Interaction Of Water And Methanol Molecules With Carboxyl Groups At The Oxidised Graphite Surface
Interaction of Water and Methanol Molecules with carboxyl groups at the Oxidised Graphite Surface
E.V. Aksenenko, Yu.I. Tarasevich
Institute of Colloid Chemistry and Chemistry of Water, Ukrainian National Academy of Sciences, Vernadsky avenue 42, 03680 Kyiv, Ukraine,
e-mail: eugene@thomascat.kiev.ua
Carboxy (−COOH) groups are the main oxide centres which exist at the surface of carbon materials [1]. However, no detailed information is available about the microscopic structure of the surface at which these centres are located; this is mainly due to the lack of the experimental method capable of providing this information. The assumptions about the atoms which constitute these centres, the microscopic geometry of a local atomic cluster which represents the centre neighbourhood etc. could be guessed only on the basis of the information concerning the preparation of the adsorbent. In this situation, one of possible approaches is the modelling of the adsorption system consisting of the portion of adsorbent containing the adsorption centre and adsorbed molecule(s) using quantum chemical methods. The results of such modelling, being compared with the experimental values of the thermodynamic functions of adsorption, could provide a reliable guide for the understanding of the structure and properties of adsorption systems.
To perform the quantum chemical studies of the interaction of water and methanol molecules with the oxidised graphite surface, this surface was simulated by a portion (cluster) of the edge of one or three two-dimensional hexagonal graphite lattice sheet(s) with the adsorption centre formed by the attachment of the carboxyl group to one of the boundary carbon atoms. The PM3 approximation [2] was employed. Calculations were performed using the MOPAC-2000 package [3].
The enthalpy of adsorption of one water molecule at the carboxyl centre was found to be −24.9kJ/mol; two hydrogen bonds are formed between the adsorption centre and the adsorbed molecule. When two water molecules are adsorbed, each of them forms one hydrogen bond with the adsorption centre; also the attraction interaction exists between the molecules, leading to the fact that the adsorption enthalpy per second molecule (−24.1kJ/mol) is lmost equal
to the enthalpy of the adsorption of single water molecule. The third water molecule is located in the second coordination hemisphere of the adsorption centre, and interacts mainly with the two molecules adsorbed in the vicinity of the centre; the enthalpy of the adsorption of the third molecule (−15.1kJ/mol) is almost twice as lower as the mean enthalpy of the adsorption per molecule. Note that the enthalpy of the water molecule adsorption at the graphite sheet edge involving no carboxyl group is still lower (−11.3kJ/mol).
The methanol molecule, adsorbed at the carboxyl centre (adsorption enthalpy −19.8kJ/mol) also forms two hydrogen bonds with the adsorption centre. The enthalpy of the adsorption of the second methanol molecule, −0.8kJ/mol, is even lower than the value for the adsorption at the model graphite sheet edge, −8.2kJ/mol. Unlike water, the second methanol molecule, being adsorbed, does not form any specific bonds with the first molecule.
The results presented above lead to an important practical conclusion. To determine the concentration of hydrophilic centres at the surface of coal adsorbents, it was proposed before (see e.g. [4]) to perform the measurements of water adsorption in the relative pressure range p/pS
Acknowledgements: This study was performed in part under the INTAS grant 00-505. Also, the support from Dr. hab. R.Miller (Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Golm, Germany) with the computation facilities and software [3] is very much appreciated.
References
1. H.P.Boehm, in: D.D.Eley, H.Pines, B.Weisz (Eds) Advances in Catalysis, Academic Press: New York/London, 1966, vol. 16, p. 179.
2. J.J.P.Stewart,J. Comput. Chem. 10(1989)209.
3. J.J.P.Stewart, MOPAC 2000 Manual, Fujitsu Ltd: Tokyo, 1999.
4. R.Sh.Vartapetyan, A.M.Voloshchuk, Uspekhi Khimiji 64(1995)1055.
5. Yu.I.Tarasevich,A.I.Zhukova,E.V.Aksenenko, S.V.Bondarenko, Adsorp. Sci. Technol. 15(1997)497.
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