The synergistic effects of surface functional groups and pore sizes on CO2 adsorption by GCMC and DFT simulations

In this work, the capture property of CO2 molecules on M-doped (M = N, P, S, and O) functionalized-graphite surfaces with different pore sizes (0.8–5.0 nm) were investigated by using grand canonical Monte Carlo simulation (GCMC) and density functional theory (DFT). The synergistic effects of surface functional groups and pore sizes on the adsorption behavior of CO2 on functionalized surfaces were elucidated. At low pressures, the surface functional groups presented a significant enhancement on CO2 adsorption performance, regardless of the pore size. At high pressures, for pore size being larger than 1.0 nm, the surface functional groups made an important contribution on the saturated CO2 adsorption capacity. Among all the surface functional groups, the P-doped functionalized-graphite surfaces had a prominent influence on CO2 uptake owning to the strong electron-gaining/donating capacity and high adsorption energies. At low pressures, G-CO─PO(OH)2 exhibited a superior CO2 capture performance (6.0 mmol/cm3 for 1.0 nm pore) at 16 kPa. At high pressures, G-C3─P has the maximum CO2 uptake (15.3 mmol/cm3 for 1.0 nm pore) at 300 kPa. Based on the comprehensive research of various polar functional groups and pore sizes, this study clarified the intrinsic enhancement mechanism in adsorption capacity of functionalized-graphite surfaces, which would pave an alternative way in the design and synthesis of carbon materials for gas capture.