Hydrogen dihydride desorption

Absorption of hydrogen by titanium metal above 400 °C gives a solid whose stoichiometry approaches TiH2, but a true dihydride does not appear to exist. This hydride has been used for the formation of glass-to-metal and ceramic-to-metal seals.1 Thermal desorption above 600 °C provides a source of very pure hydrogen.2... 

Gupta, P., Colvin, V.L. and George, S.M. (1988) Hydrogen desorption kinetics from monohydride and dihydride species on silicon surfaces , Phys. Rev. B 37, 8234. 

Further hydrogen adsorption causes rupture of the Si—Si bond between surface atoms with a hydrogen atom bonded to each additional dangling orbital so produced to give the dihydride phase. This is the sequence for room temperature adsorption. Thermally stimulated desorption from this dihydride phase occurs by the association of adjacent H atoms to give H2 as the desorption product, the surface phase remaining being the monohydride. Adsorption at a substrate temperature of 500 K produces only the monohydride phase, but if the substrate is cooled to room temperature in the presence of H, it is converted completely to the dihydride. 

These predictions are consistent with the experimental value of 58 kcal/ mol for the activation barrier to desorption, but it permits only a small barrier for the reverse adsorption reaction. Nachtigall et al. [75] pointed out that if there is a substantial activation barrier to adsorption, these calculations could not be consistent with desorption from a prepaired state. They suggested that a more complex, multistep mechanism might be responsible for desorption. For example, they drew an analogy with gas-phase elimination of H2 from disilane, in which the 1,1-elimination mechanism has a lower activation barrier than the 1,2-elimination [75]. They suggested that desorption may occur by a 1,2-hydrogen shift (to form a dihydride) followed by 1,1-elimination (desorption of the dihydride). This speculation was supported... 

For a substrate surface heated at -2°C per second, most of the dihydride surface will desorb H2 and convert to a monohydride surface at -400°C, while the monohydride will desorb H2 and yield a clean surface at -520°C. Slower heating or a static anneal will allow to desorption at lower temperatures if sufficient time is available. Hydrogen-terminated Si (100) surfaces can also be created Ifom normal air-exposed oxidized surfaces by dipping the wafer in HF. The strength of the Si-H bond is illustrated by the observation that this surface is stable in laboratory air under standard conditions for several minutes to several hours. 

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