Thermodynamics adsorbed hydrogen

On the base of this approach thermodynamics of hydrogen absorbed outside and inside the (10,10) and the (20,20) single-wall carbon nanotubes with diameters 13.56 A and 27.13 A, respectively, was calculated. The dependencies of free energy F and thermodynamical potential H on applied pressure P and temperature T were calculated. The dependencies of content of hydrogen adsorbed on nanotubes m(P,T) surface on pressure and temperature were calculated from these data. For the first time the dependencies of m(P,T) with accounting of quantum effects and van der Waals forces were calculated. 

Fedorov, A.S., Ovchinnikov, S.G. (2004) Density and thermodynamics of hydrogen adsorbed inside narrow carbon nanotubes, Physics of Solid State 46(3), 584-589. 

Another observation that is difficult to reconcile with a mechanism of topside addition from adsorbed molecular hydrogen is the uniquely high yields of trans isomers obtained with palladium catalysts, i.e., about 85% frana-decalin from either Ai- -octalin or A i -octalin. Palladium, of all the transition metals, is the one deemed most likely to dissociate adsorbed hydrogen, and therefore least likely to involve molecular hydrogen in the saturation mechanism. On the other hand, palladium is one of the most effective transition metals for olefin isomerization and could isomerize A > -octalin to the thermodynamically unpreferred but rapidly saturating A > -octalin. 

The generation of hydrous films on platinum at low potentials on the anodic sweep seems quite feasible from a thermodynamic viewpoint—especially in base. The inhibition here is evidently related to the need for six hydroxide ions to have access to coordination sites at the same platinum atom—an improbable condition for a metal atom in a regular surface site. Evidence will be presented later indicating that such hydrous oxide formation can, to a very limited extent, precede monolayer formation in the case of gold— the atoms of the latter involved in formation of the hydrous material are presumably at low lattice coordination sites on the surface. There is some evidence from recent single-crystal studies (see Section XIV) for this type of behavior in the case of platinum—it is obviously difficult to detect with polycrystalline substrates as with only a small fraction of a monolayer involved optical techniques would need to be extremely sensitive and electrochemical procedures are hampered by the fact that the redox behavior of the hydrous material coincides with that for adsorbed hydrogen. 

In electrochemistry, adsorbed hydrogen is denoted as either the underpotentially deposited hydrogen, Hypd, that is the H adlayer formed under thermodynamic equilibrium conditions where the coverage is changed reversibly with the potential applied or the over-potentially deposited hydrogen. Hqpd. as defined by Conway and co-workers [102] for the... 

Figure 6.8. The overall Born-Haber thermodynamic cycle for the free energy required to create hydro-nium ions from adsorbed hydrogen in the presence of waterP d ].

The carbon anode showed an overpotential reaction, adsorbing hydrogen and preventing gas evolution until -0.65 V versus NHE. The proton absorption could block dihydrogen evolution until it became more thermodynamically feasible [83,86]. The reversible Mn02 oxidation reactions at the cathode show oxygen overpotential to 1.4 V versus NHE, which allows the device s potential window to be extended. 

During the stationary pseudo-equilibrium (h), adsorbed atomic hydrogen penetrates into the metal by diffusion. In the internal pores, it is transformed into hydrogen gas H2(g). For diffusion equilibrium, the activity u(h) of atomic hydrogen is the same all over the metal therefore, in its final state (h), there is thermodynamic equiUbrium between H2(g) in pores and adsorbed hydrogen H(ads)... 

In the initial state (g), a corresponding thermodynamic equilibrium was established between adsorbed hydrogen and H2(g) at the standard pressure p = 1 atm, i.e. 

However, this problem does not affect the thermodynamics of electrochemical reactions. The free energy of the solvated proton can be obtained from a thermodynamic argument, that of the adsorbed hydrogen atom from standard DFT, with or without a few water molecules. In this way, the free energy balance for the reaction can be calculated, and from this, the equilibrium potential can be obtained. The same principle can be employed for complicated reactions such as oxygen reduction, which contain many possible intermediate states. Chemical steps not involving charge transfer, such as the recombination reaction H2, can be treated by pure DFT, and for... 

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