Palladium catalysts deposited on silicon nitride in the deep oxidation of methane

The physicochemical and catalytic properties of palladium catalysts were studied in the deep oxidation of methane. The catalysts were deposited on silicon nitride from aqueous (Pd/Si₃N₄-a) and toluene (Pd/Si₃N₄-t) solutions of palladium acetate. The use of aqueous and organic solutions of palladium acetate, all other preparation conditions being equal, resulted in the formation of palladium systems with different catalytic properties. The sample from Pd/Si₃N₄-t was characterized by high activity and stability. The systems studied had different structures and adsorption properties of palladium nanoparticles, which influenced the form of reagent adsorption, catalytic properties, and mechanism of surface reactions. The suggestion was made that the solvent played a key role in the formation of the active surface of Pd-containing catalytic systems.     

Palladium(II) extraction by sulfur-containing calix[4,6]arenes from hydrochloric acid solutions

The comparison of the extraction properties of calixarenes, thiacalixarenes, and calix[4,6]arene thioethers showed that methyl(thiamethyl)calix[4,6]arenes 3a and 4a have the highest extraction abilities. These extractants rapidly and completely extract palladium from hydrochloric acid solutions; regarding distribution factors achieved in the kinetic mode, they three to four orders of magnitude exceed their monodentate analogue, octylbenzyl sulfide (OBnS). Approaches are considered to enhance palladium extraction via generating mixed palladium species in low-acidity solutions and via intramolecular catalysis by the protonated oxygen atoms of alkoxy groups in the lower rim. For 1 M HCl, the kinetic order of diluent effects on palladium extraction was established. The substitution of sulfur atoms for bridging CH₂ groups was discovered to enhance palladium extraction by calix[4]arene thioether 3c.     

Palladium catalysts on porous nickel substrates for alcohol fuel cells

Parameters characterizing the active surface of catalytic palladium layers formed from mixed glycinate-chloride and ammonia complexes of palladium(II) were determined. Cyclic voltammetry on a rotating disc electrode was used to study the catalytic activity of nickel substrates and palladium layers in the reaction of methanol and ethanol oxidation in an alkaline medium. It was shown that electrodes with palladium deposited from mixed glycinate-chloride solutions have a higher catalytic activity that those formed from ammonia palladium(II) complexes.     

Spectrophotometric determination of palladium after extraction of the chloro-stannous complex by tri-n-octylamine

The complex formed between palladium(II) and tin(II) in hydrochloric acid solutions, and its extractability by tri-n-octylamine were investigated. Two different species were obtained: one a brown-red complex and the other yellow. Both were extracted into tri-n-octylamine in benzene, but the yellow species which had a definite absorption maximum at 410 mμ was more suitable for the determination of palladium. A method is described for the spectrophotometric determination of palladium in the presence of many other elements. Both species are considered to be anionic and a mechanism for their extraction is proposed.     

Properties of nitric acid palladium solutions with a high metal concentration

Nitric acid solutions (c _Pd up to 3.2 M) with variable HNO₃ concentration were studied by electronic spectroscopy, ¹⁴N and ¹⁷O NMR, acid-base titration, gravimetry, and other methods. Solid phases that precipitated on storing these solutions were studied by X-ray phase analysis, thermogravimetry, and IR spectroscopy. The conditions for stability of particular Pd(II) species were determined, and specific features of aging of such highly concentrated palladium solutions were revealed. A procedure for palladium isolation as [Pd(NH₃)₄](NO₃)₂ from nitric acid solutions in 98% yield was developed.     

Preparation and application of SBA-15-supported palladium catalyst for Heck reaction in supercritical carbon dioxide

A new method for the preparation of SBA-15-supported palladium catalyst for Heck reaction in supercritical carbon dioxide was presented.The newly formed SBA-15-supported palladium catalyst（Ph-SBA-15-PPh₃-Pd） exhibited high catalytic activity for the Heck reaction of 4-nitrobromobenzene with methyl acrylate.The catalyst can be reused several times without a loss of activity.     

Complete Oxidation of Methane Alumina Modified with Calcium, over Palladium Supported on Lanthanum, and Cerium Ions

The activity and thermal stability of Pd/Al2O3 and Pd/（Al2O3＋MOx） （M=Ca, La, Ce） palladium catalysts in the reaction of complete oxidation of methane are presented in this study. The catalyst supports were prepared by sol-gel method and they were dried either conventionally or with supercritical carbon dioxide. Then they were impregnated with palladium nitrate solution. The catalysts with unmodified alumina had a high surface area. The activity and thermal stability of the aluminasupported catalyst was also very high. The introduction of calcium, lanthanum, or cerium oxide into alumina support caused a decrease of the surface area in the way dependent on the support precursor drying method. These modifiers decreased the activity of palladium catalysts, and they required higher temperatures for the complete oxidation of methane than unmodified Pd/Al2O3. The improvement of the palladium activity by lanthanum and cerium support modifier was observed only at low temperatures of the reaction.     

Precipitation of silver/palladium alloy platelets from homogeneous solutions

Dispersed silver/palladium (Ag/Pd) nanoplatelets were prepared by delivering in parallel solutions of mixed metal nitrates and L-ascorbic acid into a nitric acid solution containing Arabic gum. The shape and size of bimetallic nanoparticles varied with the silver/palladium weight ratio and the concentration of nitric acid. The optimum conditions for platelets formation were a palladium content of ~2.0 wt.% and nitric acid concentrations above 1.0 mol dm(-3). The data presented show that both parameters play a critical role in the nucleation and growth of AgPd particles. A mechanism explaining the formation of the bimetallic nanoplatelets is proposed.     

Polypyrrole-palladium systems prepared in PdCl2 aqueous solutions

In the work, reactions of a partially deprotonated polypyrrole doped with hydroxide ions (PPyOH) in various PdCl₂ aqueous solutions which differed in acidity were studied. Using X-ray photoelectron spectroscopy, X-ray diffraction and scanning electron microscopy it was established that in the PdCl₂ solutions of lower acidity PPyOH was oxidatively doped and Pd⁰ and Pd²⁺ were incorporated into the polymer matrix. Pd²⁺ formed palladium(II) hydroxy-and/or aquochlorocomplex dopant anions and/or was coordinated by nitrogen atoms of the polymer (Pd-N bond). Additionally, deprotonation of PPyOH occurred in the PdCl₂ solutions of lower acidity. It was proposed that deprotonation of PPyOH was caused by nucleophilic attack of [PdCl₃(H₂O)]⁻ on the positively charged, doped polymer chain. By comparison of the PPyOH and chloride-doped polypyrrole (PPyCl)-palladium systems prepared in similar PdCl₂ solutions of lower acidity it was shown that the type of the counterion in the starting polymer has a decisive effect on the deprotonation process.PPyOH was less reactive towards palladium species in the PdCl₂ solutions of higher acidity where [PdCl₄]²⁻ was the dominant complex. PPy-palladium systems containing exclusively Pd²⁺ were obtained in this case. It was proposed that incorporation of palladium species in these conditions proceeded via an acid-base reaction or coordination of palladium ions by the polymer chain (Pd-N bond formation).Results of the studies may serve as the basis for the preparation of a variety of polypyrrole-supported palladium catalysts.     

Extraction of palladium from nitrate solution by triphenylphosphine or triphenylphospine oxide in chloroform

It is found that from aqueous nitric acid media, Pd is highly extracted by chloroform solutions containing triphenylphosphine (Ph₃P) and not extracted by triphenylphosphine oxide (Ph₃PO) diluted with chloroform. The effect of diluent, nitric acid molarities, Ph₃P concentration and temperature on palladium extraction by Ph₃P was investigated. Slope analysis of the experimental results indicated that the main extracted species is in the form of Pd(NO₃)₂·2Ph₃P.IR spectrum analysis of the extracted complex indicated the direct extraction of palladium as nitrate and bonding to phosphorus. Extraction of some fission product radionuclides from 2M HNO₃ showed the selective extraction of palladium by Ph₃P in chloroform.     

Carbon-supported palladium catalysts for fuel cells

Small controlled amounts of palladium were electrochemically deposited onto various carbon supports from solutions of glycinate-chloride complexes of palladium(II) in order to obtain palladium catalysts suitable for use in fuel cells. The catalytic activity of the resulting catalytic layers was studied in reactions of reduction of atmospheric oxygen and oxidation of methanol and ethanol in acid and alkaline media by measuring cyclic voltammetric curves on a rotating disk electrode.     

Complete Oxidation of Methane over Palladium Supported on Alumina Modified with Calcium,Lanthanum,and Cerium Ions

The activity and thermal stability of Pd/Al_2O_3 and Pd/(Al_2O_3 MO_x)(M=Ca,La,Ce) palladium catalysts in the reaction of complete oxidation of methane are presented in this study.The catalyst supports were prepared by sol-gel method and they were dried either conventionally or with supercritical carbon dioxide.Then they were impregnated with palladium nitrate solution.The catalysts with unmodified alumina had a high surface area.The activity and thermal stability of the alumina- supported catalyst was also very high.The introduction of calcium,lanthanum,or cerium oxide into alumina support caused a decrease of the surface area in the way dependent on the support precursor drying method.These modifiers decreased the activity of palladium catalysts,and they required higher temperatures for the complete oxidation of methane than unmodified Pd/Al_2O_3.The improvement of the palladium activity by lanthanum and cerium support modifier was observed only at low temperatures of the reaction.     

Extraction of palladium(II) from hydrochloric acid solutions with triacylated ethyleneamines

Extraction of palladium(II) from hydrochloric acid solutions with novel efficient extractants, triacylated ethyleneamines, was studied. The most effective extraction of palladium(II) was observed from 0.5–1 M HCl solutions. Extraction of palladium(II) from 1 M HCl solutions was found to occur through mixed (coordination and anion-exchange) mechanism. In the field of dominance of the anion-exchange mechanism of the extraction of palladium(II) with triacylated pentaethylenehexamine the concentration constant of palladium(II) extraction was calculated, and thermodynamic parameters of extraction were evaluated.     

Structural studies on some antimonate exchangers

The possibility of simultaneous extraction of palladium and technetium from nitrate solutions was investigated using tri-n-octylamine (TOA) solutions in carbon tetrachloride as a heavy, non-flammable diluent. Conditions of technetium extraction being essentially known, the main attention was focussed on the extraction of palladium which was studied in dependence on the concentration of nitric acid, salts (nitrates, chlorides, nitrites), urea and palladium. A strong decrease of Pd extraction with 10% TOA in CCl₄ has been found above a palladium concentration of 10⁻⁴M but in the presence of chlorides and nitrites a satisfactory high distribution can be preserved. Both Tc and Pd extracted with TOA/CCl₄ can be stripped into dilute ammoniacal solutions. An extraction procedure for the simultaneous isolation of Pd (80% yield) and Tc (99%) from fission product waste solutions (0.20 g Tc and 0.17 g Pd/dm³ 0.5–1.0M HNO₃) is proposed.     

硫物种对负载型钯催化剂上氢吸附性质的影响

用X光光电子能谱(XPS)分析了负载型Pd/Al_2O_3催化剂上硫和钯的状态。以程序升温脱附-质谱(TPD-MS)方法研究了不同硫物种及不同气氛对催化剂氢吸附性质的影响。结果表明: 硫物种对氢吸附的影响按S_2~(2-)、S~(2-)、SO_4~(2-)的顺序减小, 且前两者的影响远比SO_4~(2-)的大。这与催化剂在H_2-O_2反应中的抗硫性评价结果一致。并用原子分子轨道理论对上述结果进行了讨论。     

Palladium extraction by triphenylphosphine sulfide in benzene

The extraction of Pd(II) by triphenylphosphine sulfide (Ph₃PS) in benzene from nitric acid solutions has been investigated. The effect of different parameters on the extraction of palladium was studied. Based on the results, an equilibrium model is proposed. The stoichiometry of the extracted species was elucidated and found to be Pd(NO₃)₂2Ph₃PS. Possible interference with selected radionuclides that might exist in high level liquid waste was investigated and discussed.     

Extraction of Pd(II) by some petrochemical products from nitrate media

Liquid-liquid extraction of divalent palladium by solutions of Diesel oil (D.O.) and gas oil II (G.O.II) in benzene and lacquer petroleum from nitrate media has been studied. Palladium in concentrations of ∼10⁻³M is very effectively extracted by dilute solutions of extraction reagents and the distribution of palladium is almost independent of the acidity and nitrate concentration of the aqueous phase. Of other common salts, chlorides, thioeyanates and nitrites affect the palladium distribution. In many cases high concentrations of salts completely suppress the extraction of palladium. The rate of palladium extraction by dilute solutions of extractants is relatively small. Some substances such as dimethyl sulfoxide (DMSO) were found to accelerate the extraction. Palladium extraction from nitric acid media also has been studied from the points of view of chemical and radiation stability. Diesel oil was found to be a more stable extraction reagent in acid media than gas oil II.     

Palladium(II)-catalysed H/D allylic exchange in alkenes: An intermediacy of palladium(IV)?

Evidence is presented that the dimeric π-allylic species [(η³-allyl)PdCl]₂ is not intermediate in the Li₂Pd₂Cl₆-catalysed allylic H/D exchange in alkenes. Neither H/D exchange in α-methylstyrene, nor enrichment of [(η³-2-PhC₃H₄)PdCl]₂, was observed when the latter complex was incubated at 100°C in D₃CCOOD either in the presence or in the absence of PhC(CH₃)?CH₂, respectively. The kinetics of H/D exchange in α-methylstyrene catalysed by Li₂Pd₂Cl₆ were studied in some detail. The exchange proceeds at highest rates when reduction of palladium(II) takes place and is much slower in the presence of 1,4-benzoquinone as a palladium reoxidant. The exchange rate is directly proportional to the alkene and catalyst concentrations and independent of the reoxidant concentration. It is suggested that the palladium(II)-catalysed exchange involves an intermediate hydrid-allyl species where palladium has a formal oxidation state of IV.     

The separation of palladium and platinum by ion flotation

Differences in the ion flotation properties of palladium(II) and platinum(IV) chloro complexes in aqueous solutions are used to achieve separations of these metals. The anionic chloro complex PtCl^2-₆ is floated selectively with cationic surfactants of the type, RNR'₃Br, from solutions of PdCl^2-₄ and various concentrations of hydrochloric acid. The palladium(II) does not float from solutions of ? 3.0 M HCl and the platinum(IV) floated from these solutions can be recovered free of palladium. However, the separation is incomplete as much of the platinum(IV) is also unfloated from these solutions. Quantitative separations are obtained by conversion of the palladium(II) to the cationic ammine, Pd(NH₃)₄²⁺ with aqueous ammonia prior to flotation. The anionic chloro complex of platinum(IV) is unaffected by the presence of ammonia and is floated quantitatively with the surfactant n-hexadecyltri-n-propylammonium bromide from 0.01 M ammonia solutions.     

Extraction of gold, palladium and platinum from chloride, bromide and iodide solution with di-n-octyl sulphide (DOS) in cyclohexane

The extraction of gold, palladium and platinum from hydrochloric acid, hydrobromic acid and iodide media by solutions of di-n-octyl sulphide in cyclohexane was examined. From distribution data it was concluded that the monosolvates AuX(3).DOS and disolvates PdX(2).2DOS are extracted. Extraction of platinum was efficient only from iodide solutions; a disolvate PtI(4).2DOS was formed. The possibility of separation of gold and palladium from platinum by extraction from bromide or chloride solutions and simultaneous extraction of palladium and platinum from an iodide medium was demonstrated.