Showing papers on "Chemical decomposition published in 2000"

Dissolution and Hydrolysis of Cellulose in Subcritical and Supercritical Water

Abstract: Decomposition experiments of microcrystalline cellulose were conducted in subcritical and supercritical water (25 MPa, 320−400 °C, and 0.05−10.0 s). At 400 °C hydrolysis products were mainly obtained, while in 320−350 °C water, aqueous decomposition products of glucose were the main products. Kinetic studies of cellulose, cellobiose, and glucose at these conditions showed that below 350 °C the cellulose decomposition rate was slower than the glucose and cellobiose decomposition rates, while above 350 °C, the cellulose hydrolysis rate drastically increased and became higher than the glucose and cellobiose decomposition rates. Direct observation of the cellulose reaction in high-temperature water at high-pressure conditions by using a diamond anvil cell (DAC) showed that, below 280 °C, cellulose particles became gradually smaller with increasing reaction time but, at high temperatures (300−320 °C), cellulose particles disappeared with increasing transparency and much more rapidly than expected from the lowe...

Catalyzed alanates for hydrogen storage

Abstract: The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH{sub 4} {Leftrightarrow} 1/3Na{sub 3}AlH{sub 6} + 2/3Al + H{sub 2} {Leftrightarrow} NaH + Al + 3/2H{sub 2}. Initial work resulted in improved catalysts, advanced methods of preparation and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions observed real-time through in situ X-ray powder diffraction demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verify aluminum segregation to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 atm at 33 C, which suggest that, thermodynamically, this material is ideally suited to onboard hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at elevated temperatures (125-165 C), approaching that found in fuel cell operations, were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti-Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

Production of nanotubes by the catalytic decomposition of different carbon-containing compounds

Abstract: Carbon nanotubes were prepared in the catalytic decomposition of different carbon containing compounds over supported transition metal catalysts. Besides acetylene, ethylene, propylene, acetone, n-pentane, methanol, toluene, and methane were tested and each resulted in carbon nanotube formation. The quality of as-made nanotubes was investigated by TEM and was found to be at least as good as obtained in acetylene decomposition. Ethylene and propylene showed somewhat lower reactivity in the buckytube formation with respect to acetylene, simultaneously suppressed formation of amorphous carbon on the outer surface was found.

TiO2-mediated photodegradation of liquid and solid organic compounds

Abstract: The photocatalytic degradation of several liquid and solid organic compounds, including polymers, with molecular weights covering a wide range from 600 to 500,000 was studied on TiO2 thin films on glass under UV illumination. Nearly exact agreement was found between the weight losses of the solid compounds octadecane and stearic acid and the weights of CO2 produced during photocatalytic degradation. No other gas-phase degradation product was detected for these two compounds other than CO2, which means that potentially harmful products are not expected to pose a problem. For convenient comparison of degradation rates for various compounds and measurement methods, the values were converted to numbers of moles of carbon reacted per square centimeter per hour. Under appropriate conditions (50C, relative humidity 10% in air), octadecane was completely decomposed (<400 ng cm 2 ). In contrast, stearic acid did not decompose completely, even after more than 80 h of UV illumination. This may be due to the formation of a photocatalytically inert reaction product that blocks the TiO2 surface. The decomposition rates for all of the compounds examined spanned less than two orders of magnitude, suggesting that the photocatalytic reactions involved are rather versatile. © 2000 Published by Elsevier Science B.V.

Elucidation of the 1,4-Dioxane Decomposition Pathway at Discrete Ultrasonic Frequencies

Abstract: The sonolytic decomposition chemistry of the refractory compound 1,4-dioxane in aqueous solution has been investigated at four ultrasonic frequencies (205, 358, 618, and 1071 kHz). To maintain fully saturated solutions, argon and oxygen were used as sparge gases. Using a frequency of 358 kHz, the observed first-order kinetic rate constants for 1,4-dioxane destruction were highest with a sparge gas ratio of 75% Ar/25% O2 (k = 4.32 ± 0.31 × 10-4 s-1) and lowest in the presence of pure argon (k = 8.67 ± 0.47 × 10-5 s-1). Ethylene glycol diformate, methoxyacetic acid, formaldehyde, glycolic acid, and formic acid were found to be the major intermediates of 1,4-dioxane degradation. A reaction mechanism involving these byproducts was proposed concerning primarily reactions with oxidizing species (•OH, •OOH, •O) in and near the interfacial region of the cavitation bubble. The highest observed first-order 1,4-dioxane decomposition rate occurred at 358 followed by 618, 1071, and 205 kHz. At each frequency, approxim...

Catalytic Decomposition of Ammonia over Nitrided MoNx/α-Al2O3 and NiMoNy/α-Al2O3 Catalysts

Abstract: Ammonia decomposition over nitrided MoNx/α-Al2O3 and NiMoNy/α-Al2O3 catalysts was investigated at temperatures 600−750 °C, GHSV = 1800−3600 h-1. The conversions of ammonia decomposition over nitrid...

Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes

Abstract: Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [1]. H2O2 on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fenton's reagent, are almost as old [2]. However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent. Although the decomposition of hydrogen peroxide is often employed as a standard reaction to determine the catalytic activity of metal complexes and metal oxides [3,4], it has recently been extensively used in intrinsically clean processes and in end-of-pipe treatment of effluent of chemical industries [5,6]. Furthermore, the adoption of H2O2 as an alternative of current industrial oxidation processes offer environmental advantages, some of which are (1) replacement of stoichiometric metal oxidants, (2) replacement of halogens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by-products. On the other hand, wasteful decomposition of hydrogen peroxide due to trace transition metals in wash water in the fabric bleach industry, was also recognized [7]. The low intrinsic reactivity of H2O2 is actually an advantage, in that a method can be chosen which selectively activates it to perform a given oxidation. There are three main active oxidants derived from hydrogen peroxide, depending on the nature of the activator; they are (1) inorganic oxidant systems, (2) active oxygen species, and (3) per oxygen intermediates. Two general types of mechanisms have been postulated for the decomposition of hydrogen peroxide in the presence of transition metal complexes. The first is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)-H2O2 system [8]. The key features of this mechanism were the discrete formation of hydroxyl and hydroperoxy radicals, which can form a redox cycle with the Fe(II)/Fe(III) couple. The second is the peroxide complex mechanism, which was proposed by Kremer and Stein [9]. The significant difference in the peroxide complex mechanism is the two-electron oxidation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxygen-oxygen bond. It is our intention in this article to briefly summarize the kinetics as well as the mechanisms of the decomposition of hydrogen peroxide, homogeneously and heterogeneously, in the presence of transition metal complexes. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 643–666, 2000

Cerium dioxide as a photocatalyst for water decomposition to O2 in the presence of Ceaq4+ and Feaq3+ species

Abstract: It has been demonstrated that cerium dioxide is a potential photocatalyst that can be used to decompose water to produce oxygen in aqueous suspension containing an electron acceptor, and the optimum parameters for the reaction have been investigated The O2 yield strongly depended on the duration of irradiation, CeO2 concentration, concentration of the electron acceptor, and pH of the suspension The optimum photoproduction for O2 was obtained under the following operating conditions: Illumination time: >10 h, CeO2 concentration: 2–5 g dm−3, [Ce4+]: 4–5 mM, pH

The fate of aniline after a photo-Fenton reaction in an aqueous system containing iron(III), humic acid, and hydrogen peroxide

Abstract: The degradation of aniline (ArNH2) was facilitated by light irradiation (λ > 370 nm) of an aqueous solution, which contained Fe(III), humic acid (HA), and H2O2. The consumption of H2O2 and the reduction of Fe(III) to Fe(II) was consistent with the degradation of ArNH2 via the photo-Fenton reaction, accompanied by the generation of hydroxyl radicals (HO•). HPLC analysis of the reaction mixture indicated the presence of p-aminophenol, p-hydroquinone, and maleic and fumaric acids and the simultaneous release of NH4+ ion. However, the sum of the product concentrations, as determined by HPLC after the reaction, was much smaller than the ArNH2 concentration added initially. This can be attributed to the majority of the ArNH2 being incorporated into the polymeric structure in the HA after the reaction. The 15N NMR and pyrolysis-GC/MS studies indicated that, after the reaction, ArNH2 formed covalent bonds with quinone and the vinyl carbons in the HA, to form anilino-compounds, such as anilinoquinone and enaminone.

Chemiluminescent reaction of fluorescent organic compounds with KHSO5 using cobalt(II) as catalyst and its first application to molecular imprinting

Abstract: The decomposition of peroxomonosulfate (HSO5-) has been investigated by chemiluminescence (CL). A weak CL was observed during mixing the HSO5- solution with the Co2+ solution in unbuffered conditions. An appropriate amount of fluorescent organic compounds (FOCs), such as dansyl amino acids and pyrene, was added to the KHSO5/Co2+ solution, a strong CL was recorded. A possible CL mechanism, based on studies of the fluorescence, CL, and UV-visible spectra and comparison of Co3+ oxidation ability with the SO4.- radical ion, was discussed. The CL from HSO5-/Co2+ is the emission of singlet oxygen produced from the catalytic decomposition of HSO5-. It was suggested that the decomposition of HSO5- in aqueous solution with Co2+ proceeds via one-electron transfer to yield SO4.- radical ion. The FOC was attacked by SO4.- radical ion and oxidized to decompose into small molecules. During this proceeding, CL emission was given out. The present CL system has been developed as a flow injection analysis for FOCs. The detection limits (S/N = 3) were in the concentration range 10(-9)-10(-7) M for FOCs. Oxidation decomposition and CL emission of the analytes have been used in the molecular imprinting recognition. As an initial attempt, dansyl-L-phenylalanine was used as a template molecule and methacrylic acid and 2-vinylpyridine were used as functional monomers. The network copolymer imprinted with dansyl-L-phenylalanine exhibits an affinity for the template molecule. When the flowing streams of HSO5- and Co2+ solutions mixing through the molecularly imprinted polymer particles filled the flow cell, the template molecule, dansyl-L-phenylalanine reacted with the HSO5-/Co2+ solution and CL was emitted. The dansyl-L-phenylalanine was decomposed during the CL process, and the cavities of a defined shape and an arrangement of functional groups complementary to the template in the polymer were left for the next sample analysis.

Kinetic study of the combustion of organophosphorus compounds

Abstract: The thermochemistry and kinetics of organophosphorus compounds were studied, with BAC-MP4 method estimations as a basis and PM3 semi-empirical estimations for many new compounds. New group additivity values were proposed for enthalpies of formation at 298 K, and entropies and heat capacities of species involving pentavalent phosphorus bonded to carbon, hydrogen, oxygen, fluorine, nitrogen, and sulfur atoms. The kinetic features of unimolecular elimination were investigated by modeling pyrolysis experiments of dimethyl ethyl phosphonate (DEMP), triethyl phosphate (TEP), and di-isopropyl methyl phosphonate (DIMP). Rate constants were proposed for four- and six-center eliminations. A new combustion mechanism was developed in a systematic and comprehensive way and involved 41 phosphorus compounds in 202 reactions. All possible intermediates were taken into account. This reaction mechanism was applied to the modeling of H2/O2 flames doped with dimethyl methyl phosphonate (DMMP) and trimethyl phosphate (TMP), which are chemical warfare surrogates. The promoting effect of the agent and the species profiles were correctly reproduced. The decomposition reaction channels were studied: they included radical reactions and molecular eliminations. The cause of the increase of the reactivity of the flame was attributed to some radical combination cycles leading to an increase of heat release, which enhances the global reaction rate in spite of the radical concentration drop.

Sodium Dodecylbenzenesulfonate Removal from Water and Wastewater. 1. Kinetics of Decomposition by Ozonation

Abstract: Ozonation experiments were conducted to investigate the sodium dodecylbenzenesulfonate (NaDBS) removal from aqueous solution and domestic wastewater. The influence of pH and tert-butyl alcohol conc...

Catalytic methanol decomposition to carbon monoxide and hydrogen over nickel supported on silica

Abstract: The decomposition of methanol to carbon monoxide and hydrogen can be catalyzed at 250°C over nickel supported on silica. The activity of the catalyst prepared by a sol–gel method increases with an increase in the content of nickel up to 40 wt.% while that for the sample prepared by an impregnation technique almost reaches a plateau at the nickel content of 10 wt.%. The activity does not relate simply to the nickel surface area of the sample, but it depends on the amounts of carbon monoxide and hydrogen strongly adsorbed on the catalyst. Small nickel particles are disadvantageous in the reaction.

The behavior of Cu/ZSM-5 in the oxide and reduced form in the presence of NO and methanol

Abstract: Cu/ZSM-5 catalysts were characterized by DRS UV–Vis IR spectroscopy, CO chemisorption and NO TPD. The results indicated that the exchange level in Cu 2+ is complete. The oxidation number of copper ions after calcination is 2+, with the formation of isolated Cu 2+ species, CuO and (Cu–O–Cu) 2+ compounds, depending on the pretreatment conditions. It was found that copper atoms are arranged as small clusters, well dispersed in the zeolite framework. The results indicated the active species for catalytic decomposition of NO is Cu 2+ , which can be reduced to Cu + forming Cu + –NO 2 and Cu + –(NO) 2 complexes. The dependence of copper ions state and therefore, the pretreatment are important for the NO decomposition activity due to the formation of the (Cu–O–Cu) 2+ oxocation species, which act as catalytic sites for the disproportionation of NO to N 2 O + NO 2 . Methanol oxidation showed that the activity and selectivity are not dependent on pretreatment temperature, but they are markedly influenced by the acid site distribution. Methanol was inactive for NO reduction, since methanol can not be activated on the NO adsorption sites to form species that are active for NO reduction.

Ab Initio Calculations of Reactive Pathways for α-Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (α-HMX)

Abstract: Using the BLYP and B3LYP level of density functional theory, four possible decomposition reaction pathways of HMX in the gas phase were investigated: N−NO2 bond dissociation, HONO elimination, C−N bond scission of the ring, and the concerted ring fission. The energetics of each of these four mechanisms are reported. Dissociation of the N−NO2 bond is putatively the initial mechanism of nitramine decomposition in the gas phase. Our results find the dissociation energy of this mechanism to be 41.8 kcal/mol at the BLYP level and 40.5 kcal/mol at the B3LYP level, which is comparable to experimental results. Three other mechanisms are calculated and found at the BLYP level to be energetically competitive to the nitrogen−nitrogen bond dissociation; however, at the B3LYP level these three other mechanisms are energetically less favorable. It is proposed that the HONO elimination and C−N bond scission reaction of the ring would be favorable in the condensed phase.

Interaction of SO2 with MgO(100) and Cu/MgO(100): Decomposition Reactions and the Formation of SO3 and SO4

Abstract: Synchrotron-based high-resolution photoemission and first-principles density functional calculations (DFT-GGA) were used to study the interaction of SO2 with clean and modified (OH, Oδ-, O vacancies, or Cu adatoms present) MgO(100) surfaces. The reaction of the molecule with pure and hydroxylated powders of MgO was investigated using X-ray absorption near-edge spectroscopy (XANES). At 100 K, the main product of the adsorption of sulfur dioxide on MgO(100) is sulfite (SO2,gas + Olattice → SO3,ads). No evidence is found for bonding of SO2 to Mg sites of the surface or decomposition of the molecule. DFT calculations show that a η3-S,O,O adsorption configuration leads to a SO3-like species, and this is much more stable than configurations which involve bonding to only Mg sites or formation of SO4. On a flat MgO(100) substrate, the formation of SO4 is not energetically viable. A SO3 → SO4 transformation is observed at temperatures between 150 and 450 K with a substantial reconstruction of the oxide surface. Fr...

Si–H bending modes as a probe of local chemical structure: Thermal and chemical routes to decomposition of H2O on Si(100)-(2×1)

Abstract: Surface infrared spectroscopy and density functional cluster calculations are used to study the thermal and atomic hydrogen-induced decomposition of water molecules on the clean Si(100)-(2×1) surface. We report the first observation of the Si–H bending modes associated with the initial insertion of oxygen into the dimer and backbonds of a silicon dimer. We find that, while one and two oxygen-containing dimers are formed almost simultaneously during the thermal decomposition of water on this surface, atomic H can be used to drive the preferential formation of the singly oxidized dimer. This work highlights the sensitivity of Si–H bending modes to the details of local chemical structure in an inhomogeneous system, suggesting that the combined experimental and theoretical approach demonstrated herein may be extremely useful in studying even more complex systems such as the hydrogenation of defects in SiO2 films.