| In the present thesis, a series of atomically precise monodispersed Au25/NixAl-LDH and Au25-yPdy/Ni3Al-LDH catalysts were firstly fabricated via a modified electrostatic adsorption method. The obtained catalysts were characterized systematically and evaluated in the liquid-phase aerobic oxidation of alcohols exhibiting higher activity compared with traditional supported Au catalysts. The main points are followed.(1) NixAl-LDH supports (x=Ni/Al,2,3,4) were fabricated by coprecipitation step. Captopril (Capt)-capped water-soluble Au25Capt18 and Au25-yPdyCapt18(y= 1,3,5,7,9) clusters with different Au/Pd molar ratios were prepared by a size-focusing synthetic methodology. Atomically precise monodispersed Au25/NixAl-LDH and Au25-yPdy/Ni3Al-LDH catalysts were firstly fabricated via modified electrostatic adsorption of Au25Capt18 and Au25-yPdyCapt18 onto the positively-charged NixAl-LDH followed proper calcination based on the formation of buffer suspension (pH～7.9) of LDH predispersed in deionized water and high dispersion of Au25Capt18 and Au25-yPdyCapt18 in buffer solution.(2) Au25/NixAl-LDH (x=2,3) catalysts possess definite lamellar structure and Au25/Ni4Al-LDH shows obviously collapsed LDH layers with NiAl(O) phase. HRTEM images of Au25/NixAl-LDH show that the Au25 clusters were highly dispersed at the edge of LDH plate-like supports. Au25/NixAl-LDH (x= 2,3) show ultrafine Au25 clusters as 0.9±0.3 nm and 0.9±0.2 nm, respectively, while Au25/Ni4Al-LDH shows a little larger one as 1.1±0.4 nm.(3) Au25/NixAl-LDH catalysts exhibit excellent activity for oxidation of 1-phenylethanol to acetophenone with O2 as a sole oxidant under no base additive, and the activity in both toluene and solvent-free conditions are increased in an order Au25/Ni4Al-LDH< Au25/Ni2Al-LDH< Au25/Ni3Al-LDH. The Au25/Ni3Al-LDH shows the highest activity (TOF=6780 h-1 in toluene, 118500 h-1 in solvent-free) and can be applied for a variety of alcohols oxidation, which can be ascribed to the Au25-LDH synergistic effect between the ultrafine Au25 clusters with most negative charge on the clusters’surface and the LDH support with the best layer orderliness. The activity of 1-phenylethanol oxidation in toluene on Au25/Ni2Mn-LDH is higher than that on Au25/Ni3Mn-LDH though both lower than those of Au25/NixAl-LDH, implying that the better LDH layer structure, the higher TOF for 1-phenylethanol oxidation. The same regularity is also found in Au25/Ni3.xMnxFe-LDH systems.(4) Kinetic studies show the rate constant (k) of Au25/NixAl-LDH for 1-phenylethanol oxidation in toluene are higher than that in solvent-free, and the k values in both toluene and solvent-free are increased in an order of Au25/Ni4Al-LDH< Au25/Ni2Al-LDH< Au25/Ni3Al-LDH. The apparent activation energy (Ea) values of Au25/NixAl-LDH for 1-phenylethanol oxidation in toluene are 17.57,15.50 and 21.59 kJ/mol, respectively, while 18.29,16.87 and 22.87 kJ/mol in solvent-free, respectively. The Au25/Ni3Al-LDH exhibits the highest k value and the lowest Ea value.(5) Comprehensive characterization including XPS, CO adsorption in situ IR and TPSR results show that Au25 clusters in Au25/NixAl-LDH are electron-rich due to the electron transfer from Ni-OH octahedra in the LDH layer to Au25 clusters. The best layer orderliness of Ni3Al-LDH, corresponding to most uniform charge density in the LDH, transferred the most electrons to Au25 clusters leading to the strongest Au25-LDH interaction, contributing to O2 activation. Moreover, the LDH in Au25/Ni3Al-LDH with most -OH groups contributes to an enhanced activating ability of alcohol hydroxyl thus the highest activity for 1-phenylethanol oxidation.(6) Au25-yPdy/Ni3Al-LDH catalysts exhibit significantly enhanced activity of 1-phenylethanol oxidation compared to Au25/Ni3Al-LDH, mainly due to partial electrons transferred from Pd to Au leading to more negative Au25-yPdy clusters than Au25 clusters in favor of the enhancement of alcohols oxidation. |
PDF Full Text Request