Mesoscale Regulation Mechanism And Hydrogenation Performance Of Hydrogen-mediated Nickel And Cobalt Nanomaterials

With the crisis of global fossil resources and environmental issues,sustainable and green technologies have become research hot spots.Hydrogenation reactions play important roles on chemical industry,such as acetone hydrogenation to isopropanol and CO2 hydrogenation to CO,CH4,CH3OH,etc.Hydrogenation of acetone to produce isopropanol turns the overcapacity materials into more valuable products.It's an'atomic economic reaction' and is consistent with the strategic goal of sustainable development.CO2 hydrogenation not only reduces the harm of CO2 to environment,but also reuses carbon resources.Reasonable design of the catalysts is the key to achieving excellent hydrogenation performance.Currently,many scholars have used structure-directing agents(SDAs)to effectively control the morphologies and size of metal nanocrystals to adjust their catalytic performance.However,the mechanism of SDAs and the relationship between the shape and size of the catalyst and their catalytic performance is still unclear.As non-noble metals,cobalt and nickel have attracted research attention due to their superior hydrogenation performance and price advantage.This paper explores the mechanism of hydrogen mediated morphologies of Co and Ni nanocrystals,and studies the influence of crystal plane effects on hydrogenation performance by combining experiments and calculations,which guide the synthesis of nanometals and improve their catalytic performance.The following are main research contents:1.The roles of H2 in the structure regulation of urchin-like Co nanocrystal are studied.H2 has strong adsorption strength on the metal surface,which can affect the shape of metal nanocrystals as a SDA.Density function theory(DFT)studies show that the dissociatively adsorbed H atoms effectively reduce the Gibbs surface free energies of the Co surfaces at the synthesis temperature.The H coverage(?H)of the most stable Co surface decreases with the increase of temperature,until bare surface at high temperature.According to Wulff Construction theory,the morphologies of Co nuclei under various H coverage are predicted.The adsorption of atomic H greatly affects the composition of(111),(110)and(100)surface on the Co nuclei.In addition,the diffusion energy barriers and diffusion coefficients of Co adatom on Co surfaces with various ?H are also studied.The results suggest that adsorbed H atoms hinder the diffusion of Co adatom,except for Co(100)at ?H=0.56 ML(Molecular layer),and the surface diffusion rate on the Co(100)surface is the slowest among all surfaces The projected density of states results further reveal that adsorbed H affects the surface diffusion ability of the Co adatom by changing the electronic structure of Co surfaces.2.The kinetic roles of H2 on the formation of branched Ni(Ni-BN)are studied.H2 regulation can change the morphologies of Ni nanocrystals,Ni nanoparticles(Ni-NP)are synthesized without H2 while Ni-BN are synthesized under 14 bar H2.In this paper,we systematically study the kinetic effects of H2 regulation,including the reduction of Ni precursors and the overgrowth of Ni nuclei.Reduct experiments reveal that H2 significantly increases the reduction rate and nuclei-catalyzed reduction ratio of the Ni precursor.Molecular dynamics(MD)simulations find that H2 significantly affects the deposition rates and deposition equilibrium of the reduced Ni atoms.The faster Ni precursor reduction rates associated with enhanced Ni deposition rates contribute to the overgrowth of Ni nuclei.Ni-BN growth kinetic modeling results reveal that Ni(111)possess much higher vertical growth rate than those of other facets which leads to overgrowth of Ni nuclei along the<111>direction and finally generate the branched nanostructures.Developed MATLAB program visualizes the crystal nucleation and growth process,and successfully simulates the structure evolution of the branched structures of Ni-BN and other FCC nanocrystals in the literatures.3.The influences of exposed surfaces of Ni catalysts on acetone hydrogenation are discussed.Ni-BN mainly exposes Ni(322)facet with substantial steps and exhibits excellent acetone hydrogenation activity,while Ni-NP mainly exposes Ni(111)and Ni(100)facets with lower hydrogenation activity.As the raw material for the hydrogenation reaction and a SDA,H2 self-select to reduce the exposed area of the unfavorable Ni(111)facet and helps forming favorable high-index facet for acetone hydrogenation during the synthesis of Ni nanocrystals.DFT studies on the reaction mechanism of acetone hydrogenation on different Ni surfaces show that activative energies on the steps of Ni(322)is lower than that of Ni(111)and Ni(100).According to the MATLAB program,the proportion of each facet on the Ni nanocrystals can be calculated,which is helpful to analyze the crystal plane effect of Ni catalysts in acetone hydrogenation.Combining the kinetic growth model and the microkinetic study,the quantitative relationship between the morphology of Ni-BN and the hydrogenation performance has been established.This work not only provides detailed understanding of the shape-function relationships of Ni catalysts in acetone hydrogenation,but also provides guidance for selectively synthesizing catalysts with more favored facets by small reactant molecules self-select method.4.The effects of Ni morphologies on the performance of CO2 hydrogenation reaction are studied.In this work,Ni nanocrystals under different H2 pressures are synthesized.The Ni nanocrystals synthesized with H2 are branched structures,and their CO2 conversion and CH4 selectivity are higher than that of Ni nanoparticles synthesized without H2.DRIFTs results show that Ni catalysts with different morphologies have different reaction routes.CO2 generates methane via the formate route on Ni-BN,while via directly decomposition and hydrogenation route to generate methane on Ni-NP.In addition,the decomposed CO*desorb from Ni surfaces to formate CO(g)which reduces the selectivity of methane.DFT studies show that the activative energies of CO2 hydrogenation reaction on Ni(110)and Ni(322)facets are lower than those of Ni(111)and Ni(100)facets.Microkinetic analysis shows that the CO2 conversion rates of Ni(110)and Ni(322)facets are higher than those of other two facets.The main product on Ni(110)and Ni(322)facets is CH4,while the CO yields on Ni(111)and Ni(100)facets are higher than CH4.The exposed facets of Ni nanocrystals and microkinetic results explain the reason of the better reaction performance of Ni-BN rather than Ni-NP.This work studies the mesoscale regulation mechanism of H2 on the metal nanocrystal structure and constructs a growth kinetic model which applied to the hydrogenation reaction to study the crystal plane effects and shape-function relationships of Ni catalysts.The exploration of H2 regulation mechanism has positive significance for the synthesis of efficient catalysts which expose optimal crystal planes.