Energy-Oriented Carbon Dioxide Transformation Modulated By Surface Defects Of Metal Oxides

The overuse of fossil fuels and human activity in chemical processing lead to increased CO₂ levels in the atmosphere as well as serious climate,environment and energy issues.Therefore,the researches of low-cost catalyst with excellent catalytic performance and converting CO₂ to CO or hydrocarbons like CH₄ to realize the energy-oriented transformation by photo-/thermal-catalysis processes are of great significant.Among the various materials for catalytic CO₂ energy-oriented transformation,metal oxides represented by TiO₂ and CoO_X have become the research hotpots due to their unique properties,including low-cost,non-toxicity,high chemical stability and high catalytic activity.However,CO₂ molecules are quite stable and chemically inert with a closed-shell electronic configuration and very low electron affinity,which means that high energy or special methods are needed to accelerate the adsorption and chemical activation of CO₂ molecules on the surface of catalysts for lowering the reaction activiation energy.Previous researches indicated that surface defects like oxygen vacancy and some coordinatively unsaturated species could accelerate the adsorption and chemical activation of CO₂,and make the key reaction step of single-electron reduction of CO₂ to CO₂^-be possible.Nevertheless,most of the reported methods for introducing defects have some disadvateges,such as relatively high danger,complex process,low efficiency,etc.Therefore,many works should be taken to develop safe and efficient methods to prepare surface-defective oxides with high performance for adsorption and chemical activation of CO₂.In addition,the process of catalytic CO₂ reduction is extremely complex,but the current researches of this process and the reaction mechanism are still ambiguous,which need to be further studied.In this thesis,with the purpose of building the catalytic system with excellent adsorption and chemical activation of CO₂ molecules as well as improving the use of solar/thermal energy to realize high energy-oriented transformation of CO₂,we developed several new methods for preparing surface defective oxides with excellent performance for CO₂ molecules transformations,and further explored the process and reaction mechanism of the catalytic CO₂ conversion.The main research contents and innovative achievements are listed as follows:?1?Black Titania with numerous surface defects was prepared by Al reduction method and successfully employed to photocatalytic CO₂ reduction.In order to improve the absorption and utilization of solar energy as well as accelerate the adsorption and chemical activation of CO₂ molecules,we prepared defective black titania with surface modification by Al reduction treament.The black titania comprises a core-shell structure,in which the core is highly crystallized while the shell is disordered.The generation of the disordered shell and oxygen vacancies(or Ti³⁺)leads to the visible or even infrared light response,and also facilitates the separation and transportation of photogenerated hole-electron pair as well as the adsorption and chemical activation of CO₂ molecules.The prepared black titania obtained excellent performance for photocatalytic CO₂ reduction,among which the optimized 500-TiO_2-x exhibits high space-time yield of CH₄ of 14.3?mol g^-1 h^-1 with74%selectivity and excellent photostability for six cycles under full solar irradiation while the space-time yield and the selectivity for CH₄ of pristine TiO₂ are only 1.8?mol g^-1 h^-1 with 43%under the same conditions.?2?The hydrogenated blue titania with uniform crystalline core-disordered shell structure was prepared using a facile low-temperature solvothermal method with Li-dissolved EDA as solution,and the process as well as the reaction mechanism of photocatalytic CO₂ reduction were further studied.In order to broaden the light response of TiO₂,build the catalytic system with excellent adsorption and chemical activation of CO₂ molecules as well as avoid the disadvantages of conventional hydrogenation process,we prepared the hydrogenated blue titania with uniform crystalline core-disordered shell structure using a facile low-temperature solvothermal method with Li-dissolved EDA as solution.The generation of the Ti-H and surface defects results in the change of band structure,which efficiently enhances the light absorption as well as the fast separation and transportation of photogenerated carriers.The prepared blue titania exhibits excellent performance for photocatalytic CO₂ reduction,among which the optimized H-TiO_2-x?200?exhibits high space-time yield of CH₄ of 16.2?mol g^-1 h^-1 with 79%selectivity and excellent photostability for six cycles under full solar irradiation while the space-time yield and the selectivity for CH₄ of pristine TiO₂ are only 1.8?mol g^-1h^-1 with 43%under the same conditions.Kinetic isotope effects experiments indicated that the cleavage of C=O bond from CO₂ is the rate-determining step.And the in situ diffuse reflectance infrared Fourier transform spectroscopy verified the existence of the key intermediates CO₂^-,which has a positive correlation with the space-time yield of CH₄.?3?An efficient and cost-saving cobalt-cobalt oxide core-shell catalyst of Co@CoO-N nanochains with high efficiency of catalyzing the reduction of CO₂to CO with small amount of H₂ under a mild condition.The key innovation of the Co@CoO catalyst is the synergistic interactions between metallic Co and encapsulating coordinatively unsaturated CoO species.For developing low-cost catalysts with excellent performance for catalytic CO₂ to CO as well as accelarating the adsorption and chemical activation of both CO₂ and H₂ molecules to realize the energy-oriented transformation of CO₂,we prepared an efficient cobalt-cobalt oxide core-shell catalyst of Co@CoO nanochains to synergistically catalyze the reduction of CO₂ to CO under a mild condition by a facile direct current arc-discharge method.The catalyst nanochains are composed of large amount of nanoball,of which the core is metallic Co and the shell is coordinatively unsaturated CoO with lots of defects.With small amount of H₂,Co@CoO nanochains show excellent performance for catalytic CO₂ conversion to CO at 250 ?.The key innovation of the Co@CoO catalyst is the synergistic interactions between metallic Co and encapsulating coordinatively unsaturated CoO species,of which metallic Co can effectively activate the H₂ while the coordinatively unsaturated CoO with lots of defects can facilitate the adsorption and chemical activation of CO₂ molecules.Furthermore,with N dopant into the defective CoO shell,the presence of electron-rich N can increase the surfaced electron density,also as Lewis base modulate the adsorption ability for acidic CO₂molecules,and thus further facilitate the catalytic efficiency.The Co@CoO-N achieves the highest conversion of 19.2% and CO selectivity of 99%.