Implicit Solvation Models: Equilibria, Structure, Spectra, and Dynamics.

The influence of cobalt particle size in the range of 2.6-27 nm on the performance in Fischer-Tropsch synthesis has been investigated for the first time using well-defined catalysts based on an inert carbon nanofibers support material. X-ray absorption spectroscopy revealed that cobalt was metallic, even for small particle sizes, after the in situ reduction treatment, which is a prerequisite for catalytic operation and is difficult to achieve using traditional oxidic supports. The turnover frequency (TOF) for CO hydrogenation was independent of cobalt particle size for catalysts with sizes larger than 6 nm (1 bar) or 8 nm (35 bar), while both the selectivity and the activity changed for catalysts with smaller particles. At 35 bar, the TOF decreased from 23 x 10(-3) to 1.4 x 10(-3) s(-1), while the C5+ selectivity decreased from 85 to 51 wt % when the cobalt particle size was reduced from 16 to 2.6 nm. This demonstrates that the minimal required cobalt particle size for Fischer-Tropsch catalysis is larger (6-8 nm) than can be explained by classical structure sensitivity. Other explanations raised in the literature, such as formation of CoO or Co carbide species on small particles during catalytic testing, were not substantiated by experimental evidence from X-ray absorption spectroscopy. Interestingly, we found with EXAFS a decrease of the cobalt coordination number under reaction conditions, which points to reconstruction of the cobalt particles. It is argued that the cobalt particle size effects can be attributed to nonclassical structure sensitivity in combination with CO-induced surface reconstruction. The profound influences of particle size may be important for the design of new Fischer-Tropsch catalysts.

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Cobalt particle size effects in the fischer- : Tropsch reaction studied with carbon nanofiber supported catalysts

ion step. The reactivity of pMMO has been also explored by Chan,525 by means of DFT calculations on small models of Figure 116. Nonradical mechanism proposed in the Yoshizawa model of sMMO. Figure 117. Mononuclear and dinuclear models of the copper active sites of pMMO. CsH Bond Activation in Transition Metal Species Chemical Reviews, 2010, Vol. 110, No. 2 813

C-H bond activation in transition metal species from a computational perspective

Theoretical studies on reactions of transition-metal complexes.

Following the heme paradigm, it is often proposed that dioxygen activation by nonheme monoiron enzymes involves an iron(IV)=oxo intermediate that is responsible for the substrate oxidation step. Such a transient species has now been obtained from a synthetic complex with a nonheme macrocyclic ligand and characterized spectroscopically. Its high-resolution crystal structure reveals an iron-oxygen bond length of 1.646(3) angstroms, demonstrating that a terminal iron(IV)=oxo unit can exist in a nonporphyrin ligand environment and lending credence to proposed mechanisms of nonheme iron catalysis.

https://science.sciencemag.org/content/sci/299/5609/1037.full.pdf

Crystallographic and spectroscopic characterization of a nonheme Fe(IV)-O complex.

Solutions of lithium hexafluorophosphate, LiPF6, in propylene carbonate (4-methyl-1,3-dioxolan-2-one; denoted by PC hereafter) in the concentration range from 0.0 to 3.29 M (M = mol dm-3) have been studied regarding their conductivities, viscosities, and self-diffusion coefficients of PC by the NMR field gradient technique, Raman spectra, and NMR spectra. Walden's products are almost constant in the range up to and over 3.0 M. Therefore, Li ions are considered to be quite free from the firm interaction with anions even in such concentrated solutions. The appearance of the maximum conductivity at about 0.8 M is explained by associating with the concentration dependence of the solution viscosity. A remarkable increase in the solution viscosity was observed in a concentration beyond 2.0 M, and it can be ascribed to the cluster formation of lithium ions with PC molecules of the solvent. Such an idea of clusters can reasonably interpret some of the characteristic changes of the viscosities, the diffusion coeff...

Conductivity and Solvation of Li+ Ions of LiPF6 in Propylene Carbonate Solutions

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH2)(H2O)Fe(μ-O)2(η2-HCOO)2Fe(NH2)(H2O), I. The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two FeIV atoms, L4FeIV(μ-O)2FeIVL4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4FeIV(μ-O)(μ-OH(···CH3))FeIIIL4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an FeIII−FeIV mixed valence state. This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the...

Mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase: A density functional study

The hybrid density functional method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by the methane monooxygenase (MMO) enzyme. The key reactive compound Q of MMO was modeled by cis-(H2O)(NH2)Fe(μ-O)2(η2-HCOO)2 Fe(NH2)(H2O), I, where the substrate molecule may coordinate to the bridging oxygen atoms, O1 and O2, located on the H2O and NH2 sides, leading to two different mechanisms, O-side and N-side pathways, respectively. Previously we have detailed the N-side pathway (Basch, H.; Mogi, K.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1999, 121, 7249); here we discuss the O-side pathway, and compare the two. Calculations show that, like the N-side pathway, the O-side pathway of the reaction of I with CH4 proceeds via a bound-radical mechanism. It starts from the bis(μ-oxo) compound I and goes over the rate-determining transition state III_O for H abstraction from methane to form a weak complex IV_O between the Fe(μ-O)(μ-OH)Fe moiety and a methyl radical. This bound-ra...

Theoretical studies on the mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase: O-side vs N-side mechanisms

Theoretical study of adsorption of SO2 on Ni(1 1 1) and Cu(1 1 1) surfaces

B3LYP level optimizations were performed on the structures of the octasilsesquioxane (Si8O12H8, HT8) double four-ring (D4R) cage and single hydrogen atom-trapped HT8 (H@HT8). Moreover, the transition state in the detrapping process of the hydrogen atom from the D4R cage was examined. The basis sets used were 6-31G** for HT8 and (3 1 1/1*1*/1*) for the trapped hydrogen atom. Both HT8 and H@HT8 were structure-optimized with Oh molecular symmetry and the resulting cage conformations were similar. The trapped H atom was located at the center of the D4R cage. The weak interaction between the D4R cage and the trapped H atom in H@HT8 was determined by examining the singly occupied molecular orbital (SOMO) [8a1g] of H@HT8. The SOMO was constructed from an antibonding interaction between the lowest unoccupied molecular orbital (LUMO) [8a1g] of HT8 and the 1s orbital of the trapped H atom. For the transition state, the structure was optimized with C4v molecular symmetry. As a result, the position of the Si8 cube fr...

Koichi Mogi

Papers

Conductivity and Solvation of Li+ Ions of LiPF6 in Propylene Carbonate Solutions

Mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase: A density functional study

Theoretical studies on the mechanism of the methane → methanol conversion reaction catalyzed by methane monooxygenase: O-side vs N-side mechanisms

Theoretical study of adsorption of SO2 on Ni(1 1 1) and Cu(1 1 1) surfaces

Studies on the trapping and detrapping transition states of atomic hydrogen in octasilsesquioxane using the density functional theory B3LYP method