Reliable thermodynamic parameters for the thermal decomposition of diammonium hydrogenphosphate [(NH4)2HPO 4] may be obtained using a fluidized bed. For the same size of particle, at the same temperature, but for different carrier gases, the rate constant and activation energy increase in the following order: air, methane, hydrogen. Por the same carrier gas (air) the rate constant increases when the particle size decreases.
In order to eliminate the errors of the TG method with a static layer, and to determine the infuence of the heating rate, the composition of the carrier gas and the particle size of the solids, a method of thermal gas evolution from a fluidized bed (FTGE) with continuous and linear increase of temperature was worked out and the thermal decomposition of diammonium hydrogen phosphate was followed. This method gives reproducible results and makes possible the determination of the apparent activation energies and kinetic parameters.
Aluminium oxide gel and aluminium oxide-silica gels of different compositions were prepared and activated at different temperatures. The adsorption of NH3 was studied in a McBain type balance at 20 torr and 5x l0 -a torr at 25 ~ The thermodesorption of NH3 was studied at 5• 10 .3 torr between 25 and 550 ~ at a constant heating rate of 4~The desorption was followed by recording the thermogravimetric curve and the pressure oscillations in the system (p --T curve). The influence of composition and activation temperature is discussed. Chemisorption of a monomolecular layer is assumed in three main types of pores, together with the formation of a second monomolecular layer held by hydrogen bonds.Basic vapours (quinoline) were first used for the deternfination of the acidity of solid catalysts, by Mills and coworkers [1 ]. Trimethylamine and pyridine were later applied by Richardson and Benson [2]. Ammonia is also frequently used for the quantitative and qualitative characterization of the catalyst acidity and a considerable number of papers have recently dealt with the interaction of NH3 and acid catalysts.This interaction has been studied by means of IR spectroscopy [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], gravimetry [5,16,[18][19][20][21][22][23][24], calorimetry [25][26][27][28][29][30][31][32][33][34][35] and gas chromatography [36].In the present paper an attempt has been made to elucidate the influence of the composition and of activation temperature (Tact.) on the adsorption and thermodesorption of NH3 on aluminimn oxide-silica gel systems (A1SiG) by using a thermogravimetric method combined with pressure oscillation measurements.
ExperimentalPreparation of the systems studied. AISiG systems were obtained by co-precipitation. Solutions containing silicic acid (4 wt % SiO2) and aluminium nitrate were treated with ammonium carbonate solution at pH = 5.0-5.5. The precipitate was filtered, washed with water and dried at 105 ~ . The dried precipitate was subjected to a preliminary heat treatment at 400 for30 min in order to remove any ammonium carbonate and nitrate. Chemical analyses 'were carried out after calcination of the samples at 1100 ~ for 3 hrs. The results obtained are presented in Table 1.
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