Single-site mechanisms

In the context of this study, the extent of reaction refers to the conversion of sites from active to inactive, and is given by Equation 6 (i.e., x = 1 for no conversion, x = 0 for total conversion). For a single site mechanism it can be shown easily that F(x) reduces to 1.0. Solution of Equation 7 and substitution into Equation 3 yields the expected result ... 

Hint. For the single-site mechanism assume (as Mathur and Thodos did) that the surface reaction step can be written as ... 

Step 2. It then reacts either with another molecule on an adjacent site (dualsite mechanism), with one coming from the main gas stream (single-site mechanism), or it simply decomposes while on the site (single-site mechanism). 

The authors prefer reaction (a) as the main route to ethylene oxide and reaction (d) to combustion products. Nevertheless, they find that selectivity is not a strong function of temperature, suggesting that there is a common rate-determining step. Recently, they studied the effect of dichloro-ethane as a moderator and concluded that such a compound decreases the activity because silver sites are occupied by chloride ions. But the selectivity is higher because the single site mechanism is first order in surface species (epoxidation) and the dual site mechanism second order (combustion). 

The smface reaction may be a single-site mechanism in which only the site on which the reactant is adsorbed is involved in the reaction. For example, an adsorbed molecule of A may isomerize (or perhaps decompose) directly on the site to which it is attached ... 

The forward cmnene decomposition reaction is a single-site mechanism involving only adsorbed cumene while the reverse reaction of propylene in the gas phase reacting with adsorbed benzene is an Eley-Rideal mechanism. 

Mori et al. concluded that n-butane reformed over a Ni/Al203 catalyst at 450 °C by a single-site mechanism with the hydrocarbon and the steam competing for the nickel surface when the amount of H2O adsorbed was not... 

The first step is to develop equations for all possible controlling steps. They will include, among many possibilities, dissociation of one or more of the reactants, the common single-site mechanism for reactant decomposition, the dual-site mechanism where two sites are involved in the decomposition, and half-site mechanism where two molecules are adsorbed on a single site. Having formulated sufficient models (usually 15 to 20 are even excess of 100), various experimental and sophisticated statistical methods are available for selecting the most probable model, as described in many texts (e.g., Froment and Bischoff, 1990). A comprehensive stepwise procedure (not discussed here) is suggested by Doraiswamy (2001). 

The. single-site mechanism turns out to be the correct one. For this mechanism the rate law is... 

The authors may well be correct when concluding that the surprising weakness of inhibition by aromatics results from slow desorption of a product. However, their model and rate equation appear questionable. Like aromatics, hydrogen is also strongly adsorbed, and so is as likely a candidate as toluene for accumulation on the surface. Also, a single-site mechanism is quite improbable with two strongly adsorbed products. Moreover, the "initial" rates were measured at conversions that entailed a decrease of up to 13% in fluid density, an effect not corrected for. Lastly, the trial equation was derived only from rates at low conversion and cannot be relied upon to reflect the behavior of the reaction as it progresses. 

Bridges et al. (33) obtained lower chromia areas by CO adsorption than by O2 adsorption, and they attributed this result to a two-site adsorption mechanism for CO, and a single-site mechanism for Og. If one takes into account the ratio of the number of available double sites to single sites in the clusters of Table VI, then one may deduce the... 

In general, the molecular weight of the terpolymer is not very sensitive to the polymer composition and maintains sufBciently high (Mw 200,000 g/mol) over a wide composition range. Narrow molecular weight distribution is observed for each sample. The polydispersities (PD) of all samples in Table 4 are close to 2, indicating that the terpolymerization also follows the single-site mechanism and the monomers are uniformly distributed in the terpolymer. 

The kinetic rate law for the Langmuir-Rideal single-site mechanism is... 

If the single-site mechanism is best, then the following rearrangement of the Hougen-Watson model at the reactor inlet allows one to calculate kforward.surf.Rx and Fa via linear least-squares analysis using a first-order polynomial ... 

Single site mechanism the adsorbed butyric acid onto the catalysts reacts with non-adsorbed n-butanol in the bulk. 

Scheme 21 where X can be either a chloride or a carboxylate. These bimetallic cobalt catalysts display induction periods and poor agreement between theoretical and experimental values likely due to slow initiation relative to propagation. Polydispersities My /Mn) were 2, consistent with a single-site mechanism. ... 

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