Surface functionalization and CO2 uptake on carbon molecular sieves: Experimental observation and theoretical study

The adsorption, formation, and interaction energies between carbon dioxide (CO2) and carbon oxide functional groups on porous carbon surface were analyzed through XPS, textural analysis, CO2 gas adsorption, and theoretical study. Carbon molecular sieves (CMSs) as porous carbon were modified by several concentrations of hydrogen peroxide (H2O2) solution under atmospheric conditions in an attempt to introduce carbon oxide groups and increase their CO2 adsorption capacity. Created oxide groups on carbon surface of CMSs were determined by XPS analysis and the CO2 adsorption capacities were investigated through the CO2 adsorption isotherms at 273 and 298 K at low pressure (max. 800 mmHg). The CO2 uptake capacity on CMSs modified by H2O2 was increased compared to an unmodified CMS and increased with increasing carboxylic (-COOH) group concentration on the carbon surface of CMSs. For a theoretical approach, binding energies between CO2 and various functional groups on the surface of CMSs have been investigated using several electronic structure calculations. As the result of the computational study by the MP2 method, a carboxylic group has the highest binding energy for CO2 (-COOH····CO2) of 4.45 kcal/mol, compared to quinone (dbndO) of 3.9, phenol (-OH) of 3.2 and lactone (-O-C=O) of 3.57 kcal/mol. This work demonstrates that introducing -COOH groups on CMS by H2O2 are a suitable modification for CO2 adsorption.