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

1T 3T 5T
Figure 1: The cluster models (X = H or OH)
10T
Results
There are five possible ways for NO 2
to adsorb on Cu-ZSM-5 (Figure 2). The first possibility is for
nitrogen to bind to copper (A). Another possibility is for one of the oxygen atoms to bind to copper (B
and C). In the B structure the second nitrogen is in line with the Cu-O bond, while in the C structure
the second oxygen is located 90ˆ from the Cu-O bond. Another possibility is for both one of the
oxygen atoms and the nitrogen to bond to copper forming a Cu-O-N ring (D). Finally the last
possibility is for both of the oxygen atoms to bind to copper, thus forming a Cu-O-N-O ring (E).
Since NO 2
is a polar molecule and the dipole moment points to N from O, different binding types
should affect the bonding in NO 2
and the relative stabilities of the different adsorption complexes.
The HF method was used to obtain an initial optimization of the geometry and orbitals for each
adsorption complex. To include the correlation effects, optimizations at the DFT level were then
performed using the HF results as a starting point. The optimized DFT geometries are listed in Table 1
and the relative energies are listed in Table 2.
Although there are five possible structures, only 2 or 3 (depending on the cluster size) are stable
adsorption complexes, ie. they represent local minima on the potential energy surface. The DFT
results indicate that the E structure is the most stable adsorption complex regardless of the cluster size.
The A, B, and D structures occur as local minima, though never all three with the same cluster. The C
structure, however, never occurs as a local minimum. This is because the lone pairs of electrons on
oxygen in this structure are in orbitals, whose orientation is not favorable for this complex. If the C
structure were to exist, there would be a large strain on the Cu-O bond. If this strain were relieved by
the reduction of the Cu-O-N bond angle, then the other oxygen in NO 2
would nearly overlap with an
oxygen atom in the zeolite. Both of these situations are energetically unfavorable. Thus the C
structure cannot be a stable complex.
Meeting of Minds 2006 71