The relationship of structural defect–redox property–catalytic performance of perovskites and their related compounds for CO and NOx removal

Perovskite-type oxides La1−xAx′Co1−yBiyO3−δ (Ax′ = Ba0.2, Sr0.4; y=0, 0.2) and La1−xSrxMO3−δ (M = Co0.77Bi0.20Pd0.03, x=0, 0.2, 0.4) and perovskite-like oxides La1.867Th0.100CuO4−δ, Nd2−xAx′CuO4−δ (Ax′ = Ba0.4, Ce0.2), and YBa2Cu3O7−δ have been investigated as catalysts for CO oxidation, NO removal, and N2O decomposition, respectively. X-ray diffraction results revealed that (i) all of these materials were single phase, and (ii) the crystal structures of La1−xAx′Co1−yBiyO3−δ, La1−xSrxMO3−δ, La1.867Th0.100CuO4−δ, Nd2−xAx′CuO4−δ, and YBa2Cu3O7−δ were cubic, orthorhombic, tetragonal (T structure), tetragonal (T′ structure), and orthorhombic, respectively. The results of chemical analysis indicated that (i) there were Co4+/Co3+ ions in La1−xAx′CoO3−δ (Ax′= Ba0.2, Sr0.4), Co2+/Co3+ and Bi5+/Bi3+ ions in La1−xAx′Co0.8Bi0.2O3−δ (Ax′= Ba0.2, Sr0.4) and La1−xSrxMO3−δ, Cu2+/Cu3+ ions in La1.867Th0.100CuO4−δ, Nd2CuO4−δ, Nd1.6Ba0.4CuO4−δ, and YBa2Cu3O7−δ; and (ii) after pretreatments in H2 or helium at certain temperature, Cu+/Cu2+ ion couples appeared in these cuprate samples. Oxygen isotope exchange experiments indicated that the lattice oxygen mobility in the Bi-doped catalysts were much higher than that in the Bi-free ones. TPR results showed that lattice oxygen in the former samples could be reduced at temperatures lower than those in the latter samples. In the oxidation of CO, the Bi-incorporated catalysts performed much better than the corresponding Bi-free catalysts, the Sr-substituted perovskites showed higher catalytic activities than the Ba-substituted ones; among La1−xSrxMO3−δ, La0.8Sr0.2MO2.90 exhibited the best catalytic activity. The improved catalytic performance due to the Sr (or Ba)- and Bi-doping is believed to be associated with the enhancements in oxygen vacancy density and Con+/Co(n+1)+ (n=2, 3) and Bi3+/Bi5+ couple redox ability as well as in lattice oxygen mobility. In the elimination of NO over La1−xSrxMO3−δ, La0.8Sr0.2MO2.90 performed the best. The 300 °C-reduced La1.867Th0.100CuO4−δ catalyst that possessed dual cationic and anionic defects and Cu+/Cu2+ couple showed higher DeNO activity than the fresh one; the redox action between Cu+ and Cu2+ is an essential process for NO decomposition. In the decomposition of N2O, the 800 °C-treated Nd2−xAx′CuO4−δ (Ax′= Ba0.4, Ce0.2) and YBa2Cu3O7−δ samples were superior in catalytic performance to their fresh counterparts; oxygen vacancies were favorable for the formation of the crucial N2O22− intermediate species in N2O activation, and the redox Cup+/Cu(p+1)+ (p=1 and 2) couples involved in the N2O decomposition processes. The DeN2O activity over the Ce- or Ba-doped catalyst was much better than that over the undoped catalyst (Nd2CuO4−δ). This behavior is intimately related to the oxygen nonstoichiometry and copper ion redox properties.