A First-Principles Study Into The Properties And Activities Of Rare-earth And Tranisition Metal Materials In Envorimental Catalysis

Automotive exhaust emission serves as one of the preliminary sources in air pollution. The modern three-way catalyst can simultaneously convert the three primary pollutants CO, hydrocarbons(HCs) and NOx into CO2, N2 and H2O. The catalyst generally contains the support, washcoat and the active precious-metal component, in which washcoat is composed of ceria-based oxygen storage material and alumina, while the used metals are usually the precious metal such as Pt, Pd and Rh et al. Owing to the more stringent emission standards and demand for low-cost catalyst, there are three key problems in the current catalyst:(i) identification of composition-structure-activity relationship in ceria-based oxygen storage materials and design of higher-performance oxygen storage material; (ii) removal of NOx under lean conditions; (iii) low activity of the current catalyst in cold-start process. Aiming at these three problems, in this thesis we carried out density functional theory to study the following issues at the atomic level. They are (a) mechanism of function regulation in ceria-based oxides and transition metal-based catalyst materials; (b) shedding light on the principles related to the catalyst screening for NO oxidation which is the key reaction in NOX removal technique; (c) identification of structure-activity relationship in low-temperature CO and HCs oxidation by some kinds of typical catalyst such as gold and CO3O4 catalyst.Formation, Structures and diffusion mechanism of surface oxygen vacancy in CeO2-based materials:(i) By carrying out density functional theory calculations, we systematically studied single surface oxygen vacancy on CeO2(111). It is found surprisingly that multiple configurations with the two excess electrons localized at different positions can exist. We show that the origin of the multi-configurations of 4f electrons is a result of geometric relaxation on the surface and strong localization characteristic of 4f electrons in ceria. (ii) As for the diffusion of surface oxygen on CeO2(110) surface, we reported a new surface diffusion mechanism, i.e. the two step exchange one, which gives a kinetic elucidation into excellent OSC of ceria. It was found that the hopping mechanism, the most intuitive surface O vacancy diffusion mechanism, possesses a high barrier (1.60 eV), while the diffusion barrier of the two step exchange mechanism is much lower (0.61 eV) and expected to be favored. Based on this mechanism, we propose some doping rules to further reduce the diffusion barrier of lattice oxygen in CeO2-based materials, (iii) The interaction of NH3 with the CeGeO4(101) surface was studied, and it is found that under the experimental temperature, the surface oxygen of CeGeO4(101) can oxidize NH3 to form NOx and H2O accomplished by the formation of surface oxygen vacancies. Electronic analysis shows that in the formation of surface oxygen vacancy, Ce3+and Ge2+ will be formed by receiving two left electrons, and Ge4+shows more powerful capacity to receive electrons. When the temperature achieves the boiling point of GeO, the formed Ge2+may be gasified in the form of GeO and give rise to the further decomposition of GeGeO4 into CeO2 foam.Oxygen Storage/Release Capacity of Ce1-xZrxO2:(i) The O vacancy formation energies of CexZr1-xO2 solid solutions with a series of Ce/Zr ratios were calculated and analyzed. A model was proposed to understand the O vacancy formation energies; it consists of electrostatic and structural relaxation terms. It is found that the structural relaxation plays a vital role in affecting the O vacancy formation energies. (ii) Different arrangement of Ce/Zr cations in CexZr1-xO2 with the same composition may give different OSC. To pin down the key properties underlying the outstanding OSC ofκ-Ce2Zr2O8 at the atomic level, first principles calculations were carried out, and it is found that inκ-Ce2Zr2O8 the structural relaxation plays a key role in determining the O vacancy formation energy and it is largely localized, forming an independent local relaxation unit consisting of the six nearest neighbor OⅢions with a OⅡvacancy. To maximize both the local relaxation and the number of local relaxation units plays a crucial role for the superb OSC ofκ-Ce2Zr2O8.NO oxidation catalyzed by precious metals ans their oxides:(i) By combining DFT data and microkinetic analysis, we studied activity trend of NO oxidation on the surfaces of metal Ru, Rh, Pd, Os, Ir and Pt. Under typical experimental condition, the activity trend varies with the metal species as a volcano curve, and the optimum catalyst in terms of oxygen chemisorption energy is located between the Pt and Ir. Analysis shows that, first, the activity trend is in general determined by the competition between the reaction barrier and the coverage of surface free sites (θ). Second, since the dissociation of many important molecules, such as N2, O2 and CO, follows the same BEP relationship,θis thus usually a decisive term that affects the overall activity. Third, an equation was derived forθand its implications were discussed. (ii) Under realistic oxygen-excess condition, metal could be oxidized to form oxides. We investigated NO oxidation on the platinum group metal oxides (PtO2, IrO2, OSO2), aiming at shedding light on the activities of metal oxides and exploring the activity variation of metal oxides compared to their corresponding metals. A microkinetic model, taking into account the possible low diffusion of surface species on metal oxide surfaces, is proposed for NO oxidation. The resultant turnover frequencies of NO oxidation show that under the typical experimental condition:(a) IrO2(110) exhibits higher activity than PtO2(110) and OsO2(110); and (b) compared to the corresponding metallic Pt, Ir and Os, the activity of PtO2 to catalyze NO oxidation is lower, but interestingly IrO2 and OsO2 exhibits higher activities. The reasons for these activity differences are addressed in the thesis.Low-temperatue CO oxidation by thin Au-film and Co3O4:Gold and Co3O4 catalyst are two currently best catalysts for low-temperature CO oxidation. With respect to gold catalyst, we used density functional theory calculations to study structures and their activities of Au thin films supported at anatase TiO2(101) and Au substrate. The results show that O2 can hardly adsorb at flat and stepped Au thin films, even supported by fully-reduced TiO2(101) that can highly disperse Au atoms and offer strong electronic promotion. Interestingly, in both oxide supported and pure Au systems, wire-structured Au can adsorb both CO and O2 rather strongly, and kinetic analysis suggests its high catalytic activity for low-temperature CO oxidation. A generalized structural model based on the wire-structured film is proposed for active Au, and possible support effects are discussed:Selected oxide surfaces can disperse Au atoms and stabilize the formation of film-like structure; they may also serve as template for the preferential arrangement of Au atoms in wire structure under low Au coverage.As for Co3O4 catalyst, through analyses of elementary reactions of CO oxidation on its commonly exposed surfaces, i.e. (110)-A, (110)-B, (111) and (100), we found the following regarding the relation between the activity and structure:(i) the active site contains Co3+and surface lattice O3C coordinated with three Co3+; (ii) (110)-A is more active compared to the three other surfaces; (ii) To understand the oxides in general, we extend the investigation to other common oxides, i.e. MnO2(110), Fe3O4(110), CuO(110) and Cu(111), and proposed three properties that largely determine actitvity; CO adsorption strength, the barrier of CO reacting with lattice O and the redox capacity of oxide; (iv) H2O-induced formation of surface OH and bicarbonate are two key contributors in deactivation. Finally, as an interesting issue, namely that, contrary with the case of Co3O4, H2O is generally a promoter on late metal surfaces for CO oxidation, we identified that the significant difference of potential energy surfaces between OH on metal and metal oxides is identified to be the origin.Co3O4(110) is able to catalyze ethylene combustion at 0℃. Toward this surprising result, based on the conclusion of the low-termperatuer CO oxidation, we studied the inherent reaction mechanism and possible C-C bond cleavage pathway. It was found that the common C-C bond cleavage pathways can not work on Co3O4(110) at low temperature. Interestingly, C-C bond breakage could proceed by the CHOCO species as a result of iterative dehydrogenation and oxidation processes. With respect to the related principles, we made a brief elucidation in terms of the isolated Co3+coordination configuration and bond saturation of C atom in the intermediates. Finally, the catalytic activity of Co3O4(110) toward low temperature ethylene combustion could be rationalized in terms of its binding ability toward CH2CH2 molecule, high reactivity and reasonable basicity of surface lattice oxygen. This C-C bond cleavage strategy may enlighten the design of low-temperature catalyst for hydrocarbons oxidation.