【作者】 刘连军；

【导师】 董林；

【作者基本信息】 南京大学 ， 化学， 2010， 博士

【摘要】 铈锆固溶体具有良好的储氧释氧性能和氧化还原性,作为三效催化剂的重要成分而应用于净化机动车尾气。研究分散态铜物种在其表面的配位结构和表面性质,对设计高效消除NO催化剂具有重要意义。如何提高NO还原的低温活性和选择性,以及认清NO还原的吸附反应机理值得进一步研究。因此,本文结合热重差热分析、X-射线衍射、拉曼光谱、紫外可见吸收光谱,透射电子显微镜、光电子能谱,电子顺磁共振,氢气程序升温还原、原位红外和NO+CO模型反应等手段对铜铈基催化剂进行表征,主要目的是研究(1)载体(γ-Al2O3, t-ZrO2, CeO2, Ce0.67Zr0.33O2,定义为CZ)结构对铜基催化剂的活性和吸附行为的影响；(2)CO或/和NO分子与CuO/CZ催化剂的相互作用,催化剂反应前后的状态以及NO+CO可能的反应机理；(3)CoOx、MnOx改性对CuO/CZ催化剂的还原性质、吸附行为和活性的影响；(4)CuO/CexZr1-xO2(x=0.2,0.5,0.8)催化剂的结构特征与在NO+CO反应中催化性质之间的关系；(5)纳米二氧化铈的形貌和暴露晶面对CuO/CeO2催化剂催化还原NO的活性影响。结果表明：(一)分散态铜物种可以嵌入载体的表面空位,在氧化铈(111)面是不对称的五配位结构,在四方氧化锆(111)表面是拉伸的三角双锥构型,而在氧化铝(110)表面则处于对称八面体结构。表面结构的非相似性决定了载体和铜物种之间协同作用存在差异,所以CuO/CeO2催化剂表现出较强的低温还原能力和较高的还原NO活性。原位红外NO吸附／脱附结果表明与吸附在CuO/γ-Al2O3, CuO/t-ZrO2表面的NO物种相比,吸附在富铈相催化剂表面的螯合亚硝基、双齿和单齿硝酸盐非常活泼,很容易脱附或转换,且在高于100℃由于表面生成氧空位NO得到电子生成NO而聚集形成连二亚硝酸盐。原位红外共吸附结果表明吸附态NOx物种的类型和活性取决于载体结构和反应温度。相对于其他样品,吸附在CuO/CeO2表面的螯合亚硝基、桥式和双齿硝酸盐则非常活泼,在低温下可以和CO反应。(二)对于xCuO/CZ体系,分散态氧化铜是该反应的主要活性物种。该催化剂在低温和高温阶段表现出不同的活性和选择性,是由分散态铜物种被还原所致。这也说明了由于活性物种在高温发生变化,该反应在低温和高温会经历不同的反应机制。原位红外结果表明(1)CO会被来自载体的活性氧氧化,结果导致生成表面碳酸盐,且有部分氧化铜在15℃后会被CO还原成Cu+；(2)NO和分散态CuO作用能够形成各种结构形式的硝酸盐和亚硝酸盐物种,而晶相氧化铜却不会引起新NO吸附物种的产生；(3)在混合反应气氛中,NO分子优先同催化剂作用。这些吸附物种表现出不同的热稳定性,且在250℃会和CO发生反应。(三)对于CuO-CoOx二元氧化物体系而言,铜氧化物和钴氧化物(负载量分别小于0.5和0.32 mmol/100m2 CZ)可以高度分散在载体表面。加入钴物种可以促进铜物种和表面氧的还原,并可以提高反应活性,主要是因为铜和钴氧化物之间存在强相互作用从而形成Cu-O-Co键,取决于浸渍顺序和钻的负载量。同时,由于分散态钴和铈锆载体表面间的相互作用导致表面氧容易释放,活性氧可以将NO分子聚合体氧化成N03-1离子。对于MnOx修饰CuO/CZ体系,一方面加入锰氧化物和铜氧化物会导致铈锆载体晶格膨胀和晶格应力下降,结果诱导产生了更多氧空位。同时表面铜、锰物种和铈锆载体之间会发生电荷迁移,即Cu2++Mn2+(?)Cu++Mn3+;Ce4++Mn2+(?0)Ce3++Mn3+；另一方面,加入锰氧化物可以有效地促进催化剂的还原,协助铜物种快速改变价态和载体提供表面氧。这些还原行为主要取决于锰的负载量和浸渍顺序。此外,引入锰物种并不会改变NO分子的吸附结构,也不会引起新物种产生,但是可以活化NO吸附物种,降低其脱附温度。因此,这些因素决定了MnOx修饰可以显著提高该反应的活性和选择性。(四)CuO在CexZr1-xO2表面的分散容量受载体表面结构的影响,富铈相(假立方t’)比富锆相(四方t)更能有效地分散和锚定表面铜物种。铜物种在载体表面并没有外延生长,是通过占据载体(111)面上的空位从而嵌入表面晶格中,结果与富铈相载体产生了更强的协同作用。CuO/CexZr1-xO2催化剂的配位环境和晶格应力是影响氧化铜和载体之间协同效应差异的原因之一。在此基础上提出了铜物种嵌入在铈锆固溶体表面可能的模型结构,认为分散态氧化铜在富铈相(t″)和富锆相(t)上的配位环境有差异,其结构稳定性与晶格应力相关。同富锆相催化剂相比,富铈相具有更强的能力促进铜物种和载体表面氧的还原。原位红外结果表明分散态铜物种和假立方相之间的协同效应更容易推动CO脱附和活化NO吸附物种,但是对于四方相催化剂却相对困难。因此,CuO/Ce0.8Zr0.202催化剂比CuO/Ce0.5Zr0.502和CuO/Ce0.2Zr0.8O2表现出更高的NO还原活性。(五)和CeO2纳米多面体和立方块相比(分别主要暴露(111)和(100)面),Ce02纳米棒(优先暴露(110)和(100)面)可以更有效地分散和锚定表面氧化铜物种。同时,分散态CuO和CeO2纳米棒之间存在更强的相互作用,因而导致了该催化剂有更强的还原能力和更高的NO还原活性。根据提出的表面模型认为Cu2+离子通过占据氧化铈中的空位而嵌入到表面晶格中,且在(111)、(110)和(100)面上处于不同的配位环境。因此,这种不同的表面结构效应会导致分散态氧化铜物种的几何空间结构存在差异,从而影响了CuO和CeO2之间的协同效应。此外,CuO/CeO2纳米棒的晶格应力较大,因而五配位结构不稳定；CuO/CeO2的晶格应力较小,因而八面体配位结构更稳定,结果必然带来表面氧化铜物种催化性质的差异。更多还原

【Abstract】 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.更多还原