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使用密度泛函理论研究了Pd负载共价有机骨架COF-108上的氢溢流机理，主要包括Pd4 团簇在COF-108上的各种沉积构型，H2分子在Pd4@COF-108中Pd4团簇上的吸附和解离，H原子从Pd4团簇向COF-108上的迁移以及H原子在COF-108表面的扩散. 研究结果表明：1）Pd原子与HHTP或TBPM接触的数目越多，束缚能越大，沉积取向对束缚能的影响不大；2）H2 分子在Pd4团簇上的解离过程是自动进行的，满足氢溢流发生的条件；3）只有位于Pd4 团簇桥位上的H原子能够迁移到基底表面上，迁移过程为吸热反应，稳定性较差，这表明H原子将在基底表面进一步扩散；4）过渡金属Pd的引入可大大降低H原子在基底表面的扩散势垒，使H原子在基底表面的扩散更容易进行. 这些研究结果有助于理解氢溢流对COFs储氢性能影响的微观机理，进而为实验上定向制备性能优异的COFs基底材料提供有益指导.Hydrogen storage remains one of the main challenges in the implementation of a hydrogen-based energy economy. Among various porous materials, hydrogen storage in covalent-organic frameworks (COFs) has attracted the most significant attention since they were first synthesized due to good stability, large surface area, porosity and extremely low density. Although COFs exhibit promising hydrogen storage properties at very low temperatures, their hydrogen storage capacity is not satisfactory at room temperature, which is too low to meet the uptake target set by US-DOE, thereby being unable to have practical applications. Remarkably, hydrogen spillover has been experimentally demonstrated as an effective approach to improving the hydrogen storage capacity on porous materials at ambient temperature. In some of the most promising results the metal-organic frameworks (MOFs) and COFs have been used as substrates. However, the structures of many COFs materials are quite complex and the experimental condition is difficult to control. Furthermore, the sample preparations for these hydrogen spillover experiments are also very difficult. Therefore, only COF-1 is used in experimental study of hydrogen spillover. Although some theoretical work has contributed to understanding the hydrogen spillover mechanism of COFs, many basic problems about hydrogen spillover have not been solved, which hinders its practical application to a large extent. Based on the above reasons, the hydrogen spillover mechanism of Pd doped COF-108 is studied by using density functional theory (DFT) method, which mainly includes the various deposited configurations of Pd4 cluster on COF-108, the adsorption and dissociation of H2 on Pd4 cluster of Pd4@COF-108, the migration of H atom from Pd4 cluster toward the COF-108 and the diffusion of H atom on COF-108 surface. The results show as follows. 1) The larger the number of Pd atoms interacting with HHTP or TBPM cluster, the greater the binding energy of Pd4 deposited on them is. Deposited configuration orientation has little effect on binding energy. The binding energies of all deposition configurations for TBPM cluster are larger than those for HHTP cluster, so Pd4 cluster prefers to deposit on TBPM cluster with face-contact configuration. (2) H2 molecules spontaneously dissociated into Pd4 cluster, i.e., a barrierless H2 dissociation process takes place, which meets the first condition required by hydrogen spillover. 3) Only H atom located at the bridge site may migrate to the substrate surface, and the migration process is an endothermic reaction and less stable, which indicates that H atoms will further diffuse on the substrate surface. Although H atoms located at the top site may not migrate directly to the substrate surface, it will automatically migrate to the bridge site after the H atom on the bridge site has migrated to the substrate surface, so the migration process may proceed continuously. (4) The introduction of transition metal Pd can greatly reduce the diffusion energy barrier of H atoms on substrate surface, which makes it easier for H atoms to diffuse on substrate. These results may help us understand the microscopic mechanism of hydrogen spillover influencing the properties of hydrogen storage on COFs and provide useful guidance for targeted preparing the COFs materials with excellent hydrogen storage properties experimentally.
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Keywords:
- covalent organic frameworks /
- density functional theory /
- hydrogen spillover /
- hydrogen adsorption
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[18] Guo J H, Zhang H, Tang Y J, Cheng X L 2013 Phys. Chem. Chem. Phys. 15 2873
[19] Zhou C G, Wu J P, Nie A H, Forrey R C, Tachibana A, Cheng H S 2007 J. Phys. Chem. C 111 12773
[20] Kresse G, Hafner J 1993 Phys. Rev. B 48 13115
[21] Kresse G, Furthmuller J 1996 Comput. Mater. Sci. 6 15
[22] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[23] Blochl P E 1994 Phys. Rev. B 50 17953
[24] Kresse G, Joubert D {1999 Phys. Rev. B 59 1758
[25] Monkhorst H J, Pack J D 1976 Phys. Rev. B 13 5188
[26] Henkelman G, Uberuaga B P, Jnsson H 2000 J. Chem. Phys. 113 9901
[27] Psofogiannakis G M, Froudakis G E 2009 J. Phys. Chem. C 113 14908
[28] Wu H Y, Fan X F, Kuo J L, Deng W Q 2011 J. Phys. Chem. C 115 9241
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[1] Ct A P, Benin A I, Ockwig N W, O'Keeffe M, Matzger A J, Yaghi O M 2005 Science 310 1166
[2] Han S S, Furukawa H, Yaghi O M, Goddard II W A 2008 J. Am. Chem. Soc. 130 11580
[3] Klontzas E, Tylianakis E, Froudakis G E 2008 J. Phys. Chem. C 112 9095
[4] Furukawa H, Yaghi O M 2009 J. Am. Chem. Soc. 131 8875
[5] Klontzas E, Tylianakis E, Froudakis G E 2010 Nano. Lett. 10 452
[6] Tylianakis E, Klontzas E, Froudakis G E 2011 Nanoscale 3 856
[7] Kim D, Jung D H, Kim K H, Guk H, Han S S, Choi K, Choi S H 2012 J. Phys. Chem. C 116 1479
[8] Liu X Y, He J, Yu J X, Li Z X 2014 Chin. Phys. B 23 067303
[9] Lachawiec A J Jr, Qi G, Yang R T 2005 Langmuir 21 11418
[10] Lueking A, Yang R T 2002 J. Catal. 206 165
[11] Li Y W, Yang R T 2006 J. Phys. Chem. B 110 17175
[12] Liu X Y, Li X F, Zhang L Y, Fan Z Q, Ma X K 2012 Acta Phys. Sin. 61 146802 (in Chinese) [刘秀英, 李晓凤, 张丽英, 樊志琴, 马兴科 2012 61 146802]
[13] Li Y W, Yang R T 2006 J. Am. Chem. Soc. 128 726
[14] Li Y W, Yang R T 2006 J. Am. Chem. Soc. 128 8136
[15] Li Y W, Yang R T 2008 AIChE 54 269
[16] Suri M, Dornfeld M, Ganz E 2009 J. Chem. Phys. 131 174703
[17] Ganz E, Dornfeld M 2012 J. Phys. Chem. C 116 3661
[18] Guo J H, Zhang H, Tang Y J, Cheng X L 2013 Phys. Chem. Chem. Phys. 15 2873
[19] Zhou C G, Wu J P, Nie A H, Forrey R C, Tachibana A, Cheng H S 2007 J. Phys. Chem. C 111 12773
[20] Kresse G, Hafner J 1993 Phys. Rev. B 48 13115
[21] Kresse G, Furthmuller J 1996 Comput. Mater. Sci. 6 15
[22] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
[23] Blochl P E 1994 Phys. Rev. B 50 17953
[24] Kresse G, Joubert D {1999 Phys. Rev. B 59 1758
[25] Monkhorst H J, Pack J D 1976 Phys. Rev. B 13 5188
[26] Henkelman G, Uberuaga B P, Jnsson H 2000 J. Chem. Phys. 113 9901
[27] Psofogiannakis G M, Froudakis G E 2009 J. Phys. Chem. C 113 14908
[28] Wu H Y, Fan X F, Kuo J L, Deng W Q 2011 J. Phys. Chem. C 115 9241
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