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Toward Accurate and Efficient Predictions of Entropy and Gibbs Free Energy of Adsorption of High Nitrogen Compounds on Carbonaceous Materials

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journal contribution
posted on 06.03.2014, 00:00 by Andrea Michalkova Scott, Leonid Gorb, Elizabeth A. Burns, Sergey N. Yashkin, Frances C. Hill, Jerzy Leszczynski
The adsorption of high nitrogen compounds (HNCs) on the selected adsorption sites of carbonaceous materials from the gas phase has been investigated by ab initio quantum chemical methods at the density functional level applying both periodic and cluster approaches with M06-2X and BLYP functionals including dispersion forces (BLYP-D2). Among the possible structures of the adsorption complexes, the most stable systems possess nitrogen-containing heterocycles in a parallel orientation toward the modeled carbon surface. The adsorption enthalpies, calculated using the rigid rotor-harmonic oscillator approach (RRHO), were in good agreement with available experimental data. This approach was shown to provide sufficiently accurate adsorption enthalpies from the gas phase for the HNC–carbon systems. The vibrational, rotational, and translation contributions to the adsorption entropy were also analyzed by the approach extended beyond the RRHO scheme. The effects of anharmonic vibrations and internal rotations of the adsorbate on the adsorption sites of the modeled carbon surface were estimated. The Gibbs free energies calculated using the RRHO approach were adjusted to take into account the heterogeneity of the carbon surfaces and underestimation of the adsorption enthalpies at the BLYP-D2­(PBC) level. The corrected Gibbs free energy values of adsorption are negative for all of the investigated HNC–carbon systems, and they agree well with available experimental data. This suggests an effective adsorption of selected high nitrogen compounds on carbonaceous materials from the gas phase at 298.15 K. Partition coefficients for distribution of high nitrogen compounds on modeled carbon surfaces were also predicted in good agreement with the experimental results.

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