Bifurcation Analysis on Pt and Ir for the Reduction of NO by CO

Abstract

Oscillations and bistability in heterogeneous catalytic reactions are well known. Oxidation of CO by O 2 on a Pt surface is the renowned example in this category. Extensive literature is available for the NO-CO reaction specially related to oscillation and bistability at low pressure conditions on the Pt catalyst surface. Detailed reaction mechanisms have been proposed for the NO-CO reaction on Pt(100), which shows bifurcations, kinetic oscillations, and multiple steady states under ultra high vacuum (10 -6 -10 -7 bar) conditions due to complex surface dynamics (Fink et al., 1992;Makeeva and Kevrekidis, 2004). It was found that oscillations are observed when C NO /C CO is greater than one and in the temperature range of 100-250ºCDue to the importance of the NO-CO reaction in current catalytic converters, reduction of NO by CO on Pt group catalysts is important to study. Various reaction mechanisms have been proposed for the NO-CO reaction on Pt(100), which shows bifurcations, kinetic oscillations and multiple steady states under ultra high vacuum (UHV) conditions due to complex surface dynamics. Some experiments on supported Pt group catalysts reported in literature show oscillations and bistability under atmospheric conditions as well. Industrially relevant conditions require the modelling and detailed analysis of the system at atmospheric pressure. We have proposed a reaction mechanism for the NO-CO system on Pt group catalysts and coupled it with an isothermal PSR model to obtain solutions at atmospheric conditions with the continuation software CONTENT 1.5, at different operating conditions. Simulation results suggest that Pt(111) shows bifurcations at certain operating conditions while Ir(111) shows stable solutions at all the operating conditions studied here.En raison de l'importance de la réaction entre NO et CO dans les convertisseurs catalytiques habituels, il est important d'étudier la réduction de NO par le CO sur des catalyseurs du groupe Pt. Divers mécanismes de réaction ont été proposés pour la réaction de NO-CO sur Pt(100), qui montre des bifurcations, des oscillations cinétiques et de multiples états permanents dans des conditions de vide extrême en raison de la dynamique de surface complexe. Certaines expériences dans la littérature scientifi que sur des catalyseurs de groupe Pt supportés montrent en outre des oscillations et une bi-stabilité dans des conditions atmosphériques. Des conditions industrielles pertinentes requièrent la modélisation et l'analyse détaillée du système à la pression atmosphérique. On a proposé un mécanisme de réaction pour le système de NO-CO sur des catalyseurs de groupe Pt et on l'a couplé avec un modèle PSR isotherme afi n d'obtenir des solutions dans des conditions atmosphériques avec le logiciel de continuation CONTENT 1.5, dans différentes conditions opératoires. Les résultats des simulations suggèrent que Pt(111) montre des bifurcations à certaines conditions opératoires, tandis que l'Ir(111) montre des solutions stables à toutes les conditions opératoires étudié...

Microkinetic model for NO–CO reaction: Model reduction

“…Figure 1 shows the concentration profiles for NO, N 2 , and N 2 O obtained using PFR simulations (shown as lines) and experimental data points from Chambers et al 5 shown as symbols. Here, we have reproduced the results of Mantri and Aghalayam 14 with our PFR model. Although prediction of observed concentrations of N 2 and N 2 O in the 400–500°C temperature range is not perfect, we accept the model as such because the parameters are not data fitted and proceed further with the methodology of reduction.…”

“…At higher temperatures, the N 2 formation dominates completely. Consequently, the amount of N 2 O formed undergoes a maxima, which is well captured by the microkinetic model of 14. The dominant mechanism for reduction of NO changes about the temperature where the N 2 O concentration demonstrates a maximum.…”

“…In this work, a validated microkinetic reaction mechanism developed earlier by this group, exclusively for the NO–CO reaction on PGM catalysts, is used as a starting point 14,15. The advantages presented by this mechanism are ability to predict measured N 2 O concentrations (based on earlier experiments of Chambers et al 5); activation energies developed from independent theoretical estimates, without any data fitting; and ability to predict observed features of multiple steady states and hysteresis 16. In this reaction mechanism, the N 2 O* forms by the reaction of NO* and N*.…”

“…It desorbs to yield N 2 O in the gas phase, whereas N 2 forms directly from the recombination of two N* species. No interconversion between N 2 O and N 2 is envisaged, to maintain all the reaction steps to be LH ones, thus eliminating Eley–Rideal steps based on arguments presented earlier in 14 and 5. The aim of this work is to develop suitable global reaction rate expressions for the formation of both N 2 and N 2 O, based on this microkinetic reaction mechanism.…”

“…Since the use of microkinetic model in CFD simulations is computationally expensive, the validated microkinetic model from 14 is used to develop suitable reduced rate expressions, which overcome limitations of earlier work, for the NO–CO reaction. The reaction steps and their kinetic parameters are listed in Table I.…”