Construction Of Metal And Semiconductor Nanocomposites For Light-Driven Methane Conversion

Developing methane conversion technologies with diversified pathways is of great significance to achieve China’s energy transition into carbon neutral.However,the industrial methane conversion process requires a large number of energy and equipment inputs,leads the extra resource consumption.Therefore,exploring new techniques for methane conversion under mild condition is important to achieve the sustainable utilization of methane resource.Photocatalysis,the newly representative catalytic model,which can utilize sustainable solar energy to drive methane conversion under mild condition,has attracted extensive attention.Nevertheless,due to the multiple reaction pathways will proceed once methane is activated,and the undesired side reactions usually occurred during methane conversion process.The light-driven methane conversion technologies still suffer from unsatisfying conversion rates and products selectivity.Therefore,employing the appropriate approaches for photocatalysts optimization is necessary to achieve efficient light-driven methane conversion under mild condition.In this dissertation,we select metal-semiconductor composites,a kind of representative and widely investigated photocatalysts,as the research candidates to improve the light-driven methane conversion performance by rationally designing and modulating the parameters of electronic structure,composition and reaction interface of metal-semiconductor composites.Meanwhile,we also combine the advanced characterization methods and theoretical simulation to unveil the mechanism of lightdriven methane conversion process,and strive to understand the critical role of different components in catalysts during the reaction process at atomic/molecular levels.The research findings may provide fresh insights for designing high-performance photocatalysts toward methane conversion and offer valuable references to understand the mechanism of methane conversion process.In summary,the main results are listed as follows:1.To address the limitations of unsatisfied methane conversion rate,we used the nonoxidative coupling of methane as research object and employed TiO2 as the model catalyst,and developed the valence band modification strategy of photocatalysts through single atom loading on TiO2.The in situ measurements and theoretical simulation results indicated the photocatalytic methane conversion performance was related with structure of valence band maximum in photocatalysts.Specially,the PdO4 unit in Pd single atom modified TiO2 nanocomposite exhibited the largest contribution in valence band maximum,which displayed the accumulation of photogenerated holes.Therefore,the methane could be activated in Pd-O4 sites and were able to dissociate C-H bond for ethane formation.In the meantime,reducing the role of oxygen atom in valence band substantially suppressed the overoxidation of methane with lattice oxygen,dramatically improving the catalytic durability.Furthermore,the in situ generation of oxygen vacancies on photocatalyst during photocatalysis have been experimentally observed and assigned to the consumption of lattice oxygen for methane overoxidation.The overoxidation process could be suppressed by steadying of subsurface lattice oxygen with element doping,further enhancing the catalytic stability.2.Based on the modulation of electron structure in photocatalyst for promoting methane conversion efficiency,we employed the alloying strategy to regulate the intermediates adsorption behavior on metal cocatalysts,and further improving the ethylene selectivity during photocatalytic methane conversion process.In this section,we constructed the Pt-Bi bimetallic cocatalyst loaded TiO2 nanocomposite to modulate the charge transfer in photocatalyst,and then optimize the intermediates adsorption in the metallic sites.The experimental results showed the incorporation of Bi atom into Pt nanocrystal could increase the electron accumulation on Pt site,which further downshifted the d-band center of Pd atoms and reduced the excessive adsorption strength of methane and intermediates.Meanwhile,the in situ experiments indicated the optimized photocatalyst with rational Pd/Bi ratio could promote methane conversion to ethylene and decrease the overoxidation of methane.3.Based on the results of intermediate modulation effect on metal cocatalysts,we provided the light-driven methane conversion pathway for acetic acid synthesis by rationally integrating the catalytically active sites for methane activation and intermediates coupling on nanocomposites.The key was the construction of Pd/PdO heterostructure on WO3 support.The in situ experimental results indicated the methane could be activated by hydroxyl radical and converted to methyl intermediate and carbonyl intermediate on Pd and PdO site,respectively.As such,the methane conversion to acetic acid could be achieved through the coupling of methyl and carbonyl intermediates to form acetyl precursor,which was subsequently converted to acetic acid.Furthermore,the isotope labelling experiments demonstrated that the oxygen atom in carbonyl intermediate was derived from the lattice oxygen of PdO in nanocomposite,which enabled methane carbonylation process.According to the reaction mechanism,we designed a photochemical flow reaction device with arcshaped flow channel to promote the utilization of intermediates and enhance the efficiency and selectivity of acetic acid synthesis.