Quantum Chemical Calculation of Reactions Involving C20, C60, Graphene and H2O

Calculations of chemical reactions between C20, C60, hydrogen and water molecules are carried out using the PM3 method. Reactions with a hydrogen release at room temperature and atmospheric pressure are identified by the Gibbs energy change. The hydrogen release can be raised by increasing the number of water molecules in chlorine-assisted decomposition of fullerenes. Calculations of the Gibbs energy of chemical reactions involving water molecules between two parallel curved graphene sheets are carried out using DFT with the functional UB3LYP. During pumping between plates of an electric capacitor designed from curved graphene sheets, the water vapor with the assistance of external illumination is enriched by electrically neutral hydroxyl groups (OH).


Introduction
Interaction of hydrogen, oxygen and water molecules with carbon low-dimensional systems is in focus of many researchers. 1" 5 For example, graphene is often used as a photocatalyst 6 because of its unique two-dimensional conjugated structure and electronic properties. 7In addition, fullerenes and carbon nanotubes are useful in nanotechnology, biotechnology and medicine.The study of chemical reactions accompanied by the decomposition of water molecules on graphene sheets with the formation of electrically neutral hydroxyl groups (OH) 0 as bactericidal agent is relevant for applications in biomedicine.
The fullerene C20 can be effectively hydrogenated up to C20H20 unlike the fullerene C 60 , which is typically hydrogenated to C 60 H 36 molecule. 8The advantages of fullerene derivatives such as chlorinated C 60 Cl 30 for different applications are also studied. 9 N2 -Calculations of chemical reactions between C20, C60, hydrogen and water molecules are carried out using the PM3 method.Reactions with a hydrogen release at room temperature and atmospheric pressure are identified by the Gibbs energy change.The hydrogen release can be raised by increasing the number of water molecules in chlorineassisted decomposition of fullerenes.Calculations of the Gibbs energy of chemical reactions involving water molecules between two parallel curved graphene sheets are carried out using DFT with the functional UB3LYP.During pumping between plates of an electric capacitor designed from curved graphene sheets, the water vapor with the assistance of external illumination is enriched by electrically neutral hydroxyl groups (OH)0.AB -Calculations of chemical reactions between C20, C60, hydrogen and water molecules are carried out using the PM3 method.Reactions with a hydrogen release at room temperature and atmospheric pressure are identified by the Gibbs energy change.The hydrogen release can be raised by increasing the number of water molecules in chlorineassisted decomposition of fullerenes.Calculations of the Gibbs energy of chemical reactions involving water molecules between two parallel curved graphene sheets are carried out using DFT with the functional UB3LYP.During pumping between plates of an electric capacitor designed from curved graphene sheets, the water vapor with the assistance of external illumination is The purpose of this paper is to demonstrate possible reactions involving C 2 o and C 60 fullerenes, graphene, water, hydrogen and also chlorine, which provide release of hydrogen, hydroxyl groups (OH) 0 and associated gases (carbon monoxide, oxygen, chlorine) by estimating the Gibbs energy AG within the quantum chemical approaches.

Methodology
All calculations were carried out using the Gaussian 09 package. 11At each temperature in the range of 278-528K (5-255°C), the complete optimization of the reactant molecules was performed using the quasi-Newton ion relaxation implemented in the method of Broyden-Fletcher-Goldfarb-Shanno (BFGS). 12In our calculations, we used both the semi-empirical PM3 method and density functional theory (DFT) 13 with the hybrid exchange-correlation energy functional UB3LYP with three unrestricted mixing parameters and the 6-31G (d,p) basis set.Using the larger basis set, such as 6-311G (d,p), is impractical since it leads to energy changes of ~10~3 hartree per carbon atom.
To assess the possibility of a chemical reaction, the change in the Gibbs free energy AG = AH -TAS as a function of the absolute temperature T was estimated, where AH is the enthalpy change and AS is the entropy change.The enthalpy (or heat) of reactions AH is a function of temperature: where Hf is the heat of formation.A reaction can occur spontaneously at AG < 0. A negative sign for the heat of reaction (AH < 0) corresponds to an energy release, whereas a positive sign for the heat of reaction (AH > 0) corresponds to an energy absorption in the reaction.
Gibbs energy and enthalpy calculations for reactions with C 60 , C 6O Cl 2 o, C^dgo, H 2 O and other reactant molecules were carried out using the PM3 molecular orbital method.The DFT/UB3LYP method was used for reactions with graphene and water molecules, H°, (OH)", (H 2 O)", (2H 2 O)".
[The reactant (2H 2 O)~ is a dimer of two water molecules, which has a negative electric charge.].We have considered only those reactions that are chemically possible with high probability and are of practical interest.

Thermochemical analysis of reactions involving C 2 Q, C 60 and H 2 O
Changes of the Gibbs energy (AG) between products and reactants for different reactions with C 20 are shown in Fig. 1.Full saturation of C 20 with chlorine atoms is likely to occur since the addition of atomic chlorine to C 20 is energetically preferable (Fig. 1, reactions 2 and 5).By increasing the number of chlorine atoms attached to C 20 , the Gibbs energy change AG becomes lower and thus reaction 2 has the higher probability.

Quantum Chemical Calculation of Reactions
The Gibbs energy changes AG in the reactions 1-11 with fullerene C 60 are shown in Fig. 2. It is evident that all reactions 3-6 of interaction of C 60 Cl 60 with water are energetically favorable with the release of H 2 .However, reaction 1 occurs only at T > 378 K. Reactions 2 and 7 are also possible and show that chlorine atoms can be easily attached to Ceo even at room temperature.In reactions 5 and 6 (having approximately the same Gibbs energy changes AG), chlorinated C 60 fullerene is not decomposed, suggesting that the chlorine plays a role of a catalyst for hydrogen generation.From reactions 1, 3, 4 and 11, it can be seen that water can decompose C 60 derivatives with a hydrogen release.In reactions 5 and 6, hydrogen and oxygen can be released, and then oxygen can be added to fullerene (reaction 8), forming the C 60 O 2 molecule with a low probability by energy (cf.reaction 6 of C 2 o0 2 formation in Fig. 1).
Note instantly collapse (see reaction 1) in the presence of water vapor.In reactions 1 and 4, CO is released which can interact with water to form CO 2 in the reaction cycle with the release of hydrogen (reaction 9).Reaction 10 of C 6 oH 2 o production is energetically unfavorable (AG > 0) confirming ineffectiveness of C 60 fullerene hydrogenation. 8t can be seen that reactions 2-10 with participation of C 60 (Fig. 2) are exothermic, whereas reactions 1 and 11 are endothermic.In contrast, for chemical reactions with participation of C 2 o (Fig. 1), energy is released in reactions 2, 3, 5, 6 and 7, whereas reactions 1 and 4 are endothermic.
By common analysis of the both reaction series for C 2 o and C 60 , we may conclude that the larger the number of water molecules interacting with chlorinated fullerenes, the more energetically preferable hydrogen release becomes (see reactions 1 and 3 in Fig. 1 and reactions 3 and 4 in Fig. 2).The occurrence of reactions 3 and 4 (Fig. 2) is most probable in terms of energy.The destruction of fullerenes is unfavorable for technological processes, since new C 60 fullerenes are then needed in the cyclic process.Therefore, these reactions lead to possible hydrogen generation with consumption of chlorinated fullerenes.In order to increase the probability of reactions 5 and 6 that can preserve undestructed fullerenes and suppress reactions 3 and 4, it is necessary to limit the amount of water molecules reacting with chlorinated fullerenes.

Thermochemical analysis of reactions involving curved graphene and H 2 O
We propose the curved capacitor model (Fig. 3) consisting of two graphene sheets. 15The graphene sheets are connected by two electrodes to an external source of electrical voltage: a positive potential is applied to the concave graphene plate, whereas a negative potential is applied to the convex plate.
A local electric dipole moment can occur in this model on curved graphene sheets (see the bottom part of Fig. 3) due to asymmetrically extended 7r-bonds of carbon atoms (the direction of the electric moment is determined by the density of 7r-electron states, which is larger at the convex side of the graphene sheet). 16' 17 Therefore, the curved graphene surface can attract water molecules 16 and it can be considered as an element of straintronics. 18

Curved graphene
Water vapor Curved graphene Fig. 3. Top: scheme of generation of the hydroxyl group (OH) 0 between the convex and concave surfaces of graphene during water vapor injection.(A cross-section of curved graphene electrodes is shown.)Bottom: scheme of the occurrence of a local dipole moment on the graphene surface due to deformation of a-bonds (which determine the mechanical strength of graphene) and redistribution of electron density for 7r-orbitals of carbon atoms along the z-axis (perpendicular to the graphene surface).
Figure 4 shows the results of the thermochemical calculations of the Gibbs energy change AG in reactions 1-5.This is the closed cycle of reactions that are chemically possible using external electromagnetic radiation (photochemical processes) pre-generating free ions and atomic hydrogen.In accordance with our calculations (Fig. 4), reactions 2-4 are possible (AG < 0).They can be the primary reactions that lead to extraction of negatively charged hydroxyl groups (OH)~.Therefore, we present the roadmap of the main stages for production of hydroxyl group: (1) H 2 O molecule in the water vapor flow (Fig. 3) colliding with the negatively charged curved graphene sheet can acquire a negative charge.illumination.Then, reaction 1 can occur.Reaction 5 can proceed at room temperature leading to the formation of neutral water dimers (2H 2 O).These dimers (2H 2 O) can acquire a negative charge, by colliding with the curved and negatively charged cathode (Fig. 3), thereby increasing the number of negatively charged dimers (2H 2 O)~ between graphene electrodes that are necessary for initiating reaction 4. As a result, the yield of negatively charged hydroxyl groups is increased.Under the above conditions, reactions 1 and 5 proceed at temperatures close to the room temperature.
The Gibbs energy change AG is a criterion for the reaction possibility.According to the results shown in Fig. 4, in the entire temperature range under consideration reactions 2-4 are energetically permissible (AG < 0) with the release of (H 2 O) ãnd (2H 2 O)~ ions as well as H 2 and (OH)~.However, reaction 1 occurs only at T > 478 K, while at room temperature it can require some external energy (provided, e.g., by illumination).Reactions 2, 3 and 5 proceed with the release of thermal energy (AH < 0), and reactions 1 and 4, in contrast, are endothermic (AH > 0). 1940008-4

Conclusion
Possible chemical reactions in a gas mixture of fullerenes C 2 o or C 60 with water vapor leading to hydrogen release are revealed at temperatures close to 300 K. Reactions 2, 5, 6 and 7 in Fig. 2 constitute a closed cycle of reactions in which chlorine acts as a catalyst for hydrogen generation with low consumption of C 60 fullerenes.In reactions 1, 2, 3, 5 in Fig. 1 and 1, 3, 4,11 in Fig. 2, fullerenes C 2 o and C 60 can be decomposed with the hydrogen release.The larger the number of water molecules involved, the higher is the probability of the hydrogen release.
Reactions 1-5 in Figs. 3 and 4 constitute the closed cycle of reactions in which negatively charged hydroxyl groups (OH) ~ can be produced.Reactions 3 (with the heat release) and 4 (with the heat absorption) are main reactions resulting in the formation of (OH)~ ions at room temperature.Reactions 1 and 5 are possible at room temperature with an assistance of external illumination.Electric neutralization of (OH) ~ groups can be achieved by their contact with the positively charged graphene electrode.
Reaction 7 (hydrogenation of C 20 ) can occur at T < 1200 K (see also Ref. 14).Reaction 4 (interaction of C 20 H 20 and water with the release of H 2 ) is also possible.Figure 1 also shows that reaction 1, accompanied by reaction 2 of C 2O Cl 2 o synthesis, can occur where C 20 Cl 20 interacts with water (20H 2 O) leading to the release of CO, H 2 and Cl 2 .The hydrogen yield (20H 2 ) is lower than that in reaction 4 (30H 2 ) with hydrogenated fullerene C 20 H 20 .Reaction 3 is shown for comparison indicating the higher probability of water decomposition (40H 2 O) by chlorinated fullerene C 20 Cl 20 with higher hydrogen yield (40H 2 ).