Sequential Electroless Deposition Research Articles

Thermally stable membranes based on PdNiAu systems with high nickel content for hydrogen separation

PdNiAu-based composite membranes were obtained through sequential electroless deposition, varying their composition by modifying the Ni deposition time. After annealing at high temperature, a single phase with FCC structure was obtained in all the membranes, showing homogeneous alloy composition. A decrease in the lattice parameter was evident as the Ni content increased in the alloy. The permselective properties of the membranes were evaluated at different temperatures and pressures. The results showed that H2 permeance decreased as the Ni composition increased in the film and increased as the lattice parameter increased. The best performance was obtained with the membrane containing 29 at.% Ni (11.40 × 10−4 mol m−2 s−1 Pa−0.5), even higher than that reported for pure Pd. The membrane with higher Ni content (54 at.%) and lower permeance (0.20 × 10−4 mol m−2 s−1 Pa−0.5 at 723 K) overcame the performance of some Ni-containing membranes previously reported. The membranes showed high stability for several days at high temperatures.

New PdNiAu ternary alloys as potential material for hydrogen separation processes

Novel PdNiAu ternary samples were successfully developed by sequential electroless deposition of the pure metals. The structure, composition, and morphology of the synthesized samples were analyzed by X-ray diffraction, Scanning Electron Microscopy, and energy-dispersive X-ray Spectroscopy. XRD analysis revealed that the metal deposition order (Ni→Pd→Au or Pd→Ni→Au) strongly affects the formation of a single and homogeneous FCC phase. By means of X-ray Photoelectron Spectroscopy, Pd-enrichment to the surface was evidenced in the sample with lower Ni content. The optimized procedure allowed synthesizing the Ni29Pd64Au7_PSS composite membrane, which exhibited a hydrogen permeance slightly higher than that reported for pure Pd.

Development of catalytic membranes over PdAu selective films for hydrogen production through the dry reforming of methane

PdAu composite membranes were applied in the preparation of catalytic membranes for hydrogen production. The APTES surface modification of PdAu membranes synthesized by sequential electroless deposition was carried out. Grafted samples were analyzed by means of XPS, SEM-EDS and DRIFT. XPS data showed that surface saturation was reached when a 0.5 v/v % of APTES was employed. The morphological study of the samples showed that after APTES functionalization a slightly change on the PdAu microstructure occurred. It was observed that the APTES-functionalized samples were stable after heating to 823 K for 2 h. In addition, these membranes retained their hydrogen permeability without detrimental effect over their selectivity. Catalytic Ru and Rh-based membranes were synthesized by the dip-coating method. The membranes were evaluated in the dry reforming of methane at 823 K. Even though only low conversions were obtained, these were slightly higher than those obtained for a fixed-bed reactor with the same amount of catalyst.

Surface composition of PdCuAu ternary alloys: a combined LEIS and XPS study

PdCuAu ternary alloy samples with different composition were synthesized on top of ZrO2‐modified porous stainless steel disks by the sequential electroless deposition technique. The structure, morphology and bulk composition of the samples were characterized by X‐ray diffraction (XRD), scanning electron microscopy and energy dispersive X‐ray spectroscopy (EDX). Complete alloy formation with a pure fcc phase for the Pd71Cu26Au3, Pd70Cu25Au5 and Pd67Cu24Au9 samples and a bcc structure for the Pd62Cu36Au2 and Pd60Cu37Au3 samples were obtained upon annealing at 500 °C for 120 h as revealed by XRD. A combination of low‐energy ion scattering (LEIS) and X‐ray photoelectron spectroscopy (XPS) was used to investigate the surface properties of the PdCuAu alloys. XPS results confirmed alloy formation under the annealing conditions. XPS analysis also revealed that the near‐surface regions of the alloys became enriched in Pd with respect to the bulk composition determined by EDX. In contrast, LEIS and angle‐resolved XPS analyses showed that the top‐most surface layers in all samples were copper‐rich compared with the bulk composition. This high Cu surface concentration could impart resistance to bulk sulfide formation to the PdCuAu alloy membranes. Copyright © 2015 John Wiley & Sons, Ltd.

PdCuAu ternary alloy membranes: Hydrogen permeation properties in the presence of H2S

PdCuAu ternary alloy membranes with different component compositions were synthesized by sequential electroless deposition of components onto porous stainless steel substrates. The ternary with the highest Au content, Pd69Cu14Au17, exhibited the highest hydrogen permeation flux, comparable to that of a Pd91Au9 membrane. Upon exposure to 100ppm H2S/H2 at 673 K for 24h, all PdCuAu membranes experienced flux reductions of ~55%, followed by recovery to ~80% of the initial hydrogen flux upon reintroduction of pure hydrogen at 400°C. Complete flux recovery after H2S exposure required hydrogen treatment at 500°C. X-ray diffraction (XRD) analysis of the H2S-exposed PdCuAu membranes revealed fcc alloy structure with no evidence of bulk sulfide formation. In agreement with the XRD results, sulfur was not detected in the bulk of H2S-exposed samples by energy dispersive spectroscopy (EDS). However, analysis of H2S-exposed PdCuAu alloys by X-ray photoelectron spectroscopy (XPS) depth profiling revealed low, but measureable, amounts of sulfur in the near-surface region, about 10nm in depth. The depth profiles of samples after hydrogen recovery treatment showed significantly reduced sulfur content. These results indicate that H2S exposure causes flux loss in PdCuAu alloys through a surface-poisoning mechanism, and that the surface sulfide can be removed—and flux recovered—by high temperature treatment in hydrogen.

Pd-based binary and ternary alloy membranes: Morphological and perm-selective characterization in the presence of H2S

Pd, Pd90Ag10, Pd91Au9, Pd78Ag9Au13 and Pd75Ag16Au9 alloy membranes were prepared on vacuum-assisted ZrO2-modified porous stainless steel supports by sequential electroless deposition. The membranes were evaluated for permeability in pure H2 and in an H2S/H2 mixture. The membranes displayed a range of permeabilities in pure H2: Pd90Ag10>Pd78Ag9Au13>Pd~Pd75Ag16Au9>Pd91Au9. On exposure to 100ppm H2S/H2 at 673K for 24h, all membranes lost a significant fraction of their H2 permeabilities: Pd (lost the largest fraction, 85%)>Pd90Ag10>Pd75Ag16Au9>Pd78Ag9Au13>Pd91Au9 (60%). When H2S was removed, the membranes recovered the lost permeability to different extents. The Pd91Au9 and Pd78Ag9Au13 membranes displayed the highest fractional recovery of initial H2 permeability (~80%). But, with its higher initial pure H2 permeability, Pd78Ag9Au13 had the highest absolute H2 permeability after recovery. The microstructure, morphology and bulk composition of H2S-exposed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). XRD revealed the presence of a bulk Pd4S phase on both Pd and Pd90Ag10, the membranes that recovered the least H2 relative flux after their exposure to H2S. In contrast, bulk sulfides did not form on Pd91Au9 or on PdAgAu ternary alloys. In agreement with the XRD results, EDS did not detect sulfur in the bulk of Pd91Au9 or in either of the ternary alloy samples. Our results show that the addition of Au to the high-permeability PdAg binary membrane results in a PdAgAu ternary membrane that minimizes the permanent H2 flux loss associated with H2S exposure by preventing the formation of thick stable sulfides.

PdAgAu alloy with high resistance to corrosion by H2S

PdAgAu alloy films were prepared on porous stainless steel supports by sequential electroless deposition. Two specific compositions, Pd83Ag2Au15 and Pd74Ag14Au12, were studied for their sulfur tolerance. The alloys and a reference Pd foil were exposed to 1000H2S/H2 at 623 K for periods of 3 and 30 h. The microstructure, morphology and bulk composition of both non-exposed and H2S-exposed samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). XRD and SEM analysis revealed time-dependent growth of a bulk Pd4S phase on the Pd foil during H2S exposure. In contrast, the PdAgAu ternary alloys displayed the same FCC structure before and after H2S exposure. In agreement with the XRD and SEM results, sulfur was not detected in the bulk of either ternary alloy samples by EDS, even after 30 h of H2S exposure. X-ray photoelectron spectroscopy (XPS) depth profiles were acquired for both PdAgAu alloys after 3 and 30 h of exposure to characterize sulfur contamination near their surfaces. Very low S 2p and S 2s XPS signals were observed at the top-surfaces of the PdAgAu alloys, and those signals disappeared before the etch depth reached ∼10 nm, even for samples exposed to H2S for 30 h. The depth profile analyses also revealed silver and gold segregation to the surface of the alloys; preferential location of Au on the alloys surface may be related to their resistance to bulk sulfide formation. In preliminary tests, a PdAgAu alloy membrane displayed higher initial H2 permeability than a similarly prepared pure Pd sample and, consistent with resistance to bulk sulfide formation, lower permeability loss in H2S than pure Pd.

Alloying and surface composition of model Pd–Ag films synthesized by electroless deposition

PdAg model films and real composite membranes were prepared by sequential electroless deposition on top of porous stainless steel supports. Their surface properties were investigated by X-ray photoelectron spectroscopy (XPS), Angle resolved XPS and sputtering depth profile. It was shown that the surface of the alloy was strongly enriched in silver after the annealing treatment up to 500 °C on hydrogen stream. A relationship between the Ag 3d5/2 core-level binding energy shift and the silver surface composition was observed from the XPS data obtained with the model samples. The surface composition of real membranes after both hydrogen permeation and reaction experiments showed silver enrichment, in agreement with the data obtained from the model sample.

Evaluation of the catalytic activity of Pd–Ag alloys on ethanol oxidation and oxygen reduction reactions in alkaline medium

Pd–Ag alloys containing different amounts of Ag (8, 21 and 34at.%) were prepared in order to evaluate their catalytic activity towards the ethanol oxidation (EOR) and oxygen reduction (ORR) reactions. A sequential electroless deposition of Ag and Pd on a stainless steel disc, followed by annealing at 650°C under Ar stream, was used as the alloy electrode deposition process.From half-cell measurements in a 1.0M NaOH electrolyte at ≅20°C, it was found that alloying Pd with Ag leads to an increases of the ORR and EOR kinetics, relative to Pd. Among the alloys under study, the 21at.% Ag content alloy presents the highest catalytic activity for the EOR and the lowest Ag content alloy (8at.% Ag) shows the highest ORR activity. Moreover, it was found that the selectivity of Pd–Ag alloys towards ORR is sustained when ethanol is present in the electrolyte.

Multi‐metal shell coated with surface modification of polystyrene bead cores to improve its adhesion metal shell

AbstractWe investigated the delamination problem at the metal‐polymer interface and the mechanical buckling of the metal layer at a localized area of the metallic shell under compression between two parallel plates. First, polystyrene (PS) beads were synthesized by dispersion polymerization and then their sulfonation process. After sequential electroless deposition, the average size of multi‐metal coated sulfonated polystyrene (SPS) bead was ca 4.95 µm. Using the electromechanical indentation, the electrical resistance of a single metal‐coated SPS bead decreased with increasing compressive strain without delamination at the metal‐polymer interface, and its electrical resistance showed 5.65 Ω. Copyright © 2009 John Wiley & Sons, Ltd.

Tunable surface metal morphologies and electrical properties of monodispersed polystyrene beads coated with metal multilayers via electroless deposition

Conducting core–shell beads are composed of a polystyrene (PS) core and a metallic shell of nickel/gold, regardless of their method of preparation. This study examined conducting core–shell beads to identify the metal–polymer interface and the electromechanical response under large deformation. Firstly, monodispersed PS beads, 4.4 μm in diameter, were synthesized by dispersion polymerization. After sequential electroless deposition, the nickel content decreased after gold plating because the nickel deposit oxidized and dissolved into solution while the gold in solution was reduced to the metal on the nickel surface. When the beads were prepared using three different methods, the core–shell beads had a raspberry-like morphology, a smooth surface and an onion-like structure. The electromechanical indentation test of a single conducting core–shell bead (route 1, route 2 and route 3) showed an electrical resistance of 5.0 Ω, 5.1 Ω and 5.8 Ω, respectively. Without a sulfonation process, a noisy response of the electrical signals was observed over the entire range of stain due to the delamination of the metal–polymer interface and mechanical buckling of the metallic layer.

Fabrication of Nickel/Gold Multilayered Shells on Polystyrene Bead Cores by Sequential Electroless Deposition Processes

Core-shell conductive beads composed of a polystyrene (PS) core and a metallic shell of nickel-gold were investigated to identify the metal-polymer interface and the electromechanical response under a large deformation. Using dispersion polymerization, the average PS bead diameter was controlled to between 1.5 μm and 4.1 μm by adjusting the water and initiator concentrations, to give a monodispersed particle size distribution. In the electroless deposition processes of Ni followed by gold, the Ni-P shell was initially formed with a thickness of 43 nm, part of which was replaced by gold to give Ni-P and Au shell layers with thicknesses of 39 nm and 43 nm, respectively, as confirmed by transmission electron microscopy (TEM) observation. The electromechanical indentation test of a single core-shell bead showed that its resistance was 9.8 Ω/bead at a compressive deformation of 35%.

Deposition of palladium nanoparticles on the pore walls of anodic alumina using sequential electroless deposition

Palladium nanoparticles were deposited using a sequential electroless deposition technique on the pore walls of nanoporous anodic alumina. For the particle deposition a Pd(NH3)42+ solution was soaked in the alumina membrane and a heated air flow was applied in order to reduce the palladium complex to palladium metal nanoparticles. By repeating the deposition process the size of the nanoparticles could be tailored in this investigation between 6 and 11nm. The size of the nanoparticles was also affected by the concentration of the Pd(NH3)42+ solution, i.e., higher concentration yielded larger particle mean diameters. The samples were investigated using high resolution scanning electron microscopy, x-ray diffraction (XRD), inductively coupled plasma with a mass spectrometer, high resolution transmission electron microscopy, and energy dispersive spectroscopy (EDS). Analysis revealed narrow size distributions of the particles as well as uniform particle coverage of the pore walls. No by-products were observed with EDS, and with the XRD analysis the metallic palladium crystallinity was confirmed.

Characterizations of Pd–Ag membrane prepared by sequential electroless deposition

Palladium alloy membranes were prepared by sequential electroless plating steps on porous stainless steel (PSS). Full characterizations of Pd–Ag/PSS composite membranes were performed using scanning electron microscopy (SEM), energy dispersive spectroscopic analysis (EDS), atomic force microscopy (AFM) and X-ray diffractometry (XRD). AFM images show that in the electroless plating process lower skin layer roughness and lower deposition rate were related. Although electroless plating was conducted in a sequential process, the deposit over and inside of porous substrate was homogenous as analyzed by EDS analysis. Therefore, the deposited Pd–Ag alloy membrane retained the same composition over and through the porous substrate. The porous structure of composite membranes was also characterized by the gas permeability technique. From the experimental results, it can be concluded that a combined mechanism of Knudsen diffusion and viscous flow was conducted in the porous membrane.