Synthesis Of Ni, Au, And Ag Nanoparticles Via Microreaction And Their Application As Catalysts

Metal nanoparticles (NPs) have wide applications because of their special physical and chemical characteristics such as unique optical, electronic, magnetic, and catalytic properties. As far as the reported preparation methods were concerned, it was hard for them to realize controllable and continuous synthesis of the high-quality metal NPs without strong ligands such as thiol. Based on the enhanced heat and mass transfer efficiency as well as the stable microenvironment existed in the microchannel, the capillary microreactor was set up to synthesize monodisperse Ni NPs, Au NPs, Ag NPs, and Au-Ag alloy NPs using the weak ligand (oleylamine) as both reducing agent and surfactant. AU/CO3O4catalysts were prepared by the co-precipitation and tablet compression method, which could achieve the complete conversion of CO gas at room temperature. Furthermore, the CO thermal sensor using the catalyst as core component could decline the operating temperature to room temperature and show superior sensing ability. The main results are as follows:(1) Synthesis of monodisperse single metal NPs via capillary microreactionThe Ni, Ag and Au NPs were synthesized in oleylamine/octadecene system via microreation. At290℃, in the residence time of80s, spherical Ni NPs with an average diameter of15nm were synthesized and exhibited superparamagnetic properties. With the increaseof the residence time, triangular Ni NPs were synthesized and presented strong ferromagnetic properties. Compared with Ni block, the coercivity of Ni NPs had greatly improved. Au NPs were synthesized at120℃and their average size was about20nm. Ag NPs were obtained from120to180℃. With the increase of the synthesis temperature, the reduction rate of Ag ions was enhanced, which resulted in a high yield of90%.(2) Study of microreaction system to synthesize Au-Ag bimetal NPs at low temperatureThe homogeneous Au-Ag alloy with the smallest size of2.7nm and a standard deviation of6%could be synthesized at the temperature range from120to160℃with the molar ratio of Au3+:Ag+from1:5to1:10via microreaction system. With the increase of the residence time, the surface plasmon resonance (SPR) absorption peak of Au-Ag alloy NPs could be blue-shifted from525to410nm. With the rise of the synthesis temperature or the decrease of the molar ratio of Au3+:Ag+, the residence time became shorter and the size of alloy NPs got smaller. On the contrary, increasing the molar ratio of Au3+:Ag+, the gradient alloy NPs could be synthesized, whose SPR peak was extended to549nm. Finally, based on the data of our experiments, the synthesis mechanism of Au-Ag bimetal NPs was given. The microstructure of Au-Ag bimetal NPs was decided by the relative rate of both nucleation and diffusion of Au and Ag ions.(3) Synthesis and heat treatment of Au-Ag bimetal NPs at high temperatureAt high synthesis temperature, monodisperse Au-Ag bimetal NPs were also synthesized without Au or Ag NPs. At220℃, the process could be shortened to30s and the whole growth process of Au-Ag bimetal NPs could be devided into three sections:Au-rich gradient alloy, homogeneous alloy and Ag-rich gradient alloy. The Au-Ag gradient alloy NPs with SPR peak from410to525nm could be synthesized and their average size had no obvious change comparing with homogeneous alloy with the same SPR peak. The gradient alloy and the homogeneous alloy could be converted for each other by tuning both the reaction temperature and the residence time of the heat treatment.(4) Study of the catalyst with a high activity for CO oxidationThe Au/Co3O4tablet catalyst with a high activity was prepared by a co-precipitation and tablet compression method. As-prepared Au/Co3O4tablet catalyst showed a high temperature difference output (△T) of34℃for3vol.%CO/Air at room temperature. Even using the catalyst of only10mg, the complete conversion of3vol%CO with a flow rate of75ml/min could also be achieved at room temperature. The catalyst could display a high catalytic activity when it possessed high specific surface area, small-sized Au and Co3O4NPs, the coexistence of Au0and Au+, high molar ratio of Co3+/Co2+, and water-derived species on the surface.(5) Application of Au/Co3O4catalysts on CO sensorThe thermoelectric (TE) sensor model was built by simplifying the TE film sensor and catalytic combustion sensor. The TE sensor could work at room temperature, have a life of40h, and exhibit high repeatability. The sensor presented high selectivity at the temperature range from15to95℃. The response time became shorter with the increase of the operating temperature. At95℃, the response time and recovery time for1.28vol%CO gas were40s and26s, respectively. CO concentration and△T exhibited an approximately linear relationship. Thus, the CO concentration could be estimated from the△T signal.