Surfaces interaction with hydrogen

The last vertical column of the eighth group of the Periodic Table of the Elements comprises the three metals nickel, palladium, and platinum, which are the catalysts most often used in various reactions of hydrogen, e.g. hydrogenation, hydrogenolysis, and hydroisomerization. The considerations which are of particular relevance to the catalytic activity of these metals are their surface interactions with hydrogen, the various states of its adatoms, and admolecules, eventually further influenced by the coadsorbed other reactant species. 

Rare earth metal surface interaction with hydrogen... 

Indirect methods used can profit by the thermodynamic data of a particular metal-hydrogen system. The determination of the H/Me ratio after complete desorption of hydrogen from a sample, despite an apparent simplicity of the method, gives adequate results only when the bulk metal sample was entirely saturated with hydrogen, and that is a very rare case. The metal catalyst crystallites can be saturated in a nonuniform way, not through their whole thickness. The surface of this polycrystalline sample varies to such extent in its behavior toward interaction with hydrogen that hydride forms only in patches on its surface. A sample surface becomes a mosaique of /3-hydride and a-phase areas (85). 

Even the precovering with hydrogen is able to block the surface against electronic interaction. In Fig. 27 a nickel surface was precovered with hydrogen at 3 X 10 mm. Hg to saturation (0 = 0.39), causing an irreversible resistance decrease of 1%. After pumping off at B, carbon monoxide of 6 X 10 mm. Hg was added at Aco- At a pure nickel surface the carbon monoxide influence would have effected an increase of the resistance by 0.8%. At the surface precovered with hydrogen, neither a resistance effect nor a carbon monoxide adsorption is to be observed. 

Surface interactions with Adsorbed Moiecuies Surface hydroxyls can interact with highly polar O, N, and S atoms by means of hydrogen bonding... 

In this reaction, the catalyst surface interacts with the hydrogen molecule and in doing so weakens the H-H bond. The bond is then broken and the hydrogen atoms are bound to the catalyst. The catalyst can then interact with the alkene molecule, so weakening the pi bond of the double bond. The hydrogen atoms and the alkene molecule are positioned on the catalyst conveniently close to each other to allow easy transfer of the hydrogens from catalyst to alkene. The alkane product then departs, leaving the catalyst as it was before the reaction. 

The data points to the importance of two parameters, that of adhesive interactions between adsorbate and surface, and lateral interactions between adsorbate molecules on the surface. In general, the free energy of adsorption (at zero coverage) is of the order of 10-30 kJ mol for alcohols, carboxylic acids and esters on ferrous surfaces, consistent with hydrogen bonding between surface and adsorbate. 

The larger polymer stabilized particles also seem to be able to accommodate hydrogen on a large fraction of its surface (88%). In contrast, small particles have only about 40% of their surface atoms available to interact with hydrogen. 

ESR spectra as well as additional experiments with very pure alumina (steaming, treatment with diluted acids, interaction with hydrogen sulfide) indicate that at least two factors can influence hydrogenation activity of USY zeolites obtained via steaming contamination of parent material with Fe ions and/ or formation of new active phase of altimina inside supercages. The catalytic sites are probably coordinatively unsaturated surface species, which are poisoned by carbonaceous deposits formed as the result of prolonged interaction with olefins. 

A. Noordermeer, G. A. Kok, B. E. Nieu-wenhuys, A comparative study of the behaviour of the PdAg(lll) and Pd(lll) surfaces towards the interaction with hydrogen and carbon monoxide. Surf. Sci. 1986, 165, 375-392. 

A similar i.r. study of the interaction of secondary and tertiary amides with the surface of a silica has been carried out. It has been shown that the free —OH group of the surface interacts with the carbonyl group of the amide function, and the NH group of the secondary amide is not bound to the surface. The observed frequency shifts show that the hydrogen-bonds to... 

As many studies of gas-solid surface reactions involving rare earth inter-metallics are in some sense or other related to hydride formation, the interaction with hydrogen shall be of central interest in this section. The interaction of hydrogen with the intermetallic compounds under consideration can be represented schematically by the following sequence of processes ... 

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