The relationship of structural defect–redox property–catalytic performance of perovskites and their related compounds for CO and NO x removal

Abstract

Perovskite-type oxides La 1− x A x ′Co 1− y Bi y O 3− δ (A x ′ = Ba 0.2, Sr 0.4; y=0, 0.2) and La 1− x Sr x MO 3− δ (M = Co 0.77Bi 0.20Pd 0.03, x=0, 0.2, 0.4) and perovskite-like oxides La 1.867Th 0.100CuO 4− δ , Nd 2− x A x ′CuO 4− δ (A x ′ = Ba 0.4, Ce 0.2), and YBa 2Cu 3O 7− δ have been investigated as catalysts for CO oxidation, NO removal, and N 2O decomposition, respectively. X-ray diffraction results revealed that (i) all of these materials were single phase, and (ii) the crystal structures of La 1− x A x ′Co 1− y Bi y O 3− δ , La 1− x Sr x MO 3− δ , La 1.867Th 0.100CuO 4− δ , Nd 2− x A x ′CuO 4− δ , and YBa 2Cu 3O 7− δ were cubic, orthorhombic, tetragonal (T structure), tetragonal (T′ structure), and orthorhombic, respectively. The results of chemical analysis indicated that (i) there were Co 4+/Co 3+ ions in La 1− x A x ′CoO 3− δ (A x ′= Ba 0.2, Sr 0.4), Co 2+/Co 3+ and Bi 5+/Bi 3+ ions in La 1− x A x ′Co 0.8Bi 0.2O 3− δ (A x ′= Ba 0.2, Sr 0.4) and La 1− x Sr x MO 3− δ , Cu 2+/Cu 3+ ions in La 1.867Th 0.100CuO 4− δ , Nd 2CuO 4− δ , Nd 1.6Ba 0.4CuO 4− δ , and YBa 2Cu 3O 7− δ ; and (ii) after pretreatments in H 2 or helium at certain temperature, Cu +/Cu 2+ 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 La 1− x Sr x MO 3− δ , La 0.8Sr 0.2MO 2.90 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 Co n+ /Co ( n+1)+ ( n=2, 3) and Bi 3+/Bi 5+ couple redox ability as well as in lattice oxygen mobility. In the elimination of NO over La 1− x Sr x MO 3− δ , La 0.8Sr 0.2MO 2.90 performed the best. The 300 °C-reduced La 1.867Th 0.100CuO 4−δ catalyst that possessed dual cationic and anionic defects and Cu +/Cu 2+ couple showed higher DeNO activity than the fresh one; the redox action between Cu + and Cu 2+ is an essential process for NO decomposition. In the decomposition of N 2O, the 800 °C-treated Nd 2− x A x ′CuO 4− δ (A x ′= Ba 0.4, Ce 0.2) and YBa 2Cu 3O 7−δ samples were superior in catalytic performance to their fresh counterparts; oxygen vacancies were favorable for the formation of the crucial N 2O 2 2− intermediate species in N 2O activation, and the redox Cu p+ /Cu ( p+1)+ ( p=1 and 2) couples involved in the N 2O decomposition processes. The DeN 2O activity over the Ce- or Ba-doped catalyst was much better than that over the undoped catalyst (Nd 2CuO 4− δ ). This behavior is intimately related to the oxygen nonstoichiometry and copper ion redox properties. According to the outcome of our experiments, we conclude that there is a strong correlation either between the structural defect (mainly oxygen vacancies) and catalytic activity or between the redox [Co n+ /Co ( n+1)+ ( n=2, 3), Bi 3+/Bi 5+, and Cu p+ /Cu ( p+1)+ ( p=1 and 2) couples] ability and catalytic performance of these materials for CO and NO x removal. The generation of oxygen vacancies by A-site replacements favors the activation of O 2 and NO x ; the modification of B-site ion oxidation states by aliovalent ion substitutions in A- and/or B-sites promotes the redox process of the catalyst. Both actions influence the mobility of lattice oxygen. As a result of the combined effect, one can generate a kind of materials that show good catalytic performance for the elimination of CO and NO x .