Adsorbed dihydrogen

The rotational spectmm of undissociated dihydrogen molecules may be observed by INS on sulfide catalysts as on carbon-supported metal catalysts. On M0S2 a peak near 120 cm , assigned to the y(l —0) transition of the adsorbed dihydrogen molecule, was observed at 

On RuS2 dihydrogen was observed at 0.5 bar [130]. The peak was split into two components as a consequence of the anisotropic interaction of H2 molecules with RuS2. 

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Beside these three bands, an additional fourth, much broader, band is clearly visible at 4031 cm in the bold spectrum of Fig. 10a. In contrast to the previously assigned bands, this component, undoubtedly associated with a new molecularly adsorbed dihydrogen species characterized by an even greater weakening of the H—H bond, gradually decreased with time (solid curves. Fig. 10a). After 1 h of contact at 20 K, the 4031 cm component was totally eroded (dotted curve. Fig. 10a), demonstrating the transient nature of this fourth molecular H2 surface adduct. The progressive disappearance of the band at 4031 cm was accompanied by a... 

Physisorption involves only a weak attraction between the substrate and the adsorbent but in chemisorption a chemical reaction takes place between the adsorbent and atoms on the catalyst surface. As a result, chemisorbed species are attached to the surface with chemical bonds and are more difficult to remove. If the adsorption of hydrogen on nickel is considered as an example, the reaction involves the breaking of an H-H bond and the formation of two Ni-H bonds on the surface. As shown in Fig. 2.3, this adsorption occurs by way of an initially adsorbed dihydrogen molecule. It proceeds via a electron donation and back bonding to the a orbitals of the hydrogen molecule with the final formation of the two surface M-H species. 

AJ. Ramirez-Cuesta, P.C.H. Mitchell S.F. Parker (2001). J. Mol. Cat. A Chemical, 167, 217-224. An inelastic neutron scattering study of the interaction of dihydrogen with the cobalt site of a cobalt aluminophosphate catalyst. Two-dimensional quantum rotation of adsorbed dihydrogen. 

Kazansky V B (1999), Drift spectra of adsorbed dihydrogen as a molecular probe for alkaline metal ions in faujasites , J Mol Catal A Chem, 141, 83,... 

Sigl M, Ernst S, Weitkamp J and Kndzinger H (1997), Characterization of the acid properties of [Al]-, [Ga]- and [Fe]-HZSM-5 by low-temperature FTIR spectroscopy of adsorbed dihydrogen and ethylbenzene disproportionation , Catal Lett, 45, 27. 

It was noted that in the closed form there exists significant steric hindrance for the quinuclidine-iV lone pair to participate in interaction with pyruvate, whereas in the open conformation of the Cnd N-lone pair of quinuclidine is more readily accessible to the reactant. Scheme 5.26. shows the model of interaction of Cnd in the open conformation with methyl pyruvate as calculated by Schwalm et al. for the intermediate complex. Another important aspect is the role of adsorption of the intermediate complex on the Pt active center. In the intermediate complex the pyruvate molecule is bound to the modifier via stabilizing hydrogen bond interactions, N-H—O, between the protonated quinuclidine-N atom and the 0-atom of the alpha-cdx )ony group of pyruvate, or 0-H—N-bond for unprotonated system, such as for reactions in toluene solution. In this case, the H-atom can come from dissociatively adsorbed dihydrogen from the Pt-surface. The same... 

The IR spectra of Li-FER with adsorbed dihydrogen molecules showed a H-H stretching band located at 4090 cm" [06N1]. Three different types of Li site were found two of them adsorb H2. Periodic DFT calcirlatiorrs proved that orre of the hydrogen adsorbing sites is situated on the charmel wall and the other at the intersection of two perpendicirlar channels. These sites can adsorb one H2 and two H2 molecirles, respectively. The standard adsorption enthalpy of H2 in Li-FER was -4.1(8) kJ/mol. 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

The presence of a prominent mle = 4 peak not associated with any of the species proposed above could be due to a reaction between metallic Li and PC to yield lithium hydride. Although the same type of process was proposed for the reaction between THF and Li (see above), the temperature observed in this case seems too high for LiH decomposition. A more likely source of dihydrogen (or D2) in the TPD is the thermally induced dehydrogenation of one (or more) adsorbed reaction products. 

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