Mesoporous Pt-SiO2 and Pt-SiO2-Ta2O5 catalysts prepared using Pt colloids as templates.

Sol‐gel synthesis of silica and silica–tantalum oxide embedded platinum nanoparticles is carried out using Pt colloids as templates. These colloids are prepared by reduction with Na[AlEt₃H] and stabilized with different ligands (ammonium halide derivatives, non‐ionic surfactants with polyether chains, and 2‐hydroxy‐propionic acid). The aim of the present study is to prepare mesoporous silica embedded Pt colloids combining the “precursor concept” with the model of catalyst preparation using preformed spheres. Nanoparticles of Pt incorporated in high surface area mesoporous materials are formed after calcination. Further, it is observed that calcination of these catalysts causes partial aggregation and oxidation of the parent colloids, a process that is largely dependent on the nature of the stabilizing ligands. Several methods have been used for characterization of these materials: adsorption‐desorption isotherms at 77 K, H₂ chemisorption, X‐ray diffraction(XRD), ²⁹Si and ¹³C magic angle spinning (MAS) NMR, ammonia diffuse reflectance Fourier transform infrared spectroscopy (NH₃‐DRIFT), transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS). It is found that both metal oxide systems exhibit Brønsted acidity (weaker for silica and quite strong for silica–tantalum oxide). In addition, NH₃‐DRIFT experiments demonstrate the oxidative properties of the surface. Part of the adsorbed NH₄⁺ species is oxidized to N₂O. Testing these catalysts in the reduction of NO and NO₂ with isopentane under lean conditions indicate that the activity of these catalysts is indeed dependent on the size of the platinum particles, with those of size 8–10 nm demonstrating the best results. The support likely contributes to this effect, particularly after Ta incorporation into silica.     

Water Adsorption on Platinum Dioxide and Dioxygen Complex: Matrix Isolation Infrared Spectroscopic and Theoretical Study of Three PtO2–H2O Complexes

The interactions of water molecule with platinum dioxygen complex and dioxide molecule are investigated by means of matrix isolation infrared spectroscopy and density functional calculations. The platinum atoms reacted with dioxygen to form the previously reported Pt(O₂) complex. The Pt(O₂) complex reacted with water molecule to give the Pt(O₂)–H₂O complex, which was characterized to involve hydrogen bonding between one O atom of Pt(O₂) and one H atom of H₂O (structure A ). Upon visible light irradiation, the hydrogen bonded Pt(O₂)???HOH complex rearranged to another Pt(O₂)–H₂O isomer (structure B ), which involves (O₂)Pt???OH₂ interaction. The Pt(O₂)–H₂O complex in structure B can be isomerized to the weakly bound platinum dioxide‐water complex (structure C ) under UV irradiation.     

Effect of solvent on the exchange of ethylene for propylene on cis-PtCl2(C3H6)(TPPTS), TPPTS = P(m-C6H4SO3Na)3

To further understand the effect of water as a solvent in organometallic reactions, the lability of η²-alkenes coordinated to platinum(II) phosphine complexes has been studied in water and chloroform as a comparison of solvent effects on the exchange kinetics and alkene complex stability. ¹H NMR techniques with both deuterated chloroform and a deuterium oxide/deuterated methanol mixture as solvent systems were used at temperatures as low as ?50°C. Reaction of cis-PtCl₂L(η²-C₃H₆) [L?=?PPh₃ (triphenylphosphine) (1a), TPPTS (tris(m-sulfonatophenyl)phosphine) 1b] with ethylene to form cis-PtCl₂L(η²-C₂H₄) (2?a, b) was observed with dependence on the rate by starting platinum complex and ethylene. The role of water on this reaction, as well as its effect on the equilibrium, will be discussed. The equilibrium constant shows preference for coordination of ethylene and the temperature dependence indicates the reaction is entropy controlled.     

Confined Ultrathin Pd‐Ce Nanowires with Outstanding Moisture and SO2 Tolerance in Methane Combustion

An efficient strategy (enhanced metal oxide interaction and core–shell confinement to inhibit the sintering of noble metal) is presented confined ultrathin Pd‐CeO_x nanowire (2.4 nm) catalysts for methane combustion, which enable CH₄ total oxidation at a low temperature of 350 °C, much lower than that of a commercial Pd/Al₂O₃ catalyst (425 °C). Importantly, unexpected stability was observed even under harsh conditions (800 °C, water vapor, and SO₂), owing to the confinement and shielding effect of the porous silica shell together with the promotion of CeO₂. Pd‐CeO_x solid solution nanowires (Pd‐Ce NW) as cores and porous silica as shells (Pd‐CeNW@SiO₂) were rationally prepared by a facile and direct self‐assembly strategy for the first time. This strategy is expected to inspire more active and stable catalysts for use under severe conditions (vehicle emissions control, reforming, and water–gas shift reaction).     

Synthesis of Si-MCM-41 from ternary SiO<Subscript>2</Subscript>–CTAOH–H<Subscript>2</Subscript>O system via dry gel conversion route

Steam assisted dry gel conversion method was employed for the synthesis of Si-MCM-41 from ternary SiO₂:CTAOH:H₂O systems wherein fumed silica was used as a source of silica. The influence of synthesis time, molar ratios of CTAOH/SiO₂ in dry gel and the water content at the bottom of autoclave on the quality and formation of mesophases has been investigated. Powder XRD, N₂ adsorption–desorption, TEM and hydrothermal stability test were the techniques used for sample characterization. Keeping molar ratio of CTAOH/SiO₂ constant, shorter synthesis time, lower unit cell parameter and d spacing were observed when steam assisted dry gel conversion method was employed in place of conventional hydrothermal method. There exists an optimum lower limit for water content at the bottom of autoclave for reducing the synthesis period. Keeping synthesis temperature and CTAOH/SiO₂ molar ratio fixed, Si-MCM-41 with improved hydrothermal stability was obtained by steam assisted dry gel conversion route.     

The state of nickel in the silver modified NiMg/SiO<Subscript>2</Subscript> vegetable oil hydrogenation catalysts

Two series of silver modified Ni-Mg materials were synthesized by precipitation-deposition on SiO₂ support derived from two silica sources: diatomite activated at 800°C (Series a; Mg/Ni = 0.1 and SiO₂/Ni = 1.07) and synthetic water glass (Series b; Mg/Ni = 0.1 and SiO₂/Ni = 1.15). The modification with silver was made at three molar Ag/Ni ratios, namely 0.0025, 0.025, and 0.1. The effects of the source of the silica support and the silver presence and content on the nickel state in the silver modified reduced-passivated NiMg/SiO₂ precursors of the vegetable oil hydrogenation catalyst were established by X-ray diffraction and X-ray photoelectron spectroscopy techniques. The passivation procedure was applied in order to protect the metallic nickel particles from further oxidation. The crystallization of the formed nickel hydrosilicate phases depends on the source of the silica support, more expressed in the diatomite supported samples. It was shown that the silver modification of the NiMg/SiO₂ precursors enhances the reduction of the nickel hydrosilicates accompanied by formation of relatively smaller metallic nickel particles, more pronounced in the water glass supported precursors. The increase of the silver content in the water glass deposited samples is responsible for the metallic nickel dispersion increase. The higher content of the Ni⁰ particles on the surface of the diatomite deposited samples is in accordance with the higher stability of the larger metallic nickel crystallites to oxidation during the passivation step. On contrary, higher dispersed Ni⁰ particles on the surface of the water glass supported samples are more susceptible to the oxida The article is published in the original.     

Characterization of Fumed Alumina/Silica/Titania in the Gas Phase and in Aqueous Suspension

Fumed oxide alumina/silica/titania was studied in comparison with fumed alumina, silica, titania, alumina/silica, and titania/silica by means of XRD, ¹H NMR, IR, optical, dielectric relaxation, and photon correlation spectroscopies, electrophoresis, and quantum chemical methods. The explored Al₂O₃/SiO₂/TiO₂ consists of amorphous alumina (22 wt%), amorphous silica (28 wt%), and crystalline titania (50 wt%, with a blend of anatase (88%) and rutile (12%)) and has a wide assortment of Brønsted and Lewis acid sites, which provide a greater acidity than that of individual fumed alumina, silica, or titania and an acidity close to that of fumed alumina/silica or titania/silica. The changes in the Gibbs free energy (ΔG) of interfacial water in an aqueous suspension of Al₂O₃/SiO₂/TiO₂ are close to the ΔG values of the dispersions of pure rutile but markedly lower than those of alumina, anatase, or rutile covered by alumina and silica. The zeta potential of Al₂O₃/SiO₂/TiO₂ (pH of the isoelectric point (IEP) equals ≈3.3) is akin to that of fumed titania (pH(IEP_TiO2) ≈ 6) at pH > 6, but it significantly differs from the ζ of fumed alumina (pH(IEP_Al2O3) ≈ 9.8) at any pH value as well as those of fumed silica, titania/silica, and alumina/silica at pH < 6. The particle size distribution in the diluted aqueous suspensions of Al₂O₃/SiO₂/TiO₂ studied by means of photon correlation spectroscopy depends relatively slightly on pH in contrast to the titania/silica or alumina/silica dispersions. Theoretical calculations of oxide cluster interaction with water show a high probability of hydrolysis of Al–O–Ti and Si–O–Ti bonds strained at the interface of alumina/titania or silica/titania due to structural differences in the lattices of the corresponding individual oxides. Ab initio calculated chemical shift δ_H values of H atoms in different hydroxyl groups at the oxide clusters and in bound water molecules are in agreement with the ¹H NMR data and show a significant impact of charged particles (H₃O⁺ or OH⁻) on the average δ_H values of water droplets with (H₂O)_n at n between 2 and 48.     

Dynamics and isotherms of water vapor sorption on mesoporous silica gels modified by different salts

The mesoporous silica gels impregnated with different metal salts were prepared and studied. The pore structure and specific surface area of adsorbents were evaluated using nitrogen adsorption. Then, the sorption isotherms and dynamics of water vapor were carried out at 303 K and different relative humidity (RH). The temperature programmed desorption experiments were conducted to estimate the activation energy (E _d) of water desorption on the silica gels. The results showed that the sorption capacity for water decreased with the increase of the ionic radius (except the calcium ion) and that CaCl₂ and LiCl were particularly suitable for use in modification of the mesoporous silica gel to improve their sorption rates and capacities for water vapor at the lower and medium RH (RH < 80%). The larger the average pore diameter and pore volume of the initial silica gels, higher the accrual rates of the water vapor sorption rate and capacity were after modification with hygroscopic salts. The activation energy of the water desorption on the mesoporous silica gel modified by CaCl₂ were much higher than that on the silica gel modified by LiCl, because the polarizability of the Ca²⁺ was higher than that of Li⁺.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 Solution

The oxidation of polycrystalline platinum in 0.5 M H₂SO₄ is studied by cyclic voltammetry at potential scan rates of 5–500 mV s^–1 while varying the potential cycling range. The scheme, which is proposed for explaining the observed acceleration and deceleration of oxygen sorption at 0.75–1.0 V, accounts for the presence of oxygen in the subsurface layers of platinum (O_ss) and the formation of a barrier layer comprising complexes O_ss–Pt _n –SO₄. Cycling platinum secures certain steady-state contents of O_ss at 0.01–1.35 V. In an anodic scan, O_ss accumulates at E > 0.85 V (slow post-electrochemical stage) due to exchange of platinum and oxygen atom sites. In a cathodic scan, the desorption of most oxygen gives way to the adsorption of anions, which prevent residual O_ss from appearing on the surface. The residual O_ss disappears at E < 0.1 V after a sufficiently complete desorption of anions and the destruction of stable complexes O_ss–Pt _n –SO₄. Varying the potential cyclic limit leads, after a delay, to other steady-state O_ss contents.     

Characterization of Titania/Silica Gel by Means of Low-Pressure Nitrogen Adsorption

Adsorbents synthesized by grafting of titania onto mesoporous silica gel surfaces at different temperatures were studied by means of nitrogen adsorption–desorption and water desorption. The pore size distribution f(R_p) of titania/silica gel depends on the titania concentration (C_TiO2) and the temperature of titania synthesis. Nonuniformity of TiO₂ phase is maximal at a low C_TiO2 value (3.2 wt.% anatase deposited at 473 K), and two peaks of the fractal dimension distribution f(D) are observed at such a concentration of titania, but at larger C_TiO2 values, only one f(D) peak is seen. More ordered filling of pores and adsorption sites by nitrogen, reflecting in the shape of adsorption energy distributions f(E) at different pressures of adsorbate, is observed for adsorbent with titania (rutile+anatase) grafted on silica gel at a higher temperature (673 K).     

Mass spectrometry and UV‐VIS spectrophotometry of ruthenium(II) [RuClCp(mPTA)2](OSO2CF3)2 complex in solution

Ruthenium(II) complexes are of great interest as a new class of cancerostatics with advantages over classical platinum compounds including lower toxicity. The stability of the [RuClCp(mPTA)₂](OSO₂CF₃)₂ complex (I) (Cp cyclopentadienyl, mPTA N‐methyl 1,3,5‐triaza‐7‐phosphaadamantane) in aqueous solution was studied using spectrophotometry, matrix‐assisted laser desorption/ionization (MALDI) and laser desorption/ionization (LDI) time‐of‐flight (TOF) mass spectrometry (MS). Spectrophotometry proves that at least three different reactions take place in water. Dissolution of I leads to fast coordination of water molecules to the Ru(II) cation and then slow hydrolysis and ligand exchange of chloride and mPTA with water, hydroxide or with trifluoromethane sulfonate itself. Via MALDI and LDI of the hydrolyzed solutions the formation of singly positively charged ions of general formula RuCl_p(Cp)_q(mPTA)_r(H₂O)_s(OH)_t (p = 0–1, q = 0–1, r = 0–2, s = 0–5, t = 0–2) and of some fragment ions was shown. The stoichiometry was determined by analyzing the isotopic envelopes and computer modelling. The [RuClCp(mPTA)₂](OSO₂CF₃)₂ complex can be stabilized in dilute hydrochloric acid or in neutral 0.15 M isotonic sodium chloride solution. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxidizing Elemental Platinum with Oleum under Harsh Conditions: The Unique Tris(disulfato)platinate(IV) [Pt(S2O7)3]2− Anion

For the first time, direct oxidation of elemental platinum by a mineral acid to its tetravalent state was observed in course of the reaction of platinum with oleum (65 % SO₃) in the presence of barium carbonate. The reaction has been carried out in torch‐sealed glass ampoules at 160 °C and gave yellow single crystals of Ba[Pt(S₂O₇)₃](H₂SO₄)_0.5(H₂S₂O₇)_0.5 (triclinic, P$\bar 1$ , Z=2, a=992.05(2), b=1069.07(3), c=1114.22(3) pm, α=69.49(7), β=72.96(2), γ=72.93(1)°, V=1033.95(5) ų). The structure of Ba[Pt(S₂O₇)₃](H₂SO₄)_0.5(H₂S₂O₇)_0.5 exhibits the unique tris‐(disulfato)‐platinate anion [Pt(S₂O₇)₃]^2? with three chelating disulfate groups coordinated to the platinum atom. Charge balance is achieved by the Ba²⁺ ions, which are coordinated by (S₂O₇)^2? groups from the platinate complex and by disordered sulfuric acids and disulfuric acid molecules. Thermal decomposition of the bulk material revealed elemental platinum and barium sulfate as decomposition residual.     

Temperature dependent synthesis of micro- and meso-porous silica employing the thermo-responsive polymer of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) as structure-directing agent

Temperature dependent synthesis of micro- and meso-porous silica employing the thermo-responsive homopolymer poly(N-isopropylacrylamide) or the random copolymer poly(N-isopropylacrylamide-co-acrylic acid) as structure-directing agent (SDA) and Na₂SiO₃ as silica source is proposed. The thermo-responsive character of the SDA provides the advantages including (1) temperature dependent synthesis of microporous silica, hierarchically micro-mesoporous silica, and mesoporous silica just by changing the aging temperature below or above the low critical solution temperature of the thermo-responsive SDA, and (2) elimination of the thermo-responsive SDA from silica matrix by water extraction. The synthesis mechanism is discussed, and the effect of the aging temperature and the weight radio of SDA/Na₂SiO₃ on the synthesis of micro- and meso-porous silica are studied. Microporous silica, hierarchically micro-mesoporous silica and mesoporous silica with the surface area at 3.5−9.0 × 10² m²/g and the pore volume at 0.28−1.13 cm³/g and the average pore size ranging from 1.1 to 9.0 nm are synthesized. The strategy affords a new and environmentally benign way to fabricate porous silica materials, and is believed to bridge the gap between the synthesis of microporous and mesoporous silica materials.     

On the Reactivity of the Platina‐β‐diketone [Pt2{(COMe)2H}2(μ‐Cl)2] Towards Ph2PCH2CH2CH2SOxPh (x = 0, 2)

The reaction of the electronically unsaturated platina‐β‐diketone [Pt₂{(COMe)₂H}₂(μ‐Cl)₂] ( 1 ) with Ph₂PCH₂CH₂CH₂SPh ( 2 ) leads selectively to the formation of the acetyl(chlorido) platinum(II) complex (SP‐4‐3)‐[Pt(COMe)Cl(Ph₂PCH₂CH₂CH₂SPh‐κP,κS)] ( 4 ) having the γ‐phosphinofunctionalized propyl phenyl sulfide coordinated in a bidentate fashion (κP,κS). In boiling benzene complex 4 undergoes decarbonylation yielding the methyl(chlorido) platinum(II) complex (SP‐4‐3)‐[PtMeCl(Ph₂PCH₂CH₂CH₂SPh‐κP,κS)] ( 6 ). However, the reaction of 1 with the analogous γ‐diphenylphosphinofunctionalized propyl phenyl sulfone Ph₂PCH₂CH₂CH₂SO₂Ph ( 3 ) affords the acetyl(chlorido) platinum(II) complex (SP‐4‐4)‐[Pt(COMe)Cl(Ph₂PCH₂CH₂CH₂SO₂Ph‐κP)₂] ( 5 ). In boiling benzene complex 5 undergoes a CO extrusion yielding (SP‐4‐4)‐[PtMeCl(Ph₂PCH₂CH₂CH₂SO₂Ph‐κP)₂] ( 8 ) whereas in presence of 1 the formation of the carbonyl complex (SP‐4‐3)‐[PtMeCl(CO)(Ph₂PCH₂CH₂CH₂SO₂Ph‐κP)] ( 7 ) is observed. Addition of Ag[BF₄] to complex 5 leads to the formation of the cationic methyl(carbonyl) platinum(II) complex (SP‐4‐1)‐[PtMe(CO)(Ph₂PCH₂CH₂CH₂SO₂Ph‐κP)₂][BF₄] ( 9 ). All complexes were characterized by microanalysis and NMR spectroscopy (¹H, ¹³C, ³¹P) and complexes 4 and 6 additionally by single‐crystal X‐ray diffraction analyses.     

1H NMR in CaCl2·xH2O nanocrystals and water sorption isobars in a CaCl2-silica gel system

Mobility of water molecules in nanocrystals and other phases of CaCl₂·xH₂O (x=0–12) formed in the nanopores of silica gel during sorption of water vapor is studied by¹H NMR. The sorption properties of calcium chloride impregnated in the pores of micro- and mesoporous silica gels were measured. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. G. K. Boreskov Institute of Catalysis, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 2, pp. 261–266, March–April, 1998. This work was supported by RFFR grants No. 96-03-33069 and 97-03-33533 and NATO grant HTECH LG N 970330.     

Adsorption and desorption of toluene on nanoporous TiO2/SiO2 prepared by atomic layer deposition (ALD): influence of TiO2 thin film thickness and humidity

Adsorption and desorption of toluene on bare and TiO₂-coated silica with a mean pore size of 15 nm under dry and humid conditions were studied using toluene breakthrough curves and temperature programmed desorption (TPD) of toluene and CO₂. Two TiO₂/silica samples (either partially or fully covered with TiO₂) were prepared with 50 and 200 cycles of TiO₂ atomic layer deposition (ALD), respectively. The capacity of silica to adsorb toluene improved significantly with TiO₂-thin film coating under dry conditions. However, toluene desorption from the surface due to displacement by water was more pronounced for TiO₂-coated samples than bare samples under humid conditions. In TPD experiments, silica with a thinner TiO₂ film (50-ALD cycled) had the highest reactivity for toluene oxidation to CO₂ both in the absence and presence of water. Toluene adsorption and oxidation reactivity of silica can be controlled by modifying the silica surface with small amount of TiO₂ using ALD.     

Raman study of structural defects in SiO2 aerogels

The structure of the silica aerogels was studied by Raman spectroscopy. The spectra of the solid network resembles that of bulk silica with additional bands related to organic groups and a large amount of OH groups.The typical bands due to ring breathing also called defect bands D ₁ and D ₂ located at 490 and 610 cm^–1 are present. However, the evolution of the D ₂ band compared to that of OH band (980 cm^–1) seems apparently, in contradiction with the results previously reported in the literature. During heat treatments between 25 and 300°C the D ₂ and the OH bands increase simultaneously. Generally, in silica glass the defect band D ₂ grows at the expense of the OH groups.This result is explained by the oxidation of the organic compounds which, in this temperature range, leads to the formation of the both species (OH) and those related to siloxane rings. ²⁹Si MAS NMR results are in agreement with the Raman study.     

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.     

Isotope exchange between NO and H2O on a platinum-containing catalyst based on fiberglass

The dynamics of ¹⁸O isotope exchange between NO or H₂O and a catalyst and the dynamics of ¹⁸O label transfer from NO to H₂O have been studied under conditions of sorption-desorption equilibrium. The occurrence of a reaction of oxygen exchange between NO and water sorbed in the bulk of the catalyst was detected. This reaction occurs at platinum sites with the participation of acid sites of the glass matrix. The rate constants of the reaction of NO with platinum sites and the diffusion coefficients of water in the bulk of the glass matrix are evaluated.     

Crystal Structures of (PPh3)2Pd(N3)2, (AsPh3)2Pd(N3)2, (2‐Chloropyridine)2Pd(N3)2, [(AsPh4)2][Pd2(N3)4Cl2], [(PNP)2][Pd(N3)4], [(AsPh4)2][Pt(N3)4] · 2 H2O,and [(AsPh4)2][Pt(N3)6]

The crystal structures of the monomeric palladium(II) azide complexes of the type L₂Pd(N₃)₂ (L = PPh₃ ( 1 ), AsPh₃ ( 2 ), and 2‐chloropyridine ( 3 )), the dimeric [(AsPh₄)₂][Pd₂(N₃)₄Cl₂] ( 4 ), the homoleptic azido palladate [(PNP)₂][Pd(N₃)₄] ( 5 ) and the homoleptic azido platinates [(AsPh₄)₂][Pt(N₃)₄] · 2 H₂O ( 6 ) and [(AsPh₄)₂][Pt(N₃)₆] ( 7 ) were determined by X‐ray diffraction at single crystals. 1 and 2 are isotypic and crystallize in the triclinic space group P1. 1 , 2 and 3 show terminal azide ligands in trans position. In 4 the [Pd₂(N₃)₄Cl₂]^2– anions show end‐on bridging azide groups as well as terminal chlorine atoms and azide ligands. The anions in 5 and 6 show azide ligands in equal positions with almost local C_4h symmetry at the platinum and palladium atom respectively. The metal atoms show a planar surrounding. The [Pt(N₃)₆]^2– anions in 7 are centrosymmetric (idealized S₆ symmetry) with an octahedral surrounding of six nitrogen atoms at the platinum centers.