Microkinetic reaction model

A microkinetic reaction model was constructed to elucidate the intrinsic reaction mechanism and to provide insight into the processes occurring in the reactor. CHEMKiN software [50] was used to accommodate this model. The PrOx reaction mechanism describing the detailed gas-phase and surface chemical kinetics was constmcted from previously published work [51] and adapting the rate parameters to our experimental results. The model is composed of eight adsorption reactions, eight desorption reactions and 12 surface reactions with nine gas-phase species (N2, O, O2, CO2, H, OH, CO, H2, H2O) and eight surface species [Pt(s), 0(s), H(s), OH(s), H20(s), C(s), CO(s), CO2 (s)]. 

The microkinetic models provide quite detailed description of the transients in catalyst operation. However, the number of balanced species and reaction steps is quite high for a realistic exhaust gas composition, due to the explicit consideration of all surface-deposited reaction intermediates. The models using microkinetic reaction schemes may also exhibit quite complex non-linear dynamic behavior (cf., e.g., Kubicek and Marek, 1983 Marek and Schreiber,... 

Recent simulations by Marin and coworkers (56,57) seem to confirm Equations (12b) and (12c). A single-event microkinetics (SEMK ) model was used to analyze data characterizing Fischer-Tropsch catalysis on iron. The authors reported an activation energy of only 57 kj/mol for CO dissociation, whereas activation energies for the chain-growth reaction and termination reaction leading to alkane or alkene formation were found to be 45,118, and 97 kJ/mol, respectively. 

Thus, it is adequate to determine the energetic characteristics of the elementary reactions based on the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method developed by Shustorovich [2], while the pre-exponential factors may be estimated simply from the transition-state theory [4,26]. Here we employ, for illustrative purposes, a simplified version of a microkinetic WGSR model developed by us earlier [14],... 

The term microkinetic analysis has been applied " to attempts to synthesise information from a variety of sources into a coherent reaction model for the hydrogenation of ethene. The input includes steady-state kinetics (most importantly the temperature-dependence of reaction orders ), isotopic tracing, vibrational spectroscopy and TPD it uses deterministic methods, i.e. the solution of ordinary differential equations, for estimating kinetic parameters. It selects a somewhat eclectic set of elementary reactions, and in particular the model... 

For the heterogeneous catalytic process to be effective, the reactants present in the surrounding fluid phase must be transported to the surfece of the solid catalyst, and after the reaction, the products formed must be carried back from the surface to the bulk fluid. The path of the physical rate processes at the particle scale is divided into two parts, as depicted in the 7-step sequence of the continuous reaction model used in microkinetic analysis ... 

In the case of heterogeneous catalysis, a DCKM or microkinetic model must incorporate the added dimension of adsorbed chemical species as well as active versus non-active sites. To obtain the full predictive capability from reactant influent to product effluent, all possible reactions in the system, both heterogeneous and homogeneous, must be accounted for. To properly understand the catalytic reaction sequence, it is possible that seemingly unimportant intermediates on the surface may be what initiate gas phase reactions. To begin this elucidation, the surface chemical species and their properties must be known. 

The starting point for microkinetic modeling is the detailed reaction mechanism. Thus, while a conventional kinetic model is formulated as the rate for an apparent gas phase reaction, the surface species are explicitly included in a microkinetic model. 

The input parameters for a microkinetic model may be taken from measured adsorption and reaction rates for the catalyst, measured heats of adsorption together with thermodynamic data for the gas (or liquid-) phase above the catalyst. 

The microkinetic models in this section are built upon BEP-relations of the type described above. It will be shown that an underlying BEP-relation in general leads to the existence of a volcano relation. We shall also use the microkinetic models in combination with the universal BEP-relation to explain why good catalysts for a long range of reactions lie in a surprisingly narrow interval of dissociative chemisorption energies. 

A simple tool is described, which provides a conceptual framework for analyzing microkinetic models of heterogeneous reactions. We refer to this tool as the Sabatier Analysis . The Sabatier Analysis of the microkinetic models developed in this section suggests that the clustering of good catalysts can be explained by the combination of the universal BEP-relation and activated re-adsorption of synthesis products onto the catalyst. 

The same microkinetic model can be used to investigate how the reactivity of the optimal catalyst changes with other reaction conditions such as temperature or pressure. In Figure 4.35, the dependence of the turnover frequency on temperature is shown. For high temperatures, the optimal catalyst moves out towards the more reactive surfaces. Figure 4.36 shows the dependence of the turnover frequency on the pressure of the more important reactant. The position of the optimal catalyst for... 

In the washcoat, reaction rates are modeled via global reaction mechanisms. In such a global or macrokinetic reaction mechanism, several microkinetic adsorption, reaction and desorption steps are lumped together, reducing the overall number of kinetic parameters considerably. For some catalysts,... 

Detailed microkinetic models are available for CO, H2 and HC oxidation on noble metal(s) (NM)/y-Al203-based catalysts (cf., e.g. Chatterjee et al., 2001 Harmsen et al., 2000, 2001 Nibbelke et al., 1998). The model for CO oxidation on Pt sites includes both Langmuir-Hinshelwood and Eley-Rideal pathways (cf., e.g., Froment and Bischoff, 1990). Microkinetic description of the hydrocarbons oxidation is more complicated, particularly due to a large number of different reaction intermediates formed on the catalytic surface. Simplified mechanisms, using just one or two formal surface reaction steps,... 

Several different models were proposed for the slow NOx storage process, while only few details and approximate models are available for the highly transient NOx reduction within the rich phase, lasting only several seconds. The models can be divided into two groups, depending on whether the internal diffusion in the particles of the NOx storage material is considered explicitly, or this effect is included implicitly into the evaluated kinetic parameters. The models can be further differentiated by the level of complexity for the reaction kinetics description, i.e., either (simplified) microkinetic scheme or the global kinetics. 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

In a previous publication (7), the term microkinetics was defined to denote reaction kinetics analyses that attempt to incorporate into the kinetic model... 

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