Single Atom Catalysis in aqueous conditions: enhanced interfacial water dissociation on a Fe-porphyrin graphene defect

Single Atom Catalysis (SAC) is an expanding field of heterogeneous catalysis in which single metallic atoms embedded in different materials catalyse a chemical reaction, in order to reduce the cost and the environmental impact of the catalyst. SAC could for example replace crystalline Pt - the workhorse of electrocatalysis - but these new catalytic materials still lack fundamental understanding in their electrochemical environment. Recent synthesis and spectroscopic characterizations of metals deposited on N-doped graphitic materials have evidenced two types of metal-N$_4$ fourfold coordination, either of pyridine type or of porphyrin type. Here, we study at the quantum level two Fe-N$_4$ SAC defects in graphene that are immersed in an explicit aqueous medium. While the pyridine Fe-N$_4$ SAC is easily embedded in graphene and widely used in numerous DFT studies, the porphyrin one remains debated because of the necessary embedding of odd-membered rings in graphene. We propose an atomistic model for the porphyrin defect in a large graphene sheet at a moderate strain cost. Using spin-polarized \textit{ab initio} molecular dynamics, we show that both SAC defects spontaneously adsorb two interfacial water molecules on both sides of the catalytic site, retrieving an octahedral ligand field. Strickingly, the porphyrin defect promotes the dissociation of an adsorbed water into a hydroxide anion. To induce proton transfer reactions on the time scale of the simulations, we also apply moderate external electric fields. On the other hand, the pyridine defect preserves the water ligands intact. This unveils a different catalytic reactivity between the two SAC motives in liquid water, therefore promoting the porphyrin defect as a superior SAC electrocatalyst.