| 摘要: | 中文摘要 奈米非晶態材料結合了非晶結構與奈米尺寸的優點，它們含有較多表面原子及高度配位不飽和位置。且奈米非晶合金粒子具有特殊的等方向性結構與物化性質，因此具有特殊的性質。本研究以化學還原法，改變各種製備條件，製備一系列的Fe-P-B、Fe-P與Fe-B奈米非晶合金觸媒。觸媒的製備是以0.1M FeCl2、FeCl3和Fe(OAc)2為先驅鹽類，混和次磷酸鈉水溶液（1 M）或將硼氫化鈉水溶液（1 M）緩慢的加入金屬鹽水溶液中，並在溫度為5℃下以超音波震盪，此時即有黑色顆粒狀的Fe-P-B觸媒沈澱。所製備而成的Fe-B、Fe-P與Fe-P-B奈米非晶合金，以N2 sorption、ICP-AES、XRD、DSC、TEM和XPS等儀器鑑定其物理與表面性質。觸媒氫化活性測試，以乙醇脫氫做為測試反應；探討Fe-B、Fe-P與Fe-P-B奈米非晶合金觸媒間之催化性質的差異。結果顯示，不同的製備條件，會影響硼、磷與鐵的結合比例；進而引起其表面積、非晶結構與脫氫活性等觸媒性質的變化。在Fe-B、Fe-P與Fe-P-B奈米材料上其表面組成與整體組成相近。製備Fe-B、Fe-P與Fe-P-B材料，如製備溶劑由水改用50%的乙醇或50%異丙醇，則觸媒表面積會明顯地減少。在製備過程中，鐵鹽的來源也會影響表面積的大小；製備Fe-P-B和Fe-P材料，若鐵鹽由FeCl2改用FeCl3或Fe(OAc)2則觸媒表面積會明顯地減少；但在Fe-B材料製備上，鐵鹽的來源對表面積的影響不明顯。X-光繞射分析顯示所製備的Fe-B、Fe-P與Fe-P-B材料，均是長程無序的非晶結構。Fe-P與Fe-B觸媒的熱穩定性高於Fe-P-B觸媒。TEM的結果顯示，大部分的Fe-B、Fe-P與Fe-P-B粉末，粒徑在10~30 nm之間。然而若Fe-B、Fe-P與Fe-P-B材料，製備過程中改用Fe(OAc)2為起始原料，溶劑為IPA/H2O，則具有最大粒徑在60~150 nm之間。如果製備Fe-B、Fe-P與Fe-P-B材料，製備過程中用FeCl2為起始原料，溶劑為EtOH/H2O，則可得到均勻分佈的粒徑，粒徑大小在10~30 nm之間。觸媒表面暴露於空氣中，易被氧化。由XPS觀察得知，鐵的鹽類對於產物性質有極大影響，以FeCl3為起始原料可製備出金屬鐵，而FeCl2和Fe(OAc)2則較難得到金屬鐵。 觸媒每單位表面積的活性在乙醇的脫氫反應中，其比活性大小次序明顯不同。在所有的Fe-B觸媒中，以每單位觸媒重量的活性以Fe72.8B27.2為最大；而每單位觸媒表面積的活性也以Fe72.8B27.2最大。在Fe-B觸媒中，影響它的活性的主要原因可能是B的含量較少。在所有的Fe-P觸媒中，以每單位觸媒重量的活性以Fe82.6P17.4為最大；而每單位觸媒表面積的活性以Fe89.1P10.9最大。在Fe-P觸媒中，影響觸媒每單位重量的活性的主要原因可能是表面積較大；而影響觸媒每單位表面積的活性的主要原因可能是P的含量較少。在所有的Fe-P-B觸媒中，以每單位觸媒重量的活性以Fe82.4P1.1B16.5最大；而每單位觸媒表面積的活性以Fe78.6P8.8B12.6最大。在Fe-P-B觸媒中，影響它的活性的主要原因可能是B和P要具一定的含量所影響。在不同製備方法中，Fe-B、Fe-P與Fe-P-B奈米材料顯示不同的催化性質。 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. |
