| Summary: | 碩士 === 國立中央大學 === 化學工程與材料工程研究所 === 92 === ABSTRACT
The nanomaterials, combining the features of amorphous and nanometer powers, have more surface atoms and a higher concentration of coordinately highly unsaturated sites. Nanometer amorphous alloy powders have attracted extensive attention due to their unique isotropic structural and chemical properties. A series of ultrafine Fe-B, Fe-P and Fe-P-B amorphous alloy catalysts with various methods were prepared by a chemical reduction method. A series of ultrafine Fe-B, Fe-P and Fe-P-B samples were prepared by mixing aqueous solutions of iron salt (0.1 M), sodium hypophosphite (1 M) and/or sodium borohydride (1 M) at 5℃ under ultrasonic agitation. The solutions of FeCl3 (1000 ml, 0.1 M) and sodium hypophosphite (300 ml, 1 M) were first mixed and the solution of sodium borohydride (300 ml, 1 M) was then added dropwise into the mixture to prepared Fe-P-B materials. Similar method was used to synthesize Fe-P and Fe-B samples.The catalysts were characterized with respect to ICP-AES, X-ray diffraction, N2 sorption, DSC, TEM and XPS. Dehydrogenation of ethanol was chosen as the test reactions to the probe catalytic behaviors and to allow comparisons among these catalysts. The results concluded that the different preparation conditions significantly affected the concentration of boron and phosphorus bounded to the iron metal, resulting in the change of surface area, amorphous structure and dehydrogenation properties of the catalysts. The surface compositions were similar to the bulk compositions for Fe-B, Fe-P and Fe-P-B materials. If the solvent H2O was replaced with ethanol or isopropanol in H2O during preparation, the surface area significantly decreased for the Fe-B, Fe-P and Fe-P-B materials. The source of iron salt had significant effect on the surface area of materials. The Fe-P-B and Fe-P materials prepared with FeCl2 had higher surface areas than those of FeCl3 and Fe(OAc)2. In contrast, the Fe-B materials did not show any change. The XRD patterns of the as-synthesized Fe-B, Fe-P and Fe-P-B materials reveal an amorphous state. The Fe-B and Fe-P catalysts had a higher thermal stability than Fe-P-B. The particles size of Fe-B, Fe-P and Fe-P-B were in the range of 10 and 30 nm, whereas the Fe-B, Fe-P and Fe-P-B powders prepared with Fe(OAc)2 using IPA/ H2O as the solvent had the largest particle size in the range of 60 and 150 nm. If the Fe-B, Fe-P and Fe-P-B materials were prepared with Fe(OAc)2 using EtOH/ H2O as the solvent, the materials show narrow-distributed particles size. All the prepared catalysts were easily degraded by gaseous oxygen. The XPS data of Fe-B, Fe-P and Fe-P-B powders revealed that the starting materials of iron salt had significant influence on the metallic state of iron. One can get metallic iron species by using FeCl3 as the starting material. In contrast, the materials prepared with FeCl2 and Fe(OAc)2 did not show any elemental iron species. The catalytic activities of these catalysts were tested by dehydrogenation of ethanol. The activity per gram and the activity per surface area of the catalyst for ethanol dehydrogenation were compared. Fe72.8B27.2, prepared with FeCl3 and H2O, showed the highest activity based on gram and surface area of the catalyst among all the Fe-B catalyst. The high activity of this catalyst can be attributed to both high surface area and high turnover frequency (TOF). The high TOF of this catalyst is possibly due to its low boron content in the catalyst. The specific activity per weight of Fe82.6P17.4, prepared with Fe(OAc)2 and EtOH showed the highest activity among all the Fe-P catalyst. This catalyst had the highest surface area. In contrast, the specific activity per surface area of Fe89.1P10.9, prepared with Fe(OAc)2 and IPA showed the highest activity among all the Fe-P catalyst. The high TOF of this catalyst was possibly due to the low content of phosphorus. The specific activity per game of Fe82.4P1.1B16.5, prepared with FeCl2 and EtOH, showed the highest activity among all the Fe-P-B catalyst. The specific activity per surface area of Fe78.6P8.8B12.6, prepared with Fe(OAc)2 and IPA showed the highest activity among all the Fe-P-B catalyst. Depending on the preparation conditions, the Fe-B, Fe-P and Fe-P-B amorphous catalysts revealed significantly different catalytic properties.
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