Nano Array-like Catalysts M(Pd,Ag)_x/rGO@CoAl-LDH Via In-situ Reduction For Catalysis Application

Heck coupling reaction of aryl halides with alkenes catalyzed by transition metal Pd and catalytic reduction of 4-nitrophenol(4-NP)by Ag or Au et al.are two typical reactions in the organic synthesis.Nowadays,the catalysts for these reactions have been an ongoing research hotspot.However,conventional heterogeneous metal catalysts require the additional reducing agents during the preparation process.It is difficult to produce active species with clean crystal interfaces and full exposure,further limits the activity improvement of the catalyst.Moreover,these reducing agents are most likely to cause side effects on the environment.Therefore,rGO@CoAl-LDH hybrid-supported Pd or Ag catalysts has been fabricated via in-situ reduction method for the first time,the catalytic performance for Heck reaction and reduction of 4-NP of the prepared catalysts and structure-activity relationship are studied.The specific works are as follows:A novel 3D hierarchically structured rGO@CoAl-LDH nanohybrid with hexagonal layered double hydroxide(LDH)nanoplates interdigitated vertical to the both sides of single reduced graphene oxide(rGO)layer was prepared by a simple green citric acid assisted liquid phase coprecipitation method.A series of nanoarray-like catalysts Pdx/rGO@CoAl-LDH(x = 0.0098-1.9,refers to Pd loading in wt%on ICP)with controlled size(1.3-1.8 nm)have been synthesized for the first time through a simple lattice atomic-confined in situ reduction of oxidative Pd precursors by the evenly atomic-dispersed reductive Co(II)sites on LDH layers.The fabricated nanoarray-like catalysts Pdx/rGO@CoAl-LDH show excellent catalytic activities in the Heck coupling reaction,much higher than that of non-multilevel structured Pd0.56/rGO@CoAl-LDH-h,in particular,the Pd0.0098/rGO@CoAl-LDH with ultrafine Pd NCs(1.3±0.2 nm)exhibit the highest turnover frequency(TOF)value of 160 000 h-1.The superior catalytic activities can be ascribed to the high-dispersed tiny Pd NCs with clean surfaces,the enhanced electron transfer capacity and surface area,remarkable three-phase synergistic effect of Pd—CoAl-LDH—rGO in the present nanoarray-like Pd NCs catalysts.And that the catalyst have a wide range of substrate universality and good stability.A plausible mechanism for Heck reaction upon Pdx/rGO@CoAl-LDH catalysts is tentatively suggested.Furthermore,the nanoarray-like catalysts Agx/rGO@CoAl-LDH(x =0.045-2.56,refers to Ag loading in wt%on ICP)are prepared by in situ self-reduction of Ag20 formed through complexing Ag+ with abundant-OH group in the multi-level structured rGO@CoAl-LDH hybrid under indoor light for the first time.All the fabricated Agx/rGO@CoAl-LDH catalysts exhibit excellent catalytic performance for the reduction of 4-nitrophenol.Especially,the Ag0.12/rGO@CoAl-LDH(4.2±1.2 nm)exhibit the highest catalytic activity with an apparent rate constant of 6.25×10-3 s-1,TOF of 5393 h-1 and good recycle stability.It also show effective activity in the reduction and degradation of other nitrocompounds and organic dyes.The excellent activity of the serial catalysts can be attributed to the Ag nanoparticles with clean surface,the unique nanoarray-like structure,the enhanced adsorption of reactants through ?-? stacking effect,and the significant three-phase synergistic effect of Ag—CoAl-LDH—rGO in the present catalysts.