Significant levels of nitrate in ground water can be harmful to human health. Existing nitrate removal processes are expensive and increase the cost of the water supply, therefore it is important to find affordable methods to control nitrate levels. To this end, density functional theory (DFT) calculations were performed using the B3LYP+D2 functional on a promising homogeneous nitrate reduction electrocatalyst, Co(DIM) [DIM = 2,3-dimethyl- 1,4,8,1 l-tetraazacyclotetradeca- 1,3-diene]. Unlike other similar Co-based electrocatalysts, Co(DIM) is capable of completely reducing nitrate to ammonia, as opposed to stopping the reduction at harmful intermediates such as nitrite and hydroxylamine. DFT was used to analyze structural and electronic features of potential mechanisms for the initial reduction of nitrate to nitrite, and these results yielded many insights as to the success of the catalyst in reducing nitrate. One important feature of Co(DIM) is the role of redox non-innocence, as the DIM ligand itself is capable of acting as a reservoir for electrons at critical points in the mechanism, potentially mitigating the formation of a highly reduced metal center. In addition to DIM, lowest energy pathways for nitrite formation show formation of an unusual low-valent Co-oxyl radical species. It appears that the flexibility of the DIM ligand plays an integral part in the mechanism as well. By folding DIM concomitantly with N-O bond cleavage, nitrite is able to rebind the metal center and form a six-coordinate octahedral structure, which significantly lowers the calculated activation energy. Resulting nitrite-bound structures can then potentially re-enter the catalytic cycle leading to the further reduction of nitrite.

Role of redox non-innocence in a Co-based nitrate reduction electrocatalyst

255th National Meeting (2018)

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