Nitrate reduction on Pt single crystals with Pd multilayer

Nitrate reduction on palladium multilayers deposited on platinum single crystal electrodes was studied by cyclic voltammetry and FTIR spectroscopy in acid and alkaline media. The results are compared with those obtained with bulk palladium single crystals. The reaction is sensitive to the electrode surface structure, the reactivity depending on the solution pH. In acid solution nitrate was reduced at potentials below the potential of zero total charge (pztc), when the electrode is negatively charged. Competition between nitrate, hydrogen and anion adsorption and NO formation and accumulation at the surface are proposed as the main reasons for the slow reaction rate. On the bulk palladium single crystal electrodes, NO formation leads to a fast blockage of the surface resulting in a very low activity for nitrate reduction. In alkaline solution, nitrate is reduced at more positive potentials with significantly higher current being measured on the Pd multilayer on Pt(1 0 0) electrode.     

Isothermal solid‐state transformation kinetics applied to Pd/Cu alloy membrane fabrication

In this work, time‐resolved, in situ high‐temperature X‐ray diffraction was used to study the solid‐state transformation kinetics of the formation of the fcc Pd/Cu alloy from Pd/Cu bilayers for the purpose of fabricating sulfur‐tolerant Pd/Cu membranes for H₂ separation. Thin layers of Pd and Cu (total ～15 wt % Cu) were deposited on porous stainless steel with the electroless deposition method and annealed in H₂ at 500, 550, and 600°C. The kinetics of the annealing process was successfully described by the Avrami nucleation and growth model, showing that the annealing process was diffusion controlled and one dimensional. The activation energy for the solid‐state transformation was 175 kJ/mol, which was similar to the activation energy of Pd‐Cu bulk interdiffusion. Furthermore, the Avrami model was able to successfully describe the changes in permeance and activation energy observed in Pd/Cu alloy membranes during characterization as they were annealed at high temperatures. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

Pd-based bimetallic catalysts prepared by replacement reactions

Several Pd-based bimetallic catalysts, Pd/Co, Pd/Ni and Pd/Cu, were synthesized by replacement reactions. The catalysts were characterized by XRD and CO chemisorption and their catalytic properties were evaluated using cyclohexene self-hydrogenation. The results suggest that the high catalytic activity of Pd/Ni is most likely due to the monolayer-dispersion of Pd on the Ni surface. The results also suggest that Pd is monolayer-dispersed on the Co surface in Pd/Co, whereas Pd forms surface alloy or solid solution with Cu in Pd/Cu.     

Fabrication of vertically aligned Pd nanowire array in AAO template by electrodeposition using neutral electrolyte

A vertically aligned Pd nanowire array was successfully fabricated on an Au/Ti substrate using an anodic aluminum oxide (AAO) template by a direct voltage electrodeposition method at room temperature using diluted neutral electrolyte. The fabrication of Pd nanowires was controlled by analyzing the current–time transient during electrodeposition using potentiostat. The AAO template and the Pd nanowires were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) methods and X-Ray diffraction (XRD). It was observed that the Pd nanowire array was standing freely on an Au-coated Ti substrate after removing the AAO template in a relatively large area of about 5 cm², approximately 50 nm in diameter and 2.5 μm in length with a high aspect ratio. The nucleation rate and the number of atoms in the critical nucleus were determined from the analysis of current transients. Pd nuclei density was calculated as 3.55 × 10⁸ cm⁻². Usage of diluted neutral electrolyte enables slower growing of Pd nanowires owing to increase in the electrodeposition potential and thus obtained Pd nanowires have higher crystallinity with lower dislocations. In fact, this high crystallinity of Pd nanowires provides them positive effect for sensor performances especially.     

Preparation and characterization of silica-supported, group IB–Pd bimetallic catalysts prepared by electroless deposition methods

Electroless deposition has been used to synthesize a series of Au–, Ag–, and Cu–Pd/SiO₂ bimetallic catalysts having incremental surface coverages and compositions of each group IB metal. Thermodynamically unstable, yet kinetically stable, electroless bath(s) were developed using metal bis-cyano salts of the group 1B metal and N₂H₄ (for Au and Ag) or DMAB (for Cu) as reducing agents. The times (1–2 h) and profiles (1st order in group 1B metal concentration) observed for complete deposition indicate good kinetic control of the electroless deposition process. The bimetallic catalysts have been characterized using selective chemisorption, atomic absorption spectroscopy (AAS), Fourier transform infrared spectroscopy (FTIR) of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) techniques. Decreases in Pd surface sites with addition of IB metals confirm deposition onto the supported Pd nanoparticle surfaces. FTIR studies suggest that deposition of Cu and Ag are selective towards Pd(1 1 1) sites, while Au deposits non-discriminately on all Pd sites. Finally, XPS measurements for each family of bimetallic catalysts suggest a net electron transfer from the Pd to the deposited metal.     

Electrochemical and structural characterization of carbon-supported Pt-Pd bimetallic electrocatalysts prepared by electroless deposition

Electrochemical and structural characteristics of various Pt-Pd/C bimetallic catalysts prepared by electroless deposition (ED) methods have been investigated. Structural analysis was conducted by X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, scanning transmission electron microscopy, and energy dispersive X-ray spectroscopy (EDS). Monometallic Pt or Pd particles were not detected by EDS, indicating the ED methodology formed only bimetallic particles. The size of the Pt-Pd bimetallic particles was smaller than those of a commercially available Pt/C catalyst. The morphology of the Pt on Pd/C catalysts was identified and corresponded to Pd particles partially encapsulated by Pt.The electrochemical characteristics of the lowest Pd loading catalyst (7.0% Pt on 0.5% Pd/C) for the oxygen reduction reaction (ORR) have been investigated by the rotating ring disk electrode technique. The electrochemical activity was equal or lower than the commercially available Pt/C catalyst; however, the amount of hydrogen peroxide observed at the ring was reduced by the Pd, suggesting that such a catalyst has the potential to decrease ionomer degradation in applications. The Pt on Pd/C catalysts also show a higher tolerance to ripening induced by potential cycling. Therefore, catalyst suitability cannot be judged solely by its initial performance; information related to specific degradation mechanisms is also needed for a more complete assessment.     

Pd deposition onto Au(111) from nitrate solution

The initial stages of palladium deposition onto Au(111) from 0.1 M HNO₃ + 0.2 mM Pd(NO₃)₂ have been studied by cyclic voltammetry and in situ scanning tunnelling microscopy. It is demonstrated that nucleation starts exclusively at surface defects such as monoatomic high steps, which is at variance with recently published work. From this and our previous work it thus appears that surface defects are the preferred nucleation sites indeed for nitrate, sulphate and chloride containing solutions.     

Pd/Co Bimetallic Nanoparticles: Coelectrodeposition under Protection of PVP and Enhanced Electrocatalytic Activity for Ethanol Electrooxidation

A series of Pd–Co bimetallic nanostructures with Co compositions ranging from 0 to 13 at.% were fabricated on glassy carbon electrode by one step electrodeposition in the presence of polyvinylpyrrolidone (PVP). The roles of PVP and Co have been systematically investigated by using combined techniques such as scanning electron microscopy, energy dispersive spectrometry, cyclic voltammetry, X‐ray diffraction, and chronoamperograms. PVP was used as an additive to stabilize the Pd nanoparticles and inhibit agglomeration during their formation. The prepared Pd₁₀₀Co₁₀ bimetallic nanostructures exhibited great catalytic activity towards ethanol oxidation in alkaline, which implies that low Co doping can be a convenient way to enhance the electrocatalytic property of Pd. The present study shows that the Pd/Co bimetallic nanoparticulate can be a promising catalyst for portable applications in direct ethanol fuel cell in alkaline solution.     

Effects of incorporation of Cu and Ag in Pd on electrochemical oxidation of methanol in alkaline solution

Pd fine particles were prepared by heterogeneous reaction of PdOx with dry methanol as well as by the NaBH4 reduction method. The former method was found to give Pd nanoparticles (~5nm). Similarly f.c.c. structured, single phase nanoparticles of alloy compositions Pd0.8Cu0.2, Pd0.5Cu0.5, Pd0.8Ag0.2 and Pd0.5Ag0.5 were prepared by the heterogeneous reaction of dry methanol with intimate mixtures of PdOx+CuOx and PdOx+AgNO3. The electrochemical properties of the porous unsupported electrodes, prepared from these materials, in alkaline solutions, were investigated by cyclic voltammetry and steady-state polarization measurements. Various processes taking place during potential scanning in the presence and absence of methanol in 6m KOH solution arediscussed. Steady-state polarization data indicate that the methanol oxidation reaction (MOR) activity decreases with incorporation of Cu and Ag into the Pd lattice. The extent of decrease in the MOR activity is less for Cu ad dition than for Ag addition.     

Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd

The present work has been undertaken to tailor Pt/Al₂O₃ catalysts active for NO oxidation even after severe heat treatments in air. For this purpose, the addition of Pd has been attempted, which is less active for this reaction but can effectively suppress thermal sintering of the active metal Pt. Various Pd-modified Pt/Al₂O₃ catalysts were prepared, subjected to heat treatments in air at 800 and 830 °C, and then applied for NO oxidation at 300 °C. The total NO oxidation activity was shown to be significantly enhanced by the addition of Pd, depending on the amount of Pd added. The Pd-modified catalysts are active even after the severe heat treatment at 830 °C for a long time of 60 h. The optimized Pd-modified Pt/Al₂O₃ catalyst can show a maximum activity limited by chemical equilibrium under the conditions used. The bulk structures of supported noble metal particles were examined by XRD and their surface properties by CO chemisorption and EDX-TEM. From these characterization results as well as the reaction ones, the size of individual metal particles, the chemical composition of their surfaces, and the overall TOF value were determined for discussing possible reasons for the improvement of the thermal stability and the enhanced catalytic activity of Pt/Al₂O₃ catalysts by the Pd addition. The Pd-modified Pt/Al₂O₃ catalysts should be a promising one for NO oxidation of practical interest.     

Hydrogenation of 3,4-epoxy-1-butene over Cu–Pd/SiO2 catalysts prepared by electroless deposition

Electroless deposition has been used to prepare Cu–Pd/SiO₂ bimetallic catalysts wherein initial Cu coverages are limited only to the pre-existing Pd surface. Cu loading on the Pd surface can be systematically varied by modification of deposition kinetic parameters. In this case deposition time was used as the kinetic variable for the preparation of a series of Cu–Pd catalysts. These materials have been characterized using atomic absorption, CO chemisorption, and FT-IR (adsorption of CO), and then evaluated for the hydrogenation of 3,4-epoxy-1-butene, a functionalized olefin having many potential reaction pathways. Catalyst performance and characterization results suggest that Cu is not distributed in a monodisperse manner on the Pd surface, indicating the existence of autocatalytic deposition of Cu on Cu sites. The FT-IR results suggest that although CO adsorption on all sites is suppressed by Cu addition, initial Cu deposition occurs more readily on certain sites. The bimetallic Cu–Pd sites that are formed exhibit unusually high activity for EpB conversion and formation of unsaturated alcohols and aldehydes. This bimetallic effect on catalyst activity and selectivity is best explained, not by the existence of either ligand or ensemble effects, but rather by the bifunctional nature of the Cu–Pd sites present on the surface of these catalysts.     

化学复合镀PANI/Cu导电聚合物复合材料

采用化学复合镀技术在尼龙塑料PA66表面沉积PANI/Cu复合镀层。研究了复合镀层与基体的结合力和导电性，采用SEM观察镀层微观结构和表面形貌，并与纯铜、PANI镀层进行了比较；讨论了镀波pH值和温度等施镀工艺条件对镀层导电性能的影响。结果表明，PANI／Cu复合镀层具有良好的导电率且与基体结合紧密；增大镀液pH值和提高施镀温度，则其导电率增大。     

Catalytic combustion of methane over supported bimetallic Pd catalysts: Effects of Ru or Rh addition

A series of Pd/γ-Al₂O₃ catalysts with various amounts of Ru or Rh with, and/or without, BaO were prepared by successive incipient wetness impregnation. The catalysts were investigated for the catalytic methane combustion before, and after, H₂S poisoning in an oxygen-rich atmosphere. The addition of ruthenium enhanced the catalytic activity for methane oxidation even after H₂S poisoning while maintaining the initial catalytic activity of the fresh catalyst. These results are explained in terms of dispersion of palladium by ruthenium and poisoning resistance of ruthenium. The addition of rhodium did not improve the overall activity in methane oxidation.     

Electrochemical growth and dissolution of Ni on bimetallic Pd/Au(1 1 1) substrates

In this work we study the electrochemical growth and dissolution of a Ni on Pd-Au(1 1 1) bimetallic surfaces using in situ scanning tunnelling microscopy. We also compare Ni deposition on monometallic electrodes, i.e. Au(1 1 1) and Pd(1ML)/Au(1 1 1), using electrochemical characterizations. Results evidence that the first Ni monolayer grows preferentially on Au(1 1 1) in a wide potential range, and that a full Ni monolayer covering the entire Pd-Au surface can be selectively dissolved from Pd islands. No such selectivity is observed upon growth of subsequent Ni atomic planes. We demonstrate that the Ni-substrate interactions play a key role in the above mentioned selectivity. The binding energy of Ni to Pd is found to be 80 meV smaller than of Ni to Au. The sign and the amplitude of this difference are discussed in light of the d band filling of the Pd-Au(1 1 1) bimetallic surface and the presence of adsorbed H on Pd before deposition.     

Formation of Pd nanoparticles in surfactant-mesoporous silica composites and surfactant solutions

Highly dispersed palladium nanoparticles containing mesoporous silicas MCM-41 and MCM-48 were prepared by one-pot synthesis. The method consists of the simultaneous formation of CTA⁺ surfactant templating MCM-41 mesophase and CTA⁺ micelle-capped PdO, which was reduced by hydrogen to Pd metal with particle size ≈ 2 nm and was observed to stay inside the mesochannels of MCM-41 (pore size ≈ 3.8 nm) by TEM, XAS, and PXRD. During hydrothermal synthesis of Pd/MCM-48, Pd nanoparticles of average size ≈ 6–7 nm were deposited on the MCM-48 of pore size = 4 nm. The deposition is probably derived from ethanol reduction of Pd(II) complex generated from PdCl₂ precursor by hydrolysis of TEOS and C₁₂H₂₅(OCH₂CH₂)₄OH surfactant. The formation of Pd(0) from Pd(II) species in solid mesoporous silicas by hydrogen reduction was monitored by in situ XAS, and compared with the formation of Pd(0) from [PdCl₄]²⁻, [PdCl₃(H₂O)], and Pd(OH)₂ by sodium dodecyl sulfate surfactant and alcohol reduction in aqueous solutions.     

Structure and activity of Au-Pd/SiO2 bimetallic catalyst for thiophene hydrodesulfurization

Monometallic Au, Pd and bimetallic Au-Pd catalysts supported on SiO₂ were prepared by an impregnation method. Their activities on thiophene hydrodesulfurization (HDS) at atmospheric pressure are found to be as a function of calcination temperature of these catalysts. The bimetallic catalyst calcined in air at 400 °C gives the highest activity among them. The techniques including nitrogen physical adsorption, X-ray diffraction, transmission electron microscopy, and X-ray absorption near edge structure were employed to characterize the structure of these catalysts. The results indicate that the effect of gold particles in AuPd/SiO₂ catalyst can facilitate the reduction of PdO phase as well as inhibit the formation of less active Pd₄S phase. The promotional effect of partially oxidative gold and a little of Pd⁰ in AuPd/SiO₂ catalyst is suggested to enhance the HDS activity. The formation of Au_xPd_y alloy phase improves the resistance to sulfur-poisoning of the bimetallic catalyst. The presence of partially oxidized gold particles is considered to be due to the inter-atomic charge transfer from the Au 5d to the Pd 5d band.     

Pt/Pd alloy nanoparticles composed of bimetallic nanobowls: Synthesis, characterization and electrocatalytic activities

Bowl-like nanostructures of Pt/Pd bimetallic nanocrystals are prepared by employing Ag nanoparticles as a template and completely removing the residual Ag after the displacement. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations display that the thin walls of Pt/Pd nanobowls are composed of nanoparticles. X-ray diffraction (XRD) pattern demonstrates that Pt and Pd form the alloy in the nanobowls. The crystal structures of the nanobowls consist of [1 1 1], [2 0 0], [2 2 0] surfaces as revealed by both high-resolution TEM (HRTEM) and XRD results. The Pt/Pd nanobowls exhibit significant high electrocatalytic activities toward the methanol oxidation and oxygen reduction compared with the Pt/Pd nanospheres and Pt nanobowls.     

Complete oxidation of methane on Pd/YSZ and Pd/CeO2/YSZ by electrochemical promotion

In this work, methane combustion over Pd/YSZ and Pd/CeO₂/YSZ catalyst was investigated at a temperature range of 470–600 °C. For the first time, the feasibility of electrochemical promotion on palladium films prepared by wet impregnation was reported. The catalytic activity of palladium was found to increase over 160% via transference of oxygen ions from the solid electrolyte to the catalyst film. In addition, palladium supported over ceria and yttria-stabilized zirconia showed the highest activity. As expected, the presence of ceria allowed improving the oxygen storage capacity of the catalyst system.     

Stability of bimetallic Bi–Pd and Pb–Pd carbon-supported catalysts during their use in glyoxal oxidation

The incorporation of Bi or Pb as promoting elements in Pd-based carbon-supported catalysts drastically increases the catalytic activity in the selective oxidation of glyoxal into glyoxylic acid. Because partial dissolution of the promoter was clearly demonstrated by atomic absorption analysis of the reaction medium, experiments are performed to examine the stability of these catalysts. Dissolution tests in the presence of the individual constituents of the reaction medium (glyoxal, glyoxylate, glycolate, oxalate) were carried out in air or nitrogen to identify the factors responsible for Pb or Bi leaching. Pb- or Bi-promoted Pd/C catalysts were prepared by thermal degradation of acetate-type precursors and characterized by X-ray diffraction and X-ray photoelectron spectroscopy before and after their use in glyoxal oxidation. Promoter leaching increases with the reaction time. Monometallic Bi/C and Pb/C catalysts were found to lose smaller amounts of promoting agent than the bimetallic M–Pd/C (M=Bi, Pb) catalysts. Losses are more pronounced from Pb–Pd/C catalysts than from their Bi-based partners. Both glyoxal and glyoxylate seem to be among the main factors responsible for the promoter losses in relation to their complexing properties.     

Characterization and application of electrodeposited Pt, Pt/Pd, and Pd catalyst structures for direct formic acid micro fuel cells

In this work, we study the preparation, structural characterization, and electrocatalytic analysis of robust Pt and Pd-containing catalyst structures for silicon-based formic acid micro fuel cells. The catalyst structures studied were prepared and incorporated into the silicon fuel cells by a post CMOS-compatible process of electrodeposition, as opposed to the more common introduction of nanoparticle-based catalyst by ink painting. Robust, high surface area, catalyst structures consisting of pure Pt, pure Pd, and Pt/Pd = 1:1 were obtained. In addition, Pt/Pd catalyst structures were obtained via spontaneous deposition on the electrodeposited pure Pt structure. The catalyst structures were characterized electrochemically using cyclic voltammetry and chronoamperometry. All Pd-containing catalyst structures facilitate formic acid oxidation at the lower potentials and deliver higher oxidation currents compared to pure Pt catalyst structures. Fuel cells of these catalyst structures show that pure Pd catalyst structures on the anode exhibit the highest peak power density, i.e. as high as 28.0 mW/cm². The MEMS compatible way of catalyst electrodeposition and integration presented here has yielded catalyst structures that are highly active towards formic acid oxidation and are sufficiently robust to be compatible with post-CMOS processing.