Comparative study on the activities of different MgO surfaces in CO2 activation and hydrogenation

Abstract

The first-principles calculations based on the density functional theory (DFT) have been applied to explore the CO2 adsorption as well as the activity and selectivity of CO2 dissociation and hydrogenation on six MgO surfaces. CO2 activation can be occurred by forming carbonate species on (110), (210), (211), (111)O and (111)Mg surfaces but the direct dissociation of CO2 looks thermodynamically unfavorable due to the high reaction energies (≥3.54 eV). The occurrence of CO2 protonation to carboxyl (HOCO) by hydroxyl on MgO surfaces seems to be thermodynamically and kinetically unfavorable. The upsurge noted in the hydroxyl coverage, due to the homolytic dissociation of H2, supports the CO2 conversion to HOCO with a lower barrier (1.98 eV). However, the dissociated H2O can reversibly lead to the consumption of HOCO (HOCO*+ OH*→ H*+ OH*+ CO2*). Hydride species might be produced by the heterolytic dissociation of H2 on (111)O, (111)Mg, and (211) surfaces. Two pathways have been considered for the CO2 hydrogenation to HCOO: (i) hydride H reaction with adsorbed CO2 by Langmuir-Hinshelwood mechanism and (ii) molecular CO2 electrophilic attacking the hydride H by Eley-Rideal mechanism. CO2 hydrogenation to HCOO by hydride H is more feasible on MgO surfaces in comparison with CO2 protonation.