N2 dissociation on Fe(111), Fe(211), Fe(110), and Fe(100) surfaces does not obey the Brønsted-Evans-Polanyi relationship. It has the smallest adsorption energy and the lowest energy barrier on the Fe(111) surface because more charges are transferred from the Fe(111) surface to the absorbed antibonding orbitals (π*) of N2, leading to a weaker N≡N bond and a lower N atom adsorption energy.
Current industrial ammonia synthesis depends on the Haber-Bosch process, in which the activity of the catalyst is limited by the Brønsted-Evans-Polanyi (BEP) principle and Fe is used as a commercial catalyst. Herein, we found that the dissociation barriers of N2 on Fe(111), Fe(211), Fe(110), and Fe(100) surfaces do not follow the widely accepted BEP principle. N2 dissociation on Fe(111) surface has the smallest adsorption energy and the lowest energetic barrier. Such an abnormal phenomenon can be attributed to charge transfer from Fe surfaces to the anti-bonding orbital (π*) of the absorbed N2. More charges transferred from the Fe surface to π* of N2 leads to a weaker N≡N triple bond and a lower adsorption energy of N atoms. However, the hydrogenation of N atoms and desorption of NH3 on the four Fe surfaces follow the BEP principle. Therefore, Fe(111) is found to be the most active surface to promote ammonia synthesis, and such a conclusion is also applicable to Ni and Mo surfaces.Zum Volltext