Chemisorption kinetic models

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. 

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... 

Mechanistic studies start with determination of the kinetic rate law and the rate-limiting step information on heat and mass transfer is also needed. These studies may use such techniques as isotopic labeling, chemisorption measurements, surface spectroscopy, temperature-programmed desorption, and kinetic modeling experiments. 

A simplified kinetic model, similar to Eq. (2), was constructed to obtain estimates of the kinetic parameter values. Assuming that a, the probability for direct molecular chemisorption from the physically adsorbed state, remains relatively constant with respect to Ts, the expression for this model is shown below in Eq. (10). 

FIGURE 12.4 Top Experimentally measured exchange current, log(io), for the HER over different metal surfaces plotted as a function of the calculated hydrogen chemisorption energy per atom, AE (top axis). Single crystal data are indicated by open symbols. Bottom The result of the simple kinetic model plotted as a function of the free energy for hydrogen adsorption, AGg, = AE + 0.24 eV. (Erom Nprskov, J. K. et al., J. Electrochem. Soc., 152, J23, 2005. With permission.)... 

The adsorption of methylene blue by coir pith carbon was carried out by varying the parameters such as agitation time, dye concentration, adsorbent dose, pH and temperature. Equilibrium adsorption data obeyed Langmuir isotherm. Adsorption kinetics followed a second order rate kinetic model. The adsorption capacity was found to be 5.87 mg dye per g of the adsorbent. There was no significant change in the per cent removal with pH. The pH effect and desorption studies suggest that chemisorption might be the major mode of the adsorption process. 

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. 

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. 

KINETIC MODELS USED FOR DESCRIPTION OF CHEMISORPTION ON OXIDES SURFACE... 

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

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... 

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. 

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. 

We now turn to an illustration of the theoretical results generated by the variety of previously described theories. Since most of the results that we shall show are for clean, perfect surfaces, it is important to indicate when this assumption is valid. Serri et al. (1983) developed a kinetic model for NO (molecular) chemisorption on Pt which incorporated adsorption, desorption, and diffusion of NO on terraces, diffusion from the terraces to the steps, and... 

The chemisorbed molecules, whether on the external surface for non-porous pellets or the internal surface for porous catalyst pellets, undergo surface reaction producing chemisorbed product molecules. This surface reaction is the truly intrinsic reaction step. However, in chemical reaction engineering it is usual practice to consider that intrinsic kinetics include this surface reaction step coupled with the chemisorption steps. This is due to the difficulty of separating these steps experimentally and the ease by which they are combined mathematically in the formulation of the kinetic model. 

In this complex sequence of events the most important rate is the intrinsic rate of reaction which is composed of three rates the rate of surface reaction, the rate of chemisorption and the rate of desorption. The three processes are called the intrinsic kinetics i.e. the rate of reaction in the absence of any mass or heat transfer resistances. Intrinsic kinetics is almost always predicted experimentally in differential or integral laboratory reactors using fine powders to avoid diffusional resistances. This is not an easy task especially with regard to temperature gradients for highly exothermic reactions. Experimental difficulties associated with the development of models for intrinsic kinetics are not discussed in this book. However, the kinetic modelling of the intrinsic rates is described in chapter 3. 

Chemisorption-Surface Reaction-Desorption (CSD) Kinetic Models for Unimolecular Reactions... 

R.D. Levine and G.A. Somoijai. Kinetic Model for Cooperative Dissociative Chemisorption and Catalytic Activity via Surface Restructuring. Surf. Sci. 232 407 (1990). 

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