Elementary reaction

About: Elementary reaction is a research topic. Over the lifetime, 2972 publications have been published within this topic receiving 76110 citations.

The Catalytic Oxidative Coupling of Methane

Abstract: One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas-phase component, which includes free-radical chemistry. Ethane is produced mainly by the coupling of the surface-generated CH radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O ions. In addition to the oxidative coupling of CH4, cross-coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross-coupling reaction also appears to involve surface-generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

Kinetics and HO2 Product Yield of the Reaction C2H5O + O2 between 295 and 411 K

Abstract: The kinetics of the reaction between ethoxy (C2H5O) radicals and O2 have been studied using a combined laser photolysis/LIF technique for the generation and detection of C2H5O. In the temperature range 295–411 K the rate coefficient has been found to be . Product investigations using a chemical titration of HO2 by NO and subsequent quantitative detection of OH by LIF indicate the dominance of the HO2 formation channel, viz. with ϕ = 0.89 (−0.12+0.22). Energy correlations show that these products are formed by a direct metathesis reaction only. It is suggested, that the low pre-exponential factor in the Arrhenius expression may be the result of a more complex than linear configuration of the moiety in the transition state and/or of tunneling.

Elementary reactions of the phenyl radical, C6H5, with C3H4 isomers, and of benzene, C6H6, with atomic carbon in extraterrestrial environments

Abstract: Binary collisions of ground state carbon atoms, C( 3 Pj), with benzene, C6H6(X 1 A1g), and of phenyl radicals, C6H5(X 2 A1), with methylacetylene, CH3CCH(X 1 A1), were investigated in crossed beam experiments, ab initio calculations, and via RRKM theory to elucidate the underlying mechanisms of elementary reactions relevant to the formation of polycyclic aromatic hydrocarbons (PAHs) in extraterrestrial environments. The reactions of phenyl radicals with allene, H2CCCH2, and with cyclopropene, cyc-C3H4, as well as the reaction of benzyl radicals, C6H5CH2, with acetylene, HCCH, were also inves- tigated theoretically. The C( 3 Pj) atom reacts with benzene via complex formation to a cyclic, seven membered C7H5 doublet radical plus atomic hydrogen. Since this pathway has neither an entrance nor an exit barrier and is exoergic, the benzene molecule can be destroyed by carbon atoms even in the coldest molecular clouds. On the other hand, the reaction of phenyl radicals with methylacetylene has an entrance barrier; at high collision energies, the dynamics are at the boundary between an osculating complex and a direct pathway. Statistical calculations on the phenyl plus methylacetylene reaction demonstrate dramatic energy/temperature dependencies: at lower temperatures, the bicyclic indene isomer is the sole reaction product. But as the temperature increases to 2000 K, formation of indene diminishes in favor of substituted acetylenes and allenes, such as PhCCH, PhCCCH3, PhCHCCH2, and PhCH2CCH. Also, direct H-abstraction channels become accessible, forming benzene and C3H3 radicals, including propargyl. Similar conclusions were reached for the reactions of phenyl radicals with the other C3H4 isomers, as well as for the benzyl + acetylene reaction. The strong temperature dependence emphasizes that distinct product isomers must be included in reaction networks modeling PAH formation in extraterrestrial environments.