A Density Functional Theory Study Of Doped And Supported Ceria-Based Catalysts

Ceria-based catalysts with metal supported on or doped into ceria have high oxygen storage capacity (OSC) and have been widely used into three-way catalyst for automobile exhaust and elimination of volatile organic compounds (VOCs). Theoretical studies of the structure of supported metal nanoparticles on ceria surface, as well as doped into the ceria bulk, can explain the catalytic effects in view of electronic nature, which provide a deep understanding of catalytic reaction mechanism and give an important insight into development of catalyst with high efficiency at low temperature.The adsorption behaviors of O2 on Ce1-xZrxO2(111) surfaces were systematically investigated by density functional theory, and the corresponding electronic mechanism of different oxygen species were well explained. If O2 adsorbs on the stoichiometric Ce1-xZrxO2(111), it remains the electronic character of free molecule. When it adsorbs at the top site of Ce on the partially reduced Ce1-xZrxO2(111), a paramagnetic superoxide forms; while a diamagnetic peroxide appears if O2 fills into the surface oxygen vacancy. The low transport barrier from O2- to O22- (0.33 eV) suggests that superoxide might easily transform into peroxide with increasing temperature. CO is directly oxidized to CO2 by superoxide, indicating its high oxidative activity at low temperature; while a tridentate carbonate forms when CO reacts with peroxide, and its desorption energy barrier is 0.97 eV. Compared with ceria, one of the most important catalytic effects of doped Zr ion is to lower the oxygen vacancy formation energy and improves its oxidative activity.The structures of Agn clusters on CeO2(111), (110) and (100) surfaces, aswell as on the partial reduced CeO2(111), one and two step layers of CeO2(111), are systematically investigated by density functional theory method. The theoretical results indicate that a single Ag prefers to adsorb at hollow sites on CeO2(111), (110) and (100) surfaces; the most stable Agn cluster on the stoichiometric CeO2(111) surface is generally with a 3D structure except Ag3 and Ag7. Agn(n=2-6) clusters on CeO2(110) and (100) tend to form a 2D structure. However, it aggregates again to form 3D structure with the increase of Ag atom number. When Agn adsorbs on the partially reduced CeO2(111), strong interactions between Agn and CeO2(111) occurs with the increase of oxygen vacancy. Correspondingly, Agn (n=4,6-8) adsorbed on CeO2(111) with one vacancy inclines to form 3D structures; while it prefers to 2D configuration if three oxygen vacancies exist on CeO2(111).The adsorption behaviors of Ti on CeO2(111) are systematically investigated. It is suggested that strong interactions occur between Ti and CeO2, in which the metal Ti is oxidized to TiO2, and ceria is partially reduced simultaneously. With the increase of Ti coverage, Ti atoms inclined to adsorbed at the non-neighbor hollow sites at the stoichiometric and partially reduced CeO2(111), instead of aggregation to form metal clusters, indicating the stronger interactions between Ti and ceria compared with Ti-Ti metal atoms. Furthermore, surface oxygen atoms might migrate above Ti atoms to form a more stalbe configuration. At the same time, ceria is partially reduced. On the other hand, O2 adsorbed at Tin/CeO2 surface is apt to dissociate, which also leads to the oxidation of Ti on ceria support.