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

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+] < [Cun + 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.