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H2 Formation (h2 + formation)
Selected AbstractsPossible molecular hydrogen formation mediated by the radical cations of anthracene and pyreneJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2003Mutsumi Hirama Abstract Hydrogen molecules cannot be formed readily by the association of gaseous hydrogen atoms. Possible H2 formation mediated by the radical cations of typical polycyclic aromatic hydrocarbons (PAHs), anthracene and pyrene, was studied at the B3LYP/6-31G** level of theory. We presumed that H2 is formed by way of two elementary reactions: the addition of an H atom to a PAH molecular cation, and the H abstraction from the resulting monohydro-PAH cation (i.e., arenium ion) by a second H atom to yield H2. The first reaction takes place without any activation energy. The second reaction is also predicted to proceed along almost barrierless pathways, although it is far from being a typical ion,molecule reaction. There is a possibility that these reactions might constitute one of the mechanisms for H2 formation in extremely cold interstellar space. Deuterium enrichment in PAH cations is possibly accompanied by such H2 formation because deuteration lowers the energies of polyatomic PAH cations appreciably. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1378,1382, 2003 [source] H2 reformation in post-shock regionsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010H. M. Cuppen ABSTRACT H2 formation is an important process in post-shock regions, since H2 is an active participant in the cooling and shielding of the environment. The onset of H2 formation therefore has a strong effect on the temperature and chemical evolution in the post-shock regions. We recently developed a model for H2 formation on a graphite surface in warm conditions. The graphite surface acts as a model system for grains containing large areas of polycyclic aromatic hydrocarbon structures. Here, this model is used to obtain a new description of the H2 formation rate as a function of gas temperature that can be implemented in molecular shock models. The H2 formation rate is substantially higher at high gas temperatures as compared to the original implementation of this rate in shock models, because of the introduction of H atoms which are chemically bonded to the grain (chemisorption). Since H2 plays such a key role in the cooling, the increased rate is found to have a substantial effect on the predicted line fluxes of an important coolant in dissociative shocks [O i] at 63.2 and 145.5 ,m. With the new model, a better agreement between the model and observations is obtained. Since one of the goals of Herschel/PACS will be to observe these lines with higher spatial resolution and sensitivity than the former observations by Infrared Space Observatory -LWS, this more accurate model is very timely to help with the interpretation of these future results. [source] An electron-flow model can predict complex redox reactions in mixed-culture fermentative BioH2: Microbial ecology evidenceBIOTECHNOLOGY & BIOENGINEERING, Issue 4 2009Hyung-Sool Lee Abstract We developed the first model for predicting community structure in mixed-culture fermentative biohydrogen production using electron flows and NADH2 balances. A key assumption of the model is that H2 is produced only via the pyruvate decarboxylation-ferredoxin-hydrogenase pathway, which is commonly the case for fermentation by Clostridium and Ethanoligenens species. We experimentally tested the model using clone libraries to gauge community structures with mixed cultures in which we did not pre-select for specific bacterial groups, such as spore-formers. For experiments having final pHs 3.5 and 4.0, where H2 yield and soluble end-product distribution were distinctly different, we established stoichiometric reactions for each condition by using experimentally determined electron equivalent balances. The error in electron balancing was only 3% at final pH 3.5, in which butyrate and acetate were dominant organic products and the H2 yield was 2.1,mol,H2/mol,glucose. Clone-library analysis showed that clones affiliated with Clostridium sp. BL-22 and Clostridium sp. HPB-16 were dominant at final pH 3.5. For final pH 4.0, the H2 yield was 0.9,mol,H2/mol,glucose, ethanol, and acetate were the dominant organic products, and the electron balance error was 13%. The significant error indicates that a second pathway for H2 generation was active. The most abundant clones were affiliated with Klebsiella pneumoniae, which uses the formate-cleavage pathway for H2 production. Thus, the clone-library analyses confirmed that the model predictions for when the pyruvate decarboxylation-ferredoxin-hydrogenase pathway was (final pH 3.5) or was not (final pH 4.0) dominant. With the electron-flow model, we can easily assess the main mechanisms for H2 formation and the dominant H2 -producing bacteria in mixed-culture fermentative bioH2. Biotechnol. Bioeng. 2009; 104: 687,697 © 2009 Wiley Periodicals, Inc. [source] Evaluation of metabolism using stoichiometry in fermentative biohydrogenBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2009Hyung-Sool Lee Abstract We first constructed full stoichiometry, including cell synthesis, for glucose mixed-acid fermentation at different initial substrate concentrations (0.8,6 g-glucose/L) and pH conditions (final pH 4.0,8.6), based on experimentally determined electron-equivalent balances. The fermentative bioH2 reactions had good electron closure (,9.8 to +12.7% for variations in glucose concentration and ,3 to +2% for variations in pH), and C, H, and O errors were below 1%. From the stoichiometry, we computed the ATP yield based on known fermentation pathways. Glucose-variation tests (final pH 4.2,5.1) gave a consistent fermentation pattern of acetate,+,butyrate,+,large H2, while pH significantly shifted the catabolic pattern: acetate,+ butyrate,+,large H2 at final pH 4.0, acetate,+,ethanol,+ modest H2 at final pH 6.8, and acetate,+,lactate,+,trivial H2 at final pH 8.6. When lactate or propionate was a dominant soluble end product, the H2 yield was very low, which is in agreement with the theory that reduced ferredoxin (Fdred) formation is required for proton reduction to H2. Also consistent with this hypothesis is that high H2 production correlated with a high ratio of butyrate to acetate. Biomass was not a dominant sink for electron equivalents in H2 formation, but became significant (12%) for the lowest glucose concentration (i.e., the most oligotrophic condition). The fermenting bacteria conserved energy similarly at ,3 mol ATP/mol glucose (except 0.8 g-glucose/L, which had ,3.5 mol ATP/mol glucose) over a wide range of H2 production. The observed biomass yield did not correlate with ATP conservation; low observed biomass yields probably were caused by accelerated rates of decay or production of soluble microbial products. Biotechnol. Bioeng. 2009; 102: 749,758. © 2008 Wiley Periodicals, Inc. [source] Production of Hydrogen from Dimethyl Ether over Supported Rhodium CatalystsCHEMCATCHEM, Issue 2 2009Gyula Halasi Abstract Infrared (IR) spectroscopy revealed that dimethyl ether (DME) undergoes partial dissociation on pure and rhodium-containing CeO2 at 300,K to yield methoxy and methyl species. This process is promoted by the presence of rhodium. By means of thermal desorption measurements (TPD), the adsorption of DME on Rh/CeO2 at 300,K and subsequent decomposition of DME (Tp,370,K), releasing H2, CO, CO2, and CH4, with Tp between 420 and 673,K, were ascertained. Rh/CeO2 is an effective catalyst for the decomposition of DME to give H2 (29,35,%), CO (27,30,%) and CH4 (32,38,%) as major products with complete conversion at 673,723,K. Adding water to DME changed the product distribution and increased the selectivity of H2 formation from 30,35,% to 58,% at 723,K. In,situ IR spectroscopy showed absorption bands of CO at 2034 and 1893,cm,1 during the reaction at 673,773,K. Deactivation of the catalyst did not occur at 773,K during the time measured (approximately 10 h). Rh deposited on carbon Norit also exhibited a high activity towards the decomposition of DME, but the selectivity towards hydrogen was lower. [source] | ||||||||||