Controlled Synthesis Of Catalytic Nanomaterials For CO_x Hydrogenation Reactions And Its Catalytic Mechanism

Hydrogen can be produced from water electrolysis,which is ideally driven by renewable energy,such as wind,nuclear,marine and solar.The produced H2 can participate in CO2 hydrogenation to produce value added chemicals,representing an effective approach to remove CO2 emission in the atmosphere and prevent the energy crisis.With the increasing demand of carbon neutral and green-coal chemical process,synthetic natural gas technique has undergone a rapid research expansion.For example,with the aid of methanation technology,CO and CO2 as the gas waste from coke ovens can be converted to CH4 as green and clean energy carrier.Importantly,CO and CO2 hydrogenation reactions were typically sensitive to catalyst structures.The crystal phases,morphologies,exposed facets,and surface defects of the catalysts have great impact on the reaction performance,calling for in-depth understanding of the structureperformance relationship.In this paper,two types of catalysts,i.e.,pure metal catalysts and metalsupported catalysts,were studied in COx hydrogenation reactions.We precisely designed and prepared metallic Co catalysts with highly active facets exposed to explore the influence of COx hydrogenation performance driven by facet effects.Through the experimental and theoretical methods,the growth mechanism of Co-based catalysts and their catalytic mechanism were deeply studied.In addition to pure metal catalysts,we also studied metal-supported catalysts,such as derivatives from layered double hydroxides(LDHs)and metal-organic frameworks(MOFs).These novel catalytic nanomaterials with tunable and flexible compositions,morphologies,surface defects,and confinement effects were thoroughly investigated to correlate with the CO2 hydrogenation reaction performance.The detailed approaches and conclusions are listed in the followings:1.Two typical branched Co nanomaterials,namely nail-like Co(5 bar H2)and urchin-like Co(10 bar H2),were prepared by a H2-assisted solvothermal method with the H2 molecules used as reduction and structure-directing agents.The branch side was mainly composed of HCP Co(10-10)and HCP Co(l0-11)facets.The exposed branch top side was determined to be HCP Co(00-02)facet.The H2-promoting branches growth mechanism was proposed based on H2pressure controlled experiments and H-adsorption density functional theory(DFT)calculations.Moreover,the branch structure differences between nail-like Co and urchin-like Co could be attributed to the competitive adsorption between OAm molecules and H2 molecules,which were reconfirmed by DFT calculations.ompared with particle Co,nail-like Co and urchin-like Co both exhibited excellent catalytic performance towards CO methanation.The enhanced catalytic performance could be explained by(?)the intrinsic HCP phase of branches,as an active phase of CO hydrogenation,(?)the high specific surface area of unique branched structure,exposing more high-active sites,and(?)the exposed HCP Co(10-11)facet with the lowest energy barriers for CO dissociation.2.Branched Co and particle Co were chosen as model catalysts to study the facet effects on CO2 hydrogenation.The experimental results indicated that branched Co showed much higher CO2 methanation performance than particle Co,demonstrating the sensitive-structure characteristic of CO2 methanation.The reaction network of CO2 methanation was established,including HCOO*pathway,COOH*pathway,and CO2 direct dissociation pathway.The DFT calculations and Microkinetics analysis were carried out to study the reaction mechanism and rates on FCC Co(111)facets and HCP Co(10-11),(10-11)facets,which were exposed on particle Co and branched Co,respectively.The Microkinetics results indicated that for CO2 first activation step,the competitive reactions on forming COOH*intermediate and CO2 direct dissociation occurred on FCC Co(111)facet.In contrast,the generation of HCOO*was preferred on HCP Co(10-10)facet.Besides,CO2 dissociation was the preferential route on HCP Co(10-11)facet.At the same reaction temperature,the CH4 production rate on FCC Co(111)facet was much lower than that on HCP Co(10-10)and HCP Co(10-11)facets.At 673 K,the CH4 production rates of the three Co facets followed the trend of HCP Co(10-10)(4.43E-03 s-1)>HCP Co(10-11)(3.23E03 s-1)>FCC Co(111)(1.71E-06 s-1).Therefore,HCP Co(10-10)and(10-11)facets could be considered as the highly active facets for CO2 methanation.Facets play an important role in altering the reaction pathway and activity.The conclusion of reaction pathway analyzing from DFT results was verified by insitu Diffused Reflectance Infrared Fourier Transform Spectroscopy(DRIFTS).Hence,the theoretical results on metal high-active facets could serve as a certain guidance for the precise design and controlled preparation of catalysts.3.Ni-supported catalysts were derived from LDHs by air calcination and H2 reduction.Via a Ce-doping strategy,NiCeAl reduced metal oxide(RMO)catalysts exhibited superior CO2 methanation activity,especially at low temperature.At 250?,CO2 conversion was 78.60%with nearly 100%CH4 selectivity.Its high activity was contributed from(?)high Ni0 content resulted from the Ce-dopants in NiAl-laminates that prevented the coordination interaction of Ni-Al,inhibiting the formation of irreducible NiAl2O4 species and promoting the dispersion of reducible NiO species during calcination and(?)enhanced surface oxygen vacancies,which were related to the generation of moderate basic sites with high CO2 affinity.Furthermore,in-situ DRIFTS results proved that Ce-doping strategy could change the reaction pathway.Unlike NiAlRMO catalysts,which followed RWGS+CO hydrogenation pathway,NiCeAlRMO catalysts favored the formate pathway via generating bidentate carbonate intermediates.It has been demonstrated that the generation of bidentate carbonates intermediates as observed on NiCeAl-RMO catalysts was responsible for the enhanced low-temperature CO2 methanation performance.4.We successfully synthesized octapods with MIL-88B on MOF-5 structure via a one-step solvothermal method,utilizing the coordination difference between different metal ions and organic ligands.In the composite,Fe-MIL-88B concentrated on the eight branches of octapods,and NiZn-MOF-5 focused on cube of octapods.A phase competition driven growth(PCDG)mechanism was proposed.Additionally,various morphologies of FeNiZn-MOFs,including flowers,octapods,and cubes,were realized by adjusting the phase competition between thermodynamically stable MIL-88B and kinetically stable MOF-5.Compared with the pristine FeNiZnOx catalysts,the catalysts derived from the MOFs after H2 pyrolysis showed higher CO selectivity for CO2 hydrogenation.Wherein,the octapods FeNiZn-MOFs exhibited the highest reversed water gas shift(RWGS)reaction performance,with 16.51%of CO2 converted into CO without any by-product at 350?.The high catalytic activity can be explained from the followings:(1)the partially retained MOF structure after H2 pyrolysis benefited the adsorption of the reactant molecules;(?)the MOF-based nanomaterials with porous framework could high disperse the metallic active phase,avoiding excessive hydrogenation to CH4 due to aggregations of metal particles;and(?)more importantly,the superiority of unique octapods structure enhanced the structure stability of MOF composites,avoiding MOFs structure collapse during the high-temperature pyrolysis.