| Recent decades have witnessed rapid development of industry and prosperity of human society thanks for the exploitation and application of fossil fuels,yet it has caused many environmental problems:pollution problems,energy shortages,the greenhouse effect caused by CO2 emissions and so on.The utilization of CO2 into resources has increasingly drawn scientists’attention.It is particularly noteworthy that the reaction of CO2 and CH4 to produce such more high-value-added organic derivatives as alcohols,alkanes and alkenes,is one of the hot and difficult research topics concerning catalysis,green chemistry and chemical engineering.Whereas,considering the extremely stable chemical properties of CO2 and CH4,the activation of C-H bond and C=O bond requires high energy consumption and remains a challenge.Therefore,the research on the green and mild photocatalytic technology for this reaction has proved to be extremely significant.In this paper,starting from the adsorption and activation of reactants during heterogeneous catalytic reaction,carbon nitride with ordered mesopores and alkali metal dopants were designed to increase the amount of absorbed and activated CO2,which can provide an effective channel for both separation and transmission of photoinduced electron-hole pairs,achieving highly-efficient photoreduction of CO2 into plenty of hydrocarbons.Noble metals were used out of the consideration of breaking C-H bonds in the methane and reconfiguring them on the surface of catalyst so as to realize bimetallicly catalyzed CO2-CH4 conversion.In general,it can be divided into the following two parts:First,using urea and lanthanum acetate as precursors,La-doped g-C3N4（LGCN）photocatalyst was prepared via co-pyrolysis method,and its surface and optical properties were characterized.It was found that the prepared carbon nitride nanosheets have a high specific surface and ordered pore structure;combined with theoretical calculations,it was shown that doping La can gradually reduce the energy band gap resulting from changes in the conduction band structure,making the material partially exhibit the characteristics of conductor and form the CN-La bond,which provides a"channel"for the effective transmission of photo-generated electrons benefiting the separation effect of photo-generated carriers.When the doping amount of La reaches 55.54 wt%（0.60 LGCN）,the conpound shows the optimal performance with a variety of C2+～C3+organic produced.After 3 hours of illumination,the products of CO and CH3CH3 yielded 40.02μmol·g-1 and121.03μmol·g-1 respectively,a relatively high level reported domestically.Although La has an effective activation on C=O,the adsorption and activation performance of carbon nitride matrix on CH4 is limited.Therefore,it is necessary to introduce CH4 reaction sites to construct a high-efficiency photocatalyst with dual active sites in the CO2-CH4 reformation.Regarding that the CH4 reaction site should have double significance in both light harvesting and retaining site of holes,the noble metal Ru loading was selected to improve the catalysis performance.The bimetallic synergistic modification further improves the separation efficiency of electron holes,so that noble metals become the hole enrichment sites,and La becomes the electron enrichment active site.In the photocatalytic reaction experiment,the catalytic activity of the 0.5%Ru/0.60LGCN catalyst was three times higher than that of the unsupported catalyst,which was a high level in China.The yields of CH3OH and CH3CH（OH）CH3 were 133.33μmol·g-1·h-1,153.68μmol·g-1·h-1 and 133.33μmol·g-1·h-1,respectively.Photocatalysis is a heterogeneous process involving interfacial catalytic reaction.The improvement of photocatalytic efficiency hugely depends on the modification and regulation of active sites,where metal modification plays a vital role.In this paper,a variety of characterization and theoretical calculations are carried out to study the mechanism of catalytic reactions with a profound understanding of the dual role of metal promoters. |
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