Effectsof Sulfur-Deficient Defect and Water on Rearrangementsof Formamide on Pyrite (100) Surface.

The efficient formation of HCN/HNC fromformamide (FM) combiningthe advantages of water-assistance, self-catalyzed reactions, andthe mineral surfaces was investigated. Periodic density functionaltheory calculations with plane-wave pseudopotential basis sets wereperformed to study the interaction of FM with pyrite (100) ideal anddefect surfaces. Effects of sulfur vacancy defect and water on tautomerizationand rearrangement barriers of FM on the (100) surface were evaluated.Calculated results show that FM adsorbs more strongly on the defectsurface than on the ideal surface, with the lowest adsorption energyon the defect surface being −22 kcal/mol. The energy barriersfor rearrangements of FM on these two surfaces being close to eachother suggests that the adsorptions on the surfaces have small effectson the energy barriers. The energy barriers for formimic acid isomerformations are 44.5 and 46.0 kcal/mol, and those of aminohydroxymethyleneformations are 72.6 and 71.9 kcal/mol on the ideal and defect surfaces,respectively. A reduction of ∼30 kcal/mol in tautomerizationenergy barriers is observed in water-assisted process on the defectsurface. Because this reduction is close to that of the gas-phasereactions, the catalytic effect is clearly due to the presence ofwater molecule instead of the interaction with the surface. In thiscase, the pyrite surfaces with the ability to accumulate reactivespecies only play the role of connecting bridges between the two stepsof the proposed reaction mechanism: the water-assisted rearrangementand the self-catalyzed dehydration. [ABSTRACT FROM AUTHOR]