Effect of Fe/N-doped carbon nanotube (CNT) wall thickness on CO2 conversion: A DFT study

Abstract

Many researches on CO2 adsorption using carbon nanotubes (CNTs) have been actively studied, but experimental and theoretical studies on CO2 conversion are still in demand. In particular, the effect of CNT wall thickness on CO2 conversion is not yet established clearly. This study employed two different-walled CNT catalysts doped with iron and nitrogen, single-walled CNT (Fe-N-SWCNT) and double-walled CNT (Fe-N-DWCNT). The structural and electrical properties of these CNTs and their influences on CO2 conversion were characterized and compared using density functional theory (DFT) calculations. As a result, Fe-N-DWCNT was shown to improve catalyst stability with higher formation energy and adsorption energy for CO2 adsorption than Fe-N-SWCNT. Also, the CO2 molecules were found to be highly delocalized and strongly hybridized with Fe-N-DWCNT, leading to more active charge transfer in the catalyst. These findings demonstrate the potential of selective CO2 conversion, as wall thickness differences can lead to different electrical properties of CNTs by showing that the larger the thicknesses, the lower the energy barrier required for CO2 conversion. Specifically, Fe-N-DWCNT is easier to convert CO2 to HCOOH than Fe-N-SWCNT at lower overpotential (0.15 V) obtained with limiting potentials and free energies calculated by understanding the possible reaction pathways in the proton-electron transfer process. Therefore, these results support the hypothesis that the wall thickness of CNT influences CO2 conversion by showing that the double-walled heterogeneous CNT (Fe-N-DWCNT) is a potential catalyst to selectively produce HCOOH from CO2 conversion.