| 摘要: | 摘要 高溫燃燒觸媒可以提高燃燒效果和抑制NOx排放等優點，用來取代傳統的火焰燃燒。Arai在1989年，使用溶膠-凝膠法合成金屬取代的六鋁酸鹽觸媒，並且提到這些觸媒即使在高溫仍可維持高表面積和高催化活性。但是影響製備的參數並未詳述。本研究使用共沈澱和溶膠-凝膠法合成一系列的SLMA和BLMA觸媒，並且探討許多製備條件，包括水量，熟化時間與硝酸鹽金屬水溶液添加速度對樣品結構和表面積的影響。在鑑定樣品的物理性質上，以X光繞射分析鑑定結晶度；由氮吸附分析測定表面積；藉掃描式電子顯微鏡(SEM)與穿透式電子顯微鏡(TEM)觀察顆粒直徑及晶形。溶膠-凝膠法合成的SLMA在1000℃即可得到SrO·6Al2O3的六鋁酸鹽結構。相對的，以共沈法製備的SLMA是由SrO·6Al2O3和γ-Al2O3所組成的。具有第二相將會降低樣品在較高溫度的表面積。使用共沈法製備SLMA的表面積(20 m2/g)，明顯的比使用溶膠-凝膠法合成的表面積(23.6~105.6 m2/g)來的小。溶膠-凝膠法合成的BLMA在1000℃得到BaO·Al2O3的非六鋁酸鹽結構，將會造成BLMA在更高溫度表面積的急速減少。 Abstract High temperature combustion catalyst has attracted attention as a substitute for conventional flame combustion due to its high energy efficiency and suppressing thermal NOx emission. Arai et al. (1989) synthesized the metal-substituted hexa-aluminate catalyst by hydrolysis of metal alkoxide sol-gel method and reported these catalysts can retain high surface area and high catalytic activity at high flame temperature. The effects of preparation parameters remain unclear. A series of SLMA (Sr0.8La0.2MnAl11O19-α) and BLMA (Ba0.8La0.2MnAl11O19-α) catalysts were prepared by co-precipitation and sol-gel methods, in order to investigate the influence of the amount of water, aging time, and the feeding rate of metal nitrate on the structure and surface area of the sample. The properties of catalysts were characterized by X-ray powder diffraction, nitrogen sorption, scanning electron microscopy, and transmission electron microscopy. The crystalline structure of SLMA (Sr0.8La0.2MnAl11O19-α) prepared by sol-gel method only exhibited hexaaluminate structure, SrO·6Al2O3. It was clearly observed after calcination at 1000 ℃, which is a quite low temperature. In contrast, the crystalline structure of SLMA prepared by co-precipitation method consists of mixed phases of SrO·6Al2O3 and γ-Al2O3. The samples possess the second phase would lowered the surface area at higher calcinations temperature. The surface area of SLMA prepared by co-precipitation was 20 m2/g, which is smaller than the other samples synthesized by sol-gel process, 23.6~105.6 m2/g. One can conclude that the SLMA prepared by sol-gel method is superior to that by co-precipitation. The crystalline structure of BLMA (Ba0.8La0.2MnAl11O19-α) prepared by the sol-gel method is BaO·Al2O3, which is not a hexaaluminate structure, after calcinations at 1000 ℃.Therefore, the surface area of BLMA would decreased rapidly at higher temperature. The shape of SLMA crystalline structure is a thin plate consisted of layered particles with a thickness of about 40 nm, which is about one-fifth of its diameter, but the BLMA crystalline structure is in a taper shape. For the SLMA and BLMA prepared at H2O/alkoxide ratio of 15, the longer the aging time and the slower the feeding rate (1 cc/min) gave the highest surface area (105.6, and 135.6 m2/g) after calcinations at 1000 ℃, respectively. |
