Graphene-supported Iridium Catalysts For P-Chloronitrobenzene And Cinnamaldehyde Hydrogenation

The catalytic hydrogenation technique has been widely applied in fine chemical industry,and the development of new-type and high-performance catalysts is definitely important.The group VIII metals are most used as the active components for catalytic hydrogenation,but iridium is less researched to today.In this work,highly dispersed Ir nanoparticles(NPs)loaded on graphene were prepared,and the selective hydrogenation of p-chloronitrobenzene(p-CNB)and cinnamaldehyde(CAL)was chosen as model reactions to evaluate the catalytic performance of such graphene-supported Ir catalyst.The effects of pre-reduction process,composite support,and binary metal were investigated to recognize the structure-function relationship and to further guide the design of efficient catalysts for hydrogenation.Graphene-supported Ir catalyst was prepared in one step by a simple hydrothermal synthesis method and was treated with pre-reduction in situ at low temperature.It showed that the activity of CAL over pre-reduction catalyst(Ir/rGO-Pre)is nearly two times higher than that over Ir/rGO,but the activity of p-CNB over Ir/rGO-Pre is only half of the latter.In order to explain the catalytic phenomenon,some factors were investigated,including pre-reduction temperature,prer-eduction pressure,water contents of catalysts,and reaction solvents.At the same time,Ir/rGO-Pre and Ir/rGO catalysts were compared and studied by reaction kinetics,cycle stability,and structure characterization.Results showed that the apparent activation energy over Ir/rGO-Pre catalyst for p-CNB hydrogenation is slightly higher than that over Ir/rGO catalyst.The electronic binding energy of metal Ir 4f for Ir/rGO-Pre catalysts is 0.1 eV lower than that for Ir/rGO catalyst.It indicated that pre-reduction at low temperature effectively removed the oxygen species on the surface of Ir NPs,resulting in a weak polar metal surface which was unfavorable for the adsorption and activation of the strong polar nitro group.The graphene-carbon nanotube composite materials supported Ir catalysts were prepared by the same method above and were characterized by scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray diffraction(XRD),and N2 adsorption-desorption.Firstly,p-CNB hydrogenation was studied with different mass ratio of GO to CNT(O)(oxidized CNT).The conversion over Ir/rGO9-CNT(O)1 at 1 h was up to 99%,which is higher than those over Ir/rGO(85%)or Ir/CNT(O)(49%)under the same reaction conditions.Furthermore,CAL hydrogenation was investigated with these catalysts.When the mass ratio of GO to CNT(C)(unoxided CNT)is 5:5,the tiptop conversion at 4 h was 54%,which was higher than that over Ir/rGO(31%)or Ir/CNT(C)(18%).These results showed that graphene and carbon nanotube as composite supports have a certain synergistic effect on catalytic performance of Ir metal.Lastly,the influence of binary metal on the catalytic performance of Ir/rGO was studied.Results showed that the hydrogenation performance of Ir-M(M = Pd,Pt,Ru,Rh,Fe,Co,Ni,Cu,Zn.)catalysts were not easily modulated by a simple hydrothermal synthesis method,which implied that the synergistic effect based on bimetals had little influence on the catalytic performance of Ir nanocatalysts.