Study On Hydrothermal Conversion Of CO₂ Into Organics With Iron

CO₂,originating from fossil fuel combustion and other anthropogenic activities,has strong impact on climate change and may pose catastrophic effects to the humanity.While CO₂ is an abundant,clean,cheap,safe carbon resource,so the conversion of CO₂into organics/fuels for recycling is the fundamental way to solve the climate change and energy crisis.Thus,the study on the utilization of CO₂ has received much attention and becomes one of most potential technology.At present,many methods are applied on the utilization of CO₂,such as electrochemical reduction,catalytic hydrogenation and photocatalytic reduction of CO₂with water?artificial photosynthesis?.However,additional power for electrochemical reduction CO₂ is generally needed,the low yield and selectivity are important limiting factor.Catalytic hydrogenation requires a complex or precious catalyst and hydrogen sources,the storage,transport and security for hydrogen is a probelem.Artificial photosynthesis is regarded as one of the most promising methods for CO₂ reduction,however,the bottleneck of inefficiency has not yet been broken through.Thus,based on the idea of stimulation nature,a new innovative method of efficiently water splitting for reduction of CO₂ into organics with cheap metal iron has been proposed to avoid the shortcoming of these methods in this paper,which can combine with the technology of a solar two-step water-splitting thermochemical cycle base on the redox of metals/metal oxides and biomass hydrothermal conversion,expecting to open up a new way to break through the bottleneck of inefficient for photocatalytic reduction of CO₂with water?artificial photosynthesis?.Firstly,the efficiency of water splitting in-situ reduction of CO₂ by metallic Fe without any catalyst was investigated.The results showed that the concentration of NaHCO₃ obviously promoted Fe oxidation and achieved hydrogen production efficiently from water splitting.The yield of formic acid reached 92%,and the selectivity reached 98%at the optimized condition with 0.7 M NaHCO₃,300°C for 2h without any catalyst.In addition,the mechanism of water splitting in-situ reduction of CO₂ with metallic Fe was further investigated by XRD,Raman,XPS,SEM.These resluts indicated that hydrogen and Fe₃O₄–_x with more oxygen vacancies which were formed in-situ has a better autocatalytical to the reduction of NaHCO₃.Secondly,the catalytic activity of Ni for the reduction of CO₂ into formic acid with Fe were studied.The results showed that Ni had a good performance to decrese the reaction temperature,which was decreased 100°C.The yield of formic acid reached72%,and the selectivity reached 98%at the optimized condition with 1.0 M NaHCO₃,Fe/Ni=4:1,200°C for 3 h.The study on the mechanism of CO₂ reduction showed that Ni could help to decrease the temperature of Fe oxidation,which achieved efficiently splitting water for hydrogen production.Additionally,in-situ flower-like Fe₃O₄ also played the catalyst in this system,the repeatability of catalysts was very well,and the yield of formic acid was enhanced with the reued catalysts.Thirtly,the effectiveness of the reduction of CO₂ into methanol with Fe by Pd/C were foucus to obtain the other value-add organics.The results showed that the reduction of CO₂ into methanol with Fe by Pd/C was feasiable.The yield of methanol reached 16%,and the selectivity reached 90%at the optimized condition with 1 MPa CO₂,350°C for 3 h.We found that CO₂ obviously promoted Fe oxidation and the main oxidation product is FeCO₃.Finally,the feasibility of the reduction of CO₂ into multi-carbon compound with Fe was studied.The results showed that NaHCO₃ can be converted to acetic acid with Co₂O₃?Raney Ni?Pd/C and can be converted to phenol with Pd/C.The yield of acetic acid reached 5.7 mmol/L with Co₂O₃,the yield of acetic acid reached 21 mmol/L with Raney Ni,the yield of acetic acid and phenol reached 2.8 mmol/L and 1.9 mmol/L with Pd/C,respectively.Therefore,this work provided an efficient way to reduce CO₂ into organics by splitting water simply and rapidly and improved a highly efficient and sustainable cabon cycle,which could relieve the problem of the climate change and energy crisis.