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3Следующая задача - прогнозирование возможной утечки реального газа, например, водорода, через обнаруженный при испытаниях сквозной дефект, в процессе эксплуатации системы. И, наконец, необходимо сравнить полученные значения с теми или иными техническими требованиями и нормативами (ПДК и др.) Решение данной задачи в целом является объектом анализа и расчетов, требующих учета ряда конкретных характеристик систем.
ACCUMULATION OF HYDROGEN IN CARBON NANOSTRUCTURES
The review of the data on interaction of hydrogen with carbon nanomaterials (fullerenes, singlewall nanotubes, graphitic nanofibres and their modifications, doped by metals) is introduced.
The progress in popular use of hydrogen in the capacity of ecologically clean source of energy in many respects depends on the decision of a problem of an effective method of its storage and carriage. However, any of applied now methods of hydrogen storage (under high pressure, in liquid state, in hydrides of metals and intermetallic compounds, in an adsorbed state at decreased temperatures) (Table 1) does not satisfy to the imposed requirements, for example, of Department of Energy of USA (the mobile systems of storage, containing hydrogen on mass - more of 6.5%, on volume - more than 63 kg/m3, are necessary) or International Energy Agency (the storage systems should contain not less than 5 mass. % of hydrogen and evolve it at temperature not above 373K).
Table 1. Traditional methods of hydrogen storage
Methods of Hydrogen content Volume content of
Hydrogen storage in sorbent, hydrogen, Notices
mass. % kg/m3
Gaseous H2 100 7.7 Large mass of container, small
(300K, 10 MPa) volume capacity
Liquid H2 (20K) 100 71 Large losses, high cost
Metalhydride
TiH2 4.0 150
MgH2 7.6 120 Insufficient capacity,
LaNi5H67 1.4 85 Necessity of preheating,
TiFeH2 1.9 96 Sensitiveness to impurity
Mg2NiH4 4.0 81
Cryoadsorbed
Absorbit 0.05-2 ~1-2 Necessity of cooling and
(155K, 6.9 MPa) compression
TARASOV B.P.
Institute of Problems of Chemical Physics of RAS, 142432 Chernogolovka, Russia; E-mail: btarasov@icp.ac.ru
For use in the capacity of hydrogen-accumulating matrices now carbon materials (fig.1) are proposed most perspective, especially in connection with discovery of fullerenes, which after hydrogenation of all double bonds could contain up to 7.7 mass. % of hydrogen.
In this communication the review of data on sorption of hydrogen by carbon nanomaterials (fullerenes, nanotubes and nanofibres) is introduced.
Fig. 1. New allotrope modifications of carbon: a) Fullerenes: C60 -icosahedron («football») consists of 12 pentagons and 20 hexagons, Dout=7,1 A, d=1,675 g/cm3, C70 -ellipsoid consists of 12 pentgons and 25 hexagons, Dout=7,0 A and 8,8 A; b) Carbon nanotubes: types of nanotubes: single-wall (SWNT) and multiwall (MWNT), close and open capped, direct and spiral shapes, inset - SWNT: Dout = 1,2 - 3 nm, l = 50-1000 nm
Earlier we investigated in detail the chemical transformations in fullerene - metallic phase -hydrogen systems in a wide interval of pressures and temperatures [1-5]. The hydrogenation under pressures of 1.0-5.0 MPa and at the temperatures of 573-673K results in a formation of mixture of metal hydrides and the hydrides of fullerenes C60Hx (maximal composition is C60H36):
1-5M Pa, 570-670K
{C60 + M} + H2-> {C60Hx + MHy}
At heating of obtained mixture up to 800K the dehydrogenation takes place with a formation of fullerene-metallical compositions:
<600K ~800K {C6oHx + MHy}-> {C60Hx + M}-> {C60 + M}
At heating higher than 950K the reactions of a formation of metal carbides are exhibited in a number of cases.
At interaction of specially synthesized fullerides C60Pt and C60Pd49 with hydrogen depending on conditions of hydrogenation either hydrogen compounds of metalfullerides or mixture of hydrofullerenes and Pt or PdHy are formed:
1-3M Pa, 400-550K C60Pt + H2-> < C60PtHx >
1-3M Pa, 600-700K
C60Pt + H2-> C60Hx + Pt
1-3M Pa, 400-550K
C60Pd4.9 + H2-> < C60Pd4.9Hx >
1-3M Pa, 600-700K
C60Pd4.9 + H2-> C60Hx + PdHy
At heating up to 800K all hydrogen evolves from hydrofullerene, and the formed mixture of fullerite with metal (Pd, Pt) can be repeatedly hydrogenate with a formation of the mixture of hydrofullerene with PdHy or Pt [6, 7].
At interaction of fullerite C60 with hydrogen, evolved from metalhydrides under pressure of 1.5-5 MPa and carrying out of several cycles "heating up to 673K < = > cooling up to 300K", the crystalline hydrofullerenes C60Hx (x = 2-30) are formed [8, 9]. In the C-H bonds formed an electronic pair is offset in the direction of carbon. The constant offcc lattice of C60Hx is increased monotonously with elevation of the hydrogen content - from 1.417 nm for C60 up to 1.455 nm for C60H24. At heating of hydrofullerenes C60Hx up to 800K an evolution of hydrogen with a formation of fullerite C60 with "stretched" lattice takes place. This fullerite can be repeatedly hydrogenate. Thus, it is possible to speak about reversibility of reaction: C60 + xH2 < = > C60H2x. However at carrying out of cycles "hydrogenation - dehydrogenation" the side reaction of polymerisation begins to be exhibited.
The addition of 5-10 mass. % NH3, HJ, C2H5J to hydrogen raises essentially the rate of hydrogenation of fullerite, however thus the polymerization reaction is promoted.
Thus, our studies have shown that for use of fullerenes and their metalderivatives as the hydrogen sorbents it is necessary to increase the sorption capacity (the hydrofullerene C60H36 contains 4.5 mass. % of hydrogen), to raise the hydrogenation rate, to lower the temperature of dehydrogenation and to remove the side reactions [10].
Much more perspective for storage of hydrogen another carbon nanostructures (graphitic nanofibres, carbon nanotubes and their modifications, doped by metals) seem, since on available data their hydrogen-sorbing opportunities are exceeded much more the ones, known for another methods of
hydrogen storage, and are close to the necessary requirements. The carbon nanotubes represent the graphene sheets, rolled up in long barrels. They can be one - or multi-wall closed or opened. The opened single-wall nanotubes (SWNT), a diameter and length of which are ~1-3 nm and 1-100 microns, accordingly, are most interesting to hydrogen sorption. Graphitic nanofibre (GNF) represents the small plates of graphite (3-100 nm) located or parallelly, or perpendicularly or under angle to a fibre axis, with an increased in comparison with usual graphite interval between graphene sheets.
In Table 2 the data about hydrogen-sorbing ability of SWNT and GNF are presented. The hydrogen capacity of SWNT, obtained by laser vaporization of graphite at the presence of the metallic catalyst, is more than 8 mass. % at 80K and 7 MPa [11]. In the works [12-14] it has been shown that the hydrogen-sorbing capacity of single-wall nanotubes, obtained by electric-arc vaporization of metal-containing graphitic rods, constitutes 4-10 mass. % at an exposure of a carbon material at 133-300K and hydrogen pressure of 7-11 MPa. In the work [15] it is reported about the contents of hydrogen in SWNT in the number of 6.5-7 mass. % at room temperature and 0.1 MPa. Our studies of the hydrogen-sorbing properties of a carbon material, obtained by an electric-arc method and contained about 70 mass. % of SWNT, have shown, that such material adsorbs about 3.5 mass. % of hydrogen at 10 MPa and at repetition of cycles "colling up to 77K <=> heating up to 300K" (Table 2).
The most sensational results on an accumulation of hydrogen (11-66 mass. %) in graphitic nanofibres, obtained by a pyrolysis of hydrocarbons, are communicated in work [16]. The authors believe that at pressure of 11 MPa and room temperature hydrogen is sorbed in plane pores of GNF in width of 0.337 nm. In work [17] the high hydrogen-sorbing capacity (up to 20 mass. %) of GNF, doped by Li and K, is described. It is necessary to note, that the attempts of many explorers to reproduce results of [16, 17] have not crowned with success. Our studies of hydrogen-sorbing properties of soot containing GNF and obtained by catalytic pyrolysis of ethylene on nickel at 900K have shown that it is capable to adsorb about 2.5 mass. % of hydrogen at 5-7 MPa and repetition of cycles "cooling up to 77K < = > heating up to 300K" (Table 2).
Table 2. The sorption characteristics of carbon nanomaterials
Material Max. capacity, mass. % T, K Ph2, MPa The reference
SWNT 8.25 80 7.18 [11]
SWNT 5-10 133 0.04 [12]
SWNT 4.2 300 10-12 [13, 14]
SWNT 6.5-7 300 0.1 [15]
SWNT 3.5 300 77 5-10
GNF 11-66 300 11 [16]
GNF 0.4 298-773 0.1 [17]
GNF 2.5 300 77 5-10
Li-GNF 20 473-673 0.1 [17]
K-GNF 14 473-673 0.1 [17]
As it is visible from Table 2 the obtained by the different authors data on the content of hydrogen, accumulating by carbon nanomaterials, essentially differ. The reasons of discrepancies consist in the absence of reliable ways of obtaining clean SWNT and GNF, and also of standard techniques of their certification, for example, in the case of SWNT - on purity, extent of their "openness", on a diameter, content of metallic catalysts, which essentially influence on sorption of hydrogen. Owing to this the results, obtained at an investigation of sorption of hydrogen, characterize only concrete material and can not be utilized yet for a comparison of efficiency of carbon nanomaterials of a various type. The mechanism of uniquely high hydrogen sorption by carbon nanomaterials is not clear also. The different mechanisms (the physical adsorption and chemosorption of molecules H2 on a surface of a graphene sheet; the placement more than one layer of molecules H2 between graphene plains; the capillary condensation of hydrogen inside nanotubes and in space between graphene sheets at anomalously high temperatures - above Tcr = 33K; the charged state of hydrogen in carbon nanomaterials) are examined.
Thus, it is necessary to continue the investigations of hydrogen-sorbing properties of the carbon nanostructures and as a fundamental scientific task, and for the decision of applied problem of a development of mobile systems of hydrogen storage.
The work was carried out at support of the Russian Basic Research Foundation (grant No. 9903-32647) and the Russian Scientific Technical Program (grant No. 99005).
References
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Ю. И. Шанин
ГосНИИ НПО "ЛУЧ", Железнодорожная 24, Подольск, 142100, Россия, syi@luch.pgts.msk.ru
Yu.I Shanin
State Scientific Research Institute, Scientific Production Association "Luch", Zheleznodorozhnaya 24, Podolsk, 142100, Russia, syi@luch.pgts.msk. ru
ВЫБОР ГИДРИДОВ ДЛЯ АВТОМОБИЛЬНЫХ ГИДРИДНЫХ УСТРОЙСТВ
SELECTION OF HYDRIDES FOR AUTOMOBILE HYDRIDE DEVICES
Разработаны компьютерные программы, которые могут быть применены для быстрого начального выбора гидридов на основе табличных данных о физико-технических свойствах. Гидриды могут входить в состав различных гидридных устройств, которые могут быть использованы в автомобилестроении: аккумулятор водорода, компрессор, пусковое устройство, гидридный тепловой насос (ГТН).
В основном в работе анализируется выбор материалов гидридов (от одного до трех) при создании ГТН, работающих в диапазоне температур -50...+200°С. Набор программ по выбору гидридов предоставляет возможности быстрого перебора различных вариантов сочетаний гидридов, начиная с одного и кончая тремя гидридами (при различных видах ограничений: давление, температура, к.п.д.), применение которых гарантирует осуществление замкнутых термодинамических циклов для тепловых машин с рабочим телом "гидрид + водород". При анализе учитываются влияние наклона и гистерезиса изотерм в координатах "давление-состав". Рассматривается термодинамика циклов как в условиях равновесия, так и при наличии "движущей силы давления" между гидридами. Появляется возможность учитывать в расчетах теплоемкость сплавов, гидридов и основных конструкционных материалов, что существенно уточняет коэффициент преобразования цикла.
В работе демонстрируется выбор гидридов при создании холодильного устройства на базе металлогидридного теплового насоса, работающего либо на тепле выхлопных газов двигателя внутреннего сгорания, либо на теплоносителе системы охлаждения двигателя. Холодильное устройство может быть использовано в качестве либо металлогидридного рефрижератора на базе грузового автомобиля (авторефрижератор), либо автомобильного кондиционера. Также рассмотрен выбор гидридного материала для аккумулятора водорода, водородного компрессора и пускового разогревающего устройства.
