Synthesized Of Metal Nanoparticles Supported On Activated Carbon As An Efficient Pd-based Bimetallic Catalyst By Using Bio-reduction Method:Promoting CO₂ Hydrogenation To Formic Acid

In this paper,Pd-based bimetallic catalysts,supported on activated carbon?AC?,were prepared by a simple and eco-friendly method in which Cacumen Platycladi?CP?leaf extract was used as both reducing and protecting agents.The catalyst were characterized by a variety of techniques and successfully applied to the hydrogenation of carbon dioxide to formic acid.The main works could be summarized as follows:Firstly,herein,a simple and eco-friendly method was provided for preparing Pd-Au/AC and Pd-Cu/AC catalysts.A series of characterizations,including X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,Fourier Transform infrared spectroscopy?FTIR?,Brunner-Emmet-Teller?BET?,Thermogravimetric?TG?,High Resolution Transmission Electron Microscope?HRTEM?,Transmission Electron Microscope?TEM?,Inductively Coupled Plasma Optical Emission Spectrometry?ICP-OES?,Energy dispersive X-ray spectroscopy-Mapping?EDX-Mapping?and Gas Chromatography-Mass Spectrometer?GC-MS?were be carried out to explore the structure of synthesis materials.Combining with the catalyst evaluation results,we put forward a possible mechanism of the HCO₃^-as reaction intermediate.Secondly,the experimental results showed that bio-reduction Pd-Au/AC and Pd-Cu/AC bimetallic catalysts exhibited excellent performance in the hydrogenation of CO₂ to formic acid.The activated carbon provided a high surface area,the TEM and HRTEM was proved that the bimetallic nanoparticles were uniformly dispersed through the whole supports.The highly-dispersed Pd nanoparticles acted as the hydrogenation adsorption and dissociation sites.The XPS and XRD were indicated that the electron transfer from Au or Cu to Pd and form the bimetallic structure.More electron deficiency of Pd was beneficial for this reaction.Compared with monometallic Pd/AC catalyst,bimetallic Pd-based catalysts achieved higher reactivity,which could proved the synergistic effect between active center?Pd?and additive metals?Au or Cu?.Thirdly,optimization of bimetallic Pd-Au/AC catalyst preparation parameters and reaction conditions manifested that metal loading of 5 wt%,molar ration of metal of 2,reaction temperature of 383 K,total pressure of 10 MPa and reaction time of 2 h were optimum.Under optimal conditions,the turnover frequency?TOF?value of formic acid up to 80 h^-1.For bimetallic Pd-Cu/AC catalyst,the TOF as high as 100 h^-1was achieved under the identical condition except the total pressure was 7 MPa.This activity result proved that the catalytic activity of bimetallic Pd-Cu/AC was better than bimetallic Pd-Au/AC catalyst.The kinetic test was carried out to calculate the activation energy.The activation energy of the bimetallic Pd-Cu/AC?Ea=27 kJ/mol?was lower than the Pd-Au/AC?Ea=30 kJ/mol?catalyst,which was consistent with experiment result.This finding offered a promising approach to reduce the cost of catalyst by using transition metal Cu to replace noble metal Au,and that results has a considerable economic advantages.Finally,efforts were also made to probe the stability of the bio-synthesized catalysts.The catalysts were carefully recollected and reused in subsequent cycles as the same conditions as the fresh catalysts.There was minor variation during the consecutive cycles,which indicated remarkable stability of the bio-reduction Pd-based catalysts.In contrast,the catalytic activity has a significant decrease for the catalyst synthesized by using convention method.The TG and FTIR analysis indicated that there was residual plant biomass molecules on catalyst,which could protect the active center and avoid the agglomeration and deactivation of the catalyst.The EDX-Mapping confirmed that the metal nanoparticles were successfully synthesized by Cacumen Platycladi?CP?leaf extract and homogeneously distributed over the entire supports.In a word,the merits of excellent durability,sound productivity and preferable resuability grant the catalysts a high reaction activity and a promising future in the industrial application.