Electron Modification Of LDH/rGO Nanosheet Array Structure By Transition Metal-doping For Enhanced Hydrogenation Performance Of Nitroaromatic Compounds

For nearly a century,catalysis has been a major contributor to the world economy.Especially catalytic oxidation and hydrogenation play important roles in the production of key chemicals and intermediates.Selective reduction of nitro compounds to corresponding amines is an important reaction and a fundamental step in the production of various fine chemicals and commodities.Among them,4-nitrophenol（4-NP）is highly toxic,carcinogenic anthropogenic and difficult to degrade,which has been included in the priority control pollutant list at home and abroad.Additionally,the reduction product of 4-NP,4-aminophenol（4-AP）,is an important pharmaceutical and chemical intermediate.Clearly,the catalytic reduction of 4-NP to 4-AP shows crucial significance both industrially and environmentally.The reduction of 4-NP to 4-AP involves a complex six electron transfer process,therefore,it is crucial to design and construct a catalyst with small size,high dispersion and high exposure characteristics of active sites,as well as enhanced electron transfer ability.Noble metal-based catalysts have been widely studied,especially Pd-based catalysts with excellent hydrogen adsorption and activation performance.More importantly,by introducing abundant oxygen defect sites or special functionalized supports to construct specific Pd-support interfaces,the surface electronic structure of Pd active sites can be effectively modulated,thereby improving the activity and selectivity of the catalyst.Meanwhile,the high cost and low abundance of noble metals have made non-noble transition metal catalysts another research hotspot.In the 4-NP reduction reaction,Cu-based catalysts,especially modified with another transition metal have attracted much attention due to their excellent activity.However,the preparation of traditional Cu-based catalysts often uses hydrothermal,wet chemical reduction and precursor methods,which tends to cause the aggregation of metal or metal oxide particles leading to large size and wide distribution,thus limits the further improvement of their activities.Layered double hydroxides（LDH,also called hydotalcite）have often been used as a catalyst or support for metal nanoparticles owing to unique layered structures,atomic level confinement and dispersion of metal elements,and adjustable composition.However,due to the strong interactions between nanosized LDH plate-like particles,traditional coprecipitation method often leads to the aggregation of 2D LDH nanosheets.Therefore,carbon materials are often used to disperse LDH nanosheets and further improve the electron transfer ability of LDH materials.Reduced graphene oxide（r GO）has become one of the most attractive supports due to its high theoretical specific surface area,high conductivity,good chemical and thermal stability.Therein,the present thesis mainly focuses on the synthesis of Pd nanoclusters（NCs）catalysts loaded on hierarchical LDH/r GO supports modified with Ce species,as well as a series of non-noble 3d transition metal modified Cu-based LDH/r GO hybrids and their derivatives,to enhance the hydrogenation performance of aromatic nitro compounds.The structure-activity relationship was explored through systematic characterizations and density function theoretical（DFT）calculations.The main results of the thesis are:（1）First,a series of Ce-doped Co₃Ce_yAl_1-y-LDH/r GO（y=0.05,0.1 and 0.2） hybrid supports,synthesized by an optimized citric acid-assited coprecipitation process,display nanosheet array-like morphology along with the Co₃Ce_yAl_1-y-LDH nanosheets（～77.9 nm×12.3 nm）vertically interlaced grown on both sides of r GO layer.Meanwhile,Ce species may partially enter the Co₃Ce_yAl_1-y-LDH layer lattice,while another portion forms Ce O₂ species uniformly adhering to the LDH surface.Then,a series of catalyst precursors were obtained by loading preprepared atomically precise palladium nanoclusters Pd₁₇Capt₈（～1.1±0.3 nm）onto the surface of the supports（Pd:0.18 wt%by ICP）.The obtained precursors were properly calcined in N₂ flow to give a series of hierarchical catalysts Pd NCs/Co₃Ce_yAl_1-y（O）/r GO.The series catalysts show1.3～2 times higher catalytic reduction activity for 4-NP than that of the no Ce-doping sample,and the Pd NCs/Co₃Ce_0.1Al_0.9-LDO/r GO presents the highest activity with k_app of 0.12 s^-1 and TOF of 85122 h^-1.The DFT calculation results indicate that the presence of Co O-Ce O₂ heterojunction interface results in a positive charge on the surface of Pd NCs species(Pd^1.3324+),and the introduction of Ce O₂ significantly reduces the free energy change of the rate-determine step during the reaction.Combined with the small size and highly uniform dispersion of Pd NCs,abundant oxygen vacancies,the presence of Ce⁴⁺/Ce³⁺and Co³⁺/Co²⁺redox pairs,as well as the Pd NCs-（Co O-Ce O₂）-r GO three-phase synergistic effect,jointly endow the Pd NCs/Co₃Ce_yAl_1-y（O）/r GO catalysts excellent catalytic performance.Additionally,the as-obtained catalyst further demonstrated excellent cycling stability（15 cycles,～95%）and universality.（2）The LDH/r GO can not only serve as an excellent support for noble metals,but also as an efficient hydrogenation catalyst by introducing active metal Cu into LDH layers.More importantly,the catalytic performance of Cu-LDH/r GO hybrids can be further improved through lattice doping strategies.Therefore,a series of Ni doped bi-transition metal-based hybrids Cu_3-xNi_xAl-LDH/r GO（x=2.5,2.0,and 1.5）were synthesized by an optimized coprecipitation route.The hybrid exhibits a honeycomb-like nanosheet array morphology,with ultrathin Cu Ni Al-LDH nanosheets（～72.3×4.2 nm）vertically interlaced on both sides of the r GO substrate,displaying high specific surface area of～151.8 m²/g and abundant mesoporous structure.The catalytic activity of the series of hybrids for 4-NP reduction was significantly improved compared to the pure Cu Ni Al-LDH.Especially,Cu₁Ni₂Al-LDH/r GO showed the best activity(k_app=0.034 s^-1,196.7 h^-1),ca.1.7-folds higher than pure Cu₁Ni₂Al-LDH.It is worth noting that the activity of the present Cu₁Ni₂Al-LDH/r GO is increased by 37%(k_app=0.025 s^-1)compared to previously reported single transition metal hybrid Cu₁Mg₂Al-LDH/r GO.The enhanced activity of the bi-transition metal Cu Ni Al-LDH/r GO can be attributed to 1)the in-situ instantaneous formed～3.8 nm ultrasmall Cu₂O nanoparticles;2)The isolation,stabilization and electronic modulation effects of Ni-OH group for the active Cu species,as well as the three-phase synergy of Cu₂O-Ni-OH（Cu Ni Al-LDH）-r GO;3)Enhanced adsorption capacity for reactants throughπ-πstacking upon the honeycomb-like nanosheet array morphology.Meanwhile,the as-obtained hybrid also exhibits good cycling stability（10 cycles,～90%retention）and excellent universality towards various aromatic nitro compounds and anionic azo dyes.（3）Generally,proper calcination can significantly enhance the electron interactions between metal and support,thereby enhancing the activity and stability of the catalyst.Meanwhile,the Cu Ni Al LDH/r GO hybrid was reduced by Na BH₄ during the reduction process,in-situ generating ultrafine Cu₂O as the true active center.Therefore,based on the atomic level lattice confinement and dispersion effects of LDH layers on metal elements and the in-situ self-reduction function of r GO substrates,it is expected to obtain the Cu Ni Al-LDH/r GO calcination derived composite catalysts with greatly enhanced catalytic efficiency and cycling stability.A series of x Cu@Cu₂O-Cu Ni Al（O）/r GO（x=1.0,1.5,and 2.0）catalysts with well-maintained nanosheet array morphology were constructed by moderate calcination of the Cu Ni Al-LDH/r GO hybrid in a N₂ flow.The core@shell Cu@Cu₂O nanoparticles are all confined between the vertically adjacent Cu Ni Al（O）nanosheets and r GO substrates.The series of composites all exhibit excellent catalytic activity for 4-NP reduction,especially 1.5Cu@Cu₂O-Cu Ni Al（O）/r GO has the highest activity and maintains～91%even after continuous 25 cycles,much higher than most reported Cu-based catalysts and even comparable to some noble metal-based catalysts.The optimal performance of the 1.5Cu@Cu₂O-Cu Ni Al（O）/r GO can be mainly assigned to the electronic modification and geometric confinement effect of Ni O species which enhanced the surface electrophilicity of the uniformly dispersed core@shell-like Cu@Cu₂O nanoparticles,as well as the Cu-Cu₂O-Cu Ni Al（O）-r GO synergistic effect.The universality and fixed bed test further demonstrate the potential application of the present non-noble metal composite catalysts in practicle water restoration.（4）Given that the powder catalysts may have drawbacks such as easy aggregation,difficult recovery,and partial loss during the reaction process,developing a facile and green method to grow the powder catalysts on a rigid or flexible substrate is extremely beneficial for improving the long-term stability of the composite and further promoting the exposure of active components.Moreover,the integrated substrate-type catalyst also has the advantages of simple operation,easy separation,recovery,and reuse.Therefore,first,r GO modified nickel foam（NF）substrate was obtained by a citric acid-assisted hydrothermal treatment,then the hybrid precursor Cu Ni Al-LDH/r GO/NF was obtained via a citric acid-assisted aqueous coprecipitation-hydrothermal strategy by in-situ growth of the Cu Ni Al-LDH on r GO/NF substrate,finally the integrated catalyst Cu@Cu₂O-Cu Ni Al（O）/r GO/NF was obtained by moderate calcination of the precursor in a N₂ flow.The catalyst exhibits highly efficient catalytic activity for 4-NP reduction(TOF=408.2 h^-1)and can be continuously used for 30 cycles without activity decrease.The electrochemical impedance spectroscopy analysis shows that the present integrated Cu@Cu₂O-Cu Ni Al（O）/r GO/NF catalyst shows much lower charge transfer resistance and solution resistance than the powder one,which is extremely conducive to the improvement of the electronic transport capacity of the catalyst and the ion transfer efficiency in the solution during the catalytic process.Moreover,the obtained catalyst is easy to separate,recover,and reuse,thus enhancing the practical potential of the catalyst.（5）Based on the above research results,other 3d transition metals were further introduced into the Cu-LDH/r GO layer by lattice doping to investigate the effect of doping metals with varied 3d electronic structures on catalytic reduction performance.The results show that in the series of 3d metal（Mn,Co,Ni）doped catalysts,the catalytic reduction activity gradually increased with the increase of unpaired d electrons of the doped metals.Among them,the Mn-doped sample Cu_1.5Mn_1.5Al-LDH/r GO exhibits the highest activity with k_app of63.2×10^-3 s^-1 and TOF of 492.0 h^-1,which is closely related to the reduced coordination number of Cu-O,the most positively charged surface of active Cu species,the presence of Mn⁴⁺/Mn³⁺/Mn²⁺redox species,and reduced bandgap energy.The present lattice doping strategy based on hierarchical nanosheet array-like LDH/r GO hybrids has important reference significance for the design and construction of many other highly efficient bi/multi-transition metal-based catalysts with structural robustness,which can be widely applied in related research fields.