Metal-Nitrogen Doped Carbon Catalysts For Carbon Dioxide Reduction

With industrialization advancement,humankind's dependence on fossil energy has led to superfluous carbon dioxide(CO₂)emission,which continuously aggravates global energy and environmental problems.Existing CO₂ capture and storage technologies are not sufficient to address the growing carbon emission problem,so new strategies are needed to achieve carbon neutral.Utilizing renewable energy through photo-and electrochemical methods to realize the conversion of CO₂ to energy-dense species is a promising pathway to reduce CO₂ emissions.As a class of non-noble metal catalytic materials,the metal-nitrogen doped carbon(M-N-C)materials represented by Fe-N-C and Ni-N-C have high efficiency and selectivity for the electroreduction of CO₂,showing unique advantages in the field of CO₂ reduction.In this thesis,the porphyrin-derived M-N-C materials were used as the model catalysts.Combined with the potential application prospects,the effects of catalysts'porous structures in the CO₂ diffusion electrolyzer,the integration of chorine evolution reaction with CO₂ electroreduction systems to simultaneously produce CO,Cl₂,and KHCO₃,and the metal species of M-N-C catalysts in the CO₂ photoreduction systems on the CO₂ reduction were investigated.The main research contents are as follows:1.A series of Ni_x-N-C catalysts(x represents the percentage of nickel porphyrin ligand)were obtained by pyrolyzing metal-organic framework materials PCN-222 containing different amounts of nickel porphyrin ligand.It was found that the Ni₂₀-N-C material has the best CO₂ electroreduction performance,and the CO Faradaic efficiency in the H-type electrolytic cell is as high as 97%.Through the adsorption test and porous structure analysis,it was confirmed that the Ni₂₀-N-C catalyst has a high surface area and a hierarchically porous structure,which has a strong adsorption capacity for CO₂,and a large number of mesoporous structures improving the liquid mass transfer.Compared with a similar non-porous catalyst,the Ni₂₀-N-C catalyst significantly improves the CO₂ reduction efficiency in the gas diffusion electrolyzer system.With 6 M KOH electrolyte,the Ni₂₀-N-C catalyst achieved a high CO partial current density of 645 m A cm^-2 and a turnover frequency(TOF) of 23 s^-1 at a cathode potential of only-0.53 V vs.RHE.Besides,utilizing Ni₂₀-N-C as the CO₂ electroreduction catalyst to couple with the oxygen evolution reaction,the electrolytic production of CO and O₂ can be realized with a cell voltage of only 1.4 V,and even a 1.5 V zinc-manganese dry cell can be used to drive the CO₂ reduction electrolyzer.Besides,the catalyst continually delivered CO at 100 m A cm^-2 current density for 8 hours with CO Faradaic efficiency greater than 98%.The results exhibit that using a hierarchically porous catalyst is beneficial for improving the CO₂ electroreduction efficiency in the gas diffusion electrode and offers new strategies for the design of new types of CO₂ electroreduction catalysts.2.Ni-CB,a Ni-N-C catalytic material,was obtained by pyrolyzing nickel tetraphenylporphyrin and carbon black(CB)precursors,which achieved efficient CO₂ electroreduction.To improve the practicability of the anodic reaction in the CO₂ electroreduction system,Ni-CB catalyzed CO₂ electroreduction was integrated with the chlor-alkali process to realize the co-electrolysis of CO₂ and KCl solution,which can produce CO,Cl₂,and KHCO₃ simultaneously.The Faradaic efficiency of CO is up to 99%,while that of Cl₂ is?80%,and the production rate of KHCO₃ reaches 18 mmol h^-1.Furthermore,a continuous operating system was designed and manufactured to achieve direct separation of KHCO₃.The system operated at a current density of 100 m A cm^-2 for 5 hours at 3.8V cell voltage,with an energy efficiency of 39%.The continuous operating system can achieve a CO₂ conversion of up to 74% and has the potential to achieve negative net carbon emissions.The techno-economic analysis showed that the CO₂-chlor-alkali co-electrolysis system has obvious advantages over the existing technologies.Furthermore,the integrated system could serve as an alternative to the Solvay process for soda production.The strategy opens an avenue for techno-economically viable CO₂ reduction infrastructures and may lead to a serial of CO₂ electrolysis systems with various anodic products.3.Fe-N-C catalytic material Fe-CB was developed by pyrolyzing the mixture of iron tetraphenylporphyrin chloride and carbon black as catalyst precursor,which could achieve efficient CO₂ electroreduction with 97%CO Faraday efficiency at 0.35V overpotential.The Cd S photosensitizer and M-N-C catalysts were utilized to compose efficient heterogeneoushybrid photocatalytic systems.The feasibility of this hybrid system was verified by photo- and electro-chemical methods.Under UV-visible light(AM 1.5G),the CO₂ photoreduction system using Fe-CB as the catalyst achieved a high CO yield of 111 mmol g^-1_cat and a turnover number(TON)of 1.22×10³ in 8 hours,with a CO selectivity of 85%,exceeding the performance of most existing heterogenous photocatalytic systems.Compared with the Ni-CB catalyst,the yield of CO increased by 11.6 times under the same reaction conditions, and the selectivity of CO also increased by 2.7 times.The Fe-CB catalyst's high activity is mainly due to the Fe-N-C structure's stronger electron-donating ability,enhancing the coordination interaction between*COOH intermediate and Fe-N-C structure,reducing the energy barrier of the rate-determining step of the reaction.The results reveal that the utilization of M-N-C electrocatalyst can improve the selectivity and the efficiency of heterogeneous CO₂ photoreduction systems,offering a new strategy for the development of the CO₂ photoreduction systems.