Dr
Miroslav Rubes
(IOCB, AS, CR)
The carbon monoxide is a very sensitive IR probe molecule, and thus is quite often used to characterize adsorption sites in various materials [1]. In zeolites, the calculated CO frequency shifts are in good agreement with experimental observations[2]. However, the CO energetics (e.g. isosteric heats) still presents a challenge for computational chemistry due to the dynamical nature of the adsorption process. Experimentally, we observe about 6-7 kJ/mol difference in isosteric heats at 200 K and 300 K, respectively. This difference cannot be explained by using simple thermodynamics models and molecular dynamics (MD) simulations need to be performed. It is straightforward that the accuracy of the employed potential is of utmost importance. We developed an updated DFT/CC [3] methodology to calculate very accurate interaction energies with siliceous framework and Brønsted acid sites. The DFT/CC model was verified with “golden” standard CCSD(T)/CBS on cluster models (up to 4T) and RPA/RSE calculations on several H-FER and siliceous FER structures. The temperature effects were calculated from up to 80 ps MD trajectories for each T-position in H-FER material. Furthermore, the MD simulations indicate significant differences in isosteric heats for different T-positions in H-FER material at 200 K. This effect is partially diminished at 300 K due to the formation of less stable OC-complexes and desorption from the Brønsted site.
Summary
The CO energetics (e.g. isosteric heats) still presents a challenge for computational chemistry due to the dynamical nature of the adsorption process. Experimentally, we observe about 6-7 kJ/mol difference in isosteric heats at 200 K and 300 K, respectively. This difference cannot be explained by using simple thermodynamics models and molecular dynamics (MD) simulations need to be performed. The accuracy of the underlying computational model is of the utmost importance to interpret the experimental observations.
References
[1] Nachtigall, P.; Bulánek, R. Appl. Catal. A 2006, 307, 118–127.
[2] Bludský, O.; Šilhan, M.; Nachtigallová, D.; Nachtigall, P. J. Phys. Chem. A 2003, 107, 10381–10388.
[3] Bludský, O.; Rubeš, M.; Soldán, P.; Nachtigall, P. J. Chem. Phys. 2008, 128.
Mr
Michal Trachta
(IOCB, AS, CR)
Dr
Miroslav Rubes
(IOCB, AS, CR)
Dr
Ota Bludsky
(IOCB, AS, CR)
