Adsorption and Dissociation of Ammonia on Graphene Oxides: A First-Principles Study

The interactions of ammonia with graphene oxides (GO) were studied by density functional theory calculations. Our results indicate that the adsorption of NH3 on GO is generally stronger than that on graphene because of the presence of diverse active defect sites, such as the hydroxyl and epoxy functional groups and their neighboring carbon atoms. These surface oxygen sites can form OH center dot center dot center dot N and O center dot center dot center dot HN hydrogen bonds with NH3 and enhance charge transfers from NH3 to the graphene oxide. The dissociation of the adsorbed NH3 into the chemisorbed NH2 or NH species through the H atom abstractions leads to hydroxyl group hydrogenation and ring-opening of epoxy group. The reactions of NH3 with the hydroxyl and epoxy groups are predicted to be exothermic with different energy barriers, depending on the oxidation species and the atomic arrangement of these groups. The hydroxyl group exhibits relatively higher reactivity toward hydrogen abstraction from the adsorbed NH3 than the epoxy group in GO with a single oxygen group. The presence of a neighboring OH group may activate the oxygen groups to facilitate the surface reaction of NH3. Followed by the ring-opening of the epoxy group, the newly formed hydroxyl group can be removed by the second H atom abstraction from NH2. The calculated density of states of the adsorbed systems also reveals strong interactions between GO and NH3. The calculated results show good agreement with available experimental observations.