Metal oxide-adsorbate interactions electron transfer

It is also interesting to consider charge-transfer models developed primarily for metal surfaces. There are clear parallels to the metal oxide case in that there is an interaction between discrete molecular orbitals on one side, and electronic bands on the other side of the interface. The Newns-Anderson model [118] qualitatively accounts for the interactions between adsorbed atoms and metal surfaces. The model is based on resonance of adatom levels with a substrate band. In particular, the model considers an energy shift in the adatom level, as well as a broadening of that level. The width of the level is taken as a measure of the interaction strength with the substrate bands [118]. Also femtosecond electron dynamics have been studied at electrode interfaces, see e.g. [119]. It needs to be established, however, to what extent metal surface models are valid also for organic adsorbates on metal oxides in view of the differences between the metal an the metal oxide band structures. The significance of the band gap, as well as of surface states in it, must in any case be considered [102]. 

In addition to the acidic and basic properties mentioned previously, oxides and halides can possess redox properties. This is particularly true for solids containing transition metal ions because the interactions with probe molecules such as CO, H2, and O2 can lead to electron transfer from the surface to the adsorbed species and to the modification of the valence state of the metal centers. An important role in surface redox processes involving CO is played by the most reactive oxygen ions on the surface (e.g., those located at the most exposed positions such as corners), which can react with CO as follows ... 

While the specific surface area of carriers is often determined—as we have seen above—by using physical adsorption, the active phase of the catalysts (metal, oxide, sulphur) can be studied by selective chemisorption (with no support interaction) of an adsorbate under conditions of pressure and temperature permitting the formation of a single layer on the surface of the metal. During chemical adsorption, there is a chemical reaction between the gas molecule and the active phase, which is represented by the transfer or sharing of electrons. 

It is also interesting to note that the rates on the niobia-covered and the titania-covered surfaces are identical within the accuracy of our experiment. Since titania and niobia are chemically different, this similarity suggests that the effect these oxides have on the activity is not due to electronic interactions. While XPS results suggest some bonding occurs between the adsorbed titania and the metal, one would expect the amount of electron transfer to be different for niobia therefore, we do not believe that electron transfer is the... 

Electron transfer from metal oxide surfaces to CO can be quite facile, occurring at room temperature. This process can be important as an initial CO activation step in metal oxide catalyzed reduction schemes. We have attempted to clarify what types of metal oxides interact (MO CO MO. . . CO -) with CO in this way, and what surface features these active metal oxides possess. Only MgO, CaO, SrO, BaO, and Th02 were electron transfer active. These oxides have in common the possession of both Lewis basic sites and one electron reducing site. It appears that CO is first adsorbed on Lewis base sites followed by slow migration to electron transfer reducing sites. The studies leading to this conclusion are discussed. 

Chemisorption may also proceed by a mechanism involving an electron transfer between the adsorbate and the substrate (oxidation- reduction or redox interaction). It is the case for adsorbates such as O2 or CI2 that are strong electron acceptors. O2 can be molecularly or dissociatively adsorbed, CI2 is dissociatively adsorbed. The redox reactions that involve electronic carriers are expected to occur preferentially on semiconducting or metallic oxides. On wide-bang-gap insulators these reactions are promoted by surface defects such as ion vacancies, which may act as sources or sinks for electrons. 

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