Temperature reduction kinetic models

A key feature of PdCys as precursors of Pd(0) nanoparticles is that reduction of Pd(II) -> Pd(0) involving C-Pd bond cleavage is required. This accounts for both the high temperatures invariably required and the induction period in the absence of reductants. Rosner et al. have developed a detailed kinetic model of a Heck reaction catalyzed by dimeric palladacycles (Rosner et al. 2001 a,b). This model explains the experimental observations and is consistent with an active species... 

In practical combustion systems, such as CO boilers, the flue gas experiences spatial and temporal variations. Constituent concentration, streamline residence time, and temperature are critical to determining an efficient process design. Computational fluid dynamics (CFD) modeling and chemical kinetic modeling are used to achieve accurate design assessments and NO, reduction predictions based on these parameters. The critical parameters affecting SNCR and eSNCR design are listed in Table 17.4. 

In a recent survey [19] it was noted that a realistic model for catalytic oxidation reactions must include equations describing the evolution of at least two concentrations of surface substances and account for the slow variation in the properties of the catalyst surface (e.g. oxidation-reduction). For the synchronization of the dynamic behaviour for various surface domains, it is necessary to take into consideration changes in the concentrations of gas-phase substances and the temperature of the catalyst surface. It is evident that, in the hierarchy of modelling levels, such models must be constructed and tested immediately after kinetic models. On the one hand, the appearance of such models is associated with the experimental data on self-oscillations in reactors with noticeable concentration variations of the initial substances and products (e.g. ref. 74) on the other hand, there was a gap between the comprehensively examined non-isothermal models with simple kinetics and those for the complex heterogeneous catalytic reactions... 

Significant concentrations of NO2 have been reported in the exhaust of gas turbines and in the products of range-top burners [21]. These results are surprising because chemical equilibrium considerations reveal that the NO2/NO ratio should be negligibly small for typical flame temperatures. Furthermore, when kinetic models are modified to include NO2 formation and reduction, they show that the conversion of NO to NO2 can be neglected in practical devices. 

If the TPR profiles for the NM/Ce02 catalysts and the bare support, also included in Figure 4.3, are compared, a common high temperature feature centred at 1090 K may be noted. This peak is generally interpreted as due to the bulk reduction of ceria (61, and references there in). In agreement with several earlier studies (73,110,283), the position of this peak does not seem to be modified by the presence of any supported metal. This observation is typically interpreted in terms of a kinetic model (205) which assumes that the high temperature reduction process is controlled by the slow bulk difhision of the oxygen vacancies created at the surface of the oxide. 

The kinetic model for the gas phase reactions in the reburn and bournout zone enables the description of the influence of the reburn temperature and stoichiometry on the nitrogen species and hence is a suitable tool for the qualitative study of the influence of the main parameters. The simulation predicts a higher NO, reduction potential under ideal conditions than measured. 

The effect of temperature, contact time and reactant concentration on the kinetics of NO reduction by CsHs and by CsHg over Pt/Al203 under lean-bum conditions have been investigated and kinetic models which satisfactorily fit the data have been developed. The results suggest that with CsHs the Pt surface is dominated by carbonaceous species, while with CsHs adsorbed atomic oxygen is the main species on the Pt surface. This difference in the state of the Pt surface results in different mechanisms for NOx reduction. Thus, with CsHe, NOx reduction seems to occur via the dissociation of adsorbed NO on the Pt surface, while with CsHg, NOx reduction appears to occur via spill-over of NO2 from the Pt metal onto the AI2O3 support where it reacts with CjHs-derived species to form N2 and N2O. 

The kinetics of the selective catalytic reduction of nitric oxides (NOx) on a proprietary high temperature catalyst with diesel as the reductant have been studied. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simulation will be used when controlling the injeetion of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty. The response time and the NOx conversion level upon transient diesel injection was found to be dependent on the temperature. At temperatures below 570 K very low or no NOx conversion was observed. Above 570 K a small conversion was observed. No direct response upon diesel injection could be distinguished and the NOx conversion was independent on the hydrocarbon concentration. As the temperature was increased the response became apparent and then faster and the conversion level gradually became more dependent on the hydrocarbon concentration. Above 700 K the response was immediate (response time less than 15 s) and the conversion level was directly dependent on the hydrocarbon concentration. It was concluded that the NOx reduction proceeds via the formation of a hydrocarbon intermediate and the successive reaction between the hydrocarbon intermediate and NOx. When this reaction mechanism was modeled mtiny features of the catalyst behaviour were reproduced. 

A more attractive alternative may be a system that uses the fuel, already available, to reduce NOx emissions. In this study the kinetics of the NOx reduction on a proprietary high temperature catalyst with diesel as the reductant was examined. The objective was to derive a kinetic model that can be used for real time simulation of the catalyst. In the extension, the real time simulation will be used when controlling the injection of reductant. This is a requirement for achieving a high efficiency and a low fuel penalty. 

The majority of published research has concentrated on the preparation of the catalyst - the effect of different supports and different metals, the addition of second metals and the effect of different preparation methods on the selectivity of the catalysts for selective hydrogenation [2,3,5,6-10]. The effects of reaction conditions on selectivity have received considerably less attention. Gallezot and Richard [4] commented on the scarcity of systematic studies on the influence of reaction parameters such as pre-reduction of the catalyst, temperature, pressure, concentration of reactant and nature of the solvent for a given catalyst and reaction. Since then Singh et al. [11] have obtained quantitative kinetic data on the liquid phase hydrogenation of citral over Pt/SiOa catalysts and have used this information to present a kinetic model which fits their data. 

Willi R (1996) Low-Temperature Selective Catalytic Reduction of NOx—Catalytic Behavior and Kinetic Modeling. Dissertation, ETH Zurich... 

The second reason appears more likely. Devadas et al. [34] observed that N2O decomposes to N2 and O2 starting at 350 °C. Our results show that there was no N2O in the outlet at temperatures above 450 °C. Another possibility for N2O consumption is reaction with NH3 (reaction R16). Devadas et al. found that the presence of NH3 increased the rate of N2O decomposition. More recently, Colombo et al. [57] reported on data and kinetic modeling for N2O decomposition and N2O reduction by NH3 on Fe-zeolites. In our experiments, we obtained very high NOx conversions (>90 %) for dry feeds and temperatures >250 °C. NH3 consumption was nearly 100 % for these temperatures and hence it was difficult to determine how much NH3 was involved in the reduction of N2O (R16) and how much NH3 was oxidized to N2 (reaction R4). Similar trends for N2O production on Fe-zeolite and other catalysts were reported in the literature [2, 25, 34, 36]. 

These steps increase the coverage of surface nitrites which rapidly convert to N2. The differential rate data for temperatures below 250 °C presented earlier show clear evidence for multiple reaction pathways The differential rate of NO2 consumption exceeds that of NO at lower temperatures. This points to the formation of NH4NO3 and its inhibition of N2 formation, but also the mitigation of the inhibition by and AN reduction by NO. It can be shown that an overall rate based on the reduction of HNO3 and/or NH4NO3 as the RDS has the functional features to predict the main trends in the experimental data. Further analysis of microkinetic models that include these steps Sl lO and S19-S26 is needed. Later we describe global kinetic models that predict these data as a first step toward this goal. 

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. 

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