Preparation Of Au Nanocatalysts Based On Multifunctional Tannic Acid Hybrid Materials And Their Catalytic Performance

Au nanoparticles have been widely studied in the field of catalysis,due to their unique surface effects,catalytic performance and quantum effects.However,Au nanoparticles have the disadvantages of being easily aggregated and deactivated,uncontrollable catalytic processes,and difficult to recover.Based on tannic acid hybrid materials,a series of highly efficient and recyclable Au nanocatalysts were constructed and their catalytic performances were systematically investigated.Temperature-sensitive catalyst was prepared by coating tannic acid-ferric iron ion onto the halloysite nanotubes,grafting poly?N-isopropylacrylamide?,and followed by loading gold nanoparticles.Transmission electron microscopy,fourier transform infrared spectroscopy,and X-ray diffraction were used to characterize the chemical composition and nanostructure of Au/HTF-P.The experimental results showed that the catalyst has a high catalytic activity in the reduction of three isomeric nitrophenols,methylene blue and nitrobenzene.It is worth noting that there is a clear dependence between the catalytic activity and the ambient temperature,and the“on-off”of the catalytic reaction can be controlled by changing the temperature.In addition,the Au/HTF-P catalyst can be reused 8 times with the conversion being higher than 80%.In order to improve the metal loading and catalytic stability,tannic acid-aminopropyltriethoxysilane?TA-APTES?coating was first used to coat the halloysite nanotubes,and then poly?N-isopropylacrylamide?was grafted.Two kinds of gold nanocatalysts,Au/HTA and Au/HTA-P,were prepared after the loading of Au nanoparticles.By simply controlling the raw material ratio and reaction time,the coating thickness can be adjusted.The characterization results showed that the gold nanoparticles of the two catalysts were highly dispersed on the support,and the average particle size was about 10 nm.Compared with expensive dopamine-coated halloysite-supported gold catalysts,Au/HTA and Au/HTA-P showed a comparable catalytic activity in the reduction of 4-nitrophenol.At the same time,Au/HTA-P with a significant temperature-responsive catalytic performance has the highest catalytic efficiency at the lowest critical solution temperature of 45°C.In addition,the Au/HTA and Au/HTA-P catalysts gave high activities even after the 11th run with the coversion higher than 90%,indicating thier excellent stabilities.Based on the strong stabilizing effect of TA-APTES hybrid materials toward gold nanoparticles,APTES was used as the silicon source in a"one-pot"and reacted with tannic acid under alkaline conditions to self-assemble hybrid nanospheres.In this process,the catalyst precursor was uniformly adsorbed and dispersed in the hybrid nanospheres,and then organic matter was removed by high-temperature calcination,and finally a hierarchical porous silica-supported Au-Pd catalyst?Au₂Pd₁/h-SiO₂?was prepared.Nitrogen adsorption and desorption experiments show that the specific surface area of Au₂Pd₁/h-SiO₂ is as high as557 m²g^-1.Characterizations such as transmission electron microscopy and X-ray diffraction proved that the in situ generated Au-Pd nanoparticles were highly dispersed in the hierarchical porous silica.The results of the catalytic experiments showed that the catalyst had excellent catalytic performance in the oxidation of benzyl alcohol to benzaldehyde,and the conversion in toluene phase and the water phase was upto 87.0%and 90.2%,respectively.