Surface poisoning in the nucleation and growth of palladium atomic layer deposition with Pd(hfac)2 and formalin

Palladium (Pd) atomic layer deposition (ALD) can be performed with Pd(hfac)₂ (hfac = hexafluoroacetyl-acetone) and formalin as the reactants. For Pd ALD on oxide surfaces, the nucleation of Pd ALD has been observed to require between 20 and 100 ALD cycles. To understand the long nucleation periods, this study explored the surface reactions occurring during Pd ALD nucleation and growth on hydroxylated Al₂O₃ substrates. In situ Fourier transform infrared (FTIR) spectroscopy on high surface area nanopowders was used to observe the surface species. The adsorption of Pd(hfac)₂ on hydroxylated Al₂O₃ substrates was found to yield both Pd(hfac)* and Al(hfac)* surface species. The identity of the Al(hfac)* species was confirmed by separate FTIR studies of hfacH adsorption on the hydroxylated Al₂O₃ substrates. Isothermal loss of the Al(hfac)* species revealed second-order kinetics at 448-523 K with an activation barrier of E_d = 39.4 kcal/mol. The lack of correlation between Al(hfac)* and AlOH* species during the loss of Al(hfac)* species suggested that the Al(hfac)* species may desorb as Al(hfac)₃. After Pd(hfac)₂ exposure and the subsequent formalin exposure on hydroxylated Al₂O₃ substrates, only hfac ligands from Pd(hfac)* species were removed from the surface. In addition, the formalin exposure added formate species. The Al(hfac)* species was identified as the cause of the long nucleation period because Al(hfac)* behaves as a site blocker. The surface poisoning by Al(hfac)* species was corroborated by adsorbing hfacH prior to the Pd(hfac)₂ exposures. The amount of Pd(hfac)* species after Pd(hfac)₂ exposures decreased progressively versus the previous hfacH exposure. Pd ALD occurred gradually during the subsequent Pd ALD cycles as the Al(hfac)* species were slowly removed from the Al₂O₃ surface. Ex situ transmission electron microscopy analysis revealed Pd nanoclusters that grew in size and dispersion with increasing number of Pd ALD cycles. These nanoclusters eventually coalesced to form a continuous Pd ALD film. Surface poisoning by the hfac ligands may help to explain the nucleation difficulties for metal ALD on oxide substrates using β-diketonate reactants.     

Liquid phase hydrodechlorination of chlorophenols at lower temperature on a novel Pd catalyst

Pd catalyst supported on mesoporous silica-carbon nano-composite (Pd/MSCN) was prepared by the method of wet impregnation, and its activity for hydrodechlorination (HDC) of 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol was evaluated at 258-313 K under ordinary hydrogen pressure by using triethylamine (Et(3)N) as a base additive. XRD analysis indicates that Pd/MSCN catalyst possesses the ordered mesostructure. Meanwhile, the results from TEM and H(2) chemisorption analysis indicate the high dispersion of Pd on MSCN with Pd nanoparticles whose average size is 3.2 nm. For the first time, the high activity of nano-size Pd on MSCN for HDC of chlorophenols was observed at 258 K. In addition, it was found that the inhibition effect of Et(3)N on HDC existed obviously, and can be efficiently reduced by stepwise addition of Et(3)N. The correlations of the dielectric constants of base and the polarity of solvent to the activity of Pd/MSCN for HDC of chlorophenols were obtained.     

A facile synthesis of MPd (M = Co, Cu) nanoparticles and their catalysis for formic acid oxidation

Monodisperse CoPd nanoparticles (NPs) were synthesized and studied for catalytic formic acid (HCOOH) oxidation (FAO). The NPs were prepared by coreduction of Co(acac)(2) (acac = acetylacetonate) and PdBr(2) at 260 °C in oleylamine and trioctylphosphine, and their sizes (5-12 nm) and compositions (Co(10)Pd(90) to Co(60)Pd(40)) were controlled by heating ramp rate, metal salt concentration, or metal molar ratios. The 8 nm CoPd NPs were activated for HCOOH oxidation by a simple ethanol wash. In 0.1 M HClO(4) and 2 M HCOOH solution, their catalytic activities followed the trend of Co(50)Pd(50) > Co(60)Pd(40) > Co(10)Pd(90) > Pd. The Co(50)Pd(50) NPs had an oxidation peak at 0.4 V with a peak current density of 774 A/g(Pd). As a comparison, commercial Pd catalysts showed an oxidation peak at 0.75 V with peak current density of only 254 A/g(Pd). The synthesis procedure could also be extended to prepare CuPd NPs when Co(acac)(2) was replaced by Cu(ac)(2) (ac = acetate) in an otherwise identical condition. The CuPd NPs were less active catalysts than CoPd or even Pd for FAO in HClO(4) solution. The synthesis provides a general approach to Pd-based bimetallic NPs and will enable further investigation of Pd-based alloy NPs for electro-oxidation and other catalytic reactions.     

Uniformly dispersed Pd nanoparticles anchored Co(OH)<Subscript>2</Subscript>/Cu(OH)<Subscript>2</Subscript> hierarchical nanotube array as high active structured catalyst for Suzuki–Miyaura coupling reactions

Up to now, although tremendous effort has been made in exploring Pd-based catalysts for the Suzuki–Miyaura coupling reactions, there is still much room to enhance its catalytic performance by synthesis catalyst with define-designed and tailored structure. Herein, a new kind of Pd-based structured catalyst with hierarchical hollow structure (Pd/Co(OH)₂/Cu(OH)₂/copper foam) has been successfully synthesized by three facile steps. As framework, the hierarchical hollow nanoarray structure of Co(OH)₂/Cu(OH)₂/copper foam is fabricated by immersion and electrolytic deposition methods. Uniformly dispersed Pd nanoparticles with a narrow size distribution (1.5?±?0.2 nm) are anchored on the surface of the hierarchical nanotube array through an in situ spontaneous redox reaction between Co(OH)₂ and PdCl₄^2? without any surfactant at room temperature. Compared with other synthesis approaches, it just takes less than 1 h for the whole fabrication process in our strategy, exhibiting very high efficiency. In order to evaluate the catalytic performance of the as-prepared structured catalyst, 16 kinds of reagents were chosen as substrates for Suzuki–Miyaura coupling reaction, exhibiting excellent activity and reusability under mild conditions. We hope this simple and efficient method will open a new strategy to design and prepare structured catalysts.     

Wetting behaviors of molten melt drops on polycrystalline Al_2O_3 substrates in high magnetic fields

We measured the contact angles of Al and Sn drops on polycrystalline Al_2O_3 substrates in various high magnetic fields at different temperatures. The contact angles of both Al and Sn drops on the Al_2O_3 substrates decreased under high magnetic fields. These decreases strongly depend on the temperature,magnetic flux density, magnetic properties of the metal drops, and the reactivity of the metal drops with Al_2O_3. Our results reveal that the wetting behavior of molten metal drops on ceramics can be modified by a high magnetic field.     

Fabrication and nano-imprintabilities of Zr-, Pd- and Cu-based glassy alloy thin films

With the aim of investigating nano-imprintability of glassy alloys in a film form, Zr(49)Al(11)Ni(8)Cu(32), Pd(39)Cu(29)Ni(13)P(19) and Cu(38)Zr(47)Al(9)Ag(6) glassy alloy thin films were fabricated on Si substrate by a magnetron sputtering method. These films exhibit a very smooth surface, a distinct glass transition phenomenon and a large supercooled liquid region of about 80 K, which are suitable for imprinting materials. Moreover, thermal nano-imprintability of these obtained films is demonstrated by using a dot array mold with a dot diameter of 90 nm. Surface observations revealed that periodic nano-hole arrays with a hole diameter of 90 nm were successfully imprinted on the surface of these films. Among them, Pd-based glassy alloy thin film indicated more precise pattern imprintability, namely, flatter residual surface plane and sharper hole edge. It is said that these glassy alloy thin films, especially Pd-based glassy alloy thin film, are one of the promising materials for fabricating micro-machines and nano-devices by thermal imprinting.     

Robust enhanced hydrogen production at acidic conditions over molybdenum oxides-stabilized ultrafine palladium electrocatalysts

Electrochemical water splitting is quite seductive for eco-friendly hydrogen fuel energy production,however,the attainment of highly efficient,durable,and cheap catalysts for the hydrogen evolution reaction(HER)remains challenging.In this study,molybdenum oxides stabilized palladium nanoparticle catalysts(MoO_x-Pd)are in situ prepared on commercial carbon cloth(CC)by the facile two-step method of dip-coating and electrochemical reduction.As a self-supported Pd-based catalyst electrode,the MoO_x-Pd/CC presents a competitive Tafel slope of 45.75 mV·dec^-1,an ultralow overpotential of 25 mV,and extremely long cycling durability(one week)in 0.5M H₂S0₄electrolyte,superior to unmodified Pd catalysts and comparable to commercial Pt mesh electrode.On the one hand,the introduction of MoO_xcan inhibit the growth of Pd particles to obtain ultrafine Pd nanoparticles,thus exposing more available active sites.On the other hand,density functional theory(DFT)calculation revealed that MoO_xon the surface of Pd metal can regulate the electronic structure of Pd metal and enhance its intrinsic catalytic activity of HER.This work suggests that transitional metal nanoparticles stabilized by molybdenum oxides are hopeful approaches for obtaining fruitful hydrogen-producing electrocatalysts.     

金属/绝缘体/金属(Al/Al2O3/Au)隧道结的发光衰减机制

结合Al/Al2 O3 /Au结构MIM(metal/insulator/metal)隧道结I U特性、深度Auger谱及结发光后结面透明度的测试与观察 ,对其发光衰减机制进行了研究。结果表明 ,由于MIM结工作时 ,通过隧道电流等产生的大量焦耳热引起底电极Al膜不断氧化 ,中间栅Al2 O3 的厚度不断增加 ,从而使得隧穿电子激发表面等离极化激元 (surfaceplasmonpolariton ,SPP)的强度不断变弱 ,引起SPP耦合发光的强度不断衰减     

Al/Al2O3多层膜的研究

用真空蒸镀及自然氧化方法在玻璃基底上制备纳米量级的4、5、6、7对层的Al／Al2O3多层膜。采用称重法测定薄膜的厚度；在常温和低温下使用三点法测定多层膜的电特性；用扫描电镜（sEM）观察薄膜的表面和截面的形貌及成分。结果表明：制备的是纳米量级非晶态的Al／Al2O3多层膜，在常温和低温（77K）下均具有类似负阻的特性。     

Hydrogen Storage in Carbon-Based Nanostructured Materials

Carbon nanostructures represent a revolution in science and hold the potential for a large range of applications because of their interesting electrical, mechanical, and optical properties. Multiwall carbon nanotubes and carbon nanofibers of herringbone formation were grown by chemical vapor deposition on different catalysts from a number of hydrocarbon sources. After the total or particle removal of the catalyst system, the carbon nanostructures were analyzed for hydrogen uptake. Six samples of nanofibers grown on a Pd-based catalyst system (with a surface area of 425–455 m²/g) were controlled oxidized in air, such that they had different ratios of Pd/C varying from 0.05 to 0.9 mole ratio. The hydrogen uptake experiments were performed volumetrically in a Sievert-type installation and showed that the quantity of desorbed hydrogen (for pressure intervals ranging from 1 to 100 bars) by the carbon nanostructures free of any metal catalyst particles was between 0.04 and 0.33% by weight. For the samples of nanofibers that contained Pd in various Pd/C ratios, palladium revealed catalytic properties and supplied atomic hydrogen at the Pd/C interface by dissociating the H₂ molecules. The results show a direct correlation between the Pd/C ratio and the quantity of hydrogen absorbed by these samples. A saturation value of about 1.5 wt.% was reached for a high ratio of about 1:1 of Pd/C. The multiwall carbon nanotubes grown on a Fe:Co:CaCO₃ catalytic system and purified by acid cleaning and air oxidation showed a hydrogen uptake value of 0.1 to 0.2 wt.%.     

Prospects for using reactor palladium for <Superscript>129</Superscript>I immobilization

The efficiency and application prospects of various methods for Pd recovery from solutions of various compositions are reviewed. Particular attention is given to the problem of the recovery of reactor Pd with the aim of its use for reprocessing of radiochemical production wastes (e.g., for immobilization of ¹²⁹I and/or TPE). The results of experiments on the synthesis of Pd-based formulations incorporating PdI₂ are described. Incorporation of ¹²⁹I into a Pd matrix allows preparation of a material with a low leach rate, suitable for long-term safe storage.     

Galvanic Replacement–Mediated Synthesis of Ni‐Supported Pd Nanoparticles with Strong Metal–Support Interaction for Methanol Electro‐oxidation

Herein, well‐defined Pd nanoparticles (NPs) developed on Ni substrate (Pd NPs/Ni) are synthesized via a facile galvanic replacement reaction (GRR) route performed in ethaline‐based deep eutectic solvent (DES). For comparison, a Pd NPs/Ni composite is also prepared by the GRR method conducted in an aqueous solution. The Pd NPs/Ni obtained from the ethaline‐DES is catalytically more active and durable for the methanol electro‐oxidation reaction (MOR) than those of the counterpart derived from conventional aqueous solution and commercial Pd/C under alkaline media. Detailed kinetic analysis indicates that the unique solvent environment offered by ethaline plays vital roles in adjusting the reactivity of the active species and their mass transport properties to control over the genesis of the Pd NPs/Ni nanocomposite. The resulting Pd NPs/Ni catalyst possesses a homogeneous dispersion of Pd NPs with a strong Pd (metal)–Ni (support) interaction. This structure enhances the charge transfer between the support and the active phases, and optimizes the adsorption energy of OH^? and CO on the surface, leading to superior electrocatalytic performance. This work provides a novel GRR strategy performed in ethaline‐DES to the rational design and construction of advanced metal/support catalysts with strong interaction for improving the activity and durability for MOR.     

Aerosol based fabrication of a Cu/polymer and its application for electromagnetic interference shielding

The effect of a catalytic surface activation on the electromagnetic interference shielding of Cu deposited polymer substrates was investigated. The surface of polymer substrates was catalytically activated by different methods respectively adopted Pd aerosol nanoparticles and Sn-Pd wet chemical processes. Although both activations initiated the deposition of Cu on the substrates, differences such as morphology (Pd aerosol: ~80 nm vs Sn-Pd: ~ 140 nm, in Cu grain size) and composition (Pd aerosol: Cu and Pd vs Sn-Pd: Cu, Pd, Sn, and Cl) of Cu deposits were presented. Specimens activated using Pd aerosol nanoparticles showed a higher range of shielding effectiveness by about 4-10 dB than those activated by Sn-Pd processes in 2-18 Ghz frequencies.     

Catalytic reduction of U(VI) with hydrazine on palladium catalysts in acid solutions

The stability of finely dispersed palladium supported on silica gel with respect to various acids was studied. It was shown that palladium catalysts can be used in moderately acidic media under reducing conditions. In nitric acid solutions within a wide range of experimental conditions, the palladium catalysts do not initiate reduction of U(VI) with hydrazine. The catalytic properties of palladium catalysts differing in the size of nanocrystallites of the active metal were examined in the reduction of U(VI) with hydrazine in sulfuric acid solutions. The specific activity of Pd/SiO₂ catalysts is determined solely by the size of metal nanocrystals and is independent of the metal content on the support. The negative size effect is observed, i.e., the surface Pd atoms located on large crystallites exhibit higher catalytic activity. The results obtained were interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the mechanism of U(VI) catalytic reduction with hydrazine in the sulfuric acid solutions.     

Additional Lithium Storage on Dynamic Electrode Surface by Charge Redistribution in Inactive Ru Metal

Beyond a traditional view that metal nanoparticles formed upon electrochemical reaction are inactive against lithium, recently their electrochemical participations are manifested and elucidated as catalytic and interfacial effects. Here, ruthenium metal composed of ≈5 nm nanoparticles is prepared and the pure ruthenium as a lithium‐ion battery anode for complete understanding on anomalous lithium storage reaction mechanism is designed. In particular, the pure metal electrode is intended for eliminating the electrochemical reaction‐derived Li₂O phase accompanied by catalytic Li₂O decomposition and the interfacial lithium storage at Ru/Li₂O phase boundary, and thereby focusing on the ruthenium itself in exploring its electrochemical reactivity. Intriguingly, unusual lithium storage not involving redox reactions with electron transfer but leading to lattice expansion is identified in the ruthenium electrode. Size‐dependent charge redistribution at surface enables additional lithium adsorption to occur on the inactive but more environmentally sensitive nanoparticles, providing innovative insight into dynamic electrode environments in rechargeable lithium chemistry.     

Surface property control of semiconducting metal oxide nanoparticles

The comprehension of the underlying mechanism attributing either a reducing or oxidizing effect to humidity on the metal oxides used in gas sensors is still a challenge as the literature on this subject shows contradictory results. The high surface-to-bulk ratio of nanosized powders conferring them a much higher surface reactivity compared to their micronsized counterpart, clearly helps in a better understanding of the gas-surface interactions. A thorough FTIR surface study of a nanosized titanium dioxide powder subjected to various sequences of carbon monoxide and mixtures of carbon monoxide and water vapor is reported. The effect of acetic acid and hexamethyldisilazane grafting on TiO₂ is studied under the same conditions. It is found that water molecules have a reducing or oxidizing effect, depending on the acido-basicity of the metal oxide. Our study clearly shows that a thorough knowledge of the surface reactivity of metal oxides is a prerequisite for further major gas sensing optimization.     

双金属热轧复合的界面结合影响因素及结合机理

在综合了不同材料热轧复合的实验基础上,分析了双金属热轧复合过程中不同工艺条件对结合质量的影响.结果表明:轧制前清除材料表面的覆盖膜有助于轧制过程中形成结合点;轧制过程中适当的轧制温度和轧制压下量能大量消除轧制过程中在金属表面形成的氧化膜,从而使组元材料能形成机械结合;在热烧结过程中,原子通过界面扩散可以消除轧制过程中由于界面微观不平整形成的空洞,同时通过原子间的相互作用使组元材料间形成冶金结合.依据固相结合理论分析得出,双金属热轧复合的界面结合过程包括:金属间物理接触形成机械结合阶段,原子通过化学作用形成化学键及通过界面扩散消除空洞的冶金结合阶段,以及互扩散阶段.     

Pd–Al pillared clays as catalysts for the hydrodechlorination of 4-chlorophenol in aqueous phase

Catalysts based on pillared clays with Pd–Al were synthesized from a commercial bentonite and tested for catalytic hydrodechlorination (HDC) using 4-chlorophenol (4-CPhOH) as target compound and formic acid as hydrogen source. Stable Pd–Al pillared clays, with a strong fixation of the active phase to the solid support were obtained since no Pd was detected in the reaction media. The incorporation of Pd to the pillared clay structure yielded catalysts with high activity in the reaction studied reaching a complete removal of the 4-CPhOH under mild conditions of temperature (50–70 °C). Phenol was not the only reaction product formed, since a more hydrogenated product such as cyclohexanone was detected in the effluent, which indicates additional hydrogenation of phenol. The influence of the method of introduction of Pd in the pillared clay (ion-exchange or impregnation) and Pd concentration in the catalytic activity were studied as well as other important operating variables such as reaction temperature, catalyst concentration, 4-CPhOH initial concentration and formic acid to 4-CPhOH molar ratio. The catalysts prepared suffered deactivation after three consecutive runs, probably due to carboneous deposits formation since no appreciable Pd leaching was observed.     

An approach to prepare uniform graphene oxide/aluminum composite powders by simple electrostatic interaction in water/alcohol solution

The homogenous dispersion of graphene in Al powders is a key challenge that limits the development of graphene-reinforced metal matrix composites with high performance. Here, uniform distribution of graphene oxide (GO) coated on flake Al powders were obtained by a simply stirring and ultrasonic treatment in the water/alcohol solution. The effect of water volume content on the formation of GO/Al composite powders was investigated. The results showed that GO adsorbed with synchronous reduction on the surface of Al powders, but when the water content was higher than 80% in the solution, Al powders were totally changed into Al(OH)₃. With optimizing the water content of 60% in the solution, reduced GO was homogenously coated onto the surface of flake Al powders. The formation mechanism can be ascribed to the balance control between the liquid/solid interaction and the hydrolysis reaction.     

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O₂. Three components of the complex film formed on Al(111) can be identified: an ultrathin under‐stoichiometric Al_xO_y interface layer, an intermediate Al₂O₃ phase with γ‐alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O₂ molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al₂O₃ phase. The electron transfer takes place both from the metal‐oxide interface and the oxide surface to the adsorbed O₂ molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O₂ ⁻ species to capture protons and transform into hydrogen peroxide in a non‐activated process. H₂O₂ is further reduced to two water molecules, in a series of two‐electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al₂O₃ inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.