New insights into MnCe(Ba)O_x/TiO₂ composite oxide catalyst: Barium additive accelerated ammonia conversion

MnCeO_x/TiO₂ has been widely used in selective catalytic reduction (SCR) of NO_x at low temperature. However, it is often poisoned in the presence of water vapor and sulfur dioxide. In this work, the promotion mechanism of Ba modification was investigated. Results show that the doped BaO reacts with CeO₂ and forms BaCeO₃. This unique perovskite structure of BaCeO₃ significantly enhances NO oxidation and NH₃ activation of MnCeO_x/TiO₂ catalyst so that the NO conversion and the resistances to SO₂ improve. It is found that Ba species obviously promotes the NO adsorption ability and improve the redox properties of MnCeO_x/TiO₂ catalyst. While the acid properties of the catalyst are inhibited by Ba modification and among which Lewis acid sites are dominant for both MnCeO_x/TiO₂ and MnCe(Ba)O_x/TiO₂ catalysts. Furthermore, in situ DRIFT experiments reveal that the NO reduction upon MnCeO_x/TiO₂ and MnCe(Ba)O_x/TiO₂ catalysts follows both E-R and L-H mechanisms, in which L-H is preferred. Ba species enhances the formation of active nitrate species, which accelerates the NO reduction through L-H mechanism. It is interesting that although Ba species weakens the NH₃ adsorption, it induces the ammonia conversion to coordination ammonia, which in turn accelerates the catalytic reaction.