Activated surface complex

The intennediate species Fe(OFi) and FeOFi can be regarded as constituting tire activated surface complex. The rate constant for fonnation and decay of tlris complex, k, can be written as... 

However, we have to reflect on one of our model assumptions (Table 5.1). It is certainly not justified to assume a completely uniform oxide surface. The dissolution is favored at a few localized (active) sites where the reactions have lower activation energy. The overall reaction rate is the sum of the rates of the various types of sites. The reactions occurring at differently active sites are parallel reaction steps occurring at different rates (Table 5.1). In parallel reactions the fast reaction is rate determining. We can assume that the ratio (mol fraction, %a) of active sites to total (active plus less active) sites remains constant during the dissolution that is the active sites are continuously regenerated after AI(III) detachment and thus steady state conditions are maintained, i.e., a mean field rate law can generalize the dissolution rate. The reaction constant k in Eq. (5.9) includes %a, which is a function of the particular material used (see remark 4 in Table 5.1). In the activated complex theory the surface complex is the precursor of the activated complex (Fig. 5.4) and is in local equilibrium with it. The detachment corresponds to the desorption of the activated surface complex. 

Surface Reactions. As we have seen from the dissolution of oxides the surface-controlled dissolution mechanism would have to be interpreted in terms of surface reactions in other words, the reactants become attached at or interact with surface sites the critical crystal bonds at the surface of the mineral have to be weakened, so that a detachment of Ca2+ and C03 ions of the surface into the solution (the decomposition of an activated surface complex) can occur. 

Aagaard and Helgeson (1982) and Murphy and Helgeson (1989) propose general-rate expressions for the dissolution of silicate minerals. They suggest that the solution rate in acid water is determined by the decomposition rate of a critical activated surface complex. The rate is given by... 

In terms of transition-state theory, the rate of the surface reaction can be related to that of decomposition of an activated surface complex. The rate of production of this activated complex is rapid enough to assume that it is in equilibrium with the reactants responsible for its formation. In homogencoiM systems it is usually easy to identify the nature of the elementary steps leading to... 

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

The following assumptions are made (i) the activated complexes are in equilibrium with the reactants, (ii) the energy of a molecule is not altered when an activated complex is substituted for a nearest neighbour, and (iii) the products do not affect the course of reaction, except to define a boundary in surface processes. The various cases can be recognized from the magnitude of the pre-exponential term and calculated values [515] are summarized in Table 7. Low values of A indicate a tight surface complex whereas higher values are associated with a looser or mobile complex. 

One key aspect of SOMC is the determination of the structure of surface complexes at a molecular level one of the reasons being that our goal is to assess structure-activity relationships in heterogeneous catalysis, which requires a firm characterization of active sites or more exactly active site precursors. While elemental analysis is an essential first step to understand how the organometallic complex reacts with the support, it is necessary to gather spectroscopic data in order to understand what are the ligands and... 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

The results of the above-mentioned Langmuir analysis of the SHG responses may be interpreted in terms of a tightly packed monolayer of the SHG active cation complexes at the membrane surface. The tight layer may, however, also be a layer thicker than a monolayer in which the potential aligns the complexes to the electric field. As a consequence of the increase of the potential near the surface, the oriented complexes would on the average be nearer to the surface than the average of all complexes. 

Zirconium, titanium, and hafnium hydrides can activate the C-H bonds of several alkanes at low temperatures (even at room temperature) because they are very electrophilic and reactive. Moreover, the surface complex is immobilized by the strong metal-silica bonds, and this immobilization can prevent the coupling reactions leading to the deactivation of the complex. 

The (=Si03)3TiH surface complex was active in the skeletal isomerization of alkanes at temperatures as low as 50°C and this remarkable activity is very promising because this catalyst has the possibility to isomerize light hydrocarbons under very mild conditions.261... 

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