Macroporous carrier-free Sr-Ti catalyst for NOx storage and reduction

Abstract

A novel concept is presented, consisting of a macroporous carrier-free catalyst. As a proof of concept, a novel Cu-containing Sr-Ti NSR catalyst with a macroporous network has been synthesized, and its maximum NOx storage capacity (1500μmolNOx/gcatalyst) significantly surpasses that of conventional Pt/Ba/Al2O3 formulations (∼600–800μmolNOx/gcatalyst). This high NOx storage capacity is achieved because the active phases (mainly SrCO3 and Cu-containing Sr-Ti perovskites, as deduced by XRD) are not diluted in an inert carrier, and this can be done because the macroporous structure obtained using a polymethylmethacrylate colloidal crystal template allows the access of gases to the particles bulk. In situ DRIFTS experiments showed that NOx were chemisorbed on the novel Cu-containing Sr-Ti macroporous catalyst as a mixture of nitrite and nitro species, and this suggests that several NOx chemisorption pathways are simultaneously taking place probably involving chemisorption of both NO and NO2. Additionally, this novel catalyst is totally selective towards N2 formation as NOx reduction product, without traces of N2O nor NH3. CO2 and H2O compete with NOx for being chemisorbed on the catalyst, and this hinders the utilization of this catalyst in real diesel exhausts. However, we believe that this new concept of macroporous carrier-free catalyst could be extended to other heterogeneous catalyzed reactions or materials to avoid the diluting effect of the catalyst support.