Dissociative and Associative Attachment of CO, N2, and NO to Iron Clusters Fe4, Fe4–, and Fe4+

The lowest energy states of NFe4N, NFe4O, Fe4N2, Fe4NO, and CFe4O, along with their singly negatively and positively charged ions, are optimized using density functional theory with generalized gradient approximation for the exchange-correlation functional (DFT-GGA). It is found that NO attaches dissociatively, independent of the charge on the Fe4 cluster, while N2 attaches dissociatively to Fe4 and Fe4−, but associatively to Fe4+. CO attaches associatively independent of the cluster charge. The results of these computations, along with the results of previous calculations, are used for evaluating the energetics of Fe4A+B→Fe4C +D, where A, B, C, and D are C, O, N, their dimers, or NCO. A strong dependence on the order of attachment of reagents to the Fe4 cluster is observed. For example, the Fe4CO+O2→Fe4O+CO2 is highly exothermic (-4.48 eV), while the complimentary reaction Fe4O2 + CO → Fe4O+CO2 is endothermic (+0.43 eV).