Activation energies for the desorption of H2, H− and electron from saline hydrides heated in vacuum

To study the thermodynamic and thermionic properties of saline hydride (MHn, n=1 or 2; M=Li, Na, Mg, Ca or Sr), powders (ca. 1 mg) were deposited on a molybdenum heater, and the currents of H− and electrons (e−) desorbed directly from the hydrides were measured in vacuum as a function of temperature (T) up to 1100 K at a constant rate. A dual mass spectrometer system, in which the H2 desorbed from MHn was converted into H2+ by electron impact, was used. The temperature-programmed desorption spectra for H−, e− and H2 were analyzed using a leading edge method. The following results were obtained: (1) each desorption became appreciable at an appearance temperature, which increased in the order of H2 (ca. 700 K) and both e− and H− (above ca. 800 K); (2) the activation energies (E0, Ee and E− in kJ mol−1) for the desorption of H2, e− and H− were as follows — LiH (95, 596, 660), NaH (60, 221, 173), MgH2 (91, 335, 318), CaH2 (87, 428, 612) and SrH2 (63, 354, 308); (3) as T increased from ca. 900 to 940 K in the case of LiH, Ee corresponding to the work function increased from ca. 580 to 600 kJ mol−1; (4) such an increase was caused by a decrease in the surface density of these active sites (e.g., Li) which were created by thermal decomposition (e.g., LiH→Li+12H2) and also which promoted the desorption of e− and H−.