Comparisons between cationic polyelectrolytes and nonionic polymers for the protection of palladium and platinum nanocatalysts

Several palladium and platinum nanocatalysts protected by cationic polyelectrolytes were prepared by the in-situ reduction of palladium chloride, PdCl₂, and dihydrogen hexachloroplatinate, H₂PtCl₆. The particle sizes and size distributions were determined by transmission electron microscopy, and the colloids were further characterized by UV-vis spectroscopy. The catalytic activity of these nanoparticles was qualitatively investigated by the hydrogenation and conversion of cyclohexene as a model reaction and compared to palladium and platinum colloids protected by a selection of water-soluble, nonionic polymers. The results show that the catalytic activity is strongly influenced by the protective polymer chosen, as well as particle size and morphology. The use of cationic polyelectrolytes decreases the catalytic activities significantly, in comparison to several water-soluble, nonionic polymers investigated. The effects depend strongly on the particular metal, as illustrated in this case by differences observed between palladium and platinum. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 3151–3160, 1997     

Polymer-Protected Palladium Nanoparticles and Their Use in Catalysis

Abstract

Stable palladium colloids were prepared by the in-situ reduction of palladium chloride (PdCl₂) in the presence of protective water-soluble polymers and cationic polyelectrolytes. The particle sizes, morphologies, and particle-size distributions were determined by transmission electron microscopy and found to be in the nanometer size range. The catalytic activity of these colloidal metal-polymer systems was tested by the hydrogenation of cyclohexene as a model reaction. Most of the polymer-protected palladium nanoparticles were found to be catalytically active, and final conversions up to 100% were obtained in many cases.     

Modification of properties of ion-exchange membranes. IV. Change of transport properties of cation-exchange membranes by various polyelectrolytes

A change in electrodialytic transport properties of various cation exchange membranes was observed after the membranes had been adsorbed or ion-exchanged with various cationic polyelectrolytes. Transport properties measured in this report were the relative transport number of calcium ion to sodium ion P^Ca_Na, the current efficiency of cations, and the electric resistance of the membrane during the electrodialysis. Weakly basic and strongly basic cationic polyelectrolytes of various molecular weights were used. Though P^Ca_Na of any cation exchange membrane decreased by the adsorption or ion exchange of any cationic polyelectrolyte, the degree of the decrease in P^Ca_Na changed with species and molecular weight of polyelectrolytes and species of cation-exchange membranes. Weakly basic polyelectrolytes and low molecular weight, strongly basic polyelectrolytes were effective in producing a marked decrease in P^Na_Ca of any cation exchange membrane. The effect of strongly basic polyelectrolytes of high molecular weight on P^Ca_Na was weak in most cases. However, if it was possible to make the polyelectrolyte adhere to the surface of the membrane to form a compact coiled structure, any cationic polyelectrolyte was effective in producing a remarkable decrease in P^Ca_Na of any cation-exchange membrane.     

Functionalized multiwall carbon nanotube/gold nanoparticle composites

Multiwall carbon nanotubes (MWCNTs) were chemically oxidized in a mixture of sulfuric acid and nitric acid (3:1) while being ultrasonicated. The effect of oxidative ultrasonication at room temperature on development of functional groups on the carbon nanotubes was investigated. The dispersability and the carboxylic acid group concentration of functionalized MWCNTs (fMWNTs) varied with reaction time. The concentration of carboxylic acid groups on fMWNTs increased from 4 x 10(-4) mol/g of fMWNTs to 1.1 x 10(-3) mol/g by doubling the treatment period from 4 to 8 h. The colloidal stability of aqueous fMWCNTs dispersions was enhanced through elongated oxidation. fMWCNTs that were reacted longer than 4 h did not precipitate in aqueous media for at least 24 h. The layer-by-layer self-assembly of polyelectrolytes on fMWCNTs was characterized by zeta potential measurements. The zeta potential of fMWCNTs changed from negative charge to positive charge when cationic polyelectrolytes were self-assembled on their surface. With addition of anionic polyelectrolytes, cationic polyelectrolyte coated fMWCNTs showed the expected charge reversal as expected for multilayer self-assembly. Complex formation of positively charged gold nanoparticles and negatively charged fMWCNTs was achieved with and without polyelectrolyte coatings by electrostatic interaction. The complex formation was characterized by high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy. The here found complex formation of positively charged colloidal gold and defect sites on fMWNTs indicates the location of functional groups on carbon nanotubes. It is suggested that positively charged colloids such as gold nanoparticles could be used for detection of defect sites on carbon nanotubes.     

Interactions of monomeric and dimeric cationic surfactants with anionic polyelectrolytes: a fluorescence study

The interactions of conventional cationic, i.e. dodecyl-(DTAB), tetradecyl-(TTAB), and hexadecyltrimethylammonium bromides (HTAB), and dimeric cationic surfactants, i.e. dimethylene bis decyl-(10-2-10), and dodecyldimethylammonium bromides (12-2-12) with anionic polyelectrolytes, were studied by fluorescence measurements. The variation of I₁/I₃ ratio of the fluorescence of pyrene in aqueous solutions of polyelectrolytes was measured as a function of surfactant concentration. A three-step aggregation process involving the critical aggregation concentration (cac) and critical micelle concentration (cmc) was observed in each case. The cationic surfactants with lower hydrophobicity demonstrated higher degree of binding and vice versa.     

1H NMR studies on intermolecular association of amphiphilic cationic polyelectrolyte micelles induced by hydrophobic counteranions in water

Interactions of a series of amphiphilic cationic polyelectrolytes with various kinds of organic counteranions have been investigated in water by one- and two-dimensional ¹H NMR spectroscopy at 20 °C. The cationic polyelectrolytes were prepared by micellar homopolymerization of tail-type cationic surface-active monomers with a cationic charge with -end, (ST–C_m–AB, m=5, 7, and 9, where ST is a styrenic group, C_m, an alkylene chain at the 4-position of styrene, and AB, alkyltrimethylammonium bromide). Aliphatic monosodium salt of maleic acid (MAS) and its stereoisomer, fumaric acid (FAS), sodium benzoate (NaB), potassium hydrogen phthalate (PHK), and sodium salicylate (NaSal) were added to a salt-free aqueous solution of the polyelectrolytes and ¹H NMR measurements were carried out. Amphiphilic P(ST–C_m–AB) polyelectrolytes act as efficient hosts to strongly capture the hydrophobic counteranions B^–, PH^–, and Sal^–, but not MA^– and FA^–. The ¹H NMR signals of these hydrophobic counteranions remarkably shift upfield and broaden in water in the presence of the amphiphilic polyelectrolytes. The nuclear Overhauser effect (NOE) signals between the cationic group of the polymer and aromatic benzoate counteranion protons are clearly observed to imply cation– interaction. The capturing of hydrophobic counterions by the polyelectrolytes is likely due to electrostatic, hydrophobic, and cation– interactions between them. The reduced viscosity, _sp/C_p, for the solution at [PHK]/[P(ST–C₇–AB)]=1.0 steeply increases with increasing polymer concentration (C_p) above ca. 0.9 g/dL to show pronounced viscoelasticity.     

Ionic bonding in butadiene copolymers. II. Stress relaxation

The mechanical lability of quadrupolar links in cationic and anionic polyelectrolytes and of ion-pair links in mixed polyelectrolytes is investigated in terms of deviations from the WLF viscoelastic theory. The behavior of E(t), log A_T, and E_visc indicate that the quadrupolar links do not interchange below the second transition T_t^* found in these materials. The blended polyelectrolytes exhibit low yields of the desired ion pair linking and have transitions characteristic of quadrupolar migration of the unreacted polyelectrolytes.     

Synergetic effect in PdAu/CeO<Subscript>2</Subscript> catalysts for the low-temperature oxidation of CO

Gold-palladium catalysts supported on cerium oxide were synthesized with the double complex salts. X-ray photoelectron spectroscopy (XPS) and other physicochemical methods (TEM, TPR) were used to demonstrate that synthesis of highly active palladium catalysts requires the oxidative treatment stimulating the formation of a catalytically active surface solid solution Pd _x Ce_1?x O₂, which is responsible for the lowtemperature activity (LTA) in the reaction CO + O₂. In the case of gold catalysts, active sites for the lowtemperature oxidation of CO are represented by gold nanoparticles and its cationic interface species. Simultaneous deposition of two metals increases the catalyst LTA due to interaction of both gold and palladium with the support surface to form a Pd_1?x CexO₂ solid solution and cationic interface species of palladium and gold on the boundary of Pd-Au alloy particles anchored on the solid solution surface.     

BINDING OF IONIC SURFACTANTS ON OPPOSITELY CHARGED POLYELECTROLYTES OBSERVED BY FLUORESCENCE METHODS

Our recent studies concerning the binding of ionic surfactants on oppositely charged polyelectrolytes observedwith fluorescence techniques are reviewed. The cationic surfactants cetyltrimethylammonium bromide (CTAB),dodecyltrimethylammonium chloride (DTAC), and nonionic surfactant octaethylene glycol monododecyl ether (C_(12)E_8) wereallowed to bind on anionic poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) and its pyrene and/or naphthalenelabeled copolymers. The relative excimer emission intensity I_E/I_M of a cationic probe l-pyrenemethylamine hydrochloride(PyMeA·HCl) and the non-radiative energy transfer (NRET) I_(Py)/I_(Np) of naphthalene to pyrene for labeled polyelectrolyteswere chosen to monitor the binding process and the conformation change of surfactant-bound polyelectrolytes. The 1:1aggregation of polyelectrolyte-CTAB with respect to the charge was found as long as the CTAB concentration was slightlyhigher than its critical aggregation concentration (CAC). The intermolecular NRET indicated that the CTAB-boundpolyelectrolytes aggregated together through the hydrophobic interaction between the CTAB tails. However, neither 1:1polyelectrolyte-DTAC aggregation nor intermolecular aggregation of DTAC-bound polyelectrolyte was observed owing to itsweaker hydrophobicity of 12 carbon atoms in the tail, which is shorter than that of CTAB. As known from the fluorescenceresults, nonionic surfactant C_(12)E_8 did not bind on the anionic polyelectrolytes, but the presence of PAMPS promoted themicelle formation for C_(12)E_8 at the CAC slightly below its critical micelle concentration (CMC). The solid complex of dansyllabeled AMPS copolymer-surfactant exhibited a decrease in local polarity with increasing charge density of thepolyelectrolyte or with alkane tail length of the surfactant. SAXS suggested a lamella structure for the AMPS copolymer-surfactant solid complexes with a long period of 3.87 nm for CTAB and 3.04 nm for DTAC, respectively.     

Direct visualization of layer-by-layer self-assembled multilayers of organometallic polymers

Direct visualization of organometallic-organic and novel all-organometallic multilayer superlattices prepared by layer-by-layer assembly of cationic/anionic polyferrocenylsilane and anionic polystyrene sulfonate polyelectrolytes using a gold coating/transmission electron microscopy (TEM) technique is reported.     

One-step preparation of highly concentrated well-stable gold colloids by direct mix of polyelectrolyte and HAuCl4 aqueous solutions at room temperature

Highly concentrated, well-stable gold colloids can be prepared directly from an amine-bearing polyelectrolyte-HAuCl4 aqueous solution at room temperature. It is found that increasing molar ratio of polyelectrolyte to gold leads to increasing particle size. UV-vis spectra, transmission electron microscopy (TEM), and X-ray photoelectron spectra (XPS) were used to characterize the products thus formed.     

Thermal stability and glass transition behavior of PANI/MWNT composites

Polyaniline/multi-walled carbon nanotube (PANI/MWNT) composites were prepared by in situ polymerization. Transmission electron microscope (TEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) were used to characterize the PANI/MWNT composites. Thermal stability and glass transition temperature (T _g) were measured by thermogravimetry (TG) and temperature modulated differential scanning calorimetry (TMDSC), respectively. The TG and derivative thermogravimetry (DTG) curves indicated that with augment of MWNTs content, the thermal stability of PANI/MWNT composites increased continuously. While, T _g increased and then decreased with the MWNTs content increasing from 0 to 20 mass%.     

Self-assembled polyelectrolyte multilayer membranes with highly improved pervaporation separation of ethanol/water mixtures

Ethanol/water pervaporation through ultrathin polyelectrolyte multilayer membranes is described. The membranes were prepared by the layer-by-layer technique, i.e. by alternating sequential adsorption of cationic and anionic polyelectrolytes on a porous support. The separation capability was optimized by variation of the chemical structure of the polyelectrolytes, by variation of pH and ionic strength of the polyelectrolyte solutions used for membrane preparation and by annealing of the polyelectrolyte membranes. It was found that the separation is mainly affected by the charge density of the polyelectrolytes which is controlled by the chemical structure and the degree of ionisation of the polar groups. Selectivity for water was highest, if polyelectrolytes of high charge density such as polyethyleneimine (PEI), polyvinylamine (PVA) and polyvinylsulfate (PVS) were used and if the pH of the polyelectrolyte solutions was equal to the mean of the pK_a values of the corresponding cationic and anionic polyelectrolyte. Best results were obtained for PVA/PVS and PEI/PVS membranes which are characterized in detail with regard to their separation behavior.     

Optical Spectroscopy of Hybrid Sol-Gel Coatings Doped with Noble Metals

Sol-gel coatings in the xM (100-x) SiO₂ system, (M = Cu, Ag and Au) x =0.1–10 mol%), are deposited on soda lime glass slides by using silicon tetramethoxide Si(OCH₃)₄) and methyltriethoxysilane (SiCH₃[OCH₂CH₃]₃) as silica precursors. Anhydrous CuCl, CuCl₂ 2H₂O, Cu(NO₃)₂ 3H₂O, CuSO₄ 5H₂O, AgNO₃ and HAuCl₄ 3H₂O are used as copper, silver and gold sources. Coatings with thicknesses ranging from 100 to 900 nm are deposited on the subs trates by dip-coating and subsequently densified at 500°C for 1 h in air. Spectroscopic studies of the coatings as a function of the thicknesses and the metal concentration are carried out by photoluminescence (PL) and optical absorption (OA). In addition, direct observations of some gold coatings were performed by transmission electron microscopy (TEM). Results indicate that for silver and copper containing coatings the excitation and emission spectra arise from electronic transitions in Ag⁺ and Cu⁺ ions and no significant absorption bands due to colloidal precipitation are observed. Gold containing coatings show purple coloration due to an absorption peaking in the 520–560 nm range, which is characteristic of gold colloids. The presence of these colloids is confirmed by TEM observations.     

Salt effect on surface modification of silica by interpolyelectrolyte complexes

The effect of a low-molecular-mass salt on the properties of interpolyelectrolyte complexes formed as a result of interactions between poly(diallyldimethylammonium chloride) and copolymers of maleic acid with propylene or α-methylstyrene in their salt containing non-stoichiometric mixtures has been studied. Properties of such interpolyelectrolyte complexes were found to be determined by the chemical nature of the polyelectrolytes and by the salt concentration. The effect of salt on the surface modification of silica particles via their interactions with interpolyelectrolyte complexes has been examined. Two different ways of the surface modification of silica particles were used: (i) silica particles were contacted with previously prepared interpolyelectrolyte complexes and (ii) silica particles were contacted with cationic polyelectrolyte at first and then anionic polyelectrolyte was added. The efficiency of the surface modification was shown to be also dependent on the salt concentration and the chemical nature of polyelectrolytes. Turbidimetry, quasi-elastic light scattering, laser microelectrophoresis, and polyelectrolyte titration were used to characterize studied systems.     

Growth of poly(methyl methacrylate) particles in three-component cationic microemulsions

Polymerization of methyl methacrylate (MMA) has been studied in ternary microemulsions which were stabilized by tetradecyltrimethylammonium bromide (TTAB) or stearyltrimethylammonium chloride (STAC). The sizes of MMA-swollen polymer particles (R_h) increased continuously during polymerization. This is in contrast to that of the styrene system, where R_h increased very rapidly to a maximum and then decreased continuously towards a constant value. The continuous growth of PMMA particles at 60°C are discussed. The stability of PMMA latexes increased with increasing the hydrophobic chain length of the cationic surfactant used. Traces of the coagulations of PMMA particles in the TTAB system can be seen from TEM. © 1995 John Wiley & Sons, Inc.     

Nanopore microspheres prepared by a cationic surfmer in the miniemulsion polymerization of styrene

A polymerizable cationic quaternary ammonium surfactant (CQAS) based on 2‐(dimethylamino)ethyl methacrylate (DMAEMA) was successfully synthesized via quaternization reaction. The product was characterized by FTIR and ¹H NMR spectroscopy, and its critical micelle concentration (CMC) was obtained by surface tension measurement. The surfmer acted well as comonomer and surfactant to stabilize monomer droplets during miniemulsion polymerization. To identify whether this system undergoes miniemulsion nucleation mechanism, surface tension, particle size, and N_droplet/N_particle of the system were also measured. The effect of concentration and counter‐ion of the surfmer, and pH value of the system were systematically investigated by kinetic analysis and dynamic light scattering (DLS). The resulting nanopore microspheres were observed by transmission electron micrograph (TEM) and field emission scanning electron micrograph (FESEM) and showed the nanopore morphology with reasonable stability. Another cationic surfactant cetyltrimethylammonium bromide (CTAB) was used for comparative studies. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5800–5810, 2007     

Behaviour of cationic polyelectrolytes upon binding of electrolytes: effects of polycation structure, counterions and nature of the solvent

 The effects of polycation structure, counterions and the nature of the solvent on the interaction between low-molecular-weight salts with some cationic polyelectrolytes in water and methanol were investigated. The polyelectrolytes used in this study were cationic polymers with quaternary nitrogen atoms in the backbone with or without a nonpolar side chain (polymer type PCA₅H₁, PCA₅D₁ and PCA₅) or tertiary amine nitrogen atoms in the main chain (polymer type PEGA). LiCl, NaCl, KCl, NaBr, NaI and Na₂SO₄ were used as low-molecular-weight salts. The interaction between polycations and salts was followed by viscometric and conductometric measurements. The study of the interaction of monovalent counterions with cationic polyelectrolytes emphasized an increase in the interaction with the decrease in the radius of the hydrated counterion, both for strong polycations and for weak polycations, suggesting that counterion binding is nonspecific. In the case of SO²⁻ ₄ anions, the Λ_m−c ^1/2 curve passes through a minimum at c _p values between 1 × 10⁻³ and 3 × 10⁻³ unit mol/l; this phenomenon can be explained by the maximum counterion interaction owing to the capacity of the polyvalent counterion to bind two charged groups by intra- or interchain bridges. The investigation of the influence of the polycation structure on the counterion binding indicated an increase in charged group–counterion interactions with a decrease in the nonpolar chain length and an increase in the quaternary ammonium salt group content (charge density) in the chain. The polyelectrolyte with tertiary amine groups in the chain, PEGA, showed, on one hand, a cation adsorption order as K⁺>Na⁺>Li⁺ and, on the other hand, a stronger association between ions and PEGA chains in methanol than in water owing to the poorer solvating effect of methanol on the cations. Received: 20 February 2001 Accepted: 29 June 2001     

Cysteamine-modified ZIF-8 colloidal building blocks: Direct assembly of nanoparticulate MOF films on gold surfaces via thiol chemistry

Chemical modification of metal organic framework (MOF) nanocrystal colloids was used to endow them with chemical affinity for gold substrates. Modified nanocrystals were then used as building blocks for rapid and selective self-assembly of porous films. Cysteamine (Cys, 2-aminoetanethiol) was chosen as both chemical modulator and functionalizing agent of Zeolite Imidazolate Framework-8 (ZIF-8) MOF nanocrystals. Important parameters such as the impact of the modulator on the range of nanocrystals stability, size, polydispersity, morphology, and crystalline structure were assessed via both, small and wide angle x-ray scattering (SAXS and WAXS). Cysteamine modified ZIF-8 nanocrystals were assembled into films over conductive Au substrates and film growth was followed in-situ with Quartz Crystal Microbalance (QCM). Thiol moieties exposed out of the ZIF-8 surface after cysteamine modification, results in the formation of thiol bonds with Au conductive substrates as shown via Cyclic Voltammetry experiments. The strategy here presented allows for the synthesis of pre-designed building blocks for MOF films on metal surfaces.     

Autoxidation of 2-mercaptoethanol catalyzed by cobalt(II) phthalocyaninetetrasulfonate on colloidal particles

Cobalt(II) phthalocyaninetetrasulfonate (CoPcTS) is a highly active catalyst in aqueous media for the autoxidation of thiols to disulfides, and cationic polymers further enhance the activity. In this work we tested a variety of cationic polymer supports for the CoPcTS-catalyzed autoxidation of 2-mercaptoethanol in aqueous dispersions at 25°C. The order of activities is cationic polyelectrolytes > cationic polymer colloids > cationic colloidal silica > aqueous solution > ion exchange resins. Spectroscopic studies indicate that the high activity of the polyelectrolytes correlates with their ability to aggregate the CoPcTS. © 1992 John Wiley & Sons, Inc.