Article pubs.acs.org/JPCC
Effective Ti Doping of δ‑MnO2 via Anion Route for Highly Active Catalytic Combustion of Benzene Dongyan Li,†,‡ Wenhui Li,†,‡,§ Yuzhou Deng,†,‡,§ Xiaofeng Wu,†,‡ Ning Han,*,†,‡ and Yunfa Chen*,†,‡ †
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, P. R. China § University of Chinese Academy of Sciences, Number 19A Yuquan Road, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Due to its unique layered structure compared with those tunnel structures of α-, β-, and γ-MnO2, δ-phase MnO2 is widely investigated as pollutant degradation catalysts, supercapacitor cathodes, and so forth. However, it is still challenging to effectively dope the δ-MnO2 to improve its performance although doping of other structures has been widely reported. In this study, a facile anion route is used to dope Ti in hydrothermal process for preparation of highly active hierarchical δ-MnO2 catalyst. The Ti doping process is performed successfully at a growth temperature of 140 °C, considering keeping the active δ-phase and producing the active lattice oxygen, which is identified by X-ray diffraction patterns (XRD), X-ray photoelectron spectra (XPS), and Raman spectra. On the other hand, low temperature ( T200 (52.5 cm2 g−1) > M200 (24.2 cm2 g−1). And then the Barrett−Joyner−Halenda (BJH) pore size distributions of the samples are shows in Figure 5. It is
those pure M200 and low temperature doped T100 samples with only single pore size distribution. To examine how the Ti doping process influence the catalytic property of the hierarchical δ-MnO2, catalytic performance of the as-prepared samples are compared using oxidation of typical air pollutant benzene as shown in Figure 6. Benzene was completely converted to CO2 and H2O from GC analysis and no intermediate oxidation products were detected during the catalytic test. As shown in Figure 6a, benzene conversion increases with the reaction temperature, and the Tidoped samples (T100, T140, T200) show much higher activities than the pure MnO2 catalyst (M200), indicating that the present of Ti in MnO2 is beneficial for improving the catalytic ability of the MnO2. In specific the conversion of benzene reaches 50% at about 217 °C for the most active T140 sample, followed by the order of T100 (242 °C) > T200 (247 °C) > M200 (322 °C), which shows the effective promotion of the catalytic property by doping Ti into δ-MnO2 at 140 °C in good agreement with the Raman and TPR results in Figure 3. Besides the active oxygen, the enhanced surface hydroxyl groups might also favor benzene adsorption due to the charge interaction of the positive H ends in hydroxyl group and the negative π electrons in benzene.6 δ-MnO2 also possesses interlayer spaces of ∼0.7 nm, larger than the size of benzene (0.54−0.58 nm)34 and much larger than the size of O2, CO2, and H2O, and thus will also provide the space for the catalytic reaction of benzene with the active lattice oxygen. This is in contrast to the oxidation of CO,6 which has a rather small molecular size (0.37 nm) compared with even the tunnel of αMnO2 (0.46 nm), and thus the α and δ phases show similar activity toward CO but distinctive different for benzene degradation in this study. All these results are schematically shown in Figure 6b. In the process of benzene catalytic oxidation, benzene diffuses between particles, grains, and layer structures of the δ-MnO2 microspheres to react with the active oxygen. The consumed active oxygen is then made up by the gaseous oxygen diffused by the same pathways. In this process, the Ti-doped δ-MnO2 shows great advantages in producing active oxygen and facilitating the gases diffusion. First, the Ti dopant into the δMnO2 lattice would produce a large amount of active lattice oxygen, which will facilitate the reaction with benzene. Second, the Ti dopant will also induce abundant small pores in the range of 3−10 nm as compared with pure MnO2 due to the poor crystallization of the MnO2 grains by adding Ti dopants.
Figure 5. Pore size distribution of the as-synthesized hierarchical MnO2.
clear that all T140 possess abundant pores in the range of 3−10 nm, which is in good accordance with the TEM observation in Figure 2d, and facilitates gas diffusion and reaction with the active oxygen. However, this enhanced diffusion is limited for
Figure 6. Activities of the samples in the oxidation of benzene. (a) Catalytic performance of MnO2 and (b) schematics of the oxidation process. 10280
DOI: 10.1021/acs.jpcc.6b00931 J. Phys. Chem. C 2016, 120, 10275−10282
Article
The Journal of Physical Chemistry C
Figure 7. Characterizations of samples obtained with different amount of ammonium hexafluorotitanate. (a) and (b) SEM images of Ti-doped MnO2 with 2.0 and 0.5 g of ammonium hexafluorotitanate, and the resultant Ti doping concentrations are 0.12 atom % and 0.61 atom % from EDS analysis, (c) pore size distribution, and (d) activities of the samples obtained with different amounts of ammonium hexafluorotitanate.
Finally, the interlayer spaces of δ structure are kept by the 140 °C growth temperature as compared with the 200 °C grown αphase, which together with the pores will facilitate the reactant benzene, oxygen, and the product H2O and CO2 diffusion. To further investigate the Ti doping effect and prove the proposed mechanism in Figure 6b, the influences of synthesis temperature and precursor concentration are studied in detail. As shown in Figure S6, the sample prepared at 120 °C (T120) shows similar morphology and crystal structure as those of T100, whereas growth temperature of 160 °C (T160) changed the δ-MnO2 phase into α-MnO2. The Ti doping concentration (0.43 and 1.33 atom % for T120 and T160, respectively, from ICP) is also not as high as that of T140 (1.93 atom %), showing the optimized synthesis temperature of 140 °C. The low Ti doping concentration and the low abundance of pores shown in Figure S6 makes the T120 and T160 samples less effective in benzene degradation. On the other hand, the Ti doping is also carried out at different precursor concentrations (0.5 and 2.0 g of (NH4)2TiF6)) with growth temperature of 140 °C (0.12 and 0.61 atom % respectively from ICP), and the active oxygen species and the pore size distributions are also characterized as shown in Figure 7a−c and Figure S7. The morphology are similar in Figure 7a and b, and the O 1s peaks are the same in Figure S7c, whereas the pore size distributions are different in Figure 7c. Similar to the proposed mechanism in Figure 6b, the low doping concentration would induce more small pores (10 nm), which enhances the catalytic property compared with pure MnO2 but is still lower than the widely distributed pores of T140. The enhanced active oxygen species as well as the suitable pore size distributions play important roles in enhancing the catalytic oxidation of benzene from both thermal dynamics and kinetics viewpoints. These results show the promise of the doping approach by well choosing the dopant and the pore size control in highly active δ-MnO2 catalysts.
4. CONCLUSIONS In summary, Ti is successfully doped into layer structured δMnO2 lattice by anion route at an optimized temperature of 140 °C in hydrothermal growth, which is proved by XRD, Raman, and XPS spectra. The Ti-doped δ-MnO2 showed enhanced benzene oxidation activity as compared with pure one, as illustrated by the far lower 50% conversion temperature (∼217 °C) than that of the pure one (∼322 °C). This is mainly attributed to the induced abundant active lattice oxygen species located at the shared edges of [MnO6] and [TiO6] octahedras in Ti-doped δ-MnO2. The active lattice oxygen lowers the reaction energy barrier as verified by the higher relative intensity of peak ∼650 cm−1 in Raman spectra and the shifted O 1s to higher energy in XPS spectra. Furthermore, the hierarchically largely distributed pores in the range of 3−10 nm as well as the interlayer space of 0.7 nm in the δ-MnO2 crystal facilitate the reactant and product diffusion. These results show the promise of the anion doping method in activity tailoring of δ-MnO2 catalyst for organic air pollutant reduction.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00931. SEM images of the samples, FTIR, XPS, nitrogen adsorption−desorption isotherm curves and catalytic property of Ti-doped MnO2. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 10281
DOI: 10.1021/acs.jpcc.6b00931 J. Phys. Chem. C 2016, 120, 10275−10282
Article
The Journal of Physical Chemistry C
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Spheres: One-Step Synthesis and Catalytic Activities in Li/Air Batteries and Oxidative Chemical Reactions. Adv. Funct. Mater. 2010, 20, 3373−3382. (17) Li, D.; Wu, X.; Chen, Y. Synthesis of Hierarchical Hollow MnO2 microspheres and Potential Application in Abatement of VOCs. J. Phys. Chem. C 2013, 117, 11040−11046. (18) Su, X. H.; Yu, L.; Cheng, G.; Zhang, H. H.; Sun, M.; Zhang, L.; Zhang, J. J. Controllable Hydrothermal Synthesis of Cu-Doped DeltaMnO2 Films with Different Morphologies for Energy Storage and Conversion Using Supercapacitors. Appl. Energy 2014, 134, 439−445. (19) Lide, D. R. Crc Handbook of Chemistry and Physics, 90th ed.; CRC press: Boca Raton, 2010. (20) Portehault, D.; Cassaignon, S.; Nassif, N.; Baudrin, E.; Jolivet, J. P. A Core-Corona Hierarchical Manganese Oxide and Its Formation by an Aqueous Soft Chemistry Mechanism. Angew. Chem., Int. Ed. 2008, 47, 6441−4. (21) Genuino, H. C.; Dharmarathna, S.; Njagi, E. C.; Mei, M. C.; Suib, S. L. Gas-Phase Total Oxidation of Benzene, Toluene, Ethylbenzene, and Xylenes Using Shape-Selective Manganese Oxide and Copper Manganese Oxide Catalysts. J. Phys. Chem. C 2012, 116, 12066−12078. (22) Xia, H.; Wang, Y.; Lin, J.; Lu, L. Hydrothermal Synthesis of MnO2/CNT Nanocomposite with a CNT Core/Porous MnO2 Sheath Hierarchy Architecture for Supercapacitors. Nanoscale Res. Lett. 2012, 7, 33. (23) Julien, C.; Massot, M.; Baddour-Hadjean, R.; Franger, S.; Bach, S.; Pereira-Ramos, J. P. Raman Spectra of Birnessite Manganese Dioxides. Solid State Ionics 2003, 159, 345−356. (24) Ogata, A.; Komaba, S.; Baddour-Hadjean, R.; Pereira-Ramos, J. P.; Kumagai, N. Doping Effects on Structure and Electrode Performance of K-Birnessite-Type Manganese Dioxides for Rechargeable Lithium Battery. Electrochim. Acta 2008, 53, 3084−3093. (25) Hsu, Y. K.; Chen, Y. C.; Lin, Y. G.; Chen, L. C.; Chen, K. H. Reversible Phase Transformation of MnO2 Nanosheets in an Electrochemical Capacitor Investigated by in Situ Raman Spectroscopy. Chem. Commun. (Cambridge, U. K.) 2011, 47, 1252−4. (26) Sun, M.; Lan, B.; Lin, T.; Cheng, G.; Ye, F.; Yu, L.; Cheng, X.; Zheng, X. Controlled Synthesis of Nanostructured Manganese Oxide: Crystalline Evolution and Catalytic Activities. CrystEngComm 2013, 15, 7010−7018. (27) Luo, J.; Zhu, H. T.; Fan, H. M.; Liang, J. K.; Shi, H. L.; Rao, G. H.; Li, J. B.; Du, Z. M.; Shen, Z. X. Synthesis of Single-Crystal Tetragonal α-MnO2 Nanotubes. J. Phys. Chem. C 2008, 112, 12594− 12598. (28) Chen, R. J.; Zavalij, P.; Whittingham, M. S. Hydrothermal Synthesis and Characterization of KxMnO2 yH2O. Chem. Mater. 1996, 8, 1275−1280. (29) Gao, Y. F.; Masuda, Y.; Koumoto, K. Microstructure-Controlled Deposition of SrTiO3 Thin Film on Self-Assembled Monolayers in an Aqueous Solution of (NH4)2TiF6-Sr(NO3)2-H3BO3. Chem. Mater. 2003, 15, 2399−2410. (30) Song, K.; Jung, J.; Heo, Y. U.; Lee, Y. C.; Cho, K.; Kang, Y. M. α-MnO2 Nanowire Catalysts with Ultra-High Capacity and Extremely Low Overpotential in Lithium-Air Batteries through Tailored Surface Arrangement. Phys. Chem. Chem. Phys. 2013, 15, 20075−9. (31) Ma, X.; Suo, X.; Cao, H.; Guo, J.; Lv, L.; Sun, H.; Zheng, M. Deep Oxidation of 1,2-Dichlorobenzene over Ti-Doped Iron Oxide. Phys. Chem. Chem. Phys. 2014, 16, 12731−40. (32) Chigane, M.; Ishikawa, M. Manganese Oxide Thin Film Preparation by Potentiostatic Electrolyses and Electrochromism. J. Electrochem. Soc. 2000, 147, 2246−2251. (33) Simanova, A. A.; Pena, J. Time-Resolved Investigation of Cobalt Oxidation by Mn(III)-Rich Delta-MnO2 Using Quick X-Ray Absorption Spectroscopy. Environ. Sci. Technol. 2015, 49, 10867− 10876. (34) Li, J. R.; Kuppler, R. J.; Zhou, H. C. Selective Gas Adsorption and Separation in Metal-Organic Frameworks. Chem. Soc. Rev. 2009, 38, 1477−1504.
ACKNOWLEDGMENTS We appreciate the financial support from National High Technology Research and Development Program 863 of China (No. 2012AA062702), Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB0505000) and State Key Laboratory of Multiphase Complex Systems (MPCS-2015-A-04).
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REFERENCES
(1) Yu, Y.; Zhang, B.; He, Y.-B.; Huang, Z.-D.; Oh, S.-W.; Kim, J.-K. Mechanisms of Capacity Degradation in Reduced Graphene Oxide/αMnO2 Nanorod Composite Cathodes of Li-Air Batteries. J. Mater. Chem. A 2013, 1, 1163−1170. (2) Zhu, J.; Shi, W.; Xiao, N.; Rui, X.; Tan, H.; Lu, X.; Hng, H. H.; Ma, J.; Yan, Q. Oxidation-Etching Preparation of MnO2 Tubular Nanostructures for High-Performance Supercapacitors. ACS Appl. Mater. Interfaces 2012, 4, 2769−2774. (3) Xie, X.; Zhang, C.; Wu, M.-B.; Tao, Y.; Lv, W.; Yang, Q.-H. Porous MnO2 for Use in a High Performance Supercapacitor: Replication of a 3d Graphene Network as a Reactive Template. Chem. Commun. 2013, 49, 11092−11094. (4) Shan, J.; Zhu, Y.; Zhang, S.; Zhu, T.; Rouvimov, S.; Tao, F. Catalytic Performance and in Situ Surface Chemistry of Pure A-MnO2 Nanorods in Selective Reduction of NO and N2O with Co. J. Phys. Chem. C 2013, 117, 8329−8335. (5) Saputra, E.; Muhammad, S.; Sun, H.; Ang, H. M.; Tade, M. O.; Wang, S. Different Crystallographic One-Dimensional MnO2 Nanomaterials and Their Superior Performance in Catalytic Phenol Degradation. Environ. Sci. Technol. 2013, 47, 5882−7. (6) Liang, S.; Teng, F.; Bulgan, G.; Zong, R.; Zhu, Y. Effect of Phase Structure of MnO2 Nanorod Catalyst on the Activity for CO Oxidation. J. Phys. Chem. C 2008, 112, 5307−5315. (7) Zhang, X.; Yang, W.; Yang, J.; Evans, D. G. Synthesis and Characterization of α-MnO2 Nanowires: Self-Assembly and Phase Transformation to B-MnO2 Microcrystals. J. Cryst. Growth 2008, 310, 716−722. (8) Yang, Y.; Huang, J.; Wang, S.; Deng, S.; Wang, B.; Yu, G. Catalytic Removal of Gaseous Unintentional POPs on Manganese Oxide Octahedral Molecular Sieves. Appl. Catal., B 2013, 142−143, 568−578. (9) Devaraj, S.; Munichandraiah, N. Effect of Crystallographic Structure of MnO2 on Its Electrochemical Capacitance Properties. J. Phys. Chem. C 2008, 112, 4406−4417. (10) Ghodbane, O.; Pascal, J. L.; Favier, F. Microstructural Effects on Charge-Storage Properties in MnO2-Based Electrochemical Supercapacitors. ACS Appl. Mater. Interfaces 2009, 1, 1130−1139. (11) Wang, F.; Dai, H.; Deng, J.; Bai, G.; Ji, K.; Liu, Y. Manganese Oxides with Rod-, Wire-, Tube-, and Flower-Like Morphologies: Highly Effective Catalysts for the Removal of Toluene. Environ. Sci. Technol. 2012, 46, 4034−41. (12) Shen, X. F.; et al. Characterization of the Fe-Doped MixedValent Tunnel Structure Manganese Oxide KOMS-2. J. Phys. Chem. C 2011, 115, 21610−21619. (13) Schottdarie, C.; Patarin, J.; Legoff, P. Y.; Kessler, H.; Benazzi, E. Synthesis, Characterization and Rietveld Refinement of the Tetragonal Variant of AlPO4−16 Prepared in Fluoride Medium. Microporous Mater. 1994, 3, 123−132. (14) Poonguzhali, R.; Shanmugam, N.; Gobi, R.; Senthilkumar, A.; Shanmugam, R.; Sathishkumar, K. Influence of Zn Doping on the Electrochemical Capacitor Behavior of MnO2 Nanocrystals. RSC Adv. 2015, 5, 45407−45415. (15) Jana, S.; Pande, S.; Sinha, A. K.; Sarkar, S.; Pradhan, M.; Basu, M.; Saha, S.; Pal, T. A Green Chemistry Approach for the Synthesis of Flower-Like Ag-Doped MnO2 Nanostructures Probed by SurfaceEnhanced Raman Spectroscopy. J. Phys. Chem. C 2009, 113, 1386− 1392. (16) Jin, L.; Xu, L.; Morein, C.; Chen, C.-h.; Lai, M.; Dharmarathna, S.; Dobley, A.; Suib, S. L. Titanium Containing γ-MnO2 (TM) Hollow 10282
DOI: 10.1021/acs.jpcc.6b00931 J. Phys. Chem. C 2016, 120, 10275−10282
