dc.description.abstract | Bulk gold has been regarded as a poorly active catalyst. A theoretical calculation has explained the smooth surface of Au is noble in the dissociation adsorption of hydrogen. However, when Au is deposited as nanoparticles on metal oxides, it exhibits surprisingly high catalytic activity for CO oxidation at a temperature as low as 200K. Removal of CO in H2 stream is an important subject in fuel cell performance. The aim of this study was to investigate the effect of preparation on the characteristics of CeO2. In addition, preferential oxidation of CO in H2 stream (PROX) over gold supported on CeO2 and MnO2-CeO2 mixed oxide were investigated.
Several CeO2 supports were used in this study including NanoCeria from Degussa Company and a series of home-made CeO2 which were prepared by precipitation at constant pH using cerium nitrate and NH4OH as the starting materials. Several synthesis parameters, such as pH value (from 8 to 11) and heating temperature (from 120 to 400oC), have been varied. For the mixed oxide support, MnO2-CeO2, was prepared by impregnation method. Gold catalysts supported on CeO2 and MnO2-CeO2 were prepared by deposition-precipitation using HAuCl4 as the Au precursor. The effects of calcination temperature on the catalytic properties after loading gold were also studied.
The supported gold catalysts were characterized by powder X-ray diffraction (XRD), N2 sorption, transmission electron microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). The results show that CeO2 support prepared at pH 10 had the highest surface area (104 m2/g). The crystallinity of CeO2 increased with an increase of calcination temperature. TEM images showed that the particle sizes of CeO2 were about 20 nm. The XRD results showed that gold metal had a particle size under detection limit, which was less than 4 nm. TEM images confirmed that the particle sizes of gold for all the catalysts were less than 4 nm. The method applied in this study leaded to a fairly uniform dispersion of gold nanoparticles with diameter less than 4 nm and narrow size distribution. XPS spectra showed that metallic and oxidized gold species coexisted within nano gold particle. For the effects of calcined temperature after loading gold, XPS study showed that more metallic gold species were present in gold nanoparticles calcined at 180oC.
Activities for CO oxidation on Au/CeO2 catalysts were measured using a fixed bed continuous flow reactor. Gold catalysts supported on NanoCeria from Degussa Company demonstrated the highest activity. For gold supported on home-made CeO2 catalyst, the CeO2 support prepared at pH value 10 and calcined at 400oC showed the highest activity. For the effects of calcination temperature after loading gold, the catalyst which calcined at 180oC after loading gold had the highest activity in CO oxidation than those calcined at 120oC.
In contrast, gold catalyst which calcined at 120oC after loading gold showed a higher activity in preferential oxidation of CO in H2 stream at the reaction temperature above 50oC than those calcined at 180oC. Furthermore, the gold catalyst supported on MnO2-CeO2 mix oxide showed higher CO conversion and selectivity to CO oxidation by adding MnO2 on CeO2. By regulating the preparation and pretreatment procedure and the compositions of Au/MnO2-CeO2 catalyst, we have developed a catalyst which had high CO conversion and high selectivity to CO oxidation in PROX reaction. | en_US |