CO2 activation at atomically dispersed Si sites of N-doped graphenes: Insight into distinct electron mechanisms from first-principles calculations

Two types of single-atom Si-embedded N-doped graphene sheets, denoted as SiNxC3−x and SiNxC4−x, were designed for CO2 activation and electroreduction. The first-principles calculations show that CO2 can be chemically adsorbed at the single-atom Si sites of SiN1C2, SiN2C1, SiN3C0, SiN3C1, and SiN4C0 monolayers with quite low-energy barriers and exothermicity to some extent. Unexpectedly, CO2 activation and capture at the atomically dispersed Si sites of SiNxC3−x and SiNxC4−x follow different electron mechanisms where the three-coordinated Si in SiNxC3−x behaves as an electron donor while the four-coordinated Si acts as an electron shuttle for the electron transfer from the SiNxC4−x framework to CO2. For SiNxC4−x, the low-energy Si-pz center is a prerequisite for the Si site to capture the electron from the support framework, which is beneficial for the electron transfer to CO2. The activity of SiNxC3−x depends on both the Si-pz band center and the electron population at the three-coordinated Si, resulting in the conventional linear correlation between the activity and the p-band center not being observed. Furthermore, the SiN3C0 sheet is predicted to be quite a promising electrode material for CO2 electrochemical reduction to HCOOH, CH3OH, and CH4 with quite low limiting potentials.