Direct Coupling Of Thermocatalytic Hydrogenation And Photocatalytic Water Splitting For The Conversion Of CO₂-H₂O To Fuels

Convertion of CO₂ to fuels using solar energy not only reduces CO₂ emissions,but also produces high-energy fuels,which is of great importance to address the issues of energy and environment.Photocatalytic reduction of CO₂-H2O is one of the ways to utilize solar energy to achieve "green" conversion of CO₂ into fuels.However,it is rather difficult to efficiently convert CO₂ to fuels solely via photocatalysis due to the kinetic limitations and the thermodynamically stable CO₂.It is well known that thermocatalytic hydrogenation of CO₂ to methane is a mature process for energy storage,which readily occures under mild conditions.For example,the CO₂ conversion and CH4 selectivity can both reach-100%over Ni or Ru based catalysts at atmospheric pressure and low reaction temperature.The work reported in this thesis aims to achieve highly efficient conversion CO₂-H2O to fuel via direct coupling of photocatalytic water splitting for hydrogen production and thermocatalytic hydrogenation of CO₂ over a bifunctional catalyst.It is expected that this approach will provide a new strategy for "green" conversion of CO₂ to fuels.The results indicate that Au-Ru/TiO₂ is an excellent catalyst for thernocatalytic conversion CO₂ with H2 to alkaness even at a reaction temperature as low as 323 K.Meanwhile,Au-Ru/TiO₂ is an excellent catalyst for H2 production from water splitting under illumination.The activity of direct coupling of thermo-photocatalysis for conversion of CO₂-H2O to fuel over Au-Ru/TiO₂ is 15 times higher than that of photocatalytic water splitting to H2 at 358 K.We find that Ru or Au-Ru are the active sites for hydrogenation of carbon dioxide to methane,Au or Au-Ru is the active sites for water splitting to H2.In the process of directly coupling of photocatalytic water splitting and thermocatalytic hydrogenation of CO₂ to alkane,photo-generated electrons preferentially reduce water to produce highly active H atoms,which thermocatalytically hydrogenate CO₂ to form alkanes.As a result,the separation of photogenerated electrons and holes is promoted.Direct coupling of thermocatalytic hydrogenation and photocatalytic water splitting shows a strong synergistic effect for CO₂-H2O conversion to methane.We present here a facile strategy for highly efficient conversion of CO₂-H2O to fuels via direct coupling of photocatalytic water splitting and thermocatalytic hydrogenation of CO₂ to alkanes over Au-Ru/TiO₂.The process not only has an important impact on utilizing CO₂ for efficiently producing fuels,but also can be further extended to other hydrogenation reactions,such as ammonia synthesis etc.