Catalytic sites hydrogen deficiency

Catalytic coke is strongly hydrogen deficient, has a well developed graphitic structure and is widely spread over the surface, thus covering up the active sites to a large extent. 

The term aging generally describes a loss in the activity—or selectivity—observed in a catalytic process after a certain period of reaction time. Aging may result from some change in the nature or number of catalyst sites, or in the accessibility of the sites to reactant molecules. Thus, such factors as formation of hydrogen-deficient organic residues ( coke ), selective adsorption of impurities from the charge ad-... 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

From the results published on the hydrogenation of benzene (29, 30, 31), it appears that ruthenium and rhodium are more active than palladium. By adapting the scheme proposed by Dalla Betta and Boudart (9), we could suppose that the electron-deficient character of palladium on oxidizing sites leads to an electronic configuration very similar to that of rhodium, and, thus, to an increase in catalytic activity. 

---