| During recent years, oxide supported Au nanoparticles, such like Au/CeO2, Au/TiO2, Au/ZrO2, have been considered as promising heterogeneous catalysts in application. In spite of the outstanding performance, both basic research and application progress have been limited by the unclear interface properties of Au nanoparticles and their influence to catalytic performance, as well as the low utilization of Au nanoparticles(lower than 50%).With these two issues, sample with Au loading of 0.33 wt. % was prepared on the carrier of CeO2 by deposition precipitation. Demonstrated by ICP-AES, XRD and HRTEM techniques, Au/CeO2 sample with Au entities under nanoscale was prepared,and metal dispersion or utilization of 100 % would achieve on this sample. Au/CeO2 sample was endured different redox treatments(oxidative measuring fully-reduced sample at 300℃ in air-flow by controlling oxidation period) to obtain a series of Au/CeO2 catalysts, which were characterized by H2-TPR measurements and tested by employing CO oxidation as indicator reaction in this work. Around 20 times of hydrogen-consumptions compared to the calculated consumption(by assuming all loaded Au as AuIIIspecies to consume H2) but with the peak shifts in a low temperature range of 102-150 ℃ on these catalysts were illuminated by H2-TPR measurements, disclosing the outstanding redox feature on the interface of such Au/CeO2 catalysts. The redox state of such Au/oxide interfaces and corresponding activity were able to be simultaneously tuned by modulating the oxidation period.The catalyst obtained from 30 min oxidative measurement showed the lowest hydrogen consumption peak temperature, disclosing the most outstanding redox feature. Accordingly, various catalysts showed different catalytic performance for CO oxidation reaction, and catalysis performance of the catalyst obtained from 30 min oxidative measurement was optimal, which TOF at 30℃ for CO oxidation by up to10 times compared to the state-of-art Au/CeO2 catalyst(carrying of ca.3.0 nm Au particles). In addition, the declined activity of the typical catalyst after a 30 h continuous reaction was able to be recovered by tuning back of its redox state, furtherindicated that the metal-oxide interfacial redox property could function as dominating factor and be tuned for improving catalytic performance of such supported sub-nanoscale metal entities. Au/TiO2 and the Au/Zr O2 catalysts were prepared with the same method, which particle size was 3 nm or 5 nm, respectively, of which only Au/ZrO2 occurred gold characteristics of the diffraction peak on the XRD spectra.Under the atmosphere of the same composition, flow velocity and reduction temperature 60 mg Au/TiO2 catalyst could be fully reducted in 240 min.On this basis,sample under oxidation of 40 min showed the best catalytic activity and the lowest reaction temperature CO completely oxidation was 80 ℃. And with the Au/ZrO2 oxidation time extended, the catalytic activity showed a linear downward trend. In stability tests,the catalyst activity of both catalysts declined gently. Of course, the declined activity of the typical catalyst after a 30 h continuous reaction ware able to be recovered.These results further indicate that, for oxide supported Au catalysts, the regulation of their redox state is a facile means to improve their catalytic performance.Besides, as for the frequently used nano-Au catalysts, to limit the scale of Au catalysts into sub-nanometer and to obtain a metal/oxide interface with highly dispersed Au would be crucial to improve and tune the performance of Au catalyst. |
PDF Full Text Request