CO adsorption complexes in zeolites: How does the inclusion of dispersion interactions affect predictions made from DFT calculations? The case of Na-CHA
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Density functional theory (DFT) calculations have played a pivotal role in identifying and understanding different coordination modes of carbon monoxide adsorbed in zeolites: Previous studies combining IR spectroscopic measurements and DFT have firmly established that an adsorbed CO molecule can interact either with a single cation (single-site interaction), or with two or more cations simultaneously (dual-site or multiple-site interaction). However, one aspect that has been scarcely addressed so far is the dependence of the DFT equilibrium structures on the choice of the functional. With the ongoing development of DFT, exemplified by the more widespread use of dispersion-corrected DFT, this question becomes increasingly relevant. The present study investigates whether the inclusion of an empirical dispersion correction leads to qualitatively different predictions in comparison with dispersion-uncorrected DFT, taking CO adsorbed in sodium-exchanged chabazite having two different Si/Al ratios (Si/Al = 11:1 and Si/Al = 2:1) as a model system. Equilibrium structures obtained with the PBE functional and with the dispersion-corrected PBE-D functional are compared, revealing a tendency of dispersion-corrected DFT to favour a stronger interaction of CO with dual sites. This is indicated by a short contact between the oxygen atom of the CO molecule (already coordinated through its carbon atom to a primary Na+ cation) and a secondary Na+ cation. In addition to these qualitative findings, the quantitative agreement of calculated adsorption enthalpies and C–O stretching frequencies with experimental values obtained from variable-temperature IR spectroscopy is evaluated. While neither functional is particularly successful in predicting accurate adsorption enthalpies, the range of C–O stretching frequency values delivered by the PBE-D functional shows a better agreement with the experimental measurements.
KeywordsDensity functional theory Dispersion correction Zeolites Adsorption Carbon monoxide VTIR spectroscopy
M. Fischer acknowledges a postdoctoral fellowship by the German Research Foundation (DFG Grant Fi 1800/1-1), as well as funding by the Central Research Development Fund (CRDF) of the University of Bremen (Funding line 04 – Independent Projects for Post-Docs). M. Fischer is indebted to Dr. Rolf Arvidson and Prof. Andreas Lüttge (Marum) for generous access to the Asgard cluster and further technical support, and to Rob Bell for many insightful discussions, particularly during the initial stages of this project. The Programa Pont “La Caixa” (2014) is gratefully acknowledged for financial support to the work done at the UIB.
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