Browsing by Author "Aniagyei, Albert"

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    Ab initio investigation of O2 adsorption on Ca-doped LaMnO3 cathodes in solid oxide fuel cells
    (Royal Society of Chemistry, 2018) Aniagyei, Albert; Dzade, Nelson Y.; Tia, Richard; Adei, Evans; Catlow, C. R. A.; et. al
    We present a Hubbard-corrected density functional theory (DFT+U) study of the adsorption and reduction reactions of oxygen on the pure and 25% Ca-doped LaMnO3 (LCM25) {100} and {110} surfaces. The effect of oxygen vacancies on the adsorption characteristics and energetics has also been investigated. Our results show that the O2 adsorption/reduction process occurs through the formation of superoxide and peroxide intermediates, with the Mn sites found to be generally more active than the La sites. The LCM25{110} surface is found to be more efficient for O2 reduction than the LCM25{100} surface due to its stronger adsorption of O2, with the superoxide and peroxide intermediates shown to be energetically more favorable at the Mn sites than at the Ca sites. Moreover, oxygen vacancy defect sites on both the {100} and{110} surfaces are shown to be more efficient for O2 reduction, as reflected in the higher adsorption energies calculated on the defective surfaces compared to the perfect surfaces. We show from Lowdin population analysis that the O 2 adsorption on the pure and 25% Ca-doped ̈ LaMnO3 surfaces is characterized by charge transfer from the interacting surface species into the adsorbed oxygen pg orbital, which results in weakening of the O–O bonds and its subsequent reduction. The elongated O–O bonds were confirmed via vibrational frequency analysis.
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    A density functional theory study of the mechanisms of oxidation of ethylene by rhenium oxide complexes†
    (Elsevier, 2013-04-01) Aniagyei, Albert; Tia, Richard; Adei, Evans
    The oxo complexes of group VII B are of great interest for their potential toward epoxidation and dihy-droxylation. In this work, the mechanisms of oxidation of ethylene by rhenium-oxo complexes of thetype LReO3(L = O−, Cl, CH3, OCH3, Cp, NPH3) have been explored at the B3LYP/LACVP* level of theory.The activation barriers and reaction energies for the stepwise and concerted addition pathways involvingmultiple spin states have been computed. In the reaction of LReO3(L = O−, Cl, CH3, OCH3, Cp, NPH3) withethylene, it was found that the concerted [3 + 2] addition pathway on the singlet potential energy sur-faces leading to the formation of a dioxylate intermediate is favored over the [2 + 2] addition pathwayleading to the formation of a metallaoxetane intermediate and its re-arrangement to form the dioxylate.The activation barrier for the formation of the dioxylate on the singlet PES for the ligands studied isfound to follow the order O−>CH3> NPH3>CH3O−>Cl−> Cp and the reaction energies follow theorder CH3>O−> NPH3>CH3O−>Cl−> Cp. On the doublet PES, the [2 + 2] addition leading to the for-mation the metallaoxetane intermediate is favored over dioxylate formation for the ligands L = CH3,CH3O−,Cl−. The activation barriers for the formation of the metallaoxetane intermediate are found toincrease for the ligands in the order CH3
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    A density functional theory study of the mechanisms of oxidation of ethyleneby technetium oxo complexes
    (Elsevier B.V, 2013-01-22) Aniagyei, Albert; Tia, Richard; Adei, Evans
    The mechanisms of oxidation of ethylene by transition metal-oxo complexes of the type LTcO3(L = O , Cl,CH3, OCH3, Cp, NPH3) have been explored by computing the activation barriers and reaction energies forthe concerted and stepwise addition pathways at the density functional theory B3LYP/LACVP level oftheory. The results indicate that in the reaction of LTcO3(L = O , Cl, CH3, OCH3, Cp, NPH3) with ethylene,the formation of the dioxylate intermediate through the concerted [3 + 2] addition pathway on the singletpotential energy surface is favored kinetically and thermodynamically over its formation through thetwo-step process via the metallaoxetane intermediate. The activation barrier for the formation of thedioxylate on the singlet PES for the ligands studied is found to follow the order: O >CH3> NPH3>CH3O >Cl > Cp while the reaction energies follow the order: Cl >O >CH3> NPH3>CH3O > Cp. Onthe doublet PES, the [2 + 2] addition leading to the formation of the four-membered metallacycle inter-mediate is favored kinetically and thermodynamically for the ligands when L = NPH3. The direct [2 + 1]addition of ethylene across the oxo- ligand of doublet TcO3(CH3) to form the epoxide precursor is favoredwhen L = CH3. The activation barriers for the formation of the dioxylate intermediate are found to followthe order: Cl
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    Permanganyl chloride-mediated oxidation of tetramethylethylene: Adensity functional theory study
    (Elsevier Inc., 2020-04-08) Aniagyei, Albert; Kwawu, Caroline; Tia, Richard; Adei, Evans
    The mechanisms of the oxidation of tetramethylethylene (TME) by permanganyl chloride (MnO3Cl) havebeen explored on the singlet and triplet potential energy surfaces at the B3LYP LANL2DZ/6-31G (d) levelof theory. The results show that the pathway leading to the formation of thefive-membered dioxylatethrough concerted [3þ2] addition is favored kinetically and thermodynamically over the three otherpossible pathways, namely the [2þ2] addition via the transient metallaoxetane intermediate, epoxi-dation, and hydrogen transfer pathways. The epoxide precursor that on hydrolysis would yield theepoxide product will most likely arise from a stepwise path through the intermediacy of an organo-metallic intermediate. This pathway affords the product that is more stable (thermodynamically favor-able). However, kinetically, both the stepwise and the concerted [2þ1] addition pathways leading to theepoxide precursors are very competitive (activation barrier difference of<0.7 kcal/mol).
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    Quantum chemical studies of the addition of transition metal oxides of the type LMO3(L = O- , Cl-, NPH3, CH3, OCH3, Cp) to ethylene
    (2012-06-15) Aniagyei, Albert
    The oxo complexes of group VII (Mn, Tc, and Re) are of great interest for their potential toward epoxidation and dihydroxylation. In this work, the mechanisms of oxidation of ethylene by the group VII transition metal-oxo complexes of the type LMO3 (M= Mn, Tc, Re and L= O-, Cl, CH3, OCH3, Cp) are explored at the B3LYP/LACVP* level of theory. The activation and reaction energies for the stepwise and concerted addition pathways along spin states other than the singlet which could ultimately lead to exploration of the extent of the two state reactivity was also investigated. In the reaction of LMnO3 (L= Cp, CH3), LTcO3 (L= O- , Cl, Cp, OCH3) and LReO3 (L= Cp, O-) with ethylene, it was found that the direct [3+2] addition pathway on the doublet potential energy surfaces leading to the formation of the dioxylate intermediate is favored kinetically and thermodynamically over the two-step process via the metallaoxetane intermediate to form the dioxylate. However, in the CH3TcO3 system, the formation of the doublet epoxide precursor is kinetically the most favorable pathway. The [2+2] addition pathway leading to the formation of the four membered metallacycle was found to be the kinetically and thermodynamically most viable reaction path for the NPH3TcO3. The formation of the four membered metallacycle intermediate is kinetically and thermodynamically more favorable than the direct [3+2] addition pathway for LReO3 (L= CH3, OCH3, Cl, NPH3) on the doublet surface. However, in the ReO3(OCH3) system, the reaction occurs on the doublet potential energy surface. The activation energy for the formation of the dioxylate via the metallaoxetane intermediate and the direct [3+2] addition of the ethylene to the oxo-complex were found to be comparable along the doublet potential energy surface. The formation of the epoxide precursor will not result from the reaction of LMO3 (M= Mn, Tc, Re, L= O- , Cp) with ethylene on all the surfaces studied.
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    A theoretical study of the mechanisms of oxidation ofethylene by manganese oxo complexes†
    (Royal Society of Chemistry, 2013) Aniagyei, Albert; Tia, Richard; Adei, Evans
    The mechanisms of oxidation of ethylene by manganese–oxo complexes of the type MnO3L(L=O−, Cl, CH3, OCH3, Cp, NPH3) have been explored on the singlet, doublet, triplet and quartet potentialenergy surfaces at the B3LYP/LACVP* level of theory and the results discussed and compared with thoseof the technetium and rhenium oxo complexes we reported earlier, thereby drawing group trends in thereactions of this important class of oxidation catalysts. In the reactions of MnO3L(L=O−,Cl−,CH3, OCH3,Cp, NPH3) with ethylene, it was found that the formation of the dioxylate intermediate along the con-certed [3 + 2] addition pathway on the singlet potential energy is favored kinetically and thermodynami-cally over its formation by a two-step processviathe metallaoxetane by [2 + 2] addition. The activationbarriers for the formation of the dioxylate and the product stabilities on the singlet PES for the ligandsstudied are found to follow the order: NPH3