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Register a new userMethane Oxidation to Methanol without CO2 Emission: Catalysis by Atomic Negative Ions
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The catalytic activities of the atomic Y-, Ru-, At-, In-, Pd-, Ag-, Pt-, and Os- ions have been investigated theoretically using the atomic Au- ion as the benchmark for the selective partial oxidation of methane to methanol without CO2 emission. Dispersion-corrected density-functional theory has been used for the investigation. From the energy barrier calculations and the thermodynamics of the reactions, we conclude that the catalytic effect of the atomic Ag-, At-, Ru-, and Os- ions is higher than that of the atomic Au- ion catalysis of CH4 conversion to methanol. By controlling the temperature around 290K (Os-), 300K (Ag-), 310K (At-), 320K (Ru-) and 325K (Au-) methane can be completely oxidized to methanol without the emission of CO2. We conclude by recommending the investigation of the catalytic activities of combinations of the above negative ions for significant enhancement of the selective partial oxidation of methane to methanol.
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This paper has been withdrawn by the authors, because the authors have made abundant revisions and resubmitted the modified new version entitled Laser-induced atomic fragment fluorescence spectroscopy: A facile technique for molecular spectroscopy of spin-forbidden states to arXiv:0812.4554. Thanks for your attention.
Advanced oxidation processes that utilize highly oxidative radicals are widely used in water reuse treatment. In recent years, the application of sulfate radical (SO$_4cdot^-$) as a promising oxidant for water treatment has gained increasing attention. To understand the efficiency of SO$_4cdot^-$ in the degradation of organic contaminants in wastewater effluent, it is important to be able to predict the reaction kinetics of various SO$_4cdot^-$-driven oxidation reactions. In this study, we utilize density functional theory (DFT) and high-level wavefunction-based methods (including computationally-intensive coupled cluster methods), to explore the activation energies and kinetic rates of SO$_4cdot^-$-driven oxidation reactions on a series of benzene-derived contaminants. These high-level calculations encompassed a wide set of reactions including 110 forward/reverse reactions and 5 different computational methods in total. Based on the high-level coupled-cluster quantum calculations, we find that the popular M06-2X DFT functional is significantly more accurate for HO-additions than for SO$_4cdot^-$ reactions. Most importantly, we highlight some of the limitations and deficiencies of other computational methods, and we recommend the use of high-level quantum calculations to spot-check environmental chemistry reactions that may lie outside the training set of the M06-2X functional, particularly for water oxidation reactions that involve SO$_4cdot^-$ and other inorganic species.
We discuss the complicated resonance structure of the endohedral atom photoionization cross section. Very strong enhancement and interference patterns in the photoionization cross-section of the valent and subvalent subshells of noble gas endohedral atoms A@C60 are demonstrated. It is shown also that the atomic Giant resonance can be either completely destroyed or remains almost untouched depending on the velocity of photoelectrons that are emitted in the resonances decay process. These effects are results of dynamic modification of the incoming beam of radiation due to polarization of the fullerenes electron shell and reflection of photoelectrons be the fullerenes shell static potential. We have considered the outer np- and subvalent ns-subshells for Ne, Ar, Kr and Xe noble gas atoms. The modification of the Giant resonances is considered for a whole sequence of endohedrals with atoms and ions Xe, Ba, La, Ce+, Ce+4, Eu. The polarization of the fullerene shell is expressed via the total photoabsorption cross section. The photoelectron reflection from the static potential is taken into account in the frame of the so-called bubble potential that is a spherical -type potential.
We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+) followed by an e- transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate the LUMO of PyH+(E0calc ~ -1.3V vs. SCE) to form the solution phase PyH0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3]2+/ascorbate system. We predict that PyH0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.
Here, we report the observation of electron transfer mediated decay (ETMD) involving Mg clusters embedded in helium nanodroplets which is initiated by the ionization of helium followed by removal of two electrons from the Mg clusters of which one is transferred to the He environment neutralizing it while the other electron is emitted into the continuum. The process is shown to be the dominant ionization mechanism for embedded clusters for photon energies above the ionization potential of He. The photoelectron spectrum reveals a low energy ETMD peak. For Mg clusters larger than 5 atoms we observe stable doubly-ionized clusters. We argue that ETMD provides a new pathway to the formation of doubly-ionized cold species.
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