Mechanism of ammonia retention on graphite oxides: Role of surface chemistry and structure

Graphite oxide (GO) was synthesized from commercial graphite and modified by calcinations at 350 degrees C. The samples were used as adsorbents of ammonia at dry and moist conditions. Their surface before and after exposure to ammonia was characterized using adsorption of nitrogen, XRD, SEM, FTIR, TA, CHN analysis, and potentiometric titration. The results showed that oxidation results in the incorporation of a significant amount of epoxy, phenolic, and carboxylic groups, which is directly manifested by a decrease in surface pH. The majority of the latter is removed by calcinations. During this process, the exfoliation of graphite occurs, resulting in the formation of some micro- and mesporosity. The materials obtained have proven to be excellent adsorbents of ammonia. The NH3 reacts with carboxylic acids and is intercalated between layers or dissolved in pores with adsorbed water. The mechanism depends on the conditions of experiments and surface features of the samples. The highest capacity is obtained on GO on which a significant amount of water is adsorbed, likely on functional groups. This helps in the dissociation of carboxylic groups and their acid-base reactions with ammonia. Nevertheless, a significant amount of ammonia is intercalated between layers. Those two interactions are the strongest. When water is present in the challenge gas, the amount adsorbed decreases because of the competition between water and ammonia for active sites.