Showing papers on "Thermal decomposition published in 2005"

Thermal Decomposition of LiPF6-Based Electrolytes for Lithium-Ion Batteries

Abstract: The thermal decomposition of lithium-ion battery electrolytes 1.0 M LiPF 6 in one or more carbonate solvents has been investigated. Electrolytes containing diethyl carbonate (DEC), ethylene carbonate (EC), a 1:1 mixture of EC/dimethyl carbonate (DMC), and a 1:1:1 mixture EC/DMC/DEC have been investigated by multinuclear nuclear magnetic spectroscopy, gas chromatography with mass selective detection, and size exclusion chromatography. Thermal decomposition affords products including: carbon dioxide (CO 2 ), ethylene (CH 2 CH 2 ), dialkylethers (R 2 O), alkyl fluorides (RF), phosphorus oxyfluoride (OPF 3 ), fluorophosphates [OPF 2 OR, OPF(OR) 2 ], fluorophosporic acids [OPF 2 OH, OPF(OH) 2 ], and oligoethylene oxides. The mechanism of decomposition is similar in all LiPF 6 /carbonate electrolytes. Trace protic impurities lead to generation of OPF 2 OR, which autocatalytically decomposes LiPF 6 and carbonates. The presence of DEC leads to the generation of ethylene, while the presnce of EC leads to the generation of capped oligothylene oxides [OPF 2 (OCH 2 CH 2 ) n F].

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field.

Abstract: To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon−silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH_3 radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si−C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH_3 radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH_3 radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO_2 slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C_2 hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.

Thermal decomposition of RDX from reactive molecular dynamics

Abstract: We use the recently developed reactive force field ReaxFF with molecular dynamics to study thermal induced chemistry in RDX [cyclic-[CH2N(NO2)]3] at various temperatures and densities. We find that the time evolution of the potential energy can be described reasonably well with a single exponential function from which we obtain an overall characteristic time of decomposition that increases with decreasing density and shows an Arrhenius temperature dependence. These characteristic timescales are in reasonable quantitative agreement with experimental measurements in a similar energetic material, HMX [cyclic-[CH2N(NO2)]4]. Our simulations show that the equilibrium population of CO and CO2 (as well as their time evolution) depend strongly of density: at low density almost all carbon atoms form CO molecules; as the density increases larger aggregates of carbon appear leading to a C deficient gas phase and the appearance of CO2 molecules. The equilibrium populations of N2 and H2O are more insensitive with respect to density and form in the early stages of the decomposition process with similar timescales.

Visible-Light-Driven N−F−Codoped TiO2 Photocatalysts. 1. Synthesis by Spray Pyrolysis and Surface Characterization

Abstract: Novel photocatalytic materials, yellow-colored N−F-codoped TiO2 (NFT) powders, were synthesized from a mixed aqueous solution containing TiCl4 and NH4F by spray pyrolysis (SP). The resulting powders were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), NH3-temperature-programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS), and N2 adsorption. FE-SEM images and N2 adsorption confirm that the NFT powders possessed spherical particles with a highly porous surface morphology. The doped N and F concentrations in the NFT powders depended significantly on the SP temperature and ranged from 0.10 to 0.26 and from 0.11 to 0.40 atomic % (at. %), respectively. NH3-TPD measurement indicates that the surface of the NFT powder was strongly acidic. Moreover, by investigating the characteristics of the TiO2 precursor hydrolysis and the thermal decomposition behaviors of the other components in the starti...

Generalized synthesis of metal phosphide nanorods via thermal decomposition of continuously delivered metal-phosphine complexes using a syringe pump.

Abstract: We synthesized uniform-sized nanorods of transition metal phosphides from the thermal decomposition of continuously delivered metal-phosphine complexes using a syringe pump. MnP nanorods with dimensions of 8 nm × 16 nm and 6 nm × 22 nm sized were synthesized by the thermal decomposition of Mn−TOP complex, which was prepared from the reaction of Mn2(CO)10 and tri-n-octylphosphine (TOP), using a syringe pump with constant injection rates of 10 and 20 mL/h, respectively. When Co−TOP complex, which was prepared from the reaction of cobalt acetylacetonate and TOP, was reacted in a mixture solvent composed of octyl ether and hexadecylamine at 300 °C using a syringe pump, uniform 2.5 nm × 20 nm sized Co2P nanorods were generated. When cobaltocene was employed as a precursor, uniform Co2P nanorods with 5 nm × 15 nm were obtained. When Fe−TOP complex was added to trioctylphosphine oxide (TOPO) at 360 °C using a syringe pump and then allowed to age at 360 °C for 30 min, uniform-sized FeP nanorods with an average di...

Decomposition of Hydrogen Peroxide at Water−Ceramic Oxide Interfaces

Abstract: The thermal decomposition of hydrogen peroxide, H2O2, was determined in aqueous suspensions of SiO2, Al2O3, TiO2, CeO2, and ZrO2 nanometer-sized particles. First-order kinetics were observed for the decomposition in all cases. Temperature dependence studies found that the activation energy was 42 ± 5 kJ/mol for the overall decomposition of H2O2 independent of the type of oxide. Oxide type had a strong effect on the preexponential rate term with increasing rate in the order of SiO2 < Al2O3 < TiO2 < CeO2 < ZrO2. The rate coefficient for H2O2 decomposition increases with increasing surface area of the oxide, but the number or efficiency of reactive sites rather than the total surface area may have the dominant role. Very efficient scavengers for OH radicals in the bulk liquid are not able to prevent formation of molecular oxygen, the main H2O2 gaseous decay product, suggesting that decomposition occurs on the oxide surfaces. The decomposition of H2O2 in the γ-radiolysis of water is enhanced by the addition o...

In situ XPS investigations of Cu1−xNixZnAl-mixed metal oxide catalysts used in the oxidative steam reforming of bio-ethanol

Abstract: A series of CuNiZnAl-multicomponent mixed metal oxide catalysts with various Cu/Ni ratios were prepared by the thermal decomposition of Cu1−xNixZnAl-hydrotalcite-like precursors and tested for oxidative steam reforming of bio-ethanol. Dehydrogenation of EtOH to CH3CHO is favored by Cu-rich catalyst. Introduction of Ni leads to C C bond rupture and producing CO, CO2 and CH4. H2 yield (selectivity) varied between 2.6–3.0 mol/mol of ethanol converted (50–55%) for all catalysts at 300 °C. The above catalysts were subjected to in situ XPS studies to understand the nature of active species involved in the catalytic reaction. Core level and valence band XPS as well as Auger electron spectroscopy revealed the existence of Cu2+, Ni2+ and Zn2+ ions on calcined materials. Upon in situ reduction at reactions temperatures, the Cu2+ was fully reduced to Cu0, while Ni2+ and Zn2+ were partially reduced to Ni0 and Zn0, respectively. On reduction, the nature of ZnO on Cu-rich catalyst changes from crystalline to amorphous, relatively inert and highly stabilized electronically. Relative concentration of the Ni0 and Zn0 increases upon reduction with decreasing Cu-content. Valence band results demonstrated that the overlap between 3d bands of Cu and Ni was marginal on calcined materials, and no overlap due to metallic clusters formation after reduction. Nonetheless, the density of states at Fermi level increases dramatically for Ni-rich catalysts and likely this influences the product selectivity.

Initiated Chemical Vapor Deposition of Linear and Cross-linked Poly(2-hydroxyethyl methacrylate) for Use as Thin-Film Hydrogels

Abstract: Initiated chemical vapor deposition (iCVD) is able to synthesize linear and cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) thin films, in one step, from vapors of 2-hydroxyethyl methacrylate (HEMA), ethylene glycol diacrylate (EGDA), and tert-butyl peroxide (TBPO) without using any solvents. This all-dry technique also allows control of the cross-link density by adjusting the partial pressure of the cross-linking agent EGDA in the vapor phase. Films with specific cross-link densities and hence thermal, wetting, and swelling properties can be created in one single vacuum processing step. Through selective thermal decomposition of the initiator TBPO, films with well-defined chemical structures and full functionality retention can be deposited, which is evident in the Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. These spectroscopic methods also facilitate determination of EGDA incorporation in the cross-linked films based on the fact that HEMA contains a hydr...

Thermal decomposition of nickel acetate tetrahydrate: an integrated study by TGA, QMS and XPS techniques

Abstract: The thermal decomposition of nickel carboxylates is a feasible route to synthesize metal nanoparticles suitable for catalytic purposes. The aim of this work is the characterization of the thermal decomposition course of nickel acetate tetrahydrate, Ni(CH 3COO)2·4H2O. A thermogravimetric (TGA) decomposition study carried out in three different atmospheres (air, helium and hydrogen) showed that the dehydration of the parent salt occurs between 118 and 137 ◦ C. However, irrespective of the chosen atmosphere, the sample weight loss registered in this stage remains invariable, suggesting the formation of a an intermediate basic acetate with the formula 0.86Ni(CH 3COO)2·0.14Ni(OH)2. The dehydration step was followed at ca. 350 ◦ C by the subsequent one-step major decomposition of the acetate group, producing NiO and Ni, in treatment atmospheres of air and hydrogen, respectively, but there was some indication of an additional step when the thermolysis was conducted in helium. The conclusions possible from thermal analysis were confirmed by monitoring evolved gases employing quadrupole mass spectrometry (QMS), and a set of reactions linked to the decomposition of the acetate group has been proposed to account for most of the gas products detected. X-ray photoelectron spectroscopy (XPS) was used to investigate the solid phases obtained during the thermal decomposition of the salt in He atmosphere. © 2004 Elsevier B.V. All rights reserved.

Phase- and size-controlled synthesis of hexagonal and cubic CoO nanocrystals.

Abstract: Highly crystalline, phase- and size-controlled CoO nanocrystals of hexagonal and cubic phases have been prepared by thermal decomposition of Co(acac)3 in oleylamine under an inert atmosphere. Kinetic and thermodynamic control for the precursor formation leads to two different seeds of hexagonal and cubic phases at higher temperatures. The crystal size of both CoO phases can be easily manipulated by changing the precursor concentration and reaction temperature.

Optical and electrochemical properties of nanosized NiO via thermal decomposition of nickel oxalate nanofibres

Abstract: Nickel oxide nanoparticles with an average diameter of about 9 nm were synthesized via thermal decomposition of NiC2O4 precursor at 450 °C. The nanoparticles were investigated using XRD, TEM, TGA, and UV–vis spectrophotometry. The optical absorption spectrum indicates that the NiO nanoparticles have a direct band gap of 3.56 eV. The electrochemical tests show that the ultrafine NiO nanoparticles, as a promising electrode material, can deliver a large reversible discharge capacity of about 610 mA h g−1.

Thermodynamic Analysis of Decomposition of Thiourea and Thiourea Oxides

Abstract: Thiourea has exhibited extremely rich dynamical behavior when being oxidized either through a chemical approach or via an electrochemical method. In this study, thermodynamic properties of thiourea and its oxides are investigated by measuring their thermogravimetry (TG), differential thermogravimetry (DTG), and differential scanning calorimetry (DSC) simultaneously. Online FT-IR measurements show that products of the thermal decomposition vary significantly with the reaction temperature. In addition to the determination of their apparent activation energy (E), preexponential factor (A), and entropy (ΔS⧧), enthalpy (ΔH⧧), and Gibbs energy (ΔG⧧) of thermal decomposition, our investigation further illustrates that the decomposition kinetics of thiourea and thiourea oxides follows the Johnson−Mehl−Avrami Equation, f (α) = n(1 − α)[−ln(1 − α)]1-1/n and G(α) = [−ln(1 − α)]1/n with n equal to 2, 3.43, and 3, respectively.

Thermogravimetric analysis of organoclays intercalated with the surfactant octadecyltrimethylammonium bromide

Abstract: High resolution thermogravimetric analysis has been used to study the thermal decomposition of montmorillonite modified with octadecyltrimethylammonium bromide. Thermal decomposition occurs in 4 steps.The first step of mass loss is observed from ambient to 100°C temperature range and is attributed to dehydration of adsorbed water. The second step of mass loss occurs between 87.9 to 135.5°C temperature range and is also attributed to dehydration of water hydrating metal cations such as Na+. The third mass loss occurs between 179.0 and 384.5°C; it is assigned to the loss of surfactant. The fourth step is ascribed to the loss of OH units due to dehydroxylation of the montmorillonite and takes place between 556.0 and 636.3°C temperature range. These TG steps are related to the arrangement of the surfactant molecules intercalating the montmorillonite. Changes in the basal spacing of the clay with surfactant are followed by X-ray diffraction. Thermal analysis provides an indication of the stability of the organo-clay.

Preparation and characterization of CuO/CeO2 catalysts and their applications in low-temperature CO oxidation

Abstract: CeO 2 nanoparticles were prepared by thermal decomposition of cerous nitrate and then used as supports for CuO/CeO 2 catalysts prepared via the impregnation method. The samples were characterized by HRTEM, XRD, H 2 -TPR, and XPS. The catalytic properties of the prepared catalysts for low-temperature CO oxidation were studied by using a microreactor–GC system. The results showed that the thermal decomposition temperature affected the physical properties of the prepared CeO 2 particles, such as particle size and morphology. The loading of CuO in non-crystalline forms on CeO 2 supports was different in the CuO/CeO 2 catalysts. The loading of non-crystalline CuO was higher on the CeO 2 prepared via thermal decomposition at 500 °C than those on the CeO 2 prepared via thermal decomposition at 400, 600 and 700 °C. One part of the non-crystalline CuO in CuO/CeO 2 catalysts entered the CeO 2 lattice and the other part dispersed over the CeO 2 surface. Calcination temperature had little effect on the catalytic activity when it was lower than 600 °C. However, heating the catalysts to higher temperatures (i.e. 800 °C) had a significant impact on the catalytic activity because the crystallite size rapidly grows and more CuO and CeO 2 phase-separation occurs.

Thermolysis of RAFT-synthesized polymers. A convenient method for trithiocarbonate group elimination

Abstract: Radical polymerization with thiocarbonylthio RAFT (reversible addition-fragmentation chain transfer) agents 1-4 is arguably one of the most versatile processes for living free radical polymerization displaying superior flexibility with respect to monomers and reaction conditions. A key feature of RAFT polymerization is that the thiocarbonylthio group(s), present in the initial RAFT agent, is (are) retained in the polymeric product(s). The retention of these groups is responsible for the polymers’ living character. However, the presence of the thiocarbonylthio groups also means that the polymers are usually colored. The polymers may also, in some cases, be odorous or release an odor over time due to decomposition of the thiocarbonylthio groups and the evolution of volatile sulfur-containing compounds. The presence of such color and odor can be disadvantageous in some applications. Even though some of these issues may be largely mitigated or overcome by more appropriate selection of the initial RAFT agent, there has nonetheless been some incentive to develop effective methods for treatment of RAFT-made polymers to cleave the thiocarbonylthio end-groups postpolymerization.