Studies of the electrodeposition of palladium from baths based on [Pd(NH3)2X2] salts. I. [Pd(NH3)2Cl2] baths

Spectroscopic techniques are used to confirm the chemistry of solutions of [Pd(NH₃)₂Cl₂] in aqueous NH₄Cl at various pH. In addition, potential sweep and step methods at rotating and stationary disc electrodes (both vitreous carbon and freshly plated palladium) are used to investigate palladium deposition from a standard electroplating bath, [Pd(NH₃)₂Cl₂] in NH₄Cl/NH₃ (pH 8.9). The relative importance of oxygen reduction, hydrogen absorption and hydrogen evolution as competing cathode reactions under various conditions is defined and the advantage of strong convection for high rate plating is demonstrated.     

Selective NOx Reduction During the H2 + NO + O2 Reaction Under Oxygen-Rich Conditions Over Pd/V2O5/Al2O3: Evidence for In Situ Ammonia Generation

Under oxygen-rich conditions in H₂ + NO + O₂ mixtures, Pd/V₂O₅/Al₂O₃ catalysts are active and highly selective (～80%) for NO_x reduction to N₂. In situ DRIFT spectroscopy and reactor data show that the system operates via formation of NH_x species on the highly dispersed V₂O₅ component. Both NH₃ and NH₄ ⁺ are formed, with the latter dominant. The role of the palladium component is also discussed.     

Heterogeneous Palladium Catalysts Applied to the Synthesis of 2‐ and 2,3‐Functionalised Indoles

Heterogeneous palladium catalysts ([Pd(NH₃)₄]²⁺/NaY and [Pd]/SBA‐15) were applied to the synthesis of 2‐functionalised indoles, giving generally high conversions and selectivities (>89% yield) using only 1 mol % [Pd]‐catalyst under standard reaction conditions (polar solvent, 80 °C). For the synthesis of 2,3‐functionalised indoles by cross‐coupling arylation, the [Pd]/SBA‐15 catalyst was found to be particularly interesting, producing the expected compound with =35% yield after 12 days of reaction, which is comparable to the homogeneous catalyst, Pd(OAc)₂ (=48% yield). In the course of the study, the dual reactivity of the indole nucleus was demonstrated: aryl bromides gave clean C C coupling while aryl iodides led to a clean C N coupling.     

Palladium-lanthanum interaction phenomena in Pd-LaCl3/SiO2 and Pd-La2O3/SiO2 catalysts

The effect of La addition and the nature of the precursors on the surface properties of Pd/SiO₂ are studied. Samples containing 0.5 wt% Pd were prepared by incipient wetness impregnation and characterized by H₂ and CO chemisorption, TEM, TPR, EDX, XPS, Ar⁺-sputtering and CO/FTIR. The use of nitrate precursors improves both reducibility and dispersion of Pd. Lanthanum addition makes the metal reduction more difficult, indicating that a Pd-La interaction takes place. When starting from Cl^- precursors, Pdⁿ⁺ species are formed at the surface, whereas only Pd⁰ is found when nitrate precursors are used. The addition of LaCl₃ increases the Pd dispersion and hinders the formation of Pdⁿ⁺ species through a dilution effect. In a sample prepared from Pd(NO₃)₂₊La(NO₃)₃, the La₂O₃ formed upon calcination originates a SMSI-like effect on the palladium. This effect is suggested by the strong reduction of the H₂ and CO chemisorption capacity of Pd in this sample, in spite of the fact that the dispersion of palladium calculated from TEM increases in the presence of La. From XPS results this SMSI state appears to be due to a physical decoration of Pd by La₂O₃ rather than to an electronic Pd-La₂O₃ interaction. The ``decoration model' is further confirmed by the progressive increase of the Pd/La atomic ratio observed upon XPS/Ar⁺-sputtering at different times the sample containing La. The hypotheses of both dilution and decoration effects caused by LaCl₃ and La₂O₃, respectively, are strengthened by the CO/FTIR results. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of residual Cl− derived from metal precursors on catalytic activity in the hydrogenation of naphthalene over supported Pd catalysts

Hydrogenation of naphthalene was performed over supported Pd catalysts prepared from precursors that contained Cl⁻ and from precursors that were free of Cl⁻. The activity of catalyst prepared from PdCl₂ was much higher than that prepared from Pd(NH₃)₂(NO₂)₂. The species of Pd precursor strongly affected the activity of Pd/Al₂O₃, whereas no significant influence was observed in the effect of Pd/SiO₂, because residual Cl⁻ was easily removed by calcination and/or reduction. Catalytic activity was increased by addition of NH₄Cl to Cl⁻-free Pd/Al₂O₃. Residual Cl⁻ was concluded to enhance the catalytic activity of the supported Pd catalysts.     

Highly Active Catalyst from [PdCl2(NH2(CH2)12CH3)2] on NH4ZSM-5

A palladium catalyst highly active for the cyclohexene hydrogenation has been obtained by heterogenisation of [PdCl₂(NH₂(CH₂)₁₂CH₃)₂] on zeolite NH₄ZSM-5. TOF is more than twenty times higher than for the homogeneous catalyst or the activated carbon heterogenised complex. Changes in the electronic state of palladium have been observed by XPS analysis. Palladium reduction is produced upon heterogenisation on the NH₄ZSM-5 zeolite.     

Efficient Reduction of NO x by H2 Under Oxygen-Rich Conditions over Pd/TiO2 Catalysts: An in situ DRIFTS Study

The H₂/NO/O₂ reaction under lean-burn conditions has been studied by means of in situ DRIFTS, reactor measurements and temperature-programmed desorption with the aim of understanding the very different behavior of Pd/TiO₂ and Pd/Al₂O₃ catalysts. The former deliver very high NO _x conversions (70-80%) with good N₂ selectivity whereas the latter show very low activity. In addition, PdTiO₂ exhibits two distinct NO _x reduction pathways, thus greatly extending the useful temperature range. It is shown that the PdTiO₂ low-temperature channel involves adsorption and subsequent dissociation of NO on reduced (Pd⁰) metal sites. The low activity of PdAl₂O₃ is a consequence of palladium remaining in an oxidized state under reaction conditions. The high-temperature NO reduction channel found with PdTiO₂ is associated with the generation and subsequent reaction of NH _x species.     

Electrodeposition of zinc–nickel alloys from ammonia-containing baths

This paper describes the use of ammonia-containing baths for Zn–Ni alloy electrodeposition. Buffering properties of the ammonia/ammonium couple limit the local change in pH in the vicinity of the electrode surface caused by simultaneous hydrogen evolution. In addition, it is shown that the divalent zinc and nickel species exist in the form of Zn(NH₃) ₄ ²⁺ and Ni(NH₃) ₆ ²⁺ complexes over a large pH range. The electrochemistry of the deposition at pH 10 was investigated by galvanostatic experiments and cyclic voltammetry, and compared with deposition from ammonium chloride baths at pH 5. The Ni content in the alloys were found to be 40–60% higher from the ammonia-containing bath than from the acidic baths. Reduction of divalent ions and hydrogen evolution were shown to occur at potentials 250mV more cathodic than with baths at pH 5; the deposition mechanism may be affected by complexation of the metal cations by ammonia.     

Electrodeposition of Co–Pd alloys from ammonia solutions and their catalytic activity for hydrogen evolution reaction

Investigations of the influence of electrolysis parameters such as the concentration of metal ammonia complexes, working electrode potential and temperature on the composition, structure and catalytic activity of synthesized alloys for water molecule reduction reaction in 2 M NaOH (T = 25 °C) were conducted. The alloys were deposited under potentiostatic conditions within potential range from ?0.7 to ?1.1 V in electrolytes of pH 9.5, containing ammonia complexes of cobalt(III) and palladium(II), [Co(NH₃)₆]³⁺ and [Pd(NH₃)₄]²⁺, of different concentration ratio. Structural changes in electrodeposited alloys were discussed based on results of X-ray diffraction measurements. An elemental analysis was performed using the energy-dispersive X-ray spectroscopy technique. Finally, based on results of galvanostatic measurements, the Tafel slope within the range of activation control for hydrogen evolution reaction was determined and mechanism of the process was discussed. The alloys presented low Tafel slope value, from 25.4 to 88.7 mV dec^?1. The alloy of the highest activity for hydrogen evolution reaction contained 31.2 at.% of Pd.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

Plasma Chemical Modification of Polycarbonate Surfaces for Electroless Plating

The preliminary steps of the “electroless” metallization of polycarbonate are investigated by XPS. They consist of the chemisorption of a catalyst (Pd) on the surface to be metallized. The corresponding surface can be activated either by chemical etching or by reactive or non-reactive gas plasma treatment. Therefore, the surface treatment of polycarbonate determines the palladium adsorption. It is shown here that a surface carrying oxygenated functions adsorbs palladium through Sn²⁺ ions which are themselves bonded to oxygen atoms. On the other hand, a surface on which nitrogenated groups have been grafted (by NH₃ or N₂ plasma treatment) chemisorbs palladium directly on these nitrogen atoms. Reaction mechanisms are proposed in both cases and a new and simplified process for making the surfaces catalytic is proposed.     

Surface plasma functionalization of polycarbonate: Application to electroless nickel and copper plating

Electroless metallization of polymers requires different steps including (i) the preconditioning of the surface to render it chemically reactive, (ii) the chemisorption of a catalyst (Pd) to activate the so-modified surface towards subsequent metallization, and (iii) the metallization itself. In the present work, electroless metallization was carried out on poly(carbonate) substrates, and step (i) was performed using plasma treatments in various gaseous atmospheres (NH₃, N₂, O₂). When a sufficiently high surface concentration of nitrogenated groups is grafted it is shown from X-ray photoelectron (XPS) analyses that the so-functionalized surfaces can chemisorb Pd²⁺ ions directly from a simple PdCl₂ solution. Thus, step (i) allows simplification of the conventional electroless processes commonly used up to now. Moreover, XPS analyses indicate chemical reduction of Pd²⁺ ions to Pd(0) during the more or less long initiation time before the start of metallization. This reduction is easily performed in nickel plating baths thanks to the hypophosphite reducer. On the other hand, this reduction is possible with formaldehyde, the reducer of copper plating baths, only if Sn²⁺ ions (brought by the conventional processes using SnCl₂ and PdCl₂ solutions) are present.     

Electrochemical properties of two-component polymers of palladium and pyrrolidine and piperazine derivatives of C60

Redox-active films have been generated via electrochemical reduction in a solution containing palladium(II) acetate and [C₆₀]fullerene, or derivatives of C₆₀. The C₆₀ derivatives include piperazine (piperazine-C₆₀), pyrrolidine (CH₃-pyr-C₆₀), and a pyrrolidine salt, [(CH₃)₂-pyr-C₆₀]⁺ attached to the fullerene unit. In these films, fullerene moieties are covalently bonded to palladium atoms to form a polymeric network. The polymer yields involving the piperazine and pyrrolidine derivatives of C₆₀ are significantly lower than the yield of the C₆₀/Pd film. The CH₃-pyr-C₆₀/Pd and [(CH₃)₂-pyr-C₆₀]⁺/Pd films are electrochemically active in the negative potential region due to the reduction of the fullerene moiety. Reduction of the CH₃-pyr-C₆₀/Pd film is accompanied by the transport of supporting electrolyte cations from the solution into the film. In the first reduction step of the [(CH₃)₂-pyr-C₆₀]⁺/Pd film, both cations and anions of the supporting electrolyte are involved. The piperazine-C₆₀/Pd film exhibits electrochemical activity at both negative and positive potentials. In the negative potential region, reduction of the fullerene cage takes place. Oxidation of the piperazine moiety is responsible for the observed current in the positive potential range. Here, the oxidation process of this polymer is significantly influenced by the presence of metallic palladium particles in the film.     

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO₂) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO₂ exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO₂ catalyst is superior to Pd/ZrO₂, Pd/Al₂O₃, Pd/TiO₂, Pd/ZSM-5 and Pd/SiO₂ catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H₂-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO₂ inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

PdO/Al2O3–(Ce1-X Zr X )O2 catalysts: effect of the sol-gel support composition

The contribution of (Ce_1-X Zr _X )O₂ additives to alumina supports prepared by sol-gel and the catalytic properties of PdO/Al₂O₃–(Ce_1-X Zr _X )O₂ catalysts (~0.3 wt% Pd, ~5 wt% (Ce_1-X Zr _X )O₂) in CO oxidation was herein investigated. The addition of (Ce_1-X Zr _X )O₂ to the support enhanced the surface area and decreased the size of Al₂O₃ particles. The UV–Vis bands of PdO particles and Pd²⁺ ions indicate that zirconia in (Ce_1-X Zr _X )O₂ promotes palladium-support interactions by forming highly dispersed PdO particles. Temperature-programmed reduction (TPR) in hydrogen revealed that ceria enhanced the redox capacity of the supports while zirconia lowered the reduction temperature of palladium oxide species. The comprehensive study revealed that the Ce/Zr ratio was a key factor influencing the catalytic activity of samples in CO oxidation, because palladium oxide-support interactions had a significant effect in changing of the reducibility of samples. So, the PdO/Al₂O₃–(Ce_0.5Zr_0.5)O₂ exhibited the highest catalytic activity.     

Electrodeposition of cobalt-nickel alloys from ammoniacal single-complex baths

The electrodeposition of cobalt-nickel alloys has been studied from ammoniacal single-complex baths containing 5–50 CoSo₄·7H₂O, 5–50 NiCl₂·46H₂O, 20–50 g dm^?3 (NH₄)₂SO₄ and 250–350 cm³ dm^?3 NH₄OH (20.5% solution). The effects of the plating current density and the concentrations of the bath constituents on the cathodic polarisation, current efficiency and composition of the alloy were investigated. A wide range of alloy compositions with a current efficiency of 60–80% could be obtained from the main baths examined. The alloy plating system interchanged between the normal and the anomalous types depending upon the plating current density and the concentration of NH⁺₄ ion in the bath. Electron microscopic and X-ray diffraction studies proved that the structure of the electrodeposited alloy was controlled by the alloy composition.     

INFLUENCE OF THE AMINE NATURE ON THE COMPOSITION OF PALLADIUM COMPLEXES IN SOLVENT EXTRACTION SYSTEMS

ABSTRACT

The extraction of palladium chloro complexes by di-n-octylamine and diamines of various structure as function of the composition of the aqueous and organic phases has been studied. The compositions of the extracted species are indicated and the mechanisms of their distribution are described. It was shown that from 1 to 3 M HC1 solutions. complexes such as (R₂NH₂)₂PdCl₄ solvated by molecules of dioctvlamine chloride are extracted. With increasing the initial concentrations of palladium or decreasing acidity of the aqueous phase, a direct coordination takes place, first of one and then two molecules of amine to atom of palladium with the formation of extracted compounds such as HC1 solutions by salts of amines, diamines and QAB ionic associates such as (AmH)₂PdCl₄, (AmH)₂)PdCl₄ and (R₄N)₂PdCl₄, respectively, are recovered into the organic phase. In systems containing trioctylamine, tetraoctylalkylenediamines (n = 4, 6) and QAB. ionic associates containing the dimeric complex anion, Pd₂Cl₆ ^2-, are also formed. When primary (n-octylaniline - OA) and secondary amines are used as extractants the formation of dimeric species in the organic phase is not observed.

The extraction of palladium from weakly acidic and neutral solutions can proceed through a combination of anion-exchange and coordination mechanisms with the formation of (AmH)[Pd(Am)Cl₃] in systems with primary and secondary amines, and through a coordination mechanism with the formation of complexes such as Pd(Am)₂Cl₂ in systems with primary, secondary and tertiary amines. When palladium is extracted by diamines with a short hydrocarbon chain (n=2) the formation of coordination compounds also takes place.     

Low-temperature catalytic combustion of methane over Pd/CeO2 prepared by deposition–precipitation method

Ceria supported 2 wt% Pd catalysts for low-temperature methane combustion were prepared by the impregnation (IM) and deposition–precipitation (DP) methods, which are denoted as Pd–IM and Pd–DP, respectively. DP was found to be an available method for achieving high activity and stability of the Pd/CeO₂ catalyst. The temperatures for methane ignition (T_10%) and total conversion (T_100%) over Pd–DP are 224 and 300 °C at GHSV of 50,000 h⁻¹, which are 83 and 110 °C lower than the corresponding temperatures of Pd/Al₂O₃. X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses show that palladium species in Pd–DP is highly dispersed, positively charged and difficultly reduced. Raman spectra disclosed that the largest concentration of defects and/or oxygen vacancies was formed in Pd–DP catalyst. A kind of cationic PdO^δ+ sites with higher binding energies than PdO are in close vicinity to the oxygen vacancies in the CeO₂ support and might act as the active centers for methane oxidation. Furthermore, the deactivation and steam aging tests for Pd–DP showed that the performance of this type of palladium was very stable and could be repeatedly recovered after several long time aging tests.     

New dinuclear Pd(II) complex with pyrazolate bridges. Synthesis and crystal structure of [Pd(μ-pz)(pzH)2]2(BF4)2 (pzH=pyrazole)

Dinuclear pyrazolate-bridged Pd(II) complex [Pd(μ-pz)(pzH)₂]₂(BF₄)₂ has been prepared by treatment of pyrazole (pzH), sodium ethoxide, and [Pd(CH₃CN)₄](BF₄)₂. The new complex has been characterised by elemental analyses, IR, ¹H NMR, ¹³C{¹H} NMR, and single crystal X-ray diffraction methods. This structure shows two palladium atoms bridged by two pyrazolate ligands. The square-planar geometry of each Pd atom is completed by two pyrazole ligands. The six-membered dipalladacycle formed by the two Pd atoms and the two bridging pyrazolate ligands adopt a distorted boat-like conformation. This compound is the first example of a dinuclear Pd(II)–pyrazolate homoleptic complex.