PVP封端剂对Pd纳米晶电催化氧化甲醇和乙醇性能的影响

Direct alcohol fuel cells (DAFCs) have attracted considerable research interest because of their potential application as alternative power sources for automotive systems and portable electronics. Pd-based catalysts represent one of the most popular catalysts for DAFCs due to their excellent electrocatalytic activities in alkaline electrolytes. Thus, it is of great importance to understand the structure-activity relationship of Pd electrocatalysts for alcohol electrocatalysis. Recently, size- and shape- controlled Pd nanocrystals have been successfully synthesized and subsequently used to study the size and shape effects of Pd electrocatalysts on alcohol electrocatalysis, in which the Pd (100) facet exhibited higher electrocatalytic oxidation activity for small alcohol molecules than the Pd (111) and (110) facets. Although it is well known that capping ligands, which are widely used in wet chemistry for the size- and shape-controlled synthesis of metal nanocrystals, likely chemisorb onto the surfaces of the resulting metal nanocrystals and influence their surface structure and surface-mediated properties, such as catalysis, this issue was not considered in previous studies of Pd nanocrystal electrocatalysts for electrocatalytic oxidation of small alcohol molecules. In this study, we prepared polyvinylpyrrolidone (PVP)-capped Pd nanocrystals with different morphologies and sizes and comparatively studied their electrocatalytic activities for methanol and ethanol oxidation in alkaline solutions. The chemisorbed PVP molecules transferred charge to the Pd nanocrystals, and the finer Pd nanocrystals had a higher coverage of chemisorbed PVP, and thus exposed fewer accessible surface sites, experienced more extensive PVP-to-Pd charge transfer, and were more negatively charged. The intrinsic electrocatalytic activity, represented by the electrochemical surface area (ECSA)-normalized electrocatalytic activity, of Pd nanocubes with exposed (100) facets increases with the particle size, indicating that the more negatively-charged Pd surface is less electrocatalytically active. The Pd nanocubes with average sizes between 12 and 19 nm are intrinsically more electrocatalytically active than commercial Pd black electrocatalysts, while the activity of Pd nanocubes with an averages size of 8 nm is less. This suggests that the enhancement effect of the exposed (100) facets surpasses the deteriorative effect of the negatively charged Pd surface for the Pd nanocubes with average sizes between 12 and 19 nm, whereas the deteriorative effect of the negatively charged Pd surface surpasses the enhancement effect of the exposed (100) facets for the Pd nanocubes with average sizes of 8 nm due to the extensive PVP-to-Pd charge transfer. Moreover, the Pd nanocubes with average sizes of 8 nm exhibit similar intrinsic electrocatalytic activity to the Pd nanooctahedra with (111) facets exposed and average sizes of 7 nm, indicating that the electronic structure of Pd electrocatalysts plays a more important role in influencing the electrocatalytic activity than the exposed facet. Since the chemisorbed PVP molecules block the surface sites on Pd nanocrystals that are accessible to the reactants, all Pd nanocrystals exhibit lower mass-normalized electrocatalytic activity than the Pd black electrocatalysts, and the mass-normalized electrocatalytic activity increases with the ECSA. These results clearly demonstrate that the size- and shape-dependent electrocatalytic activity of Pd nanocrystals capped with PVP for methanol and ethanol oxidation should be attributed to both the exposed facets of the Pd nanocrystals and the size-dependent electronic structures of the Pd nanocrystals resulting from the size-dependent PVP coverage and PVP-to-Pd charge transfer. Therefore, capping ligands on capped metal nanocrystals inevitably influence their surface structures and surface-mediated properties, which must be considered for a comprehensive understanding of the structure-activity relationship of capped metal nanocrystals.     

Oxygen reduction reaction on carbon-supported palladium nanocubes in alkaline media

Carbon-supported Pd nanocubes with the size of 30, 10 and 7 nm were prepared and their electrocatalytic activity towards the oxygen reduction reaction (ORR) in alkaline solution was studied. For comparison carbon-supported spherical Pd nanoparticles and commercial Pd/C catalyst were used. The catalysts were characterised by transmission electron microscopy, electro-oxidation of carbon monoxide and cyclic voltammetry and the ORR activity was evaluated using the rotating disk electrode method. The ORR on all studied Pd/C catalysts proceeded via four-electron pathway where the rate-limiting step was the transfer of the first electron to O₂ molecule. The specific activity of Pd nanocubes was more than two times higher than that of spherical Pd nanoparticles and increased with increasing the particle size.     

Selective Semihydrogenation of Alkynes on Shape‐Controlled Palladium Nanocrystals

A systematic study on the selective semihydrogenation of alkynes to alkenes on shape‐controlled palladium (Pd) nanocrystals was performed. Pd nanocrystals with a cubic shape and thus exposed {100} facets were synthesized in an aqueous solution through the reduction of Na₂PdCl₄ with L ‐ascorbic acid in the presence of bromide ions. The Pd nanocubes were tested as catalysts for the semihydrogenation of various alkynes such as 5‐decyne, 2‐butyne‐1,4‐diol, and phenylacetylene. For all substrates, the Pd nanocubes exhibited higher alkene selectivity (>90 %) than a commercial Pd/C catalyst (75–90 %), which was attributed to a large adsorption energy of the carbon–carbon triple bond on the {100} facets of the Pd nanocubes. Our approach based on the shape control of Pd nanocrystals offers a simple and effective route to the development of a highly selective catalyst for alkyne semihydrogenation.     

电解液中的乙二胺四甲叉膦酸对甲酸在炭载Pd催化剂上电氧化性能的影响

利用X射线能量色散(EDS)谱、X射线衍射(XRD)谱、透射电子显微镜(TEM)和电化学等技术研究了在电解液中添加乙二胺四甲叉膦酸(EDTMP)对甲酸在Pd/C催化剂上电氧化性能的影响. 结果表明, 当EDTMP添加的浓度为0.5 mmol/L时, Pd/C催化剂对甲酸氧化的电催化活性和稳定性最好. 这主要归结于吸附在Pd/C催化剂表面的EDTMP不但能通过基团效应降低CO的吸附量, 还能抑制Pd/C催化剂催化甲酸分解的速率, 从而减少了CO的毒化作用. 但当EDTMP的浓度大于0.5 mmol/L时, 吸附过多的EDTMP反而会占据Pd的活性位点, 降低催化作用.     

钯纳米晶体在电催化甲酸氧化反应中的形貌效应

本文基于课题组前期工作,选用适当的金属前驱物、还原剂、稳定剂和保护剂,通过调控氧化刻蚀和反应动力学等,成功合成了形貌和尺寸均不相同的Pd纳米晶.经过认真的纳米粒子清洗和电极修饰组装,考察了它们在电催化甲酸氧化反应中的形貌与性能的关系.研究结果表明,Pd纳米晶样品的最大电流密度以纳米八面体（nanooctahedra）、纳米线（nanowires）、纳米立方体（nanocubes）、纳米瓜子（nanotapers）、凹面纳米立方体（concave nanocubes）的顺序递增,催化甲酸氧化反应的起始氧化电位均小于0.2V.研究结果印证了Pd纳米晶催化甲酸氧化反应的催化性能在尺寸效应上主要受活性表面积的影响,扣除表面积效应后的催化性能与其尺寸没有明确关系.该系列Pd纳米晶的催化性能主要取决于其表面结构,得出Pd纳米晶催化甲酸氧化反应遵循{111}晶面〈{100}晶面〈高指数晶面的性能活性顺序.综合最大电流密度和最小操作电位因素发现,Pd凹面纳米立方体和Pd纳米瓜子具有相对较好的商用价值.     

Shape-dependent electrocatalysis: formic acid electrooxidation on cubic Pd nanoparticles

The electrocatalytic properties of palladium nanocubes towards the electrochemical oxidation of formic acid were studied in H(2)SO(4) and HClO(4) solutions and compared with those of spherical Pd nanoparticles. The spherical and cubic Pd nanoparticles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The intrinsic electrocatalytic properties of both nanoparticles were shown to be strongly dependent on the amount of metal deposited on the gold substrate. Thus, to properly compare the activity of both systems (spheres and nanocubes), the amount of sample has to be optimized to avoid problems due to a lower diffusion flux of reactants in the internal parts of the catalyst layer resulting in a lower apparent activity. Under the optimized conditions, the activity of the spheres and nanocubes was very similar between 0.1 and 0.35 V. From this potential value, the activity of the Pd nanocubes was remarkably higher. This enhanced electrocatalytic activity was attributed to the prevalence of Pd(100) facets in agreement with previous studies with Pd single crystal electrodes. The effect of HSO(4)(-)/SO(4)(2-) desorption-adsorption was also evaluated. The activity found in HClO(4) was significantly higher than that obtained in H(2)SO(4) in the whole potential range.     

Morphology syntheses and properties of well-defined Prussian Blue nanocrystals by a facile solution approach

Large-scale syntheses of Prussian Blue (PB) uniform nanocubes and nanospheres, together with an interesting PB micro-frame structure, have been achieved by direct dissociation of a single-source precursor K(3)[Fe(CN)(6)] with polyvinylprrolidone (PVP) as a capping and reducing agent. The reaction temperature has been proved to be a key factor for morphology development of PB nanocrystals, and the size of PB nanocrystals can be tuned by adjusting the feed ratio of K(3)[Fe(CN)(6)] to PVP and the concentration of K(3)[Fe(CN)(6)]. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and high resolution transmission electron microscopy (HRTEM) were used to characterize the resulting nanocrystals and investigate their growth mechanisms. Optical and magnetic studies showed size- and shape-dependent optical and magnetic properties in the PB nanocrystals. An excellent electro-catalytic property to hydrogen peroxide reduction was observed at the PB nanocube-modified electrode.     

Synthesis of palladium concave Nanocubes with high‐index facets and their catalytic properties

Palladium nanocrystals with a variety of shapes have received particular interest in recent years due to their unique properties in catalysis. Herein, Pd concave nanocubes with high‐index facets (Pd‐CNs) was synthesized by a simple water‐based route without seeds using L‐ascorbic acid (AA) as the reduction agent in the presence of CTAB. X‐ray diffraction and transmission electron microscopy were employed to demonstrate the formation of concave structures with high‐index facets of the Pd‐CNs with an average size of 17.5 nm. The as‐prepared Pd‐CNs presented significantly higher catalytic activity than commercial Pd/C (an average particle size of 4.7 nm) in the electro‐oxidation of methanol, but exhibited weaker property in Suzuki coupling reaction, which provided an evidence for the effect of shape and size on different reactions.     

UV-ozone cleaning of supported poly(vinylpyrrolidone)-stabilized palladium nanocubes: effect of stabilizer removal on morphology and catalytic behavior

Poly(vinylpyrrolidone) (PVP)-stabilized Pd nanocubes were synthesized, deposited on a carbon-based support, and subsequently treated with UV-ozone (UVO) in order to eliminate the traces of PVP still present on the surface. Cubes, being a thermodynamically unfavorable shape, are very prone to restructuring to minimize the interfacial free energy and thus allow the assessment of their morphological stability during UVO cleaning. The process of PVP removal was monitored by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ attenuated total reflection infrared spectroscopy (ATR-IR). High-resolution scanning electron microscopy (SEM) imaging was used to evaluate the morphology of the nanocubes. The effect of PVP removal was also studied in the hydrogenation of acetylene, showing a 4-fold increase of activity. This method can be applied to nanoparticles of other common shapes, which expose different crystal planes, in order to study the structure sensitivity of chemical reactions.     

纳米Pd的形貌和尺寸可控制备及其1,4-丁炔二醇加氢性能

采用化学还原法合成Pd纳米立方体,并将其作为晶种,进一步合成大尺寸的纳米Pd立方体以及具有不同{100}和{111}晶面比例的纳米Pd多面体.将形貌和尺寸可控的纳米Pd溶胶应用于1,4-丁炔二醇催化加氢的反应中,反应结果表明,纳米Pd的催化性能取决于其尺寸和形貌.{111}晶面的催化活性高于{100}晶面,PVP稳定的Pd胶体对1,4-丁烯二醇均具有较高选择性,具有适当{100}和{111}晶面比例的纳米Pd多面体对1,4-丁烯二醇的选择性可达96%.     

一氧化碳辅助钯、铂纳米晶的形貌控制

形貌控制对调控贵金属纳米晶的催化和光学性能至关重要．近年来,在发展铂、钯纳米晶的形貌控制的方法过程中,一氧化碳（CO）不仅作为合成铂、钯纳米晶的优良还原剂,还可通过在特定晶面的选择性吸附辅助铂、钯纳米晶的形貌控制．CO辅助铂、钯纳米晶形貌控制的方法正逐步展现出独特的优越性,甚至帮助我们制备了一些目前其他方法所无法制备的纳米晶．该综述文章首先从表面科学的角度分析讨论CO分子在铂、钯单晶面上的不同吸附行为,然后总结分析了CO调控铂、钯纳米晶形貌的几个典型例子（超薄钯纳米片、介晶钯纳米花、钯四角叉／四面体以及铂纳米立方体、铂钴削角八面体）,讨论了CO在控制铂、钯纳米晶的形貌控制作用及其化学本质,最后提出CO在辅助贵金属纳米晶的形貌控制中的挑战和展望．     

Structural dependence of oxygen reduction reaction on palladium nanocrystals

We have synthesized sub-10 nm Pd cubic and octahedral nanocrystals and then evaluated their activities towards oxygen reduction reaction (ORR). The ORR activity of Pd nanocubes was one order of magnitude higher than that of Pd octahedra, and comparable to that of the state-of-the-art Pt catalysts.     

Cu3PdN nanocrystals electrocatalyst for formic acid oxidation

We report a hot-injection method to fabricate monodispersed Cu₃PdN nanoparticles (NPs). The crystal structure, morphology, and chemical composition of the as-synthesized Cu₃PdN have been characterized by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopy. The as-developed Cu₃PdN NPs electrocatalysts show improved activity and enhanced stability for formic acid oxidation compared with the corresponding Pd and Cu₃Pd NPs.     

How to Remove the Capping Agent from Pd Nanocubes without Destructing Their Surface Structure for the Maximization of Catalytic Activity?

We report a robust method for effectively removing the chemisorbed Br⁻ ions, a capping agent, from the surface of Pd nanocubes to maximize their catalytic activity. The Br⁻ ions can be removed by simply heating the sample in water, but the desorption of Br⁻ ions will expose the underneath Pd atoms to the O₂ from air for the formation of a relatively thick oxide layer. During potential cycling, the oxide layer evolves into detrimental features such as steps and terraces. By introducing a trace amount of hydrazine into the system, the Br⁻ ions can be removed by heating without forming a thick oxide layer. The as-cleaned nanocubes show greatly enhanced activity toward formic acid oxidation. This cleaning method can also remove Br⁻ ions from Rh nanocubes and it is expected to work for other combinations of nanocrystals and capping agents.     

Aqueous‐Phase Synthesis of Single‐Crystal Pd Seeds 3 nm in Diameter and Their Use for the Growth of Pd Nanocrystals with Different Shapes

This article describes an aqueous method for the synthesis of Pd seeds with a single‐crystal structure and a uniform diameter of 3 nm and their use for the growth of Pd nanocrystals with a variety of shapes. We have also investigated the effects of a number of parameters, including the temperature, reducing power of the reductant, and capping agent on the reduction rate of a Pd precursor, and thus the final size, size distribution, and morphology of the Pd seeds. By taking advantage of the coordination effect of Br^? ions with Pd²⁺ ions and their selective adsorption on the Pd(100) surface, Pd nanocrystals with a number of distinct shapes could be conveniently produced by varying the concentration of KBr added into the growth solution. This work provides a general and facile method for the green synthesis of Pd nanocrystals with controlled shapes, especially for the preparation of Pd nanocrystals with sizes in the sub‐10 nm regime.     

Cu2O nanocrystals with various morphology: Synthesis,characterization and catalytic properties

Cu₂O nanocubes, octahedra, spheres and truncated rhombic dodecahedral were prepared and their structural, morphological, and electronic properties were investigated by X-ray diffraction analysis, X-ray absorption near edge structure, scanning electron microscope and transmission electron microscope and X-ray absorption near edge structure. Cu₂O nanocrystals were successfully employed to catalyze the 1,3-dipolar cycloaddition reaction for the synthesis of 1,4-disubstituted triazoles. Cu₂O nanocubes and octahedral showed the superior catalytic performance in the cycloaddition reaction. These results reveal that crystal-plane engineering of oxide catalysts is a useful strategy for developing efficient catalysts for organic reaction.     

Controllable Synthesis of Nearly Monodisperse Spherical Aggregates of CeO2 Nanocrystals and Their Catalytic Activity for HCHO Oxidation

Herein, we report a facile surfactant‐assisted solvothermal synthetic method to prepare nearly monodisperse spherical CeO₂ nanocrystals. A good control of the size of CeO₂ nanocrystals in the range of 100–500 nm was achieved by simply varying the synthetic parameters such as reaction time, volume ratio of ethanol to water (R), molar ratio of PVP, and concentration of Ce(NO₃)₃?6 H₂O in solution. A possible mechanism for the growth of spherical CeO₂ nanocrystals is proposed. The obtained CeO₂ nanocrystals with a surface area of up to 47 m²g^?1 were then employed as a catalyst support. By loading Au‐Pd nanoparticles (about 3 wt. %) onto the CeO₂ support, an Au‐Pd/CeO₂ catalyst was prepared that exhibited high catalytic activity for HCHO oxidation. At the low temperature of 50 °C, the percentage of HCHO conversion was 100 %, suggesting potential applications in preferential oxidation and other catalytic reactions. These Au‐Pd/CeO₂ catalysts may also find applications in indoor formaldehyde decontamination and industrial catalysis. The facile solvothermal method can be extended to the preparation of other metal oxide nanocrystals and provides guidance for size‐ and morphology‐controlled synthesis.     

Using a precursor in lamellar structure for the synthesis of uniform ZnS nanocrystals

Uniform ZnS nanocrystals of about 15?nm were prepared through a low temperature hydrothermal approach by treating Zn-PhPO nanosheets with Na₂S aqueous solution. Both the precursor and the final product were studied by the means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The photo-luminescent spectrum of the synthesized ZnS nanocrystals showed their good crystalline nature. Based on this study, the precursor structure-controlling effect was discussed, and in addition, the relevant factors possibly affecting the particle formation and the growth possessed were applied in the discussion to interpret the transformation mechanism. Further research showed that both the structure characters of the precursors and the mass transportation which occurred during the synthesis greatly affected the morphology and organization state of the final products. This research may provide some facts on the structure-controlling approaches along with a general method for the preparation of uniform sulfide nanocrystals.     

Aqueous-Phase Synthesis of Size-Tunable PbSe Nanocubes at Room Temperature for Optical Property Characterization

A simple aqueous-phase synthesis of PbSe nanocubes with tunable sizes has been developed by first preparing a Na₂SeSO₃ stock solution through dissolution of selenium powder in a solution of Na₂SO₃ at 90–100 °C for 30 min, and adding part of this solution to a mixture of lead acetate and acetic acid at room temperature with stirring for only 5–8 min to complete the nanocrystal growth. Adjusting the volume of acetic acid and Na₂SeSO₃ solution added enabled the size of the nanocrystals to be tuned, with average edge lengths of 13 to 121 nm attained. Changes in solution color revealed very different crystal growth rates for the 13 and 121 nm nanocubes. The PbSe cubes exhibit size-dependent absorption bands in the ultraviolet and visible-light region; the band positions show progressive redshifts with increasing particle size. Slight photocatalytic activity upon 532 nm laser irradiation of the nanocubes suggests the presence of higher energy levels in the band structure of PbSe. The synthetic conditions can be easily scaled up to obtain a large quantity of PbSe nanocubes for applications.     

A Quantitative Analysis of the Reduction Kinetics Involved in the Synthesis of Au@Pd Concave Nanocubes

Surface capping has been shown to play a pivotal role in controlling the evolution of metal nanocrystals into different shapes or morphologies. With the synthesis of Au@Pd concave nanocubes as an example, here we demonstrate that the capping agent can also impact the reduction kinetics of a precursor, and thereby its reduction pathway, for the formation of metal nanocrystals with distinct morphologies. A typical synthesis involves the reduction of a Pd^II precursor by ascorbic acid at room temperature in the presence of Au nanospheres as seeds, together with the use of hexadecyltrimethylammonium chloride (CTAC) or hexadecyltrimethylammonium bromide (CTAB) as the capping agent. In the case of CTAC, the Pd^II precursor prevails as PdCl₄²⁻, leading to the formation of Au@Pd concave nanocubes with a rough surface because of the fast reduction kinetics and thus the dominance of solution reduction pathway. When switched to CTAB, the Pd^II precursor changes to PdBr₄²⁻ that features slow reduction kinetics and surface reduction pathway. Accordingly, the Au@Pd concave nanocubes take a smooth surface. This work demonstrates that both reduction kinetics and surface capping play important roles in controlling the morphology of metal nanocrystals and these two roles are often coupled to each other.