Effect of Magnetic States on the Reactivity of an FCC(111) Iron Surface

We have carried out calculations to compare the catalytic activity of ferromagnetic (FM) and antiferromagnetic (AFM) states of the (111) surface of an FCC iron film by evaluating the reaction energy and activation barrier for the dissociation of H-2 and CO. The calculations were carried out using density functional theory and a meta-GGA approximation of the energy functional. The results show significant sensitivity to the magnetic state of the surface. We find that charge transfer upon adsorption is more efficient at the AFM2 surface. For H-2, this leads to barrierless dissociative adsorption on the AFM2 surface, while a molecular adsorbed state is metastable at the FM surface, separated by an activation energy of 0.35 eV from the dissociated state. CO adsorbs more strongly on the AFM2 surface, but the activation energy for dissociation is 0.27 eV lower on the FM surface mainly because the energy of the transition state is lower. The difference in reactivity is analyzed in terms of integrated density difference and spin-dependent projected density of states. It is concluded that empty, back-bonding states offer a more favorable overlap at the FM surface leading to a more facile CO dissociation. The results indicate that catalytic activity can be strongly affected by the application of a magnetic field when it can induce a change in the magnetic state of the catalyst.