Au/ZrO2催化剂中ZrO2的尺寸效应:1,3-丁二烯加氢反应

在不同温度(673～1073K)下,于流动N2气中焙烧ZrO(OH)2醇(乙醇)凝胶,制备了不同尺寸的ZrO2-AN纳米晶(6～30nm).采用沉积-沉淀方法制备了相应的质量分数为0.7%的Au/ZrO2-AN催化剂.用XRD,XRF,TEM/HRTEM,EDS,N2吸附和1,3-丁二烯加氢反应对ZrO2-AN和Au/ZrO2-AN催化剂进行了表征.结果表明,在所有的Au/ZrO2-AN样品中,Au粒子的平均尺寸为4～5nm,ZrO2-AN的颗粒大小没有因为负载Au粒子而发生改变.1,3-丁二烯在Au/ZrO2-AN催化剂催化下能以100%的选择性进行加氢反应生成单烯烃.随着Au/ZrO2-AN催化剂中ZrO2-AN纳米晶尺寸的增加或“载体”焙烧温度的升高,1,3-丁二烯的转化率明显降低;1-丁烯的选择性先增加后减小,2-丁烯中反/顺异构体的摩尔比在0.5～1.0的范围内逐渐增大,TEM/HRTEM表征结果清楚地表明,Au/ZrO2-AN催化剂中Au粒子与ZrO2-AN颗粒接触界面/周边随ZrO2-AN颗粒尺寸的减小而明显增加,这很可能是含有更小尺寸ZrO2-AN纳米粒子的Au/ZrO2-AN催化剂具有更高的催化活性的重要原因.     

Au/ZrO2催化CO氧化反应中ZrO2纳米粒子的尺寸效应

从同一ZrO(OH)₂出发制备了三种不同尺寸的ZrO₂纳米颗粒(ZrO₂-CP: 40～200 nm, ZrO₂-AN: 18～25 nm, ZrO₂-AD: 10～15 nm), 采用沉积-沉淀方法制备了相应的Au/ZrO₂催化剂. 用XRD, XRF, TEM和低温N₂吸附对ZrO₂和Au/ZrO₂进行了表征. XRD和TEM分析表明Au/ZrO₂样品中Au粒子的平均尺寸为4～5 nm, 而ZrO₂的晶相和颗粒大小没有因为“负载”Au粒子而发生变化. CO催化氧化反应的结果表明, Au/ZrO₂催化活性随着ZrO₂纳米粒子尺寸的减小活性明显增加. TEM/HRTEM结果表明, Au/ZrO₂催化剂中Au粒子与ZrO₂颗粒接触界面随ZrO₂颗粒尺寸的减小而明显增加, 这很可能是含有更小尺寸ZrO₂纳米粒子的Au/ZrO₂催化剂具有更高催化活性的重要原因.     

A New Concept for Advanced Heterogeneous Metal Catalysts

Oxide-supported metal catalysts, having always nano-sized structures in which the metal catalysts are prepared as highly dispersed nano-crystals (typically 1-20 nm) on support oxide particles that are often one to several orders of magnitude larger than the metal nano-particles, are an important class of heterogeneous metal catalysts that finds many applications in chemical/petrochemical industries, in environmental protection, in chemical sensors and in the manufacture of fine and special chemicals. It is believed that catalysis by supported metals is the oldest application of nanotechnology. The literature has been rich in nano-size effect of metal nanoparticles in the metal/oxide catalysts. However, it is until recently that the development of size-controlled synthesis of oxide nanoparticles has made it possible to study the nano-size effect of oxide-support particles. When the particle sizes of an oxide support are reduced to become comparable to the sizes of the active metal nanoparticles, the oxide could deviate dramatically from its function as a conventional support. Such metal/oxide catalysts consisting of comparably sized metal and oxide nanocrystals are better called metal/oxide nanocomposite catalysts or catalytic nanoarchitectures.In this presentation, several attempts with reducing the particle size of oxide supports (ZrO2, TiO2,MgO, Al2O3) to approach the metal/oxide nanocomposite concept will be discussed to emphasize the importance of the support size effect. Examples will be given on characteristics of nanocomposite Ni/oxide catalysts for the reforming of natural gas with CO2 and/or steam, and on Au/oxide catalysts for CO oxidation and hydrogenation of unsaturated organic compounds. It will be emphasized that systematic investigations into the size effects of both the metal and oxide nanoparticles approaching the metal/oxide nanocomposite concept can lead to advanced heterogeneous metal catalysts.Moreover, intensive practice of the nanocomposite concept would also lead to discovery of special size-dependent metal-oxide interaction and catalysis, which may provide new opportunity for performance enhancement of potential and existing catalysts.     

Microstructured Au/TiO2 model catalyst systems

The preparation of microstructured Au/TiO2 model catalysts as a first step toward micrometer-scale parallel studies on model catalysts and toward studies of mesoscopic effects in catalytic reactions was investigated by atomic force microscopy and X-ray photoelectron spectroscopy. The model systems, which consist of micrometer-size active areas covered with Au nanoparticles that are separated by similarly sized inactive areas free of Au particles, are fabricated by combining optical lithography methods for microstructuring and ultrahigh vacuum evaporation for Au nanoparticle deposition and by applying suitable cleaning steps. It is demonstrated that practically perfect microstructures with Au nanoparticles of catalytically relevant sizes (2-3-nm diameter) on a clean TiO2 substrate can be produced this way and that the processing steps do not affect the deposited Au nanoparticles, neither in size nor in lateral distribution.     

Interaction of Au with thin ZrO2 films: influence of ZrO2 morphology on the adsorption and thermal stability of Au nanoparticles

The model catalysts of ZrO(2)-supported Au nanoparticles have been prepared by deposition of Au atoms onto the surfaces of thin ZrO(2) films with different morphologies. The adsorption and thermal stability of Au nanoparticles on thin ZrO(2) films have been investigated using synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The thin ZrO(2) films were prepared by two different methods, giving rise to different morphologies. The first method utilized wet chemical impregnation to synthesize the thin ZrO(2) film through the procedure of first spin-coating a zirconium ethoxide (Zr(OC(2)H(5))(4)) precursor onto a SiO(2)/Si(100) substrate at room temperature followed by calcination at 773 K for 12 h. Scanning electron microscopy (SEM) investigations indicate that highly porous "sponge-like nanostructures" were obtained in this case. The second method was epitaxial growth of a ZrO(2)(111) film through vacuum evaporation of Zr metal onto Pt(111) in 1 × 10(-6) Torr of oxygen at 550 K followed by annealing at 1000 K. The structural analysis with low energy electron diffraction (LEED) of this film exhibits good long-range ordering. It has been found that Au forms smaller particles on the porous ZrO(2) film as compared to those on the ordered ZrO(2)(111) film at a given coverage. Thermal annealing experiments demonstrate that Au particles are more thermally stable on the porous ZrO(2) surface than on the ZrO(2)(111) surface, although on both surfaces, Au particles experience significant sintering at elevated temperatures. In addition, by annealing the surfaces to 1100 K, Au particles desorb completely from ZrO(2)(111) but not from porous ZrO(2). The enhanced thermal stability for Au on porous ZrO(2) can be attributed to the stronger interaction of the adsorbed Au with the defects and the hindered migration or coalescence resulting from the porous structures.     

以水热法合成的ZrO_2负载Au催化剂上的低温水煤气变换反应

采用一种简便的水热法合成了一系列ZrO2,并采用沉积-沉淀法制得相应1.0%Au/ZrO2催化剂,在模拟甲醇重整气气氛下评价了它们的低温水煤气变换(WGS)反应催化性能.结果发现,于150oC水热合成的ZrO2负载的Au催化剂活性最佳,240oC反应时CO转化率达87%,明显高于相同反应条件下Au负载量较高的Au/Fe2O3,Au/CeO2及Au/CeZrO4催化剂.采用X射线衍射、原子吸收光谱、N2物理吸脱附及扫描电子显微镜等手段对样品进行了表征.结果表明,Au/ZrO2催化剂的总孔体积及平均孔径越大、圆形片状形貌越规整,其低温WGS催化活性就越高.     

Hydrotalcite-supported gold catalyst for the oxidant-free dehydrogenation of benzyl alcohol: studies on support and gold size effects

Gold nanoparticles with uniform mean sizes (≈3 nm) loaded onto various supports have been prepared and studied for the oxidant-free dehydrogenation of benzyl alcohol to benzaldehyde and hydrogen. The use of hydrotalcite (HT), which possesses both strong acidity and strong basicity, provides the best catalytic performance. Au/HT catalysts with various mean Au particle sizes (2.1-21 nm) have been successfully prepared by a deposition-precipitation method under controlled conditions. Detailed catalytic reaction studies with these catalysts demonstrate that the Au-catalyzed dehydrogenation of benzyl alcohol is a structure-sensitive reaction. The turnover frequency (TOF) increases with decreasing Au mean particle size (from 12 to 2.1 nm). A steep rise in TOF occurs when the mean Au particle size becomes smaller than 4 nm. Our present work suggests that the acid-base properties of the support and the size of Au nanoparticles are two key factors controlling the alcohol dehydrogenation catalysis. A reaction mechanism is proposed to rationalize these results. It is assumed that the activation of the β-C-H bond of alcohol, which requires the coordinatively unsaturated Au atoms, is the rate-determining step.     

Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals

A new and simple method has been developed to synthesize large quantities of highly monodisperse tetragonal zirconia nanocrystals. In this synthesis, a nonhydrolytic sol-gel reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride at 340 degrees C generated 4 nm sized zirconia nanoparticles. A high-resolution transmission electron microscopic (HRTEM) image showed that the particles have a uniform particle size distribution and that they are highly crystalline. These monodisperse nanoparticles were synthesized without any size selection process. X-ray diffraction studies combined with Rietveld refinement revealed that the ZrO(2) nanocrystals are the high-temperature tetragonal phase, and very close to a cubic phase. When zirconium(IV) bromide is used as a precursor instead of zirconium chloride, zirconia nanoparticles with an average size of 2.9 nm were obtained. The UV-visible absorption spectrum of 4 nm sized zirconia nanoparticles exhibited a strong absorption starting at around 270 nm. A fluorescence spectrum with excitation at 300 nm showed a broad fluorescence band centered around 370 nm. FTIR spectra showed indication of TOPO binding on the ZrO(2) nanoparticle surface. These optical studies also suggest that the nanoparticles are of high quality in terms of narrow particle size distribution and relatively low density of surface trap states.     

Active and Recyclable Gold Metal Nanoparticles Catalyst Supported on Nitrogen-Doped Mesoporous Carbon for Chemoselective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol

Several supported gold metal catalysts with different Au nanoparticles sizes were prepared and evaluated for the chemoselective hydrogenation of cinnamaldehyde (CA) to cinnamyl alcohol (CAL). To investigate the structure-activity relationship, stability of catalyst, heterogeneity and recyclability, the structural characteristics of materials and Au catalysts (fresh and spent catalysts) were studied by employing variety of physico-chemical techniques. The interrelationship among Au nanoparticles size (nm) with turnover frequency (h⁻¹) of Au catalysts has also been explored. Among the various Au catalysts tested, nitrogen-doped mesoporous carbon (NMC) supported Au catalyst having homogeneously dispersed (78.8%) Au nanoparticles (1.6 nm) synthesized by sol-immobilization method (Au-NMC-SI) demonstrated improved catalytic activity affording 78% CAL selectivity and 94.2% CA conversion without using any promoter. Moreover, Au-NMC-SI catalyst exhibited good recyclability and stability. The catalyst synthesis approach described in this investigation opens up a novel strategy for the design of highly efficient metal nano-catalysts supported on NMC materials.     

Supported Au nanoparticles as efficient catalysts for aerobic homocoupling of phenylboronic acid

Au nanoparticles with small sizes (1-4 nm) were effectively formed on Mg-Al mixed oxides (Au/MAO), which showed superior catalytic performances and good recyclability in aerobic homocoupling of phenylboronic acid.     

用辛烷基硫醇单层保护Au纳米粒子制备CO氧化催化剂Au/γ-Al2O3

采用两相法合成出含活性组分Au的辛烷基硫醇单层保护Au纳米粒子(C8AuNPs)的正己烷溶胶, 用“逐次浸润”法将C8AuNPs负载在γ-Al2O3上, 经真空干燥及活化处理制得Au/γ-Al2O3催化剂. 所制得的Au催化剂前体C8AuNPs/γ-Al2O3表面Au粒子平均粒径可控制在2-3 nm范围内, 且分布比较单一; 催化剂活性评价600 h后, 其表面Au的粒径仍主要分布在2-4 nm范围内; 真空干燥温度影响Au催化剂的粒子尺寸和催化活性, 随着真空干燥温度的提高, Au纳米粒子的粒径增大. 将所制备的催化剂用于低温CO氧化反应, 催化活性评价结果表明, 经25 ℃真空干燥制得的2.5%(质量分数, w)Au/γ-Al2O3具有较高的活性和长期稳定性, 其催化CO完全转化的最低温度为-19 ℃, 在15 ℃下CO完全转化时Au/γ-Al2O3的单程寿命至少900 h; 4.0%(w) Au/γ-Al2O3在15 ℃和进料中含水条件下对CO完全氧化的单程寿命不低于2000 h, 可见催化剂具有强的抗潮湿中毒特性. 综合上述实验结果, 讨论了影响Au/γ-Al2O3催化剂活性的可能因素.     

CO2重整甲烷反应高效稳定Ni/ZrO2催化剂的纳米结构特点

分别通过在常压流动N2气中加热处理ZrO(OH)2醇凝胶和在空气中焙烧ZrO(OH)2水凝胶制备了含不同晶相组成和不同尺寸ZrO2纳米粒子的Ni/ZrO2催化剂.Ni/ZrO2催化剂上CO2重整CH4反应的活性和稳定性以及多种催化剂表征(XRD,TEM,TPR及TPD等)数据表明,高效稳定的Ni/ZrO2催化剂必须具有“金属/氧化物”纳米复合物的特征.ZrO2纳米粒子的晶相组成对CO2重整甲烷反应中纳米复合物型Ni/ZrO2催化剂的稳定性没有明显影响.     

Oxidation of CO on gold supported catalysts prepared by laser vaporization: direct evidence of support contribution

Gamma-Al2O3, ZrO2, and TiO2 gold supported model catalysts have been synthesized by laser vaporization. Structural characterization using Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy experiments have shown that the gold clusters deposited on the different supports have similar distribution of size centered around 3 nm and are in the metallic state. However, X-ray photoemission measurements also indicate lower binding energies than the usual Au 4f(7/2) at 84.0 eV for both alumina and titania supported catalysts, indicating a modification of the electronic structure of the metal. One has taken benefit of these features to study the influence of the nature of the support toward CO oxidation activities without being hindered by particle size or gold oxidic species effects. By comparing the activities of the different catalysts, it is concluded that the nature of the support directly affects the activity of gold. The following tendency is observed: titania and zirconia are superior to alumina as supports, titania being slightly better than zirconia. From XPS and activity results we can conclude that the existence of negatively charged clusters is not the key point to explain the high activity observed for Au/ZrO2 and Au/TiO2 catalysts and also that metallic Au is the major catalytically active phase. Hence, due to their very nature, titania and to a less extent zirconia should participate to the catalytic process.     

Preparation of uniform Au@SiO2 particles by direct silica coating on citrate-capped Au nanoparticles

Preparation of Au@SiO₂ particles by direct silica coating on three different sized citrate-capped Au nanoparticles (17, 25 and 33 nm) with Stöber method was investigated in this work. It was found that the uniformity of the resulting Au@SiO₂ particles was related to both the sizes of the Au nanoparticles and the concentration of citrate during the particle synthesis. When the citrate concentration during the particle synthesis was low, the 25 and 33 nm Au nanoparticles could be well dispersed in the Stöber system, thus resulting the formation of uniform Au@SiO₂ particles containing single core. However, small Au nanoparticles (17 nm) were identified to show poor stability in the Stöber system even under low citrate concentration, the silica coating must be performed in a pre-hydrolyzed Stöber system to get the uniform Au@SiO₂ particles. This approach was also applicable to citrate capped Ag nanoparticles. After removal of the excess citrate in the Ag prepared by citrate reduction, uniform Ag/SiO₂ particles containing single core colloids could also be prepared by the direct silica coating.     

Applied magnetic field rejects the coating of ferromagnetic carbon from the surface of ferromagnetic cobalt: RAPET of CoZr2(acac)2(OiPr)8

We present the results of the RAPET (reaction under autogenic pressure at elevated temperatures) dissociation of CoZr(2)(acac)(2)(O(i)Pr)(8) at 700 degrees C in a closed Swagelok cell under an applied magnetic field of 10 T. It produces a mixture of carbon-coated and noncoated metastable ZrO(2) nanoparticles, bare metallic Co nanoparticles, and bare carbon. The same reaction in the absence of a magnetic field produces spherical Co and ZrO(2) particles in sizes ranging from 11 to 16 nm and exhibiting, at room temperature, metastable phases: fcc for cobalt and a tetragonal phase for zirconia. The metastable phases of Co and ZrO(2) are manifested because of a carbon shell of approximately 4 nm thickness anchored to their surfaces. The effect of an applied magnetic field to synthesize morphologically different, but structurally the same, products is the key topic of the present paper.     

Platinum-catalyzed synthesis of water-soluble gold-platinum nanoparticles

The ability to control composition and size in the synthesis of bimetallic nanoparticles is important for the exploitation of the bimetallic catalytic properties. This paper reports findings of an investigation of a new approach to the synthesis of gold-platinum (AuPt) bimetallic nanoparticles in aqueous solution via one-phase reduction of AuCl(4-) and PtCl(4)(2-) using a combination of reducing and capping agents. Hydrogen served as a reducing agent for the reduction of Pt(II), whereas acrylate was used as a reducing agent for the reduction of Au(III). The latter reaction was found to be catalyzed by the formation of Pt as a result of the reduction of Pt(II). Acrylate also functioned as capping agent on the resulting nanocrystals. By controlling the feed ratios of AuCl(4-) and PtCl(4)(2-) and the relative concentrations of acrylate, an effective route for the preparation of AuPt nanoparticles with bimetallic compositions ranging from approximately 4 to 90% Au and particle sizes ranging from 2 to 8 nm has been demonstrated. The composition, size, and shell properties were characterized using transmission electron microscopy, direct current plasma-atomic emission spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. Implications of the results to the exploration of bifunctional catalysts are also briefly discussed.     

Facile preparation of Au nanoparticle-embedded polydopamine hollow microcapsule and its catalytic activity for the reduction of methylene blue

Abstract

Development of novel supported catalysts with high activity and stability is still a challenge. In this study, the Au-polydopamine (Au-PDA) hollow microcapsules with Au nanoparticles embedded into the PDA microcapsule shell have been synthesized through a simple template-induced covalent assembly method, where polystyrene (PS) nanospheres were used as templates to form core/shell structured PS/Au-PDA composites, followed by core removal through tetrahydrofuran etching. Their morphology and composition were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectra (FT-IR), UV-Vis spectrophotometer and X-ray diffraction (XRD), respectively. Results showed that the Au-PDA microcapsules possessed well-fined hollow structure and uniform sizes with inner diameter of about 385?nm, shell thickness of about 30?nm, and Au nanoparticles with diameter of about 17?nm incorporated. The catalytic performance of Au-PDA hollow microcapsules was evaluated through the reduction of methylene blue (MB) dye with NaBH₄ as a reducing agent. Compared to PDA/Au composites with Au nanoparticles loaded on the surface of PDA microspheres, as-prepared Au-PDA hollow microcapsules show good stability and recyclability in the catalytic experiments as the Au nanoparticles were firmly wrapped in PDA matrix, which makes the Au-PDA hollow microcapsules a practicable catalyst candidate for advanced catalytic systems.     

Ensemble effects in Cu/Au ultrasmall nanoparticles control the branching point for C1 selectivity during CO2 electroreduction

Bimetallic catalysts provide opportunities to overcome scaling laws governing selectivity of CO₂ reduction (CO₂R). Cu/Au nanoparticles show promise for CO₂R, but Au surface segregation on particles with sizes ≥7 nm prevent investigation of surface atom ensembles. Here we employ ultrasmall (2 nm) Cu/Au nanoparticles as catalysts for CO₂R. The high surface to volume ratio of ultrasmall particles inhibits formation of a Au shell, enabling the study of ensemble effects in Cu/Au nanoparticles with controllable composition and uniform size and shape. Electrokinetics show a nonmonotonic dependence of C1 selectivity between CO and HCOOH, with the 3Au:1Cu composition showing the highest HCOOH selectivity. Density functional theory identifies Cu₂/Au(211) ensembles as unique in their ability to synthesize HCOOH by stabilizing CHOO* while preventing H₂ evolution, making C1 product selectivity a sensitive function of Cu/Au surface ensemble distribution, consistent with experimental findings. These results yield important insights into C1 branching pathways and demonstrate how ultrasmall nanoparticles can circumvent traditional scaling laws to improve the selectivity of CO₂R.

Bimetallic catalysts provide opportunities to overcome scaling laws governing C1 selectivity of CO₂ reduction (CO₂R).     

Binary Assembly of Au Nanoparticles with Controllable Two‐dimensional Architecture Directed by Phosphate Backbone Modified Oligonucleotide

A three complementary strand oligonucleotide system was employed to assemble two different‐sized, 15 and 25 nm, Au nanoparticles into binary two‐dimensional (2D) structures. First, two non‐complementary strands of phosphate backbone modified oligonucleotides (PM oligo‐DNA) were loaded on the surface of the 15 and 25 nm Au nanoparticles, respectively. Upon the addition of the third linker DNA, which was half complementary to each of the modified DNA, the Au nanoparticles would be assembled to binary 2D aggregates. The number of the 15 nm Au nanoparticles around a 25 nm Au naoparticle can be readily controlled by the length of the DNA helix used.     

甲醇在铁掺杂Pt-ZrO_2/C催化剂上的电氧化

应用溶胶-凝胶法制备不同铁掺杂量的ZrO2粉体,以ZrO2/C为载体制备Pt-Zr1-xFexO2/C催化剂电极.X射线衍射(XRD),透射电镜(TEM)表征载体及催化剂的表面形貌和晶体结构.结果表明,催化剂中Pt和ZrO2颗粒在活性炭表面均匀分布,铂颗粒尺寸为2~4 nm,ZrO2颗粒尺寸为3~7 nm.循环伏安法和计时电流法测定催化剂对甲醇电氧化作用,显示在ZrO2晶体中掺杂铁可以提高催化剂的甲醇电氧化活性.其性能与铁的掺杂量有关.