Mechanism Of CO₂ Photocatalytic Reduction Over Morphology Controlled Metal/TiO₂ Nanomaterials

CO₂ photocatalytic reduction by using Ti O2 catalyst is one of the most potential renewable technologies,which can reduce the emission of CO₂ and provide valuable fuels.However,TiO2 catalyst still suffers from the low photocatalytic efficiency,low utilization of solar energy,and poor selectivity.In order to solve these problems,several kinds of metal modified TiO2 catalysts with different morphologies were developed by using surface modification combined with morphology control technologies.The photocatalytic CO₂-reduction properties and reaction mechanism of these catalysts were investigated systematically.We hoped that these studies can provide theoretical basis and technical supports for the development of novel photocatalyst and the application of CO₂ photoreduction technology.In this research,Pt modified TiO2 nanoparticles were prepared by using a sol-gel method.The physicochemical properties of the catalysts were characterized and their photocatalytic activities under UV and visible light irradiation were analyzed.A potential reaction mechanism of the catalyst was proposed based on the experimental results.The results show that TiO2 existed in anatase and Pt existed as Pt0 and Pt2+.Pt0 nanoparticles deposited on the surface of TiO2 and Pt2+ ions were doped into the lattice of TiO2.H2,CH4,and CO were found to be the main products of Pt/TiO2 catalyst.The loading of Pt0 and Pt2+ can enhance the production of H2 and CH4,while has no obvious influence on the formation of CO.Pt/TiO2-3 catalyst shows the strongest activity,the H2 and CH4 yield reached 2763.1 和 264.5μmol g-1 after 7h UV light irradiation.In the case of visible light irradiation,the CH4 yield reached 138.6μmol g-1 after 7h irradiation.The high visible light activity of Pt/TiO2 catalyst is because Pt nanoparticles can inhibit the recombination of photogenerated charges and Pt2+ can extend the light response region of TiO2.Compared to the light absorption property,the effect of the recombination rate of photogenerated electron-hole pairs on the visible light activity of Pt/TiO2 is more remarkable.To enhance the activity of TiO2,Pt nanoparticles deposited mesoporous TiO2 catalyst（Pt/M-TiO2）was synthesized by using a hard-templete method combined with a NaBH4 reduction method.The influence of Pt nanoparticles deposition and reaction mode on the photocatalytic activity of the catalyst was analyzed.The results show that Pt/M-TiO2 has a higher surface area than that of Pt/TiO2 nanoparticles.H2,CH4 and CO were found to be the main products and appropriate loading of Pt can obviously enhance the activity of mesoporous TiO2 catalyst.Under gas-solid reaction mode,0.5%Pt/M-TiO2 has the highest activity,whose H2 and CH4 yield were 13.4 and 0.73μmol g-1 h-1,respectively.This is because Pt nanoparticles can promote the sepration of photogenerated charges and the mesoporous structure of TiO2 has large surface area,enhancing the absorption capacity of the catalyst for CO₂ and water and accelerating the transfer of charges.The reaction mode also has great influence on the activity of the catalyst.The liquid-solid reaction mode promotes the the splitting of water,but gas-solid reaction mode is helpful for the reduction of CO₂.Based on this,a novel reaction mode was developed to use the H2 produced from gas-solid reaction mode to enhance the CO₂ hydrogenation in gas phase.This reaction mode can enhance the activity of the catalyst because the novel reaction not only guarantees the contact of CO₂ and catalyst but also provide strong reductant for CO₂ reduction.To enhance the selectivity of CO₂ photoreduction over TiO2 catalyst,Pt and Cu co-loaded TiO2 nanocrystals with co-exposed {001} and {101} facets were synthesized by using a solvothermal method combined with a chemical reduction method.The photocatalytic activity of the catalyst was investigated systematically,and effect of the ratios of {001}/{101} facets and the loading of Pt and Cu on the structure,morphology,optical property,and photocatalytic CO₂ reduction activity and selectivity of the catalyst was explored.A potential reaction mechanism for selective photocatalytic reduction of CO₂ was proposed.The results show that appropriate ratios of exposed {001} and {101} facets can promote the spatial separation of photogenerated charges over different facets,which effectively inhibit the recombination of photogenerated electron-hole pairs,resulting enhance photocatalytic activity.The deposition of Pt prefers the reduction of H2 O while the loading of Cu2 O tends to reduce CO₂.The co-loading of Pt and Cu2 O can obviously inhibit the production of CO and H2 during photocatalytic process,while reduce CO₂ into CH4 selectivity（96.6%）.The low generation of H2 can be attributed to the stronger CO₂ abosrption caused by Cu2 O deposition,resulting in weak aborption of water vapor on the catalyst.The co-loading of Pt and Cu2 O can effectively promote the separation of photogenerated charges,accelerating the transfer of electron to Cu2 O.This can provide more electrons for the reduction of CO₂,which is helpful to the selective reduction of CO₂ into CH4.A simple model incorporating the coupled effect of the adsorptive photocatalytic reduction and oxidation was developed,based on the assumption that the process is chemisortion and the controlling equation is a Langmuir-Hinshelwood type of kinetic equation.It is then analysed using experimental data of Pt/TiO2 and Pt-Cu/TF1.2 catalysts.The experimental data fitted very well with our propsed model.The model could be effectively used to estimate the reaction rate and the product formation from most of the photocatalytic reduction of CO₂ with gaseous water,which is helpful to understand the photocatalytic reaction process,control reaction tendency,and optimize reaction conditions.LCA and ELCA were used to analyze the CO₂ emission and energy comsuption of the oxyfuel combustion and oxyful-photocatalysis power generation systems throughout their overall life cycle.The results of LCA showed that,for the two power generation systems,the CO₂ emission during the operation stage were both dominated in the life cycle。The CO₂ emission of oxyfuel-photocatalysis power generation system in the whole life cycle was obviously lower than that of oxyfuel combustion power generation system,only 63.2% of the latter one.The results of ELCA indicatated that the exergy input in the operation stage was highest among the whole life cycle for the both power generation systems.And the overall exergy input of oxyfuel-photocatalysis power generation system was only 62.97% of that of the oxyfuel combustion power generation system.This is because the photocatalytic conversion equipment can reduce the coal consumption,resulting in decreased CO₂ emission and energy/resource consumption.