A theoretical study of the CO and NO chemisorption on Cu2O(111) (original) (raw)
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The Journal of Physical Chemistry B, 2005
Interactions of the CO and NO molecules with the Cu(II) and Cu(I) isolated sites on the amorphous silica surface are investigated by means of density functional theory (DFT) methods within the finite cluster model approach. The clusters of silica of increasing nT size (T = Si) are used, with n from 2 to 6. The Cu(II) sites are characterized by calculated g-tensors and hyperfine coupling constants (HFCCs) and compared with experiment. On this basis, the three-coordinated complexes are the most plausible. Due to the charge transfer from the silica the metal charge shrinks and the spin density is distributed over silanol and siloxy groups up to 50%. The reduced sites are exclusively two-coordinated. Strong interaction of CO with Cu(I)-nT sites (31-39 kcal/mol) gives rise to the formation of carbonyl adducts with planar coordination around copper. The population of the ligand pi system shifts downward the stretching frequency in agreement with experiment. Reaction with a second CO molecule gives a geminal dicarbonyl of very uniform structure independent of the site. Carbonyl complexes with Cu(II) are less stable and of tetrahedral coordination of the metal. Accumulation of the positive charge on the complex along with sigma overlap with d orbitals locates the calculated CO stretching frequency above free molecule value. NO molecule is preferably bound to the Cu(II)-nT sites, forming a tetrahedral complex with tilted adsorbate and NO stretching frequency blue-shifted with respect to the free molecule value. The full set of electron paramagnetic resonance (EPR) parameters and vibrational frequencies for the copper(I) mononitrosyl, {CuNO}(11), though not observed experimentally, are predicted and compared to the same magnetophore inside the ZSM-5 zeolite. The interaction energies show that in the CO/NO reaction mixture adsorption is selective and allows discrimination between Cu(I) and Cu(II) sites. However, for the Cu(I) complex, formation of mixed-ligand structures of the {Cu(CO)(NO)}(11) type is possible.
CuO Surfaces and CO 2 Activation: A Dispersion-Corrected DFT+U Study
We have used computational methodology based on the density functional theory to describe both copper(I) and copper(II) oxides, followed by the investigation of a number of different low index CuO surfaces. Different magnetic orderings of all the surfaces were studied, and reconstructions of the polar surfaces are proposed. A detailed discussion on stabilities, electronic structure, and magnetic properties is presented. CuO(111) and CuO(111) were found to have the lowest surface energies, and their planes dominate in the calculated Wulff morphology of the CuO crystal. We next investigated the adsorption of CO 2 on the three most exposed CuO surfaces, viz., (111), (111), and (011), by exploring various adsorption sites and configurations. We show that the CO 2 molecule is activated on the CuO surfaces, with an adsorption energy of −93 kJ/mol on the (011) surface, showing exothermic adsorption, while (111) and (111) surfaces show comparatively weak adsorption. The activation of the CO 2 molecule is characterized by large structural transformations and significant charge transfer, i.e., forming a negatively charged bent CO 2 −δ species with elongated C−O bonds, which is further confirmed by vibrational analyses showing considerable red shift in the frequencies as a result of the activation.