Reaction Dynamics And Energy Conversion Of Fuel Production By Catalyzing H₂O And CO₂ Splitting With Photo-Thermal Synergy

The sun is a natural giant nuclear fusion reactor and the largest and cleanest renewable energy source that humans can currently use effectively.Solar energy utilization is an important part of the clean energy revolution in the fourth scientific and technological revolution.The starting point of solar fuel is to store low-energy,unstable solar energy in the form of chemical energy,which can well match the modern energy consumption system.The complete solar fuel with water and carbon dioxide as raw material contains the idea of material circulation,which is a sustainable way of energy conversion and utilization.The two-step thermochemical cycle uses thermal energy to convert carbon dioxide and water into carbon monoxide,hydrogen and oxygen through a series of chemical reactions,which in turn synthesizes the synthesis gas into a hydrocarbon fuel such as methanol,which is reduced compared to direct pyrolysis of carbon dioxide and water.The reaction temperature is easy to use with nuclear energy and solar energy,and has high energy conversion efficiency.However,the required temperature is still very high,and a series of difficulties caused by high temperature make it a great challenge for further development.Photochemical preparation based on semiconductor materials.The complete solar fuel is based on the photoresponse characteristics of the material,and the photogenerated electron holes are excited by sunlight to directly perform a redox reaction or form a current,and then an electric current is reacted with the current.Photochemistry can be carried out at room temperature,but there is a problem that the solar spectrum energy utilization is low and the electrode cost is high.More and more researchers are beginning to explore new ways to implement photothermal coordination,coupling light and heat in solar energy to achieve full utilization of spectral energy.In this paper,based on the idea of photothermal synergy,a new photo-thermochemical cycle?PTC?method for decomposing water and carbon dioxide is proposed.The high-quality UVvisible part of the solar spectrum is used to generate photo-induced oxygen vacancies at room temperature instead of two-step heat-induced VOs at high temperature.In the second step,the visible infrared portion of the light having a thermal effect is used to generate heat,and at a certain temperature,the photo-induced oxygen vacancies are reduced to carbon dioxide or water to obtain carbon monoxide or hydrogen.The main reaction process is as follows:M_xO_y?M_xO_y?-1?+1/2O₂?g??photochemistry,room temperature?M_xO_y?-1?+CO₂?g??M_xO_y+CO?g??thermochemistry,>373 K?M_xO_y?-1?+H₂O?g??M_xO_y+H2?g??thermochemistry,>373 K?In this paper,TiO₂ was used as a circulating material to realize PTC decomposition of carbon dioxide,and a preliminary cyclic reaction mechanism was proposed.In order to further reveal the decomposition mechanism of water and carbon dioxide in PTC,TiO₂ and 0.5 wt% Fe-TiO₂ experimental samples were prepared by sol-gel method.During the photoreaction process,Fe³⁺ became an electron capture center,which reduced the recombination rate of photogenerated electron hole pairs and improved the photoreaction activity.The red shift phenomenon of 0.5 wt% Fe-TiO₂ and the narrower band gap indicated that it can absorb a wider range of spectra,which would generate more photogenerated electrons and holes.According to the first principle method,the simulation calculation was carried out on the basis of the experiment,and the PTC mechanism was deeply explored.The oxygen vacancy formation of the Fe-doped surface was lower than the undoped clean surface,indicating that the Fe-TiO₂ surface was more prone to generate more oxygen vacancies.Through the optimization calculation of the adsorption configuration of CO₂ and H₂O on the surface of TiO₂?101?and Fe-doped TiO₂?101?,the adsorption heat release was larger,the optimized configuration was stable,and the oxygenabsorbing adsorption was easier to form.The results of the simulation calculations can be consistent with the experimental results,and finally a deeper PTC mechanism model of water and carbon dioxide decomposition was obtained.Theoretical calculations can not only assist the experimental mechanism research,but also guide the material design.In order to provide nanostructure design guidance,three M elements?Zn,Ni,and Cu?with similar radii were doped into TiO₂ as representative elements for DFT calculation and experimental verification.Good agreement between DFT calculations and experimental verification provided an effective guide to nanostructure design,indicating that the oxygen vacancy formation energy and optical properties were two key factors in the PTC for nanostructures for CO₂ conversion.The experimental results showed that since M incorporated into TiO₂ acts as an electron trapping agent,a lighter-induced EHP with a lower recombination rate was formed.The XRD and ESR results supported the presence of Ti-O-Cu-O-Ti bonds on the surface of Cu-doped TiO₂.The formation of Ti-O-Cu-O-Ti bonds was believed to be the key to enhancing CO₂ reduction in PTCs.Finally,the performance of Cu-doped surface guided by DFT calculation was better than that of other doped TiO₂,and a nanostructure design and enhancement mechanism for improving PTC conversion of CO₂ was proposed.The PTC has both a photoreaction step and a thermal reaction step.From the viewpoint of photothermal coupling,the Pd nanoparticle-loaded titanium dioxide material achieved both photothermal response.The reaction mechanism has been deeply studied.PNT samples had good optical response performance in the UV-visible band due to their unique LSPR properties,which were reflected in different degrees in simulation and various characterization methods.At the same time,in the thermal reaction stage,in-situ infrared spectroscopy and theoretical calculations showed that CO₂ was more likely to form active intermediate reactive groups CO₂-on PNT,and a special Pd-CO₂--VO structure would be formed near PN.The thermal synergistic coupling factor greatly promoted the reaction,and its maximum CO yield was nearly 9 times higher than that of P25.This indicated that the photothermal synergy had possibility of further increasing the PTC fuel yield,and could also help the PTC to decompose CO₂ and H₂O synthetic hydrocarbon fuels.From the perspective of conversion efficiency between solar energy and chemical energy,the concept of energy quality and the considered problems were clarified based on energy carriers to discuss the energy conversion process.Starting from the second law of thermodynamics,we tried to introduce the idea of energy quality in thermophysics,and deal with the three energies of light energy,heat energy and chemical energy in the process of energy conversion.Based on the two treatment methods of light and heat,the light reaction and the thermal reaction were treated separately due to the special mode of PTC.The photocatalysis principle based on the energy band theory was used to explain the photoreaction process?electron level?,and the activation energy theory was used to interpret the thermal reaction process?molecular level?.The electron-molecule with two different layers were connected by chemical reaction,and finally the semi-empirical theoretical efficiency calculation model of PTC was obtained.