| As the environmental pollution and energy crisis deepening, elimination of pollutants and turns them to clean resources has become one of the most pressing problem. Therefore, in order to relieve the energy crisis and reduce the environment pollution, the scientific research workers are looking for available ways to translate the pollutants in the air (such as poisonous gas containing elements of C, N) into the renewable energy. In recent years, with the continuous development of theoretical calculation method and computer technology, there have been more and more researchers doing theory research rather than just imitating experiments, because it can provide experimental research with reasonable microcosmic forecast and theoretical guidance. One of the main problems of the transformation pollutants is the choice and synthesis of high efficiency catalysts. For this difficulty, a new and very effective synthesis method is surface microstructure control. In this work, we will use density functional theory method to study the two main ways of surface microstructure control (exposure effective surface and loading co-catalyst). It can also provide good theoretical guidance for the scientists synthesising of such high efficiency catalyst. For the method of exposure effective surface, we choose the small molecules (H2O, CO2and NH3) adsorbed and dissociated on two kinds of typical spinel catalysts (ZnGa2O4and MgAl2O4as model for research. The purpose of this work is to explore how the microcosmic surface atomic and electronic structure (especially the surface states) effect on the catalytic activity. For the method of loading co-catalyst, we choose the CO oxidation process on the Au16supported Mg(OH)2catalyst as the model for research. This study is aim at exploring the carrier’s influence on the catalytic activity of catalyst. Our main research contents and conclusions are shown as follows: In chapter one, we mainly introduced the research background and significance of our work, including the mechanisms of semiconductor photo-catalysis technology and the present situation and deficiency of surface controllable catalysts and precious metal load catalysts etc. Finally, we elaborated the reason we select this topic, as well as the research train of thought and the general content of this work.In chapter two, we mainly described the density functional theory, the local density approximation theory and the generalized gradient theory in detail, and also introduced the main quantitative calculation software package used in this work.In chapter three, we mainly studied the H2O and CO2separate and common adsorption behaviors on the perfect and oxygen defective low index ((100),(110) and (111) surface) ZnGa2O4catalysts. Our calculations show that on the stoichiometrically perfect surface, the most stable molecular adsorption that could take place involved the generation of hydrogen bonds. For dissociative adsorption, the adsorption energy of the (111) surface was more than four times the energies of the other two surfaces, indicating it to be the best surface for water decomposition. A detailed comparison of these three surfaces showed that the primary reason for this observation was the special electronic state of the (111) surface. When water dissociated on the (111) surface, the special Ga3c-4s and4p hybridization states at the Fermi level had an obvious downshift to the lower energies. This large energy gain greatly promoted the dissociation of water. Because the generation of O3c-vcancy defects on the (100) and (110) surfaces could increase the stability of the dissociative adsorption states with few changes to the energy barrier, this type of defect would make the decomposition of water molecules more favorable. However, for the (111) surface, the generation of vacancy defects could decrease the stability of the dissociative adsorption states and significantly increase their energy barriers. Therefore, the decomposition of water molecules on the oxygen vacancy defective (111) surface would be less favorable than the perfect (111) surface. After discussed the adsorption behavior of water, now we will turn to the discussion of the adsorption behavior of CO2. On the perfect (100) surface, the most stable adsorption state involved the Zn-O-Ga bridge site, with an adsorption energy of-0.16eV. In the case of the (110) and (111) surfaces, the strongest binding occurred on the Zn-O bridge sites, with much lower adsorption energies of0.22eV and0.35eV, respectively. In addition, the perfect surfaces showed CO2activation ability, but dissociation adsorption could not proceed. The oxygen vacancies on these three surfaces (1) made the metal sites beside them carry less positive charge and further reduced the adsorption energies on these metal sites, and (2) created efficient adsorption sites that allowed even dissociative adsorption. The most favorable molecular and dissociative adsorption states both involved the O30vacancy site of the (100) surface, and these two processes were spontaneous with adsorption energies of-0.74eV and-0.80eV, respectively. When H2O molecules are present on the perfect and defective surfaces, the generation of hydrogen bonds between H2O and CO2would slightly enhance the stability of adsorption (except for that on the (111)-Vo3c surface), making it energetically favorable. However, the co-adsorption of H2O could also increase the energy barriers for the decomposition reactions on the defective surfaces, making them kinetically unfavorable. Furthermore, the oxygen vacancy defects showed good activity for H2O adsorption and decomposition, as well. Thus, when H2O and CO2were both present in the adsorption system, H2O would compete with CO2for the oxygen vacancy sites and further decrease the amount of CO2adsorption and decomposition, and further reduce the reactivity of the catalyst. This means that if we want to use ZnGa2O4catalyze the decomposition of H2O, we need to let the catalyst mainly exposed the (111) surface and generate less oxygen defects; if we want to use ZnGa2O4catalyze CO2decomposition into CO and O2, we should be sure that the reaction system is in a dry environment; if we want to use ZnGa2O4to catalyze the reaction between CO2and H2O yielding hydrocarbons, we need to separate and remove water from the product stream to maintain the high activity of the catalysts. Our research can provide a new design idea for the synthesis of high efficient and surface controllable ZnGa2O4catalysts.In the fourth chapter, we further studied the adsorption and decomposition behavior of small molecule NH3on another typical spinel catalyst (MgAl2O4). Adsorption and dissociation of NH3on the low-index (100),(110) and (111) surface were investigated using density functional theory. On the perfect and defective surfaces, different configurations were achieved for NH3molecules on various sites. Comparing these three surfaces, we found that on the (100) and (110) surfaces the NH3molecular adsorption is more favorable than the dissociative adsorption, while on the (111) surface we obtained the opposite consequence. Our further analysis indicates that this is a surface-structure-sensitive reaction. The most stable adsorption state on Mg atom occurs on the (100), while the most stable adsorption state on Al atom occurs on the (111) surface. Our results shows that the main reason for the different stability of the adsorption states on different metal atoms is caused by the surface special electronic states at the Fermi level which emerged on the Mg2c atom of (100) surface and the Al3c atom of (111) surface. For molecular adsorption, the special Mg2c-2s state of (100) surface and Al3c-2s and2p states of (111) surface at the Fermi level will have a distinct downshift to the lower energies and significantly enhance the stability of the adsorption states. In addition, the dissociative adsorption states on the (100) and (111) surfaces which have special surface states are much more stable than the (110) surface as well. Because the special surface state on the (111) surface is generated by the hybridization of Al3c-2s and2p states, the enhancement of the adsorption states on the (111) surface is larger than the (100) surface whose special surface state is generated by only one state (Mg2c-2s). The generation of oxygen vacancy will make the adsorption states less stable, especially for the (111) surface. This means that the perfect (111) surface will be the most favorable surface for NH3adsorption and decomposition. These findings have an important implication for the decomposition or synthesis of NH3on the MgAl2O4surfaces and can provide theoretical guidance for chemists to synthesize high-efficiency MgAl2O4catalysts. These findings have an important implication for the decomposition or synthesis of NH3on the MgAl2C4surfaces and can provide theoretical guidance for chemists to synthesize high-efficiency MgAl2O4catalysts.In the fifth chapter we mainly studied the catalyst load effect on the catalytic activity. Through the calculation of the separate and common adsorption of CO and O2on the Aui6supported perfect Mg(OH)2(0001) surface, we found that on the perfect surface, CO and O2can be adsorbed and activated both on the surface of the Au clusters and the interface between Au clusters and Mg(OH)2(0001) surface (CO molecule is more easily adsorbed on the interface; O2molecular adsorption is more likely to happen on the Au clusters and dissociative adsorption is more likely to happen on the interface; when they co-adsorbed, the molecule adsorption states and oxidation products on Au clusters are both stable than the that on the interface, which will make the reaction energy barrier and the CO2activation energy become higher). Through the calculation of the separate and common adsorption of CO and O2on the Aui6supported defective Mg(OH)2(0001) surface, we found that on the surface with hydroxyl defects, the defect can reduce CO adsorption compared to the perfect surface (the CO adsorption on the Vo-2sites even can not happen), O2adsorption on the Vo-1and Vo-3sites have very big improvements, however, on the Vo-2and Vo-4sites the adsorption states are much less stable than that on the perfect surface, the co-adsorption of CO and O2is similar to O2adsorption (on the Vo-1and Vo-3site the adsorption states have very big promotion and on the Vo-2and Vo-4site the adsorption states are very unstable). As a result, some hydroxyl defects (the hydroxyl defects near the Au clusters) can promote the adsorption and activation of O2and CO, but the generation of the hydroxyl defects that are far away from the Au clusters will make the adsorption state become unstable. Our results showed that the oxidation reaction of CO and the adsorption of O2and CO on the Au clusters supported Mg(OH)2(0001) surface has a strong surface-dependence, and the formation of surface hydroxyl space has great influence on it. Our research can have very good guidance for us to understand the Au clusters supported Mg(OH)2catalysts, the atomic and electronic structure of the surface and the oxidation reaction of CO, the adsorption behavior of O2and CO, and further guide the researchers to synthesis the morphology controlled Au clusters supported Mg(OH)2catalysts.Finally, in chapter six we summarized the main conclusions and the innovation points of our work, and put forward the outlook in this research direction. |
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