Steps in a Catalytic Reaction

DtfTerem shapes and sizes of catalyst. (Courtesy of the Engelhard 

A reaction takes place on the surface, but the species involved in the reaction must get fo anifrom the surface. 

The overall rate of reaction is limited by the rate of the slowest step in the mechanism. When the diffusion steps (1, 2,6, and 7 in Table 10-2) are very fast compared with the reaction steps (3, 4. and 5), the concentraticHis in the immediate vicinity of the active sites are indistinguishable from those in the bulk fluid. In this situation, the transport or diffusion steps do not affect the overall rate of the reaction, In other situations, if the reaction steps are very fast compared with the diffusion steps, mass transport does affect the reaction rate. In systems where diffusion from the bulk gas or liquid to the catalyst surface or to the mouths of catalyst pores affects the rate, changing the flow conditions past the catalyst should change the overall reaction rate. In porous catalysts, on the other hand, diffusion within the catalyst pores may limit the rate of reaction and. as a result, the overall rate will be unaffected by external flow conditions even though diffusion affects the overall reaction rate. 

Mass transfer (diffusion) of the reaciant(s) (e.g., species A) from the bulk fluid to the e.xterna] surface of the catalyst pellet 

Diffusion of the reactant from the pore mouth through the catalyst pores to the immediate vicinity of the internal catalytic siuface 

The overall rate of reaction is equal to the rate of the slowest step in the mechanism. When the diffusion steps (1,2, 6, and 7 in Table 10-2) are very fast compaired with the reaction steps fd, 4, and 5), the concentrations in the immediate vicinity of the active sites are indistinguishable from those in the bulk fluid. In this situation, the transport or diffusion steps do not affect the overall rate of the reaction. In other situations, if the reaction steps are very fast 

Diffusion of the products from the interior of the pellet to the pore mouth at the external surface 

Mass transfer of the products from the external pellet surface to the bulk fluid 

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Finally, we note that all transfers to alcohol-water mixtures or additions of alcohol to crystal mother liquor involve changes in the proton activity of the solution. Care must be taken to ensure that the pH does not change too much, or the crystal may be disrupted. Worse still, the enzymatic activity may be abolished. Control of proton activity in mixed solvents is discussed in Section III,D. If dielectric effects are controlled and pH is properly adjusted, the microenvironment of a crystalline protein will correspond closely to that of aqueous solution at room temperature. Such correspondence is essential for temporal resolution of individual steps in a catalytic reaction. 

Dissociation is often a crucial step in a catalytic reaction chain. However this is not always the case. Metals active for the O2 + H2 reaction are the noble metals of group 8-10, especially Pt, a metal with a low O-metal bond strength. The same metals are active for the CO + O2 reaction. The most active metals do not easily break the C-O bond and do not form oxides under reaction conditions. Both reactions are Langmuir-Hinshelwood reactions between adsorbed O and adsorbed H or CO. Desorption of products of the reaction intermediates here is reaction rate controlling. 

Table 7.13 Steps in a Catalytic Reaction (Source adapted from Ref. 

The oxide surface becomes reduced in this process if this were a step in a catalytic reaction, some other O-containing species would have to donate an O atom back to the substrate in order for the reaction to continue. Like the other types of reaction, 0-transfer adsorption can be either molecular or dissociative. 

As with the other surface reactions discussed above, the steps in a catalytic reaction (neglecting diffusion) are as follows the adsorption of reactant molecules or atoms to form bound surface species, the reaction of these surface species with gas phase species or other surface species and subsequent product desorption. The global reaction rate is governed by the slowest of these elementary steps, called the rate-determining or rate-limiting step. In many cases, it has been found that either the adsorption or desorption steps are rate determining. It is not surprising, then, that the surface stmcture of the catalyst, which is a variable that can influence adsorption and desorption rates, can sometimes affect the overall conversion and selectivity. 

In the oxidative addition of an A-B bond to a metal, new M-A and M-B bonds are formed as the A-B bond is cleaved (Eq. 2.1). The reverse reaction, reductive elimination, leads to the extrusion of an A-B molecule from a precursor M(A)(B) complex this is often the product forming step in a catalytic reaction. In the oxidative direction, we break the A-B bond and form an M-A and an M-B. Since A and B are always considered as le X-type (anionic) ligands, the oxidation state, the electron count, and coordination number of the metal all increase by two units during the reaction. The change of +2 in the formal oxidation state gives the reaction the name oxidative addition. These terms as well as the conceptual basis of organometallic chemistry are discussed in a previous work [4]. 

Since A can be obtained from thermodynamic data, and a a can be determined if V and V are known, a a plays a very important role in verifying reaction mechanism. If there is no rate determining step in a catalytic reaction, the concept of average chemical stoichiometric number can be introduced. The kinetics of reverse reaction can be determined from that of forward-reaction kinetics by crj or... 

More time is spent on adsorption since it is often the controlling step in a catalytic reaction. 

In a stoichiometric reaction, the passage through M.TS would be the slow, or rate-determining, step. In a catalytic reaction the cyclic nature of the system means that the rates of all steps are identical. On a circular track, on average the same number of Crains must pass each point per unit lime. The slow step in a catalytic process is called the turnover limiting step. Any change that lowers the barrier for this step will increase the turnover frequency (TOF). Changes in... 

Adsorption of reactants on the surface of a catalyst represents the first elementary step in a catalytic reaction cycle. Chemisorption and physisorption are two kinds of adsorption, and differ according to the type and strength of bond. In physisorption, the adsorption bond is due to the rather weak van der Waals interactions between permanent or induced dipoles. Chemisorption occurs if a real chemical bond is formed between the substrate and the adsorbate. Molecules may adsorb intact or dissociate on the surface. Catalytic reactions almost always involve the dissociation of at least one of the reacting molecules. In certain highly stable molecules, such as methane or ethane, chemisorption is not possible without the rupture of a C-H bond. Dissociation of molecules on metals leads to predominantly neutral fragments (homolytic bond splitting), whereas on oxides, the dissociation... 

The first step in a catalytic reaction is the adsorption of a molecule from the gas phase. When a molecule collides with the surface it can either adsorb or bounce back to the gas phase. We distinguish between two cases, trapping and sticking. If the attraction between the molecule and the surface is due to van der Waals forces, the molecule is physisorbed and the process is called trapping. If the attraction is stronger, a chemisorption bond may form. The sticking coefficient, 5q, is the probability that a molecule becomes chemisorbed at the surface. 

Describe the steps in a catalytic reaction and in chemical vapor deposition (CVD)... 

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