In this study numerical solutions are obtained using quasi steady state and unsteady state conditions to predict the reduction in concentrations of polluting hydrocarbons. Before their release to the atmosphere these gases undergo catalytic after-treatment in a converter, causing a decrease in their concentrations. Both homogenous as well as heterogeneous reactions are considered for hydrocarbons propylene and propane. Quasi steady and unsteady state models are developed to simulate heat and mass transfer between the exhaust gas and the catalyst surface, convective heat and mass transport, chemical reactions and the related heat release along with heat conduction in the substrate.