Coordinatively Unsaturated Metal–Organic Frameworks M₃(btc)₂ (M = Cr, Fe, Co, Ni, Cu, and Zn) Catalyzing the Oxidation of CO by N₂O: Insight from DFT Calculations

Abstract

The oxidation of CO by N₂O over metal-organic framework (MOF) M₃(btc)₂ (M = Fe, Cr, Co, Ni, Cu, and Zn) catalysts that contain coordinatively unsaturated sites has been investigated by means of density functional theory calculations. The reaction proceeds in two steps. First, the N-O bond of N₂O is broken to form a metal oxo intermediate. Second, a CO molecule reacts with the oxygen atom of the metal oxo site, forming one C-O bond of CO₂. The first step is a rate-determining step for both Cu₃(btc)₂ and Fe₃(btc)₂, where it requires the highest activation energy (67.3 and 19.6 kcal/mol, respectively). The lower value for the iron compound compared to the copper one can be explained by the larger amount of electron density transferred from the catalytic site to the antibonding of N₂O molecules. This, in turn, is due to the smaller gap between the highest occupied molecular orbital (HOMO) of the MOF and the lowest unoccupied molecular orbital (LUMO) of N₂O for Fe₃(btc)₂ compared to Cu₃(btc)₂. The results indicate the important role of charge transfer for the N-O bond breaking in N₂O. We computationally screened other MOF M₃(btc)₂ (M = Cr, Fe, Co, Ni, Cu, and Zn) compounds in this respect and show some relationships between the activation energy and orbital properties like HOMO energies and the spin densities of the metals at the active sites of the MOFs.