Theoretical Study of the Reaction Mechanism of Fe Atoms with H2O, H2S, O2 and H+

doi:10.1021/jp011324s

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Title:	Theoretical Study of the Reaction Mechanism of Fe Atoms with H2O, H2S, O2 and H+
Authors:	黃德彥;Hwang, Der-yan;Mebel, Alexander M.
Contributors:	淡江大學化學學系補正完畢
Date:	2001-08-09
Issue Date:	2009-12-01
Publisher:	American Chemical Society (ACS)
Abstract:	Density functional B3LYP/6-31G, B3LYP/6-311G, B3LYP/6-311+G(3df,2p), and ab initio CCSD(T)/6-311G calculations showed the reaction of free iron atoms with water in the ground quintet electronic state to proceed by the formation of a weakly bound Fe−OH2 molecular complex. The complex is slightly unbound at the CCSD(T)/6-311G level but stable according to density functional calculations and can isomerize to the HFeOH molecule, overcoming a barrier of 15−33 kcal/mol (with respect to the reactants), but further decomposition of HFeOH to FeO and H2 is hindered by a high barrier. In the presence of protons (in acidic environment), iron atoms can easily attach H+ with formation of the quintet FeH+ molecules. The reaction of these molecules with water, q-FeH+ + H2O → q-HFeOH2+ → q-FeOH+ + H2, is exothermic and occurs without activation barrier. In solution, q-FeOH+ may attach another proton (if the Coulomb repulsion barrier between the two ions can be overcome) and dissociate to q-Fe2+ and H2O, so the water molecule assists oxidation of a neutral iron atom to Fe2+, and two protons can be converted into molecular hydrogen transferring their charge to Fe. The FeH+ molecules are also shown to readily react with molecular oxygen, producing FeOOH+ without energy barrier. The FeH+ + O2 reaction is more facile than the reaction of FeH+ with water due to higher overall exothermicity (68−88 kcal/mol vs 20−34 kcal/mol for FeH+ + H2O → FeOH+ + H2) and a lower barrier for the intermediate reaction step (14−17 vs 35−46 kcal/mol), which can be rate-determining if the reaction occurs in solution. The reaction mechanism involving sequential Fe(5D) + H+ → q-FeH+, q-FeH+ + O2 → q-HFeO2+ → q-FeOOH+ reactions, followed by dissociation of q-FeOOH+ in solution yielding Fe2+, may be relevant to the first step of rusting. The calculations showed that electronically excited triplet iron atoms are more reactive with H2O. The triplet Fe + H2O → Fe−OH2 → HFeOH reaction is exothermic and has its transition state lying lower in energy than the reactants. No triplet-quintet intersystem crossing was found along the reaction pathway. The mechanism for the Fe + H2S reaction in the ground quintet electronic state is found to be similar to that for the reaction with water, but the critical barrier for the formation of the HFeSH intermediate is lower. Because of the reduced endothermicity of the Fe + H2S → FeS + H2 reaction and lower reaction barriers, the reaction of iron atoms with H2S is more likely to yield iron sulfide and molecular hydrogen than the reaction with water to produce FeO + H2.
Relation:	Journal of physical chemistry A 105(31), pp.7460-7467
DOI:	10.1021/jp011324s
Appears in Collections:	[Graduate Institute & Department of Chemistry] Journal Article

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