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7ХРАНЕНИЕ ВОДОРОДА
HYDROGEN STORAGE
Статья поступила в редакцию 06.08.12. Ред. рег. № 1389
The article has entered in publishing office 06.08.12. Ed. reg. No. 1389
УДК 544.723; 546.112
СОРБЦИЯ ВОДОРОДА В ПОРИСТЫХ ОКСИДАХ
Л. Гринберга, А. Шиварс, Я. Клеперис
Институт физики твердого тела при Латвийском университете Латвия, Рига, LV-1063, ул. Кенгарага, д. 8 Тел.: +37167262145, +3716132778; e-mail: Liga.Grinberga@cfi.lu.lv
Заключение совета рецензентов: 20.08.12 Заключение совета экспертов: 25.08.12 Принято к публикации: 30.08.12
Обычно оксиды (стекло, цеолиты) не поглощают заметный объем водорода, однако допирование этих материалов небольшим количеством палладия может значительно увеличить этот показатель. В данной работе рассматриваются сорбционные свойства композитов, состоящих из высокодиспергированного порошка оксида, покрытого наночастицами палладия при помощи экстракционно-пиролитического метода. Для изучения взаимодействий между полученными композитами и водородом использован объемный или метод Сиверта. Результаты измерения динамической сорбции показывают, что композиты на основе стекла пирекс достигают высокого содержания водорода уже через несколько минут. Композиты на основе силикагеля поглощают меньшее количество водорода, и после 10 минут поглощение существенно замедляется. Количество поглощенного водорода для стекла пирекс превышает аналогичный показатель для силикагеля примерно в 5,6 раза.
Ключевые слова: сорбция водорода, пористые оксиды, гидрид металла, силикагель.
In a regular circumstances oxides (glass, zeolite) are not absorbing notable amount of hydrogen, however doping small quantities of palladium in these materials can provide a capturing of significant amount of this gas. The hydrogen sorption properties in composites made of palladium nano-particles coated on the surface of highly dispersed powder of oxides by using extractive-pyrolytic method are studied. To investigate an interaction of hydrogen with Pd/oxide composites the Sievert or volumetric method was used. The results obtained of the study on dynamic sorption experiments showed that Pyrex glass based composite sample reaches high hydrogen concentration in the material in few minutes. Silica gel based composite material does not reach the same hydrogen load and after 10 minutes the amount of hydrogenation increases slowly. The overall amount of absorbed hydrogen for Pyrex glass based material exceeds the amount of absorbed hydrogen by silica gel based material approximately 5.6 times.
Keywords: hydrogen sorption, porous oxides, metal hydride, silica gel.
HYDROGEN SORPTION OF POROUS OXIDES
L. Grinberga, A. Sivars, J. Kieperts
Institute of Solid State Physics, University of Latvia 8 Kengaraga str., Riga, LV-1063, Latvia Tel.: +37167262145, +3716132778; e-mail: Liga.Grinberga@cfi.lu.lv
Referred: 20.08.12 Expertise: 25.08.12 Accepted: 30.08.12
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Organization: Institute of Solid State Physics, University of Latvia. Education: University of Latvia, Faculty of Physics and Mathematics (1995-2007). Experience: Institute of Solid State Physics, University of Latvia, Engineer (1999-2003). Assistant (2003-2005), Researcher (2005-2008), Leading researcher (2008...). Participated in 9 Scientific research projects, currently in 4.
Main range of scientific interests: hydrogen storage, alternative energy, electronic noises. Publications: 21 papers in international scientific journals, 1 section in 1 monograph.
Liga Grinberga
Organization: Institute of Solid State Physics, University of Latvia. Education: University of Latvia, Faculty of Physics and Mathematics, BSc (2009-2012). Experience: Institute of Solid State Physics, University of Latvia, Engineer (2010.). Participating in 2 Scientific research projects.
Main range of scientific interests: material science, electronics, process automation. Publications: 2 papers in international scientific journals.
Andris Sivars
Introduction
The science of gas sorption has become particularly important in the development of materials for hydrogen storage. To test the adequacy of the new materials for hydrogen economy the characterization of gas absorption, adsorption and desorption kinetics, capacity, thermodynamic properties, as well as cycling performance of reversible materials is crucial.
There are many metals known which form hydrides, since 1886, when Graham [1] firstly reported that palladium metal could absorb a large amount of hydrogen. The character of the metal-hydrogen bond is very dependent on the properties of the metal atoms due to low electron affinity of hydrogen. Unfortunately, none of the metals fulfil the requirements for commercial use as hydrogen storage materials - high reversible storage capacity and fast kinetics at ambient temperature.
Several techniques are employed for gas sorption measurements but the most common ones are gravimetric and volumetric methods. In gravimetric methods, the amount of sorbed gas is determined by measuring the apparent mass change of the sample. In volumetric methods, the amount of gas sorbed is typically determined by a change in pressure within a calibrated volume containing the sample. The measurements described in this paper were performed using PCTPro-2000, which is a volumetric instrument that offers the best state-of-the-art measurements [2].
The hydrogen storage in solid materials proceeds through at least three mechanisms: adsorption, absorption and chemical reactions. Adsorption process
has two forms - physisorption and chemisorption, as the case from the bond between a hydrogen molecule and material. Adsorptive processes typically require highly porous materials with a high surface area available for hydrogen sorption to occur. It is well known, that noble metals like Pd can adsorb and diffuse hydrogen in reactive forms over relatively large distances, therefore the sorption mechanism is explained as follows. Hydrogen is adsorbed and dissociated by the Pd particles and then spread in atomic form inside the silica structure by spill-over effect [3].
Previous experiments showed that it is possible to increase amount of stored hydrogen in solid materials using lanthanum-nickel alloy and Pyrex glass composite [4]. Main argument in favour of the silica and related materials (zeolite, gel, glass, etc.) is their wide prevalence, light weight and stable chemical properties.
In this paper the hydrogen sorption properties of the composites based on silica material and hydride forming metals are studied.
Experimental part
The composite material used in this research consists of 90 wt% silica based materials - Pyrex glass (PG), silica glass (SiO2), silica gel (SG), zeolite (ZE) - and 10 wt% hydride forming metal (palladium) were prepared by extractive-pyrolytic method [5].
Silica materials before the preparation of the composite were ground in the tungsten carbide (WC) crucibles using ball mill Retsch MM200. The weight ratio of 2 WC balls vs. silica material was 4.4:1, milling time 5 min, frequency 25 Hz.
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Водородная экономика. Хранение водорода
Palladium nanopowder (< 20 nm, Aldrich), HCl (35%) and HNO3 (65%) (Lachema), trioctylamin C8HnN (95%) (Fluka) and analytical grade toluene СбН5СН3 (Stanchem) were used as precursors for preparation of the composite material. Firstly, an organic precursor was prepared by extracting palladium from 20 ml HCl solution using 50 ml trioctylamin solution in toluene.
A carrier (silica material) was boiled in HCI (1:1) solution, washed by distilled water and dried. After keeping in alkaline solution it was repeatedly washed, dried and impregnated with the precursor solution.
After the impregnation the materials were dried at 90-110 °C for 20 min, heated from room temperature to 300 °C and annealed. The process of carrier impregnation, drying and thermal treatment were carried out intermittently 10 times followed by pyrolysis. Composites consisting of Pd and silica based materials with nominal metal loading of 10 wt% were obtained.
To study a surface morphology the Scanning Electron Microscope (SEM) of Carl Zeiss brand, model EVO 50 XVP was used. The SEM images were taken in secondary electron (SE) mode; the acceleration voltage was equal to 30 kV, and the emission current was between 0,5pA and 500nA. The energy dispersive detector for X-rays (EDX) was used for composite determination (Table 1).
Таблица 1
Результаты анализа элементов в используемых материалах: SG - силикагель; ZE - цеолит;
SiO2; PG - пирекс
Table 1
Elemental analysis of used silica materials: SG -silica gel; ZE - zeolite; SiO2; PG - Pyrex
Element Weight, %
SG ZE SiO2 PG
Al 4,01 5,78 1,90 1,64
Si 42,67 32,14 30,90 22,99
O 52,46 46,82 57,12 49,54
Ti 0,17 0,49 - -
Fe 0,51 2,69 - -
Ca 0,19 5,67 - -
K - 6,41 1,04 0,73
Na - - 4,31 3,30
Cl - - 0,74 0,58
Ba - - 3,98 2,30
C - - - 19,19
The prepared composites were characterized by X-ray diffraction method (XRD) using diffractometer D-8 Advance (Bruker AXS) with CuKa radiation (X = 1.5418 Â) in a wide range of Bragg angles (100 < 20 < 750) with a scanning rate of 0,020/s. The average size of Pd crystallites on the SiO2, PG, SG, ZE particles was determined to be
between 40-50 nm. That was evaluated from the XRD pattern (Fig. 1) using Scherrer's equation [6].
The specific surface area of the powders was measured using a Sorptometer KELVIN 1042 (COSTECH Instruments) by BET method using nitrogen gas as an adsorbate with an accuracy better than 97%. Table 2 shows comparison of two samples (SG and SiO2), where a surface and volume correlation between pure samples, layered with 10wt% of Pd and after treatment with hydrogen can be found.
400 300
■
о
u200 100
" 11 20 30 40 SO 60 70
2 Thcta
Рис. 1. Результаты ДРА для образцов SG, покрытых 10 масс% Pd Fig. 1. XRD pattern of the SG sample covered with 10 %wt Pd
Таблица 2
Удельная площадь поверхности и объем пор измеренных образцов: SG - силикагель; SiO2 -
кварцевое стекло; SGPd и SiO2Pd - после покрытия 10 масс% Pd; SGPd+H и SiO2Pd+H -композиты после нескольких циклов сорбции-десорбции водорода
Table 2
Specific surface areas and pore volumes of the measured samples: SG - silica gel; SiO2 - silica glass; SGPd and SiO2Pd after coverage with 10 wt% of the Pd; SGPd+H and SiO2Pd+H -composites after several cycles of hydrogen sorption-desorption
Образец BET Surface Area, m2/g Langmuir Surface Area, m2/g Total pore volume, mm3/g
SG 83,97 111,50 190,82
SGpd 58,05 81,69 95,74
SGPd+H 53,45 74,74 102,43
SiO2 1,49 2,04 1,48
SiO2pd 2,13 2,95 2,27
SiO2Pd+H 2,04 2,95 3,05
Hydrogen sorption experiments were performed with PCTPro-2000 (SETARAM) that is fully automated Sievert's type instrument for measuring gas sorption properties of materials. The measurement sequences for each sample were identical. Before the measurements all the samples were vacuumed and annealed at 200 °C for 1h to remove oxides and gaseous enclosures. Afterwards cooling to 26 °C during vacuuming was performed.
International Scientific Journal for Alternative Energy and Ecology № 09 (113) 2012
© Scientific Technical Centre «TATA», 2012
Hydrogen sorption kinetics was measured at 26 °C, applying different hydrogen gas pressures: 2.5 bar, 5 bar, 10 bar, 30 bar, 100 bar. Experimental mode defined that initially the set pressure is applied and the change of pressure and temperature is measured. The quantity of absorbed or desorbed hydrogen gas is calculated knowing the initial and final pressure and the volume of a sample chamber. The result is used to calculate the weight of absorbed hydrogen accordingly to the active mass of the sample.
Hydrogen sorption experiments were performed with PCTPro-2000 (SETARAM) that is fully automated Sievert's type instrument for measuring gas sorption properties of materials. The measurement sequences for each sample were identical. Before the measurements all the samples were vacuumed and annealed at 200 °C for 1h to remove oxides and gaseous enclosures. Afterwards cooling to 26 °C during vacuuming was performed.
Hydrogen sorption kinetics was measured at 26 °C, applying different hydrogen gas pressures: 2.5, 5, 10, 30, 100 bar. Experimental mode defined that initially the set pressure is applied and the change of pressure and temperature is measured. The quantity of absorbed or desorbed hydrogen gas is calculated knowing the initial and final pressure and the volume of a sample chamber. The result is used to calculate the weight of absorbed hydrogen accordingly to the active mass of the sample.
Results
Initial hydrogen sorption kinetic measurements were performed at the different applied hydrogen gas pressures (2.5, 5, 10, 30, 100 bar). Obtained results showed that the only noticeable amount of absorbed hydrogen for these materials occurs at the pressure 2.5 bar; therefore this particular pressure was used for further measurements.
Рис. 2. Кинетика сорбции водорода для SiO2, цеолита (ZE), силикагеля (SG) и пирекса (PG), покрытого 10 масс%
палладия (2,5 бар, 26 °С) Fig. 2. Hydrogen sorption kinetics of SiO2, zeolite (ZE), silica gel (SG) and Pyrex (PG) covered with 10 wt% of palladium (2.5 bar, 26 °C)
Fig. 2 shows amount of absorbed hydrogen in the weight % according to the weight of sample. It can be seen that SiO2 sample reaches high hydrogen
concentration in the material in ~ 6 min, while SG, PG and ZE samples attains just a half of the hydrogen load at the same time. However SiO2 sample approximately after 15 minutes starts to absorb hydrogen again and overall amount of absorbed gas is higher than that for PG.
Conclusions
XRD and scanning electron microscope analysis of silica/Pd based composite materials showed that not only the pure nanocrystalline Pd but also PdO are located on the external surfaces of silica based material nanoparticles. BET surface analysis showed that porous silica gel and zeolite has at least 35 times larger specific surface area than Pyrex and SiO2 glass.
It can be assumed that the material with larger surface area will absorb more hydrogen due to the greater amount of potential interaction sites that would be exposed to hydrogen. However, hydrogen sorption kinetic measurements showed that the material with the less specific surface area (SiO2, PG) absorbs more hydrogen than the other one (SG, ZG).
Potential explanation is related with the spill-over effect. The spill-over of hydrogen involves electron transfer to acceptors within the support, modifying the chemical nature of the support. It may also activate a previously inactive material and induce subsequent hydrogen physisorption. The spill-over of atomic hydrogen is highly dependent on the chemical bridges formed at the interface; thereby less porous material can provide better media for active hydrogen particle transport and storage.
This work demonstrates the possibility to use silica based materials doped with palladium for hydrogen storage. The interest in these materials, besides the ubiquity of silicates, is related to opportunities to design better and safer hydrogen storage media.
Acknowledgements
Authors acknowledge the Research project 09.1553 of Latvian Council of Science
References
1. Graham T. On the absorption and dialytic separation of gases by colloid septa // Phil. Trans Roy. Soc. 1866. Vol. 156. P. 415.
2. Gross K.J. The PCTPro-2000 - The Ultimate Tool for Gas Sorption Analysis // Material Matters. 2007. Vol. 2, No 2: 26-28.
3. Grinberga L., Kleperis J., Bajars G., et. al. Estimation of hydrogen transfer mechanisms in composite materials // Solid State Ionics. 2008. Vol. 179. P. 42-45.
4. Grinberga L. and Kleperis J. Development of new composite materials for hydrogen storage. The AB5 type hydride alloy with silica glass support // Journal of Physics: Conference Series. 2007. Vol. 93. 012024 doi:10.1088/1742-6596/93/1/012024.
5. Kholkin A.I. and Patrusheva T.N. Extractive pyro lytic method. Moscow: Com Book, 2006.
6. Jenkins R. and Snyder I.R. Introduction to X-ray powder diffractometry. New York: Wiley-Interscience, 1996.
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