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) m2·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.