Fabrication And Catalytic Performances Of Bimetallic Au-Pd Catalysts

In materials science, due to "synergistic effect", alloy formed by mixing and fusing different metals has some unique properties and has been widely applied in catalysis, optical and magnetic field. Bimetallic gold-palladium (Au-Pd) nanostructures have a wide range of applications in the field of catalysis. For example, in conventional catalysis, they are commonly used in the oxidation of alcohols, hydrogenation of alkynes and oxidation of carbon monoxide; in photocatalysis, they are commonly used in alcohols and aldehydes oxidation and the degradation of organic matters; in electrocatalysis, they are commonly used in alcohol fuel cells, formic acid fuel cells and sensors.In this thesis, we prepared and characterized Au and Au-Pd nanoparticles supported on TiO2 nanobelt (TiO2-NB), and applied them into the aerobic oxidation of benzyl alcohol under visible light irradiation. We also focused on the fabrication, characterization and catalytic performance towards methanol and ethanol electrooxidation of porous Au-Pd alloy. The main information is as follows:1. Fabrication of Au/TiO2-NB and its photocatalytic performanceWe synthesized H2Ti3O7 nanobelts via hydrothermal process using P25 as the precursor. By controlling the calcination temperature of H2Ti3O7 nanobelts, single-crystal anatase and TiO2(B) nanobelts, as well as double-crystal TiO2(B)/anatase heterostructured nanobelts were obtained. Au particles were deposited on TiO2 nanobelts by deposition-precipitation method and Au/TiO2-NB photocatalyst was fabricated. Transmission electronic microscope observed that nanosized Au particles were uniformly dispersed over the surface of TiO2 nanobelts. X-ray photoelectron spectroscopy and UV-vis spectra indicated that Au nanoparticles were metallic state and the crystal structure of TiO2 nanobelt scarcely affected the electronic and optical absorption properties of Au/TiO2-NB nanostructures. The Au/T-2 based on TiO2(B)/anatase heterostructured nanobelts exhibited much higher photocatalytic activity than either Au/T-1 based on TiO2(B) nanobelts or Au/T-3 based on anatase nanobelts. Under visible light irradiation, Au/T-2 produced 12.5 μmol of benzaldehyde which was much higher than that of Au/T-1 and Au/T-3. Bare TiO2 nanobelts did not produce any photocurrent signal, while all Au/TiO2-NB nanostructures exhibited a significant anodic photocurrent response under visible light irradiation. This result demonstrated that bare TiO2 nanobelts cannot absorb visible light with wavelengths above 450 nm and produce photo induced charge carriers. Therefore, the photocurrent responses of Au/TiO2-NB nanostructures can be attributed to the injection of hot electrons from Au nanoparticles into the conduction band of TiO2 nanobelt over the schottky barrier at Au/TiO2 interface, moreover, the transient photocurrent response of Au/T-2 is much stronger than those of Au/T-1 and Au/T-3. We believed that TiO2(B)/anatase heterojunction facilitated the injection of hot electrons from Au nanoparticles into TiO2 nanobelt and contributed to the much higher photocatalytic activity of Au/T-2 samples.2. Fabrication of Au-Pd/TiO2-NB and its photocatalytic performanceUsing TiO2(B)/anatase heterostructured nanobelts as the support, a range of bimetallic Au-Pd/TiO2-NB nanostructures were synthesized by a similar deposition-precipitation method. Small-sized metal nanoparticles were highly dispersed on the surface of TiO2 nanobelts. A narrow size distribution with an average diameter of 2.0 nm was obtained. X-ray photoelectron spectroscopy and UV-vis spectra results were indicative of Au-Pd alloy nanoparticles. Compared with monometallic structure (Au/T-2 and Pd/T-2), Au-Pd/T-2 exhibited enhanced catalytic activity, and the amount of benzaldehyde formed over Au-Pd/T-2 is three times as more as Au/T-2 and five times as more as Pd/T-2. A weaker photocurrent response was measured for Au-Pd/T-2 compared to the value obtained for Au/T-2, therefore, the remarkably improved photocatalytic activity of Au-Pd/T-2 was due to the "synergistic effect". The hot electrons excited in Au can transfer to Pd, making Pd as new active sites and accelerating the reaction.3. Fabrication of porous Au-Pd alloy and its electrocatalytic performancePorous Au-Pd alloy film was successfully constructed on anti-corrosion AuPdAg alloy wire surface via an alloying/dealloying process. The fabrication process involved electrodeposition of silver on the substrate, high temperature calcination to produce a new silver-rich alloy phase and electrochemical dealloying in 1 M perchloric acid. Scanning electron microscopy coupled with X-ray diffraction showed that the porous structure on alloy surface was composed of nanosized Au-Pd alloy particles. Electrochemical characterization confirmed that the structure and composition of the films could be easily adjusted by controlling the time-dependent corrosion process.The surface porous-structured conductive metal is a monolithic material which can be directly employed as electrode without coating. We applied it to the electrooxidation of methanol and ethanol in alkaline media. The results showed that it had excellent electrocatalytic performances which specifically performed as high current density, low onset potential of oxidation, superior poisoning tolerance and catalytic stability. Onset potentials of methanol and ethanol oxidation over dealloyed sample were -0.58 and -0.69 V, and the maximum oxidation current densities were 575.8 and 623.9 mA/cm2, which were superior to the reported nanoporous Au-Pd electrocatalysts.