Carefully design hollow MnxCo3-xO4 polyhedron derived by In-situ pyrolysis of MOFs for the outstanding low temperature catalytic oxidation performance Jiuhu Zhao1,2, Weiliang Han1, Zhicheng Tang1*, Jiyi Zhang2 (1. State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China

2. School of petroleum and chemical, Lanzhou University of Technology, Lanzhou 730050, China) *Corresponding author. Tel.: +86-931-4968083, Fax: +86-931-4968019, E-mail address: [email protected] (Z.Tang).

Abstract In this paper, three different structures of MnxCo3-xO4 were successfully synthesized by optimizing the heating decomposition conditions of Mn@Co-ZIFs precursors to form three types MnxCo3-xO4 catalysts with different morphology, including the hollow MnxCo3-xO4 polyhedron (HW-MnxCo3-xO4), Box-In-Ball MnxCo3-xO4 polyhedron (BIB-MnxCo3-xO4) and nanoparticle MnxCo3-xO4 polyhedron (NP-MnxCo3-xO4). Interestingly, the structure effect of MnxCo3-xO4 polyhedron on the catalytic oxidation of toluene was systematically investigated. It could be noted that the HW-MnxCo3-xO4 sample exhibited superior catalytic performance, and the complete conversion temperature of toluene (T100) was 195 °C. Furthermore, the toluene conversion of HW-MnxCo3-xO4 sample had no significant decrease at 188 °C for 30 h, indicating that it exhibited excellent stability for toluene oxidation reaction.

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Through a series of characterizations, it was concluded that the morphology and structures of MnxCo3-xO4 catalysts could evidently alter the surface atomic ratio of Co2+/(Co3++Co2+), BET surface area, the number of surface adsorbed oxygen, the interaction between Mn and Co3O4 and so on. Especially, we discovered that the catalytic activity of MnxCo3-xO4 polyhedron was obviously improved with the increase of surface atomic ratio of Co2+/(Co3++Co2+). In addition, large BET surface area, lots of surface adsorbed oxygen, strong interaction between Mn and Co3O4 would speed up the catalytic oxidation of toluene. Key word: ZIF-67; MnxCo3-xO4; hollow structure; toluene catalytic oxidation; morphology effect


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Introduction Volatile organic compounds (VOCs) are mainly conducive to form photochemical smog, which are considered as great contributors to the atmospheric pollution, the impact on human health is dangerous.[1] Because of their toxic properties and malodorous, it is urgent to reduce their negative impacts on environment and human health.[2-4] Among those conventional control processes, thermal incineration is a convenient approach to deal with high concentration VOCs. But incineration operaton costs is high, and produces undesirable by-products when oxidation process of VOCs is incomplete or poorly.[5-7] In addition, adsorption process is one of the most effective methods with the advantages of high processing performance. However, the adsorption process merely transfers pollutants from one stage to another, rather than removing them from the environment.[8] By comparison, the catalytic oxidation of VOCs is one of the most promising technologies for VOCs elimination, due to the low cost and could avoid the generation of secondary pollution.[9-16] Transition metal oxides are an active catalyst for VOCs elimination due to high thermal stability and low price.[17] Among them, cobalt oxide is the most widely used for catalytic oxidation material.[18-20] The high activity of Co3O4 may be due to the relatively low ∆H of O2 vaporization.[21] Thus, cobalt oxide is one of the most effective catalysts for VOC removal due to high activity and low cost.[22,23] In recent reports, appropriate incorporation of metal oxides may be more active and thermally stable than the single oxides. For example, Co3-xFexO4,[24] MnxCo3-xO4,[25-26] Cu-Mn/MCM-41,[27] ZnxCo1-xCo2O4[28] and NiOx, CrOx and Bi2O3 modified



Co3O4[29-31] exhibited higher activity than pure Co3O4. Recently, metal organic frameworks (MOFs) are a kind of ordered porous materials composed of organic ligand and metal ions, which have gained extensive consideration to synthesize porous materials.[32] MOFs has been studied as a new multifunctional materials and applied in many fields because of its unique architecture and versatility.[33,34] For instance, MOF-derived Co3O4 hollow dodecahedrons derived by Co based zeolite imidazole framwok (ZIF-67) are applied for CO oxidation, which exhibited outstanding catalytic activity.[35] In addition, it is a new method to assemble nanoparticle catalysts into core-shell structure to greatly improve their activity and stability. The core-shell nanostructure is composed of nuclear nanoparticles wrapped in porous materials, which ensure accessible and active metal surfaces of reactant molecules and could increase the durability of the catalyst.[36] For example, W. Zhao synthesized Co-Mn oxides with structures of box-in-box hollow nanocages, and the use of them as catalysts for the oxidation of toluene. The Co1Mn1BHNCs exhibited superior catalytic performance due to high mobility of lattice oxygen, low toluene desorption temperature.[37] As a new functional material with changeable properties, it is also used in many other fields, for example, gas storage[38] and electrocatalytic properties.[39] As we know, almost few research on the application of MnxCo3-xO4 with different morphology synthesized by optimizing the heating decomposition conditions of Mn@Co-ZIFs as the catalyst is applied for VOCs. In the work, three samples of MnxCo3-xO4 with different morphology were synthesized by controlling different temperatures and heating rates. Moreover, the


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catalytic performances were investigated by toluene oxidation. The effect of morphology was extensively characterized on the catalytic performance. Results and discussion Morphology of the catalysts The morphology of catalysts was investigated by SEM (Fig. 1a-e) and TEM (Fig. 2a-g). Fig. 1a was SEM image of ZIF-67 nanocrystals. Obviously, ZIF-67 had a regular dodecahedron appearance with high uniformity, the average size was about 1 µm. Fig. 1b was SEM image of Mn@ZIF-67 nanocrystals. It could be observed that Mn@ZIF-67 nanocrystals also had a regular dodecahedron appearance. Unlike ZIF-67, the surface of Mn@ZIF-67 was rough. The average size of Mn@ZIF-67 was about 1 µm, as well. Fig. 1c-e and Fig. 2a-c were SEM and TEM images of three MnxCo3-xO4 catalyst, respectively. After the calcination treatment at 350 °C in air for 2 h with a heating rate of 1 °C min-1 in air, it was discovered that the HW-MnxCo3-xO4 sample exhibited the polyhedron morphology and the average size was about 900 nm(Fig. 1c). Furthermore, the HW-MnxCo3-xO4 sample presented concave and plicated appearance. As shown in Fig. 2a, it could be seen that the HW-MnxCo3-xO4 sample exhibited hollow structure. This might be due to heterogeneous contraction process when heat treating progress. When the heating rate went up to 10 °C min-1 in air, the SEM image of BIB-MnxCo3-xO4 sample (Fig. 1d) presented polyhedron structure and the average size was 900 nm as well. From the TEM images of BIB-MnxCo3-xO4 sample (Fig. 2b), by the dark edges and bright regions, we discovered that the box and ball could be clearly distinguished. It could be seen that



the external still remained dodecahedron and the interior contained near-spherical cores, apparently. Thus, the TEM images provided visual evidence for the formation of Box-In-Ball structure. This was due to two forces (cohesive and adhesive forces) acted in the opposite direction at the interface, because of the gradual decomposition of Mn@ZIF-67 during the heterogeneous heating process. The adhesive force might prevent the inward contraction, causing an outer box to form. On the contrary, the cohesive force produced the inward contraction and inner ball was generated. As the temperature went up to 600 °C, the SEM and TEM images presented nanoparticle morphology (Fig. 1e and Fig. 2c), the average size was about 100 nm, which might be due to the internal vacancy with high thermal energy and the diffusion ofthe high thermal energy to the outer surface, and caused the collapse of hollow structure to form a large number of nanopartilces.[40] Fig. 2d and Fig. 2e showed HRTEM image of the as-prepared HW-MnxCo3-xO4 sample. The HW-MnxCo3-xO4 showed the Co3O4 (111) and Co3O4 (220) crystalline plane with 0.47 and 0.29 nm, respectively. Simultaneously, (111), (220), (311), (400), (511) and (440) plane of Co3O4 were detected according to the diffraction ring (JCPDS no. 42-1467). Interestingly, there was no detection of lattice fringes attributable to manganese oxide over the HW-MnxCo3-xO4 sample. As shown in Fig. 2f and Fig. 2g, the elemental mapping image of HW-MnxCo3-xO4 sample manifested that C, O, Mn and Co elements were uniformly distributed in the polyhedrons. Meanwhile, the distribution of elements of HW-MnxCo3-xO4 sample also verified the formation of hollow structure.


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Texture properties of the catalysts XRD analysis was conducted to study the structural properties of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts. According to the literature, the diffraction peaks at 19.0°, 31.3°, 36.9°, 44.8°, 59.4°, 65.2° and 77.3° were corresponding to the (111), (220), (311), (400), (511) and (440) of Co3O4.[41] As shown in Fig. 3, however, there was no obvious diffraction peaks for Mn or MnOx phases were detected in those of MnxCo3-xO4, and the detectable reflections were not consistent with standard data of cubic phase Co3O4. It suggested that the Mn incorporate into the Co3O4 lattice. Contacted to the SEM, the HW-MnxCo3-xO4 and the BIB-MnxCo3-xO4 samples well inherited the dodecahedron morphology of ZIF-67, suggesting that the anchoring of Mn NPs did not break the structure of Co-ZIF. By applying the Scherrer equation, the crystallite size could be also calculated from the Co3O4 (311) diffraction peak.[42] The results were shown in Table 1. The crystallite size of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts were 19.90 nm, 21.23 nm and 29.06 nm, respectively. The

N2

adsorption-desorption

isotherms

of

the

HW-MnxCo3-xO4,

BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 was shown in Fig. 4. By the IUPAC, the N2 adsorption-desorption

isotherms

of

HW-MnxCo3-xO4,

BIB-MnxCo3-xO4

and

NP-MnxCo3-xO4 could be divided to type Ⅳ with H3 hysteresis loop. It suggested that three catalysts had partly mesoporous structure. Table 1 showed the average pore diameters, pore volumes and BET surface areas of HW-MnxCo3-xO4, BIB-MnxCo3-xO4, and NP-MnxCo3-xO4. The BET surface areas of HW-MnxCo3-xO4, BIB-MnxCo3-xO4,



and NP-MnxCo3-xO4 were 59.7, 7.3 and 13.5 m2 g-1, respectively. Easy to see, the BET surface areas of HW-MnxCo3-xO4 was larger than that of BIB-MnxCo3-xO4 and NP-MnxCo3-xO4. Generally speaking, the increase of specific surface area was beneficial to improving the activity of the catalyst. In addition, BIB-MnxCo3-xO4 sample showed the larger average pore diameter (38.4 nm) than that of HW-MnxCo3-xO4 sample (22.1 nm) and NP-MnxCo3-xO4 sample (11.5 nm). In catalytic oxidation of toluene process, these porous structures could be beneficial for toluene molecules to quickly penetrate into the pores and contact to active sites. The TG curves of Mn@Co-ZIFs in air was shown in Fig. 5. Easy to see, Mn@Co-ZIFs had a sharply weight loss in the temperature range of 350-400 °C, the total weight loss in the decomposition process from 350 °C to 400 °C was 59.1%, which well represented by the weight change in the transformation from Mn@Co-ZIFs to MnxCo3-xO4. Such a large weight loss meant that large amounts of CO2, H2O and NOx would be released in the decomposing process of the Mn@Co-ZIFs precursors. Chemical states and redox behavior XPS characterization of three catalysts was carried out and the Co 2p, Mn 2p and O 1s XPS spectra were exhibited in Fig. 6. It could be clearly seen that the Co 2p peaks exhibited a slight broaden and shift to lower binding energy (Fig. 6a). In the Co 2p spectrum of the three catalysts, two main peaks at 780 eV and 795 eV were observed, corresponding to Co 2p3/2 and Co 2p1/2, respectively. The fitting peaks at binding energies of 782.6 eV and 798 eV were ascribed to Co2+, while another fitting


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peaks of 780.9 eV and 796.3 eV were related to Co3+.[43] Meanwhile, it could be seen that the satellite peaks were located on 789.3 eV and 804.9 eV. Because of the coexistence of Co3+ and Co2+ species, all the spectra presented typical components in three catalysts. Table 2 exhibited the Co2+/(Co3++Co2+) ratio. The relative ratio of Co2+/(Co3++Co2+) that was calculated using the curve-fitted data decreased, which were 0.56, 0.44 and 0.39 respectively. It was found that the ratio of Co2+/(Co3++Co2+) conformed the order of HW-MnxCo3-xO4 > BIB-MnxCo3-xO4 > NP-MnxCo3-xO4. Fig. 6b displayed the Mn 2p XPS spectra of the three samples. Generally, The peak of Mn 2p consisted of two main spin orbitals, Mn 2p3/2 and Mn 2p1/2 peaking at 641.1 and 652.4 eV. By restraining the FWHM of Mn4+ and Mn3+ to 2.81 and 2.19 V, the

deconvolution

of

Mn

2p

was

executed.[44,45]

Table

2

showed

the

Mn3+/(Mn3++Mn4+) ratio. The relative ratio of Mn3+/(Mn3++Mn4+) that was calculated using the curve-fitted data decreased, which were 0.42, 0.41 and 0.40 respectively. There are no difference in ratio of Mn3+/(Mn3++Mn4+). O 1s XPS spectra was shown in Fig. 6c. The O 1s XPS of three samples could be fitted by three peaks, which at binding energies of around 529.6, 530.6, 532.1 eV. They were assigned to the lattice oxygen (O ɑ ), chemical adsorption oxygen (Oβ), adsorbed water/OH (Oγ), respectively. It was generally believed that the charge of oxide ions was affected by their surrounding chemical environment, and the nature of dopant ions would decide the shifts of O 1s binding energy to either side.[46] A slight shift of binding energy for O 1s was found over BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 compared with that of HW-MnxCo3-xO4. It was rationally deduced that there existed



the strong interaction of two oxides in MnxCo3-xO4. Furthermore, the chemical adsorption oxygen species played a significance part in catalytic oxidation. The more Oβ species, the better catalytic activity. Table 2 exhibited the Oβ/(O ɑ +Oβ+Oγ) ratio. For HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4, the surface atomic ratio of Oβ/(Oɑ+Oβ+Oγ) were 0.58, 0.33 and 0.22, respectively. The Raman spectra of MnxCo3-xO4 samples was shown in Fig. 7. Four distinct peaks were located at 479, 519, 617 and 686 cm-1, which were consistent with the position and width of Co3O4[47]. According to the literature[48], Raman spectra of Mn3O4 sample exhibited five peaks, A1g symmetry mode at 660 cm-1, T2g symmetry modes at 479 cm-1 and 375 cm-1, Eg and T2g symmetry mode at 320 cm-1 and 290 cm-1, respectively. In the range of 200-800 cm-1, the normal spinel structure of Co3O4 possessed four Raman active modes of A1g + Eg + 2F2g. The band at 686 cm-1, corresponding to the A1g mode is due to the octahedral sites (CoO6) in the O7h symmetry. The weak band at 617 cm-1 was associated with the F2g mode, nevertheless the Raman modes at 519 cm-1 and 479 cm-1 was respectively symmetries of F2g and Eg mode. Compared with BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts, the strongest symmetry, A1g, in the HW-MnxCo3-xO4 sample slightly shifted to lower wave-numbers as well as described red shift. HW-MnxCo3-xO4 sample slightly shifted to lower wavenumbers, indicating it was more likely to incorporate manganese into the lattice. Generally speaking, red shift narrow peaks indicated that the sample had a highly defective structure. It was vitally significant to activate absorbed oxygen molecules to active oxygen species[49].


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Fig. 8 showed FT-IR spectra of three catalysts. The absorption band at 567 and 665 cm-1 could be ascribed to the vibrations of Co-O bands derived from Co3O4.[50] Easy to see, the two bands of HW-MnxCo3-xO4 exhibited a small shift toward lower wavenumbers from the partially enlarged picture. According to previous literature,[51] the two bands were characteristic of (Mn-O) modes of Mn3O4 at around 600 cm-1 and 500 cm-1. Hence, the shift come from substitution of manganese for cobalt. The broad peak at 3423 cm-1 could be related to the vibrations of O-H bands of molecular water and of hydrogen-bound groups. It could be observed that there were significantly different strength in the strength of oxygen functional groups of the three samples. Compared with BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts, HW-MnxCo3-xO4 had the strongest characteristic peaks of Co-O and -OH groups. The oxidation-reduction properties of the MnxCo3-xO4 samples were studied by H2-TPR. Fig. 9 illustrated the results of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts. It was observed that they had two peaks for the reduction of Co3O4 in these three catalysts. The two peaks of Co3O4 were reduced to CoO and CoO to metallic cobalt, respectively. For three catalysts, the peak position was different. The peak at 278 °C of HW-MnxCo3-xO4 sample, corresponding to the reduction steps of Co3O4 to CoO, and the peak at 477 °C was assigned to the transformation of CoO into Co. Nevertheless, the reduction peaks of BIB-MnxCo3-xO4 samples shifted to 346 and 533 °C, respectively. For NP-MnxCo3-xO4 catalysts, these temperatures changes was more obvious, 384 and 587 °C, which was shifted more significantly to the high temperature. In general, a catalyst with small crystallite size



showed superior reduction capacity. Thus, the HW-MnxCo3-xO4 sample displayed excellent reducibility due to smallest crystallite size. Meanwhile, A weak reduction peak for the HW-MnxCo3-xO4 samples could be observed at about 120 °C. This peak was attributed to very active species on the surface formed during the preoxidation treatment. In other words, the molecular oxygen species adsorbed on the oxygen vacancies. Therefore, it could be concluded that the synthesis of cobalt oxides having a small crystallite size favoured the formation of easily reducible sites, which are less abundant on more highly crystalline Co3O4 catalysts. The result was consistent with the XPS analysis.[52] In the HW-MnxCo3-xO4 sample, the anionic vacancies close to the Mn cations might be the appropriate position to trap oxygen species, which could stretch the distance between the two Co cations, forming Co-Mn-O system. As shown in fig. 8b, it could be noted that the pure Co3O4 sample exhibited lowest reducibility, thus the HW-MnxCo3-xO4 sample tend to induce a systematic shift of lower temperature due to the presence of Mn. Because of more adsorbed oxygen species, it could clearly be noticed that the HW-MnxCo3-xO4 catalyst might exhibit superior catalyst activity. Catalytic performance of Toluene oxidation Catalytic activity The catalytic activity curves of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts was shown in Fig. 10 (a), the temperatures for 50%, 90% and 100% toluene conversion (T50, T90 and T100) of three MnxCo3-xO4 catalysts were listed in Table 3. T50 of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4


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catalysts were 159, 207 and 227 °C, while T90 of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 catalysts were 188, 218 and 238 °C, respectively, indicating that the catalytic performance was the order of HW-MnxCo3-xO4 > BIB-MnxCo3-xO4 > NP-MnxCo3-xO4 catalysts. The total conversion temperature (T100) of toluene on the three catalyst was 195, 240 and 255 °C, respectively. So, it could be concluded that the morphology of MnxCo3-xO4 catalysts played an significant part in determining the activity in toluene oxidation. Table 4 summarized the comparation of catalytic activity on these samples and the former literatures for toluene oxidation. For example, Co3O4-0.01 (T90 =226 °C),[53] Co3O4 microspheres (T90 =285 °C),[53] 7.4 Au/Co3O4 microspheres (T90 =250 °C),[54] Co3O4-KIT6 (T90 =233 °C),[55] Co3O4-HT (T90 =260 °C),[56] Co-Mn (1:1) (T90 =240 °C),[57] L-12 (T90 =269 °C).[58] Compared to the previous

literature,

HW-MnxCo3-xO4

catalyst

presented

excellent

catalytic

performance. Fig. 10b showed the influence of weight hourly space velocity (WHSV) for the toluene reaction over HW-MnxCo3-xO4 catalyst. Because of the increase of retention time of toluene in the catalyst bed, the complete conversion temperature of toluene increased with the decrease of WHSV from 60000 to 30000 mL g-1 h-1. When the WHSV was 30000 and 60000 mL g-1 h-1, the total conversion temperature (T100) was 195 and 280 °C, respectively. The HW-MnxCo3-xO4 sample exhibited superior catalytic performance at the lower WHSV, suggesting that the WHSV greatly influenced on the performance of HW-MnxCo3-xO4 catalyst. The phenomenon about different on catalytic activity of three MnxCo3-xO4



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catalysts might be explained by catalysts synthesis and characterization. From N2 adsorption-desorption analysis, the HW-MnxCo3-xO4 catalyst exhibited larger specific surface area, thus it showed better catalytic activity. Furthermore, the higher surface Co2+ species and surface chemical adsorbed oxygen of HW-MnxCo3-xO4 catalyst were advantageous to the reaction of toluene oxidation. Thus,

these above factors usually

leads to the superior catalytic activity of HW-MnxCo3-xO4 catalyst in reaction of toluene oxidation. According to XRD, XPS and Raman, active centers of oxidation should form small cobalt oxide spinel-type crystallites. Including species from tetrahedral and Co3+ species from octahedral positions, cobalt spinel appeared to an excellent candidate, it could boost electron transfer in the catalytic oxidation cycle.[59] Thus Co2+ which located at a relatively opened coordination position could be a center of oxygen adsorption and formed of active oxygen species which were a precondition for catalytic oxidation. The HW-MnxCo3-xO4 catalyst had the higher surface Co2+ species, thus it exhibited superior catalytic activity. The surface chemical adsorbed oxygen was treated as an extremely important role on toluene oxidation. It could be observed that the HW-MnxCo3-xO4 catalyst owned the more surface Oβ species and Oβ/(Oα+Oβ+Oγ) atomic ratio was directly proportional to catalytic activity. Therefore, this was a reason that HW-MnxCo3-xO4 possessed excellent catalytic activity in reaction of toluene oxidation. In addition, a weak reduction peak was observed from H2-TPR, it attested the molecular oxygen species was attached to the oxygen vacancies, which consistent with XPS. The oxygen vacancy could active the O2 of reaction gas and promote catalytic oxidation, simultaneously. Thus, the catalytic


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performance was also associated with the interaction between MnxO4 and Co3-xO4. Besides, the HW-MnxCo3-xO4 catalyst had smallest crystallite size, leading to strong reducibility. Low temperature reducibility could enhance the catalytic performance. All of these factors determined that the HW-MnxCo3-xO4 catalyst presented best catalytic performance for toluene catalytic oxidation. Stability The best HW-MnxCo3-xO4 catalyst was selected to evaluate its thermal stability, and the results were exhibited in Fig. 10. Three consecutive toluene transformations were performed using HW-MnxCo3-xO4 and thermal stability tests at 188 °C were determined. As shown in Fig. 10c, the activity of catalyst was investigated three times and the total conversion temperature (T100) was 195, 200, 205 °C, respectively. It was discovered that there was almost no differences between the three curves, suggesting that the HW-MnxCo3-xO4 presented similar catalytic activity. Therefore, it was concluded that the HW-MnxCo3-xO4 catalyst had favorable catalytic cycling performance for toluene oxidation. To probe the thermal stability of HW-MnxCo3-xO4 catalyst, the thermal lifetime experiments of the HW-MnxCo3-xO4 were investigated. As shown in Fig. 10d. It was obvious that the curve fluctuates in 85 % ranges. Hence, the HW-MnxCo3-xO4 catalyst at 188 °C was stable in 30 h, perfectly. In summary, HW-MnxCo3-xO4 had excellent thermal stability. It was well known that the rate of chemical reaction was associated with the activation energy. By reducing the activation energy, some slow reactions would be



promoted. The Arrhenius plots for toluene oxidation over three samples reaction were studied. The activation energy (Ea) could be calculated from the slope of linear ln r versus 1000/T. As shown in Fig. 11, all profiles exhibited outstanding linear relations between ln r and 1000/T and the Ea of the MnxCo3-xO4 catalysts were listed in Table 3. The HW-MnxCo3-xO4 catalyst had the lower Ea (Ea = 57.4 kJ mol-1) than BIB-MnxCo3-xO4 catalyst (Ea = 62.4 kJ mol-1) and NP-MnxCo3-xO4 (Ea = 74.9 kJ mol-1). Obviously, the catalytic activities followed an inverse trend with respect to the Ea values. Hence, the results confirmed that among these samples, the HW-MnxCo3-xO4 had the superior catalytic activity. The texture and chemical properties after toluene oxidation reaction XPS results of HW-MnxCo3-xO4 catalyst after three consecutive runs (used HW-MnxCo3-xO4) were exhibited in Fig. 12. As shown in Fig. 12a, the Co 2p XPS results of the catalyst could also be curve-fitted into 2 components, which were characteristic of Co3+ (low energy) and Co2+ (high energy). The above work confirmed that the ratio of Co2+/(Co3++Co2+) could influence the catalytic activity. It was found that there was no difference between the fitting peaks of used and fresh HW-MnxCo3-xO4 catalyst at binding energies. Moreover, the ratio of Co2+/(Co3++Co2+) were 0.56 and 0.51, indicating that Co2+ cations were the dominant species in Co3O4 spinel oxide. As shown in Fig. 12b, the O 1s XPS of used HW-MnxCo3-xO4 catalyst could be fitted by three peaks as well. According to Table 2, the ratio of Oβ/(O ɑ +Oβ+Oγ) were 0.58 and 0.50, indicating that the surface adsorption oxygen was not much less. Thus, the HW-MnxCo3-xO4 catalyst had favorable catalytic cycling


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performance for toluene oxidation. Proposed reaction mechanism Generally, Langmuir-Hinshelwood (L-H) mechanism was preferentially considered for the toluene oxidation process. The Langmuir-Hinshelwood (L-H) mechanism deemed that the reaction taked place between the adsorbed molecular, including the adsorbed VOCs and the surface adsorbed oxygen. Therefore, it was essential for both the VOCs and oxygen molecule to be adsorbed on the surface of the catalyst. The controlling step was the surface reaction between two adsorbed molecules at analogous active sites. The reaction pathway of toluene oxidation may proceed in the consecutive steps. As shown in Fig. 13, the toluene molecule was adsorbed onto catalysts surface, and reacted with the chemically adsorbed oxygen to form benzaldehydic species, eventually, formed CO2 and H2O. At the same time, the catalysts produced oxygen vacancies, and then O2 molecular gas would replenish in the reaction gas to form the new chemically adsorbed oxygen. So far, it completed the redox cycle.[60]



Conclusion In summary, we synthesized three kinds of MnxCo3-xO4 catalysts with different morphology through controlling the pyrolysis conditions of Mn@Co-ZIFs, and the morphology effect of MnxCo3-xO4 on the toluene oxidation was investigated in detail. Notably, the MnxCo3-xO4 presented hollow dodecahedron at 350 °C with heating rate of 1 °C min-1, box-in-ball dodecahedron at 350 °C with heating rate of 10 °C min-1 and nanoparticle morphology at 600 °C with heating rate of 1 °C min-1, respectively. Among, the HW-MnxCo3-xO4 catalyst exhibited superior catalytic activity and the complete conversion temperature (T100) was 195 °C. Meanwhile, the HW-MnxCo3-xO4 catalyst also exhibited good cycling at three consecutive runs and the excellent thermal stability. Through a series of characterizations, it is concluded that the excellent catalytic performance of HW-MnxCo3-xO4 catalyst was attributed to the higher atomic ratio of Co2+/(Co3++Co2+) on the surface, lots of surface adsorbed oxygen largest specific area and mimimum crystallite size. In addition, this novel strategy could also open a door for the application of MOF materials to VOCs removal.

ASSOCIATED CONTENT Supporting information Experimental parts.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Z.Tang) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21707145, 51808529), Key Science and Technology Program of Lanzhou City (2017-4-111, 2018-RC-65), Province Natural Science Foundation of GanSu (18JR3RA383, 17JR5RA317) and West Light Foundation of The Chinese Academy of Sciences.

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Table 1 Crystallite size and pore structure parameters of HW-MnxCo3-xO4, BIB-MnxCo3-xO4, and NP-MnxCo3-xO4. Samples

Crystallite size

BET surface area

Pore volume

Average pore diameter

(nm)

(m2 g-1)

(cm3 g-1)

(nm)

HW-Mn/Co3O4

19.9

59.7

0.33

22.1

BIB-Mn/Co3O4

21.2

7.3

0.25

38.4

NP-Mn/Co3O4

29.1

13.5

0.04

11.5


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Table 2 Surface element compositions in MnxCo3-xO4 materials. Sample

Co2+/(Co3++Co2+)

Oβ/(Oɑ+Oβ+Oγ)

Mn3+/(Mn3++Mn4+ )

HW-MnxCo3-xO4

0.56

0.58

0.42

BIB-MnxCo3-xO4

0.44

0.33

0.41

NP-MnxCo3-xO4

0.39

0.22

0.40

Used HW-MnxCo3-xO4

0.51

0.50

0.31



Page 30 of 46

Table 3 Catalytic activities of the catalysts at WHSV=30000 mL g-1 h-1.

samples

Reaction temperature

Ea

(°C)

(kJ/mol)

T50

T90

T100

HW-MnxCo3-xO4

159

188

195

57.4

BIB-MnxCo3-xO4

207

218

240

62.4

NP-MnxCo3-xO4

227

238

255

74.9


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Table 4 The oxidation of toluene over Co-based catalysts. samples Co3O4-0.01 Co3O4 microspheres Au/Co3O4 microspheres Co3O4-KIT6 Co3O4-HT Mn-Co (1:1)

Surface area (m2 g-1) 56 17.4 22.4 102 41.9 27.9

VOC VOC conc. type (ppm) Toluene 1000 Toluene 1000 Toluene 1000 Toluene 1000 Toluene 1000 Toluene 500

WHSV (mL g-1 h-1) 15000 20000 20000 20000 20000 96000

T50% (°C) 217 266 242 228 241 226

T90% (°C) 226 285 250 233 260 240

Ref. no. 53 53 54 55 56 57

L-12

28.37

Toluene

1000

10000

263

269

58

HW-MnxCo3-xO4

59.68

Toluene

3000

30000

159

188

Present work



Scheme 1 Schematic showing the synthetic procedures of MnxCo3-xO4 with various morphology.


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Fig. 1 SEM images of the as-prepared ZIF-67 (a), Mn@ZIF-67 (b), HW-MnxCo3-xO4 (c), BIB-MnxCo3-xO4 (d) and NP-MnxCo3-xO4 (e).



Fig. 2 TEM images of the as-prepared HW-MnxCo3-xO4 (a), BIB-MnxCo3-xO4 (b), NP-MnxCo3-xO4 (c) HRTEM image of HW-MnxCo3-xO4 (d,e) and EDS mapping images of HW-MnxCo3-xO4 (f, g).


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Fig. 3 XRD pattern of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 (a) and diffraction ring of HW-MnxCo3-xO4 (b).



Fig. 4 N2 adsorption-desorption isotherms of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4.


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Fig. 5 TG curve of the Mn@Co-ZIFs in air.



Fig. 6 The Co 2p (a), Mn 2p (b), O 1s (c) XPS spectra of three MnxCo3-xO4 catalysts.


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Fig. 7 Raman spectra of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4.



Fig. 8 FT-IR spectra of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4.


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Fig. 9 H2-TPR profiles of HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4 (a); NP-MnxCo3-xO4 and Co3O4 (b).



Fig. 10 (a) Catalytic performance for toluene oxidation (b) Effect of weight hourly space velocity on toluene oxidation over the HW-MnxCo3-xO4 sample. (c) Stability tests on HW-MnxCo3-xO4. (d) The toluene conversion during three consecutives runs using HW-MnxCo3-xO4 sample.


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Fig. 11 Arrhenius plots of toluene catalytic oxidation reaction over HW-MnxCo3-xO4, BIB-MnxCo3-xO4 and NP-MnxCo3-xO4.



Fig. 12 Co 2p and O 1s XPS spectra of HW-MnxCo3-xO4 catalyst before and after reaction.


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Fig. 13 Schematic of oxidation of toluene on MnxCo3-xO4.



Page 46 of 46

For Table of Contents Use Only

Carefully design hollow MnxCo3-xO4 polyhedron derived by In-situ pyrolysis of MOFs for the outstanding low temperature catalytic oxidation performance Jiuhu Zhao1,2, Weiliang Han1, Zhicheng Tang1*, Jiyi Zhang2 (1. State Key Laboratory for Oxo Synthesis and Selective Oxidation, National Engineering Research Center for Fine Petrochemical Intermediates, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China

2. School of petroleum and chemical, Lanzhou University of Technology, Lanzhou 730050, China)

Three morphologies of MnxCo3-xO4 were synthesized by optimizing the heating decomposition,

including

Box-in-Ball,

Hollow

and

nanoparticle.

The

HW-MnxCo3-xO4 catalyst exhibited good cycling at three consecutive runs and the excellent thermal stability for the catalytic oxidation of toluene.


Carefully design hollow MnxCo3-xO4 polyhedron derived by In-situ

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