CO adsorption and oxidation at the catalyst-water interface: an investigation by attenuated total reflection infrared spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases.     

Solvation‐Induced Changes in the Mechanism of Alcohol Oxidation at Gold/Titania Nanocatalysts in the Aqueous Phase versus Gas Phase

Gold/titania catalysts are widely used for key reactions, notably including the selective oxidation of alcohols in the liquid phase. Our large‐scale ab initio simulations disclose that the liquid‐phase reaction mechanism is distinctly different from that in the gas phase because of active participation of water molecules. While concerted charge transfers related to O₂ splitting and abstraction of both protonic and hydridic hydrogens are enforced under dry conditions, stepwise charge transfer is preferred in the condensed phase. Dissociation of reactive water molecules and subsequent Grotthuss migration of protonic defects, H⁺(aq), allows for such a decoupling of the oxidation process, both in time and space. It is expected that these observations are paradigmatic for heterogeneous catalysis in aqueous phases.     

THE PHOTOCHEMISTRY OF PIPERAZINE-2,5-DIONE IN AQUEOUS SOLUTION

Abstract The aqueous photochemistry of deoxygenated solutions of piperazine-2,5-dione was investigated and three products were isolated in low yield using preparative high-performance liquid chromatography: 3,3'-bis(piperazine-2,5-dione) ( 1 ), oxamide and N-methyloxamide. The structure of 1 was established by infrared, mass and high-field proton magnetic resonance spectroscopy, and by the synthesis of its meso diastereoisomer. A 1:2 mixture of meso and racemic diastereoisomers of 1 was formed in good yield in thermally or photochemically initiated reactions of piperazine-2,5-dione with t -butyl peroxide in aqueous t -butanol.
The rate of photolysis of aqueous piperazine-2,5-dione was markedly enhanced in the presence of iron(II) ions, copper(II) ions or powdered titanium dioxide. A diastereomeric mixture of 1, oxamide and N-methyloxamide were isolated from the reaction with iron(II) ions. The possibility that oxamides were formed in these reactions via reaction with atmospheric oxygen was eliminated in a series of control experiments.
The results suggest α-carbon radicals are important in the photolysis of peptides in fluid aqueous media, possibly as a consequence of photoinduced electron transfer. No significant adduct formation could be detected when mixtures of piperazine-2,5-dione and uracil or thymine were irradiated in frozen aqueous solution.     

Interfacial organic synthesis in a simple droplet-based microfluidic system

A spherical liquid-liquid interface can be obtained by dispersing one liquid phase into another to form droplets, which will facilitate the two-phase reactions between the immiscible participating fluids. The phase transfer catalysts assembled at the droplet "wall" catalyze the reactions between the aqueous and organic phases. The study illustrates an interfacial synthetic approach which is ideal for the biphasic reaction by taking advantage of the droplet-based microdevice. The improved reaction efficiency can be attributed to the high surface-to-volume ratio and internal flow circulation in the droplets.     

尿嘧啶水助质子转移反应机理的研究

用密度泛函理论,在B3LYP/6-311++G**计算水平下分别对尿嘧啶所有的气相、液相、过渡态和质子转移异构体的结构进行全优化,获得它们在气相和水相中的几何结构和电子结构,PGM反应场溶剂模型用于水相计算.结果显示:在气相和水相中,水参与反应降低了互变异构质子迁移的反应活化能,对互变异构质子迁移的反应起到催化作用,但...     

碳纳米复合物作为催化剂和乳化剂用于水/有机两相体系反应(英文)

This mini-review summarizes some novel aspects of reactions conducted in aqueous/organic emulsions stabilized by carbon nanohybrids functionalized with catalytic species. Carbon nanohybrids represent a family of solid catalysts that not only can stabilize water-oil emulsions in the same fashion as Pickering emulsions, but also catalyze reactions at the liquid/liquid interface. Several exam-ples are discussed in this mini-review. They include (a) aldol condensation-hydrodeoxygenation tandem reactions catalyzed by basic (MgO) and metal (Pd) catalysts, respectively; (b) Fischer-Tropsch synthesis catalyzed by carbon-nanotube-supported Ru; and (c) emulsion polymerization of styrene for the production of conductive polymer composites. Conducting these reactions in emul-sion generates important advantages, such as increased liquid/liquid interfacial area that consequently means faster mass transfer rates of molecules between the two phases, effective separation of products from the reaction mixture by differences in the water-oil solubility, and significant changes in product selectivity that can be adjusted by modifying the emulsion characteristics.     

别嘌醇质子迁移过程的理论研究

别嘌醇(Allopurinol)是次黄嘌呤的位置异构体,是唯一在临床上应用的黄嘌呤氧化酶抑制剂.     

A method for measuring infinite-dilution partition coefficients of volatile compounds between the gas and liquid phases of aqueous systems

Vejrosta, J., Novák, J. and Jönsson, J.Å., 1982. A method for measuring infinite-dilution partition coefficients of volatile compounds between the gas and liquid phases of aqueous systems. Fluid Phase Equilibria, 8: 25–35.A method has been developed for measuring the partition coefficients of volatile compounds between the gas and liquid phases of aqueous systems, based on the direct analysis of both phases. A gas mixture containing a known proportion of a volatile compound is drawn through the liquid (water) until equilibrium is established. A defined volume of the liquid phase is then withdrawn through a porous-polymer trap while maintaining the system at equilibrium. The residual water in the trap is then expelled by a stream of nitrogen gas, and the deposit remaining is thermally desorbed and analyzed by gas chromatography. This approach, together with an experimental technique for producing gas mixtures containing an accurately known concentration of hydrocarbon at low values, makes it possible to determine accurately the partition coefficients of low-solubility compounds, such as for hydrocarbons in aqueous systems, at very low solute concentrations in the system. The method has been verified by measuring the partition coefficient of hexane between the gas and liquid phases of an aqueous system at various concentrations and temperatures.     

Heterogeneous catalysis on solids of gases diffusing through a liquid layer,studied by inverse gas chromatography

Physicochemical parameters for heterogeneous catalytic reactions when the catalytic bed was under a liquid phase have been determined, using a non-linear adsorption isotherm by the reversed-flow version of inverse gas chromatography (RF-GC). The mathematical analysis developed in heterogeneous catalysis, mass transfer across gas-liquid boundaries, and diffusion coefficients of gases in liquids was associated with a non-linear adsorption isotherm to find the relevant equations pertaining to the problem. These equations were then used to calculate the adsorption/desorption rate constant, the rate constant for the first-order catalytic reaction and the equilibrium constant for the non-linear adsorption isotherm. The diffusion coefficients of the reactant in the liquid and gaseous phases and the partition coefficients for the distribution of the reactant between the gaseous and liquid phase were also determined.     

密度泛函研究不同价态金属离子与谷胱甘肽相互作用

采用密度泛函DFT/B3LYP方法,研究了在气相和生物环境内稳定存在的2种构型的还原型谷胱甘肽（GSH）与不同价态金属铬离子(Cr2+,Cr3+,Cr6+)相互作用。 金属离子的电荷越高、半径越小,与GSH结合能越大,使GSH的变形程度也越大。 金属Cr6+在气相和液相条件与GSH作用均促使了GSH的骨架断裂,末端羧基发生脱羧。 Cr3+和Cr2+与气相中性和液相两性离子结构的GSH分子相互作用均形成了9种稳定的复合物,与气相计算结果相比,考虑溶剂化效应之后,金属离子与GSH两性离子作用的结合能要比与在气相条件下中性的GSH相互作用能大大降低。     

Surface-directed boundary flow in microfluidic channels

Channel geometry combined with surface chemistry enables a stable liquid boundary flow to be attained along the surfaces of a 12 microm diameter hydrophilic glass fiber in a closed semi-elliptical channel. Surface free energies and triangular corners formed by PDMS/glass fiber or OTS/glass fiber surfaces are shown to be responsible for the experimentally observed wetting phenomena and formation of liquid boundary layers that are 20-50 microm wide and 12 microm high. Viewing this stream through a 20 microm slit results in a virtual optical window with a 5 pL liquid volume suitable for cell counting and pathogen detection. The geometry that leads to the boundary layer is a closed channel that forms triangular corners where glass fiber and the OTS coated glass slide or PDMS touch. The contact angles and surfaces direct positioning of the fluid next to the fiber. Preferential wetting of corner regions initiates the boundary flow, while the elliptical cross-section of the channel stabilizes the microfluidic flow. The Young-Laplace equation, solved using fluid dynamic simulation software, shows contact angles that exceed 105 degrees will direct the aqueous fluid to a boundary layer next to a hydrophilic fiber with a contact angle of 5 degrees. We believe this is the first time that an explanation has been offered for the case of a boundary layer formation in a closed channel directed by a triangular geometry with two hydrophobic wetting edges adjacent to a hydrophilic surface.     

Liquid crystal polymers. XVIII. Comparison of the properties of aromatic polyesters with different mesogenic units and a common flexible spacer

Four closely related polyesters, with aromatic ester triads and decamethylene spacers, were prepared and characterized for their thermotropic liquid crystalline properties. Two of the polymers, which were based on linear biphenols, formed stable nematic phases, whereas one of the two based on nonlinear biphenols formed a poorly defined nematic phase; the other was not liquid crystalline. Several polymerization reactions and methods were evaluated and structure-property relationships of the polymers are discussed. The concept of “degree of liquid crystallinity” for polymers with poorly defined thermotropic behavior is considered.     

Surfactant-mediated water transport at gelatin gel/oil interfaces

We studied spontaneous emulsification (SE) at Water/Oil (W/O) interfaces, using several types of aqueous reservoirs immersed in dodecane plus Span80 surfactant. Above a threshold surfactant concentration C(SE), aqueous satellite droplets are formed at the W/O interface. Varying the aqueous reservoir size, from below 100 microm (droplets) to centimeters (macroscopic phases), allowed investigating SE with complementary techniques. Release (rates) and size distributions for SE droplets were measured with microscopy. For gelled aqueous phases, water expulsion due to SE was quantified. Values for C(SE) were measured and were found to be higher for aqueous phases containing gelatin and/or NaCl. We also studied water exudation during network building and syneresis in aqueous gelatin gels immersed in dodecane/Span80. Below C(SE) (i.e., in the absence of SE) this process is still responsible for significant physico-chemical changes at the W/O interface. To study these in more detail, we performed atomic force microscopy experiments (in force-distance mode) on macroscopic gels. Both changes in the local elastic response and in the wettability of the AFM tip were detected. Together they suggest the formation of "water pockets" after prolonged (gel) setting times, along with a densification of the interfacial gelatin network.     

Uptake and UV-photooxidation of gas-phase PAHs on the surface of atmospheric water films. 1. Naphthalene

The adsorption and photochemical reaction of naphthalene vapor at the air-water interface of water films (22 microm and 450 microm) were studied in a horizontal flow reactor. Experiments were conducted in the regime where gas-phase mass transfer resistance did not limit the uptake. The equilibrium uptake was dependent on water film thickness only below 1 microm. Bulk water-air and air-to-interface partition constants were estimated from the experiments. The equilibrium partition constant between the water film and air decreased with increasing temperature. Photochemical reaction products were isolated in the water film after exposure to UV light. Four main oxygenated products were identified (1,3-indandione, 1(3H)-isobenzofuranone (phthalide), 2H-1-benzopyran-2-one (coumarin), and 1-naphthol). The initial rates of product formation were 46 to 154% larger for the thin film (22 microm) compared to both a thick film (450 microm) and bulk aqueous phase photooxidation. The atmospheric implications of reactions in water films are discussed.     

Gas permeability of combined membrane systems with mobile liquid carrier

Gas transfer in a membrane system called a selective membrane valve (SMV) is studied. The SMV is a system consisting of two mobile gas phases, one mobile liquid phase, and two membranes acting as interfaces between gas and liquid. Such a membrane system has supplementary variable parameters and is designated for the separation of multicomponent gas mixtures. System permeability for individual gases (CO₂, O₂, and H₂) and its dependence on a flow rate of a liquid phase are studied. Time dependences of the non-steady state transfer of CO₂ through the immobile layer of chemisorbent (aqueous K₂CO₃ solution) at its different concentrations are studied for the first time. Two theoretical models are developed: the model of gas transfer through a selective membrane valve system with a mobile liquid absorbent (in the absence of chemical interaction) and the model of a non-steady-state transfer of CO₂ through the immobile layer of aqueous potassium carbonate solution. The first model makes it possible to determine gas-to-liquid diffusion coefficients; the second model permits us to plot kinetic permeability curves and to calculate system permeability with allowance for the CO₂ transfer accompanied by reversible chemical reaction with the carrier. The model dependences agree well with the experimental data.     

Minimum energy pathways for proton transfer between adjacent sites exposed to water

The capacity to transfer protons between surface groups is an innate property of many proteins. The transfer of a proton between donor and acceptor, located as far as 6-7 A apart, necessitates the participation of water molecules in the process. In a previous study we investigated the mechanism of proton transfer (PT) between bulk exposed sites, a few ?ngstr?ms apart, using as a model the proton exchange between the proton-binding sites of the fluorescein molecule in dilute aqueous solution.1 The present study expands the understanding of PT reactions between adjacent sites exposed to water through the calculation the minimum energy pathways (MEPs) by the conjugate peak refinement algorithm2 and a quantum-mechanical potential. The PT reaction trajectories were calculated for the fluorescein system with an increasing number of water molecules. The MEP calculations reveal that the transition state is highly strained and involves a supramolecular structure in which fluorescein and the interconnecting water molecules are covalently bonded together and the protons are shared between neighboring oxygens. These findings are in accord with the high activation energy, as measured for the reaction, and indicate that PT reactions on the surface proceed by a semi- or fully concerted rather than stepwise mechanism. A similar mechanism is assumed to be operative on the surface of proteins and renders water-mediated PT reactions as highly efficient as they are.     

Pyrolysis of liquefied petroleum gas assisted by radicals desorbed from mesh catalyst surface

The purpose of this study is to understand the reactions on the catalyst surface and in the gas phase during the catalytic pyrolysis of light hydrocarbons. To avoid the complexity of internal pore diffusion and heat transfer limitation, nickel mesh without pore structure was used as a catalyst for the catalytic pyrolysis of a commercial liquefied petroleum gas (LPG) sample in a quartz tube reactor and in a wire‐mesh reactor over a temperature range of 600–850°C. With a Ni mesh catalyst, no catalyst deactivation associated with coke formation was observed at high gas flow rate. Our experimental results indicate that the desorption of radicals from the catalyst surface is an important process in the catalytic pyrolysis of LPG using the Ni mesh catalyst. The desorption of radicals across the gas–catalyst interface is greatly facilitated by increasing gas flow rate passing through the mesh. The desorbed radicals would initiate and/or enhance the gas‐phase radical chain reactions and lead to improved reaction rates for the pyrolysis of LPG although the product selectivities remained unchanged. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 637–646, 2003     

Determination of acrylamide formed in asparagine/D-glucose maillard model systems by using gas chromatography with headspace solid-phase microextraction

A gas chromatographic method, along with a headspace solid-phase microextraction (HS-SPME), was developed for the determination of acrylamide formed in Maillard reaction model systems. The developed method was validated by liquid chromatography/mass spectrometry. A headspace sample was collected from an aqueous acrylamide solution (100 microg/mL) by SPME and directly injected into a gas chromatograph equipped with a nitrogen-phosphorus detector. The recovery of acrylamide from an aqueous solution was satisfactory, i.e, >93% under the conditions used. Acrylamide formed in an asparagine/D-glucose (molar ratio, 1/2) Maillard reaction model system heated at 150 and 170 degrees C for 20 min was collected and analyzed by the newly developed method using gas chromatography with nitrogen-phosphorus detection and HS-SPME. The amounts of acrylamide were 318 +/- 33 microg/g asparagine from a sample heated at 150 degrees C and 3329 +/- 176 microg/g asparagine from a sample heated at 170 degrees C. Addition of cysteamine or glutathione to the above model system reduced acrylamide formation. Acrylamide formation was not observed when cysteamine or glutathione was added to asparagine in the above model systems to obtain equimolar concentrations of both compounds. This newly developed method is simple and sensitive, and requires no solvent extraction.     

2,6-二巯基嘌呤互变异构体热力学稳定性的密度泛函理论研究

利用密度泛函(DFT)B3LYP/6-311G(d,p)方法,水相计算采用自洽反应场(SCRF)中的Onsager模型,对气相和水相中可能存在的13种2,6-二巯基嘌呤互变异构体进行了全优化,并计算了各异构体的热力学参数、偶极矩及原子净电荷。计算结果表明,不论是气相还是水相,二硫酮DTP(1,3,7)是最稳定的异构体。溶剂化效应使各异构体的稳定性均增强,偶极矩大者其稳定性显著增大。溶剂化吉布斯自由能与异构体在两相中偶极矩之差存在相关性。二硫酮DTP(1,3,7)在水相中与致癌物BPDE进行亲核取代反应时,二硫酮DTP(1,3,7)中的S10原子优先进攻亲电试剂BPDE.     

Cyclic voltammetry of highly hydrophilic ions at a supported liquid membrane

Cyclic voltammetry has been used to study the coupling of ion transfer reactions at a liquid membrane. The liquids are either supported by a porous hydrophobic membrane (polyvinylidene difluoride, PVDF) when the organic solvent is non-volatile (o-nitrophenyloctylether) or are merely a free standing organic solvent layer such as 1,2-dichloroethane comprised between two hydrophilic dialysis membranes supporting the adjacent aqueous phases. The passage of current across the liquid membrane is associated with two ion transfer reactions across the two polarised liquid liquid interfaces in series. It is shown that it is possible to study the transfer of highly hydrophilic ions at one interface by limiting the mass transfer of the other ion transfer reaction at the other interface. Indeed, for systems comprising an ion M in one aqueous phase and a reference ion R partitioned between the membrane and the other aqueous phase, the observed and simulated cyclic voltammograms have a half-wave potential determined by the Gibbs energy of transfer of M transferring at one interface and by the limiting mass transfer of R at the other interface. This new methodology opens a way to measure the Gibbs energy of transfer of highly hydrophilic or hydrophobic ions, which usually limits the potential window at single liquid liquid interfaces (ITIES).