Enhanced adsorption of CO2 at steps of ultrathin ZnO: the importance of Zn–O geometry and coordination

Abstract

The interaction between CO₂ and ultrathin ZnO supported on Au(111) has been studied using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. We find that CO₂ binds weakly on the planar ZnO bilayer and trilayer surfaces, desorbing at T = 130 K. CO₂ binds more strongly at the steps formed between ZnO bilayers and trilayers, desorbing at T = 285–320 K depending upon the CO₂ exposure. The adsorption energies determined from DFT calculations for CO₂ on the ZnO planar surfaces and at the steps are ∼5.8 and 19.0 kcal mol⁻¹, respectively, agreeing with the apparent activation energies of desorption (E_d) estimated based on the TPD peaks at the limit of low CO₂ exposures (7.7 and 19.5 kcal mol⁻¹, respectively). The DFT calculations further identify that the most stable adsorption configuration of CO₂ at the steps of ultrathin ZnO is facilitated by the geometry and coordination of the Zn cations and O anions near the step region. Specifically, the enhanced adsorption takes place via bonding of both the C and O atoms of the CO₂ molecule to the tri-fold coordinated O anions at the trilayer edge and to the neighboring Zn cations on the bilayer terrace, respectively, leading to CO₂ bending and formation of a carbonate-like species.