从头开始理解形貌依赖的CuOx-CeO2相互作用

CuOx/CeO2催化剂在CO氧化反应中表现出高催化活性和显著结构敏感性.文献报道中CuOx/CeO2催化剂体系的合成条件差异较大,从而导致观察到的CuOx-CeO2相互作用存在较大争议.因此,系统研究并阐明CuOx/CeO2催化剂中CuOx-CeO2相互作用对于理解复杂的CuOx-CeO2界面催化作用具有重要的研究意义.近期发现,氧化物纳米晶的形貌可作为一种新的结构参数,在不改变氧化物催化剂组成的条件下实现其结构和性能的调控.本文以不同形貌CeO2纳米晶为载体,包括优先暴露{110}+{100}晶面的CeO2纳米棒、优先暴露{100}晶面的CeO2纳米立方体和优先暴露{111}晶面的CeO2纳米多面体,采用等体积浸渍方法合成了Cu担载量为0.025%~5%的CuOx/CeO2纳米晶催化剂,结合谱学和电镜表征方法,以及CO吸附原位红外光谱,系统研究了CuOx物种在不同形貌CeO2纳米晶上的结构演化及其催化CO氧化的构-效关系.结构表征结果表明, CuOx物种结构不仅依赖于Cu的担载量,也依赖于载体CeO2的形貌.随着Cu担载量的增加, CuOx物种优先沉积在CeO2的表面缺陷位,然后聚集和长大;同时伴随着CuOx物种从孤立Cu离子到与载体强/弱相互作用的CuOx团簇,高分散Cu O颗粒和大尺寸Cu O颗粒.孤立Cu^+离子和与载体弱相互作用CuOx团簇主要形成于CeO2纳米立方体的表面,这可能与CeO2纳米立方体暴露的氧终止CeO2{100}晶面相关.CO吸附原位红外结果表明, CuOx团簇与不同CeO2表面相互作用的强度顺序为:CeO2纳米棒暴露的{110}面>CeO2纳米多面体暴露的{111}面>CeO2纳米立方体暴露的{100}面.CeO2纳米立方体与Cu2+离子间相互作用弱于与Cu^+之间的,因此CeO2纳米立方体负载的CuOx物种在CO还原过程中容易停留在稳定的Cu^+中间物种;而CeO2纳米棒与Cu2+离子之间的相互作用强于与Cu^+之间的相互作用,因此CeO2纳米棒负载的CuOx物种在CO还原过程中容易形成金属铜.因此CO吸附原位红外光谱观察到CeO2纳米立方体负载CuOx催化剂中吸附在Cu^+的CO物种远远多于CeO2纳米棒负载CuOx催化剂.CO氧化反应结果表明, CuOx/CeO2催化剂表现出同时依赖于CuOx物种结构和CeO2形貌的结构敏感性.CuOx/CeO2催化剂活性表现出与CuOx/CeO2催化剂的CO还原性能的正相关性,说明中CuOx/CeO2催化CO氧化反应遵循Mv K反应机理.这些结果系统地关联了CeO2形貌, CuOx-CeO2相互作用, CuOx物种结构和CeO2还原性能, CuOx/CeO2催化CO氧化反应活性.     

CO氧化反应中的二氧化铈晶面效应

二氧化铈(CeO2)因其具有较强的储放氧能力,被用作氧化还原反应的催化材料.自2005年,研究者制备出形貌可控的CeO2纳米棒、纳米立方块和纳米多面体,在CeO2形貌控制及构效关系研究方面取得长足发展.各种结构表征手段包括原位拉曼(in situ Raman)、原位傅里叶变换红外光谱(in situ DRIFTS)、核磁共振(NMR)和电镜等被用来研究不同形貌CeO2的表面结构和在催化反应中的活性差异.一般的活性规律为CeO2纳米棒({110}/{100})>纳米立方块({100})>纳米多面体({111}/{100}).近年来,负载型CeO2催化剂因其能稳定分散金属,通过金属-载体相互作用调控界面电子结构并表现出优异的催化活性而引起广泛关注,其中晶面效应在负载型CeO2催化体系中显得较为复杂.铜铈催化剂被认为是非常经济有效的CO氧化催化剂,然而由于制备和测试条件差异导致的CeO2晶面对铜铈催化剂催化CO氧化活性的影响规律并不统一.我们之前的研究工作发现纳米棒CeO2-{110}晶面上的Cu-[Ox]-Ce结构不利于形成Cu((40)),而纳米颗粒CeO2-{111}晶面上的CuOx团簇很容易形成Cu((40)),从而对CO催化氧化极为有利,这与纯载体CeO2的规律并不一致.与此同时,对于铜负载的CeO2纳米棒(NR)及纳米立方体(NC)所体现的性质及活性差异缺少系统深入的研究.在上述工作基础上,我们采用沉积沉淀法在CeO2 NR及CeO2 NC上负载1%wt的铜分别得到1Cu CeNR和1Cu CeNC,并对所合成催化剂的结构和吸附性能进行了表征.高分辨透射电镜(HRTEM)照片显示,CeO2纳米棒主要暴露{110}晶面,而CeO2纳米立方体以{100}晶面为主.催化测试结果表明,1Cu CeNC在130℃时CO已完全转化为CO2,而相同温度下1Cu Ce NR只有50%转化.进一步通过氢气程序升温还原(H2-TPR)和一氧化碳程序升温脱附(CO-TPD)分析发现, 1Cu Ce NC催化剂具有较强的还原性且表面氧物种含量高.此外, X射线光电子能谱(XPS)和in situ DRIFTS研究表明, 1Cu Ce NC促进Cu((40))位点生成,导致活性Cu((40))-CO物种增多,这些优异的化学性质导致其具有较高的催化CO氧化活性.     

CeO2改性Cu/Al2O3催化剂上甲醇水蒸气重整制氢

研究CeO2改性Cu/Al2O3催化剂上甲醇水蒸气重整制氢反应过程,得到低温活性、氢选择性和稳定性较好的催化剂.Cu/Al2O3催化剂中添加CeO2提高了催化剂的活性和稳定性,当CeO2质量分数为20％时,催化剂活性表现最佳.在反应温度250 ℃,水醇摩尔比为1.0,液体空速为3.28 h －1条件下,甲醇转化率为95.5％,氢气选择性为100％.此外,CeO2通过促进水气转化反应降低了重整气中CO的含量.Cu/CeO2/Al2O3催化剂在200 h的寿命实验中,活性仍保持在90.0％以上,而Cu/Al2O3催化剂在100 h的寿命实验中,活性已很快下降.XRD和TPR分析及表面元素分布结果表明,铜和铈相互作用促进了铜在催化剂表面的高度分散,阻止了铜晶粒团聚、烧结,促使铜晶粒细小化,促进了铜的还原,改善了Cu/CeO2/Al2O3催化剂的性能.     

CeO2对CuO／Al2O3分散状态及催化性能的影响

用色谱－液反流动法技术,考察了ＣｕＯ催化剂对ＣＯ的性及添加ＣｅＯ２对催化活性的影响,并运用ＸＲＤ,ＴＰＲ和ＸＰＳ技术,研究了催化剂的物相结构和还原牧场 生。实验结果表明,ＣｅＯ２的添加增加了ＣｕＯ的分散性,使ＣｕＯ的特征衍射峰消失,从而改善了催化剂的氧化活性。     

不同方法制备的CeO2载体对CuO/CeO2催化剂水煤气变换活性和稳定性的影响

分别采用沉淀法(A),水热法(B)和柠檬酸溶胶-凝胶法(C)制备了三种CeO2材料,并以其为载体采用沉积-沉淀法制备了CuO/CeO2催化剂.运用N2物理吸附、粉末X射线衍射(PXRD)、原位粉末X射线衍射(in situ PXRD)、氢气-程序升温还原(H2-TPR)和循环伏安法(CV)等技术对其进行了表征,考察了不同方法制备的CeO2载体对CuO/CeO2水煤气变换(WGS)催化剂的结构、氧化-还原性能、催化活性和稳定性的影响.结果表明,它们的催化活性和稳定性顺序都是CuO/CeO2-A＞CuO/CeO2-B＞CuO/CeO2-C.联系表征结果,CuO/CeO2催化剂的活性与催化剂中CuO的颗粒度、CuO的微观应力和中等大小且与二氧化铈相互作用的CuO的数量等有关,而这些因素很大程度上受CeO2载体本身的热稳定性的影响.根据CV中扫描次数的增加,Cu2+←→Cu0氧化还原峰面积减小,推断CuO/CeO2催化剂在一定条件下氧化还原是不可逆的,这可能是其在反应气氛下经受温度循环之后活性降低的原因.     

CeO2负载的Cu, Ir和Pd催化剂上的醇转移脱氢反应

 研究了CeO2负载的Cu, Ir和Pd催化剂上的醇转移脱氢反应. 利用催化剂体系脱氢和加氢功能的偶合,在较温和的反应条件下实现了二级脂肪醇的高选择性转移脱氢. 其中, Ir/CeO2催化剂表现出较高的催化活性,其TOF值达16.0 h-1.     

不同方法制备的CeO2载体对CuO/CeO2催化剂水煤气变换活性和稳定性的影响

分别采用沉淀法(A), 水热法(B)和柠檬酸溶胶-凝胶法(C)制备了三种CeO2材料, 并以其为载体采用沉积-沉淀法制备了CuO/CeO2催化剂. 运用N2物理吸附、粉末X射线衍射(PXRD)、原位粉末X射线衍射(in situ PXRD)、氢气-程序升温还原(H2-TPR)和循环伏安法(CV)等技术对其进行了表征, 考察了不同方法制备的CeO2载体对CuO/CeO2水煤气变换(WGS)催化剂的结构、氧化-还原性能、催化活性和稳定性的影响. 结果表明, 它们的催化活性和稳定性顺序都是CuO/CeO2-A>CuO/CeO2-B>CuO/CeO2-C. 联系表征结果, CuO/CeO2催化剂的活性与催化剂中CuO的颗粒度、CuO的微观应力和中等大小且与二氧化铈相互作用的CuO的数量等有关, 而这些因素很大程度上受CeO2载体本身的热稳定性的影响. 根据CV中扫描次数的增加, Cu2+←→Cu0氧化还原峰面积减小, 推断CuO/CeO2催化剂在一定条件下氧化还原是不可逆的, 这可能是其在反应气氛下经受温度循环之后活性降低的原因.     

TPR, TPO, and TPD examinations of Cu0.15Ce0.85O(2-y) mixed oxides prepared by co-precipitation, by the sol-gel peroxide route, and by citric acid-assisted synthesis

Temperature-programmed reduction (TPR), oxidation (TPO), and desorption (TPD) studies were performed on three copper-ceria mixed oxide samples having the same nominal composition, Cu0.15Ce0.85O(2-y), but prepared in three different ways: by co-precipitation, the sol-gel peroxide route, and the sol-gel citric acid route. The obtained results reveal that despite a drastic initial drop in specific surface area after consecutive redox cycles, the hydrogen consumption remains constant. This is because CuO is highly dispersed over the surface of CeO2 nanocrystallites and remains highly dispersed even after the agglomeration of CeO2 nanocrystallites in a denser secondary structure. The dispersed CuO is reduced to Cu(0) during the TPR, forming agglomerated metal particles on the surface of partially reduced CeO2. However, after subsequent temperature-programmed oxidation all the Cu(0) is oxidized back into CuO and redispersed over the CeO2 crystallites.     

草酸盐热分解法制备CeO_2负载CuO催化剂用于CO优先氧化反应

采用草酸盐热分解-浸渍法制备了一系列不同CuO负载量的CuO/CeO_2催化剂,并将其用于CO优先氧化(PROX)研究.当CuO负载量为10%时催化剂活性最高,具有温区最宽且温度最低的CO完全转化窗口(96~160℃),并且当反应温度低于131℃时,产物中CO2选择性始终保持100%.研究结果表明,当负载少量CuO时,Cu~(2+)离子会进入CeO_2晶格形成固溶体;进一步提高CuO负载量会导致CuO在CeO_2表面聚集.对于CuO/CeO_2催化剂,形成Cu-Ce固溶体会在催化剂表面生成大量的表面氧空位和Ce3+;Ce3+则与Cu~(2+)作用产生更多的表面Cu~+,而Cu~+是CO PROX的活性中心,因此表面Ce3+含量的提高和Cu-Ce之间相互作用的增强是活性提高的主要原因.与普通沉淀-浸渍法制备的CuO/CeO_2催化剂相比,草酸盐热分解-浸渍法制备的催化剂更有利于Cu-Ce固溶体的形成,从而具有更多的表面Ce3+和更强的表面Cu-Ce相互作用,因此具有更高的CO优先氧化活性.     

A new type of strong metal-support interaction and the production of H2 through the transformation of water on Pt/CeO2(111) and Pt/CeO(x)/TiO2(110) catalysts

The electronic properties of Pt nanoparticles deposited on CeO(2)(111) and CeO(x)/TiO(2)(110) model catalysts have been examined using valence photoemission experiments and density functional theory (DFT) calculations. The valence photoemission and DFT results point to a new type of "strong metal-support interaction" that produces large electronic perturbations for small Pt particles in contact with ceria and significantly enhances the ability of the admetal to dissociate the O-H bonds in water. When going from Pt(111) to Pt(8)/CeO(2)(111), the dissociation of water becomes a very exothermic process. The ceria-supported Pt(8) appears as a fluxional system that can change geometry and charge distribution to accommodate adsorbates better. In comparison with other water-gas shift (WGS) catalysts [Cu(111), Pt(111), Cu/CeO(2)(111), and Au/CeO(2)(111)], the Pt/CeO(2)(111) surface has the unique property that the admetal is able to dissociate water in an efficient way. Furthermore, for the codeposition of Pt and CeO(x) nanoparticles on TiO(2)(110), we have found a transfer of O from the ceria to Pt that opens new paths for the WGS process and makes the mixed-metal oxide an extremely active catalyst for the production of hydrogen.     

Comparative Study of CuO Species on CuO/Al2O3, CuO/CeO-Al2O3 andCuO/La2O-Al2O3 Catalysts for CO Oxidation

CuO/Al2O3, CuO/CeO2-Al2O3, and CuO/La2O3-Al2O3 (denoted as Cu/Al, Cu/CeAl, and Cu/LaAl) catalysts were prepared by an impregnation method. CuO species and CuO/Al2O3 thermal solid-solid interaction were characterized by in situ XRD, Raman spectroscopy and H2-TPR techniques. For the Cu/Al catalyst, a CuAl2O4 phase exists between the CuO and Al2O3 layer and the CuO phase exists on the surface in both highly dispersed and bulk forms. For the Cu/CeAl catalyst, there is highly dispersed and bulk CuO on the surface, but most of the CuO has transferred into the internal layer of CeO2 as bulk CuO and CuAl2O4. For the Cu/LaAl catalyst, only bulk CuO is present on the surface of the catalyst and no CuAl2O4 is formed. The catalytic activity order for CO oxidation is Cu/CeAl>Cu/Al>Cu/LaAl. The highly dispersed CuO on the catalyst surface may be the active phase for CO oxidation. The results show that the addition of CeO2 not only promotes both the transference of CuO and the formation of CuAl2O4 but also favors the CO oxidation due to the association of highly dispersed CuO with CeO2, while La2O3 hinders the transference of CuO and the formation of CuAl2O4.     

Cux (x=1-4)团簇在CeO2(111)表面的吸附

用基于密度泛函理论的第一性原理方法研究了Cu团簇(Cu_x, x=1-4)在CeO₂(111)表面的吸附. 研究发现当团簇比较小时(x=2, 3), 倾向于平铺表面; 当x=4时, Cu团簇在CeO₂(111)表面以三维的四面体结构吸附较为稳定, 从Cu 3d到Ce 4f的电荷转移使Cu团簇带正电荷. 由二维的菱形结构到三维的四面体结构的转变势垒为1.05 eV, 并且其中一个Cu原子直接迁移到另外三个Cu原子的空位顶部的转变路径比较有利. 在Cu团簇与CeO₂的相互作用过程中, Cu-O和Cu-Cu相互作用的竞争最终决定了Cu团簇在CeO₂上的形貌. 这种CeO₂(111)负载的带正电的三维Cu团簇将对水分解, 进而对水煤气反应具有高的催化活性.     

Effect of CeO2 preparation method and Cu loading on CuO/CeO2 catalysts for methane combustion

CeO2 was synthesized by sol-gel, hydrothermal, nitrate thermal decomposition methods, respectively, and used as support to prepare CuO/CeO2 catalysts. According to characterization and reaction results, preparation method of CeO2 had a great influence on the physicochemical properties and activities of CuO/CeO2 catalysts. CuO with high dispersion and strong interaction with CeO2 was highly active in methane combustion, while CuO particles less associated with CeO2 showed less activity. The CuO catalyst supported on CeO2 which was prepared via nitrate thermal decomposition method showed the largest area, the smallest particle size, the highest dispersion of copper species and strong support metal interactions. Therefore, it presented the highest redox ability and activity for methane combustion. Activities of the catalysts with different copper content kept increasing until 5% Cu loading and from then on kept constant. Moreover, methane conversion decreased as methane space velocities increased on CuO/CeO2 catalyst. Addition of CO2 to the feed did not produce a significant effect on the catalytic activity, but the presence of H2O provoked a remarkable decrease on the activity of CuO/CeO2 catalyst.     

La-Ce-Cu系列催化剂SO2中毒机理研究

采用NH_4NO_3共熔法合成La－Ce－Cu系列样品，并通过XRD分析了样品的相组成。XRF证明了系列样品组成均为氧缺陷型化合物．用TEM研究了样品的表面结构，发现随着SO_2中毒的加深，Cu向表面偏析与S结合生成新的化合物，导致了原晶体结构的变形，同时证明纯CeO_2不发生S中毒．XPS研究表明：样品La_(.029)Ce_(0.57)Cu_(0.14)O_x中毒后Cu从La_2CuO_4中分解出来并与S结合生成CuSO_4；相应的CeO_2参与了中毒反应，生成Ce_2O_3,整个中毒反应如下：La_2CuO_4＋2CeO_2＋SO_2＝CuSO_4＋La_2O_3＋Ce_2O_3     

Cu-CeO2基阳极直接甲烷SOFC的制备及其性能

采用干压法制备了NiO-YSZ(氧化钇稳定氧化锆)/(ZrO2)0.89(Sc2O3)0.1(CeO2)0.01(10ScSZ-1CeO2)半电池, 经还原-酸溶法除去NiO制备了多孔YSZ负载致密10ScSZ-1CeO2双层结构, 通过浸渍法在多孔YSZ阳极基体中引入Ce、Cu的硝酸盐制备Cu-CeO2-YSZ复合阳极, 结合La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)阴极构建了Cu-CeO2-YSZ/10ScSZ-1CeO2/LSCF单元电池. 通过X射线衍射(XRD)和场发射扫描电镜(FESEM)等手段对电池单元的物相、微观结构进行表征. 结果表明: 还原-酸溶法制备的YSZ/10ScSZ-1CeO2双层结构的YSZ基体具有孔隙率高(>64%)、孔洞连通性好的微观结构, 有助于采用浸渍法引入Ce、Cu硝酸盐; 10ScSZ-1CeO2电解质薄膜致密无缺陷, 厚约30 μm. 电性能测试表明所构建单元固体氧化物燃料电池(SOFC)具有良好的电性能输出, 在650 ℃以湿H2和CH4为燃料时的最大功率密度分别为0.29和0.09 W·cm-2; 在700 ℃以湿H2和CH4为燃料时的最大功率密度分别达到0.48 和0.21 W·cm-2. 优良的电性能主要归功于小的电解质内阻和阴极极化电阻以及良好的阳极微观结构.     

CuO／CeO_2／γ－Al_2O_3催化剂的结构及氧物种

应用ＸＲＤ、ＥＳＲ及ＸＰＳ方法研究了ＣｕＯ／ＣｅＯ２／γ－Ａｌ２Ｏ３催化剂的体相及表面结构，并对催化剂表面氧物种进行了表征．实验结果表明，ＣｅＯ２使Ｃｕ２＋还原为低价钢离子和增加钢化合物的分散性．催化剂中ＣｅＯ２含量高时，表面有Ｏ２－和Ｏ－物种，而ＣｅＯ２含量低时，只有Ｏ２－物种．     

Preferential oxidation of CO in excess H2 over CeO2/CuO catalyst:Effect of calcination temperature

Different from the classical configuration CuO/CeO2 catalyst,the inverse configuration CeO2 /CuO catalyst (atomic ratio of Ce/Cu=10/100) was prepared by impregnation method.Five calcination temperatures were selected to investigate the interaction between CeO2 and CuO support.It is found that as calcination temperature increased from 500 to 900 C,sintering of CeO2 particles on the support occurred together with the diffusion of a portion of Ce 4+ ions into CuO crystals,forming solid solution.Formation of interface complex Ce-O-Cu was suggested by TPR measurements.The catalyst calcined at 700 C gives the highest activity for preferential oxidation of CO in excess H2 stream.     

Thermodynamic, electronic and structural properties of Cu/CeO2 surfaces and interfaces from first-principles DFT+U calculations

The thermodynamic, structural and electronic properties of Cu-CeO(2) (ceria) surfaces and interfaces are investigated by means of density functional theory (DFT+U) calculations. We focus on model systems consisting of Cu atoms (i) supported by stoichiometric and reduced CeO(2) (111) surfaces, (ii) dispersed as substitutional solid solution at the same surface, as well as on (iii) the extended Cu(111)/CeO(2)(111) interface. Extensive charge reorganization at the metal-oxide contact is predicted for ceria-supported Cu adatoms and nanoparticles, leading to Cu oxidation, ceria reduction, and interfacial Ce(3+) ions. The calculated thermodynamics predict that Cu adatoms on stoichiometric surfaces are more stable than on O vacancies of reduced surfaces at all temperatures and pressures relevant for catalytic applications, even in extremely reducing chemical environments. This suggests that supported Cu nanoparticles do not nucleate at surface O vacancies of the oxide, at variance with many other metal/ceria systems. In oxidizing conditions, the solid solutions are shown to be more stable than the supported systems. Substitutional Cu ions form characteristic CuO(4) units. These promote an easy and reversible O release without the reduction of Ce ions. The study of the extended CeO(2)(111)/Cu(111) interface predicts the full reduction of the interfacial ceria trilayer. Cu nanoparticles supported by ceria are proposed to lie above a subsurface layer of Ce(3+) ions that extends up to the perimeter of the metal-oxide interface.     

The synergistic effects of the Cu-CeO2(111) catalysts on the adsorption and dissociation of water molecules

The interaction of water molecules with the Cu-CeO(2)(111) catalyst (Cu/CeO(2) and Cu(0.08)Ce(0.92)O(2)) is studied systematically by using the DFT+U method. Although both molecular and dissociative adsorption states of water are observed on all the considered Cu-CeO(2)(111) systems, the dissociation is preferable thermodynamically. Furthermore, the dissociation of water molecule relates to the geometric structure (e.g. whether or not there are oxygen vacancies; whether or not the reduced substrate retains a fluorite structure) and the electronic structure (e.g. whether or not there is reduced cerium, Ce(3+)) of the substrate.In addition, the adsorption of water molecules induces variations of the electronic structure of the substrate, especially for Cu/CeO(2-x)(111)-B (a Cu atom adsorbed symmetrically above the vacancy of the reduced ceria) and highly reduced Cu(0.08)Ce(0.92)O(2)(111), i.e. the Cu(0.08)Ce(0.92)O(2-x)(111)-h. The variations of electronic structure promote the dissociation of water for the highly reduced system Cu(0.08)Ce(0.92)O(2-x)(111)-h. More importantly, the improvement of WGS reaction by Cu-ceria is expected to be by the associative route through different intermediates.     

甲醇水蒸汽重整制氢Cu/Zn/Ce/Al催化剂的XPS研究

利用XPS技术，研究了在甲醇水蒸汽重整制氢过程中，添加稀土元素Ce对Cu/Zn/Al催化剂表面结构的影响。发现，催化剂表面存在Cu^2 和Cu^ 物种；CeO2的加入促进表面Cu^ 物种的生成，提高了表面Cu^ 物种的浓度。Cu60/Zn30/Al5/Ce5催化剂表面具有较高的Cu^ 浓度，是催化剂长时间保持高活性的重要原因之一。