La2−xCexCu1−yZnyO4 perovskites for high temperature water-gas shift reaction

The performance of La_2−xCe_xCu_1−yZn_yO₄ perovskites as catalysts for the high temperature water-gas shift reaction (HT-WGSR) was investigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites exhibited the La₂CuO₄ orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the La_1.90Ce_0.10CuO₄ perovskite alone, when calcined at 350/700 °C, also showed a (La_0.935Ce_0.065)₂CuO₄ perovskite with tetragonal structure, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those calcined at 350/700 °C and, among these, La_1.90Ce_0.10CuO₄ was outstanding, probably because of the high surface area associated with the presence of the (La_0.935Ce_0.065)₂CuO₄ perovskite with tetragonal structure and orthorhombic La₂CuO₄ phase.     

La1-xCaxMnO3纳米颗粒的制备及氧还原催化性能

本文采用溶胶凝胶法制备了一系列不同Ca含量的钙钛矿型氧化物La_1-xCa_xMnO₃(x=0~0.4)纳米颗粒, X射线粉末衍射及精修、扫描电镜表征显示其相纯度和结晶度高, 颗粒平均粒径约40 nm。在0.1 mol·L^-1 KOH水溶液中进行的氧还原电催化性能测试显示, La_0.7Ca_0.3MnO₃样品催化活性最高, 表观电子转移数接近4, 还原电流密度与Pt/C催化剂相当, 而催化稳定性优于Pt/C。进一步研究了La_1-xCa_xMnO₃样品中Mn价态、晶胞参数的改变对氧还原催化活性的影响, 结果表明当x=0.3时, 催化剂中Mn处于混合价态, Mn-O键长适中, 最有利于电催化反应。     

合成方法对Rh/CeO2-ZrO2-La2O3催化剂的结构、表面性能及DeNOx活性的影响

用沉积沉淀法合成两种不同系列的CeO2-ZrO2-La2O3混合氧化物(ZrO2和La2O3沉积CeO2粒子(标记为A-x)以及CeO2和La2O3沉积ZrO2粒子(标记为B-x)),并用作Rh催化剂的载体。XRD、拉曼、TPR、XPS和O2脉冲等表征结果显示出不同的沉积顺序将导致不同的结构和氧化还原性能,且B-x具有更高的氧迁移性、储氧能力和表面Ce浓度。当其负载Rh后,Rh/B-x催化剂具有更高的NO和CO转化率及N2选择性,且Ce的最佳含量为50at%。这可能归因于Rh负载于富铈表面形成更多有利于NO分解的表面Ce3+活性位。     

La1-xCaxMnO3纳米颗粒的制备及氧还原催化性能

本文采用溶胶凝胶法制备了一系列不同Ca含量的钙钛矿型氧化物La1-xCaxMnO3(x=0~0.4)纳米颗粒,X射线粉末衍射及精修、扫描电镜表征显示其相纯度和结晶度高,颗粒平均粒径约40 nm。在0.1 mol.L-1KOH水溶液中进行的氧还原电催化性能测试显示,La0.7Ca0.3MnO3样品催化活性最高,表观电子转移数接近4,还原电流密度与Pt/C催化剂相当,而催化稳定性优于Pt/C。进一步研究了La1-xCaxMnO3样品中Mn价态、晶胞参数的改变对氧还原催化活性的影响,结果表明当x=0.3时,催化剂中Mn处于混合价态,Mn-O键长适中,最有利于电催化反应。     

La2-xCexCu1-yZnyO4 perovskites for high temperature water-gas shift reaction

The performance of La2-xCexCu1-yZnyO4 perovskites as catalysts for the high temperature water-gas shift reaction (HT-WGSR) was inves-tigated. The catalysts were characterized by EDS, XRD, BET surface area, TPR, and XANES. The results showed that all the perovskites exhibited the La2CuO4 orthorhombic structure, so the Pechini method is suitable for the preparation of pure perovskite. However, the La1.90Ce0.10CuO4 perovskite alone, when calcined at 350/700℃, also showed a (La0.935Ce0.065)2CuO4 perovskite with tetragonal struc-ture, which produced a surface area higher than the other perovskites. The perovskites that exhibited the best catalytic performance were those calcined at 350/700 ℃ and, among these, La1.90Ce0.10CuO4 was outstanding, probably because of the high surface area associated with the presence of the (La0.935Ce0.065) 2CuO4 perovskite with tetragonal structure and orthorhombic La2CuO4 phase.     

Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) perovskites used in methane combustion

A study on the synthesis of La_1−xAg_xMnO_3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La_0.8Ag_0.2MnO_3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts.     

Mn-Mo-W-O_x脱硝催化剂活性组分的配伍优化

采用等体积浸渍法制备系列Mn-Mo-W-O_x/堇青石和Mn-Mo-W-O_x/TiO_2催化剂,用于选择性催化还原NO.通过Mn、Mo、W 3种元素不同配比对催化剂配伍进行优化,确立Mn-Mo-W-O_x最佳配比.采用XRD、N_2-BET、PyIR、SEM以及XPS等表征分析催化剂的固相结构、比表面积、酸量、表面形貌和表面元素.结果表明:当Mn/Mo/W元素摩尔比为10∶0.5∶1,载体为TiO_2时,催化剂的催化性能最优.适量Mo掺入Mn-W-O_x催化剂可以增大其比表面积,提高催化剂表面L酸酸量以及Mn~(4+)离子浓度,从而有效提高了催化剂高温活性.载体替换为TiO_2时催化剂的比表面积和酸量明显提高,从而增强了催化剂的脱硝性能.     

Characterization and catalytic behavior of La2 - x (Sr, Th)xCuO4 ± λ in reaction NO+CO

Two mixed oxide systems La₂-_xSr_xCuO_{4± λ} (0.0⩽x⩽1. 0) and La_2-xTh_xCuO_{4± λ} (O. O⩽x⩽ 0.4) with K₂NiF₄ structure were prepared by varyingx values. Their crystal structures were studied by means of XRD and IR spectra. The average valence of Cu ion at B site, nonstoichiometric oxygen (λ) and the chemical composition in the bulk and on the surface of the catalysts were measured by means of chemical analysis and XPS. The catalytic behavior in reaction CO+NO was investigated under the regular change of average valence of Cu ion at B site and nonstoichiometric oxygen (λ). Meanwhile, the adsorption and activation of the small molecules NO and the mixture of NO+CO over the mixed oxide catalysts were studied by means of MS-TPD. The catalytic mechanism of reaction NO+CO over these oxide catalysts were proposed; and it has been found that, at lower temperatures the activation of NO is the rate determining step and the catalytic activity is related to the lower valent metallic ion and its concentration, while at higher temperatures the adsorption of NO is the rate determining step and the catalytic activity is related to the oxygen vacancy and its concentration. Project supported by the National Natural Science Foundation of China.     

Mn负载量对nMnO_x/TiO_2催化剂NH_3-SCR催化性能的影响

MnO_x/TiO_2催化剂由于具有优异的低温脱硝性能,已成为SCR催化剂的研究热点之一.我们通过浸渍法制备了一系列不同Mn负载量的nMnO_x/TiO_2(n=2.5%, 5%, 10%, 15%)(质量分数)催化剂,考察Mn负载量对催化剂脱硝性能的影响.利用N_2物理吸附, X-Ray Diffraction (XRD), Scanning Electron Microscope(SEM),Temperature Programmed Reduction with H_2(H_2-TPR),Temperature Programmed Desorption with NH_3(NH_3-TPD)和X-Ray Photoelectron Spectroscopy (XPS)对其结构进行表征.结果表明,催化剂的脱硝性能随着Mn负载量(2.5%～15%)(质量分数)的变化呈现"火山型"曲线,当Mn负载量为10%(质量分数)时,催化剂的脱硝性能最佳. H_2-TPR和XPS结果表明nMnO_x/TiO_2催化剂上表面氧比例和表面Mn~(4+)浓度均随着Mn负载量的增大,先增大后减小,具体顺序为10MnO_x/TiO_(2 ) 15MnO_x/TiO_(2 )5MnO_x/TiO_(2 ) 2.5MnO_x/TiO_2,与脱硝性能顺序完全一致.进一步关联表面氧的比例与T_(50)发现,催化剂的表面氧的比例与T_(50)呈线性关系,即表面氧比例越高, T_(50)越小,脱硝活性越高. NH_3-TPD结果表明,弱酸酸量的增加有助于低温脱硝活性的提高.这些结果揭示了Mn负载量影响脱硝性能的作用规律,为今后开发高效的锰基低温脱硝催化剂提供了技术支撑.     

Critical covalence and superconductivity in Ln2−xCexCuO4

The variations of superconductive properties with x of the n-type Ln_2−xCe_xCuO₄ (Ln = La_0.5Nd_0.5, Nd, or Gd) systems have been investigated. As the size of Ln³⁺ decreases, (i) the solubility limit x of Ce decreases, (ii) the value of x at which a transition from antiferromagnetic semiconductor to superconductor occurs increases, and (iii) the width Δx of the superconductive region decreases. The decreasing solubility of Ce with decreasing size of Ln³⁺ is due to decreasing tensile strain in the CuO₂ sheets. The progressive shift of the semiconductor to superconductor transition to higher x values with decreasing size of Ln³⁺ is explained on the basis of increasing electrostatic Madelung energy E_M caused by decreasing Cu---O bond length. A larger E_M means a larger charge transfer gap Δ and a smaller covalent-mixing parameter λ and bandwidth W; so a decreasing size of Ln³⁺ necessitates a higher level of Ce-doping in order to achieve a critical covalence essential for superconductivity to occur.     

Atomic‐Scale Insights into Surface Lattice Oxygen Activation at the Spinel/Perovskite interface of Co3O4/La0.3Sr0.7CoO3

Surface lattice oxygen in transition‐metal oxides plays a vital role in catalytic processes. Mastering activation of surface lattice oxygen and identifying the activation mechanism are crucial for the development and design of advanced catalysts. A strategy is now developed to create a spinel Co₃O₄ /perovskite La_0.3Sr_0.7CoO₃ interface by in situ reconstruction of the surface Sr enrichment region in perovskite LSC to activate surface lattice oxygen. XAS and XPS confirm that the regulated chemical interface optimizes the hybridized orbital between Co 3d and O 2p and triggers more electrons in oxygen site of LSC transferred into lattice of Co₃O₄ , leading to more inactive O^2? transformed into active O^2?x. Furthermore, the activated Co₃O₄/LSC exhibits the best catalytic activities for CO oxidation, oxygen evolution, and oxygen reduction. This work would provide a fundamental understanding to explain the activation mechanism of surface oxygen sites.     

A new approach of CeO_2 and La_2O_3 effects on the three-way catalysts containing low precious metals

A series of three-way catalysts (TWCs),containing a small amount of precious metals (PMs,including Pt,Pd and Rh) and a large amount of promoters CeO2 and La2O3,were prepared with different precursor compounds and various doped manners.Crystal phases,dispersion of cerium and lanthanum,textural structure and thermal stability of the catalysts were investigated by XRD,XPS and pore parameters determination.The catalytic performance was studied by the measurements of CO,C3H6 and NO conversions on dependence of temperature at stoichimetric number point (S=1.00),and from S=0.75 to 1.30 at 280℃ or 340℃ for fresh or aged samples,respectively.The correlation between the catalytic performance and the characteristics of fresh and aged samples were discussed.The results show that the sample,in which CeO2 and La2O3 are doped with mixed oxide powders,possesses poor dispersion and less thermal stability,and the conversions of NO and C3H6 are apparently lower than those of the samples aged at 850℃ The main reason is     

Promotional effect of Ce on the SCR of NO with NH<Subscript>3</Subscript> at low temperature over MnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> supported by nitric acid-modified activated carbon

Different amounts of Mn and Ce oxides were loaded onto nitric acid-modified activated carbon (ACN) by wet impregnation. The series of catalysts were employed for the selective catalytic reduction of NO _x by NH₃ at temperatures between 100 and 250 °C. Cerium-modified catalysts exhibited higher de-NO _x performance than those modified with Mn/ACN, even with the same total loadings. The precursor solution with a molar ratio for Ce/(Mn + Ce) of 0.4 exhibited the highest catalytic activity. Enhanced resistance to SO₂ and H₂O and better stability were observed for 10%Mn–Ce(0.4)/ACN relative to 10%Mn/ACN. The catalysts were further characterized by N₂ physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H₂-TPR), and temperature-programmed desorption of ammonia (NH₃-TPD). The N₂ physisorption and XRD results suggested that co-doping Ce with Mn increased the surface area and promoted the dispersion of Mn–Ce binary metal oxides. H₂-TPR the NH₃-TPD results demonstrated that the interaction between manganese oxide and cerium oxide species enhanced the redox and surface acidity of 10%Mn–Ce(0.4)/ACN.     

铈铬比例对CeO2-CrOx复合氧化物催化剂上1,2-二氯乙烷催化氧化性能的影响

采用共沉淀法制备了不同铈铬比例的CeO₂-CrO_x复合氧化物(Ce/Cr 摩尔比分别为9/1、4/1、2/1、1/1、1/2、1/4、1/8)以及单纯的CeO₂和Cr₂O₃, 并研究了各催化剂对1,2-二氯乙烷(DCE)的催化氧化性能. 结果表明,相较于单纯的CeO₂, 不同铈铬比例的复合氧化物催化剂对DCE的催化氧化活性有明显提高, 其中Ce/Cr 摩尔比为2/1的CeO₂-CrO_x复合氧化物上DCE的氧化活性最好, 且只有极微量的含氯等副产物产生; 随着Ce/Cr 摩尔比减小, 对HCl的选择性有下降的趋势. 通过X射线衍射(XRD)、N₂吸附/脱附(BET)、紫外拉曼(UV-Raman)光谱、H₂程序升温还原(H₂-TPR)、NH₃程序升温脱附(NH₃-TPD)等实验技术, 研究了铈铬比例对铈铬复合氧化物的物理化学性质的影响. 结果表明, 适当比例Ce-Cr的复合, 形成了结构较稳定的Ce-Cr-O固溶体, 提高了催化剂活性氧物种的流动性, 催化剂表面酸量及强弱酸比例, 从而有利于DCE的吸附活化, 进一步脱氯降解以及深度氧化.     

助剂对Ni/SiO2催化剂微观结构及二硝基甲苯催化加氢性能的影响

采用等体积浸渍法制备了负载型Ni/SiO₂催化剂,研究了Ce、Zr、La、Co和Fe助剂对催化剂微观结构及其催化二硝基甲苯（DNT,C₆H₃CH₃（NO₂）₂）加氢制备甲苯二胺（TDA,C₆H₃CH₃（NH₂）₂）性能的影响。通过XRD,BET,H₂-TPD、H₂-TPR和XPS技术对催化剂进行了表征。结果表明,助剂的引入促进了Ni物种在载体表面的分散,减小了Ni晶粒的尺寸,使得NiO晶粒更易还原。添加La、Fe和Zr助剂增加了有效的Ni活性中心数,有利于催化活性的提高,其中,添加La助剂制备的催化剂催化性能最优,DNT转化率和TDA选择性分别为98.1%和99.1%。但Co和Ce助剂的加入降低了化学氢吸附量,使得有效的Ni活性中心数降低,降低了催化剂的催化活性。     

助剂对Ni/SiO_2催化剂微观结构及二硝基甲苯催化加氢性能的影响

采用等体积浸渍法制备了负载型Ni/SiO2催化剂,研究了Ce、Zr、La、Co和Fe助剂对催化剂微观结构及其催化二硝基甲苯(DNT,C6H3CH3(NO2)2)加氢制备甲苯二胺(TDA,C6H3CH3(NH2)2)性能的影响。通过XRD,BET,H2-TPD、H2-TPR和XPS技术对催化剂进行了表征。结果表明,助剂的引入促进了Ni物种在载体表面的分散,减小了Ni晶粒的尺寸,使得NiO晶粒更易还原。添加La、Fe和Zr助剂增加了有效的Ni活性中心数,有利于催化活性的提高,其中,添加La助剂制备的催化剂催化性能最优,DNT转化率和TDA选择性分别为98.1%和99.1%。但Co和Ce助剂的加入降低了化学氢吸附量,使得有效的Ni活性中心数降低,降低了催化剂的催化活性。     

铂-氢钨青铜电极的制备及对氢氧化的电催化

采用电化学还原法在表面改性的碳布上,通过改变催化剂沉积顺序及氢钨青铜沉积时间制备铂-氢钨青铜复合催化剂,所得电极作为质子交换膜燃料电池(PEMFC)阳极。利用X射线衍射(XRD)、热重分析(TG)、扫描电子显微镜(SEM)、循环伏安(CV)及单电池极化性能测试研究了催化剂的组成、沉积量、分散性及其对氢氧化的电催化活性。实验结果表明,氢钨青铜沉积时间及催化剂沉积顺序对电极催化性能有显著影响,当氢钨青铜沉积时间为10 min,先沉积氢钨青铜、后沉积铂所得Pt/HxWO3电极对氢氧化具有最佳的催化活性。适量的氢钨青铜才能与铂形成较好的协同催化效应。     

Catalytic methanol decomposition to carbon monoxide and hydrogen over Pd/CeO2-ZrO2-La2O3 with different Ce/Zr molar ratios

The catalytic behaviors of Pd (1.4 wt%) catalysts supported on CeO₂-ZrO₂-La₂O₃ mixed oxides with different Ce/Zr molar ratios were investigated for methanol decomposition. Nitrogen adsorption-desorption (BET), X-ray photoelectron spectroscopy (XPS), H₂ temperature-programmed reduction (H₂-TPR), X-ray diffraction (XRD) and Pd dispersion analysis were used for their characterization. Pd/Ce_0.76Zr_0.18 La_0.06O_1.97 catalyst showed the highest BET surface area, best Pd dispersion capability and strongest metal-support interaction. Moreover, XPS showed that there was lattice defect oxygen or mobile oxygen. According to the result of O 1s measurements the lattice defect oxygen or mobile oxygen helped to maintain Pd in a partly oxidized state and increased the activity for methanol decomposition. The Pd/Ce_0.76Zr_0.18La_0.06O_1.97 catalyst exhibited the best activity. A 100% conversion of methanol was achieved at around 260 °C, which was about 20-40 °C lower than other catalysts     

Surface composition in perovskite-like cuprates as probed by X-ray photoelectron spectroscopy

Complex cuprates La_0.85Sr_0.15CuO_2.5?δ having an anion-deficient perovskite structure and La_2?x Sr _x CuO_4?δ (x = 0.15, 0.6, 1.0) having a K₂ NiF₄ layered structure have been prepared by ceramic technology. X-ray powder diffraction verified that single-phase samples were obtained. X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of compacted samples. It was found that both the photoionization cross-section and the photoelectron mean free path should be taken into account when calculating the surface composition. The surface was enriched in strontium as a result of segregation, regardless of the bulk composition of the cuprate sample.     

Structural Transition in the La2−xNdxCuO4System

The structural transition in the La_2−xNd_xCuO₄system is studied through thex=0.45, 0.5 compositions, using neutron powder diffraction. Both compositions could be refined as biphasic systems represented by theI4/mmmand theAbmaspace groups. The presence of T and T′ phases could be observed in both compositions. The apical oxygen atom in the copper coordination octahedron can be regarded as the key for structural distortion causing the transition from T to T′ structures.