(202c) Sp-D Orbital Hybridization Driven Metal-Graphene and Metal-Graphene-Metal Catalyst for Direct and Electrochemical Synthesis of Hcooh from CO2: First-Principles Approach

The efficient transformation of easily available CO₂ into renewable energy sources is a crucial for the decarbonization. Formic acid (FA), the simplest combination of hydrogen and CO₂, is one of the most promising hydrogen storage compounds for its high hydrogen density, non-toxicity, and great stability under ambient temperature and pressure. However, direct synthesis of CO₂ to FA on the noble metal and metal alloy catalysts has been a challenge for a decade since the reaction is thermodynamically unfavorable and kinetically difficult due to the nature of relatively weak _­adsorption of CO₂ on the metal surfaces in comparison to HCOO, and thus requires very high pressure/temperature. Electrochemical conversion of CO₂ on the conventional carbon black supported noble metal and metal alloy catalysts, in contrast, operates at ambient conditions, but suffer low stability through dissolution and agglomeration, and requires high overpotentials to overcome a competitive hydrogen evolution reaction (HER). In this context, we investigate a highly active and stable catalysts obtained by depositing a transition metal monolayer on graphene (M/Gr) using DFT method. It is found that the hybridization of sp and d orbitals between graphene and metal significantly reduces the local density of states of d-band of metals and increases s-and p-orbital of graphene near the fermi level at the same time, which leads to a strong covalent bonding between metal monolayer and graphene. Especially, the calculation of the orbital-resolved density of states for d band of M unravels that sharp d_z², d_yz, and d_xz orbital peaks below the fermi level play a decisive role in the formation of covalent bonding between M and Gr. In addition, it is found that the charge polarization on graphene in Pt/Gr and Pd/Gr not only enables a deposition of another metallic monolayer on the graphene, thus forming Pt/Gr/M or Pd/Gr/M sandwich structures, but also induces a transition from physisorption to chemisorption for M = Ag, Au, and Cu cases. Finally, it is discovered that the direct synthesis of FA from CO₂ is the most energetically favorable on Pt/Gr catalyst, and by evaluating the overpotentials required for the electrochemical CO₂ reduction to FA, among M/Gr, Pt/Gr/M, and Pd/Gr/M electrocatalysts Ni/Gr, Pd/Gr, Pt/Gr/Ag, and Pt/Gr have excellent activity and selectivity toward electrosynthesis of FA.