Exploring different reaction mechanisms for oxidation of CO over a single Pd atom incorporated nitrogen-doped graphene: A DFT study

Single-metal catalysts have attracted particular interests in recent years due to their outstanding catalytic activity in CO oxidation reaction. In the present study, periodic DFT calculations are performed to investigate possible reaction mechanisms for oxidation of CO over a single Pd atom incorporated nitrogen-doped graphene. According to our results, the formation energy of a single-vacancy decreases by incorporation of pyridinic nitrogen atoms in graphene. The Pd atom can be stably anchored over the vacancy site, as evidenced by the large energy barriers for diffusion of the Pd atom to its neighboring sites. It is found that the incorporation of nitrogen atoms makes a significant increase in the adsorption energy of O-2 and CO molecules over the supported Pd atom. The latter can be attributed to the shift in the bonding states of the Pd towards the Fermi level, which facilitates the charge-transfer from the 4d orbitals of this atom to the 2 pi* orbitals of O-2 and CO molecules. The CO oxidation over the Pd atom supported nitrogen-doped graphene can proceed via three different mechanisms; namely, Eley-Rideal (ER), Langmuir-Hinshelwood (LH) and termolecular Eley-Rideal (TER) mechanisms. Our calculations reveal that the TER mechanism is preferred over the LH and ER, due to its smaller activation energy. The CO oxidation via the TER mechanism is a thermodynamically favorable process over the supported Pd atom, due to a large negative change in the Gibbs free energy of this reaction. The findings of this study can be useful to design more efficient single-atom catalysts to remove toxic CO molecules.