dc.description.abstract | Abstract
Supported gold catalyst has been a subject of intense investigation since the report of its exceptionally high low-temperature CO oxidation activity by Haruta et al. In spite of these efforts, however, there is still great uncertainty of the cause of the high activity and there is a wide variation in the activities reported among different laboratories. Superficially, all results suggest that the catalytic activity depends on the support. The aim of this study was to develop a method to prepare iron hydroxide support which has a higher surface area and abundant hydroxyl groups on the surface. It was used as a support for gold in CO oxidation. Several parameters have been investigated for the synthesis of iron hydroxide, such as iron salt (FeCl3.6H2O, Fe(NO3)3.6H2O and FeCl2.4H2O), pH value (from 8 to 12), calcination temperature (from 120℃ to 300℃), feeding rate, etc. The iron hydroxide support was characterized by powder X-ray diffraction, TEM, XPS and N2 sorption. The XRD patterns of the iron hydroxide appeared only wide, without any definite XRD peaks, suggesting that the material was either amorphous or of a particle size too small (< 10 nm). TEM images show that the particle diameters were less than 20nm. XPS Fe 2p3/2 spectra showed the phase transition of iron oxide from Fe3O4 to FeO. The N2 sorption analysis indicated that the surface area of iron hydroxide was greater than 300 m2/g under suitable preparation conditions. The pH value and the feeding rates played the important roles to obtain iron oxide with high surface area. The higher activity of FexOy nanoparticles was attributed to a small particle size, high surface area and more densely populated surface coordination unsaturated sites. These nanosized iron oxide samples also demonstrated to have high activity even at room temperature. The higher the surface area of the iron oxide is, the higher the CO conversion is. Obviously, in this work, nanosized iron oxide was characterized both as a catalyst and as a support of Au catalysts. Supported gold catalysts were prepared by deposition-precipitation using HAuCl4 as the Au precursor. The as-synthesized iron hydroxide was used as the support. The in-situ chemical reduction method was applied to reduce gold compound to metallic state. The gold catalysts were characterized by N2 adsorption, XRD, TEM, TEM-EDS and XPS. The N2 adsorption analyses indicated that the surface areas of Au catalysts were greater than 250 m2/g. The XRD results demonstrated that gold metal had a particle size under detection limit, which is less than 4 nm. TEM images clearly showed that the particle diameters of gold for all the samples were less than 4 nm. TEM-EDS showed that gold is present on the Au catalysts in the form of metallic
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particles. XPS spectra present the Au 4f7/2 peaks of the Au catalysts at binding energy below 84.2 eV. Therefore, Au is in metal state Au0. The method applied in this study leads to a fairly uniform dispersion of gold nanoparticles with diameter less than 4 nm and narrow size distribution. Effects of catalysts preparation conditions on the performance of gold catalysts for low-temperature CO oxidation was carried out. The catalytic activity was measured using a fixed bed continuous flow reactor. At first, for CO oxidation at room temperature, the iron oxide-supported gold catalysts were prepared by reduction process. The materials were synthesized at pH value of 9 and calcined at 120 ℃or 180.℃ A specific emphasis was on the effect of surface area of the support on the catalytic performance. This research clearly showed that higher surface area of FexOy support gave a higher catalytic activity. It should be pointed out that increasing the surface area of iron oxide support resulted in an enhancement over the CO oxidation and retard decay of catalyst. Secondly, CO oxidation of Au/FexOy catalysts prepared by deposition -precipitation method, synthesized at pH value of 9 and 10.5 and calcined at 120 ℃and 180℃ were tested. We focused on the effect of pH values on the CO oxidation over the Au/FxOy catalysts. It can be seen that the apparent increase in catalytic activity of the catalyst was due to the proper pH value. Obviously, the suitable pH value is an important factor, and the optimum value is 9. It also demonstrated that suitable selection of pH value and calcination temperature play a key role in creating high catalytic performance for low-temperature CO oxidation which are more effective than the high surface area of support. For the iron oxide-supported gold catalysts synthesized at different pH values and calcined at 120℃, the catalyst pretreated by in-situ reduction process was much less active than those by deposition- precipitation method, providing the same high surface area of support. Comparing the samples prepare by the deposition-precipitation and in-situ reduction, using high surface area support and calcined at 180℃ after preparation, all the samples demonstrated 100 % conversion of CO. In other words, if the pH value during deposition of Au was 9 and calcinations temperature was 180℃, there is no significant effect on the other preparation parameters. In this study, the Au/FxOy catalyst, prepared by either deposition-precipitation method or reduction process, supported on high surface area of iron oxide, synthesized at pH 9 and calcined at 180℃, shows the best performance of CO oxidation. Full conversion was kept at ambient temperature over 90 min on stream. II | en_US |