A DFT + U study of structure and reducibility of CenO2n−x (n ⩽ 4, 0 ⩽ x ⩽ n) nanoclusters

► We model small ceria clusters with different reduction states and sizes. ► The capped-like structures are energetically more favorable than the open ring-like structures. ► The planar Ce2O2 rhombus structure is the building block for the most stable CenO2n−x clusters. ► The reduction energy of CenO2n−x clusters increases with the size and the extent of reduction.