Detailed microkinetic analysis of reactions in the C/H/O/N system over platinum

Abstract

The present PhD thesis considers an alternative theoretical framework based on Variational Transition State Theory (VTST) for estimating Arrhenius parameters for adsorption reactions. The estimation of rate data suitable for microkinetic analysis of reaction pathways in heterogeneous catalytic systems typically requires sticking coefficients. The use of VTST enables the construction of chemical mechanisms without the dependency on experimental data for sticking coefficients, for the final determination of the rate parameters. The energetics of the surface reactions are derived using the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method. A comprehensive chemical mechanism is developed for the description of the surface reactions associated with the catalytic removal of the three principal oxides of nitrogen (NO, NO2 and N2O) from automotive exhaust gases through the use of supported platinum catalysts. The reference five-step Pt catalytic mechanism [1] for NO removal has been updated to 21 reversible reaction steps featuring all principal oxides of nitrogen. Gas phase reactions are considered, via a comprehensive C/H/O/N mechanism that includes ammonia. The NOx reduction mechanism was applied under nearly stoichiometric conditions, various metal loadings and coat amounts. A rigorous comparison with experimental data [2], obtained by Toyota Motor Europe, for nine different Test Cases was performed along with the analysis of modifications to the energetics and rate parameters. The outcomes obtained for CO, NO and C3H6 conversion against temperature present a promising agreement with the experimental data [2]. The same framework, with the absence of the nitrogen and propene containing catalytic chemistry, is implemented to study the ethane auto-thermal catalytic cracking in a quartz lined tubular reactor. The validation of the mechanism was carried out via experimental measurements provided by BP Chemicals [3]. The absence and presence of CO in the feed stream were studied to further explore the catalytic activity of the chemical system. The work provides a framework for the development of heterogeneous reaction mechanisms, without the reliance on experimental sticking coefficients, focusing on the reduction of NOx emissions from the automotive exhaust gases. The proposed method can be further augmented and used for the generation of novel heterogeneous reaction mechanisms.