Investigation Of The Preparation Of Supported Noble Metal Catalysts And The Mechanism Of Catalytic CO Oxidation Reaction

Catalyst plays an important role in energy conversion,chemical production and environmental protection.Among them,supported noble metal catalysts have attracted extensive attention due to their high reactivity.Gold was seen as an“inert”metal with no catalytic properties.It is not until Haruta et al.found highly dispersed gold catalysts exhibited excellent catalytic performance in CO oxidation reaction that intensive researches on gold catalysts are conducted.However,there is a lack of consensus on the active site of gold(positively charged gold,Au^δ+;metallic gold,Au⁰ or negatively charged gold,Au^δ-)and the reaction mechanism.Based on the above background,the main results in this thesis were as follows:The highly dispersed Au/SBA-15 catalyst was prepared by cationic([Au（en）₂]³⁺,en=ethylenediamine)ion exchange method and the ethylenediamine ligands was removed by ozone treatment.In situ diffuse reflectance infrared spectroscopy（in situ DRIFT）was used to characterize the charge state of gold sites and the activity during CO oxidation reaction.The results show that there are two sites on Au/SBA-15:metallic gold（Au⁰）and negatively charged gold(Au^δ-)after reduction-ozonation-reduction treatment.The adsorption of CO molecule on Au^δ-is weaker,and the activity of Au^δ-in the oxidation reaction of CO is higher.Next,Au/SBA-15 was prepared by conventional high-temperature calcination to verify whether the production of Au^δ-is related to the ozone treatment.It was found that the two kinds of gold sites on Au/SBA-15 catalyst were independent of the preparation methods,and the catalytic properties did not vary with different preparation methods.Room temperature ozone oxidation was applied to remove the organic ligands on colloidal nanoparticles.Pt-PVP/Al₂O₃（PVP:poly（N-vinyl-2-pyrrolidone））and Au-PVA/Ti O₂（PVA:poly（vinyl alcohol））were prepared.Transmission electron microscopy（TEM）combined with CO–FTIR were applied to characterize the treatment efficiency.It was found that room temperature ozone treatment method showed certain advantages in keeping the size and the coordination number of Pt nanoparticle sites.The colloid synthesis method provides a feasible method to study the effects of different supports.Colloidal ligand-free Pt nanoparticles were used as the precursors supported on reducible（Pt/CeO₂）and non-reducible oxide（Pt/Al₂O₃）to investigate the support effect in CO oxidation reaction.The results indicate that on Pt/Al₂O₃,none of the carbonate intermediate species can be converted into CO₂.The CO oxidation reaction follows the traditional L-H mechanism and ignited at a higher temperature（150℃）.The decomposition of bidentate carbonate(b-CO₃^2-)intermediate on Pt/CeO₂ into CO₂ at low temperature（75℃）is the main reaction channel for its low temperature reactivity.