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.     

Synthesis of platinum colloids sterically stabilized by poly(N-vinylformamide) or poly(N-vinylalkylamide) and their stability towards salt

Colloidal dispersions of nanometer-sized platinum colloids were prepared by ethanol reduction of PtCl₆ ²⁻ in the presence of poly(N-vinylformamide) (PNVF), poly(N-vinylacetamide) (PNVA) or poly(N-vinylisobutyramide) (PNVIBA) and analyzed by UV-vis spectroscopy and transmission electron microscopy. The dispersion stability of each colloid to the presence of added KCl was determined by a stirring and centrifugation procedure. The platinum colloid stabilized by PNVF (PNVF-Pt) was the most stable and its critical flocculation concentration was not observed up to the highest electrolyte concentration employed (4.0 M). The stability of the platinum colloids stabilized by poly(N-isopropylacrylamide) (PNIPAAm) and poly(vinylpyrrolidone) (PVP) was also examined. The sequence of polymer-stabilized platinum colloids in increasing order of dispersion stability was found to be PNIPAAm-Pt < PNVIBA-Pt < PVP-Pt < PNVA-Pt < PNVF-Pt. Received: 25 August 1998 Accepted in revised form: 14 January 1999     

Low-temperature oxidation of carbon monoxide on Pd(Pt)/CeO2 catalysts prepared from complex salts

Catalysts containing cerium oxide as a support and platinum and palladium as active components for the low-temperature oxidation of carbon monoxide were studied. The catalysts were synthesized in accordance with original procedures with the use of palladium and platinum complex salts. Regardless of preparation procedure, the samples prepared with the use of only platinum precursors did not exhibit activity at a low temperature because only metal and oxide (PtO, PtO₂) nanoparticles were formed on the surface of CeO₂. Unlike platinum, palladium can be dispersed on the surface of CeO₂ to a maximum extent up to an almost an ionic (atomic) state, and it forms mixed surface phases with cerium oxide. In a mixed palladium-platinum catalyst, the ability of platinum to undergo dispersion under the action of palladium also increased; as a result, a combined surface phase with the formula Pd _x Pt _y CeO_{2 ? δ}, which exhibits catalytic activity at low temperatures, was formed.     

Determination of trace elements in AR grade alkali salts after preconcentration by column solid-phase extraction on TiO2 (anatase)

 Column solid-phase extraction using TiO₂ (anatase) as a solid sorbent was applied to preconcentrate traces of Cd, Co, Cu, Fe, Mn, Ni and Pb from AR grade alkali salts prior to their measurements by atomic absorption spectrometry (AAS). Multi-element preconcentration was achieved from NaCl, KCl, KNO₃, NaNO₃, CH₃COONa, NaHCO₃ and Na₂CO₃ solutions, whereas the sorption of trace elements from phosphates and sulfates is not quantitative. Optimal conditions (recoveries of the analytes >95%) for solid-phase co-extraction of the most common heavy metal ions are proposed. The conditions for quantitative and reproducible elution and subsequent AAS are established. A method of determination of trace elements in different salts is proposed. It is characterized by precision, reproducibility and a high preconcentration factor. The solid-phase extraction by TiO₂, combined with ETAAS allows the determination of 0.1 ng g^-1 Cd, 2 ng g^-1 Co, 1 ng g^-1 Cu and Ni, 0.5 ng g^-1 Mn and 0.4 ng g^-1 Pb. Received: 1 April 1996/Revised: 24 June 1996/Accepted: 9 July 1996     

Polymerization of styrene by Novel Ni(a)‐ and Pd(II)‐based complexes

The catalytic activities of nine neutral nickel and palladium α‐acetylide complexes [M= (C=CR)₂(PR'₃)₂, M=Ni, Pd; R = Ph, CH₂OH, CH₂OOCH, CH₂OOCPh, CH₂OOCPhOH‐o; R' = Ph, Bu] are compared. Among them, Ni(C‐CPh)₂(PBu₃)] shows the highest catalytic activity and gives the polystyrene with high molecular weight (M_w= 188800) and a syndio‐rich microstructure. The catalytic behavior of transition metal acetylides is related to metal, phosphine, and alkynyl ligands bonded to the metal atoms.     

Template synthesis of tin-doped indium oxide (ITO)/polymer and the corresponding carbon composite hollow colloids

SnO₂, In₂O₃, and Sn-doped In₂O₃ (ITO)/polymer and the corresponding carbon composite hollow colloids are template synthesized. It is essential that the sulfonated gel shell of the cross-linked polystyrene hollow colloid can favorably induce adsorption of target precursors. After being calcined in air to remove the template, SnO₂, In₂O₃, and ITO hollow colloids are obtained. Because the cross-linked polymer gel can be transformed into carbon in nitrogen at higher temperature such as 800 °C, metal oxide/carbon hollow colloids are consequently derived, whose shells are mesoporous. The SnO₂-, In₂O₃-, and ITO-containing polymer or carbon composite hollow colloids will be promising in sensors, catalysts, and fuel cells as electrode materials. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Simultaneous Determination of Palladium, Platinum and Rhodium by On-Line Column Enrichment and HPLC with 4-Hydroxy-1-Naphthalthiorhodanine as Pre-Column Derivatization Reagents

In this paper, 4-hydroxy-1-naphthalthiorhodanine (HNTR) was synthesized, and a new method for the simultaneous determination of palladium, platinum and rhodium ions as metal-HNTR chelates was developed using rapid column high-performance liquid chromatography combined with on-line enrichment. The palladium, platinum and rhodium ions were pre-column derivatized with HNTR to form colored chelates. The Pb-HNTR, Pt-HNTR and Rh-HNTR chelates could be absorbed onto the front of the enrichment column when they were injected into the injector and sent to the enrichment column [ZORBAX Stable Bound, 4.6 × 10 mm, 1.8 μm] with a buffer solution of 0.05 mol L⁻¹ sodium acetate-acetic acid (pH 4.0) as mobile phase. After enrichment, and by switching the six-ports switching valve, the retained chelates were back-flushed by mobile phase and traveling towards the analytical column. Separation of these chelates on the analytical column [ZORBAX Stable Bound, 4.6 × 50 mm, 1.8 μm] was satisfactory with 68% acetonitrile (containing 0.05 mol L⁻¹ of pH 4.0 sodium acetate-acetic acid buffer salt and 0.1% of tritonX-100) as mobile phase. Palladium, platinum and rhodium were separated completely within 2 min. The detection limits (S/N = 3) of palladium, platinum and rhodium are 1.2 ng L⁻¹, 1.5 ng L⁻¹ and 1.8 ng L⁻¹, respectively. This method was applied to the determination of palladium, platinum and rhodium in water, urine and soil samples with good results.     

Monodisperse,submicrometer-scale platinum colloidal spheres with high electrocatalytic activity

Monodisperse, submicrometer-scale platinum (Pt) colloidal spheres were prepared through a simple direct chemical reduction of p-phenylenediamine (PPD)-chloroplatinic acid (H₂PtCl₆) coordination polymer colloids. It was found that the prepared Pt colloids had the similar size and morphology with their coordination polymer precursors, and the prepared Pt colloids with rough surfaces were three-dimensional (3D) structured assemblies of high-density small Pt nanoparticles. The electrochemical experiments confirmed that the prepared Pt colloids possessed a high electrocatalytic activity towards mainly four-electron reduction of dioxygen to water, making the prepared Pt colloids potential candidates for the efficient cathode material in fuel cells.     

Synthesis and characterization of single-source molecular precursors for the preparation of metal chalcogenides

Metal chalcogenides constitute an important family of functional materials. Subtle changes in shape, size and phase of these materials result in variations in physical properties (e.g. electronic and optical), which can be exploited for various technological applications. Several strategies have evolved recently for controlling shape, size and phase of these materials. This work discusses design and synthesis of single-source molecular precursors for the preparation of metal chalcogenides both in bulk and nano-size regime. Precursors for palladium chalcogenides, indium sulphides andII–VI materials are presented. Synthesis of a variety of palladium(II)/platinum(II) complexes with internally functionalised chalcogenolate ligands, selenocarboxylates; gallium and indium dithiolate complexes and zinc/cadmium/ mercury complexes with N,N′-dimethylaminoalkylselenolate ligands and their characterization by NMR and X-ray crystallography are also discussed. Data on thermal behaviour of a few representative complexes, [Pd(SeCOAr)₂(PR₃)₂], [PdCl(E∩N)(PR₃)], [InMe₂(S∩S)], [In(S∩S)₃] and [M(E(CH₂)_nNMe₂)₂] (M = Zn, Cd, Hg;n = 2 or 3) are presented.     

Catalytic Oxidation of Small Molecules in a Gas Phase in the Presence of Heterogenized Platinum and Palladium Complexes with Chemically Different Ligands

This work is devoted to the study of catalytic properties of the metal complexes of platinum and palladium with acetylacetone and N-allyl-N"-propylthiourea heterogenized on the surface of silica in the oxidation reactions of H₂ and CO in a gas phase. We found that the acetylacetonate complexes were not degraded under catalytic reaction conditions, whereas the metal complexes withN-allyl-N"-propylthiourea exhibited a high activity only after partial degradation of the ligand. We demonstrated that the catalytic activity of the grafted metal complexes was higher than that of traditional supported platinum and palladium catalysts with the same metal content. Taking into account the structure of active centers in Pt and Pd complexes grafted on SiO₂ and the interaction of these centers with reactants, we proposed a detailed mechanism for the catalytic action, which adequately describes the entire set of experimental data.     

Characteristics of the Oxidation of Carbon Monoxide on Platinum and Palladium Acetylacetonates Immobilized on a Silica Surface

Characteristics of the kinetics of the oxidation of carbon monoxide on acetylacetonates of palladium and platinum immobilized on a silica surface have been studied. The bound metal complexes show no hysteresis in the dependence of the rate of reaction on the concentration of CO and O₂ and have a higher catalytic activity than Pt/SiO₂ and Pd/SiO₂. A mechanism is proposed for the oxidation of carbon monoxide on platinum and palladium complexes bound to a SiO₂ surface.     

Synthesis and electrochemical characterization of aPEO-based polymer electrolytes

In this paper, the preparation and purification of an amorphous polymer network, poly[oxymethylene-oligo(oxyethylene)], designated as aPEO, are described. The flexible CH₂CH₂O segments in this host polymer combine appropriate mechanical properties, over a critical temperature range from −20 to 60 °C, with labile salt-host interactions. The intensity of these interactions is sufficient to permit solubilisation of the guest salt in the host polymer while permitting adequate mobility of ionic guest species. We also report the preparation and characterisation of a novel polymer electrolyte based on this host polymer with lithium tetrafluoroborate, LiBF₄, as guest salt. Electrolyte samples are thermally stable up to approximately 250 °C and completely amorphous above room temperature. The electrolyte composition determines the glass transition temperature of electrolytes and was found to vary between −50.8 and −62.4 °C. The electrolyte composition that supports the maximum room temperature conductivity of this electrolyte system is n = 5 (2.10 × 10⁻⁵ S cm⁻¹ at 25 °C). The electrochemical stability domain of the sample with n = 5 spans about 5 V measured against a Li/Li⁺ reference. This new electrolyte system represents a promising alternative to LiCF₃SO₃ and LiClO₄-doped PEO analogues.     

Platinum–polytyramine composite material with improved performances for methanol oxidation

Polytyramine (PTy) is shown to be a possible alternative to other conducting polymers as a support material for fuel cell electrocatalysts such as platinum. In this work, a Pt–PTy composite was prepared via potentiodynamic deposition of polytyramine on graphite substrate, followed by the electrochemical deposition of Pt nanoparticles. The material obtained by this straightforward method exhibited, for platinum loadings as low as ca. 0.12 mg cm⁻², a specific electrochemically active surface area of the electrocatalyst of ca. 54 m² g⁻¹, together with a good electrocatalytic activity for methanol oxidation in acidic media, thus ensuring better efficiency of Pt utilization. The system Pt–PTy appears to be worthy of development for methanol fuel cell applications also because the results suggested that, when deposited as small particles in a PTy matrix, platinum is less sensitive to fouling during CH₃OH oxidation.     

Investigation of polymer-protected noble metal nanoparticles by transmission electron microscopy: control of particle morphology and shape

 Noble metal nanoparticles were prepared by the in situ reduction of the respective metal salt precursors in the presence of various protective polymers. Transmission electron microscopy (TEM) has been used to determine the particle shapes and morphologies. These are strongly influenced by the reduction methods and conditions chosen, but the choice of the protective polymer is equally important for controlling the particle morphologies and for the stabilization of the colloids. A whole spectrum of nanoparticle morphologies and shapes was obtained, ranging from nanoagglomerates which are nevertheless well-defined and well-stabilized to nanosized single crystals with triangular shape. Received: 2 February 1998 Accepted: 29 May 1998     

Influence of Pd precursors on the catalytic performance of Pd–H4SiW12O40/SiO2 in the direct oxidation of ethylene to acetic acid

The effects of palladium precursors (PdCl₂, (NH₄)₂PdCl₄, Pd(NH₃)₂Cl₂, Pd(NO₃)₂ and Pd(CH₃COO)₂) on the catalytic properties in the selective oxidation of ethylene to acetic acid have been investigated for 1.0 wt% Pd–30 wt% H₄SiW₁₂O₄₀/SiO₂. The structures of the catalysts were characterized using X-ray diffraction, N₂ adsorption, H₂-pulse chemical adsorption, infrared spectrometry of the adsorbed pyridine, H₂ temperature-programmed reduction and X-ray photoelectron spectroscopy. The present study demonstrates that the different palladium precursors can lead to the significant changes in the dispersion of palladium. It is found that Pd dispersion decreases as follows: PdCl₂ > (NH₄)₂PdCl₄ > Pd(NO₃)₂ > Pd(NH₃)₂Cl₂ > Pd(C₂H₃O₂)₂, which is nearly identical to the catalytic activity. This indicates that the dispersion of palladium plays an important role in the catalytic activity. Furthermore, density of Lewis (L) and Brönsted (B) acid sites are also strongly dependent on the palladium precursors. It is also demonstrated that an effective catalyst should possess a well combination of Brönsted acid sites with dispersion of palladium.     

Identification of reaction products of mild oxidation of H2S in solution and in solid state by UV-VIS spectroscopy

 UV-VIS spectroscopic investigations were carried out on several sulphur-compounds to attribute the absorption bands (290, 302, 330 nm) observed by in-situ reflectance spectroscopy to particular S_xO^2- _y and S^2- _x species. It was possible to assign the absorption bands to S₂O^2- ₃, S^2- ₂ and S₂O^2- ₄. These species are relevant in the catalytic oxidation of H₂S over alumina. Received: 11 January 1996/Revised: 4 April 1996/Accepted: 9 April 1996     

Effect of the nature of support and Cu precursor on the activity of copper-containing catalysts in CO oxidation

We have studied the catalytic activity of supported copper-containing catalysts based on ZSM-5, Al₂O₃, and SiO₂ in oxidation of CO. We have established that the difference between the activities of systems with 1.8% copper content obtained from different precursors is determined by the different reducibilities of their active sites, the number of such sites, and the distribution of the metal ions. The fact that the activity is highest for 1.8% Cu-ZSM-5 obtained from copper acetate is due to the relatively higher number of associated Cu²⁺ cations in square planar coordination in a non-lattice oxygen environment, which have high reducibility, and the higher overall oxygen content O_ads + O_OH in the surface layer of the catalyst.     

Specific features of the interaction of components in the DMSO−HBr−Pd0 system

The products of interaction of components in the donor-acceptor electron-transport (DAET) DMSO−HBr system and their complex formation with the metallic palladium surface were studied. H₂O and Me₂S (main reaction products) and CO, CS₂, C₂H₆, MeBr, H, and CH₄ (minor reaction products) were found in the gas phase by mass spectrometry (MS). The samples of metallic palladium treated with the DAET system with a components ratio corresponding to the minimum and maximum rates of metal dissolution were studied by the methods of thermoprogrammed desorption with MS detection (TPD-MS) and XPS. According to the TPD-MS data, two forms of Me₂S are present on the metal surface, whereas the XPS method detected two complexes with the molecular compositions Pd^IIBr₄S_1.26 and Pd^IIBr_3.86S_1.42. The addition of an aqueous solution of NaOH to the system results in the formation of HCOONa, which indicates that compounds (CH₂O, HCOOH) capable of reducing the palladium complexes are present in the DAET system. Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1740–1743, October, 2000.     

The nature of colloidal platinum in aqueous solution: features of catalytic reactions

The specific features of the catalytic reduction of methylviologen by dihydrogen in water in the presence of platinum colloids synthesized by various methods are studied. The colloids prepared by the radiation-chemical reduction of PtCl₄ ^2– in the presence of polyacrylate or polyphosphate as stabilizers and colloids prepared by the reduction with dihydrogen efficiently catalyze the reaction. The citrate colloids synthesized by the reduction of PtCl₆ ^2– with citric acid are characterized by a prolonged induction period after which these colloids also gain the catalytic activity. We assume that the citrate platinum colloids are giant clusters with the close-packed metal core containing the magic number of platinum atoms (Pt₅₆₁), which are coordinated by the ligand molecules blocking the surface of the metal particle. In the presence of H₂, the ligand molecules are decomposed or removed from the surface, which is accompanied by the appearance of the catalytic activity.