A global kinetic model for NOx storage and reduction on Pt/BaO/Al2O3 monolithic catalysts

A global kinetic model for NOx storage and reduction for the case of anaerobic regeneration with hydrogen is developed, based on parallel experimental studies [R.D. Clayton, M.P. Harold, V. Balakotaiah, Appl. Catal., B 84 (2008) 616; R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal., B 90 (2009) 662.]. The existence of two different types of BaO storage sites on the catalyst is proposed, which differ in their storage as well as regeneration activity. The two-site model explains the close to complete NOx storage at the start of the storage phase and the gradual emergence of NO and NO2 during later storage times. The effluent concentrations and concentration fronts of the reactants and products within the monolith are predicted by the model, providing insight into the mechanisms of regeneration and storage. The H2 front velocities are predicted to increase as the H2 front propagates down the length of the monolith, thus showing the presence of more stored NOx in the front of the reactor. The simulations show that even though regeneration is fast, H2 concentration fronts are not very steep, which is attributed to the lower regeneration activity of the “slow” sites. The model captures the formation of NH3 and the NH3 concentration fronts, which reveal the reaction of NH3 formed upstream with the stored NOx downstream of the H2 front. The lower diffusivity of NH3 as compared to H2 is shown to be responsible for the wider width of the NH3 front and earlier appearance of NH3 in the effluent than H2.