(190ak) Evaluation of Density Functional Theory (DFT+U) to Describe Doped Cerium Oxide Surfaces

Density functional theory (DFT) has trouble representing systems which contain cerium atoms, evidenced by improper delocalization of 4f electrons in reduced ceria. The use of DFT+U, with the Hubbard U correction term, localizes the 4f electrons in systems which contain reduced ceria. Though, this correction has been widely applied to investigate ceria surface chemistry, producing results in agreement with experiment. However, the applicability to metal-doped ceria surface, with a surface Ce atom replaced with another metal, has not been established.  Here, we investigate the electronic structure of doped ceria systems, specifically investigating whether ceria or the doped atoms reduce upon oxygen vacancy formation or hydrocarbon adsorption. To determine which atoms reduce a number of different techniques can be used including Bader charge analysis, site projected atomic charges, density and partial density of states plots, and orbital imaging. We investigate a series of transition metal dopants, and report periodic trends in reducibility of doped ceria surfaces.  We also investigate whether dopants serve as a reduction center or alter the reducibility of nearby ceria atoms, and test the sensitivity of surface reducibility to the U parameter.  Comparisons are made with X-ray adsorption spectroscopy experiments.