Understanding the multiple interactions in vanadium-based SCR catalysts during simultaneous NOx and soot abatement

Abstract

The recently proposed 2-way SCRonDPF systems, which consist of selective catalytic reduction (SCR) catalysts coated on diesel particulate filters (DPFs), are promising to simultaneously remove NO_x and soot emissions. However, such multifunctional systems are very demanding due to the presence of various interacting phases as well as concurrent reactions competing for the same active sites. In the present study, a simple to complex strategy was employed to understand the multiple interactions during combined NO_x and soot removal over a V-based SCR catalyst. First the effect of NO₂, NO, H₂O and NH₃ on soot oxidation was studied on soot alone and then in loose and tight contact with the V-based catalyst. In a next step, the effect of various SCR gas mixtures was investigated. For the gas–soot interplay, NO₂, NO and NH₃ were found to exhibit a promotional, non-inhibitory and inhibitory effect on soot oxidation, respectively. Ammonia–soot interaction dominates the soot oxidation in a standard SCR gas feed if no catalyst is present, while the co-presence of NO₂ in the fast SCR gas mixture results in a faster soot oxidation. For triple-phase systems involving also the V₂O₅–WO₃/TiO₂ catalyst, the oxidation of soot begins only after the activation of the standard SCR gas components over the catalyst. In contrast, NO₂ directly interacts with the soot, irrespective of the presence or absence of the V-catalyst. Water was found to promote soot oxidation for all investigated reaction conditions. Interestingly, a small amount of soot was identified to enhance the NO_x conversion at high temperatures. All in all, a full picture of the promotional and inhibitory effects of SCR gases on soot oxidation could be developed, which is important for further improving 2-way SCRonDPF systems.