Kinetic surface parameter

They focus on the ID simulation of an urea SCR system. The system includes a model for N02 production on a DOC, a model for urea injection, urea decomposition and hydrolysis catalyst, a model for a vanadium-type SCR catalyst and a model for NH3 decomposition on a clean-up catalyst. The catalyst models consist of a ID monolith model with global kinetic reactions on the washcoat surface, kinetic parameters have been taken from literature or adjusted to experimental data from literature. The complete model was implemented in AVL BOOST (2006). AVL BOOST is an engine cycle and gas exchange simulation software tool, which allows for the building of a model of the entire engine. 

Temperature programmed desorption (TPD) is an experimental technique to measure surface kinetic parameters. The most straightforward analysis of TPD is due to Redhead [331], Assuming that the surface has some fractional coverage 0 of adsorbed A molecules, the desorption rate of A from the surface r(j (1/s) is taken to be... 

A simple dynamic model is discussed as a first attempt to explain the experimentally observed oscillations in the rate of propylene oxide oxidation on porous silver films in a CSTR. The model assumes that the periodic phenomena originate from formation and fast combustion of surface polymeric structures of propylene oxide. The numerical simulations are generally in qualitative agreement with the experimental results. However, this is a zeroth order model and further experimental and theoretical work is required to improve the understanding of this complex system. The in situ use of IR Spectroscopy could elucidate some of the underlying chemistry on the catalyst surface and provide useful information about surface coverages. This information could then be used to either extract some of the surface kinetic parameters of... 

The TPD data presented in Figure 2 and the TPR data for coadsorbed A and B (not shown) were then concurrently optimized to provide a consistent set of uniform surface kinetic parameters which were best able to reproduce these observations. In this fitting procedure, the... 

Determining the surface kinetic parameters using oxygen isotopic exchange can be more complicated for oxide catalysts or catalysts with oxide supports. Oxygen exchange between the gas phase, the catalyst surface, and the catalyst bulk may result in the failure of the transient to relax... 

The effect of partial pressure of hydrogen on the surface kinetic parameters for methanation on Ru/Si02 was shown in the study by Bajusz and Goodwin. The rate of reaction can be expressed as... 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

The above considerations show that the rate of a corrosion reaction is dependent on both the thermodynamic parameter and the kinetic parameters rjj and rjj. It is also apparent that (q) the potential actually measured when corrosion reaction occurs on a metal surface is mixed, compromise or corrosion potential whose magnitude depends on E, and on the Ej, -I and Ej, -I relationships, and (b) direct measurement of 7 is not possible when the electrodes are inseparable. 

A consequence of this theoretical approach which includes kinetic parameters is the establishment and coupling of certain ion fluxes across the phase boundary (equality of the sum of cathodic and anodic partial currents leading to a mixed potential). If a similar approach can be applied to asymmetric biological membranes with different thermodynamic equilibrium situations at both surfaces, the active ion transport could also be understood. 

The charge-discharge reactions occur at the phase boundary between the active material and the electrolyte. To make sure that a sufficient rate of reaction is achieved, the surface of the reacting materials has to be large. Otherwise, the kinetic parameters would reduce the reaction rate too much. Table 5 shows the surface areas of the active materials in the positive and the negative electrode. 

In actual experiments we do not usually observe directly the desorbed amount, but rather the derived read-out quantities, as is the time dependence of the pressure in most cases. In a closed system, this pressure is obviously a monotonously increasing function of time. In a flow or pumped system, the pressure-time dependence can exert a maximum, which is a function of the maximum desorption rate, but need not necessarily occur at the same time due to the effect of the pumping speed S. If there are particles on the surface which require different activation energies Ed for their desorption, several maxima (peaks) appear on the time curve of the recorded quantity reflecting the desorption process (total or partial pressure, weight loss). Thereby, the so-called desorption spectrum arises. It is naturally advantageous to evaluate the required kinetic parameters of the desorption processes from the primarily registered read-out curves, particularly from their maxima which are the best defined points. 

How do we derive the activation energy of desorption from TPD Data Unfortunately, the differential equation in (12) can not be solved analytically. Hence, analyzing TPD curves can be a cumbersome task, in particular because the kinetic parameters usually depend on surface coverage. 

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care ... 

Figure 7.14. The temperature-programmed reaction and corresponding Arrhenius plot based on rate expression (21) enables the calculation of kinetic parameters for the elementary surface reaction between CO and O atoms on a Rh(lOO) surface.

Based on surface science and methods such as TPD, most of the kinetic parameters of the elementary steps that constitute a catalytic process can be obtained. However, short-lived intermediates cannot be studied spectroscopically, and then one has to rely on either computational chemistry or estimated parameters. Alternatively, one can try to derive kinetic parameters by fitting kinetic models to overall rates, as demonstrated below. 

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

Table 10.7. Kinetic parameters for the reactions (9)-(14), as used in Fig. 10.16. Prefactors are in s for surface reactions and s bar for steps invoiving gaseous species.

Explain how the kinetic parameters of an elementary step can be derived from temperature-programmed experiments with surfaces on which the reacting species have been preadsorbed. 

Give at least two reasons why it is important to know the kinetic parameters of elementary surface reactions in catalytic mechanisms. 

It is basically irrelevant in steady-state measurements in which direction the polarization curves are recorded that is, whether the potential is moved in the direction of more positive (anodic scan) or more negative (cathodic scan) values. But sometimes the shape of the curves is seen to depend on scan direction that is, the curve recorded in the anodic direction does not coincide with that recorded in the cathodic direction (Eig. 12.3). This is due to changes occurring during the measurements in the properties of the electrode surface (e.g., surface oxidation at anodic potentials) and producing changes in the kinetic parameters. 

Transient measnrements (relaxation measurements) are made before transitory processes have ended, hence the current in the system consists of faradaic and non-faradaic components. Such measurements are made to determine the kinetic parameters of fast electrochemical reactions (by measuring the kinetic currents under conditions when the contribution of concentration polarization still is small) and also to determine the properties of electrode surfaces, in particular the EDL capacitance (by measuring the nonfaradaic current). In 1940, A. N. Frumkin, B. V. Ershler, and P. I. Dolin were the first to use a relaxation method for the study of fast kinetics when they used impedance measurements to study the kinetics of the hydrogen discharge on a platinum electrode. 

For a solution of Eq. (18.12), we must also know the dependence of current density on polarization. First we consider the simpler case of low values of polarization when the linear function (6.6) with p as the kinetic parameter is valid. Solving the dilferential eqnation for these conditions, we arrive at the following expression for the distribntion of local current densities in the electrode in a direction normal to the surface ... 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

A survey of the mathematical models for typical chemical reactors and reactions shows that several hydrodynamic and transfer coefficients (model parameters) must be known to simulate reactor behaviour. These model parameters are listed in Table 5.4-6 (see also Table 5.4-1 in Section 5.4.1). Regions of interfacial surface area for various gas-liquid reactors are shown in Fig. 5.4-15. Many correlations for transfer coefficients have been published in the literature (see the list of books and review papers at the beginning of this section). The coefficients can be evaluated from those correlations within an average accuracy of about 25%. This is usually sufficient for modelling of chemical reactors. Mathematical models of reactors arc often more sensitive to kinetic parameters. Experimental methods and procedures for parameters estimation are discussed in the subsequent section. 

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