Bonding adsorption energy

A term that is widely used (and sometimes abused) in discussions about metal-water interactions is hydrophilicity. By this term is meant the strength of interaction between a metal surface and water molecules in contact with it, and the term usually implies chemical bond strength. However, there is a problem with the way hydrophilicity scales are built up. Various quantities (capacitance, adsorption energy, etc.) are used to rank the metals, and the hydrophilicity scale may differ for different parameters. 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Whereas the adsorption energies of the adsorbed molecules and fragment atoms only slightly change, the activation barriers at step sites are substantially reduced compared to those at the terrace. Different from activation of a-type bonds, activation of tt bonds at different sites proceeds through elementary reaction steps for which there is no relation between reaction energy and activation barrier. The activation barrier for the forward dissociation barrier as weU as for the reverse recombination barrier is reduced for step-edge sites. 

Upon adsorption, there is again a strong interaction of the 5a and 2jt orbitals and the metal sp electrons, resulting, as above, in a downward shift and broadening of these two levels. Also, in this case the variation of the adsorption energy is accounted for by the interaction with the d band of the metal, which will cause the levels to split into bonding and antibonding parts. The result is shown in Fig. 6.32, which should be seen as a realistic alternative to the more qualitative representation of Fig. 6.25. 

With respect to the adsorption energy, the interaction of the 5cr and 2jt orbitals with the sp band again gives a large and negative (i.e. stabilizing) contribution, E p, to the bond. The hybridization Afj.hyb can be estimated in a similar way to that in the case of atomic adsorbates. 

As mentioned above, interaction with water may affect the adsorption energy, especially for species that form hydrogen bonds. The most accurate way of including the effect of water is to explicitly add water molecules into the simulations. At the temperatures and pressures relevant for an electrochemical experiment, the water-containing electrolyte will be liquid. However, since in this context we are mainly interested in the effect of water on adsorption energies and not so much the actual structure of hquid water itself, we can probably simplify the problem. 

The CO adsorption energy (the energy required to break the Pt-CO bond), ads(CO/Pt), was related to ASCLS by the following equation [Tregha et al., 1981 ... 

Depending on the difference in adsorption energies (see Section 5.4) dinitrosyl complexes are formed either concomitantly or subsequently with the mononitrosyl complexes. Those processes have been widely investigated for selected TMIs and can be followed easily by IR technique [57], The appearance of a characteristic doublet due to the collective antisymmetric and symmetric vibrations of the M(NO)2 moiety growing at the expanse of the NO valence band is usually taken as a confirmation of the dinitrosyl formation. As discussed below in more detail, they play important role in the inner-sphere route of the N—N bond making (see Section 6.2.1). 

When the calculations were repeated for copper, it was found that essentially all the bonding is due to the metal 4 s orbitals. The binding energy is close to the value found for nickel (experimental adsorption energy = 2.4 eV). 

Due to the presence of low-temperature desorption peak a new desorption site was included to phenomenological model of TPD experiments previously used for the description of the Cu-Na-FER samples [5], The fit of experimental TPD curves was performed in order to obtain adsorption energies and populations for individual site types sites denoted A (A1 pair), B (sites in P channel (A1 at T1 or T2)), C (sites in the M channel and intersection (A1 at T3 or T4)) [3] and D (newly introduced site). The new four-site model was able to reproduce experimental TPD curves (Figure 1). The desorption energy of site D is cu. 82 kJ.mol"1. This value is rather close to desorption energy of 84 kJ.mol"1 found for the site B , however, the desorption entropy obtained for sites B and D are rather different -70 J.K. mol 1 and -130 J.K. mol"1 for sites B and D , respectively. We propose that the desorption site D can be attributed to so-called heterogeneous dual-cation site, where the CO molecule is bonded between monovalent copper ion and potassium cation. The sum of the calculated populations of sites B and D (Figure 2) fits well previously published population of B site for the Cu-Na-FER zeolite [3], Because the population of C type sites was... 

NEP and PVP. In the polymer where monomers are linked in a chain, hydrophobic parts are largely screened from interactions with the solvent. For free monomers such screening is not possible so that they experience more unfavourable interactions with the solvent. The adsorption energy parameter xs is not affected by the different chemical surrounding of free monomer and polymer segments, since the mechanism for interaction with the surface is hydrogen bonding in both cases (8).)... 

Molecular modeling work performed by Sasol researchers on fee cobalt (100) shows that increased coverage of 50% atomic carbon will induce a clock type reconstruction (Figure 4.3) similar to that observed for the classic case of Ni (100).28 The adsorption energy of the carbon is stabilized by 15 kJ/mol compared to the unreconstructed surface, resulting in a more stable surface.28 The reconstruction results in a shorter distance between the carbon and cobalt but also an increase in coordination of the cobalt atoms and, thus, fewer broken bonds. The barrier for the carbon-induced clock reconstruction was found to be very small (1 kJ/mol), which suggested that the process is not kinetically hindered. The... 

TDS also gives information on the strength of the bond between adsorbate and substrate. An important check is obtained from desorption of Ag from a thick layer Here the activation energy of desorption should be equal to the heat of vaporization of Ag, 254 kJ/mol. Of course, the more interesting information is in the adsorption energies of Ag on Ru. This requires quite a bit of effort as we shall see. 

The difference in adsorption energy between a single Ag atom on Ru(001), 240 kJ/mol, and an Ag atom at the edge of an island, 290 kJ/mol, may be considered as the two-dimensional heat of Vaporization of Ag on Ru, and amounts to about 50 kJ/mol. Thus, the desorption energy depends not only on the strength of the bond between adsorbate and substrate, but also on interactions between the adsorbate atoms. Both contributions can be estimated from TDS. 

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