Cu exchanged SSZ-13 is one of the most promising catalysts in the
selective catalytic reduction of NOx using ammonia. However, the true
nature of the active sites under realistic conditions (i.e. finite
temperature and presence of water) is still not fully understood. In
this work we explore these effects using ab-initio molecular dynamics
calculations and thermodynamic analysis.
A key technique in understanding the chemical activity of transition
metal centers in zeolites is infrared spectroscopy of adsorbed tracer
molecules. In this work we model vibrational spectra of CO adsorbed to
Cu(I) sites and NO adsorbed to Cu(II) sites in SSZ-13 using molecular
dynamics calculations. For both cases finite temperature induces a
movement of the cation. For NO, changes in the Cu-N-O angle lead to
additional accessible local minima. A combination of those two
phenomena leads to complex multi-peak vibrational spectra for one
molecule adsorbed to one active site, which then allows a clear
assignment of experiment and the identification of the distribution of
active sites present in realistic systems. Linking this distribution
to experimental activity measurements furthermore allows the
identification of active and inactive species for the given reaction.
Under realistic conditions also water is present, which leads to a
reconstruction of the active sites. We use thermodynamic analysis to
imitate realistic conditions. Interestingly not all active sites
reconstruct similarly, i.e. not the same amount of water molecules are
adsorbed to them under the same conditions. This allows closely
reproducing experimentally observed Cu-O vibrations and their
variation with hydration.
The results in this work suggest that under reaction conditions Cu
cations are hydrated and finite temperature leads to regular changes
of coordination. We expect that these two effects lead to significant
changes for reaction paths and barriers, which explains the
experimentally observed catalytic behavior.