Two-dimensional detailed modeling of H2 fuel rich combustion over Rh/Al2O3 catalyst

The previous analysis of experiments of H2 fuel-rich combustion over a Rh catalyst—reported in Maestri et al. [2007. Role of gas-phase chemistry in the rich combustion of H2 and CO over a Rh/Al2O3 catalyst in annular reactor. Chemical Engineering Science 62, 4992– 4997]—has been extended using a detailed surface kinetic model coupled with a detailed kinetic scheme for H2 gas-phase combustion. The model is now able to investigate possible interactions between the surface and homogeneous chemistries. Results suggest that the homogeneous chemistry played a role, especially at high temperatures. In the intermediate range of temperatures, where the experimental conversion data are systematically higher than those corresponding to the diffusive limit, model predictions revealed that a very weak interaction between the gas-phase and surface kinetics occurs. In fact, the fraction of radicals that desorbs from the surface is significantly lower than that needed in order to partially activate the homogeneous process. Finally, we accounted for the presence of zones upstream and downstream of the main catalytic bed with a low catalytic activity. These sections could be related to non-uniformity of the catalytic washcoat at the boundaries of the catalytic bed, uncertainty of the beginning and ending of the washcoat layer, and possible evaporation and redeposition of the catalyst. We show that such low activity sections could significantly increase the conversion above the diffusive limit and provide a plausible mechanism to rationalize the experimental data. Moreover, dynamic simulations further revealed that under isothermal conditions the ignition of the homogenous process is governed by a build-up of a “radical pool” at the reactor back end followed by upstream propagation: axial diffusivity is crucial in order to model appropriately the experimental data.