Atomic-level Regulation Of Fe-Pt Based Catalysts For Alkaline Hydrogen Evolution Reaction

Atomic-level design and structural regulation of catalysts is very crucial to solve the problems of energy shortage and environmental pollution.Platinum-based（Pt）catalysts are widely used in catalytic fields（e.g.,heterogeneous catalysis,electrocatalysis,photocatalysis,etc）because of their remarkable catalytic performances.However,due to their high cost and scarce reserves,the widespread application of Pt remains limited.Iron（Fe）is listed as the second to aluminium（Al）in the earth’s metal reserves,holding a great potential for application.In addition,the introduction of Fe species into Pt to form Fe-Pt based catalysts（such as alloys,intermetallic compounds（IMCs）and other supported catalysts）enables optimizing their catalytic activity while reducing the usage of Pt contents.Because the introduction of Fe optimizes the electronic structure of Pt and regulates the d-band center.IMCs have special coordination structures and stoichiometrics due to the ordered arrangement of various elemental atoms,displaying even better performance than alloys,especially in terms of stability.However,the effects of phase structure,surface structure and strain-coordination on the catalytic performance of Fe-Pt based catalysts remain unclear,and it is crucial to identify the active sites and thereof establish more efficient reaction routes.Thus,it is very necessary to establish a clear structure-performance relationship for design and preparation of efficient Fe-Pt based catalysts.In this dissertation,we focus on a variety of Fe-Pt based catalysts,by using advanced techniques of spherical aberration transmission electron microscopy,X-ray absorption near edge structure and in-situ diffuse reflectance infrared Fourier transform spectroscopy（DRIFTS）,made a full play of atomic-level structural design,surface structure regulation,strain-coordination based structural optimization and further identification of active sites.The main investigation contents and conclusions are as follows:1.Based on Fe-Pt binary phase diagram,a series of Fe-Pt based catalysts were designed and prepared,and successfully applied to alkaline hydrogen evolution reaction.For the synthesis of fcc-Pt,L1₂-type FePt₃,L1₀-type FePt,A1-type FePt alloy,bcc-Fe and their mixed phases,our study shows that the introduction of appropriate Fe species can significantly improve the performance of Pt-based catalysts.For activity aspect,the current density of Fe₀Pt_1.0/CNTs was 10 m A cm^-1at53 m V overpotential.In case of Fe_0.25Pt_0.75/CNTs,the overpotential decreased to 36m V,and Fe_0.1Pt_0.9/carbon nanotubes（CNTs）containing minor Fe reached 10 m A cm^-1at 29 m V overpotential.In terms of stability,Fe₀Pt_1.0/CNTs were rapidly inactivated within 20 h,even though Fe_0.1Pt_0.9/CNTs containing a small amount of Fe that affords a significantly improved stability.The unique Fe/Pt coordination is responsible for the obtained excellent catalytic performance due to.Mechanistically,it enables regulating the electronic structures of catalysts,and optimizing their activity and stability of the catalyst while the amount of Pt was reduced.2.The crystal structure and surface atomic arrangement of FePt₃cataysts and the structure-activity relationship in alkaline hydrogen evolution reaction were fully investigated.A series of FePt₃-x H catalysts（x represents a variety of thermal treatment temperature）supported on CNTs were synthesized by using simple impregnation method,and we adjusted the order degree of Fe/Pt atomic arrangement in FePt₃catalysts by a series of heat treatment processes（air annealing,reduction process）.Through advanced microscopy and spectroscopy techniques,it was found that when Fe/Pt in the catalyst nanoparticles adopts an ordered arrangement（L1₂IMCs）,a small number of（111）and Pt-riched（200）/（220）planes are exposed.When Fe/Pt locates in a disordered arrangement（A1 alloy）,randomly-distributed Fe/Pt facets of（111）and（200）are exposed.The former has better activity and stability in alkaline hydrogen evolution reaction.The excellent catalytic performance is attributed to the unique Fe/Pt coordination,and the Pt-riched（200）facets protecting Fe from further oxidation.Therefore,we have successfully achieved atomic-level regulation of surface and subsurface of the Fe-Pt based catalysts,to thereof optimize their catalytic performance.3.Structural optimization based on atomic-level strain and coordination is critical for catalyst design and performance improvement.Due to the unique arrangement of Fe/Pt in the L1₀structure,we regulated the strain of Fe-Pt based catalyst by doping different types of elements,and tunned the coordination environment of Fe-Pt by changing the doping contents.Based on the design of strain and coordination structure,we made further exploration on the optimal conditions of the L1₀-type FePt catalyst in the alkaline hydrogen evolution reaction.4.Benefited from on the aforementioned systematic studies on the atomic-level structural design and surface regulation of the Fe-Pt based catalysts,we also identified the active site and reaction mechanism of a relavently extreme case-single Pt atom catalysts on an Fe-based support.Our design of single-atom catalysts can maximize the atomic utilization of noble Pt.Combining the systematic theoretical calculations and experimental design,Ni Fe-layered double hydroxide supported single Pt atom catalysts was successfully preparted.Then we monitored the reaction process and the involved intermediates by using advanced electron microscopy and in-situ DRIFTS.As a result,we confirmed the ative sites and the relavent pathways of the involved catalytic reactions.