Pyridinic-N Coordination Effect on the Adsorption and Activation of CO2 by Single Vacancy Iron-Doped Graphene

Graphene doped with different transition metals has been recently proposed to adsorb CO2 and help reduce the greenhouse effect. Iron-doped graphene is one of the most promising candidates for this task, but there is still a lack of full understanding of the adsorption mechanism. In this work, we analyze the electronic structure, geometry, and charge redistribution during adsorption of CO2 molecules by single vacancy iron-doped graphene by DFT calculations using the general gradient approximation of Perdew, Burke, and Ernzernhof functional (PBE) and the van der Waals density functional (vdW). To understand the impact of the pyridinic-N coordination of the iron atom, we gradually replaced the neighboring carbon atoms by nitrogen atoms. The analysis indicates that chemisorption and physisorption occur when the molecule is adsorbed in the side-on and end-on orientation, respectively. Adsorption is stronger when pyridinic-N coordination increases, and the vdW functional describes the chemical interactions and adsorption energy differently in relation to PBE without significant structural changes. The development of the chemical interactions with the change of coordination in the system is further investigated in this work with crystal overlap Hamilton population (COHP) analysis.


Table of contents
Methodology for the calculation of BSSE Table S1 with the BSSE calculated parameters Table S2 with the comparison between 4x4 and 5x5 supercells Main physical characteristics of the substrates (Table S3) PDOS of the substrates (Figure S1) PDOS of Fe-0N and Fe-3N substrates using GGA+U (Figure S2) Initial configurations of the CO2(s)@Fe-0N system (Figure S3) Molecular dynamics simulation of CO2(s)@Fe-0N system at 500 K (Figure S4) Total density of states of the CO2 molecule (Figure S5) PDOS of CO2 and Fe atom on Fe-0N with vdW (Figure S6) COHP curves in the CO2(e)@Fe-0N with vdW (Figure S7) PDOS of CO2 and Fe atom on Fe-3N with vdW (Figure S8) COHP curves in the CO2(s)@Fe-0N with vdW (Figure S9)

S1.1 Metodology for basis set superposition error calculation
To calculate the error produced by BSSE we use the counterpoise correction.To do this, we are going to define two systems that we will call A and B. A in our case is the substrate (Fe-0N, Fe-1N, Fe-2N and Fe-3N) and B is the molecule (CO2).We define the notation of the energies in the following way: in the superscript the basis set used is indicated, in the subscript the geometry is indicated and in parentheses the system considered.
For example, the term    () is the energy corresponding to the calculation of system B (CO2 only) with the geometry it has when it is activated and using its own basis functions together with the "ghosts" basis functions of the substrate.They are called ghost basis functions because they are functions of system A (the substrate in our example) that are added to the functions of system B (CO2 in our example) to perform the single point calculation but without modifying the number of electrons, that is, they are "empty" functions basis.Then the BSSE-corrected adsorption energy is given by The following table shows the energies (eV) established in the equation for the CO2(e)@Fe-0N and CO2(s)@Fe-3N systems.
Configu ration  S3. binding energy, geometric parameters, charge transfer to the iron atom and magnetic moment.

Figure S5 .
Figure S5.Total density of states of the CO2 molecule.

Figure
Figure S6.PDOS curves of CO2 (a) and the Fe atom (b) in the CO2(e)@Fe-0N configuration with vdW.PDOS curves of CO2 (c) and the Fe atom (d) in the CO2(s)@Fe-0N configuration with vdW.

Table S2 .
Comparison of binding energy (eV), transferred charge (e) and geometric characteristics between the 4x4 and 5x5 supercell in the end-on configuration