Study of Dissociative Hydrogen Adsorption on Lithium Oxide Terraces and Steps.

Hydrogen adsorption on the lithium oxide surfaces occurs due to local environment around adsorption site. Local environment encompasses the local coordination of oxygen and lithium sites in the surfaces, the thickness of the slab, and the adsorbate-adsorbate and step-step interactions. We have analyzed the effects of local environment using an analysis of the change in the density of states as a function of the change in the local environment. This turns out to be a very powerful way of providing an understanding of the surface reactivity. Dissociative hydrogen chemisorption on terraces and step structures of the rm Li_2O surface has been investigated with ab initio Hartree -Fock calculations. Terraces are atomically flat planes on a crystalline surface and may terminate at structures called "steps". Calculations on the unrelaxed (100), (110) and (111) rm Li_2O terraces show that except for the (100) terrace, the (110) and (111) terraces are stable. Results on the heterolytic sites of n-layer (110) (where n >= 2) surface and three-layer (111) surface suggest that dissociative hydrogen chemisorption is endothermic. For a one-layer (110) surface at 100% surface coverage, the chemisorption is exothermic, forming rm OH^- and rm Li^+H^-Li^+ bonds. Reactive oxygen site is mainly responsible for the occurrence of this chemisorption process due to its lower coordination (four-fold) on the surface. Density of states unambiguously show that the occupied O rm 2p_{z} band strongly interacts with the incoming dissociative hydrogen. This band which is perpendicular to the terrace constitutes the uppermost energy levels facilitating the chemisorption. On the other hand, the bands, O rm 2p_{x} and rm 2p_{y}, parallel to (110) terrace, do not participate in the chemisorption process. For the step structure type II, dissociative hydrogen chemisorption is also exothermic with the same final product as in the case of a one-layer (110) slab. On the homolytic sites of the (110) and (111) terraces or step structures, no hydrogen chemisorption is found to occur on the rm Li_2O surface. The tritium released in the form of HT and HTO can be qualitatively deduced as follows. According to our calculations, the hydrogen, which is presence in the helium purge gas, dissociates and is chemisorbed on the lithium oxides surface in the form of rm OH^- and rm Li^+H^ -Li^+. Initially, the reaction between tritium, rm T^+, and rm H^- from rm Li^+H^ -Li^+ is energetically favorable to occur. However, as irradiation proceeds, the lithium being converted into rm T^+ causes stoichiometric unbalance. Since rm Li_2O is a line compound, the only way to keep its stoichiometry is by releasing the rm OH^- in the form of HTO.