Ammonia adsorption-desorption kinetics

Langmuir kinetics have usually been considered for ammonia adsorption-desorption kinetics [48, 51, 52]. On the other hand, Andersson et al. ruled out that... 

The kinetic parameters of ammonia oxidation were fitted by multiresponse nonlinear regression, while the parameter estimates for the ammonia adsorption-desorption kinetics were kept unchanged with respect to those obtained from the fit in the previous section. Notably, in this case both the NH3 and the N2 outlet concentrations were regarded as regression responses. 

A simple approach to capture the ammonia adsorption/desorption kinetics is the single-site approach, where NH3 is assumed to adsorb on a global single-surface site. A nonactivated ammonia adsorption process is considered while a Temkin-type coverage dependence of the activation energy is assumed for the desorption process [24]. The reaction rate expression of adsorption is given in Eq. (3.21) ... 

The dynamic study of the ammonia SCR reactions was typically addressed by investigating the adsorption-desorption kinetics of ammonia and then the surface reactions. 

Lietti and co-workers studied the kinetics of ammonia adsorption-desorption over V-Ti-O and V-W-Ti-O model catalysts in powder form by transient response methods [37, 52, 53[. Perturbations both in the ammonia concentration at constant temperature in the range 220-400 °C and in the catalyst temperature were imposed. A typical result obtained at 280 °C with a rectangular step feed of ammonia in flowing He over a V2O5-WO3/TiO2 model catalyst followed by its shut off is presented in Figure 13.5. Eventually the catalyst temperature was increased according to a linear schedule in order to complete the desorption of ammonia. 

To describe the NH3 + NO/NO2 reaction system over a wide range of temperatures and NO2 NOxfeed ratios in addition to ammonia adsorption-desorption, ammonia oxidation and standard SCR reaction with the associated kinetics already discussed in Section 2.3.2, the following reactions and kinetics have been considered by Chatterjee and co-workers [79] ... 

The reaction of ammonia with oxygen over V-based catalysts produces mainly nitrogen, according to the stoichiometry of R5 in Table V. Analogously to the case of the ammonia adsorption-desorption, specific runs were carried out in order to extract the intrinsic kinetics of ammonia oxidation and at the same time to validate the previously fitted kinetics of the ammonia adsorption-desorption process. 

Specific runs were carried out in order to extract the intrinsic kinetics of ammonia oxidation and at the same time to validate the kinetics of the ammonia adsorption-desorption process, previously fitted. 

It is worthy of note that the kinetic model was capable to capture both the light-off temperature of ammonia oxidation and the slope with which it proceeds upon increasing the temperature. Moreover, the ammonia adsorption-desorption... 

The same estimates of kinetic parameters for ammonia adsorption/desorption and oxidation as previously obtained were retained with no further adjustment. 

To describe the full NH3-NO/NO2-O2 reacting system in the whole range of temperatures and NO2/NOX feed ratios, other reactions apart from those that describe the redox cycles, resulting in the Standard and Fast SCR reactions, had to be incorporated in the kinetic model. Such additional reaction steps included ammonia adsorption/desorption and oxidation, nitrates adsorption and desorption, NO2 SCR, and N2O formation (see Table 10.1). 

Data, not reported, indicated that ammonia does not appreciably adsorb onto the PGM catalyst. However, literature reviews on the ammonia oxidation mechanism over PGM catalysts unanimously consider the adsorption of both reactant molecules (NH3 and O2) [16], with ammonia adsorbing in on-top position on Pt [17]. Indeed, the lack of a detectable storage capacity for ammonia during our runs is not sufficient to mle out the adsorption of the same species on the catalyst surface accordingly, notwithstanding the experimental evidence, ammonia adsorption/desorption steps were included in the developed kinetic model (R.16 and R.17 in Table 18.2). 

Analysis of the dynamics of SCR catalysts is also very important. It has been shown that surface heterogeneity must be considered to describe transient kinetics of NH3 adsorption-desorption and that the rate of NO conversion does not depend on the ammonia surface coverage above a critical value [79], There is probably a reservoir of adsorbed species which may migrate during the catalytic reaction to the active vanadium sites. It was also noted in these studies that ammonia desorption is a much slower process than ammonia adsorption, the rate of the latter being comparable to that of the surface reaction. In the S02 oxidation on the same catalysts, it was also noted in transient experiments [80] that the build up/depletion of sulphates at the catalyst surface is rate controlling in S02 oxidation. 

This empirical rate expression considers the active sites of the catalyst as only a fraction of the total adsorption sites for ammonia and is consistent vfith the presence of a reservoir of ammonia adsorbed species which can take part in the reaction. The ammonia reservoir is likely associated vfith poorly active but abundant W and Ti surface sites, which can strongly adsorb ammonia in fact, nhs roughly corresponds to the surface coverage of V. Once the ammonia gas-phase concentration is decreased, the desorption of ammonia species originally adsorbed at the W and Ti sites can occur followed by fast readsorption. When readsorption occurs at the reactive V sites, ammonia takes part in the reaction. Also, the analysis of the rate parameter estimates indicates that at steady state the rate of ammonia adsorption is comparable to the rate of its surface reaction with NO, whereas NH3 desorption is much slower. Accordingly, the assumption of equilibrated ammonia adsorption, which is customarily assumed in steady-state kinetics, may be incorrect, as also suggested by other authors [55]. 

To describe the NH3 + NO + O2 (standard SCR) reacting system, NH3 adsorption-desorption, ammonia oxidation to nitrogen and standard SCR have been considered with the kinetics already presented in the previous section. 

The reactions of adsorption desorption of NH3 and ammonia oxidation to N2 were considered with the kinetic expressions shown in Section V.A.2.b. 

By combining such complexes with an ammonia decomposition catalyst (see above) one obtains a very versatile hydrogen source (see Fig. 6.40). Christensen et al. [243] have shown that the kinetics of ammonia adsorption and desorption are reversible and fast, even at moderate temperatures. 

Standard SCR reaction (10.13), the reactions of adsorption-desorption of NH3, and ammonia oxidation to N2 (10.8) were considered and the kinetic expressions discussed in the previous sections were used jointly with each one of the SCR rate expressions introduced in the following. 

A detailed kinetic model was developed [11-13] and constructed out of several submodels previously described in this chapter. The different sites used in the model is discussed in Sect. 12.2.2. The submodels include (i) ammonia adsorp-tion/desorption, (ii) water adsorption/desorption, (iii) ammonia oxidation, (iv) NO2 storage, and (v) NO oxidation. The results of these smdies were used in developing... 

This chapter is a review of the state of the art in kinetic modeling of ammonia/urea SCR over copper containing zeolites. Both fundamental detailed kinetic models as well as more globalized models are discussed. Several submodels are studied for the SCR system (i) ammonia adsorption and desorption, (ii) NO2 adsorption and desorption, (iii) water adsorption and desorption, (iv) ammonia oxidation, (v) NO oxidation, (vi) standard SCR, (vii) rapid SCR, (viii) slow NO2 SCR, (ix) N2O formation, and (x) urea decomposition and hydrolysis to produce ammonia. As can be seen from this large number of steps, this is a complex system. 

A good description of the ammonia storage and desorption is critical in order to describe transient features of the SCR system, and usually a Temkin type of kinetics is used that considers the adsorbate-adsorbate interactions. The parameters for these reactions are usually fitted to TPD experiments, but also microcalorimetry studies are presented. The most common approach is to consider one ammonia adsorption site, but more detailed kinetic models use several adsorption sites. Ammonia oxidation is a reaction occurring at high temperatures, which unfortunately decreases the selectivity of the NOx reduction in SCR. It is therefore crucial to include this reaction in kinetic models for this system. 

Nonactivated ammonia adsorption is assumed on both sites (/ ads-site-i and ads-site-2) while different rate expressions are used to describe NH3 desorption. Since Site-1 includes different types of Lewis acid sites and also ammonia physisorbed on the catalyst surface, Temkin-type coverage dependent adsorption is adopted in order to take such a site heterogeneity (/ des-site-i) into account. On the contrary, the nature of Br0nsted acid sites is well defined for zeolites, being indeed associated with the so-called bridging hydroxyls, thus it is reasonable to assume that these sites are homogeneous in terms of ammonia adsorption strength. Based on this assumption, simple Arrhenius kinetics are adopted for the NH3 desorption process from Site-2. ... 

The parameters of the reaction rates, consisting of pre-exponential factors and activation energies have to be calibrated in order to attain an optimal fit between simulated and experimental data. A step-by-step procedure is followed based on experiments which are specifically designed to minimize the number of parameters that have to be calibrated simultaneously. The sequence can be organized as follows First, parameters related to ammonia adsorption and desorption processes are identified, in the absence of SCR reactions. Second, NO oxidation parameters are calibrated without ammonia in the feed stream. Finally, the calibration of the remaining parameters is performed all together [39]. Different experimental methods and procedures involved in the derivation of global SCR reaction kinetics will be presented in Sect. 13.5. 

At such a high degree of complexity of modern exhaust after treatment systems, modeling and simulation of the catalyst performances play an important role as part of the total system simulation in the automotive development process. The processes occurring on the SCR catalysts are already well understood and modeled [6-12], whereas this is not the case for the ASC, for which only a few literature surveys exist [2, 3, 13-15]. Scheuer et al. [3] presented a mechanistic kinetic model for ammonia oxidation over a PGM catalyst. Such a model was derived from previous literature works [16, 17] and includes the following reactions NH3, O2 and NO adsorption/desorption from the catalytic sites, NH3 activation and N2, NO and N2O formation, with the last three species being the main NH3 oxidation products. The model consists thus of seven reactions that are assumed to proceed... 

CI2 evolution reaction, 38 56 electrochemical desorption, 38 53-54 electrode kinetics, 38 55-56 factors that determine, 38 55 ketone reduction, 38 56-57 Langmuir adsorption isotherm, 38 52 recombination desorption, 38 53 surface reaction-order factor, 38 52 Temkin and Frumkin isotherm, 38 53 real-area factor, 38 57-58 regular heterogeneous catalysis, 38 10-16 anodic oxidation of ammonia, 38 13 binding energy quantification, 38 15-16 Haber-Bosch atrunonia synthesis, 38 12-13... 

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