Controllable Synthesis Of Cobalt-containing Nanosheet Array-like Ternary Copper-based LDH/rGO Hybrids And Derivaties To Boost The Catalytic Reduction Performance For 4-nitrophenol

With the rapid development of economy,chemical wastewater pollution has become one of the urgent problems to be solved.Among them,aromatic nitro compounds pose a serious threat to humans and animals.Therefore,the creation of facile,high-efficient and environmentally friendly removal methods has attracted more and more attention.The catalytic hydrogenation by Na BH₄ is a hot spot because of mild conditions and no secondary pollution.Aiming at the dispersibility and recycling of catalysts in the catalytic reduction process of nitrophenol,the Co-doped Cu-LDH/r GO hybrids were constructed.The catalytic performance and structure-activity relationship of the catalysts were investigated via the 4-NP reduction as a model reaction.Moreover,a possible mechanism was tentatively proposed through systematically characterizations.The paper focuses on:（1）A series of Co-doped ternary Cu_xCo_3-xAl-LDH/r GO hybrids were prepared by citric acid-assisted aqueous phase coprecipitation method.The Cu_xCo_3-xAl-LDH/r GO hybrids are featured as hexagonal Cu Co Al-LDH nanosheets（73.4～78.2 nm×12.1～13.5 nm）with ab-plane perpendicularly interlaced grown on the surface of r GO layer in both sides.The series of hybrids showed excellent catalytic activity in the reduction of 4-NP,especially Cu_1.5Co_1.5Al-LDH/r GO showing the highest activity(apparent rate constant k_app value of 49.2×10^-3 s^-1 and TOF of 232.8 h^-1),which is much better than most reported Cu-based catalysts and even comparable to some precious ones.The obtained hybrids can serve as an efficient reservoir for in situ formed Cu₂O.These findings can be attributed to i)in situ generated ultra-small and highly dispersed Cu₂O nanoparticles（～4.3nm）based on lattice confinement of LDH layer,ii)the atomically dispersed Co²⁺species promoting the formation of electron-rich Cu species,iii)the enhanced Cu₂O-LDH-r GO three-phase synergy upon the 3D nanosheet array-like morphology with high S_BET,and iv)the improved adsorption of the substrate throughπ-πinteraction between r GO in the hybrids and the benzene ring.The universality and cycling stability of the hybrids were investigated.It was found that regardless of the presence of electron withdrawing or electron donating substituents in the aromatic nitro compounds,the Cu_1.5Co_1.5Al-LDH/r GO hybrid has excellent catalytic hydrogenation activity and has good catalytic reduction performance for anionic organic dyes.Even after 10 runs,the Cu_1.5Co_1.5Al-LDH/r GO hybrid can still completely catalyze the reduction of 4-NP to 4-AP in 2 min without the removal of the product,suggesting the good cycling stability.In addition,the hybrid Cu_1.5Co_1.5Al-LDH/r GO not only can be used for the efficient catalytic reduction of highly concentrated 4-NP solution（20 m M）,but also exhibits excellent reduction performance in fixed-bed system,implying the potential applications of the current Co-doped hierarchical ternary Cu-based LDH/r GO hybrids in water remediation.（2）A series of sandwich-type r GO/Cu_1.5Co_1.5Al-LDH/r GO-y nanohybrids were prepared by the impregnation-reduction method using the optimal Cu_1.5Co_1.5Al-LDH/r GO sample as precursor.Among them,r GO/Cu_1.5Co_1.5Al-LDH/r GO-1 had a better catalytic reduction activity for the 4-NP with a k_app value of 159.8×10^-3 s^-1,which is nearly 3.2 times that of the corresponding precursor.The results can be explained that an ultrathin r GO layer anchored on the hybrid surface significantly improves the electron transfer capability of the nanohybrid.