Dissociation hydrogen molecule

Note that this is the equilibrium constant for one hydrogen molecule dissociating into two atoms. Had we defined the process as being half a molecule of H2 forming one H atom we would have had an equilibrium constant that was the square root of the one given by Eq. (80). 

On the other hand, the hydrogen molecule dissociates on active sites in either a homolytic or heterolytic way, and the reactivity or selectivity of the dissociated hydrogen atom with olefins is strongly influenced by whether the dissociation mode is homolytic or heterloytic. 

Further we studied ability of chemical hydrogen adsorption on a SWCNT surface by hydrogen molecule dissociation to atoms. With the help of LDA calculations it was found that energy of thermal dissociation of H2 molecule changed insignificantly at molecule movement to SWCNT or fullerene surface. This energy had a value about 6 eV. Therefore the probability of hydrogen chemisorption on SWCNT surface is very low at common temperatures without a catalyst influence. 

It has been generally accepted that hydrogen molecules dissociate into two hydrogen atoms when they dissolve into metals,9 Figure 1 illustrates the periodic supercell models for hydrogen occupation at interstitial sites in bee (V, Nb, Ta, Cr, Mo and W) and fee (Ni, Pd, Pt, Cu, Ag and Au) metals. The models consist of four metal atoms and one hydrogen atom (H/M = 0.25). The initial positions of metal atoms and the initial values of lattice constants were those from experimental values of pure metals. The initial position of the hydrogen atom was either the octahedral site (O-site) or the tetrahedral site (T-site). 

Carbon monoxide is produced from fuel processing of hydrocarbons, e.g. reforming, and is strongly adsorbed as a surface metal carbonyl, inhibiting the dual-site hydrogen molecule dissociation step. 

These trends explain the important fact that H2 dissociates on the group VIII metals, hydrogen molecule dissociation is activated on Cu, but will not dissociate on Ag or Au. 

I. Langmuir (48) showed that if hydrogen gas were introduced into a bulb containing a heated tungsten filament, and the walls cooled in liquid air, a continuous decrease in pressure was observed. He supposed hydrogen molecules dissociated on the filament to give chemisorbed atoms, which evaporated and were trapped on the cold walls. 

The third class of hydrides is the metallic, or interstitial, hydrides, which are formed when transition metal crystals are treated with hydrogen gas. The hydrogen molecules dissociate at the metal s surface, and the small hydrogen atoms migrate into the crystal structure to occupy holes, or interstices. These metal-hydrogen mixtures are more like solid solutions than true compounds. Palladium can absorb about 900 times its own volume of hydrogen gas. In... 

Amorphous metals do not have the high free volumes found in oxide and organic glasses. Their structures are so dense that the inert gases cannot pass through the doorways between interstices. Since hydrogen molecules dissociate on absorption into metals, they can diffuse as very small protons through these materials. The permeability of amorphous metals, just as for crystalline metals, appears to be primarily controlled by the nature of oxide films on their surface. 

Indeed, recent studies of Kay et al. [30, 31] showed that the probability of hydrogen molecule dissociation on the surface of palladium is close to unit. This circumstance excludes that this step is the slow step of the absorption process. By thermal desorption measurements, the above-mentioned authors argued that the phenomenon speed is governed by atomic hydrogen diffusion from the surface towards the massive phase. 

When dissolving into a metal, hydrogen molecules dissociate into atoms ... 

While hydrogen molecule dissociates on the catalyst surface, two H atoms used for hydrogenation of one reactant molecule originate from one hydrogen molecule. 

---