Mechanism And Property Of Composite Metal Oxide For Catalytic Removal Of NO_x

Nitrogen oxide (NO_X) is an environmental pollutant because it contributed to catalytic destruction of stratospheric ozone and does harm to human being. Compared with perovskites, selected metal exchanged zeolites (principally Cu, Co, Rh or Pd-ZSM-5), noble metals, and metal oxides prepared by traditional methods, the mixed oxides obtained from calcinations of hydrotalcite-like layered compounds have high surface areas, strong alkalescence and good catalytic activity. Therefore, hydrotalcite-like compounds are potential catalyst precursors for the chemical absorption and reduction of nitrogen oxide.CuAl, CuFeAl, NiFe hydrotalcite-like layered compounds (CuAl-CO₃-LDHS, CuFeAl-CO₃-LDHs, NiFe-CO₃-LDHs) were synthesized by coprecipitation, which were converted into CuAl, CuFeAl, NiFe mixed metal oxides after calcinations at 473 K to 1173 K. XRD, SEM, and FT-IR were used to characterize the structure and morphology of the samples. The hydrotalcites with diameter of 2～7 μm could synthesized at pH=10¹1, and its mixed oxides with diameter of 0.25～0.4 μm were obtained after calcinations at about 773 K. When CuAl-CO₃-LDHs and CuFeAl-CO₃-LDHs are prepared, there existed CuO in resultants because of "Jahn-Teller" effect and its content increased with the increase of molar ratios of Cu/Al or Cu/(Fe+Al).The oxidative adsorption of nitrogen dioxide has been carried out on CuAl and CuFeAl mixed oxides obtained by calcinations of CUAI-CO₃-LDHS and CuFeAl-CO₃-LDHs at about 500℃. The mechanism of NO₂ oxidative adsorption is investigated by means of XRD and FTIR. The results show that NO₂, O₂ and water vapor is adsorbed on the surface of mixed metal oxides, and reacts on each other to form HNO3; Then, HNO3 reacts with CuAl and CuFeAl mixed oxides, resulting in the formation of Cu(NO₃)₂. After placing the samples obtained in the desiccator for 15 days, the layered structure is reconstructed due to "memory effect" of calcined hydrotalcites, leading to the formation of CuAl-NO₃-LDHs and CuFeAl-NO₃-LDHs with diameter of 1.0-2.0 μm. However, without water vapor in the reagent bottle, CuAl and CuFeAl mixed oxides only physically adsorb NO₂, and no new species come into being.