Dissociation dihydrogen

Dihydrogen dissociates on the surface into hydrogen atoms that act independently. Mixtures of 50 50 H2 D2 produce equilibrated mixtures of H2 HD D2. Portals for dissociation-recombination are edges and corners. 

Homoexchange proceeds as a result of dihydrogen dissociation and allows to get information on the intensity of the latter. 

Recently [5], we already found differences in the dihydrogen dissociation rates between fresh and used samples at temperatures between 100 and 300°C and about 1 mbar hydrogen pressure. 

Now we measured the dihydrogen dissociation rate in situ and compared it with the DHC rate. To this end we had to choose experimental conditions, which led toan only low degree of conversion, in order to estimate the DHC... 

Fig. 7 Dependences of n-octane DHC and on the catalyst surface. dihydrogen dissociation on time on stream...

The comparison of the relative rates (related to the first n-octane conversion measurement 5 min after the DHC start) in Fig. 7 demonstrates clearly that the dissociation and DHC rates change to the same degree with time. The identical behaviour of DHC and dihydrogen dissociation during deactivation confirms our former considerations based on low pressure of line exchange experiments [5]. The deactivation process must be caused by a decreased number and/or activity of active sites responsible for the dehydrogenation. Increasing mass transport limitations are improbable. 

Our objective proved elusive in the early days of quantum chemistry. The major problem is electron correlation. As I have pointed out on several occasions through the text, the dissociation energy of dihydrogen into its lowest-energy products... 

The mechanism is dominated by the remarkable stability of the Fe( 72-H2) bond, which is one of the most stable 72-H2 complexes reported in the literature [8, 10]. Remarkably, the free coordination site for the incoming alkyne is provided by the reversible dissociation of one of the phosphine moieties of the PP3 ligand rather than dissociation of the dihydrogen ligand (see Scheme 14.1). The coordinated alkyne subsequently inserts into the Fe-H bond and the emerging Fe-vinyl bond is... 

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