Oxygen Reduction Reaction To Hydrogen Peroxide And Mechanism Of Electrocatalysis By Metal-Organic Layers

Hydrogen peroxide is thought to be one of the greenest oxidants and is widely used in industry and daily life.H2O2 is mainly produced through the oxidation process of anthraquinone.However,the transport,storage and handling of bulk H2O2 are hazardous and therefore expensive,making the distributed on-demand production of H2O2 highly desirable.Thus,electrochemical routes to H2O2 production through the oxygen reduction reaction(ORR)have attracted attention because of the afforded advantages,including low energy utilization and cost-effectiveness.Metal-Organic Frameworks(MOFs)represent a class of hydrid porous materials that are constructed from organic ligands and metal ion nodes.Metal-Organic Layers(MOLs)with monolayer thickness is a mesh-like thin film that can be used to modify electrode surface.MOLs are constructed from organic linkers and metal connecting nodes and are two-dimensional analogue of MOFs with rich chemical tunability.A well designed MOL can provide specific secondary interaction with metal aquo complexes.On the other hand,the metal atoms/ions can still reach the electrode surface through the meshes on the MOLs.Firstly,we report modification of glassy carbon electrode by a thin layer of an electrochemically inactive MOL constructed from benzene-1,3,5-tribenzoate(BTB)and Hf6(?3-O)4(?3-OH)4(HCO2)6 secondary building unit(SBU).As the proton on the?3-OH of the Hf6 SBU can dissociate under neutral pH,the MOL is negatively charged.A[Ni?(H2O)6]2+ complex can thus adsorb onto the MOL at the electrode surface via coulombic attraction as well as hydrogen bonding for O2 reduction.Over-reduction of H2O2 is prevented due to detachment of the reduced[Ni?(H2O)4(OH)(OOH)]0 species from the electrode surface.As a result,the dynamic site-isolation of Ni? leads to an efficient and selective catalyst for O2 reduction to H2O2.Furthermore,calculations using density functional theory provide a possible pathway for electrochemical reduction of O2 to H2O2.Molecular dynamic simulation also provides evidence of the dynamic nature of Ni2+ centers on the MOL that is close to the electrode for efficient electron transfer and able to transiently dissociate in the reductive form to avoid over-reduction.Secondly,we designed two different MOLs based on reactions between the metal salts ZrOC12·8H2O and AICl3·6H2O and the ligand molecule 2,3,6,7,10,11-hexahydroxytriphenylene hydrate(HHTP)using LB film method.We studied the ultra-thin MOL for their activity in oxygen reduction to H2O2 as tested using a rotating ring-disk electrode set-up in 0.5 M KNO3 electrolyte under oxygen atmosphere.We find that Faradaic efficiency of hydrogen peroxide can be as high as 80%for Zr-HHTP,while the Al-HHTP only gave a Faradaic efficiency of 20%.The interaction between Zr and HHTP must be very important for selective generation of H2O2.Compared to the sintered carbon materials doped with Zr element,Zr-HHTP has well-defined structure,which is suitable for mechanistic study.