Theoretical Study on Carbon Monoxide Adsorption on Unsupported and γ‑Al₂O₃‑Supported Silver Nanoparticles: Size, Shape, and Support Effects

Nanoparticles (NPs) supported on metal oxides exhibit high catalytic activities for various reactions. The shape and oxidation state of such NPs, which are related to the catalytic activity, are often determined by the support. Herein, we conducted a density functional theory study on isolated silver (Ag) NPs and two types of Ag-NPs supported on gamma-aluminum oxide (γ-Al₂O₃). First, carbon monoxide (CO) adsorption on the isolated Ag NPs was investigated for decahedra (D_5h), icosahedra (I_h), and cuboctahedra (O_h) of various sizes. I_h and O_h NPs showed moderate size dependence, whereas D_5h NPs showed high size dependence when the height was below 1.4 nm. The enhancement of CO adsorption on D_5h NPs was attributed to the presence of superatomic states. Next, we performed geometrical optimization of Ag₅₄/γ-Al₂O₃(110) with a decahedral shape. Two types of structures were obtained: amorphous Ag₅₄(A) and locally fivefold symmetrical Ag₅₄(B) structures. Both NPs on γ-Al₂O₃(110) were found to be positively charged, but electron transfer to the support occurred only from the Ag atoms at the two bottom layers, and the upper part of NPs was relatively neutral. The enhancement of CO adsorption on Ag₅₄(B) disappeared due to loss of the high symmetry. In turn, the moderate size dependence of neutral isolated NPs can be applied.