Chemisorption kinetics

In contrast to the case of acceptors these expressions differ by the time of chemisorption kinetics Ntit). 

Various approximation of this model (e.g. those of molecular splitting, pseudo-chemical, chaotic and molecular field approximations) were reported and analyzed by Tovbin and Fedyanin [88, 89]. In particular, the pseudochemical approximation for chemisorption kinetics is of the form... 

A main purpose of proposed models and solutions presented in the review is to provide feasibility to describe the chemisorption kinetics in a wide range of experimental conditions and to determine the optimal conditions and structure of reactants for the oxides surface modification. 

Majority of studies concerning organic compounds chemisorption kinetics on oxides surface are performed for the reaction mechanisms described by Schemes 1, 8 or 15 and mainly, for the first mechanism of organosilicon compounds interaction with OH groups of non-porous silica surface. 

Already at 1 = 2, this model describes well enough the kinetics of chemisorption of CH3OH, CH3NH2, H2O and NH3 on the dehydroxylated silica surface at a constant temperature [26]. The use of Eq. 32 to describe the organosilicon compounds chemisorption kinetics on the silica surface is presented in Section 6.2. 

The lateral interactions between chemisorbed particles or between these particles and activated complexes should be taken into consideration describing the chemisorption kinetics on the solid surface. This interaction is determined by a distance between neighbouring active sites (r). The account for the lateral interaction is possible on a single crystal surface in quasi- chemical or mean-field approximations [30,32]. Let us write the common expression for chemisorption kinetics on the homogeneneous surface with the account for lateral interactions between chemisorbed particles ... 

The following models are used for description of organic compound chemisorption kinetics on oxides surface. 

Parameters for Eq. (41) describing the organosiloxanes chemisorption kinetics on non-porous siUca surface [108,109]... 

The exponential distribution function on chemisorption activation energy conforms to Howar and Taylor s, Banham and Burt s as well as to Kwan s equations. They were used long ago for the description of small molecule chemisorption kinetics on oxide surfaces [116]. It is suggested that apparent rate constant depends on the coverage heterogeneous surface as follows [117] ... 

At high values of 7 > 3RT, i.e., for the broad distribution on, the approximate solution of the corresponding integral equation for chemisorption kinetics including the reaction of the first-order with active sites and power distribution function on E obtained by the Roginskij method [119], can be used to determine ko, 7 and u parameters of Eq. (50) ... 

It is apparent that such dependence is possible at varying surface coverage of the heterogeneous surface, since reactivity of the sites reduces as 0 increases. An approach for calculation of the surface distribution functions on In A and on values from chemisorption kinetic isotherms measured at different temperatures has been developed in [70,130,131]. Distribution functions on Ink at different T can be found, for example, using the regularization procedure or expressions from Table 4. If In A and E values are connected by Eq. (60), the expression connecting cumulant of Ink distribution with the sum of cumulants of In A and E distributions is changed for cumulants of the second order as [132] ... 

A main task of chemisorption kinetics theory is prediction of activation parameters on the basis of physicochemical properties of reactant and surface active sites. One of the most widespread methods used for homogeneous media is application of the linear models... 

Surface of most oxides is heterogeneous, and properties of its active sites should be described by means of the distribution on parameters a. Approach to calculation of distribution functions p(a-,) on the basis of chemisorption kinetic data on heterogeneous surface was developed in [132]. 

This approach was applied to the determination of silica surface distribution functions on reaction constants of organosilicon compounds chemisorption using the Gamma distribution on k (Table 4). The distribution functions on Ink were calculated on the basis of chemisorption kinetic isotherms for methylchlorosilanes (CH3)4 nSiCln (n from 1 to 4) and for the compounds (CH3)3SiX X = Cl, CN, N3, NCO, NCS, 0Si(CH3)3 on the non-porous dehydrated silica surface [4,22]. As shown in [57] the correlation is observed between the logarithms of rate constants with the sum of inductive constants of substituents at Si atom in (CH3)4 nSiCln compounds ([Pg.269]

Thus, the proposed approach for determination of heterogeneous oxides surface distribution functions on the coefficients of linear models may be useful for the description of chemisorption kinetics of organic compounds. 

Earlier, the chemisorption kinetic isotherms of alkylchlorosilanes [4,57,133,140], cyclic and linear organosiloxanes [108,109] and trimethylsilylpseudohalogenides [4,58-61] on the pyrogenic silica surface have been obtained. 

Influence of oxides structure and their temperature pretreatment on chemisorption kinetics of organosilicon compounds... 

The comparison of Eqs. (55) and (56) with Eqs. (14) and (15), respectively, shows that if o(t) is constant with t (in particular, if Vr 00(7") = 0) the chemisorption kinetics have the same formal expressions as the desorption kinetics so that the considerations of Section 5 apply here. 

A time-logarithm behaviour can be recognized neither in Eq. (55) nor (56), so that the Elovich kinetics cannot be understood in terms of chemisorption kinetics on ideally homogeneous surfaces. 

In the induced-heterogeneity model the chemisorption process is assumed to be an activated process and the clean surface is considered as homogeneous and characterized by chemisorption kinetics with activation energy Eq. 

As the process progresses the chemisorption activation energy is then assumed to increase from Eo to a value E Q) depending on the coverage 0. If one stipulates that chemisorption kinetics are described by the rate equation ... 

Ho YS, McKay G. A comparison of chemisorption kinetic models applied to pollutant removal on various sorbents. Process Safety Environ Protect 1998 76 332 0. 

Catalytic reactions occur on active centers at surfaces distinction between physisorption and chemisorption Kinetic mechanisms of heterogeneous catalytic reactions Langmuir-Hinshelwood kinetics Basis of non-equilibrium thermodynamics Potential energy surface for the reaction H + H2 Transition-state theory... 

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