Selectively Adsorbed CO and O₂ on Transition-Metal-Incorporated Porphyrin

We have investigated the adsorption of CO and O₂ on transition-metal (TM)-incorporated porphyrin. Our calculations show that one or more CO molecules can be adsorbed on the Sc-, Ti-, V-, Cr-, or Mn-incorporated porphyrin, while only one CO molecule is adsorbed on the Fe- or Co-incorporated porphyrin. In the case of O₂, only one O₂ molecule prefers to adsorb on the TM-incorporated porphyrin, regardless of TM. Moreover, O₂ is strongly bound to the Sc-, Ti-, V-, Cr-, or Mn-incorporated porphyrin compared with that of CO, while it is weakly bound to the Fe- or Co-incorporated porphyrin compared with that of CO. Such different binding behaviors can be explained by variations of the highest occupied molecular orbital (HOMO) level in the d-orbitals of TM-incorporated porphyrin. We find that the HOMO levels are shifted downward in the cases of O₂ adsorption on the Sc-, Ti-, and V-incorporated porphyrin and CO adsorption on the Fe- and Co-incorporated porphyrin, indicating the stronger binding energies between them. These studies can be useful to understand the capture of CO and O₂ by the TM-incorporated porphyrin.