Photocatalytic H₂O₂ Production By Acetylene And Diacetylene Functionalized Covalent Triazine Frameworks

Photocatalytic H2O2 production from H2O and O2 utilizing sunlight as the only energy input is a potentially viable approach in terms of green and sustainable synthesis of H2O2.Recent studies indicate that metal-free polymer photocatalysts have shown great promise for photocatalytic H2O2 production via two-electron/two-proton reduction of molecular O2.They usually show much higher activity and selectivity toward H2O2 production by forming suitable intermediates and eliminating side reactions compared to inorganic photocatalysts.On the other hand,photogenerated holes in certain polymer photocatalysts can oxidize H2O via the one-electron process to form OH· which then combines with each other to produce H2O2.However,this stepwise one-electron water oxidization process requires high concentrations of OH· to form desired H2O2,limiting the H2O2 production yields.Hence,the other half-reaction,which is the two-electron oxidation of water toward H2O2 production,still remains elusive.This can be ascribed to the fact that four-electron water oxidation to form O2(+1.23 eV vs NHE)is thermodynamically more favourable than two-electron water oxidation to form H2O2(+1.78 eV vs NHE).Meanwhile,chemical structures in polymer networks that can facilitate the formation of H2O2 intermediates in hole-involved two-electron pathway still require further elucidations.Nevertheless,it is critically important to directly utilize the photogenerated holes to produce H2O2 in order to increase the H2O2 production yield and reach 100%atom utilization efficiency.Covalent triazine frameworks(CTFs)are a unique c.lass of two-dimensional(2D)polymer photocatalysts that exhibit tunable chemical and electronic structures and can be precisely controlled by using different nitrile precursors.Here we show that introducing acetylene(-C?C-)or diacetylene(-C?C-C?C-)moieties into covalent triazine frameworks(CTFs)could remarkably promote photocatalytic H2O2 production.This enhancement is inherent to the incorporated carbon-carbon triple bonds which are essential in modulating the electronic structures of CTFs and suppressing charge recombinations.Furthermore,first-principles calculations reveal that the acetylene and diacetylene moieties can facilitate O2 adsorption during the two-electron oxygen reduction reaction to form H2o2 and also significantly reduce the energy associated with OH*formation which is a crucial step in two-electron oxidation of water.The solar-to-chemical conversion(SCC)efficiency can reach 0.14%using CTF-BDDBN,surpassing that of natural synthetic plants(globally average?0.10%).Our study unveils an important reaction pathway toward photocatalytic H2O2 production,reflecting that precise control over the chemical structures of polymer photocatalysts is vital to achieve efficient solar-to-chemical energy conversion.