Adsorption and activation of CO2 on Zrn (n = 2–7) clusters

Abstract

The first step in the conversion of CO₂ to useful chemicals involves the adsorption of this molecule on a catalyst accompanied with its high degree of activation. In this paper, we explore the efficacy of small sized zirconium clusters, Zr_n (n = 2–7), in the adsorption and activation of the CO₂ molecule by using the density functional theory based ab initio method. The results of our calculations provide compelling evidence for the chemisorption and very high degree of activation of CO₂ with the elongation of the C–O bond in the range of 1.27–1.42 Å compared to 1.16 Å for free CO₂ and the deformation of the O–C–O bond angle from linear to 115–136°. This activation takes place through a charge migration from the Zr_n cluster to the CO₂ molecule resulting in the formation of CO₂^δ− species. To assess the catalytic potential of Zr-clusters for CO₂ conversion, we also analyse the reaction pathways and the transition barrier heights for the dissociation of CO₂ (CO₂ → CO + O) on all the Zr_n clusters. Our results for the dissociation of CO₂ to CO and O fragments reveal that the transition barrier is small for all the Zr_n clusters except for Zr₂ and Zr₄ and it attains a minimum value of 0.11 eV for an isomer of the Zr₆ cluster. The present work clearly demonstrates that small-sized monometallic Zr-clusters are highly efficient in activating and dissociating a CO₂ molecule adsorbed on these clusters.