Study On Metal Catalysts Confined By MOF/COF Materials And The Catalytic Conversion Of Small Molecules

MOF(Metal-Organic Framework)and COF(Covalent Organic Framework)materials have controllable pore size and designed and functionalized structure,which can serve as ideal supports for preparing metal catalysts.Metal catalysts confined by MOF and COF materials have well-define structures,which can provide explicit catalytic active sites for studying structure-property relationships and reaction mechanism for photocatalytic and electrocatalytic activation of small molecules,such as H2O,H2,O2,N2,and CO2.In this work,a systematic study was carried out on the preparation of single atom catalysts and nanoparticle catalysts supported on MOF and COF materials for small molecule activation.The main contents are given as follows.(1)A general strategy for preparing MOF-based single atom catalyst was developed.The ethylenediaminetetraacetic acid(EDTA)ligands,which can chelate single metal ion,were grafted to MOF-808(MOF-808-EDTA).After metal ion loading,MOF-based single atom catalysts are acitved by reduced atmosphere.Upon activated in hydrogen atmosphere at 200?,the stable MOF-808-EDTA encapsulated single-atom metal catalysts with well-defined atomic positions were synthesized.The synthesized single-atom metal catalysts were confirmed using various experimental characterization techniques including HAADF-STEM,EXAFS,and DFT calculations.As a demonstration case,it has been found that the as-synthesized MOF-808-EDTA encapsulated single-atom Pt catalyst shows excellent photocatalytic hydrogen evolution activity(68.33 mmol g-1 h-1)and high stability.This work also provides a general synthesis approach for preparing MOF anchored single-atom metal catalysts such as Pd,Rh,Ru,Cu,Co,and Ni.(2)MOF-808-EDTA can effectively capture metal ions,which also can be used to develop a strategy for controlling metal nanopariticle sizes.MOF-808-EDTA can capture single Pt2+ ions by EDTA sites;while the excess and uncaptured Pt2+ ions will form PtO2 nanoclusters with various sizes inside MOF-808-EDTA pores as precursors.Upon pyrolysis,the sizes of ultrafine Pt nanoparticles can be effectively controlled in the range of 2.1?4.1 nm by the initial Pt2+ion concentrations via both migration/coalescence and Ostwald ripening aggregation mechanisms.Compared with the commercial Pt/C(20 wt%of Pt),the Pt nanoparticles with the size of 4.1 nm exhibits a lower overpotential of 42.1 mV at a current density of 10 mA cm-2 and a Tafel plot of 24.45 mV dec-in 0.5 M H2SO4.In addition,it has a higher stability although it has lower amount of Pt(9.58 wt%).The proposed synthesis strategy in this work has been found to be a suitable method for preparing the controllable sizes of ultrafine metal catalysts with high catalytic activity.(3)The coordination structure of single atom catalysts is vital to explore the catalytic reaction mechanism.The well-defined single-atom Pt anchored at the-N3 sites of stable and ultrathin covalent triazine framework(CTF)nanosheets(?1.0 nm)have been successfully synthesized(Pt-SACs/CTF).The well-defined coordination structure of Pt-N3 sites in the Pt-SACs/CTF catalyst have been characterized using HAADF-STEM and EXAFS,as well as ab initio molecular dynamics simulations.The photocatalytic ammonia production rate over the as-synthesized Pt-SACs/CTF catalyst is 171.40 ?mol g-1 h-1 in the absence of sacrificial agent.On the basis of DFT calculations,it has been found that alternating mechanism is energetically more favorable than distal mechanism over the well-defined Pt-N3 sites.The significance of the present work is that it demonstrates that the single-atom metal catalysts are anchored at the two-dimensional stable CTF nanosheets for photocatalytic nitrogen fixation to ammonia.(4)A metal-free 2D/2D heterostructured catalyst with black phosphorus(BP)and CTF is developed for photoreduction of CO2 to CH4.Compared with the pristine BP or CTF,the photoreduction CO2 to CO(4.60 ?mol·g-1·h-1)and CH4(7.81 ?mol·g-1·h-1)rate over the CTF-BP catalyst have been improved.The results show that the CTF-BP heterostructure can change the reaction path,which dramatically enhances the photocatalytic selectivity for CO2 reduction to CH4.The present work not only develops a metal-free highly efficient and selective catalyst for photoreduction of CO2,but also provides a new heterostructure engineering route for designing and synthesizing highly active and metal-free catalysts applied in the sustainable solar-to-chemical energy conversion and environmental remediation.(5)A continuous self-sacrifice template strategy is introduced to prepare the bifunctional electrocatalyst for achieving the step-by-step construction of multicomponent catalytic active sites.ZIF-67 as a continuous self-sacrifice template was used to prepare bifunctional NiCo2O4/Co,N-CNTs electrocatalysts.The as-prepared NiCo2O4/Co,N-CNTs NCs display remarkable electrocatalytic activity toward ORR(E1/2=0.862 V)and OER(Ej10=1.569 V)simultaneously,outperforming commercial catalysts like Pt/C and RuO2.The reversible oxygen electrode index(?E=Ej10(OER)-E1/2(ORR))is?0.707 V in alkaline electrolyte.Interestingly,NiCo2O4/Co,N-CNTs NCs act as a cathode electrocatalyst in a primary Zn-air battery system with a high power density of 173.7 mW cm-2.The continuous self-sacrifice template strategy proposed in this work is expected to inspire the design of high-performance bifunctional nonprecious electrocatalysts for application in sustainable energy conversion devices.