Adsorption of CO2 on triamine-grafted pore-expanded mesoporous silica, TRI-PE-MCM-41, was investigated from very low pressure to 1 bar at four temperatures (298, 308, 318, 328 K) using gravimetric measurements. TRI-PE-MCM-41 exhibited one of the highest equilibrium capacities compared to other typical CO2 adsorbents such as zeolites, activated carbons, and metal-organic frameworks (MOFs). In contrast, under the same pressure and temperature conditions, TRI-PE-MCM-41 exhibited very small uptakes of N-2, CH4, H-2, and O-2. Column-breakthrough measurements of CO2 in mixtures with other species showed exceedingly high selectivity of CO2 over N-2, CH4, H-2, and O-2 even at very low CO2 concentrations, indicating that TRI-PE-MCM-41 is suitable adsorbent for gas purification applications. Moreover, water vapor was found to have a beneficial effect on CO2 adsorption capacity even at very low CO2 partial pressure, e.g. 400 ppm, without adverse effect on CO2 selectivity.