  Understanding the atomic mechanism of low-temperature CO oxidation on a
heterogeneous catalyst is challenging. We performed density functional theory
(DFT) calculations to identify the surface structure and reaction mechanism
responsible for low-temperature CO oxidation on Pd/CeO2 (100) surfaces. DFT
calculations reveal the formation of a unique zigzag chain structure by the
oxygen and Ce atoms of the topmost surface of CeO2(100) with Pd atoms located
between the zigzag chains. O2 adsorbed on such Pd atoms is stable in the
presence of CO but plays a very important role in lowering the activation
barrier for low-temperature CO oxidation by forming a square-planar PdO4
structure and facilitating further O2 adsorption. In-situ Raman spectroscopy
studies confirm the adsorbed oxygen species to be peroxides. The calculated
activation barrier for CO oxidation, based on the mechanism suggested by these
unique structures and peroxides, is 31.2 kJ/mol, in excellent agreement with
our experimental results.
