Thermal desorption of H2, H− and electron by temperature-programmed heating of saline hydrides in vacuum

To clarify the thermochemical and thermionic properties of saline hydrides, a small amount (ca. 1 mg) of powdery NaH or LiH deposited on a molybdenum ribbon was heated up to ca. 1000 K either stepwise at ca. 10 K intervals or continuously at a constant rate (ca. 2–20 K/s) in vacuum (ca. 10−4 Pa), and the desorption rates of H2, electron (e−) and/or H− were measured mass spectrometrically as a function of the sample temperature (T), the introduced hydrogen gas pressure (PH) or the time (t) after a change in T or PH. Theoretical analysis of the data thus achieved yields the following results: (1) In the temperature-programmed desorption spectra observed with NaH, both e− and H− showed a single peak at ca. 800 K while a broad peak of H2 appeared around ca. 750 K. (2) The activation energies (E− and E0) for the desorption of H− and H2 from NaH were 172 ± 18 and 61 ± 7 kJ/mol, respectively, whilst the work function (φ) of NaH at those temperatures corresponding to the leading edge of an electron desorption peak was 261 ± 19 kJ/mol. (3) In the case of LiH, E−, E0 and φ were 940 ± 89, 97 ± 12 and 747 ± 41 kJ/mol, respectively. (4) By the thermal dissociation such as LiH(solid) → Li(solid) + H2(gas)/2, φ was decreased by 20 kJ/mol or much more depending upon t or T, but the active spots (mainly Li) thus produced was destroyed by admission of H2 up to ca. 10−1 Pa. (5) The deactivation (Li + H2/2 → LiH) depending upon both T and PH was readily recovered (reactivated) after stopping the admission. (6) Our new method of monitoring e− was very convenient and useful for studying the thermal decomposition of saline hydrides.