Spatiotemporal patterns in a porous catalyst during light-off and quenching of carbon monoxide oxidation

• Published in: Chemical engineering science 2004-10, Vol.59 (19), p.3983-3991
• Main Author: Herz, Richard K.
• Format: Article
• Language: English
• Subjects: Catalysis; Detailed kinetics; Dynamic simulation; Light-off
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2004.03.044

A detailed numerical model was used to simulate the behavior of carbon monoxide oxidation within a porous platinum/alumina catalyst during temperature ramps. The model was validated in previous work by fitting step-response experiments which were performed over a range of temperatures and in which concentration gradients over the catalyst layer were directly measured. As a result of the low CO and O 2 concentrations used, the catalyst layer could be considered isothermal. The numerical experiments performed with the model in this work reveal complex spatial patterns of species and local reaction rate which change with time and temperature. As temperature is increased, CO desorbs and reaction rapidly increases, reacting adsorbed CO off the Pt surface and producing a peak in CO 2 production during catalyst light-off. Over a nonporous surface of the same material, the reaction rate would be an order-of-magnitude lower and no CO 2 peak would be produced. At steady state after reaction light-off has been obtained, reaction occurs in a narrow zone below the external face of the layer which is exposed to the constant feed gas composition. As temperature is then decreased, the CO 2 production rate decreases gradually as the front of the region covered with adsorbed CO penetrates further and pushes the reaction zone deeper into the catalyst layer. When the adsorbed CO front reaches the internal face, the CO 2 production rate drops abruptly as the reaction “quenches”. Catalyst layer thickness was changed over the range 0.06–1.0 mm at constant total Pt content. As the layer thickness was decreased, the steady-state CO 2 production rate after light-off increased, however the range of temperatures in which the catalyst was active decreased. Three qualitatively different sets of spatiotemporal patterns were obtained as the layer thickness was changed from relatively thin, to medium, to thick. Analysis of the patterns provides understanding of the temperature-dependent behavior of the catalyst and how this behavior varies with catalyst layer thickness.