| dc.description.abstract | 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. | en_US |