Letter pubs.acs.org/JPCL
Tribochemical Decomposition of Light Ionic Hydrides at Room Temperature Roman Nevshupa,† Jose Ramón Ares,*,‡ Jose Francisco Fernández,‡ Adolfo del Campo,§ and Elisa Roman∥ †
Spanish National Research Council, Institute “Eduardo Torroja” (IETCC−CSIC), C/Serrano Galvache 4, Madrid 28033, Spain Department of Physics of Materials, Autónoma University of Madrid, Madrid 28049, Spain § Spanish National Research Council, Institute of Ceramic and Glass (ICV-CSIC), C/Kelsen 5, Madrid 28049, Spain ∥ Spanish National Research Council, Institute of Material Science of Madrid (ICMM-CSIC), C/Sor Juana Inés de la Cruz 3, Madrid 28049, Spain ‡
S Supporting Information *
ABSTRACT: Tribochemical decomposition of magnesium hydride (MgH2) induced by deformation at room temperature was studied on a micrometric scale, in situ and in real time. During deformation, a near-full depletion of hydrogen in the micrometric affected zone is observed through an instantaneous (t < 1 s) and huge release of hydrogen (3−50 nmol/s). H release is related to a nonthermal decomposition process. After deformation, the remaining hydride is thermally decomposed at room temperature, exhibiting a much slower rate than during deformation. Confocal-microRaman spectroscopy of the mechanically affected zone was used to characterize the decomposition products. Decomposition was enhanced through the formation of the distorted structure of MgH2 with reduced crystal size by mechanical deformation.
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position under mechanical deformation of MgH2, which has been used as a model light hydride. The phenomenon has been characterized in situ and in real time during deformation of MgH2 on the micrometric scale using a novel technique of MSGE spectrometry.13,18 For this purpose, cylindrical pellets, 13 mm in diameter and 2 mm thick, of MgH2 were obtained by cold compacting of commercial powder (Alfa Aesar, >95%) in a die under a uniaxial pressure of 75 MPa. The pellets were further sintered under hydrogen atmosphere (10 bar and 210 °C) in a Sieverts system during 24 h to improve their mechanical strength. To prove that H2 desorption is a result of MgH2 decomposition, several partially deuterated pellets were prepared from MgD2 powder in the same way as those of MgH2. MgD2 powder was obtained by thermal dehydriding of commercial MgH2, followed by charging with D2 under 10 bar and 250 °C. Structural characterization of the pellets was carried out by Xray diffraction with Cu Kα radiation at Θ/2Θ configuration. (See the Supporting Information.) Microscopic mechanical deformation of the pellet surface was produced in vacuum (p ≈ 10−8 hPa) by reciprocating motion of an alumina ball, 2 mm in diameter, under various normal loads in sub-Newton range. The desorption rate (H2
ight ionic hydrides, mainly MgH2, are considered to be the leading candidates for safe portable and on-board hydrogen storage due to high gravimetric capacity (>5%); however, slow dehydriding kinetics and high formation enthalpy are precluded from their implementation at near-room-temperature (RT) applications.1 The research to date2,3 has tended to solve those drawbacks by incorporation of catalysts, tuning the chemical composition and exploring the effects of nanostructuring. Thermodynamic and kinetics constraints are usually overcome by using thermal energy. Recently, nonthermal energy sources including microwave radiation and sonication4,5 have been considered. The latter is a kind of mechanochemical process that appears to be quite effective for H2 desorption because mechanical force can be directly transduced into chemical reactions.6,7 In particular, it was confidently demonstrated that mechanical stress and deformation can lead to the release of small gas moleculesthe phenomenon known as mechanically stimulated gas emission (MSGE)from metals,8−10 minerals,11,12 amorphous carbon,13 ionic liquids,14 and polymers.15−17 So far, the studies of MSGE have not dealt with compounds having large energy barrier for hydrogen desorption such as stable light hydrides. Although mechanical milling is widely applied for decreasing the desorption temperatures of light hydrides, this effect has usually been attributed to nanostructuring rather than to mechanical stress.3 This work is aimed at closer investigating into H2 desorption associated with tribochemical decom© XXXX American Chemical Society
Received: May 14, 2015 Accepted: June 30, 2015
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DOI: 10.1021/acs.jpclett.5b00998 J. Phys. Chem. Lett. 2015, 6, 2780−2785
Letter
The Journal of Physical Chemistry Letters
parameters (Figure 1A,B) but not linearly and tending to saturation (Table 1). Saturation can be linked to depletion of
equivalent) was determined from the measurements of the total pressure using an ionization gauge and following the procedure previously described.9 The amount of desorbed gases was obtained by integration of pressure time series,9 whereas the total mechanical energy was determined from integration of friction force over the sliding distance. After MSGE experiments the samples were brought to atmosphere, where newly formed Mg phase could react with air. Confocal-microRaman spectrometry was used for characterizing the chemical transformation induced in MgH2 due to mechanical deformation in vacuum, followed by oxidation in air. Massive gas desorption was observed at room temperature even under slight mechanical deformation through surface rubbing (Figure 1A). Under similar experimental conditions,
Table 1. Supplied Mechanical Energy (SME) and Quantity of Emitted Gas (H2 eq.)a Load, FN (N) Number of cycles, NC SME (mJ) Mode 1. Instantaneous emission Mode 2. Retarded emission Total a
0.21 7 4.11 38.1 (34%)
0.42 7 8.20 88.9 (37%)
0.42 50 60.4 213 (42%)
74.5 (66%) 113
154 (63%) 243
297 (58%) 510
Amounts are given in nmol and percentage of the total emission.
hydrogen in mechanically affected zone (MAZ) under repetitive deformation cycles. In addition, the amount of fresh material involved in MAZ in each cycle gradually decreases with the number of cycles, as can be seen in stabilization of the width of the deformed zone after certain number of cycle. The increase in SME also led to decrease in the time constant of desorption rate decay during the second desorption mode (Figure 2A).
Figure 1. MSGE from MgH2. (A) Emission rate (H2 eq.) at three different loads: 0.21, 0.42, and 0.63 N and seven sliding cycles. The inset shows slow desorption decay after the end of deformation. Lines are linear fit of the experimental data. (B) Evolution of the emission rate (H2 eq.) for 50 sliding cycles. Normal load 0.42 N. Figure 2. (A) Time constant of retarded emission as a function of the mechanical energy. (B) Confocal-microRaman x−z cross-section showing distribution of MgO and Mg(OH)2 (0.21 N, 7 cycles). (C) Confocal-microRaman x−z cross-section showing distribution of MgO and Mg(OH)2 (0.42 N, 50 cycles).
the rate of gas emission from MgH2 was much more intensive (2 to 3 orders of magnitude higher) as compared with pure metals containing interstitial dissolved H that afforded evidence of ionic hydride decomposition.19,20 Two desorption modes could be distinguished. The first one, quite fast (time constant
