Thermochimica Acta, Vol.371, No.1-2, 155-161, 2001
Activation energies for the desorption of H-2,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 H-2 desorbed from MHn, was converted into H-2(+) by electron impact, was used. The temperature-programmed desorption spectra for H-, e(-) and H-2 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 H-2 (ca. 700 K) and both e(-) and H- (above ca. 800 K); (2) the activation energies (E-0, E-e and E- in kJ mol(-1)) for the desorption of H-2, 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, E-e 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 + 1/2H(2)) and also which promoted the desorption of e(-) and H-.