Oxygen surface species characterization

Ammonia and pyridine are frequently used as probe molecules for the characterization of acidic surfaces, but they also adsorb on strongly basic sites. Tsyganenko et al. (54) proposed various species resulting from NH3 adsorption on basic solids (Scheme 1). The formation of species I corresponds to hydrogen bonding to a basic surface oxygen, and species II, formed by dissociation to give NH2 and hydroxyl species, involves an acid-base site. Such adsorption requires... 

Our group reported IR and UV-VIS spectra characterizing the adsorption of CS2 on MgO (197). Substitution of surface O2 ions by S2 was observed, resulting in a tailored modification of the basicity of the surface. The CS2 molecules adsorb on the low-coordinated oxygen anions, 02 /3c + CS2 -> [CS2O]2. The [CS20]2- surface species are decomposed in vacuo at 650 K... 

A number of oxides can be modeled by vacuum-grown thin oxide films and oxide nanoparticles which are amenable to atomic level characterization with the application of surface-sensitive (surface science) methods. This is crucial for the advancement of the field because in many cases oxide surfaces are not simple truncations of the bulk oxide structure. Defects such as oxygen vacancies, despite being minority surface species, play an important role in the catalytic properties of an oxide surface. In this chapter, we have presented results on the synthesis, characterization and catalytic properties of Ga Oj, In Oj, and V Oj, Nb Oj, Pd O and PdO vacuum-grown thin oxide films and oxide nanoparticles. The catalytic activity of the model systems can be examined at atmospheric pressure and compared to those... 

From the above discussion it becomes apparent that some conflicting experimental evidence exists on hydrocarbon adsorption and on surface intermediates. This arises primarily from the use of electrocatalysts of varying histories and pretreatments. It should be stressed that many adsorption studies were performed on anodically pretreated platinum. The removal of surfaces oxides from such electrodes may have not been always accomplished when the surface was cathodically reduced in some experiments, as outlined in Section IV,D. Obviously, different surface species could exist on bare or on oxygen-covered electrocatalysts. Characterization of surface structure and activity and of adsorbed species using modern spectroscopic techniques would provide useful information for fuel cell and selective electrocatalytic oxidations and reductions. 

The above discussion exemplifies the existing uncertainties and the conflicting postulates on surface intermediates of oxygenated carbonaceous species. It amplifies once more the need for a synthetic evaluation of such reactions on well-characterized, standard catalyst and electrocatalyst surfaces using a variety of techniques, possibly in situ. 

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