Bimolecular catalytic reactions

A bimolecular catalytic reaction can be expressed by the following equation ... 

The rate expression developed above is for a monomolecular catalytic process. There are also two types of bimolecular catalytic reactions. One proceeds by the reaction of an adsorbed A with a gas phase B ... 

A deeper understanding of reaction mechanisms in catalysis is highly recommended to practitioners in this field, despite the difficulty of identifying a unique rate expression for many, if not all, bimolecular catalytic reactions, as illustrated in the rather simple reaction considered here. In particular, the difficulty of mechanism identification is overwhelming if just one feed composition has been studied. Even when one has available the wealth of data from a TS-PFR experiment, the best fit is hard to find on the basis of one experiment. Regardless of this complication, the rewards of understanding the details of the reaction mechanism and its kinetics are substantial. 

A simple illustrative example can be considered an irreversible, bimolecular, catalytic reaction, A + B = C, obeys the rate expression... 

Acid/base catalysis is probably the oldest type of homogenous catalytic reactions. Following the definitions by Bronstedt [6] and Lowry [7], the acids are proton donators and the bases are proton acceptors. Let us consider a bimolecular catalytic reaction with the equation presented as following ... 

Bimolecular Catalytic Reactions Supposing that the surface reaction is the rate-determining step, we obtain for an irreversible bimolecular reaction the following relations ... 

Example 2.9 Maximum rate of bimolecular catalytic reaction. 

This equation is derived on the basis of a theoretical analysis of the mechanism and velocity of the bimolecular catalytic reaction. In the temperature range 100-130 the results of calculations according to this equation agree satisfactorily with experimental data obtained at atmospheric pressure in a flow system. 

The desire to understand catalytic chemistry was one of the motivating forces underlying the development of surface science. In a catalytic reaction, the reactants first adsorb onto the surface and then react with each other to fonn volatile product(s). The substrate itself is not affected by the reaction, but the reaction would not occur without its presence. Types of catalytic reactions include exchange, recombination, unimolecular decomposition, and bimolecular reactions. A reaction would be considered to be of the Langmuir-Hinshelwood type if both reactants first adsorbed onto the surface, and then reacted to fonn the products. If one reactant first adsorbs, and the other then reacts with it directly from the gas phase, the reaction is of the Eley-Ridel type. Catalytic reactions are discussed in more detail in section A3.10 and section C2.8. 

The proposed mechanism for the DD process is not intended to represent that of any actual catalytic reaction, but to simulate a generic bimolecular reaction. Monte Carlo simulations of the reaction mechanism described by Eqs. (21)-(25) have shown the existence of IPTs exhibiting a rich variety of critical behavior. 

The major problem in accomplishing water splitting via the pathway of Scheme 4 is how to suppress the back recombination reaction + A -> D + A, which is a simple exothermic bimolecular process and therefore typically proceeds much more rapidly than complex catalytic reactions of H2 and O2 evolution. 

Once thermal reactions begin, the coal undergoes structural changes through spontaneous bimolecular reactions between coal constituents or solvent species (75) or through catalytic reactions accelerated by added catalysts or the inherent mineral components (76). Hence, the reactivity changes with the progress of these reactions. 

It is perhaps necessary to mention a possible objection to the conclusion that the catalytic reactions are truly unimolecular. If there were a complete layer of, say, nitrous oxide molecules on a surface, and reaction were determined by the impact of a second nitrous oxide molecule on one of these then the reaction would appear to be unimolecular although essentially bimolecular. But there are certain strong objections to such an hypothesis. 

Quite often it is found that one of the above-mentioned steps is much slower than the others and therefore controls the rate of the catalytic reaction. As an example, the rate expression for the bimolecular reaction... 

The main difficulty in accomplishing reactions (25) and (26) is that it is difficult for the complex and hence relatively slow multielectron catalytic reactions (25) and (26) to compete with the simple exothermic bimolecular reaction of the reverse recombination of light-separated charges... 

The molecular size pore system of zeolites in which the catalytic reactions occur. Therefore, zeolite catalysts can be considered as a succession of nano or molecular reactors (their channels, cages or channel intersections). The consequence is that the rate, selectivity and stability of all zeolite catalysed reactions are affected by the shape and size of their nanoreactors and of their apertures. This effect has two main origins spatial constraints on the diffusion of reactant/ product molecules or on the formation of intermediates or transition states (shape selective catalysis14,51), reactant confinement with a positive effect on the rate of the reactions, especially of the bimolecular ones.16 x ... 

Such redox reactions are frequently catalysed by platinum [3], other noble metals [232], silver [126-128], and carbons [233] which are all electronconducting solids. This fact points to a simple catalytic mechanism whereby the electron is transferred from Redi to Ox2 through the solid phase, as depicted in Fig. 19. In contrast to other bimolecular catalytic mechanisms (Sect. 1.5.3), the two reactants do not need to occupy neighbouring sites. Since the catalytic rate depends upon the coupled transfers of an electron from Red] to the solid and from the solid to Ox2, the kinetics are best treated in electrochemical terms. 

The catalytic reaction is simply a bimolecular reaction between B and R, with boundary conditions given by ce,m lz=0+ = cxBm, cj >m z=0+ = c] m. The yield of S increases monotonically as the Damkohler number of the catalytic reaction, Das, increases, and finally attains an asymptotic value when the catalytic reaction reaches its mass transfer limited asymptote. This feature is illustrated in Fig. 19, where the variation of Ys with Das is shown. It is interesting to note from Fig. 19, that the value of the mass transfer limited asymptote depends on the micromixing limitation of the homogeneous reaction. Larger is the micromixing limitation (rj) of the homogeneous reaction, more is the local... 

Violation of the stability condition (3.5) is allowed in bimolecular auto catalytic reactions, where the reaction groups of the initial reactants and of transformation products are interrelated. Consider the simplest bimolec ular reaction of the autocatalytic formation of an intermediate Aj by the process similar to the considered one ... 

A similar, but bimolecular, photoinduced reaction was observed on the basis of the nickel complex (28), p-toluene thiolate, and thioanisole reactants to generate methane and disulfide. The thiyl radical and Ni(I) complex was prepared by the photolysis of the Ni(II) complex (28) and j -toluene-thiolate anion in acetonitrile solution. Upon irradiation (A, = 350 nm) of the mixture of complex (28), j -toluene-thiolate ion, and thioanisole in acetonitrile under argon, gas chromatography-mass spectral analysis showed the formation of methane, ditolyl disulfide (TolS)2, and a mixed disulfide TolSSPh. The proposed catalytic mechanism is depicted in... 

Disproportionation reaction rates depend on carbenium ion reactivities, which are determined by catalyst site acid strength. Carbenium ions produced at strong acid sites are less likely to undergo P-scission or desorption. Compared with HY, the smaller pores in HZSM-5 inhibit bimolecular disproportionation reactions. In contrast, the low paraf-fin/olefin volatile product ratio for the PE-MCM-41 sample is likely due to the low acidity of the catalyst. Although the MCM-41 pore size is large enough to facilitate disproportionation, catalytic site acidity is too low for this reaction pathway to be dominant. 

Dagonnier et al. (295) also investigated the inclusion of the catalyst temperature into the set of differential equations describing a catalytic reaction. These authors, however, specified an a priori variable, the surface temperature, the meaning of which is not well-defined. The reaction mechanism they present consists of bimolecular reactions as described by Eqs. (44)-(46) ... 

Thus, it can be reasonably supposed that, at low time factor, in the initial conditions (first hour on stream), just a few C4+ ions are formed. Most of the catalytic sites are free and available and the DIS is the prevailing reaction, either with a Sn2 or with a SnI mechanism. When the time factor increases, the other reactions also occur and the consequent increase of the number of R+ ions justifies the predominance of the Sn2 mechanism (m/p = 2.0). However, the catalyst deactivates with t-o-s and the amount of caUdytic sites available for the bimolecular disproportionation reaction decreases. Under these conditions the SnI mechanism should be more important. Expectedly, the observed value of the m/p ratio at 24 hours on stream is about 1.5. 

The examples of new and unexpected catalytic reactions are probably more interesting. Surfaces are able to stabilize organometallic fragments which have no equivalent in molecular organometallic chemistry. This situation is due to the fact that surfaces, due to their rigidity, prevent bimolecular interactions. Thus highly reactive species are stable and exhibit quite unexpected reactions in the field of C-H and C-C bond activation such as alkane metathesis, Ziegler-Natta depolymerization, methane activation, and several others. 

As most of the acid sites are located in pores of molecular size the rate and the selectivity of catalytic reactions depend not only on the intrinsic properties of the sites but also on the pore structure. A zeolite catalyst selects the reactant or the product by their ability to diffuse to and from the active sites (reactant and product selectivity). Steric constraints in the environment of the sites limit or inhibit the formation of intermediates or transition states (restricted transition state selectivity) [24,25]. The strong polarizing interaction between zeolite crystallites and adsorbed molecules leads to an unusually high concentration of the reactants in the pores. This concentration effect causes an enhancement of the rates of bimolecular reaction steps over monomolecular reaction steps [26]. 

Carbonaceous deposits result from the transformation of reactants, reaction products, impurities of the feed, etc., on acid sites through various successive bimolecular steps condensation, hydrogen transfer, etc. Therefore, their rate of formation depends on the following parameters which usually affect the rate of catalytic reactions, namely ... 

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