Adsorbed intermediate structure, selectivity

Apart from the above mentioned redox type reactions, we like to consider (in connection with work to be published by us elsewhere) another type of relaxations, due to the possible reorganisations of sorption intermediates on the catalyst surface, as suggested by some investigations in our laboratory. This structuring on the catalyst surface is equivalent to a change in the entropy of the system catalyst surface / adsorbed intermediates and seems to be responsible e.g. for the selectivity change under transient conditions in the oxidation of hydrocarbons. Actually this structural organization of the surface intermediates is also a rate process which can be observed under transient conditions. 

CO2 production time series, 39 88 equation structure, 39 87-88 Kurtanjek s mechanism, 39 91 oxide models, 39 89-92 subsurface oxygen model, 39 90-91 selective, 30 136-137 small organic molecules, chemical identity of adsorbed intermediates, 38 21 states... 

In the investigation of hydrocarbon partial oxidation reactions the study of the factors that determine selectivity has been of paramount importance. In the past thirty years considerable work relevant to this topic has been carried out. However, there is yet no unified hypothesis to address this problem. In this paper we suggest that the primary reaction pathway in redox type reactions on oxides is determined by the structure of the adsorbed intermediate. When the hydrocarbon intermediate (R) is bonded through a metal oxygen bond (M-O-R) partial oxidation products are likely, but when the intermediate is bonded through a direct metal-carbon bond (M-R) total oxidation products are favored. Results on two redox systems are presented ethane oxidation on vanadium oxide and propylene oxidation on molybdenum oxide. 

Somorjai offers the picture presented in Table V. Although the order of is rather well established experimentally, there is no definitive theoretical explanation of it. However, we have already mentioned two factors, probably not independent of each other, which might be of importance here. A high hydrogenolytic selectivity seems to be related to the following factors (1) the ease with which a metal forms multiple bonds (159) and (2) the depth of the dehydrogenation of adsorbed intermediates at the given temperatures (216). The second factor is also supported by data reviewed by Tetenyi (222). Most likely, a third factor can also be added to the list (3) the extent and structure of the carbon layer on the surface. This third factor will be discussed further in Section V. 

Characterization is a central aspect of catalyst development [1,2], The elucidation of the structures, compositions, and chemical properties of both the solids used in heterogeneous catalysis and the adsorbates and intermediates present on the surfaces of the catalysts during reaction is vital for a better understanding of the relationship between catalyst properties and catalytic performance. This knowledge is essential to develop more active, selective, and durable catalysts, and also to optimize reaction conditions. 

Principal Adsorbent Types. Commercially useful adsorbents can be classified by the nature of their structure (amorphous or ciystalline). by the sizes of their pores (micropores, mesopores, and macropores), by the nature of their surfaces (polar, nonpolar, or intermediate), or by their chemical composition, All of these characteristics are important in the selection of the best adsorbent for any particular application. However, the size of the pores is the most important initial consideration because if a molecule is to be adsorbed, it must not be larger than the pores of the adsorbent. 

We have shown how the band structure of photoexcited semiconductor particles makes them effective oxidation catalysts. Because of the heterogeneous nature of the photoactivation, selective chemistry can ensue from preferential adsorption, from directed reactivity between adsorbed reactive intermediates, and from the restriction of ECE processes to one electron routes. The extension of these experiments to catalyze chemical reductions and to address heterogeneous redox reactions of biologically important molecules should be straightforward. In fact, the use of surface-modified powders coated with chiral polymers has recently been reputed to cause asymmetric induction at prochiral redox centers. As more semiconductor powders become routinely available, the importance of these photocatalysts to organic chemistry is bound to increase. 

Researchers attempted to find correlations between the composition of the unit cell and the selectivity for paraxylene. In this respect, D. Barthomeuf (9) proposed an approach based on Sanderson s intermediate electronegativity which allows us to estimate the basicity of zeolite oxygens, and hence the strength of the acid-base interaction between xylene molecules and zeolites. It should be noted that these calculations provide an insight into the interactions between the zeolite structure and the molecules at low loading only, i.e. when the interactions between adsorbed molecules are negligible. 

Adsorbed moleeules and intermediates at high pressures can be detected by vibrational speetroseopies provided they can differentiate between gas phase and surfaee signals. For example, Fig. 4 shows a (conventional) IRAS spectrum of CO at 50mbar on Pd(l 11) at 300 K (113,114). The signal of adsorbed on-top CO at approximately 2060 cm is nearly obscured by the rovibrational absorption spee-trum of the CO gas phase. In contrast, as shown below, SFG and PM-IRAS selectively probe the adsorbed surface species and thus provide surface-sensitive information, even in the presence of a gas phase. Investigations of the catalyst structure and composition under working conditions can be earried out by high-pressure (HP-) STM and (HP-) XPS, provided that the instruments are properly modified (9,115). 

Experimental results showed that carbonmineral composites are much better than others adsorbents, for example, the mineral one. A good selectivity of carbon materials made us to assume that it is a carbon substance is responsible both for selectivity and synergistic effect of adsorption too. From our point of view one of the reason of such a behavior could be specially organized carbon structures such as intermediate complexes (clusters), which possess peculiar electron properties only to them. Probably similariy toxic substances are adsorbed, such as phenols, cresols, quaiacol, aldehydes, polyatomic alcohols, ethers etc. (Table 1). 

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