First-Principle Study On Some Ceria Related Systems

Ceria has been widely used in three-way catalytic (TWC) converters and fuel cells. In this thesis, adsorption of SO2, sulfur and noble metal (Pd, Pt and Rh) on the ceria surface and the properties of the Cu-doped ceria bulk systems are investigated by using density functional theory calculations.Chapter 1, a brief introduction to the density functional theory (DFT) is presented. The basic idea of DFT is that the ground state of a system is determined by the ground state electron charge density, while it is difficult to obtain the exact exchange—orrelation potential. The two most widely used approximations for the exchange correlation are the local density approximation (LDA) and the generalized gradient approximation (GGA). The Vienna Ab-initio Simulation Package (VASP), and the projector augmented wave (PAW) and DFT+U method are descripted.Chapter 2, we present an introduction for the application of the three-way catalytic (TWC) converters and solid oxide fuel cells (SOFC) as well as a review of the prevous theoretical studies on the ceria. The sulfur poisoning is one of the biggest problems for ceria applied in TWC and SOFC. According to the previous tests, GGA+U (U=5 eV for Ce4f) method will be used in the current study.Chapter 3, results from first-principles calculations present a rather clear picture for the interaction of SO2 with the unreduced and partially reduced ceria surfaces. The Ce3+/Ce4+ redox couple, together with the many oxidation states of S, give rise to a multitude of SOX species, with oxidation states from+â…¢to+â…¥. SO2 adsorbs either as a molecule or attaches via its S-atom to one or two surface oxygens to form sulfite(SO32-) and sulfate(SO42-) species, forming new S-O bonds. Molecular adsorption is found on the(111) surface. While SO32 structures are found on both (111) and (110) surfaces, SO42- structures are only observed on the (110) surface together with the formation of two reduced Ce3+ cations. SO2 can also partially heal the ceria oxygen vacancies by weakening one of S-O bonds, while significant electron transfer from the surface (Ce4f) into the Lowest Unoccupied Molecular Orbital of SO2 adsorbate takes place and oxidizes the surface Ce3+ cations. Furthermore, we propose a mechanism that could lead to monodentate sulfate formation at the (111) surface.Chapter 4, the interaction of sulfur with the ceria(111) and (110) surfaces are studied by using DFT calculations. Two sulfoxy species are identified:oxy sulfate species (SO2-) on both the CeO2(111) and (110) surfaces and hyposulfate(SO22-) on the(110) surface. S2- is clearly formed when surface or subsurface oxygen ions are replaced by sulfur. These sulfide species are most stable at the surface Furthermore, sulfite(SO32-) structures are found when a sulfur atom replaces one Ce in the ceria surfaces. The calculated sulfur diffusion barriers are larger than 1.4 eV for both surfaces and thus sulfur is essentially immobile. The study provides a possible explanation for the sulfidation phenomena of the ceria-based catalysis. Thus we find three different species from interaction of S with Ceria which are all, due to the strong binding, capable of poisoning the surface, reduced or unreduced.Chapter 5, results from first-principles calculations present a rather clear picture of the interaction of single noble metals (NM:Pd, Pt and Rh) and the corresponding NM4 clusters with the CeO2(111) surface. The most preferable adsorption sites for both the Pd and Pt adatoms are the surface O bridge sites, while the Rh adatom prefers to stay at the O-hollow site. The Rh adatom shows much stronger interaction with the CeO2(111) surface than do the Pd and Pt adatoms, while the Pd adatom has the smallest adsorption energy. The small clusters show stronger interaction than the corresponding single NM adatoms on the CeO2(111) surface. The reaction of NM+Ce4+â†'NM8+/Ce3+ was found for both the single noble metal adatoms and the small clusters adsorbate, indicating that NM adsorbates are mainly oxidized by the surface Ce ions with obvious charge transfer from NM to the CeO2(111) surface. The three base atoms of the small clusters that bind with the CeO2(111) surface shows positive charges, while the top metal atoms of the NM4 clusters has a little negative charge.Chapter 6, the atomic and electronic structures of the Cu-doped ceria have been presented using the DFT+U method. It is found that the Cu dopant prefers to bind to four oxygen neighbors in the form of Cu and forms CuO phase in all of the Cu-doped ceria systems. The effects of Cu dopant on the O vacancy formation energy (Evac) in CeO2 show that the Evac are lowered in the Cu-doped ceria systems by 2.98 eV and 0.68 eV for the first and the second O vacancies, respectively. The big reduction of the Evac for the first O vacancy is due to the structural relaxation and the charge balance requirement, while the electronic relaxation for the transition of Ce4+â†'Ce3+ is mainly resulted from the easy creation of the second O vacancy.