Effect of catalyst on regioselectivity and vinyl/H exchange on silicon in the hydrosilylation of vinyl-siloxanes by T8 hydrogen silsesquioxane

We have recently reported the synthesis of octopus molecules of defined shape and size with molecular weights well into the thousands. These octopus molecules were made by placing eight pendant groups symmetrically about a central silsesquioxane core via the H₂PtCl₆ catalyzed hydrosilylation of 1-alkenes as well as vinyl- and allyl-siloxanes by T₈ hydrogen silsesquioxane, (HSiO₃₂)₈. The chemistry of addition was studied and it was found that while the addition of the 1-alkenes to T₈ was regioprecise with only -addition being observed, both - and -addition occurred with vinyl-siloxane. In addition, H-vinyl exchange on silicon was observed to occur with addition of vinyl-siloxane to T₈. In the current studies, the effect of the hydrosilylation catalyst. homogeneous and heterogeneous, on the regioselectivity of addition and on the extent of exchange on silicon was evaluated. It was found that the heterogeneous catalysts Pt–C, sulfided Pt–C, and Rh–C required higher temperature and longer times to get complete reaction than the homogeneous catalysts. H₂PtCl₆ and the tetramethyldivinyldisiloxane complex of Pt. Pd supported catalysts were not effective catalysts for this hydrosilylation. The extent of exchange on silicon and the degree of the second mode of addition occurring were higher with the heterogenous catalysts and may be a result of the higher reaction temperatures.     

Biocidal polymers active by contact. I. Synthesis of polybutadiene with pendant quaternary ammonium groups

Hydroxytelechelic polybutadienes carrying covalently bound quaternary ammonium salts were successfully synthesized in three steps. The first one was the preparation of 1-(N, N-dimethylaminopropyl) 1,1,3,3-tetramethyldisiloxane (M′₂A) by hydrosilylation. The addition occurred mainly in the terminal position of the double bond, but isomers were formed in small proportion (an isomer resulting from an inverse addition and an isomer resulting from an isomerization of the double bond). The proportion of these isomers increased with the concentration of catalyst (H₂PtCl₆). Similar results were obtained with a Pt (0)-di-vinyltetramethyldisiloxane complex (Pt,DVDS). The second step was the grafting of M′₂A onto the 1,2-units of a hydroxytelechelic polybutadiene by hydrosilylation. The yield of the reaction was higher than 90% and the OH functionality decreased slightly. Here again, different platinum catalysts were compared: initial rates were higher with Pt,DVDS, but the final yields were better with H₂PtCl₆. The last step was the quaternization of the pendant tertiary amino groups in methanol with alkyl bromides from C₈H₁₇Br to C₁₆H₃₃Br. The yield of the reaction was higher than 90% and the OH functionality was not modified. © 1993 John Wiley & Sons, Inc.     

Pt catalysts supported on zeolitized-pumice for the selective hydrogenation of campholenic aldehyde: A characterization and kinetic study

A detailed characterization study of Pt catalysts supported on a novel zeolitized-pumice (Z-PM) support is reported. Two catalysts, named Pt(Cl)/Z-PM and Pt(Ac)/Z-PM, prepared by impregnation from H₂PtCl₆ and Pt(acac)₂ as precursors and subsequent reduction under H₂ at 623 K, were investigated in terms of microstructure, chemical and surface properties by using X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS).Characterization data showed that the microstructural stability of the support was preserved by using Pt(acac)₂ as precursor. It was verified instead that, by using H₂PtCl₆, the microstructure of the support changed during the thermal activation step under hydrogen, favouring a strong interaction between Pt and support as well as the formation of bimetallic Pt–Fe alloys. Moreover, a moderate increase of Pt d-band vacancies with respect to metallic Pt was observed in this catalyst by XPS.The above catalysts were tested in the selective hydrogenation of campholenic aldehydes to the corresponding unsaturated alcohol, naturanol. In comparison to the Pt(Ac)/Z-PM sample, the ex-chloride Pt(Cl)/Z-PM catalyst showed a higher selectivity to naturanol. This behaviour was interpreted on the basis of the different microstructural and electronic properties as evinced by the characterization data.     

Effect of Preparation Conditions of Pt/C Catalysts on Oxygen Electrode Performance in Proton Exchange Membrane Fuel Cells

Supported Pt/C catalyst with 3.2 nm platinum crystallites was prepared by the impregnation—reduction method. The various preparation conditions, such as the reaction temperature, the concentration of precursor H₂PtCl₆ solution and the different reducing agents, and the relationship between the preparation conditions and the catalyst performance were studied. The carbon black support after heat treatment in N₂ showed improved platinum dispersion. The particle size and the degree of dispersion of Pt on the carbon black support were observed by transmission electron microscopy (TEM). The crystal phase composition of Pt in the catalyst was determined by X-ray diffraction (XRD). The surface characteristics of the carbon black support and the Pt/C catalyst were studied by X-ray photoelectron spectroscopy (XPS). The electrochemical characteristics of the Pt/C catalysts were evaluated from current—voltage curves of the membrane electrode assembly (MEA) in a proton exchange membrane fuel cell.     

Preparation and Characterization of Pt/C and Pt/TiO2 Electrocatalysts by Liquid Phase Photodeposition

The preparation of carbon and titanium dioxide supported Pt catalysts through a photochemical and photocatalytic routes were investigated. The catalysts were prepared by irradiation with UV-light (365 nm) at room temperature using H₂PtCl₆ and C₁₀H₁₄O₄Pt (Pt(acac)₂) as platinum precursors. The kinetic studies revealed that H₂PtCl₆ produced metallic platinum faster than Pt(acac)₂ and also showed that the amount of platinum deposited on TiO₂ was higher than on carbon. The samples were characterized by X-ray diffraction, SEM/EDS and cyclic voltammetry. X-ray diffraction permitted to identify the crystallographic (111) and (200) planes from platinum metal on the catalysts synthesized, the intensity of peaks depends of the amount of platinum deposited. SEM/EDS test confirmed what it was found by the kinetics studies. The electrocatalytic activity was compared with a commercial Pt E-Tek catalyst (10 wt%). The electrochemical results showed that Pt/C-AA catalyst synthesized by liquid phase photo-deposition method has stability in acid media and high distribution of the actives sites on the electrode surfaces. It could be considered as a candidate for electro-catalyst for polymer electrolyte fuel cell. The Pt/TiO₂ catalysts did not present electrochemical activity.     

Ethylene Hydrogenation over Pt/Ga2O3/Al2O3 Catalysts

The effects of gallia addition on the Pt dispersion and the activity for ethylene hydrogenation at 0 °C were studied for Pt-supported catalysts as a function of the reduction temperature (350, 450, and 550 °C). The catalysts contained 0.5 wt.% Pt and were prepared by successive incipient wetness impregnations with Ga(NO₃)₃ and H₂PtCl₆ aqueous solutions. CO and H₂ chemisorption data indicated that, the addition of small amount of Ga₂O₃ caused an increase of the Pt dispersion and a decrease of ethylene conversion. Both of them decreased appreciably when Ga₂O₃ addition was increased, particularly, for the case where β-Ga₂O₃ was used as a support. The increase in reduction temperature magnified the negative effects of the gallia addition on dispersion and activity, although the addition of small amount of gallia improved the resistance to metal sintering. Results were interpreted in terms of the presence of reduced Ga species, which can encapsulate Pt particles.     

Preparation and catalytic use of platinum in magnetic core/shell nanocomposites

Platinum (Pt) nanoparticles show high activity as catalysts in various chemical reactions. The control of the morphology of Pt nanostructures can provide an opportunity to improve their catalytic properties. The preparation of Pt‐loaded iron‐oxide polyvinylbenzyl chloride nanocomposites was done in several stages: first by the formation of the core consisting of magnetite nanoparticles and second by the polymerization of vinylbenzyl chloride in the presence of the magnetic core particles. The third step is the amination of the chlorine group with ammonia, which leads to an ion exchange resin. Then, the Pt precursor (H₂PtCl₆) is attached by ion exchange. Finally, the Pt ions are reduced to Pt metal with NaBH₄. The obtained material can be dispersed easily and be used as a catalyst which can be separated after the reaction by magnetic fields. Characterization of the resulting metallic nanocomposites is evaluated by atomic absorption spectroscopy, thermal gravimetric analysis, transmission electron microscopy, infrared spectroscopy, and gas chromatography. The activity of Pt at magnetic core/shell nanocomposites was measured for the reduction reaction of cinnamaldehyde to cinnamyl alcohol. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells

Pt/TiO_x/C nanocomposites have been synthesized by depositing hydrated titanium oxide on carbon-supported Pt (Pt/C), reducing H₂PtCl₆ with sodium formate on carbon-supported hydrated titanium oxide (TiO₂/C), and simultaneously depositing hydrated titanium oxide and reducing H₂PtCl₆ with formate on carbon support, followed by heat treatment at 500 and 900 °C in 90% Ar-10% H₂ mixture. The catalytic activity for oxygen reduction was evaluated in half cells with sulfuric acid electrolyte and in single direct methanol fuel cells (DMFC). Tolerance to methanol was studied with half cells containing sulfuric acid mixed with methanol. Charge transfer resistance and electrochemical active surface area of the Pt/TiO_x/C catalysts were studied with impedance and cyclic voltammetry measurements. Both the synthesis methods and heat treatments influence the catalytic activity, and some of the Pt/TiO_x/C composites exhibit higher catalytic activity than Pt/C. The Pt/TiO_x/C catalysts also exhibit better methanol tolerance than Pt/C. The mechanism for the enhanced catalytic activity of Pt/TiO_x/C is discussed.     

Influence of the nature of the platinum precursor on the surface properties and catalytic activity of alumina-supported catalysts

The influence of the nature of the metal precursor — platinum acetylacetonates and chloroplatinic acid — on the surface properties and catalytic activity of Pt/Al₂O₃ catalysts is reported. The obtained results indicate that the catalysts prepared by the organometallic route present higher metal dispersion and lower acidity compared with those prepared from H₂PtCl₆. On the other hand, XPS results showed that the state of platinum is essentially Pt⁰ in the catalysts obtained from Pt(acac)₂ while in the solids prepared by impregnation of H₂PtCl₆ there exists an important contribution of Pt⁺ species which plays a positive role in the hydrogenation of toluene. An additional hydrogen spillover due to the presence of more acidic support is also suggested as an explanation of the observed catalytic results.     

Silver/Platinum Supported on TiO<Subscript>2</Subscript> P25 Nanocatalysts for Non-photocatalytic and Photocatalytic Denitration of Water

Denitration of water was investigated by non-photocatalytic and/or photocatalytic processes (UV-A irradiation at 365 nm) using a mixture of Ag/P25?+?Pt/P25 monometallic catalysts and Ag–Pt(Pt–Ag)/P25 bimetallic catalysts (2 wt% Ag; 4 wt% Pt) prepared by drop-wise wetness impregnation of TiO₂ P25 support. In the bimetallic samples, the influences of the Pt precursor (H₂PtCl₆·6H₂O; K₂PtCl₆) and of the impregnation order of the metallic salts were examined. The highest N₂ yield (42.3%) in the non-photocatalytic process was achieved with the Ag/P25?+?Pt/P25 mixture but with ca. 12.6% NO₂ ^? yield. Photocatalytic activity was enhanced in presence of H₂ in comparison to H₂-free condition. Ag/P25 is the most active photocatalyst, however high NO₂ ^? yield is obtained (32.5%). The bimetallic samples exhibit high versatility, being active both in the non-photocatalytic and the photocatalytic processes. Low NO₃ ^? conversion and high NO₂ ^? selectivity results were obtained from impregnation of Ag first. In contrast, impregnation of Pt precursor from K₂PtCl₆ first effectively promoted NO₃ ^? reduction towards N₂ yield of 36% and particularly low NO₂ ^? yield of 2.7%, due the presence of metallic nanoparticles of different sizes and interaction with TiO₂ with a peculiar strong Pt and Ag interaction. Best results obtained in non-photocatalytic and photocatalytic processes are almost similar.     

Preparation of platinum-ruthenium alloys supported on carbon by a sonochemical method

Pt and Pt-Ru alloys with several Pt/Ru ratios supported on carbon (Vulcan) were prepared using high-intensity ultrasound by reduction of H₂PtCl₆ and RuCl₃ precursors in an aqueous solution. This method of catalyst preparation was performed in absence of any surfactant or organic addictive. The particles formed were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDX) and transmission electron microscopy (TEM). From the XRD studies, a decrease of metal particle size and of the lattice parameters was observed with the increase of the Ru content. The electroactivities were tested for the methanol oxidation reaction in acid electrolyte, and it was found that Pt-Ru catalysts were more activity than pure Pt.     

Study of Ni and Pt catalysts supported on α-Al2O3 and ZrO2 applied in methane reforming with CO2

Ni and Pt catalysts supported on α-Al₂O_3, α-Al₂O₃-ZrO₂ and ZrO₂ were studied in the dry reforming of methane to produce synthesis gas. All catalytic systems presented well activity levels with TOF (s⁻¹) values between 1 and 3, being Ni based catalysts more active than Pt based catalysts. The selectivity measured at 650 °C, expressed by the molar ratio H₂/CO reached values near to 1. Concerning stability, Pt/ZrO₂, Pt/α-Al₂O₃-ZrO₂ and Ni/α-Al₂O₃-ZrO₂ systems clearly show lower deactivation levels than Ni/ZrO₂ and Ni or Pt catalysts supported on α-Al₂O₃. The lowest deactivation levels observed in Ni and Pt supported on α-Al₂O₃-ZrO₂, compared with Ni and Pt supported on α-Al₂O₃ can be explained by an inhibition of reactions leading to carbon deposition in systems having ZrO₂. These results suggest that ZrO₂ promotes the gasification of adsorbed intermediates, which are precursors of carbon formation and responsible for the main deactivation mechanism in dry reforming reaction.     

Nature and catalytic performance of PtSAPO-5 molecular sieve catalyst synthesized by one step

Pt catalysts supported on silicoaluminophosphate molecular sieves were synthesized by adding platinum salts directly to the gel mixture of SAPO. The synthesized products were marked as PtSAPO-5, in order to distinguish with the SAPO-5 supported platinum catalyst prepared by impregnation, which was marked as Pt/SAPO-5. The as-synthesized and calcined PtSAPO-5 samples were characterized by XRD,²⁹Si MAS NMR,¹²⁹XeNMR and NH₃-TPD, and their catalytic activities were evaluated by the hydroisomerization of n-hexane. The results revealed that in PtSAPO-5, the platinum could catalyze the removal of the templates entrapped in the as-synthesized samples. The presence of Pt in the gel mixture can increase silicon content in the framework of SAPO-5, and the acid sites. In the activated PtSAPO-5 catalyst, Pt was highly dispersed in the channel of SAPO-5 molecular sieve. Pt exists in the form of big particles outside of the channel of Pt/SAPO-5 prepared by impregnating method, and impregnation with H₂PtCl₆ improved the strength of acid sites. PtSAPO-5 has a higher activity and selectivity than Pt/SAPO-5 in the hydroisomerization of n-hexane. This is because the former has medium acid sites and high dispersion of Pt, while the channel of Pt/SAPO-5 is somewhat retarded by the big platinum particles, and the acid sites are stronger than the former.     

长链α-烯烃的硅氢加成反应研究

以聚三氟丙基乙烯基甲基硅氧烷为保护剂制备了102白色担体负载的铂络合物催化剂,用于催化长链α-烯烃与甲基二氯硅烷的硅氢加成反应。考察了铂催化剂用量、反应温度、不同长链烯烃原料、催化剂回收利用等因素对长链烷基甲基二氯硅烷收率的影响,结果表明,102白色担体负载的铂络合物催化剂对该硅氢加成反应具有较高的活性和稳定性,在常温下无需加热,无诱导期即可发生加成反应。在密闭体系中,1-十二烯烃与甲基二氯硅烷的摩尔比为1∶1.1,铂络合物与甲基二氯硅烷的摩尔比为6.5×10-5∶1时,十二烷基甲基二氯硅烷的收率可达88%;敞开体系中,相同条件下,十二烷基甲基二氯硅烷的收率可达77%。催化剂重复使用8次不失活。     

New organoplatinum (IV) complex with quaterpyridine ligand: Synthesis,structure and its catalytic activity in the hydrosilylation of styrene and terminal alkynes

The reaction of ligand L with PtCl₂ leads to a novel structural motif of octahedral ortho-metalated complex of formula [Pt(L_-H)Cl₃] 1. This result is a consequence of drastic conditions of reaction and preferred Pt(IV) octahedral coordination geometry. The new compound has been characterised on the basis of the spectroscopic data in solution, and its structure confirmed in the solid state by X-ray crystallography (1a—[Pt(L_-H)Cl₃]∙MeOH, 1b—[Pt(L_-H)Cl₃]∙C₆H₅CH₃). This article reports organoplatinum (IV) complex 1 as effective and highly selective catalyst precursor in the hydrosilylation of styrene and terminal alkynes.     

Morphology controlled 1D Pt nanostructures synthesized by galvanic displacement of Cu nanowires in chloroplatinic acid

One-dimensional platinum (Pt) nanostructures with different shape, size, and morphology were synthesized by galvanic displacement of sacrificial Cu nanowires in chloroplatinic acid baths. By increasing the concentration of H₂PtCl₆, the morphology of Pt nanostructures greatly altered from Pt nanoparticles decorated Cu nanowires, Pt coated Cu core-shell nanowires, dense Pt nanotubes to porous Pt nanotubes. At low concentration of H₂PtCl₆ (<1 mM), the Pt content monotonically increased with an increase in [H₂PtCl₆] (upto approx. 80 at.%). The Pt content became independent of [H₂PtCl₆] at higher concentration. The average external diameter and wall thickness of Pt nanotubes decreased with increasing [H₂PtCl₆] suggesting that the dissolution of copper is strongly dependent on [Cl⁻], which led to changes in the morphology of Pt nanostructures.     

Platinum—tin/alumina catalyst: Modification of the metallic phase after successive oxidation—reduction cycles

PtSn/Al₂O₃ catalysts, prepared by different deposition techniques and submitted to successive oxidation—reduction cycles at 500°C, were characterized by TPR and test reactions (cyclohexane dehydrogenation and cyclopentane hydrogenolysis), and compared with fresh samples. Interpretation of the results is based on the fact that in catalysts obtained by both coimpregnation and successive impregnation, a higher quantity of PtSn alloys was produced as a consequence of the oxidation—reduction cycles. In catalysts prepared by impregnating the support with the [PtCl₂(SnCl₃)₂]²⁻ complex, the cycles produced changes in the metallic phase probably due to either a surface enrichment of tin in alloy particles or a modification of the alloy phase composition.     

Facile synthesis of a carbon-rich SiAlCN precursor and investigation of its structural evolution during the polymer-ceramic conversion process

A liquid carbon-rich SiAlCN precursor is facilely synthetized by hydrosilylation between liquid polyaluminocarbosilane (LPACS) and 1,3,5,7-tetravinyl- 1,3,5,7-tetramethylcyclotetrasilazane {[CH₃(CH₂CH₂)SiNH]₄} (TeVSZ). The structural evolution during the polymer-ceramic conversion process is investigated by various methods. The results show that the main cured mechanism is β-addition on hydrosilylation, although α-addition on hydrosilylation, polymerization of vinyl groups and dehydrocoupling reaction between N–H bonds also occur during the cured process. During the pyrolysis process, dehydrogenation and dehydrocarbonation condensation reactions, transamination reactions occur, leading to formation of a three-dimensional network inorganic structure at 400–800 °C, where part of Al–O bonds convert to Al–N bonds. Then the network inorganic structure undergoes demixing and separation into amorphous SiAlCN(O) phase, where the amorphous turbostratic free carbon phase also form at 800–1200 °C. With demixing and decomposition of the amorphous carbon-rich SiAlCN(O) phase, the crystalline β-SiC and graphitic carbon start to form at about 1400 °C, the crystalline sizes of them both enlarge with increasing temperature. However, the crystal growth of β-SiC is distinctly inhibited due to existence of the rich carbon phase, tiny amounts of Al₂O₃ and AlN. In addition, a small amount of AlN can promote the formation of α-SiC at 1800 °C.     

Isomerization of n-heptane on Pt/MOR/Al2O3 catalysts

Pt/MOR/Al₂O₃ catalysts with mordenite zeolite contents of 10 to 50 wt % are prepared. Solutions of H₂PtCl₆ and [Pt(NH₃)₄]Cl₂ are used as precursors of Pt. It is shown by means of transmission electron microscopy (TEM) that the localization of platinum on a MOR/Al₂O₃ mixed support depends directly on the nature of the metal’s precursor. The catalysts are tested in the isomerization of n-heptane. It is shown that the best samples of catalysts provide yields of the target products (dimethyl and trimethyl substituted isomers of heptanes) on the level of 21 wt % at a temperature of 280°C, while those of a C₅₊ stable catalyzate are on the level of 79–82 wt %. The catalysts can be used to improve the environmental friendliness of gasolines by employing them in the isomerization of the 70–105°C fraction of directly distilled gasoline.     

Initial stages of Pt deposition on Au(111) and Au(100)

The deposition of Pt onto unreconstructed Au(111) and Au(100) was studied with cyclic voltammetry and in-situ STM. The latter revealed that in [PtCl₄]²⁻ containing electrolytes, both surfaces are covered by an ordered adlayer of the complex. For the adsorbed [PtCl₄]²⁻ a slightly compressed (√7×√7) R19.1°-structure was assumed for Au(111) and a (3×√10) for Au(100). In both cases, a rather high overpotential for Pt deposition was observed, most probably due to the high stability of the [PtCl₄]²⁻ complex. Nucleation of Pt starts mainly at defects like step edges for low deposition rates and three-dimensional clusters are formed. Due to the high overpotential, some nuclei appear also on terraces at random sites. Higher coverages of Pt lead to a cauliflower like appearance. It is not possible to dissolve the platinum clusters at positive potentials without severely roughening the gold surface. The [PtCl₄]²⁻ complex is oxidized to the [PtCl₆]²⁻ complex at about 0.7 V, when metallic Pt is on the surface.