MOM 2006 journal for pdf.pmd - University of Michigan-Flint

The relative energies of the NO adsorption complexes at the HF level show little variation among
all the clusters (Table 4). There seems to be an anomaly with the 5T –OH HF energies. Looking at the
DFT results, complex H is not a stable structure. Therefore the energy difference between complexes
F and G is in the same range as the rest of the clusters. The calculated energies at the HF level
indicate that complex F is the most stable and, therefore, the most likely adsorption type. The G and
H complexes are relatively closer in energy. DFT energies, however, differ significantly from the HF
energies. Most of the clusters still indicated complex F as the most stable, and G is significantly
higher in energy than H. The substantial differences in energies at the DFT level demonstrate the
importance of electron correlation in the calculations. The 5T –H cluster provides contradictory
results, though. The trend continues, however, with –OH termination. This seems to indicate that
hydrogen termination is not as accurate a model of zeolite as the hydroxyl terminated clusters. Given
the DFT energies, complex G is clearly the least stable of the adsorption complexes. The cluster size
also makes the difference in the DFT calculations. In general, the cluster models indicate that the F
complex is the most stable, followed by the G complex. When the cluster size reaches 5T and 10T,
the H structure can no longer be obtained. This indicates that the H structure is not a minimum on the
potential energy surface. As such, this might be a reasonable initial guess for a transition state, though
that is not the focus of this study.
Table 4: Relative Energies ( kcal mol) of the NO adsorption complex
1T 3T -H 3T -OH 5T -H 5T -OH 10T
HF
F 0.0 0.0 0.0 0.0 18.41 0.0
G 5.52 4.27 3.94 3.86 21.96 3.06
H 4.56 6.34 7.99 6.77 0.0 -
DFT
F 0.0 0.0 0.0 4.45 0.0 0.0
G 28.49 24.89 25.06 19.46 21.39 22.33
H 6.55 4.89 4.96 0.0 - -
Summary
The most obvious conclusion from this data is that the cluster size has a significant impact on the
energies of the complexes as well as on the geometries. The H complex is a local minimum with
smaller clusters but not in the large clusters. The 5T–H cluster provides results that are contrary to the
general trends. This data displayed the H complex as the most stable, while F was the most stable in
all other cases. Additionally, the NO 2
complexes are also greatly influenced by the cluster size. The
A, B, and D complexes occur with certain clusters and not with others. The reason for this is as yet
unclear. In all cases, though, the DFT calculations indicate that the E complex is the most stable NO 2
complex.
In addition to the effects of the cluster size, the level of theory also impacts the results. Electron
correlation from the DFT calculations contributes to a large increase in the relative energy of the G
complex. From HF energies alone, it is not clear whether the G or H complex is more stable. DFT
results, however, show a clear difference in energy between these two adsorption complexes.
Complex G, then, is clearly the least stable complex, while H is not a local minimum, and thus not a
stable adsorption complex, in the large clusters. The HF energies also do not clearly indicate the most
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