Modeling spatially resolved profiles of methane partial oxidation on a Rh foam catalyst with detailed chemistry

Using spatially resolved measurements of temperature and concentration, we critically analyzed the chemistry and transport limitations in the partial oxidation of methane (POM) reaction carried out on Rh, supported on a foam catalyst. The analysis was based on two models, both sharing a detailed surface chemistry but with different gas–surface transport processes. The simulation neglecting transport limitations correctly predicts the outlet concentrations, apparently because of the approach to equilibrium, but significant disagreement was found along the catalysts, particularly in the initial region, demonstrating the existence of regions in which strong diffusive limitations prevail. We developed a pseudo-1D model that can differentiate the species and temperature in the bulk of the gas and at the surface and describe heat (including radiation) and mass transport through correlations with ad hoc parameters based on experimental studies. With this model, we correctly predicted the profiles along the reactor for all species. Only CO2 had a relevant relative error, but its composition was very low. The solid temperature was well reproduced as well, whereas the gas temperature was somewhere higher than the experimental temperature, possibly due to overestimation of the heat transport coefficient. Analysis of the transport limitations found that O2 and H2O had large concentration gradients between gas and surface due to their involvement in the total oxidation, which is a very fast reaction. The analysis thus demonstrated that production and consumption rates at the catalytic surface were frequently sufficiently high so as to enter a diffusive regime. Accordingly, we highlight the need to augment the implementation of detailed surface chemistry with some accounting of the transport processes of both mass and heat. In addition, we show that the Chilton–Colburn analogy can be seriously misleading under these conditions of locally fast heterogeneous kinetics.