NO_x Storage Performance at Low Temperature over Platinum Group Metal-Free SrTiO₃-Based Material

Pt-based catalysts are commonly employed as NO_x-trapping catalysts for automobiles, while perovskite oxides have received attention as Pt-free NO_x-trapping catalysts. However, the NO_x storage performance of perovskite catalysts is significantly inferior at low temperatures and with coexisting gases such as H₂O, CO₂, and SO₂. This study demonstrates that NO_x storage reactions proceed over redox site (Mn, Fe, and Co)-doped SrTiO₃ perovskites. Among the examined catalysts, Mn-doped SrTiO₃ exhibited the highest NO_x storage capacity (NSC) and showed a high NSC even at a low temperature of 323 K. Moreover, the high NO_x storage performance of Mn-doped SrTiO₃ was retained in the presence of poisoning gases (H₂O, CO₂, and SO₂). NO oxidation experiments revealed that the NSC of Co-doped SrTiO₃ was dependent on the NO oxidation activity from NO to NO₂ via lattice oxygen, which resulted in an inferior NSC at low temperatures. On the other hand, Mn-doped SrTiO₃ successfully adsorbed NO molecules onto its surface at 323 K without the NO oxidation process using lattice oxygens. This unique adsorption behavior of Mn-doped SrTiO₃ was concluded to be responsible for the high NSC in the presence of poisoning gases.