Interaction of Carbon Monoxide with Small Gold Clusters Inside TS-1 Pores

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We report a two-layer, quantum mechanics/molecular mechanics analysis of CO adsorption on Au 1 - 5 clusters located near T6-Ti-nondefect, T6-Si-nondefect, T6-Ti-defect, and T6-Si-defect sites in TS-1, augmented with thermochemistry at 298.15 K and 1 atm. The BPW91/LANL2DZ:UFF level of theory overestimates the adsorption energies compared to the BPW91/LANL2DZ/6-311+G(d, p):UFF level. In all the cases cluster−CO geometries are planar, and CO adsorption is thermodynamically favorable. The adsorbed CO is anionic, and the red-shift in the C−O stretching frequency, due to back-donation of electron density to the 2π* antibonding orbital in CO, is consistent with the elongation of the C−O bond. The Mulliken charge on adsorbed CO versus Δ E ads of CO shows approximate linear correlation when both the quantities are plotted on a per Au atom basis, suggesting that the charge-transfer to adsorbed CO contributes significantly to Δ E ads . The total charge on the cluster−CO adduct matches almost quantitatively with the negative charge on bare (without CO) Au clusters adsorbed inside the TS-1 pores. We found the following interesting charge-transfer mechanism for CO adsorption on Au clusters. The TS-1 lattice loses electron density to adsorbed Au clusters. Upon exposure to CO, these clusters donate their entire excess negative charge to the 2π* antibonding orbital in CO and acquire somewhat cationic (and in some cases only slightly anionic) character. The already electron-deficient TS-1 framework (containing Au clusters) does not lose any additional electron density upon exposure to CO. We also advance the possibility of using CO to probe supported, few-atom Au clusters. Concurrent use of IR spectroscopy will be helpful in deconvoluting CO adsorption on Au from that on the support, which adsorbs CO relatively weakly. The calculated trend in the CO stretching frequency (υ CO ) is as follows: υ CO,Au 5 < υ CO,Au 1 < υ CO,Au 3 < υ CO,Au 2 ∼ υ CO,Au 4 < υ gas-phase-CO ≤ υ CO,TS-1 . Since adsorption of a second CO molecule on all the Au clusters except Au 1 is significantly weaker than that of the first CO molecule, CO uptake data as a function of temperature and CO pressure, along with IR analysis, would be needed to characterize the cluster-size distributions.