Hierarchically structured carbon:: Synthesis of carbon nanofibers nested inside or immobilized onto modified activated carbon
Hierarchically structured carbon:: Synthesis of carbon nanofibers nested inside or immobilized onto modified activated carbon
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DOI：
10.1002/anie.200500685
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发表时间：
2005-01-01
影响因子：
16.6
通讯作者：
Schlögl, R
中科院分区：
文献类型：
--
作者：
Su, DS;Chen, XW;Schlögl, R
5488 2005 Wiley-VCH Verlag GmbH & Co. KGaA，魏因海姆Angew. Chem.Int.Ed.2005，44，5488-5492中所述的方法，并且避免了纳米碳与非碳载体结构的组合，因为所产生的传输和化学性质的不连续性将导致整体化合物性质的劣化和化学不稳定性。理想的基质是来自天然来源的活性炭。这种通常含有木质素和硅酸盐的生物无机聚合物是坚固的，在几个维度上结构良好，可用于化学改性，并且可大量获得。[20]活性炭的天然前体[21]利用生物细胞结构来预设大孔网络的布置，并且它们受益于无机添加剂作为催化剂的锚定位点，从而用于纳米碳。至关重要的是，纳米碳和活性碳载体之间的化学相互作用是强的和惰性的，以在延长的操作期间保持分级结构。本文介绍了从棕榈油生产的生物废弃物中获得的活性炭的改性方法，以作为纳米碳生长的支撑。我们得到了一个家庭的固定CNFs在各种修改。可以在活性炭的外表面上获得纳米结构的碳，从而产生用于粘合剂-填料应用的有用几何形状。或者，将大孔烧入活性炭的本体结构中并随后用碳纳米结构填充孔导致活性炭内部具有嵌套CNF的分级结构的碳材料，其高度适用于吸附和催化应用。这些分级结构的碳材料的生产背后的概念示于方案1中。在宏观水平上，通过选择性地去除生物聚合物的“软”部分，将散装活性炭聚集体转化为大孔支架。这是通过碳化过程中或碳化后的催化燃烧来实现的。然后通过将用于产生孔隙的催化剂的氧化形式转化为金属形式，将催化剂颗粒有意地嵌入碳基质中。金属颗粒在还原气氛下局部气化载体。然后，这种形式用于通过化学气相分解（CVD）有机分子在一代或几代中生长纳米碳;如果需要，则嵌入额外的催化剂。然后，通过在活性炭颗粒的外部生长纳米碳以使NAC颗粒彼此牢固地结合，可以将所得的固定化纳米碳-活性炭复合物（NAC）组装成更大的固体聚集体。基质介质的传输可以通过改变纳米结构的密度和通过改变多孔碳载体的填充因子来很好地控制。可能需要的化学表面改性的最后阶段未在方案1中示出。通过改变催化剂前体的沉积条件（从离子交换到均匀沉淀），可以改变催化剂在活性炭支架内部或外部的优先沉积位置，通过改变单元操作（1）催化剂浸渍，2）载体碳的煅烧，3）催化剂的还原和纳米碳的生长，可以得到一族嵌套或固定化的CNT/CNFs。我们使用CVD方法生长碳纳米管/碳纳米纤维，因为它有几个优点，电弧放电和激光烧蚀。例如，CVD方法不仅导致单壁或多壁碳纳米管，而且当不同的碳纳米管排列在一起时，也导致碳纳米管排列在一起。
5488 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2005, 44, 5488–5492 used for the construction and that the combination of nanocarbon with the non-carbon support structures is avoided as the resulting discontinuities in transport and in chemical properties would lead to deterioration of the overall compound properties and chemical instability. An ideal substrate is activated carbon from natural sources. Such bioinorganic polymers containing typically lignins and silicates are strong, well-structured in several dimensions, accessible for chemical modifications, and available in large amounts.[20] Natural precursors to activated carbons [21] utilize the biological cell structure to preset the disposition of the macropore network, and they benefit from inorganic additives as anchoring sites for the catalyst and therefore for the nanocarbon. It is essential that the chemical interaction between the nanocarbon and the activated carbon carrier is strong and inert to preserve the hierarchical structure during extended operation. The present paper describes ways to modify the activated carbon obtained from the biowaste of palm oil production to serve as the support for the growth of nanocarbon. We obtained a family of immobilized CNFs in various modifications. Nanostructured carbon can be obtained on the outer surface of the activated carbon yielding a useful geometry for binder–filler applications. Alternatively, burning large pores into the bulk structure of the activated carbon and subsequently filling the pores with carbon nanostructures leads to hierarchically structured carbon materials with nested CNFs inside the activated carbon which are highly suitable for sorption and catalytic applications. The concept behind the production of these hierarchically structured carbon materials is illustrated in Scheme 1. On the macroscopic level, bulk activated carbon aggregates are transformed into a macroporous scaffold by selectively removing the “soft” parts of the biopolymer. This is achieved by catalytic burning during or after carbonization. The catalyst particles are then deliberately embedded in the carbon matrix by transforming the oxidic form of the catalyst used for creating porosity into a metallic form. The metal particles gasify the support locally under a reducing atomosphere. This form is then used for growing nanocarbon by chemical vapor decomposition (CVD) of organic molecules in one or several generations; if needed additional catalyst is embedded. The resulting immobilized nanocarbon-activated carbon composite (NAC) can then be assembled into larger solid aggregates by growing nanocarbon on the outside of the activated carbon grains to bind NAC particles firmly to each other. The transport of matrix media can be well controlled by changing the density of the nanostructures and by varying the filling factor of the porous carbon support. A final stage of chemical surface modification that may be necessary is not shown in Scheme1. The preferential location of catalyst deposition inside or outside of the activated carbon scaffold can be varied by changing the deposition conditions of the catalytic precursor from ion exchange to homogeneous precipitation.By varying the unit operations—1) impregnation with the catalyst, 2) calcinations of the support carbon, 3) reduction of the catalyst and growth of nanocarbon—a family of nested or immobilized CNTs/CNFs can be obtained. We use the CVD method for growing the CNTs/CNFs as it has several advantages over electric-arc discharge and laser ablation. For example, the CVD method leads not only to single-walled or multiwalled CNTs but also to aligned CNTs when different …