Determination of diffusion coefficients of hydrogen in fused silica between 296 and 523 K by Raman spectroscopy and application of fused silica capillaries in studying redox reactions

Diffusion coefficients (D) of hydrogen in fused silica capillaries (FSC) were determined between 296 and 523 K by Raman spectroscopy using CO2 as an internal standard. FSC capsules (3.25 x 10(-4) m OD, 9.9 x 10(-5) M ID, and similar to 0.01 m long) containing CO2 and H-2 Were prepared and the initial relative concentrations of hydrogen in these capsules were derived from the Raman peak-height ratios between H2 (near 587 cm(-1)) and CO2 (near 1387 cm(-1)). The sample capsules were then heated at a fixed temperature (T) at one atmosphere to let H-2 diffuse out of the capsule, and the changes of hydrogen concentration were monitored by Raman spectroscopy after quench. This process was repeated using different heating durations at 296 (room T), 323, 375, 430, 473, and 523 K; the same sample capsule was used repeatedly at each temperature. The values of D (in m(2) s(-1)) in FSC were obtained by fitting the observed changes of hydrogen concentration in the FSC capsule to an equation based on Fick's law. Our D values are in good agreement with the more recent of the two previously reported experimental data sets, and both can be represented by: In D = -(16.471 +/- 0.035) - 44589 +/- 139/RT (R-2 = 0.99991) where R is the gas constant (8.3145 J/mol K), Tin Kelvin, and errors at 1 sigma level. The slope corresponds to an activation energy of 44.59 +/- 0.14 kJ/mol. The D in FSC determined at 296 K is about an order of magnitude higher than that in platinum at 723 K, indicating that FSC is a suitable membrane for hydrogen at temperature between 673 K and room temperature, and has a great potential for studying redox reactions at these temperatures, especially for systems containing organic material and/or sulphur. (C) 2009 Elsevier Ltd. All rights reserved.