Controlled Synthesis And Photo-/Electro-catalytic Properties Of Porphyrinic MOF-based Derivatives

Metal-organic frameworks(MOFs),as a class of relatively new porous crystalline materials,possess high specific surface areas,versatile structures and functionalizable porosity,making them different from traditional porous materials.In recent years,MOFs have been widely used in gas storage and separation,drug delivery,catalysis,etc.Porphyrin and metalloporphyrin are widely existed in the environment and playessential roles in life activities.They exhibit promising applications in catalysis,optoelectronics and so on.By combining the merits of porphyrin and MOFs,the porphyrinic MOFs possess paticular application potential in many aspects.In this dissertation,three porphyrin-based MOFs have been selected as templates/precursors.Their pyrolyzed composites have been obtained by controlled synthetic routes and photocatalytic or electrocatalytic properties are further investigated.The main research results are summarized as follows:1.MOFs have shown semiconductor-like characters in photocatalysis and have been used for water splitting into hydrogen,photo-reduction of carbon dioxide and so on.In order to improve the efficiency of catalyst,the introduction of co-catalyst,such as Pt nanoparticles,is an effective way as they not only trap electrons but also provide effective proton reduction sites,and thus dramatically improving the photocatalytic activity.Given the limited storage and high cost,downsizing the Pt nanoparticles into single Pt atoms,would be a good solution.The selected porphyrinic MOF,Al-TCPP,which possesses plenty of coordination sites to anchor single metal atoms,is employed to be support to stabilize and give single Pt atoms after reduction.Under light irradiaton,the singe-atom Pt/Al-TCPP composite exhibits good photocatalytic H2 production efficiency,high turnover frenquency and high recyclability.Ultrafast transient absorption spectroscopy further unveils that the single Pt atoms confined into the MOF provide highly efficient electron transfer channels and density functional theory(DFT)calculations indicate that the introduction of single Pt atoms into Al-TCPP improves the hydrogen binding energy,thus greatly boosting the photocatalytic H2 production activity.2.With increasing energy demand and environmental concerns,highly efficient electrochemical energy storage and conversion devices,such as fuel cells,are considered as promising power sources.But the sluggish kinetics of oxygen reduction reaction(ORR)at cathode limits the development.Presently,platinum(Pt)and its derivatives are the most efficient catalysts for ORR.However,the commercialization of Pt suffers from high cost,limited quantity available,poor stability and susceptibility to methanol crossover.A porphyrin-MOF,PCN-224,have been recognized as suitable precursors/templates to give porous carbons with high surface area and homogeneous FeNx and CoNx active species for efficient ORR through pyrolysis with alternating Fe(Ⅲ)and Co(Ⅱ)centered in the porphyrin linkers.Benefiting from the unique hollow porous nanostructure,along with the synergetic effect between the highly dispersed FeNx and CoNx sites,the optimized catalyst exhibits excellent ORR electrocatalytic activity,favorable reaction kinetics,long-term stability and superior methanol tolerance under both alkaline and acidic conditions,surpassing the state-of-the-art Pt/C and most of the non-noble metal electrocatalysts.3.The anodic oxygen evolution reaction(OER),a thermodynamically uphill reaction with slow kinetics,hampers the rate of the overall water splitting and it is ofgreat importance to develop efficient and low-cost OER catalysts.A representative metalloporphyrinic MOF,PCN-600-Ni,integrating with graphene oxide(GO),is served as an ideal precursor and template to afford bimetallic iron-nickel phosphide/reduced graphene oxide composite via pyrolysis and subsequent phosphidation process.Thanks to the highly porous structure,synergetic effect of Fe and Ni elements in bimetallic phosphide as well as the good conductivity endowed by rGO,the optimized Fe-Ni-P/rGO-400 exhibits remarkable catalytic performance for OER in 1.0 M KOH,affording an extremely low overpotential of 240 mV at a current density of 10 mA/cm2,which is far superior to the state-of-the-art IrO2 and among the best in all non-noble metal-based electrocatalysts.