Characteristics of ammonia adsorption on activated alumina

Adsorption of ammonia was measured volumetrically on activated alumina at temperatures of (273, 298, and 323) K and gas pressures up to 108 kPa. It was found that the final adsorption amounts at the terminal pressure point were (3.13, 2.53, and 1.89) mmol·g^-1 at (273, 298, and 323) K, respectively. The Langmuir, Freundlich, Sips, and Toth isotherm models were employed to correlate the adsorption isotherms. Ammonia diffusivities in these adsorbents at (298 and 323) K and at five different pressure points were calculated from the adsorption kinetic uptake curves by using a classical diffusion model. It was found that the average diffusivity values were (4.02·10^-14 and 4.71·10^-14) m ²·s^-1, respectively, at (298 and 323) K. The heat of adsorption values are between ( - 35.6 and - 15) kJ·mol^-1 for adsorption loadings between (1.3 and 2.3) mmol·g^-1. The Gibbs energies were calculated as ( - 14.01, - 13.23, and - 9.07) kJ·mol^-1 at (323, 298, and 273) K, respectively. The value of the entropy lies within ( - 65 to - 3) J·mol^-1·K ^-1, ( - 73 to - 6) J·mol^-1·K^-1, and (-95 to - 21) J·mol^-1·K^-1 for adsorption amounts of (1.3 to 2.3) mmol·g^-1 and at temperatures of (323, 298, and 273) K, respectively.