Abstract
Density functional theory is employed to model the chemisorption of ammonia on epoxy-containing polycyclic aromatic hydrocarbons (PAHs) and understand the reaction mechanism of ammonia addition on partially reduced graphene oxide flakes. Coronene (C24H12) and ovalene (C32H14) based four-epoxy group containing molecules are used to mimic the RGO surface properties. The reaction mechanism changing effect of a second ammonia molecule as well as explicit water molecules is considered. The proposed reaction mechanism consists of two steps: the migration of one epoxy group out of the modelled four-epoxy group formation to a thermodynamically less stable one and the nucleophilic addition of the ammonia molecule. The second step involves forming an amine group and reducing an epoxy group to a hydroxyl one. Interestingly, the forming amine group bonds to the carbon atom with the smallest bond order among the available ones and not necessarily to the carbon atom of the opening epoxy ring. Incorporating a second ammonia molecule has a negligible effect on the overall reaction mechanism, while in the presence of one water molecule, the reaction goes through a different pathway involving a trimolecular state during the nucleophilic addition. Including more than one water molecule or applying an implicit solvent model does not cause further changes in the reaction.
Highlights
IntroductionGraphene has initiated great research activities due to its beneficial properties, such as large surface area, high chemical stability, and novel electronic properties.[1,2,3,4,5,6] An effective route to tune graphene’s properties is chemical modification, which enables us to use it for potential applications, including graphene nanoribbon field-effect transistors,[7,8] composite materials,[9] adsorbents and sensors.[10,11,12,13,14] Previous studies show that sp[2] carbon-based nanomaterials and their derivatives could react with nitrogen-containing molecules such as NO2 and NH3, resulting in a local change in carrier concentration, after which physical properties are changing.[15] only weak adsorption was found when they reacted with pristine graphene, a growing amount of defects and active sites on the surface effectively improved the adsorption.[14,16] Graphene oxides (GOs), containing oxygen-based active sites, are essential precursors of graphene preparation
We investigated the possible reactions of ammonia on a partially reduced graphene oxide model by optimising possible reaction pathways for three basic ammonia-epoxy nucleophilic substitution scenarios on an ovalene based fourepoxy group-containing polycyclic aromatic hydrocarbon
Instead of the supposed one-step nucleophilic substitution between the partially reduced graphene oxide model and the ammonia, the overall reaction mechanism consists of two steps: the migration of an epoxy group and the nucleophilic addition of the ammonia molecule
Summary
Graphene has initiated great research activities due to its beneficial properties, such as large surface area, high chemical stability, and novel electronic properties.[1,2,3,4,5,6] An effective route to tune graphene’s properties is chemical modification, which enables us to use it for potential applications, including graphene nanoribbon field-effect transistors,[7,8] composite materials,[9] adsorbents and sensors.[10,11,12,13,14] Previous studies show that sp[2] carbon-based nanomaterials and their derivatives could react with nitrogen-containing molecules such as NO2 and NH3, resulting in a local change in carrier concentration, after which physical properties are changing.[15] only weak adsorption was found when they reacted with pristine graphene, a growing amount of defects and active sites on the surface effectively improved the adsorption.[14,16] Graphene oxides (GOs), containing oxygen-based active sites, are essential precursors of graphene preparation. Above a certain amount, it reduces the accessibility of epoxy and –COOH groups, and limits the amount of ammonia adsorbed.[21]
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