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Despite the small differences in activation energies, there is a
dramatic difference in absorption/desorption reaction rates that
is much greater than what would be expected due to the size
differences between the Mg nanocrystals. Figure 3 is a plot of the
relative H2 absorption rates for different size samples at each
temperature, relative to the largest samples, versus diameter.
The rates for 25 nm particles are over seven times higher than
those for 38 nm particles.19 This cannot be attributed to increas-
ing surface area or decreasing diffusion distance alone, since the
rate does not follow the expected inverse dependence on particle
diameter, even if the TEM estimate of 72 nm is assumed for
the largest particles. Defect sites have been cited as important
components of Mg-based materials to improving the kinetics
of H2 sorption.5,8 Hence, we hypothesize that there is an
increase in the density of defect sites formed through the
low-temperature solution synthesis described here, as the particle
size decreases.
(10) Paskevicius, M; Sheppard, D. E.; Buckley, C. E. J. Am. Chem.
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(11) Kalidindi, S. B.; Jagirdar, B. R. Inorg. Chem. 2009, 48, 4524.
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ꢀ
(16) Bogdanoviꢀc, B. Int. J. Hydrogen Energy 1984, 9, 937.
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(19) The less dramatic difference in desorption reaction rates
between the different samples may be due to agglomeration effects that
are likely to occur during desorption at higher temperatures.
We have demonstrated that the solution synthesis of Mg
nanoparticles with controlled size can provide a simple route to
dramatically enhanced H2 sorption kinetics. We anticipate that
the addition of low mass percent quantities of catalyst in the
future will act as grain-growth inhibitors, additionally providing a
route toward reducing the activation energies for absorption/
desorption in an effort to reduce the temperatures required.
’ ASSOCIATED CONTENT
S
Supporting Information. Additional X-ray powder dif-
b
fraction patterns of Mg nanocrystals, and Arrhenius plots with
notes on calculations for activation energies. This material is
’ AUTHOR INFORMATION
Corresponding Author
Present Addresses
†Lawrence Berkeley National Laboratory, Berkeley, CA.
‡Toyota Research Institute, Ann Arbor, MI.
’ ACKNOWLEDGMENT
We thank Colorado State University for start up funds and the
CSU Microscope Imaging Network (TEM).
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dx.doi.org/10.1021/ja201791y |J. Am. Chem. Soc. 2011, 133, 10679–10681