Role of aluminum oxycations in retention of ammonia on modified activated carbons

Highly porous wood-based activated carbon was modified by impregnation with Keggin Al-13 oxycations and calcined at 300 degrees C. This resulted in the formation of new acidic centers highly dispersed on the surface. The carbons were used as adsorbents of ammonia from the gas phase with different contents of water either on the carbon surface or in challenge air. The materials before and after exposure to ammonia were characterized using adsorption of nitrogen, potentiometric titration, XRF, XRD, FTIR, and thermal analysis. Even though modification with oxycations significantly increased the ammonia removal capacity of the carbons, a significant part of ammonia was weakly adsorbed and easily desorbed from the surface by air purging. The presence of water and Bronsted acidic centers favored ammonia binding on the surface as NH4+ ions. Alternatively, on the surfaces of calcined samples, where Lewis centers were formed, the interaction of ammonia via lone pairs of electrons with these centers seems to be the important mechanism of adsorption. This kind of interaction is apparently weaker than the Bronsted interaction.