The roles of surface structure, oxygen defects, and hydration in the adsorption of CO2 on low-index ZnGa2O4 surfaces: a first-principles investigation

Abstract

The effects of the surface atomic and electronic structures, oxygen defects, and hydration on CO₂ adsorption on ZnGa₂O₄(100), (110), and (111) surfaces were studied using density functional theory (DFT) slab calculations. For the perfect (100) surface, the most stable adsorption state involved the Zn–O–Ga bridge site, with an adsorption energy of 0.16 eV. In the case of the (110) and (111) surfaces, the strongest binding occurred on the Zn–O bridge sites, with much lower adsorption energies of −0.22 eV and −0.35 eV, respectively. In addition, the perfect surfaces showed CO₂ activation ability, but dissociation adsorption could not proceed. The oxygen vacancies on these three surfaces (1) made the metal sites beside them carry less positive charge and further reduced the adsorption energies on these metal sites, and (2) created efficient adsorption sites that allowed even dissociative adsorption. The most favorable molecular and dissociative adsorption states both involved the O_3c vacancy site of the (100) surface, and these two processes were spontaneous with adsorption energies of 0.74 eV and 0.80 eV, respectively. When H₂O molecules are present on the perfect and defective surfaces, the generation of hydrogen bonds between H₂O and CO₂ would slightly enhance the stability of adsorption (except for that on the (111)-Vo_3c surface), making them energetically favorable. However, the co-adsorption of H₂O could also increase the energy barriers for the decomposition reactions on the defective surfaces, making them kinetically unfavorable. Furthermore, the oxygen vacancy defects showed good activity for H₂O adsorption and decomposition, as well. Thus, when both H₂O and CO₂ were present in the adsorption system, H₂O would compete with CO₂ for the oxygen vacancy sites and further decrease the amount of CO₂ adsorption and decomposition. These findings have important implications for the decomposition of CO₂ on the ZnGa₂O₄ surfaces and can provide theoretical guidance for chemists to efficiently synthesize ZnGa₂O₄ catalysts.