The Reaction Mechanism Of The Photocatalytic CO₂ Reduction On Transition Metal Modified Covalent Triazine Frameworks

The increasing amount of CO2 in the atmosphere and the consumption of fossil fuels lead to global warming and energy shortage.Utilizing the endless solar energy to convert CO2 into useful chemicals can effectively alleviate these two problems.However,the achieved CO2 conversion efficiency of current-usage photocatalyst was.far lower than the requirements of practical application.As a result,it is urgent to explore the high-efficient photocatalytic reduction of CO2 catalysts.Covalent triazine frameworks(CTFs),as a special class of organic porous framework,possess larger specific surface area and more open pore structure,which can provide more sites for the adsorption and reaction of CO2,and then accelerate the photocatalytic reaction.In previous work of our group,we use a simple synthetic method to prepare covalent triazine polymer(CTF-T1)with a high nitrogen content.The interaction between CTF-T1 and CO2 molecules can provide convenient condition for the CO2 transformation.However,the drawbacks such as the narrow photo-response range,high recombination rate of photo-induced carriers and so on limited the enhancement of photocatalytic activity of CTF-T1.Considering transition metals were the active component for reducing CO2,the thesis was mainly focused on the research about transition metal modified covalent triazine frameworks for photocatalytic conversion of CO2.The physicochemical structures and the photoelectric properties were characterized and tested in details.The mechanism of the photocatalytic reduction of CO2 was also revealed to provide basis for the construction of high quantum efficiency photocatalytic system.The main research contents were as follows:(1)The fabrication of Ni(OH)2 loaded CTF-T1 composites and the photocatalytic mechanism were studied;(2)The construction of cobalt modified CTF-T1 and the research of the adsorption characteristics,the essential reason for the improved photocatalytic CO2 conversion rates;(3)The correlations among photoelectric properties,CO2 adsorption characteristics and photocatalytic activity on CuCo2O4 modified CTF-T1 polymer.The chief results and conclusions of this paper were as below:(1)Employing noble-metal free materials Ni(OH)2 as the cocatalyst to construct Ni(OH)2/CTF-T1 composites.The highly efficient photocatalytic reduction of CO2 was realized by controlling the loading amount of Ni(OH)2 accurately;(2)The cobalt modified CTF-T1 has been successfully prepared by impregnation method.Notably,the highest photocatalytic activity obtained at Co1/CTF-T1 was about 44 times higher than that of pristine CTF-T1.The XPS analysis indicated that cobalt was introduced into CTF-T1 frameworks in the form of Co-N coordination bond,which might be good to improve the catalytic reduction of CO2.ESR analysis showed that cobalt modification increased the number of single electron CTF-T1 conjugation system,which provided more electrons involved in the reduction reaction,thus enhancing the photocatalytic activity of CTF-T1;(3)The CuCo2O4/CTF-T1 heterojunction photocatalysts have been successfully synthesized via thermal annealing method using CTF-T1 and CuCO2O4 as precursors.The structure could not only efficiently accelerate the charge carrier separation but also reduce the energy barrier to modify the surface reaction kinetics on CTF-T1,and promote the photocatalytic behaviors.