Abstract
Allophane is a hollow spherical aluminium silicate with some holes, and inner and outer surfaces of the hollow sphere can adsorb cations, anions and molecules. In this study, two natural allophane samples with different Si/Al atomic ratio (KyP: 0.67, KnP: 0.99) and a montmorillonite sample were used to adsorb ammonia and pyridine gases. The amount of adsorption of ammonia and pyridine at lower equilibrium concentrations increased in the order, montmorillonite < KnP < KyP, and the strength of adsorption of the two adsorbates estimated by Langmuir K and Freundlich n values increased in the same order. For all the samples, the amount and strength of adsorption of ammonia were greater than those of pyridine. Ab initio/hybrid (ONIOM) method was used to simulate the formation of five-coordinated Al in the pore region of allophane, and the adsorption of ammonia and pyridine on the Al was also simulated. The five-coordinated Al was speculated to form easily with heating. Binding energy of ammonia to the five-coordinated Al was calculated to be stronger than that of pyridine, and this was in good agreement with the experimental results.
