Adsorbate bond activation and formation

In the first sections of this chapter, we discuss the nature of the surface chemical bond of adsorbed atoms and molecules. Computational access to the transition states of surface reactions has given reactivity rules of surface reactions, which we discuss in later sections deahng with adsorbate bond activation and formation. 

The mechanism involves a metal atom insertion into the O—H bond, thus resulting in the formation of an adsorbed metal—OH species (at the same or similar binding site) and a new metal—H bond. This is a classic bond activation process, which involves a significant stretch of the O—H bond in order to lower the antibonding ooh orbital to enable it to accept electron density from the metal. The reaction has been calculated by DFT to be endothermic by +90 kJ/mol over Pt(lll) surfaces with an activation barrier of +130 kJ/mol [Desai et al., 2003b]. 

In all above mentioned applications, the surface properties of group IIIA elements based solids are of primary importance in governing the thermodynamics of the adsorption, reaction, and desorption steps, which represent the core of a catalytic process. The method often used to clarify the mechanism of catalytic action is to search for correlations between the catalyst activity and selectivity and some other properties of its surface as, for instance, surface composition and surface acidity and basicity [58-60]. Also, since contact catalysis involves the adsorption of at least one of the reactants as a step of the reaction mechanism, the correlation of quantities related to the reactant chemisorption with the catalytic activity is necessary. The magnitude of the bonds between reactants and catalysts is obviously a relevant parameter. It has been quantitatively confirmed that only a fraction of the surface sites is active during catalysis, the more reactive sites being inhibited by strongly adsorbed species and the less reactive sites not allowing the formation of active species [61]. 

EXAFS analysis provided structural parameters (bond distance and coordination number) for Cu-O and Cu-Cu. The small coordination numbers of the Cu-Cu (0.9) and Cu-O bonds (2.4) indicate that the hydrothermal synthesis prohibits the growth of Cu species and produced small Cu + -oxide clusters, which did not significantly change in size after the PROX reaction [90]. CO, 5.75 x 10 4 mol adsorbed on 1 g of Cu/Ce-CTAB (0.49 CO/Cu) was present, but no C02 formation was observed. The results indicate that neither the water-gas shift reaction nor CO oxidation with lattice oxygen proceeded on the Cu/Ce-CTAB catalyst. In contrast, with 2.40 x 10 4 mol 02 adsorbed on 1 g of the fresh Cu/Ce-CTAB catalyst (0.20 02/Cu) a stoichiometric amount of C02 (0.39 C02 per Cu) was produced when this surface was subsequently exposed to CO, which suggests the high oxidation activity of the Cu+-oxide cluster species on the Ce02 surface. XRF analysis showed that the small amount... 

If counter ions are adsorbed only by electrostatic attraction, they are called indifferent electrolytes. On the other hand, some ions exhibit surface activity in addition to electrostatic attraction because of such phenomena as covalent bond formation, hydrogen bonding, hydrophobic and solvation effects, etc. Because of their surface activity, such counter ions may be able to reverse the sign of because the charge of such ions adsorbed exceeds the surface charge. 

The results of the DFT calculations for various stable C2H.V species and transitions states on Pt(lll) and Pt(211) are summarized in Table V, which also shows entropy changes for the various steps, as estimated from DFT calculations of the vibrational frequencies of the various adsorbed C2H species and transition states on 10-atom platinum clusters (55). Table V also includes estimates of the standard Gibbs free energy changes for the formation of stable C2H surface species and activated complexes responsible for C-C bond cleavage at 623 K. These estimates were made by combining... 

In the CP-02 complex the CP surface is an electron density donor. For example, in the case of PANI the bond orders in adsorbed O2 molecules decrease by about 30%, and the bond lengths L increase by about 24%. So, the adsorbed O2 molecules have a fairly high degree of activation and can readily interact with the protons in a solution. Further calculations show that in such case H2O2 compound forms even inside of adsorption complex. So, it is not necessary to spent high additional energy for formation of hydrogen peroxide. 

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