Correlating The Structure-intrinsic Activity Relation Of Aunanoparticles Supported On Inert SiO₂ Support

Catalysis is one of the most important technologies to solve the environment problems and resource shortages during the social development. Recently, nanocatalysis, an inter-discipline of catalysis and nanoscience, has emerged as one of the research frontiers. As a pioneering system in nanocatalysis, supported Au nanocatalysts have received extensive investigations and have been demonstrated to be efficient for catalyzing various reactions; however, the relevant fundamental understanding is still ambiguous, even for the simple CO oxidation reaction. This situation could be partly attributed to the fact that many factors cooperatively control the catalytic activity of supported Au nanocatalysts, which makes it difficult to unambiguously correlate the structure-activity relation.. SiO₂ is an inert support that does not participate in the catalytic reaction, and Au/SiO₂ catalyst is not so active in CO oxidation as Au/MOx catalysts (MOx = CeO₂, Fe3O4, TiO₂, Co3O4 et al); however, Au/SiO₂ catalyst provide a system to investigate the structure-intrinsic activity of supported Au nanoparticles without the contribution from the support. In this thesis, we have systematically investigated Au/SiO₂ catalysts for CO oxidation and successfully elucidated several important fundamental issues related to Au nanocatalysis in CO oxidation. Our main findings are summarized as follow:1. The employed atmosphere during the treatment of catalyst precursor has been found to exert great influence on the structure of Au nanoparticles in Au/SiO₂. Calcination in air forms large Au nanoparticles inactive in CO oxidation at low temperatures whereas H2 production can form fine Au nanoparticles active in CO oxidation at low temperatures. It was found that the Au precursor can be directly reduced to metallic Au by H2 at low temperatures whereas the Au precursor is transformed into Au(I) at low temperatures and then into metallic Au at high temperatures. The difference between the temperature where metallic Au forms during H2 reduction and that during calcination in air might result in the formation of Au nanoparticles with different sizes. These results provide some insights in the preparation of catalytically active Au nanoparticles on the inert SiO₂ support.2. A series of gold nanoparticles with different size distributions have been successfully prepared on inert SiO₂ support and their catalytic activity in CO oxidation has been investigated. It seems that 3-4 nm is a critical size for Au nanoparticles to exhibit intrinsic activity in catalyzing CO oxidation at low temperatures, but ultrafine Au nanoparticles (< 2 nm) is not catalytically active. These results demonstrate a volcano-shape dependence of the intrinsic activity of Au nanoparticles in low temperature CO oxidation on their size. In-situ XANES and DRIFT results reveal that ultrafine Au nanoparticles lose the characteristics of metal and thus their reactivity towards CO greatly weakens. These results well correlate the size, electronic structure and intrinsic activity in low temperature CO oxidation of Au nanoparticles.3. The NaNO₃ additive can induce large Au nanoparticles supported on SiO₂ inert in low temperature CO oxidation to exhibit certain an activity in catalyzing CO oxidation at low temperatures. After the addition of NaNO₃ additive, the size and electronic structure of large Au nanoparticles do not change, but their surfaces roughen. NO₃- anions undergoes decomposition during the calcination process. We proposed that the interaction between NO3- anions and Au nanoparticles leads to the decomposition of NO₃- anions and also the surface restructuring of Au nanoparticles. This eventually forms low-coordinated Au atoms on large Au nanoparticle surfaces capable of catalyzing low temperature CO oxidation. These results manifest the important role of low-coordinated Au atoms in the catalytic activity of Au nanoparticles.4. The NaOH additive can substantially enhance the catalytic activity of"inert"Au/SiO₂ catalyst in catalyzing CO oxidation at low temperatures, and the structure of Au nanoparticles were found to be similar in Au/NaOH/SiO₂ and corresponding Au/SiO₂ catalysts. Au/NaOH/SiO₂ catalysts are stable at 90°C but slowly deactivate at 60°C. The accompanying density functional theory calculation results reveal that hydroxyls on Au(111) can activate molecular O₂ and catalyze CO oxidation with relatively low activation energy, in which COOH (a) plays a determining role. The di-CO3H species whose formation is thermodynamically favorable at low temperatures on Au(111) is responsible for the deactivation of Au/NaOH/SiO₂ at 60°C. These results provide unambiguous evidence for the role of hydroxyls in the activation of O₂ and catalytic activity of supported Au nanoparticles without the involvement of support.5. The interaction between Au nanoparticles and active oxides (MOx) has been studied by the addition of a small amount of MOx to Au/SiO₂. Under the same preparation condition, fine Au nanoparticles active in low temperature CO oxidation can be stabilized on MOx surface. Au nanoparticles were found to nucleate on the oxygen vacancy sites on ZnO, providing the first evidence for the Au-oxygen vacancy interaction in supported powder catalysts. It was also found that the structure of MOx strongly affects the structure of prepared Au nanoparticles. The distribution of hydroxyls on the MOx precursor plays a decisive role in the Au-MOx interaction in Au/MOx/SiO₂ catalysts prepared by DP method. The gold precursor preferentially deposits and interacts with hydrogen-bonded hydroxyls and then with isolated hydroxyls in Co(OH)2 on SiO₂, eventually forming large and fine Au nanoparticles in the catalysts, respectively. The structure of Au nanoparticles can be modified by modifying the distribution of hydroxyls on the MOx precursor. These results shed some light on the preparation process of supported Au nanoparticles prepared by DP method.6. The intrinsic activity of supported Au nanoparticles in CO oxidation were found to increase, decrease and increase again with the increase of reaction temperature. This trend indicates that CO oxidation catalyzed by Au nanoparticles follows different reaction mechanisms at low reaction temperature region and high reaction temperature region, and the intrinsic activity of Au nanoparticles in low temperature CO oxidation exhibit a volcano-shape dependence on the reaction temperature. We proposed that a weakly chemisorbed surface species in involved in the low temperature CO oxidation catalyzed by Au nanoparticles, whose subsequent surface reaction on Au surface and desorption from Au surface compete with the increase of reaction temperature, giving rise to the observed volcano-shape dependence. These results provide additional insight in the fundamentally understanding of CO oxidation mechanisms catalyzed by Au nanoparticles.In summary, employing Au/SiO₂ catalysts, we have successfully clarified some important issues in the fundamental understanding of structure-intrinsic activity in CO oxidation of supported Au nanoparticles, including the influence of their size, the presence of low-coordinated Au atoms, their electronic structure and the presence of hydroxyls chemisorbed on their intrinsic activity, the interaction between Au nanoparticles and active oxide support (MOx), and the mechanisms of CO oxidation catalyzed by Au nanoparticles. The results reported in the thesis deepen the fundamental understanding of Au nanocatalysis.