Dissociative adsorption catalytic mechanism

Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. As explained in Chapters 1 and 2, catalysis is a cyclic event that consists of elementary reaction steps. Hence, to determine the kinetics of a catalytic reaction mechanism, we need the kinetic parameters of these individual reaction steps. Unfortunately, these are rarely available. Here we discuss how sticking coefficients, activation energies and pre-exponential factors can be determined for elementary steps as adsorption, desorption, dissociation and recombination. 

Linear mechanisms are rather common for heterogeneous catalytic reactions. Examples are given and examined by Cornish-Bowden [43] and Ker-nevez [44]. Non-linear mechanisms, i.e. those including interactions of several molecules of the same or different surface substances, however, are more frequent. For example, a widely spread step of dissociative adsorption is non-linear. 

Various types of neutron scattering can be utilized to extract data on structure and dynamics for novel catalytic materials. By selectively deuterating an SSZ-13 zeolite, Cheetham and others" used ND performed on the Dual Beam Neutron Spectrometer (DUALSPEC) diffractometer at the Chalk River Laboratories and found that two acid sites are present in the unit cell of the zeohte. INS can be used to probe the mechanism of the catalytic reaction by looking at the change in the vibrational modes of the adsorbed molecules on the surface. Lennon et alP found that the interaction of HCl with a ]-alumina catalyst results in the dissociative adsorption of HCl, in which the hydroxyl groups terminally bound to A1 are replaced by chlorine. INS spectra reveal an in-plane deformation mode, 5 (OH), that can be resolved into two bands located at 990 and 1050 cm. ... 

It has been postulated f11-12) that the formation of CH by the heterogeneous catalytic reduction of CO gas with gaseous hydrogen proceeds via carbon atoms on the surface from CO. Such a mechanism involving dissociative adsorption of CO may operate during the electrochemical reduction of CO2 in aqueous solution. This leads us to a tentative conclusion that the deactivation of the electrode occurs because of polymerization of surface carbon atoms to an... 

Fits of two principal reaction mechanisms, both of which have the above general form, were made, after initial trials of rate expressions corresponding to mechanisms with other forms of rate expression had resulted in the rejection of these forms. In the above equation the Molecular Adsorption Model (MAM) predicts n=2, m=l while the Dissociative Adsorption Model (DAM) leads to n=2, m=l/2. The two mechanisms differ in that MAM assumes that adsorbed molecular oxygen reacts with adsorbed carbon monoxide molecules, both of which reside on identical sites. Alternatively, the DAM assumes that the adsorbed oxygen molecules dissociate into atoms before reaction with the adsorbed carbon monoxide molecules, once more both residing on identical sites. The two concentration exponents, referred to as orders of reaction, are temperature independent and integral. All the other constants are temperature dependent and follow the Arrhenius relationship. These comprise lq, a catalytic rate constant, and two adsorption equilibrium constants K all subject to the constraints described in Chapter 9. Notice that a mechanistic rate expression always presumes that the rate is measured at constant volume. 

The feed stream is stoichiometric in terms of the two reactants. Diatomic A2 undergoes dissociative adsorption. Components B, C, and D experience single-site adsorption, and triple-site chemical reaction on the catalytic surface is the rate-controlling feature of the overall irreversible process. This Langmuir-Hinshelwood mechanism produces the following Hougen-Watson kinetic model for the rate of reaction with units of moles per area per time ... 

Diatomic gas A2 undergoes dissociative adsorption, gas B does not require an active site on the catalytic surface because it attacks adsorbed atomic A from the gas phase, and gas C experiences single-site adsorption. Chemical reaction on the catalytic surface is rate limiting in the three-step mechanism. Stoichiometric proportions of A2 and B are present initially (i.e., a 1 2 feed of A2 and B). 

Figure 22-1 Pressure dependence of the best pseudo-first-order kinetic rate constant ki with units of min" is calculated from 0.03 to 100 atm at 325 K for the synthesis of methanol from carbon monoxide and hydrogen. The original heterogeneous catalytic mechanism is postulated as five-site chemicai reaction rate controiiing, whrae H2 unda--goes dissociative adsorption and CO and CH3OH each adsorb on singie active sites. In each case, all adsorption/desorption equilibrium constants are either 0.25 or 2.5 atm. ...

The catalytic mechanism of PtRu has been interpreted in terms of a so-called bifunctional effect of the surface in which Pt sites adsorb and dissociate methanol-forming CO and Ru atoms adsorb and dissociate water molecules, thus providing, at low potentials, oxygen atoms needed to complete the oxidation of adsorbed CO to CO2 [75]. The facts above, showing an increased rate of adsorption of methanol in the presence of Ru, indicate that the bifunctional mechanism alone does not fully describe the catalytic action of ruthenium. 

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