States of Adsorbed Hydrogen and Their Effect on the Reaction of CO Oxidation on Pd and Ta

Various states of hydrogen are identified on the foil and film surfaces of palladium and tantalum by photoelectric, conductivity, and thermal desorption methods. They are formed in the course of ₂ diffusion through a membrane and in the course of adsorption from the gas phase. The effect of an ethylene pyrolysis product, pyrocarbon, on the activity in CO oxidation on the palladium surface with and without _ads is determined. The presence of hydrogen is found to weaken the effect of pyrocarbon. A study of hydrogen adsorption on the tantalum foil showed that hydrogen adsorption drastically declines in the presence of chemisorbed CO, but the H–Ta binding strength doubles. The fact that the sorption ability of tantalum is completely restored upon CO adsorption and partially restored upon ₂ chemisorption is achieved by thermochemical treatment in hydrogen.     

Dual mechanism of hydrogen desorption from palladium alloys postulated on the basis of cyclic voltammetric studies

The process of hydrogen absorption/desorption in Pd-Pt and Pd-Pt-Rh alloys has been investigated using cyclic voltammetry. Hydrogen absorbed at constant potential was electrooxidized at various scan rates. The charge of hydrogen oxidation has been found to be dependent on the scan rate. The decrease in the oxidation charge observed for low scan rates indicates that, under these conditions, some amount of hydrogen may be removed via a non-electrochemical recombination reaction. The results suggest that the dual mechanism of hydrogen desorption, involving electrochemical oxidation and non-electrochemical recombination, confirmed for pure palladium, is valid also for palladium alloys.Contribution to the 3rd Baltic Conference on Electrochemistry, GDASK-SOBIESZEWO, 23–26 APRIL 2003.Dedicated to the memory of Harry B. Mark, Jr. (February 28, 1934–March 3rd, 2003)     

Acyclic tetrachalcogenoether ligands tethered with bulky substituents: Their syntheses and coordination chemistry

New o-phenylene-bridged tetrachalcogenoether ligands tethered with an extremely bulky substituent, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) group, [TbtE(o-phenylene)Se]₂(o-phenylene) (3: E = S, 4: E = Se), were synthesized. Complexation reactions of 3 and 4 with Na₂PdCl₄ gave the corresponding dichloropalladium(II) complexes 7 and 8. X-ray structural analysis of 7 and 8 indicated that the central palladium metals are in a distorted octahedral environment, where the two inner selenium atoms and the two chlorine atoms form a square planar arrangement around the palladium metal and the two terminal chalcogen atoms weakly coordinate to the palladium center at the axial positions. The AIM calculations also supported the existence of the interaction between the palladium and the terminal chalcogen atoms in the crystalline state. On the other hand, the ⁷⁷Se NMR spectra suggested that there are no or very weak interactions between the palladium and the terminal chalcogen atoms in solution. The UV/Vis spectra of 7, 8, and related compounds imply the possibility of weak interaction between the terminal selenium atoms and the palladium center in solution.     

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.     

Kinetics study of two-channel hydrogen and deuterium atom reactions with interhalogen molecules

Rate constants and the ratio of rates of two available reaction channels (branching ratios) for the reactions of hydrogen and deuterium atoms with FCl, ICl, and BrF molecules were measured using a fast-flow reactor with RF discharge as source of atoms and with superheterodyne ESR spectrometer as detector. For the reaction with FCl a substantial difference was found in branching ratios when substituting hydrogen atoms with deuterium ones: _D+FCl/_H+FCl = 3.3±0.2. The results are compared with the known experimental data and theoretical calculations; in particular, the possible influence of light atom (H or D) migration in collision complex on reaction mechanism is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1733–1740, October, 1994.     

Pyrido[2,3-d]pyrimidines 7. Reactions of 1,3-dimethyl-5,7-dichloro-6-nitropyrido-[2,3-d]pyrimidine-2,4-dione with amines. Synthesis of derivatives of triazolo(4′,5′:4,5)pyrido[2,3-d]pyrimidine

The reaction of 5,7-dichloro-6-nitropyrido[2,3-d]-pyrimidine-2,4-dione with amines gave products of the substitution of one or two chlorine atoms by ammo groups. The catalytic reduction of 6-nitro-5,7-disubstituted pyrido[2,3-d]pyrimidines with hydrogen over palladium oxide on carbon leads to 5,6-diamino- and 5,6,7-triaminopyridopyrimidines, the reaction of which with amyl nitrite in acetic acid gives triazolo(4,5:4,5) pyrido[2,3-d]pyrimidines.For communication 6 see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 398–401, March, 1993.     

Extraction of palladium from nitric acid medium and its spectrophotometric determination using arsenazo III

Summary Di-octyl sulphoxide in xylene was successfully employed for extracting palladium from 0.5 to 1.5 mol/l nitric acid medium. The extracted species was found to be Pd(NO₃)₂·2DOSO. The palladium was back extracted into a mixture of 2 mol/l sodium carbonate and 0.05 mol/l ammonia. The recovery was found to be quantitative. A spectrophotometric method using Arsenazo III was developed in nitric acid medium for the determination of palladium. The colour development was found to be maximum in the acid range of 2.5 to 5 mol/l. Beer's law was found to be obeyed in 1 to 100 g range of palladium. The molar extinction coefficient was found to be 2.26×10⁴ l/mol/cm. The RSD obtained at 16 g of palladium was 5%.     

Indirect complexometric determination of palladium(II) using sodium nitrite as a selective masking reagent

A selective complexometric method is described for the determination of palladium, sodium nitrite being used as masking reagent. Palladium(II) in a given sample solution is initially cornplexed with an excess of EDTA and the surplus EDTA is titrated with zinc sulfate solution at pH 4.5–5.5 (acetic acid-sodium acetate buffer), using xylenol orange as indicator. An excess of sodium nitrite is then added, the mixture is shaken well and the EDTA released from the Pd-EDTA complex is titrated with a standard zinc sulfate solution. Results are obtained for 2.5–27.5 mg of Pd with relative errors 0.5% and standard deviations 0.05 mg. The interferences of various ions are studied. The method is applied for the determination of palladium(II) in alloys and complexes.     

A new cataluminescence sensor for carbon tetrachloride using its catalytic reduction by hydrogen on palladium/carbon surface

A new cataluminescence (CTL) sensor was developed based on the chemiluminescence (CL) emission from the catalytic hydrodechlorination of carbon tetrachloride on the surface of palladium/carbon catalyst. The factors influencing the CTL signal, such as the catalyst, carrier gas, gas flow rate, temperature and the CL wavelength, were investigated in detail. Under the optimal conditions, the linear range of the CTL intensity versus concentration of carbon tetrachloride was 4.7–235 μg/mL (R = 0.9944, n = 7), with a limit of detection of 0.7 μg/mL (σ = 3). GC/MS results suggest that the possible CTL mechanism of the reduction is the formation of CCl₃ radicals. The CCl₃ radicals combine with H free atoms or capture hydrogen atoms from H₂ molecules to form excited CHCl₃ intermediates, which decay from the excited-state to the ground giving CTL emission for the detection. It is also found that some benzene derivatives with α-H of branched-chain, such as toluene, ethylbenzene and xylenecan, can play a role of catalyst in the reaction.     

Preparation and catalytic properties of NR-stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt₃H^?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR₄X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)₄Br to Pd(ac)₂ in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)₄]₂PdCl₂Br₂ with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10³ catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10³ catalytic cycles.     

Self-Consistent-Field potential-energy surfaces for hydrogen atom pairs within small palladium clusters

An ab initio electronic structure study is presented of hydrogen–hydrogen interactions in an electronic environment perturbed by the presence of palladium atom clusters. In particular, we investigated changes in the interatomic potential when the atomic centers are trapped inside an fcc palladium octahedral hole and when they are separated from each other by a (111) plane of palladium atoms. The (111) plane was modeled with a cluster of three palladium atoms. Self-consistent field (SCF ) level calculations were performed, and palladium atom pseudopotentials were employed to make the systems studied computationally tractable. For pairs of atoms placed within the octahedral hole, various lines of approach were considered [along the (100), (110), and (111) directions]. Lattice deformations and electronic excitations were examined for their effect on the interatomic potential.     

Synthesis,spectral characterization,X-ray crystal structure,electrochemical studies,and DNA interactions of a Schiff base pro-ligand and its homobimetallic complexes containing the cysteamine moiety

A Schiff base, N,N′-(3,4-dithiahexane-1,6-diyl)bis(5-methylsalicylideneimine), was synthesized and characterized by X-ray crystallography. Dimeric complexes of nickel(II), palladium(II), and vanadium(IV) were synthesized by the reactions of the Schiff base with nickel(II) acetate, palladium(II) acetate, and vanadyl acetylacetonate in 1:1 molar ratio. In all three complexes, the thiol group was deprotonated and coordinated to the metal. The X-ray structure of the Schiff base showed that in the crystalline form, the SH groups were oxidized to the corresponding disulfide. In the dimeric complexes, coordination took place through the azomethine nitrogen, enolic oxygen, and sulfur atoms. The metal-to-ligand ratio was 1:1, and molar conductance data revealed that the metal complexes were nonelectrolytes. The free Schiff base and its complexes showed photoluminescence in methanol at room temperature. The redox behavior of the compounds was studied by cyclic voltammetry in DMF, which showed both quasi-reversible and irreversible processes. The interaction of the complexes with DNA was investigated by electronic absorption spectroscopy.     

Intensification of the Nitrate Anion Reduction on a Membrane Palladium Electrode

Polarization curves of electroreduction of nitrate anions in sulfuric acid aqueous solutions on a Pd/Pd palladium electrode with the opposite side kept at different constant potentials in the region of Pd–H -phase formation are compared. The reduction rate of nitrate anions is shown to be substantially higher on a membrane electrode, provided the conditions of hydrogen supply from the membrane's opposite side to the reaction zone are realized. This phenomenon is caused by the reduction of a chemisorbed intermediate (a certain N(III) form) by hydrogen diffused through the membrane. It is shown that the measurements of hydrogen diffusion currents through a membrane can be used in plotting hydrogen sorption isotherms in the Pd–H -phase.     

An infrared study of species from ethylene,hydrogen and carbon monoxide adsorbed on palladium/silica catalysts

Two different procedures, A and B, for preparing palladium on silica catalysts give different infrared spectra, and to a large measure different surface species, from the adsorption of ethylene, hydrogen or carbon monoxide. A π-species (I) from ethylene is found on both catalyst preparations. On preparation A the other dominant species is probably the σ-diadsorbed species (II). On preparation B additional strong bands are tentatively interpreted in terms of a doubly π - bonded non-associatively adsorbed species (III). Only preparation B gives absorptions in the accessible region above 1300 cm^?1 which can be attributed to adsorbed hydrogen.

     

Crystal structure of tetra(μ-acetato)-bis{[1-ethyl-3-(pyridine-2-yl)carbamide]copper}

The crystal structure of tetra(μ-acetato)-bis{[1-ethyl-3-(pyridine-2-yl)carbamide]copper} Cu₂(L)₂(CH₃COO)₄ (I), where L is 1-ethyl-3-(pyridine-2-yl)carbamide, is determined. The asymmetric unit cell of the crystal structure of I contains a copper complex with two acetate ions and a monodentate molecule of 1-ethyl-3-(pyridine-2-yl)carbamide, which is coordinated via the pyridine nitrogen atom. Due to the symmetry center, binuclear complexes form in the crystal, in which the acetate ions act as bridges between the metal atoms. In them, the coordination polyhedron of the central copper atoms represents an almost ideal tetragonal pyramid. Its base is formed from the oxygen atoms of acetate ions. In the crystal of the binuclear complex, hydrogen bonds form between the acetate ions and the L ligand along with an intramolecular hydrogen bond, which stabilize the conformation of the organic L molecule. Between the neighboring complexes in the crystal, the van der Waals interaction occurs.     

Theoretical study of the semihydrogenation of alkynes catalyzed by Pd(0) complexes: Is a zwitterionic pathway possible?

Density functional theory B3LYP calculations have been carried out for the Pd(diimine)(C₂H₂)+H₂Pd(diimine)(C₂H₄) reaction, which is the key reaction in the semihydrogenation of alkynes homogeneously catalyzed by (diimine)palladium(0) complexes. The results show that a H₂ heterolytic addition across one Pd–N bond opens a feasible zwitterionic pathway for the catalytic process, and accounts for the pairwise transfer of the two hydrogen atoms inferred from parahydrogen induced polarization NMR experiments.Proceedings of the 11th International Congress of Quantum Chemistry satellite meeting in honor of Jean-Louis Rivail     

Evaluation of Lighting Systems,Carbon Sources,and Bacteria Cultures on Photofermentative Hydrogen Production

Fluorescent and incandescent lighting systems were applied for batch photofermentative hydrogen production by four purple non-sulfur photosynthetic bacteria (PNSB). The hydrogen production efficiency of Rhodopseudomonas palustris, Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodospirillum rubrum was evaluated using different carbon sources (acetate, butyrate, lactate, and malate). Incandescent light was found to be more effective for bacteria cell growth and hydrogen production. It was observed that PNSB followed substrate selection criteria for hydrogen production. Only R. palustris was able to produce hydrogen using most carbon sources. Cell density was almost constant, but cell growth rate and hydrogen production were significantly varied under the different lighting systems. The kinetics study suggested that initial substrate concentration had a positive correlation with lag phase duration. Among the PNSB, R. palustris grew faster and had higher hydrogen yields of 1.58, 4.92, and 2.57 mol H₂/mol using acetate, butyrate, and lactate, respectively. In the integrative approach with dark fermentation effluents rich in organic acids, R. palustris should be enriched in the phototrophic microbial consortium of the continuous hydrogen production system.     

Oxygen dissolution in polycrystalline palladium at O2 pressures of 0.1 to 100 Pa

Oxygen dissolution in polycrystalline palladium Pd(poly) at O₂ pressures ( $P_{O_2 } $ ) of 0.1 to 100 Pa and a temperature of 600 K has been investigated by temperature-programmed desorption. The dissolution process under these conditions includes O₂ chemisorption on the oxide film surface, the insertion of Oads atoms under the oxide layer, and their diffusion into the subsurface layers of palladium. During chemisorption, a structure ensuring that the O_ads coverage of the surface increases with increasing $P_{O_2 } $ forms on the surface of the oxide film. This is favorable for O_ads penetration through the oxide film and increases the amount of absorbed oxygen. The O_ads coverage of the surface calculated via the Langmuir equation at an O₂ desorption activation energy of E _des = 125 kJ/mol correlates with the number of absorbed oxygen monolayers (n). At n ≥ 1, oxygen absorption by Pd(poly) is due to the diffusion of O atoms in the palladium lattice. After the accumulation of 14–18 oxygen monolayers in the subsurface layers of palladium, oxygen absorption practically stops depending on $P_{O_2 } $ . Thus, the acceleration of oxygen dissolution in palladium is due to the formation of the surface oxide film and the increase in the O_ads coverage of this film, which facilitates the insertion of O_ads atoms into the subsurface layers of palladium.     

Molecular structure of mono- and dicarbonyls of rhodium and palladium

Summary Density Functional Theory has been applied to the study of the molecular structure of neutral and positively charged mono- and dicarbonyls of rhodium and palladium. The calculated optimized geometries, dissociation energies and normal frequencies are reported for the MCO, MCO⁺, M(CO)₂ and M(CO) ₂ ⁺ systems (where M=Rh and Pd), and the trends are discussed in detail. For neutral carbonyls, we interpret the M–C bond strength in terms of repulsion, which must be avoided, and attraction. These are related to the metal atom properties, such as the atomic splittings and the atomic ionization energies. In ionic carbonyls, the bonding is characterized by electrostatic attraction and repulsion. The rhodium carbonyls are generally found to be more stable than the corresponding palladium carbonyls. The palladium dicarbonyls are found to be linear, while both linear and bent structures are stable for rhodium dicarbonyls. An interpretation of these trends is made.