以不同方法製備稻殼灰分-氧化鋁擔載鎳觸媒之研究

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题名:	以不同方法製備稻殼灰分-氧化鋁擔載鎳觸媒之研究
作者:	郭茂穗;Maw-Suey Kuo
贡献者:	化學工程與材料工程研究所
关键词:	稻殼灰分;鎳觸媒;甲烷化;methanation;nickel catalyst;rice husk ash
日期:	2003-06-28
上传时间:	2009-09-21 12:16:43 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	本研究是以精製稻殼灰分(RHA)含浸硝酸鋁經煆燒製成矽-鋁氧化物(RHA-Al2O3)做為觸媒擔體，再以含浸法及沈澱固著法製備擔體鎳觸媒(Ni/RHA-Al2O3)。實驗中使用氮吸附法、感應偶合電漿原子發射光譜儀(ICP-AES)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)、X-射線繞射儀(XRD)、X-射線光電子分析儀(XPS)、熱重分析(TGA)、微分掃瞄式熱量分析儀(DSC)、氫氣程溫脫附(H2-TPD)、氨吸附、及程溫還原(TPR)作觸媒特性分析，並在常壓下進行二氧化碳甲烷化反應以探討製備條件及操作變數對觸媒活性的影響。再依實驗結果分析建立Ni/RHA-Al2O3擔體鎳觸媒的模型。在稻殼灰分精製部份，實驗結果發現：在台灣的稻殼中矽含量因產地而異，以SiO2計含量為9~14wt.%。在台灣南部地區水稻成長期短，稻殼中的矽含量比較低，反之亦然。稻殼經酸洗(3N HCl)、煆燒可得純度高達99.9%以上的多孔性高表面積的SiO2。稻殼灰分中的SiO2純度與製程有關，先酸洗後煆燒可得高純度SiO2，反之若先煆燒再酸洗則無法得到高純度SiO2。SiO2的表面積隨煆燒溫度的提高而降低。稻殼灰分是屬於非晶形(amorphous)中孔洞(mesopores)，有利於作為觸媒擔體。在複合擔體(RHA-Al2O3)製程部份，實驗結果發現：含浸的鋁鹽有堵塞稻殼灰分孔洞的現象，會慢慢的阻塞大孔洞，使RHA-Al2O3的孔洞更均一性，將有助於化學反應的選擇性。RHA-Al2O3的表面積隨Al2O3的含量增加而降低，而固體酸的酸量也與RHA-Al2O3的BET比表面積有密切的關係。在含浸法製備複合擔體鎳觸媒(I-Ni/RHA-Al2O3)方面，實驗結果發現：在鎳載量10wt.%以下，鎳呈現良好的分散效果，鎳載量10wt.%以上則逐漸形成bulk-NiO。含浸法複合擔載鎳觸媒有3種形態的氧化鎳，即bulk-NiO及2種NiAl2O4-like。NiAl2O4-like之一是Ni-ions在Al2O3的四面體(tetrahedral)晶格內，另一是Ni-ions在Al2O3的八面體(octahedral)晶格內，前者很難還原，後者較易還原。在低載量時只有tetrahedral的NiAl2O4-like，在高載量時3種成分都有，bulk NiO會聚集使晶粒長大。因此，氧化鎳的晶粒大小隨鎳載量的增加而成長。含浸硝酸鎳乾燥後形成的觸媒先驅物為網狀結構的硝酸鋁鎳複合物，硝酸鋁鎳複合物的熱分解溫度至少要400℃。先驅物煆燒後鎳離子進入氧化鋁晶格內形成非常難還原的鋁酸鎳NiAl2O4尖晶石(spinel)。由TPR可以看出在煆燒時產生固溶現象，Ni-ions逐漸從bulk-NiO擴散到Al2O3的octahedral晶格內，再由octahedral晶格擴散到tetrahedral晶格內。因此，煆燒溫度越高越難還原。由XRD可以看出隨著煆燒溫度的提高，NiO逐漸轉變為spinel的過程中氧化鎳與spinel的晶粒相互消長。含浸法複合擔體鎳觸媒(I-Ni/RHA-Al2O3)的表面結構屬於mesopore，表面積隨鎳載量的增加而增加，一直到15wt.%鎳載量時表面積為最大(200m2/g)，然後下降。雖然表面積比商用擔體鎳觸媒I-Ni/SiO2-Al2O3小(396m2/g)，但孔徑比商用複合擔載鎳觸媒(I-Ni/SiO2-Al2O3)大，深度較淺，因此孔隙阻力較小。經CO2甲烷化反應測試證實：I-Ni/RHA-Al2O3的活性及選擇性優於I-Ni/SiO2-Al2O3。含浸法複合擔載鎳觸媒有較佳的熱穩定性。因此CO2甲烷化反應的觸媒活性不受煆燒溫度及還原溫度的影響。由實驗證實觸媒的活性會隨著反應溫度的增加而提升，直至500℃，再漸次下降。因此，CO2甲烷化反應的最佳溫度為500℃。在沈澱固著法製備複合擔載鎳觸媒(P-Ni/RHA-Al2O3)方面，提高溶液中的Ni-ions濃度或延長沈澱時間都可以增加鎳載量，而延長沈澱時間則可提高鎳的固著量。實驗結果發現：若沈澱時間固定為24h，以Ni-ions濃度控制鎳載量則鎳載量26.4wt.%以下，鎳有良好的分散效果，鎳載量26.4wt.%以上則逐漸形成bulk-NiO。若Ni-ions濃度固定為0.14M，以沈澱時間控制鎳載量，則鎳載量在12.0wt.%以下，鎳呈現良好的分散效果，鎳載量12.0wt.%以上則逐漸形成bulk-NiO。因此，將沈澱固著時間設定在24小時以上，再以Ni-ions濃度控制觸媒的鎳載量可得最佳的分散效果。由於沈澱物(Ni(OH)2 group或Ni(OH)2CO3 group)的穩定性比Ni-ions高，只與擔體表面的氫氧化物交互作用，不會擴散到擔體的晶格內。因此，沈澱法製備的複合擔載鎳觸媒(P-Ni/RHA-Al2O3)只有2種形態的氧化鎳，即bulk-NiO及nickel aluminate。在低載量時只有nickel aluminate，在高載量時2種成分都有。如同含浸法鎳觸媒，氧化鎳的晶粒大小隨鎳載量的增加而成長。但沈澱時間超過48 h以上，氧化鎳的晶粒反而有減小的跡象。沈澱固著的鎳觸媒先驅物亦為網狀結構，而煆燒所需的溫度約在500℃。先驅物煆燒後形成非常難還原的鋁酸鎳尖晶石(spinel，NiAl2O4)。由XRD可以看出，隨著煆燒溫度的提高，NiO逐漸轉變為spinel，但晶粒小，因此X-ray的繞射圖譜比較不明顯。沈澱固著法複合擔載鎳觸媒(P-Ni/RHA-Al2O3)的表面積隨沈澱時間的增加而增加(151~176 m2/g)，但隨Ni-ions的濃度提高而減少。其表面結構屬於中孔洞，孔徑大小可由Ni-ions濃度控制。由實驗證實： CO2甲烷化反應的最佳溫度為500℃。經CO2甲烷化反應性能測試比較：I-Ni/RHA-Al2O3的活性及選擇性優於P-Ni/RHA-Al2O3。本實驗所製備的稻殼灰分-氧化鋁複合擔體具高促進效果。無論是含浸法或沈澱固著法，所製備的鎳觸媒其CO2甲烷化反應的選擇率都可高達90%以上，是一種性能優越的觸媒。 Rice husk ash (RHA) was impregnated with aluminium nitrate and calcined to form silica alumina oxides (RHA-Al2O3) and used as catalyst support; it is further loaded with nickel by impregnation and deposition-precipitation. Characterization was made with nitrogen adsorption, ICP-AES, SEM, TEM, XRD, XPS, TGA, DSC, H2-TPD, ammonia adsorption, and TPR while the catalytic performance was determined by the hydrogenation of carbon dioxide under normal pressure. Based on the experiment results, model is established to describe the preparation of Ni/RHA-Al2O3 from RHA. Silica content is different for RHA of different origin and ranges from 9 to 14wt.% in terms of SiO2. Highly porous RHA with SiO2 content greater than 99.98% can be obtained by hydrolysis following leaching with 3N HCl. Such obtained RHA is of amorphous and mesopore enriched and merits attention of being used as catalyst support. For the composite support (RHA-Al2O3), it is found that impregnation of aluminum salt modified the pore size resulting in uniform pores, which is believed in favor of selectivity. In the mean time, the specific surface area of RHA-Al2O3 decreased with increasing Al2O3 content. The acid amount of RHA-Al2O3 is proportional to the surface area。 In the preparation of I-Ni/RHA-Al2O3 catalysts by incipient wetness impregnation method, three types of nickel oxide can exist, namely, bulk-NiO and two types of NiAl2O4-like, one with Ni-ion deposited on tetrahedral lattice of Al2O3 and the other on octahedral lattice. That on octahedral lattices was reduced easily, that on tetrahedral lattice was much difficult to be reduced. NiAl2O4-like of tetrahedral was observed in lower nickel loading below 10wt.%, when nickel loading went beyond 10wt.% bulk-NiO formed gradually and all three types of nickel oxide were observed. The crystal size of nickel oxide grew with increasing of nickel loading. The dried precursor, complex of nickel aluminum nitrate from incipient wetness impregnation of nickel nitrate, was of net work structure and decomposed at 400ºC and above. From TPR, the solid dissolution was observed; Ni-ions diffused from bulk-NiO to octahedral lattices of Al2O3 then to tetrahedral lattices and became more difficult to be reduced. It is also seen from XRD, the NiO converted to spinel along with the increasing of calcination temperature. The specific surface area of I-Ni/RHA-Al2O3 increased with nickel loading during impregnation to the maximum of 200 m2/g at 15wt% nickel loading and then decreased. Though the specific surface area of I-Ni/RHA-Al2O3 is lower than that of commercial I-Ni/SiO2-Al2O3, its pores are larger than that of I-Ni/SiO2-Al2O3 but shallower giving less pore resistance. Results of methanation experiment confirm that I-Ni/RHA-Al2O3 exhibits better activity and selectivity as well than I-Ni/SiO2-Al2O3. The supported nickel catalyst possess good thermal stability, hence the activity of catalysts for the CO2 methanation was not affected by calcination and reduction temperature. However, the catalyst activity increases with reaction temperature until 500 ºC then decreases, therefore, the optimum temperature for CO2 methanation is 500 ºC. For the P-Ni/RHA-Al2O3 catalysts prepared by deposition- precipitation, both increasing the Ni-ion concentration and deposition- precipitation time increased the nickel loading. Increasing the deposition- precipitation time also increased the amount of nickel-support interaction. With constant deposition-precipitation time of 24 hours, controlling the nickel loading by adjusting Ni-ion concentration, nickel dispersed very well when nickel loading below 26.4wt.% but gradually formed bulk-NiO when go beyond 26.4wt.%. When keeping the Ni-ion concentration at 0.14M and controlling the nickel loading by deposition-precipitation time, it gave good dispersion when nickel loading was below 12.0wt.%. Therefore, controlling the nickel loading by adjusting Ni-ion concentration gives better dispersion of nickel on catalyst. The stability of precipitate from deposition-precipitation is better than Ni-ion, it react only with hydroxides on the support surface and will not diffuse into the support lattice. P-Ni/RHA-Al2O3 from deposition-precipitation method only presented two types of nickel oxides, bulk-NiO and nickel aluminate. With low nickel loading, only nickel aluminate was observed. Both types of nickel oxides were found at higher nickel loading. Similar to the incipient wetness impregnation method, crystal size of nickel oxide increased with nickel loading. But when the deposition-precipitation time extended over 48 hours, the crystal size tended to shrink slightly. The catalyst precursor after drying also presents net work structure and converts to nickel oxide at the calcination temperature of 500 ºC. Calcination of the catalyst precursor gave NiAl2O4 spinel which is very difficult to reduce. From XRD it is found that NiO transformed to spinel gradually along with increasing of calcination temperature but with smaller crystal size than that found in incipient wetness impregnation method. It shows that the X-ray diffraction spectrum of phase change is more obvious with incipient wetness impregnation method than that of deposition-precipitation method. The BET specific surface area of P-Ni/RHA-Al2O3 from deposition-precipitation increased with precipitation time ranging from 151 to 176 m2/g, but decreased with increasing of Ni-ions concentration. The surface structure is of mesopore and the pore size can be controlled by Ni-ion concentration. Experimental result shows that the activity of catalyst increases with reaction temperature until 500 ºC and then decreases. Hence the optimum reaction temperature for CO2 methanation is also 500 ºC. Based on the reaction performance on the CO2 methanation, I-Ni/RHA-Al2O3 gives both better activity and selectivity than P-Ni/SiO2-Al2O3 does. The RHA-alumina composite oxide prepared in this experiment is a catalyst support with highly promotive. Whether the supported nickel catalyst made from incipient wetness impregnation method or deposition-precipitation, all gives better than 90% yield in the CO2 methanation and is a superior catalyst.
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