Single Pd Atom Embedded in CeO2(111) for NO Reduction with CO: A First-Principles Study

Environmental and economic constraints necessitate further improvement of the activity and selectivity of dispersed Pd, Rh, and Pt metals for use in NOx reduction. We present here a density functional theory plus U study of NO reduction with CO catalyzed by a single Pd-1 atom embedded in CeO2(111) (noted as Pd-1/CeO2(111)). The complete catalytic cycle for NO + CO -> CO2 + 1/2N(2), including competitive adsorption of reactants, generation of the oxygen vacancy by CO, formation of N2O2* intermediates, scission of N-O bonds, and formation of N-2, is mapped out. The calculations indicate that Pd-1/CeO2(111) is active and selective toward N-2, in agreement with available experimental literature. The key intermediate N2O2* toward N-2 is identified, and the rate-limiting step is the first deoxygenation step of N2O2* with a barrier of 1.43 eV. The Pd-1-O-V pair embedded in CeO2(111) is proposed to be the active site, responsible not only for the formation of N2O2* by the reaction of two NO molecules but also for the subsequent two deoxygenation steps to make N-2. Detailed electronic structure analysis indicates that the formation of the Pd,Ov pair and the synergetic effect between Pd 4d electron and reducibility of CeO2 are responsible for the catalytic activity of single Pd atom embedded in ceria.