Can Dynamics Be Responsible for the Complex Multipeak Infrared Spectra of NO Adsorbed to Copper(II) Sites in Zeolites?

Copper-exchanged SSZ-13 is a very efficient material in the selective catalytic reduction of NO_x using ammonia (deNO_x-SCR) and characterizing the underlying distribution of copper sites in the material is of prime importance to understand its activity. The IR spectrum of NO adsorbed to divalent copper sites are modeled using ab initio molecular dynamics simulations. For most sites, complex multi-peak spectra induced by the thermal motion of the cation as well as the adsorbate are found. A finite temperature spectrum for a specific catalyst was constructed, which shows excellent agreement with previously reported data. Additionally these findings allow active and inactive species in deNO_x-SCR to be identified. To the best of our knowledge, this is the first time such complex spectra for single molecules adsorbed to single active centers have been reported in heterogeneous catalysis, and we expect similar effects to be important in a large number of systems with mobile active centers. Theory and practice: The IR spectra of NO adsorbed on Cu centers in a copper-containing zeolite were modeled using molecular dynamics simulations. The spectra are complex, which is due to the thermal motions of the cations and the adsorbates, and are in excellent agreement with the experimental spectra.