The Woodlands, TX, United States
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Gao J.,Stevens Institute of Technology | Zhao H.,Huntsman Advanced Technology Center | Yang X.,Brookhaven National Laboratory | Koel B.E.,Princeton University | Podkolzin S.G.,Stevens Institute of Technology
ACS Catalysis | Year: 2013

Acetylene reactivity as a function of Sn concentration on Pt catalytic surfaces was studied by comparing adsorption and reactions of regular and deuterated acetylene at 90-1000 K on three surfaces, Pt(111), Pt 3Sn/Pt(111), and Pt2Sn/Pt(111), using high-resolution electron energy loss spectroscopy, temperature-programmed desorption, and density functional theory calculations. The strongly adsorbed di-σ/π-bonded acetylene species, which dominate on pure Pt, were not detected on the Pt-Sn surfaces. The presence of Sn is also shown to suppress acetylene decomposition and, as a result, to maintain adsorbed acetylene in the molecular form as weakly adsorbed π- and di-σ-bonded species. The destabilization of adsorbed acetylene makes associative reactions with the formation of dimers (C4 hydrocarbons) and trimers (benzene) progressively more energetically favorable with increasing Sn concentration. Acetylene adsorption modes and reactions on Pt catalytic surfaces can, therefore, be controlled with Sn alloying. The concentration of Sn needs to be an optimal level for catalytic activity since all hydrocarbon species bind preferentially only to Pt sites. © 2013 American Chemical Society.

Gao J.,Stevens Institute of Technology | Zhao H.,Princeton University | Zhao H.,Huntsman Advanced Technology Center | Yang X.,Princeton University | And 3 more authors.
Angewandte Chemie - International Edition | Year: 2014

Vibrational spectroscopic measurements and density functional calculations were used to identify a preferential catalytic mechanism for the transformation of acetylene, HC-CH, to vinylidene, C-CH2, on surfaces of Pt-Sn ordered alloys. In this mechanism, two adjacent Pt atoms adsorb an acetylene molecule and a third neighboring Pt atom is required for stabilizing the reacting H atom during the transformation. Therefore, unlike a direct H shift along the C-C bond in organometallic compounds with a single transition-metal atom, this mechanism has a geometric site requirement of three adjacent Pt atoms in the form of a three-fold site. The same geometric site requirement is identified for preferential C-H bond cleavage of acetylene with the formation of adsorbed C-CH and H species. In the absence of three-fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C-H bond cleavage are suppressed. A preferential catalytic mechanism has been identified for the transformation of acetylene to vinylidene on Pt-Sn surfaces. Unlike a direct H shift along the C-C bond in organometallic compounds, this mechanism requires three adjacent Pt atoms. The same requirement is identified for C-H bond cleavage. Without three-fold Pt sites, the reaction mechanism changes, and reactions of H transfer and C-H bond cleavage are suppressed. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Harikrishnan G.,University of Minnesota | Singh S.N.,Huntsman Advanced Technology Center | Kiesel E.,University of Minnesota | Macosko C.W.,University of Minnesota
Polymer | Year: 2010

Nanodispersions of polyurethane components with a three dimensional gelled network of filler is formed by the addition of a very small quantity of vapor grown carbon nanofiber (CNF). Reactive foaming of these nanodispersions produced polyurethane foams with superior properties. The kinetic profiles of polymerization and foaming reactions are not affected by the addition of filler. The cellular structure of nanocomposite foam becomes more uniform. Thermal conductivity and fire retarding tendency of the nanocomposite foams are superior at a very low loading of filler (1% by weight in components which corresponds to <0.5% by weight in foam). The filler did not open cells or induce structural defects. © 2010 Elsevier Ltd.

Harikrishnan G.,University of Minnesota | Harikrishnan G.,Indian Institute of Technology Kharagpur | Singh S.N.,Huntsman Advanced Technology Center | Lindsay C.I.,Huntsman Polyurethanes | MacOsko C.W.,University of Minnesota
Green Chemistry | Year: 2012

The property of osmotic swelling of a hectorite clay suspension in water is exploited for dispersing it in a polymeric foam matrix. This green route eludes the surface modification of clay as well as the mediation by organic solvents for clay dispersion during foaming. The route follows the initial swelling and partial delamination of clay galleries by water molecules, followed by further dispersion in an oligomeric isocyanate foaming component, before conducting reactive foaming. A nominal mass fraction of clay provides an effective barrier, restricting the gas phase mass transport in the foam. Electron microscopic investigations show that clay particles are preferentially located on the lamellar part of the microscopic foam cell, rather than on plateau borders. © The Royal Society of Chemistry 2012.

Ippalapalli S.,Indian Institute of Technology Kharagpur | Ranaprathapan A.D.,Indian Institute of Technology Kharagpur | Singh S.N.,Huntsman Advanced Technology Center | Harikrishnan G.,Indian Institute of Technology Kharagpur
ChemPhysChem | Year: 2013

Two-way multicomponent diffusion processes in polymeric nanocomposite foams, where the condensed phase is nanoscopically reinforced with impermeable fillers, are investigated. The diffusion process involves simultaneous outward permeation of the components of the dispersed gas phase and inward diffusion of atmospheric air. The transient variation in thermal conductivity of foam is used as the macroscopic property to track the compositional variations of the dispersed gases due to the diffusion process. In the continuum approach adopted, the unsteady-state diffusion process is combined with tortuosity theory. The simulations conducted at ambient temperature reveal distinct regimes of diffusion processes in the nanocomposite foams owing to the reduction in the gas-transport rate induced by nanofillers. Simulations at a higher temperature are also conducted and the predictions are compared with experimentally determined thermal conductivities under accelerated diffusion conditions for polyurethane foams reinforced with clay nanoplatelets of varying individual lamellar dimensions. Intermittent measurements of foam thermal conductivity are performed while the accelerated diffusion proceeded. The predictions under accelerated diffusion conditions show good agreement with experimentally measured thermal conductivities for nanocomposite foams reinforced with low and medium aspect-ratios fillers. The model shows higher deviations for foams with fillers that have a high aspect ratio. In and out: Two-way multicomponent diffusion in polymer nanocomposite foams is investigated by simulations and thermal conductivity measurements. Variations in the aspect ratio of the filler, filler-polymer interfacial interactions, and the volume fraction of the filler are considered. Simulations of the thermal conductivity are in good agreement with experiments for low and medium aspect-ratio fillers but show deviations at higher aspect ratios. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Barman B.N.,Huntsman Advanced Technology Center
Journal of Chromatography A | Year: 2014

Carbonyl compounds, specifically aldehydes, present in amine catalysts or amines are determined by reversed-phase liquid chromatography using ultraviolet detection of their corresponding 2,4-dinitrophenylhydrazones. The primary focus has been to establish optimum conditions for determining aldehydes accurately because these add exposure concerns when the amine catalysts are used to manufacture polyurethane products. Concentrations of aldehydes determined by this method are found to vary with the pH of the aqueous amine solution and the derivatization time, the latter being problematic when the derivatization reaction proceeds slowly and not to completion in neutral and basic media. Accurate determination of aldehydes in amines through derivatization can be carried out at an effective solution pH of about 2 and with derivatization time of 20. min. Hydrochloric acid has been used for neutralization of an amine. For complete derivatization, it is essential to protonate all nitrogen atoms in the amine. An approach for the determination of an adequate amount of acid needed for complete derivatization has been described. Several 0.2. M buffer solutions varying in pH from 4 to 8 have also been used to make amine solutions for carrying out derivatization of aldehydes. These solutions have effective pHs of 10 or higher and provide much lower aldehyde concentrations compared to their true values. Mechanisms for the formation of 2,4-dinitrophenylhydrazones in both acidic and basic media are discussed. © 2014 Elsevier B.V.

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