Mechanisms of Complete Dissociation of CO₂ on Iron Clusters

Dissociation of CO₂ on iron clusters was studied by using semilocal density functional theory and basis sets of triple-zeta quality. Fe₂ , Fe₄ , and Fe₁₆ clusters were selected as the representative host clusters. When searching for isomers of Fe_n CO₂ , n=2, 4 and 16 corresponding to carbon dioxide attachment to the host clusters, its reduction to O and CO, and to the complete dissociation, it was found that the total spin magnetic moments of the lowest energy states of the isomers are often quenched with respect to those of initial reagents Fe_n +CO₂ . Dissociation pathways of the Fe₂ +CO₂ , Fe₄ +CO₂ , and Fe₁₆ +CO₂ reactions contain several transition states separated by the local minima states; therefore, a natural question is where do the spin flips occur? Since lifetimes of magnetically excited states were shown to be of the order of 100 fs, the search for the CO₂ dissociation pathways was performed under the assumption that magnetic deexcitation may occur at the intermediate local minima. Two dissociation pathways were obtained for each Fe_n +CO₂ reaction using the gradient-based methods. It was found that the Fe₂ +CO₂ reaction is endothermic with respect to both reduction and complete dissociation of CO₂ , whereas the Fe₄ +CO₂ and Fe₁₆ +CO₂ reactions are exothermic to both reduction and complete dissociation of carbon dioxide. The CO₂ reduction was found to be more favorable than its complete dissociation in the Fe₄ case.