Hydrogen Transfer on Metal Surfaces

While one could further expand greatly on the specifics of H bonds, one of their prominent characteristics is that the H atom involved can easily be transferred from one electronegative heteroatom (nitrogen, oxygen, chlorine...) to another, and the question arises as to how this transfer process proceeds and where the H atom is actually located or which electronegative center it is associated with. 

Hydrogtn-Tmnsfer Reactions. Edited by J. T. Hynes, J. P. Klinman, H. H. Limbach, and R. L. Schowen Copyright 2007 WILEY-VCH Veriag GmbH Co. KGaA, Weinheim ISBN 978-3-527-30777-7 

Sparsely H-covered surface dashed line fully H-covered surface (consideration of coverage-dependent interaction potentials, cf Fig. 25.5) 

A brief outline as to how this chapter is organized may be helpful. In our attempt to consider H bonding and related effects on and at metal surfaces we will largely exclude the aforementioned complications such as surface reconstruction, subsurface-site population or hydrogen sorption effects, since they may obscure the essential H transfer and bonding phenomena. 

The Principles of the Interaction of Hydrogen with Surfaces Terms and Definitions 

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Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

Catalytic hydrogenation takes place on the surface of the metal. The metal must therefore be finely divided, and is usually dispersed on the surface of an inert support. This is what Pd/C means—finely divided palladium carried on a charcoal support. The first step is chemical absorption of hydrogen onto the metal surface, a process that results in breakage of the H—H bonds and distributes hydrogen atoms where they can react with the organic substrate. Now the alkene can also bond to the metal, and hydrogen can be transferred from the metal to the alkene. 

Similar mechanisms are postulated for commercial alkene/arene, carbonyl and nitrile hydrogenations on metal surfaces in particular, individual metal atoms are involved. In contrast hydrogenolysis, the cleavage of C—C or C—O (N, S, etc.) bonds, appears to need two or more adjacent sites and can sometimes be reduced by alloying the main component (addition of copper to nickel, for example). The stability of supported metal (especially platinum) catalysts permits their use at high temperatures, to promote hydrogen transfers between alkanes, alkenes and arenes or dehydrogenation processes. 

Under anaerobic conditions, it is assumed that the protective effect is linked to the ability of microorganisms to transfer electrons and protons to and from the metal surface. This may lead to the formation of a passivating layer of adsorbed atomic hydrogen (H ) on the surface. In contrast to detrimental organisms such as SRB, these bacteria act as anode and the metal as cathode. It was found that SRB develop very slowly under such conditions [14]. However, the exact mechanism of electron transfer between cells and metal surfaces is still not fully understood [1]. 

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Polymerization occurs at active sites formed by interaction of the metal alkyl with metal chloride on the surface of the metal chloride crystals. Monomer is chemisorbed at the site, thus accounting for its orientation when added to the chain, and propagation occurs by insertion of the chemisorbed monomer into the metal—chain bond at the active site. The chain thus grows out from the surface (31). Hydrogen is used as a chain-transfer agent. Chain transfer with the metal alkyl also occurs. 

Hydrogen evolution at metal electrodes is one of the most important electrochemical processes. The mechanisms of the overall reaction depend on the nature of the electrode and solution. However, all of them involve the transfer of proton from a donor molecule in the solution to the adsorbed state on the electrode surface as the first step. The mechanism of the elementary act of proton transfer from the hydroxonium ion to the adsorbed state on the metal surface is discussed in this section. 

Transfer hydrogenation in the alcohol-ketone system on metal catalysts was investigated by Patterson et al. In particular, by studying the reaction between 2-propanol and butanone on Cu they concluded that it must be a direct surface reaction (11), the mechanism being essentially a proton transfer in the adsorbed phase (Scheme 2). 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

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