Reactive surface intermediate species

The measurement of the amount of reactive surface intermediate species in order to quantitatively determine the density of surface active sites... 

Saturation of the Oxide Surface with Reactive Surface Intermediate Species... 

Metal alkylidyne fragments are frequently invoked as intermediates in the transformation of hydrocarbons on metal surfaces. These species are usually formulated as triply bridging alkylidynes however, terminal surface alkylidynes may be considered as reactive surface intermediates (30). Evidence for metal carbyne intermediates on Pt—Co bimetallic surfaces was found in a study of the isomerization and hydrogenolysis of alkanes (3]). 

A key question in selective oxidation catalysis is whether the reactive surface-oxygen species is used to activate C-H bonds or is used, instead, to aid in the addition of oxygen to the reactive surface intermediates to form oxygenated products. We analyze the results for studies carried out over both M0O3 or V2O5 surfaces. 

The authors observed that the surface coverage decreases with increasing temperature due to the increase on the reactivity of the surface intermediate species (typical results are presented in Figure 11.7). In fact, the authors demonstrated that the surface decomposition rate constant, Arrds, is directly influenced by the nature of the oxide support and accounts for the ligand effect. ... 

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

During the catalytic cycle, surface intermediates include both the starting compounds and the surface metal atoms. This working site is a kind of supramolecule that has organometallic character, and, one hopes, the rules of the organometallic chemistry can be valid for this supramolecule. The synthesis of molecular models of these supramolecules makes it possible to study the elementary steps of the heterogeneous catalysis at a molecular level. Besides similarities there are, of course, also differences between the reactivity of a molecular species in solution and an immobilized species. For example, bimo-lecular pathways on surfaces are usually prohibited. 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

From the work reported in literature it can be thus concluded that there will be various forms of carbonaceous species, which vary in reactivity, that exist on the catalyst or support during FTS. Some forms of this carbon are active (atomic surface carbide and CHX species) and even considered as intermediate species in FTS. However, it is also clear that especially during extended runs there may be a build up/transformation to less reactive forms of carbon (e.g., polymeric carbon). The amounts of these species may be small, but depending on their location, they may be responsible for a part of deactivation observed on cobalt-based FTS catalysts. The electronic interaction of carbon with the catalyst surface may also result in decreased activity. 

The assumption of reactive chemisorption may be useful for the surface intermediate of C5 cyclic reactions. It may well be possible that a competition occurs between a reactive and a dissociative chemisorption the former giving C5 the latter cyclic products. There is a thermodynamic relationship between these two surface species (see Section II,A,2). Scheme XIII summarizes all the above-mentioned facts about hydrogen effects and various surface intermediates (31). 

In addition, aided by profound knowledge of the nature and reactivity of some surface organometallic species, it was possible to identify the various steps and the nature of intermediates involved in the nucleation processes occurring on the surface in the selective growth of very large clusters such as for instance in the case of [Os5C(CO)i4] and [OsioC(CO)24] [52]. As this subject is treated in detail elsewhere in this book it is not covered here. 

Until now, the detailed mechanism involved in the MTG/MTO process has been a matter of debate. Two key aspects considered in mechanistic investigations are the following the first is the mechanism of the dehydration of methanol to DME. It has been a matter of discussion whether surface methoxy species formed from methanol at acidic bridging OH groups act as reactive intermediates in this conversion. The second is the initial C—C bond formation from the Ci reactants. More than 20 possible mechanistic proposals have been reported for the first C-C bond formation in the MTO process. Some of these are based on roles of surface-bound alkoxy species, oxonium ylides, carbenes, carbocations, or free radicals as intermediates (210). 

The existence of several adsorbed states of an olefin on metal surfaces is shown by infrared spectroscopic studies [68]. This technique has the advantage that it yields direct information regarding the chemical identity of the various adsorbed species, although there are limitations to its use. One of the main limitations is that the presence of surface intermediates may not be revealed if the appropriate band intensities are too weak [69]. In this context, it has been suggested [70] that the C—H bands associated with carbon atoms which are multiply bonded to the surface are too weak to be observed. Pearce and Sheppard [71] have also proposed the operation of an optical selection rule, similar to that found with bulk metals [72], in determining the bands observed with adsorbed species on supported metal catalysts. In spite of these limitations, however, the infrared approach has contributed significantly to the understanding of the nature and reactivity of adsorbed hydrocarbons. 

---