Characterization of alumina gel catalysts by emanation thermal analysis (ETA)

Alumina gels made from the metal alkoxide is known to have high catalyst activity for the selective reduction of NOx by hydrocarbons. It is also reported that the fine structure of the gels effects the activity. In this study, the effect of the preparation method on the fine structure and catalyst activity of the gels was investigated. Monolithic gels were obtained by hydrolysis of Al(sec-C4H9O)(3). The wet gels were dried at 90 degreesC (xerogels), supercritically dried (aerogels), or dried after immersion in an ethanol solution of methyltrimethoxysilane (modified xerogels). The changes in the microstructure during heating were discussed using the results of TG-DTA, ETA and N-2 adsorption. The ETA curves show the Rn-220-release rate, E, of the samples, previously labelled with Th-228 and Ra-224, during heating. The decrease in E of the xerogel at temperatures higher than 400 degreesC indicates a gradual decrease in the surface area and porosity. A remarkable decrease in the BET surface area of the xerogel was found after heat-treating at 500 degreesC. On the other hand, constant E of the aerogels and modified xerogels above 450 degreesC suggests high thermal stability. The pore radii, estimated by BJH method, and the catalyst activities at 500 degreesC of the aerogels and the modified xerogels were higher than those of the xerogels. The temperature range in which the alumina gels are applicable as catalysts was determined.