Fundamental Study On Ceria-zirconia Solid Solution Supporting Copper Oxide Nanocatalyst For NO Reduction

Ceria-zirconia solid solution is one of the most important components in the three-way catalyst for elimination of exhaust gas, proably due to its oxygen storage and release capacity and unique redox property. Therefore, fundamental study on the coordination structure and surface property of copper oxide dispersed on its surface is significant to the design of efficient catalysts for NO reduction. The purpose of present work focused on exploring (1) the influence of support (γ-Al2O3, t-ZrO2, CeO2, Ce0.67Zr0.33O2, hereafter denoted as CZ) structure on the activity and adsorption behavior of copper-based catalysts; (2) the CO or/and NO interaction with CuO/CZ catalyst, and the possible reaction mechanism for NO+CO; (3) the effect of CoOx and MnOx modification on the activity and adsorption of CuO/CZ; (4) the correlation of structual characteristics with catalytic performance over CuO/CexZr1-xO2; (5) the morphology and crystal-plane effects of nanoscale CeO2 on the activity of CuO/CeO2 catalyst. These mentioned catalysts were comparatively studied by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman spectroscopy, high solution transmission electron microscopy (HR-TEM), electron paramagnetic resonance (EPR), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS) and H2-temperature programmed reduction (H2-TPR), in situ Fourier transform infrared spectroscopy (FT-IR) and NO+CO model reaction. It was suggested that(1) The incorporated copper species on ceria (111) surface were in an unstable five-coordination structure, and on t-ZrO2 (111) surface in the elongated environment, whereas onγ-Al2O3 (110) surface were in a symmetrical and stable octahedral coordination. These dissimilarities naturally influenced the synergistic interaction between copper and supports, thus CuO/CeO2 catalyst showed the higher reducibility and activity for NO reduction. In situ FT-IR of NO adsorption/desorption results revealed that compared with those adsorbed species on CuO/t-ZrO2 and CuO/γ-Al2O3, the chelating nitro, bidentate and monodentate nitrates over the ceria-rich phase catalysts were more active to desorb or transform. Hyponitrites were identified on its surface above 100℃due to the formation of oxygen vacancy. Co-interaction of NO+CO results suggested that the adsorption type and reactivity of NOX species were dependent on the supports structure and temperature. The chelating nitro, bidentate and bridge nitrates over CuO/CeO2 surface were more active to react with CO at low temperatures due to its superior redox activity.(2) The dispersed CuO species were the main active components for this reaction. The catalysts showed different activities and selectivity at low and high temperatures, which should be resulted from the reduction of dispersed copper oxide species. This reaction went through different mechanisms at low and high temperatures possibly due to the change of active species. FT-IR results suggested 1) CO was activated by oxygen originating from CZ support, which led to surface carbonates formation, and partial dispersed CuO was reduced to Cu+ species above 150℃.2) NO interacted with the dispersed CuO and formed several types of nitrite/nitrate species, whereas crystalline CuO made little contribution to the formation of new NO adsorbates.3) NO was preferentially adsorbed on CuO-CZ catalysts compared with CO in the reactants mixture. These adsorbed nitrite/nitrate species exhibited different thermal stability and reacted with CO at 250℃. As a result, a possible mechanism was tentatively proposed to approach NO reduction by CO over CuO-CZ catalyst.(3) For CuO-CoOx binary metal oxides system, both copper oxide and cobalt oxide (loadings≤0.5 and 0.32 mmol/100m2 CZ, respectively) were highly dispersed on the surface of CZ support. The addition of cobalt species promoted the reduction of the dispersed copper oxide, and improved the activity of copper oxide supported on CZ due to the strong interaction between dispersed copper oxide and cobalt oxide that resulted in the formation of Cu-O-Co bond, which was dependent on the preparation procedure and cobalt oxide content. The introduction of cobalt oxide altered the adsorption type of NO and CO on these catalysts, and oxidized the NO dimers into ionic NO3-1.For the MnOx modification system, the incorporation of copper and manganese species resulted in the lattice expansion and the decease of microstrain of ceria-zirconia, thus inducing the formation of oxygen vacancies. There was a strong interaction between surface copper, manganese and support via charge transfer (Cu2++Mn2+?Cu++Mn3+; Ce4++Mn2+?Ce3++Mn3+). The addition of manganese species promoted the reduction of the resultant catalysts, and could assist copper oxide in changing the valence and support in supplying oxygen. These reduction behaviors were dependent on the loading amounts of MnOx and impregnation procedure. In addition, the introduction of MnOx cannot change the adsorption type of NO, but readily helped to activate the adsorbed NO species. As a result, these factors were responsible for the enhancement of activity and selectivity through MnOx modification.(4) The ceria-rich (pseudocubic t'') phase could disperse and stabilize the copper species more effectively, and resulted in the stronger interaction with copper than zirconia-rich (t) phase. Furthermore, compared with zirconia-rich phase, the synergistic interaction of copper with ceria-rich phase easily promoted the reduction of copper species and support surface oxygen, as well as the activation of adsorbed NO species. The rapid change in copper velence was an important part of this reaction mechanism. Therefore, CuO/Ce0.8Zr0.202 catalyst exhibited the higher activity for NO reduction than CuO/Ce0.5Zr0.5O2 and CuO/Ce0.2Zr0.8O2. A surface model was proposed to discuss these catalytic properties. The copper species at the interfacial area did not maintain an epitaxial relationship with CexZr1-xO2, while could penetrate into the CexZr1-xO2 surface lattice by occupying the vacant site on the exposed (111) plane. The type and coordination environment of copper species were different in ceria-rich and zirconia-rich phase surface, and their stabilities were related to the microstrains.(5) CeO2 nanopolyhedra were enclosed by (111) and (100) planes, respectively. Nanocubes showed the only (100) planes. Nanorods predominately exposed (110) and (100) planes. Nanorods showed the greater superiority than polyhedra and cubes for dispersing and stabilizing copper oxide species. Moreover, there was a stronger synergistic interaction between dispersed CuO and CeO2 nanorods. CuO/CeO2 polyhedra followed, in sequence and cubes showed the least interaction. This in turn led to a higher surface reducibility and activity of CuO/CeO2 nanorods for NO reduction at low temperatures. Moreover, a proposed surface model suggested that Cu+ ions could penetrate into the surface lattice by occupying the vacant sites in the nanostructure CeO2. As a result, the site geometry and coordination environment of dispersed copper oxide were naturally different on these (111), (110) and (100) planes. The structure stability was related to the lattice strain of CuO/CeO2 catalysts. This surface structure effect brought out the differences in the catalytic properties.