Thermal decomposition mechanism of levoglucosan during cellulose pyrolysis

Levoglucosan (1,6-anhydro-β-d-glucopyranose) decomposition is an important step during cellulose pyrolysis and for secondary tar reactions. The mechanism of levoglucosan thermal decomposition was studied in this paper using density functional theory methods. The decomposition included direct CO bond breaking, direct CC bond breaking, and dehydration. In total, 9 different pathways, including 16 elementary reactions, were studied, in which levoglucosan serves as a reactant. The properties of the reactants, transition states, intermediates, and products for every elementary reaction were obtained. It was found that 1-pentene-3,4-dione, acetaldehyde, 2,3-dihydroxypropanal, and propanedialdehyde can be formed from the CO bond breaking decomposition reactions. 1,2-Dihydroxyethene and hydroxyacetic acid vinyl ester can be formed from the CC bond breaking decomposition reactions. It was concluded that CO bond breaking is easier than CC bond breaking due to a lower activation energy and a higher released energy. During the 6 levoglucosan dehydration pathways, one water molecule which composed of a hydrogen atom from C3 and a hydroxyl group from C2 is the preferred pathway due to a lower activation energy and higher product stability. © 2012 Elsevier B.V. All rights reserved.

The Involvement of Phycocyanin Pigment in the Photodecomposition of the Cyanobacterial Toxin, Microcystin-LR

While investigating the destruction of the cyanobacterial hepatotoxin microcystin-LR in the presence of phycocyanin pigment via semiconductor photocatalysis, it became apparent that the pigment was catalysing the toxin decomposition. The mechanism of this process in terms of phycocyanin acting as a photo-oxygenation sensitizer via singlet oxygen and superoxide attack is explored. The absorption and fluorescence spectra of phycocyanin have been obtained and data on the properties of the excited state calculated. The established photo-oxygenation sensitizer rose bengal was also used as a catalyst for the photolytic decomposition of microcystin-LR to help elucidate the decomposition mechanism. 

The effect of H-2 and the presence of hot-O-(ads) during the decomposition of N2O on platinum

The decomposition of N2O was studied using a silica-supported Pt catalyst. The catalyst was found to exhibit short-lived activity at low temperatures to yield N-2 and O-(ads), the latter remained adsorbed on the surface and poisoned the active sites. Creation of hot-O-(ads) atoms during N2O decomposition is proposed to allow O-2 desorption at intermediate temperatures. Inclusion of H-2 as a reducing agent greatly enhanced the activity and suppressed low temperature deactivation. Simultaneous and sequential pulsing of N2O and H-2 showed that H-2 inclusion with the N2O gas stream produced the greatest activity. A mechanism involving H-(ads) addition to

TAP studies of ammonia decomposition over Ru and Ir catalysts

The Temporal Analysis of Products (TAP) technique has been used to investigate the mechanism involved in the catalytic decomposition of NH3 over a series of catalysts consisting of activated carbon supported Ru (promoted and non-promoted with Na) and over an activated carbon supported Ir. An extensive study of the role played by both the support and the promoter in the

DRIFTS/MS/Isotopic Labeling Study on the NO-Moderated Decomposition of a Silica-Supported Nickel Nitrate Catalyst Precursor

In the preparation of silica-supported nickel oxide from nickel nitrate impregnation and drying, the replacement of the traditional air calcination step by a thermal treatment in 1% NO/Ar prevents agglomeration, resulting in highly dispersed NiO. The mechanism by which NO prevents agglomeration was investigated by using combined in situ diffuse reflectance infrared fourier transform (DRIFT) spectroscopy and mass spectrometry (MS). After impregnation and drying, a supported nickel hydroxynitrate phase with composition Ni(3)(NO(3))(2)(OH)(4) had been formed. Comparison of the evolution of the decomposition gases during the thermal decomposition of Ni(3)(NO(3))(2)(OH)(4) in labeled and unlabeled NO and O(2) revealed that NO scavenges oxygen radicals, forming NO(2). The DRIFT spectra revealed that the surface speciation evolved differently in the presence of NO as compared with in O(2) or Ar. It is proposed that oxygen scavenging by NO depletes the Ni(3)(NO(3))(2)(OH)(4) phase of nitrate groups, creating nucleation sites for the formation of NiO, which leads to very small (similar to 4 nm) NiO particles and prevents agglomeration.

Density Functional Theory Studies of Ethanol Decomposition on Rh(211)

Density functional theory calculations were carried out to examine the mechanism of ethanol decomposition on the Rh(211) surface. We found that there are two possible decomposition pathways: (1) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(3)CO -> CH(3) + CO -> CH(2) + CO -> CH + CO -> C + CO and (2) CH(3)CH(2)OH -> CH(3)CHOH -> CH(3)COH -> CH(2)COH -> CHCOH -> CHCO -> CH + CO -> C + CO. Both pathways have a common intermediate of CH(3)COH, and the key step is the formation of CH(3)CHOH species. According to our calculations, the mechanism of ethanol decomposition on Rh(211) is totally different from that on Rh(111): the reaction proceeds via CH(3)COH rather than an oxametallacycle species (-CH(2)CH(2)O- for Rh( 111)), which implies that the decomposition process is structure sensitive. Further analyses on electronic structures revealed that the preference of the initial C(alpha)-H path is mainly due to the significant reduction of d-electron energy in the presence of the transition state (TS) complex, which may stabilize the TS-surface system. The present work first provides a clear picture for ethanol decomposition on stepped Rh(211), which is an important first step to completely understand the more complicated reactions, like ethanol steam reforming and electrooxidation.

A further investigation of the kinetic demixing/decomposition of La0.6Sr0.4Co0.2Fe0.8O3- δ oxygen separation membranes

Kinetic demixing and decomposition were studied on three La_0.6Sr_0.4Co_0.2Fe_0.8O_{3- δ} oxygen-separation hollow fibre membrane modules, which were operated under a 0.21/0.009bar oxygen partial pressure difference at 950°C for 1128, 3672 and 5512h, respectively. The post-operation membranes were characterized by Secondary Ion Mass Spectrometry, Scanning Electronic Microscope, Energy Dispersive Spectrum and X-ray Diffraction. The occurrence of kinetic demixing and decomposition was confirmed through the microstructural evolution of the membranes. Secondary-phase grains were found on the air-side surface of the membranes after the long-term operation and Co and Fe enrichment as well as La depletion was found on the surface and in the bulk at the air side. Cation diffusivities were found to be in the order Co>Fe>Sr>La. Kinetic demixing and decomposition rates of the membranes at the air side were found to be self-accelerating with time; the role of A-site deficiency in the perovskite lattice in the bulk near the air side surface is implicated in the mechanism. The oxygen permeability was not affected by the kinetic demixing and decomposition of the material during long-term operation (up to 5512h), however, we may expect permeability to be affected by secondary phase formation on the air-side surface at even longer operational times. © 2010 Elsevier B.V.