Reactant inhibition

Reaction of a i solid with a solid] As with solid and gas Normally the immobility of reactants inhibits reaction and barrier layer for-1 tion is more common... 

Even though the reaction is bimolecular, reactant inhibition does not occur for this type of reaction. 

Remarkably, DMF functions both as a solvent and reactant inhibition and trapping experiments support a SET mechanism producing difluorocarbene, which on further reaction with fluoride, generated by reaction of difluorocarbene with DMF, eventually leads to the trifluoromethylzinc reagent (Scheme 7). 

The rate of reaction is approximately linear with cyclohexene partial pressure, however there are systematic deviations at higher partial pressures. The first order behavior with respect to hydrogen and the slight reactant inhibition of cyclohexene suggest the following kinetic correlation ... 

Negative orders with respect to products are fairly common. A negative order with respect to a reactant (reactant-inhibited reaction) is unusual. An example is olefin hydroformylation with a reaction order between zero and minus one in CO, a reactant (see eqn 6.12). This behavior results from a first step in which one CO ligand is displaced from the catalyst, and the fact that one of the two later steps in which CO re-enters the pathway occurs after an irreversible step. 

Various other forms of inhibition are possible. The simplest of these is reactant inhibition, in which an inhibitor competes with the catalyst for the reactant, reducing the latter s likelihood to enter the catalytic cycle. As an example, consider reactant inhibition in a three-membered cycle whose last step is irreversible ... 

In some reactions, the rate increases rather than decreases as conversion progresses. This is loosely called autocatalysis, although no genuine catalysis may be involved. The acceleration may stem from promotion by a product or major early intermediate, or from consumption of a reactant that functions as inhibitor. In product-promoted reactions, the kinetics order with respect to a product (or early intermediate) is positive. This causes the rate to increase to a maximum and then to decline as the effect of consumption of the reactant or reactants begins to overcompensate that of promotion by the product. In reactant-inhibited reactions, the order with respect to a reactant is negative. The rate may increase until the respective reactant is used up and, in some cases, may theoretically approach infinity. The latter behavior is, of course, physically impossible, and another mechanism or event necessarily takes over. 

In a reactant-inhibited reaction, slow mass transfer of the inhibiting reactant from another phase accelerates the rate and may cause instability. 

In product-promoted reactions in which the accelerating species exits into another phase, or in reactant-inhibited reactions in which the respective reactant enters from another phase, slow mass transfer may boosts rather than depresses the reaction rate. In reactant-inhibited reactions, slow supply of the inhibiting reactant may cause the system to become unstable There may be a sharp stability limit beyond which catastrophic selfacceleration occurs until the phase has become depleted of the reactant or reactants or some other phenomenon has come into play. 

The kinetics of oxidation over noble metals is complex as the reactants inhibit the adsorption rate of each other [2]. By variation of the reactant gas mixture, the surface coverage of different species can be changed and thus the activity for oxidation of CO and hydrocarbons (HC) at low temperatures can be affected. Pre-conditioning, to achieve the most active surface state for oxidation of CO and HC, can be performed by pre-reduction. Low-temperature activity for CO oxidation has been reported for Pt/Ce/ALOa and Pd/Ce/ALOa after exposing the catalyst to reducing atmospheres at temperatures above 300° [3]. 

Unusual reaction orders are found in product-promoted or reactant-inhibited ("autocatalytic") reactions, the former with positive apparent order with respect to a product, the latter with negative apparent order with respect to a reactant (see Section 8.9). An example of a product-promoted reaction is acid-catalyzed ester hydrolysis. An example of a reactant-inhibited reaction has already been encountered, namely, olefin hydroformylation, whose order with respect to CO is negative (see eqn 6.12 in Section 6.3). Such behavior is also not uncommon in heterogeneous catalysis (see Section 9.3.2) and enzyme catalysis ("substrate-inhibited" reactions in biochemistry lingo, Section 8.3). A reaction having an order with respect to a silent partner—CO in a homogeneous hydrogenation—will be examined in some detail later in this chapter (see Examples 7.3 and 7.4). 

Rather than because of promotion by a product or intermediate, the rate may accelerate because a reactant that acts as inhibitor is consumed. For example, a small amount of inhibitor present initially may depress the rate of a chain reaction until used up (see Section 10.8). More interesting are reactions inhibited by one of the principal reactants (called substrate-inhibited in biochemistry parlance). An example is hydroformylation, in which CO is a reactant with negative reaction order (see Example 6.2 in Section 6.3). There is a subtle but important difference between product-promoted and reactant-inhibited reactions The rate of a product-promoted reaction builds up to a maximum and then declines as reactant depletion overpowers product promotion. In contrast, the rate of a reactant-inhibited reaction keeps escalating, possibly catastrophically, until the respective reactant is almost completely exhausted. Typically, some other mechanism then takes over. [The negative apparent reaction order of the respective reactant arises from an additive denominator term in a one-plus rate equation, but the other terms may be small or insignificant by comparison.] Possible mass-transfer implications of such behavior will be examined in Section 13.3. 

The acceleration and chance of instability in a reactant-inhibited reaction are less pronounced if mass transfer of the respective reactant is fast. Thus, the very high mobility of hydrogen makes the effects unlikely to occur in the most common situation of reactant inhibition, that is, in heterogenous hydrogenation (see Example 9.4 in Section 9.3.1). 

This describes the deactivation of the catalyst due to the coverage of active sites of the catalyst by either reactants (reactants inhibition, non-monotonic kinetics discussed in chapter 3), or products (product inhibition discussed in chapter 3). 

Finally notice in Figure 4.10 that for n < 0, the rate decreases with increasing reactant concentration the reactant inhibits the reaction. Inhibition reactions are not uncommon, but care must be exercised in using this kinetic model when the concentrations are small. Notice the rate becomes unbounded as ca approaches zero, which is not physically realistic. When using an ODE solver to compute solutions that can reach zero in finite time, it is often necessary to modify the right-hand sides of the material balance as follows... 

The principles underlying this treatment are capable of extension to cover a greater number of reactants, inhibition by products, poisoning by adventitious impurities, dissociation of reactants upon adsorption (Section 3.2.4) and many other situations. The relevant rate expressions were collected and comprehensively evaluated many years ago by O. A. Hougen and K. M. Watson,and monographs on chemical kinetics ° often contain a fuller presentation than is thought necessary here. 

An effectiveness factor higher than one is obtained for reaction with reactant inhibition (negative reaction order)... 

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