Adsorption of Copper Clusters in TS-1 Pores:  Ti versus Si and Gold versus Copper

We present a quantum mechanics/molecular mechanics (QM/MM) analysis of interaction of Cu_1-5 clusters with nondefect and metal-vacancy defect versions of T6-Ti and T6-Si sites in TS-1. The clusters inside the TS-1 pores are significantly stabilized due to long-range interactions such as van der Waals forces. We predict stronger adsorption of Cu clusters on Ti sites versus that on Si sites and on defect sites versus that on nondefect sites. A graph of adsorption energy (ΔE_ads) versus cluster size (n) shows a peak around n = 3. While these findings are consistent with previous results on Au clusters, we have identified significant differences in the Au/TS-1 and Cu/TS-1 systems, especially in the charge-transfer interactions. Moreover, the binding configurations and cluster−support interactions are different in Cu/Ti and Cu/Si systems. While Cu_1-5 clusters adsorbed on Si sites are negatively charged, the Cu₃ and Cu₅ clusters adsorbed on Ti sites are significantly positively charged. Due to relatively low ionization potential, these clusters lose electron density to cationic Ti sites through negatively charged lattice oxygens. The attractive coulomb interactions between positively charged Cu atoms and negatively charged lattice oxygens are consistent with the X₂O₂ and X₃O₃ (X_mO_n:  m metal atoms and n oxygen atoms) binding configurations of Cu₃/Cu₅ clusters adsorbed on Ti sites, which make the lattice-oxygen-mediated charge-transfer energetically favorable. The X₁O_n (n = 1 or 2) binding configurations are preferred by the Cu_1-5 clusters adsorbed on Si sites and also by the Au_1-5 clusters adsorbed on both Ti and Si sites. Because the Au−O interactions are weaker than the Cu−O interactions, the X₂O₂ and X₃O₃ binding configurations are not preferred by the Au₃/Au₅ clusters adsorbed on Ti sites. The differences in cluster−support interactions in Cu/Ti versus Cu/Si and Au/Ti versus Cu/Ti systems can be explained on the basis of geometric and electrostatic effects.