纳米钯膜电极的制备、结构表征和特殊反应性能

用循环伏安方法制备纳米钯膜电极,运用扫描隧道显微镜和原位红外光谱等方法研究其结构和反应性能.STM图像表明,制备的纳米钯膜具有特殊的层状结构,纳米级厚度的层状晶体由直径6nm左右的Pd微晶聚集而成.发现当钯膜厚度为几个纳米时,CO的吸附表现出异常红外效应,即红外谱峰反向和红外吸收显著增强(增强因子可达42.6).纳米钯膜电极对氢的反应也具有特殊的性能,与氢向钯晶格扩散吸收过程相比较,氢吸脱附的表面过程成为主要反应.研究结果还指出,纳米钯膜电极的异常红外效应和对氢反应的特殊性能与钯膜厚度密切关联,并可归结为钯膜材料的纳米尺度效应.     

Relationship between the reactions of hydrogen sorption/desorption and methanol oxidation on bifunctional Ni/Pd electrode in alkaline solution

The present paper is aimed at studying the influence of the hydrogen sorption/desorption process occurring on the layered nickel–palladium (Ni/Pd) electrode on the kinetics of the reaction of methanol oxidation in strong alkaline KOH solution. The electrodes were prepared by chemical deposition of a thin layer of porous palladium on a nickel foam support. A scanning electron microscope was used for studying the morphology of both the nickel support and the porous palladium layer. The mechanism of the anodic desorption of hydrogen changes depending on whether or not 6 M KOH electrolyte is admixed with methanol. It was shown that, in the first cycle of the cyclic voltammetry (CV) measurements, the anodic peak current and peak charge related to the oxidative desorption of hydrogen significantly decrease due to the presence of methanol in KOH. This effect is attributed to the obstacles in hydrogen sorption due to the formation of a passivating layer on the Pd surface composed of both adsorbed methanol molecules and the intermediate products involving adsorbed CO. On the other hand, hydrogen desorbing from Pd electrode exerts influence on the kinetics of the reaction of methanol oxidation. Ni/Pd electrode undergoes considerable reactivation due to the potentiostatic saturation with hydrogen at ?1.1 V, followed by the ease in hydrogen desorption. The CV measurements proved that, after such a treatment, the peak of hydrogen desorption partially overlaps the double peak of methanol oxidation and, in consequence, the rate of methanol oxidation is enhanced. The positive effect of hydrogen releasing from the electrode on the kinetics of the reaction of methanol oxidation is ascribed to the anti-poison behavior consisting in the reaction of hydrogen radicals with intermediates adsorbed on the Pd surface.     

Unexpected deprotection of silyl and THP ethers induced by serious disparity in the quality of Pd/C catalysts and elucidation of the mechanism

Commercial Pd/C catalysts show different catalytic activity toward the deprotection of silyl and THP ethers. The Pd/C purchased from Merck and ACROS exhibits marked tendency to cleave these protective groups unexpectedly without hydrogen conditions although Aldrich's Pd/C (20,569-9) is inactive in the absence of hydrogen. It was proved that the Pd/C disparity toward the deprotection of TES and THP ethers results from residual acids and/or palladium chloride in the production process of Pd/Cs. Although a TES ether cleavage reaction in the absence of hydrogen and a THP ether cleavage reaction in the presence of hydrogen using 10% Pd/C were recently published, we could conclude they were only an acid-catalyzed solvolysis, the acid being released from the catalyst. Hydrogen is essential for the actual 10% Pd/C-catalyzed cleavage of TES ethers and THP ethers which must be stable under the true Pd/C-catalyzed hydrogenation conditions.     

Facet‐Dependent and Light‐Assisted Efficient Hydrogen Evolution from Ammonia Borane Using Gold–Palladium Core–Shell Nanocatalysts

Au–Pd core–shell nanocrystals with tetrahexahedral (THH), cubic, and octahedral shapes and comparable sizes were synthesized. Similar‐sized Au and Pd cubes and octahedra were also prepared. These nanocrystals were used for the hydrogen‐evolution reaction (HER) from ammonia borane. Light irradiation can enhance the reaction rate for all the catalysts. In particular, Au–Pd THH exposing {730} facets showed the highest turnover frequency for hydrogen evolution under light with 3‐fold rate enhancement benefiting from lattice strain, modified surface electronic state, and a broader range of light absorption. Finite‐difference time‐domain (FDTD) simulations show a stronger electric field enhancement on Au–Pd core–shell THH than those on other Pd‐containing nanocrystals. Light‐assisted nitro reduction by ammonia borane on Au–Pd THH was also demonstrated. Au–Pd tetrahexahedra supported on activated carbon can act as a superior recyclable plasmonic photocatalyst for hydrogen evolution.     

Hydrogen evolution and Cu UPD at stepped gold single crystals modified with Pd

The evolution of hydrogen on Au(332) and Au(665) surfaces modified with Pd was studied by cyclic voltammetry in hydrogen-saturated sulfuric acid. A strong catalytic activity of Pd decorating the steps, and even monoatomic rows, reflected in the exchange current density for the hydrogen evolution reaction, was found. In comparison, the activity of Pd at terrace sites is negligible. This is explained by the previously observed weak adsorption of hydrogen at monoatomic Pd rows according to the Sabatier principle. For Au(665)/Pd electrodes where the Pd steps have been blocked with Cu, the catalytic activity decreases to values in the same order of magnitude of those for Au(665) surfaces modified with more than a full monolayer of Pd. No direct evidence of hydrogen spillover from Pd-covered areas to the Au substrate was found. Cu underpotential deposition measurements also suggest that no alloy formation takes place between the Cu atoms and the underlying Pd film, nor between Pd and the gold substrate. Dedicated to Professor Dr. Algirdas Vaskelis on the occasion of his 70th birthday.     

Effect of carbon nanofiber microstructure on oxygen reduction activity of supported palladium electrocatalyst

Palladium electrocatalysts supported on carbon nanofibers (CNFs) with controlled microstructure or on activated carbon (AC) are prepared, and the effects of the carbon materials microstructure on the oxygen reduction reaction (ORR) properties of the electrocatalysts are investigated. The physical properties of the CNFs with different microstructure, i.e. platelet CNF (p-CNF) and fish-bone CNF (f-CNF), are characterized by high resolution transmission electron microscope and N₂ physisorption. From cyclic voltammetric studies, it is found that Pd/p-CNF and Pd/f-CNF are more active than Pd/AC. The effects of CNF microstructure on the ORR activities of Pd/f-CNF and Pd/p-CNF are discussed. The p-CNF has a higher ratio of edge atoms to basal atoms, and therefore Pd/p-CNF has more positive ORR onset reduction potential and ORR peak potential than Pd/f-CNF. The supports also have influences on the reaction process. The ORR is surface reaction controlled when Pd/AC is used, while it becomes diffusion control when Pd/f-CNF is used.     

Durable High-Temperature Proton Exchange Membrane Fuel Cells Enabled by the Working-Temperature-Matching Palladium-Hydrogen Buffer Layer

The durability degradation during stack-operating conditions seriously deteriorates the lifetime and performance of the fuel cell. To alleviate the rapid potential rise and performance degradation, an anode design is proposed to match the working temperature of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) with the release temperature of hydrogen from palladium. The result is significantly enhanced hydrogen oxidation reaction (HOR) activity of Pd and superior performance of the Pd anode. Furthermore, Pd as hydrogen buffer and oxygen absorbent layer in the anode can provide additional in situ hydrogen and absorb infiltrated oxygen during local fuel starvation to maintain HOR and suppress reverse-current degradation. Compared with the traditional Pt/C anode, the Pd/C also greatly improved HT-PEMFCs durability during start-up/shut-down and current mutation. The storage/release of hydrogen provides innovative guidance for improving the durability of PEMFCs.     

Hydrogen uptake mechanism of a silicone‐rubber DEB getter mixture

The kinetics of the hydrogen getter 1,4‐bis(phenylethynyl)benzene (DEB) blended with carbon‐supported Pd (DEB‐Pd/C) dispersed uniformly in silicone [DEB‐Pd/C‐poly(dimethyl siloxane)] were studied with a thermogravimetric method as a function of the hydrogen pressure and temperature. A diffusion‐controlled reaction model was developed to explain the experimental results. The diffusion coefficient, solubility coefficient, and permeability of hydrogen through silicone rubber were determined. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 425–431, 2001     

甲烷在清洁 Pd(111) 及氧改性的 Pd(111) 表面解离的密度泛函理论研究

 采用广义梯度近似 (GGA) 的密度泛函理论 (DFT) 并结合平板模型, 研究了 CH4 在清洁 Pd(111) 及 O 改性的 Pd(111) 表面发生 C朒 键断裂的反应历程. 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能. 结果表明, CH4 采用一个 H 原子指向表面的构型在 Pd(111) 表面的顶位吸附, CH3 的最稳定的吸附位置为顶位, OH, O 和 H 的最稳定吸附位置均为面心立方. CH4 在清洁 Pd(111) 表面裂解的活化能为 0.97 eV, 低于它在 O 原子改性 (O 没有参与反应) 的 Pd(111) 表面的活化能 1.42 eV, 说明表面氧原子抑制了 CH4 中 C朒 键的断裂. 当亚表面 O 原子和表面 O 原子 (O 参与反应) 共同存在时, C朒 键断裂的活化能为 0.72 eV, 低于只有表层氧存在时的活化能 (1.43 eV), 说明亚表面的 O 原子对 CH4 分子的活化具有促进作用. CH4 在 O 原子改性的 Pd(111) 表面裂解生成 CH3 和 H, 以及生成 CH3 和 OH 的反应活化能分别为 1.42 和 1.43 eV, 说明 CH4 在 O 原子改性的 Pd(111) 表面发生这两种反应的难易程度相当.     

Hydrogen generation by plasma-assisted electrochemical pumping

We demonstrated that the reaction between water vapor and sulfur dioxide (SO₂) can be catalyzed plasma-chemically and hydrogen species, products of the reaction, can be pumped out electrochemically as hydrogen molecules (H₂) with the help of palladium (Pd) bipolar electrode. The plasma-energizing effect on the reaction between water vapor and SO₂ is solely played by non-thermal electrons generated by atmospheric pressure electrical microdischarge. Of the reaction products the hydrogen atoms are dissolved in the Pd membrane, transferred to the interface contacting a proton conducting medium through diffusion and eventually anodized at the interface. This type of electrolysis does not require platinum catalyst and opens a possibility of increasing the energy efficiency for hydrogen generation.     

Kinetic Evidence for a Non‐Langmuir‐Hinshelwood Surface Reaction: H/D Exchange over Pd Nanoparticles and Pd(111)

The mechanism of hydrogen recombination on a Pd(111) single crystal and well‐defined Pd nanoparticles is studied using pulsed multi‐molecular beam techniques and the H₂/D₂ isotope exchange reaction. The focus of this study is to obtain a microscopic understanding of the role of subsurface hydrogen in enhancing the associative desorption of molecular hydrogen. HD production from H₂ and D₂ over Pd is investigated using pulsed molecular beams, and the temperature dependence and reaction orders are obtained for the rate of HD production under various reaction conditions designed to modulate the amount of subsurface hydrogen present. The experimental data are compared to the results of kinetic modeling based on different mechanisms for hydrogen recombination. We found that under conditions where virtually no subsurface hydrogen species are present, the HD formation rate can be described exceptionally well by a classic Langmuir–Hinshelwood model. However, this model completely fails to reproduce the experimentally observed high HD formation rates and the reaction orders under reaction conditions where subsurface hydrogen is present. To analyze this phenomenon, we develop two kinetic models that account for the role of subsurface hydrogen. First, we investigate the possibility of a change in the reaction mechanism, where recombination of one subsurface and one surface hydrogen species (known as a breakthrough mechanism) becomes dominant when subsurface hydrogen is present. Second, we investigate the possibility of the modified Langmuir–Hinshelwood mechanism with subsurface hydrogen lowering the activation energy for recombination of two hydrogen species adsorbed on the surface. We show that the experimental reaction kinetics can be well described by both kinetic models based on non‐Langmuir–Hinshelwood‐type mechanisms.     

Hydrolysis Batteries: Generating Electrical Energy during Hydrogen Absorption

The hydrolysis reaction of aluminum can be decoupled into a battery by pairing an Al foil with a Pd‐capped yttrium dihydride (YH₂–Pd) electrode. This hydrolysis battery generates a voltage around 0.45 V and leads to hydrogen absorption into the YH₂ layer. This represents a new hydrogen absorption mechanism featuring electrical energy generation during hydrogen absorption. The hydrolysis battery converts 8–15 % of the thermal energy of the hydrolysis reaction into usable electrical energy, leading to much higher energy efficiency compared to that of direct hydrolysis.     

氢气存在下一氧化碳偶联反应的化学和性能

实验研究了在氢气存在下，一氧化碳与亚硝酸乙酯偶联合成草酸二乙酸反应体系在典型操作条件下的化学反应和反应性能，结果表明氢气只与反应体系中的亚硝酸乙酯反应生成乙醇。由于氢气与一氧化碳在催化剂表面竞争吸附，并且氢的存在打破了生成草酸二乙酯所必需的正常的氧化还原过程：Pd^0→Pd^2 →Pd^0，因此氢的引入使一氧化碳转化率及草酸二乙酯选择性下降。此外，化学吸附测试结果表明氢气和一氧化碳是在催化剂表面同一活性中心上吸附。     

Influence of atmospheric oxygen on hydrogen detection on Pd using Kelvin probe technique

Influence of oxygen concentration in the measurement atmosphere on detection of hydrogen using Kelvin probe was studied. The studied material was a 100-μm-thick palladium foil, which was mounted in a 3D printed electrochemical flow cell. The used setup enables hydrogen loading with in-situ contact potential measurement of the hydrogen exit side of the Pd electrode. The hydrogen loading and unloading procedure, including insertion of different amounts of hydrogen into the Pd membrane and recording resulting values of contact potential difference, was performed at distinct oxygen concentrations ranging between 1 and 80 vol%. An increasing amount of oxygen in the atmosphere surrounding the hydrogen-loaded Pd electrode resulted in an accelerated removal of hydrogen from the Pd. The kinetics of this reaction was studied based on Kelvin probe measurements, and a reaction mechanism is discussed.     

pH effect on electrocatalytic reduction of CO2 over Pd and Pt nanoparticles

Adsorbed hydrogen participates in electrocatalytic reduction of CO₂ and competitive hydrogen evolution reaction (HER) simultaneously, and its reaction pathway greatly affects the activity and selectivity of CO₂ reduction. In this work, we investigate pH effect on electrocatalytic reduction of CO₂ over Pd and Pt nanoparticles (NPs) with a similar size in a pH range from 1.5 to 4.2. Pt NPs completely contribute to HER in the pH range. Over Pd NPs, Faradaic efficiency for CO production at − 1.19 V (vs. reversible hydrogen electrode) varies from 3.2% at pH of 1.5 to 93.2% at pH of 4.2, and current density for CO production reaches maximum at pH of 2.2. The significant enhancement of Faradaic efficiency and current density for CO production over Pd NPs at high pH values is attributed to decreased kinetics of hydrogen evolution reaction by increasing hydrogen binding energy and lowered adsorption affinity of CO-like intermediate compared to Pt.     

Pd(111), Pd(100)及Pd(110)表面H2和O2直接合成H2O2的密度泛函理论研究

采用周期性密度泛函理论研究了H₂和O₂在Pd(111),Pd(100)及Pd(110)表面上直接合成H₂O₂的反应机理,对反应的主要基元步骤进行了计算和分析.结果表明,Pd(111)表面对H₂O₂直接合成的催化选择性最好,表面原子密度较低的Pd(100)表面和Pd(110)表面上含有O-O键的表面物种解离严重,不利于H₂O₂的生成.H₂O₂的选择性与含有O-O键表面物种的O-O键能和表面物种的结合能有关.含有O-O键的表面物种在表面的结合能越大,越容易发生解离,不利于形成H₂O₂.     

Pure hydrogen production from methylcyclohexane using a new high performance membrane reactor

A high efficiency membrane reactor (Pd based) has been developed for hydrogen generation from methylcyclohexane, which is able to produce a pure hydrogen stream with a reaction yield close to 100%.     

Synthesis of seven-membered ring diazepin-2-ones via palladium-catalyzed highly regioselective cyclization of 2-vinylpyrrolidines with aryl isocyanates

The first palladium-catalyzed ring-expansion reaction of 2-vinylpyrrolidines with aryl isocyanates to form seven-membered ring heterocycles is described. This regioselective reaction requires 5 mol % of Pd(2)(dba)(3).CHCl(3) and 10 mol % of dppp at 40-60 degrees C in THF and results in the formation of 1,3-diazepin-2-ones in good isolated yields. When Pd(OAc)(2) and PPh(3) were utilized in the reaction, an intramolecular hydrogen migration occurs resulting in the formation of conjugated diene derivatives of urea.     

Preparation of silica–alumina composite membranes for hydrogen separation by multi-step pore modifications

In the present paper, a silica–alumina composite membrane for hydrogen separation was prepared within an α-alumina support by the multi-step pore modification. The α-alumina support has an asymmetric structure composed of a thin dense skin layer and a thick coarse layer and the average pore size of its skin layer is 80 nm. The composite membrane layer was formed in the vicinity of the interphase between the two layers of the support by two consecutive steps; namely, in situ silica sol–gel reaction and soaking and vapor deposition. In order to enhance the hydrogen selectivity, palladium (Pd) particles were impregnated in the final step utilizing Pd-acetate as a Pd precursor. Although both silica and Pd induced the surface diffusion, Pd was more effective for selective hydrogen adsorption than silica. This multi-step method produced a porous membrane with moderate hydrogen selectivity and satisfactory hydrogen permeance at high temperature and at high transmembrane pressure. The separation factor of hydrogen relative to nitrogen was maintained at about 10 even when the transmembrane pressure was as high as 110 kPa, and the hydrogen permeance was still much higher than that of non-porous polymeric membranes. In addition, the microstructural distributions of Si and Pd within the intermediate membrane layer were examined by a scanning electron microscopy (SEM) and an energy dispersive X-ray analysis (EDX)