Hydrogen in molecular form

Calculations were also performed for OL1H2 and OBH2, in order to determine whether similar minima existed for these molecules. The corresponding LiH and BH2 bind hydrogen in molecular form, just like BeH, while their hypersurfaces also have the unusual volcano form. 

Some metals such as Pd and Nb can dissolve hydrogen in atomic form in their lattice. For other metals as Cu, Ag, Au, Pt, Rh, this phenomenon is usually absent. If these latter metals contain traces of hydrogen (10-100 ppm, due to the production process), there is the formation of small gas bubbles with a typical diameter around 10 4mm [21]. The pressure of hydrogen, which is in the molecular form, inside the bubbles, is very high, and hydrogen becomes liquid or solid when the metal is cooled. Hence also in this case, a heat release due to the ortho to para conversion takes place [22,23]. The thermal release is of the order of 1 nW/g nevertheless it may be important in experiments at extremely low temperatures. 

Many transition metal complexes catalyze homogeneous activation of molecular hydrogen in solution, forming hydrido complexes. Such complexes include pentacyanocobaltate(II) anion, [Co(CN)5], many metal carbonyls, and several complexes of rhodium, iridium, and palladium. 

Transition metals can contain, within their coordination field, hydrogen in molecular (H2) and/or atomic (hydride) form. These types of metal-ligand complexes are unquestionably the most dynamic molecular systems known in terms of structural variability and atom motion/exchange processes. Until about 20 years ago, metals were known to contain only atomically bound hydrogen, that is, metal hydrides and metal hydride complexes (L MH,., where L is an ancillary ligand). However the discovery by Kubas and coworkers [1] in 1983 of side-on coordination of a nearly intact dihydrogen molecule (H2) to a metal complex has led to a new paradigm in chemistry that is the subject of many review articles [2-10]. 

Here we shall derive the rate equation for a general case where an organic molecule (A) adsorbs on a cluster consisting of m primary sites. The small molecule (B, typically hydrogen) adsorbs either in molecular form or dissociatively. 

A second assumption is that CO and dihydrogen absorb competitively in molecular form on the remaining surface sites. Because Sexton and Somorjai s temperature-programmed desorption studies (12) show that molecularly chemisorbed CO is desorbed rapidly from the rhodium surface only above 250°C at 10" Torr, virtual saturation of these associ-atively adsorbing sites by carbon monoxide can be assumed to occur at 300°C and 50 atm. The fact that molecular hydrogen does not compete... 

That portion of the hydrogen content of a ferritic steel which will not be released by diffusion at ambient temperature (more particularly, at 25 5 °C), but may be extracted at higher temperatures, e.g. 650 °C. It is believed that this hydrogen is trapped in molecular form, either in the lattice, in voids (particularly in association with non-metallic inclusions) or in chemical combination with other elements, such as carbon (when methane is formed). 

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