Mapping Vibrational Spectra to the Structures of Copper Species in Zeolites Based on Calculated Stretching Frequencies of Adsorbed Nitrogen and Carbon Monoxides

Abstract

In this work, adsorption of nitrogen monoxide (NO) and carbon monoxide (CO) probe molecules on various copper sites in a range of zeolites is studied. The structures of copper sites, binding energies, and vibrational frequencies of adsorbed probe molecules are calculated using density functional theory (DFT). This allows mapping vibrational spectra regions to specific copper species as a function of the zeolite topology and Si/Al ratio. CO can adsorb on Cu⁺ions by forming mono- and dicarbonyls or on copper ions bonded to methoxy species by forming methoxy-monocarbonyls, which exhibit a blue shift in wavenumbers. The stretching frequencies of adsorbed NO generally increase in the following order: [CuOH]⁺< [Cu₂O]²⁺/[Cu₂O₂]²⁺< [Cu²⁺] < [Cu_n + 1On]²⁺/[CunOn]²⁺(n> 3) < [Cu₃O₂]²⁺/[Cu₃O₃]²⁺. The shift values between different species vary between 5 and 20 cm^-1, showing the possibility for structure assignment based on infrared frequencies. Zeolite frameworks with smaller pores exhibit a shift of vibrational bands of adsorbed NO toward lower frequencies because of the confinement effect of the zeolite pore structure. Zeolites with larger pores stabilize the copper species of higher nuclearity. Our data indicate that the tabulated infrared frequencies of adsorbed CO and NO may be used to assign zeolitic copper speciation from experimental data.