| Reducing CO2 into high value-added chemicals with semiconductor photocatalytic technology is considered to be a “kill two birds with one stone” method to solve the two problems of energy crisis and greenhouse effect caused by the burning of fossil fuels.Among numerous of photocatalysts,the n-type Ti O2 semiconductor is regarded as promising photocatalytic materials for CO2 reduction due to its low cost,high stability and environmental friendliness.Among Ti O2 photocatalysts with various morphologies,Ti O2 nanotube and Ti O2 nanobelt have attracted much attention due to their highly ordered structure,rapid electron transport performance and high specific surface area.However,Ti O2 also has the defect of high recombination rate of charge carriers,the wide band gap of Ti O2 also results in its low utilization rate of sunlight.These defects seriously affect the photocatalytic activity of Ti O2.Therefore,we carried out the following works in this research,the methods of construction of molecular-bridged Z-type heterojunction composite,simultaneous regulation of photogenerated holes and electrons,and construction of p-n heterojunction composite and introducing precious metals were carried out to strengthen the optical absorption performance and the photogenerated charge separation of Ti O2;The strategy of construction of microbial electrochemical system with a photocathode coupled with the chemical energy in waste water and light energy was used to improve the photogenerated charge separation and photocatalytic performance of CO2 reduction of Ti O2,and the transfer mechanism of the photogenerated charge was analyzed.To the problems that the poor separation of photogenerated charge and low utilization of sunlight of Ti O2,P-O molecular-bridged Z-scheme g-C3N4/Ti O2-nanotube heterojunction composite was first constructed.The results showed that the photocatalytic activity for CO2 reduction of Ti O2 nanotubes was significantly improved by combination with g-C3N4 and the introduction of P-O molecular bridges.The highest generated efficiencies for the reduced products of acetic acid,formic acid and methanol were 6.7 ± 0.11 mg L-1 h-1 cm-2,4.1 ± 0.09 mg L-1 h-1 cm-2 and 5.5 ± 0.10 mg L-1 h-1 cm-2,respectively,3.3 times,2.9 times and 3.5 times of that produced by TNTs.The enhanced photocatalytic activity was mainly attributed to the combination of g-C3N4 which improved the visible light absorption ability of the composites,the construction of Z-scheme heterojunction not only improved the separation of photogenerated electrons and holes,but also preserved the photogenerated electrons with strong reduction ability in the composites.The introduction of P-O bridges accelerated the electron transfer in the interface of Ti O2 and g-C3N4,and further improved the separation of photogenerated electrons and holes;The Al-O molecular-bridged Z-scheme porous g-C3N4/Ti O2-nanotube heterojunction composite was prepared using Na HCO3 as the pore former.The results confirmed that the highest generated efficiencies of CO2 reduction of acetic acid,formic acid and methanol of the Al-O molecular-bridged porous g-C3N4/Ti O2-nanotube composite were higher than that produced by the P-O molecular-bridged g-C3N4/Ti O2-nanotube composite.The main reason for the improved performance was the porous structure of g-C3N4 further enhanced the visible light absorption of the composite,and the effect of accelerating the electron transfer in the interfaces by Al-O molecular bridges was stronger than that by P-O molecular bridges.Because four holes are needed to be involved in the reaction of water oxidation during the process of photocatalytic reduction of CO2,in addition to the regulation of photogenerated electrons,the regulation of photogenerated holes is also of great significance.Therefore,the strategy of simultaneous regulation of photogenerated electrons and holes was proposed.The Ti O2 nanotubes co-modified with Mn Ox and r-GO were first prepared to improve the photocatalytic activity of Ti O2 nanotubes.The results showed that the co-modification of Mn Ox and r-GO could effectively improve the photogenerated charge separation of Ti O2 nanotubes,which was mainly attributed to the simultaneous regulation of photogenerated electrons and holes,that was the capture of photogenerated holes by Mn Ox and the transfer of photogenerated electrons by r-GO.The modified Ti O2 nanotubes had a selectivity for the synthesis of acetic acid and formic acid,the highest generated efficiencies for acetic acid and formic acid of the modified Ti O2 nanotubes were 5.2 ± 0.13 mg L-1 h-1 cm-2 and 3.1 ± 0.11 mg L-1 h-1 cm-2,respectively,2.5 times and 2.2 times of that produced by TNTs;The Ti O2 nanotubes co-modified with Mn Ox and Pd were further prepared,the photogenerated charge separation of Ti O2 nanotubes was improved by the simultaneous regulation of photogenerated holes and electrons with Mn Ox and Pd.The modified Ti O2 nanotubes also had a selectivity for acetic acid and formic acid,the highest production yields of acetic acid and formic acid of the Ti O2 nanotubes co-modified with Mn Ox and Pd were higher than that produced by the Ti O2 nanotubes co-modified with Mn Ox and r-GO,which was mainly attributed to that the Pd nanoparticles,as the co-catalysts,can not only transfer the photogenerated electrons,but also provide surface active sites to promote the activation of CO 2 molecules.The Ti O2 nanobelt was prepared by hydrothermal method,and then combined with Co3O4 to construct Co3O4/Ti O2 nanobelt p-n heterojunction composite.The combination of Co3O4 effectively improved the absorption of visible light by Ti O2 nanobelt while the introduced self-built electric field by the construction of p-nheterojunction improved the spatial separation of photogenerated electrons and holes of Ti O2,and improved the photocatalytic CO2 reduction of Ti O2 nanobelt.The Pd nanoparticles were introduced into the Co3O4/Ti O2 nanobelt p-n heterojunction composite to transfer the photogenerated electrons,and the photogenerated charge separation of the heterojunction composite was further improved.The highest generated efficiencies of acetic acid,formic acid and methanol obtained by the optimized 7Pd/0.27Co3O4/TNBs sample were 7.2 ± 0.13 mg L-1 h-1 cm-2、5.2 ± 0.08 mg L-1 h-1 cm-2 and 6.0 ± 0.07 mg L-1 h-1 cm-2,respectively,5.5,8.7 and 6.7 times of that produced by Ti O2 nanobelt.In addition to modifying the catalyst itself,the construction of microbial electrochemical system with a photocathode coupled the chemical energy contained in waste water with light energy is also an effective means to improve the photocatalytic performance of photocatalysts.Hence,the synthesized modified Ti O2 nanobelt and nanotube Ti O2 photocatalysts were made into photoelectrodes and used as photocathodes,and then coupled with microbial anodes to construct electrochemical systems with photocathodes.The results showed that the potentials of the photocathodes made with 0.27Co3O4/TNBs or 7Pd/0.27Co3O4/TNBs could be well matched with the potential of microbial anode to realize the construction of systems.Compared with the samples prepared by different modification methods,the generated efficiencies of acetic acid,formic acid and methanol of 9.5 ± 0.28 mg L-1 h-1 cm-2,7.5 ± 0.22 mg L-1 h-1 cm-2 and 8.1 ± 0.18 mg L-1 h-1 cm-2 obtained by the system constructed by coupling of 7Pd/0.27Co3O4/TNBs photocathode with microbial anode were highest,1.3,1.4 and 1.4 times,respectively,of that produced by bare 7Pd/0.27Co3O4/TNBs photocathode.Compared with the single photocathode,the improved CO2 reduction performance of system under light irradiation was mainly attributed to the electrons generated by the microbial anode oxidizing organic matter in wastewater flowing to the photocathode through the external circuit,which improved the separation of photogenerated electrons and holes of photocathode.The development of this work provides a new method for designing and synthesizing photocatalyst materials with high photogeneraed charge separation to deal with the issues of environmental pollution and energy crisis. |
PDF Full Text Request