First-Principles Study Of The Catalytic Activities Of Fe-CeO₂（111） Surface And CO Adsorption And Oxidation At The Doped Surface

This paper studies Fe adding of ceria with the aim of theoretical understanding how Fe adding modifies the catalytic activities and the interaction of the CO molecule with ceria (111) surface. We apply first principles density functional theory to investigate the pure ceria(l 11) surface, the Fe/CeO2(111) system, MxCe1-xO2(111)(M=Fe, Pt, Rh, Ir, Sn, Zr) systems and the Fe doped CeO2 (111) surface, the structures and electronic properties of these surfaces as well as the adsorption and oxidation of CO at the doped surface are calculated. We also calculated and analyzed the oxygen vacancy formation energies of various sites and the adsorption energies of a CO molecule at different trial adsorption sites. The bader charge and valence state have also been calculated. The paper includes four parts:The first, we studied the pure ceria(lll) surface and the MxCe1-xO2(111) (M= Fe, Pt, Rh, Ir, Sn, Zr) systems. For the pure CeO2(111) system, there is only one unique surface O atom and one unique subsurface O atom. We can see the formation energy of a surface O vacancy and a subsurface O vacancy are 2.95eV and 2.86eV, respectively. For the MxCe1-xO2(111) systems, We calculated and analyzed the geometric parameters, the oxygen vacancy formation energies of various sites and the density of states (DOS) of the MxCe1-xO2(111)(M=Fe, Pt, Rh, Ir, Sn, Zr) systems. The order of the MxCe1-xO2(111) (M= Fe, Pt, Rh, Ir, Sn, Zr) systems for facilitating the oxygen vacancy formation is: Fe> Pt> Rh> Ir> Sn>Zr.The second, the Fe doped Ceo.92 Fe0.08O2(111) system was investigated at length. The oxygen vacancy formation energies at different sites of the doped system are much smaller than those for the pure CeO2(111) system, this means vacancy formation is very much facilitated by Fe doping. The improvement of the activity of the surface oxygen after Fe doping compared with the pure case in which the subsurface O vacancy formation energy is smaller. And the Fe dopant is likely to be the center of the oxygen vacancy clusters. From the optimized structures aspect, we see the structural changes of doped system are much larger than those of the undoped system, and the structural changes with a surface oxygen vacancy are larger than those changes with a subsurface oxygen vacancy. From the electronic structure aspect, after Fe doping, there are new metal induced gap states (MIGS) situated at the Fermi level. We also found that the left hole states move toward the Fermi level, which may be used to accommodate extra electrons and there by facilitate the formation of oxygen vacancies. Indeed, the DOS for the Ceo.92Feo.o802(111) surface with one vacancy shows these states are occupied.The third, we studied the Fe/CeO2(111) system, the oxygen vacancy formation energies at different sites for the corresponding optimized structures are bigger than 2.95eV, and the formation energy of a surface O vacancy are smaller than those of a subsurface oxygen vacancy. That means vacancy formation is restrained. From the optimized structures and Electronic structures aspects, we got the similar results.The last, To theoretically understand whether or why the Fe dopant is helpful to improve the oxidation of CO on the doped surface, a CO molecule was placed at a number of different positions on the Ceo.92Feo,os02(111) surface (O-top, Ce-top, O-bridge, and O-hollow sites, etc.). Two types of adsorption are found:CO physisorbed and CO2 formed. CO physisorption is observed at the Ce-top sites and the O1/02-bridge site, and CO2 formation is found on O-top sites.