Gold adsorption and diffusion on reduced CeO2(111)

R. PÉREZ; P. G. LUSTEMBERG; Y. PAN; D. KOSMIDER; V. BRÁZDOVÁ; G. THORTON; M. V. GANDUGLIA-PIROVANO; N. NILIUS

Osnabrueck

Encuentro; COST Action CM1104-Reducible oxide chemistry, structure and functions; 2016

p { margin-bottom: 0.25cm; line-height: 120%; }a:link { color: rgb(0, 0, 255); }a.western:link { }a.cjk:link { }a.ctl:link { font-family: "Times New Roman"; }Surfaceoxygen defects are believed to govern the adsorption characteristicsof reducible oxides such as CeO2.We address the adsorption and diffusion of Au atoms on CeO2(111)and challenge this perception on the basis of a combined scanningtunneling microscopy and DFT (PBE)+Ustudy.Surfaceoxygen vacancies are clearly thermodynamically preferred for Auadsorption, but incoming Au atoms at moderate temperature are foundto occupy mostly regular bridge surface sites, and notthe vacant site. The observation is explained with the strongpolaronic nature of the Au-ceria system, which imprints a dominantdiabatic character to the diffusive motion of Au atoms. Au adatoms,moving froma terrace to an oxygen vacant site,would change their oxidation state from Au+to Au0to Au.The associated diabatic barriers for the Au+ Au0step of 0.6 to 1.0 eV are insurmountable, and the vacant sites remainunoccupied. Moreover,fora reduced surface with subsurface oxygen vacancies, Ce3+sites were suggested to be the preferred location for the binding ofAuspecies[1], based on DFT calculations with a (2 × 2) unit cell (def=1/4)and the HSE06 hybrid functional. Using the same computational set-up,we find that a hollow site centered at a subsurface oxygen atom,where the 4f →6scharge transfer occurs from a Ce3+ion in a deeper layer, is energetically more favorable by 0.36 (0.34)eV with HSE06 (PBE+U)than on-top of a Ce3+ion in the outermost cerium layer [2]. The preference is explained interms of the reduction of both lattice strain and Coulomb repulsion.We note that neitherthe hollow northe Ce3+sites are stable in the isolated defect or dilute limit [e.g.,def=1/16,(4 × 4)], reflecting the impact of lattice strain in reduced ceriaon the Au adsorption behavior.Theseresults underline the importance of polaronic lattice distortions andcharge-transfer processes for adsorption and diffusion phenomena onreducible oxides.[1]Y.Pan, N. Nilius,H.-J.Freund, J. Paier,C.Penschke,and J.Sauer,Phys.Rev. Lett. 111,206101 (2013)[2]K.Kośmider,V.Brázdová,M.V. Ganduglia-Pirovano,andR. Pérez,J. Phys. Chem. C 120,927 (2016)