Catalytic conversion of CO2 to liquid fuels
or valuable
chemicals is an attractive alternative to geological sequestration.
In the present study, we applied density functional theory slab calculations
in the investigation of the adsorption and hydrogenation of CO2 on the (110) surface of In2O3. Our
results indicate that the adsorbed CO2 is activated, forming
a surface carbonate species by combining with surface oxygen, and
has an adsorption energy of −1.25 eV. Heterolytic dissociative
adsorption of H2 results in a surface hydroxyl from H binding
the surface O site and a hydride from H binding the In site. The migration
of H from the In site to the neighboring O site is energetically favorable
but has a significant activation barrier of 1.32 eV. Water may adsorb
on the surface either molecularly or dissociatively, with adsorption
energy of −0.83 eV and −1.19 eV, respectively. Starting
from CO2 coadsorbed with the H adatoms on the In2O3 surface, we examined two possible conversion pathways
for CO2: (a) CO2 is hydrogenated by the H adatom
on the In site to form a surface formate species (HCOO); (b) CO2 is protonated by the H adatom on the O site to form a surface
bicarbonate species (COOH). Reaction a is endothermic by +0.33 eV,
whereas b is exothermic by −0.78 eV. Although the formation
of the bicarbonate species is energetically favorable, the subsequent
step to form CO and OH is highly endothermic, with a reaction energy
of +1.07 eV. Furthermore, the bicarbonate species can react with a
surface hydroxyl easily, resulting in coadsorbed H2O and
CO2. These results indicate that hydrogenation of CO2 to the formate species is favorable over protonation to the
bicarbonate species on the In2O3 surface. These
results are consistent with the experimental observations that the
indium oxide based catalyst has a high CO2 selectivity
and H2O resistance.