Adsorption of NO on Cu-SAPO-34 and Co-SAPO-34: A Periodic DFT Study

The location of Cu(I), Cu(II), and Co(II) cations at the extraframework sites of metal cation exchanged SAPO-34 as well as their adsorption properties toward NO are studied by periodic DFT calculations with the PW91 functional. The relative stability of configurations with variable positions of the Si → P substitutions and cation sites was examined; structures with the divalent cation located in proximity to closely spaced negative framework charges were of highest stability. The highest adsorption capacities of the divalent Cu and Co cations were determined for an environment with Si → P substitutions separated in the unit cells, but the interaction between the cation and the more distant negative charges is important for the formation of a stable adsorption complex. The Cu(I)−NO configuration in SAPO-34 is bent, with an angle ∠Cu(I)−N−O of 141.0°. This enables an interaction of the dz² state of the cation with both the σ and π* molecular orbitals. Backdonation to the π* state leads to a red-shift of the NO stretching frequency by ∼100 cm^-1, in good agreement with experiment. For divalent metal cations Cu(II) and Co(II), the highest adsorption energies are related to wider ∠M(II)−N−O angles, shorter M(II)−N bonds, and a slightly weakened N−O bond, indicating a significant electrostatic contribution (ion-dipole interactions) to the bonding and a much less pronounced d_π → π* back-donation, as reflected by an almost unchanged NO stretching frequency.