Theoretical Study on Carbon Monoxide Adsorption on Unsupported and γ-Al 2 O 3 -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-Al 2 O 3 ). First, carbon monoxide (CO) adsorption on the isolated Ag NPs was investigated for decahedra ( D 5 h ), icosahedra ( I h ), and cuboctahedra ( O h ) of various sizes. I h and O h NPs showed moderate size dependence, whereas D 5 h NPs showed high size dependence when the height was below 1.4 nm. The enhancement of CO adsorption on D 5 h NPs was attributed to the presence of superatomic states. Next, we performed geometrical optimization of Ag 54 /γ-Al 2 O 3 (110) with a decahedral shape. Two types of structures were obtained: amorphous Ag 54 (A) and locally fivefold symmetrical Ag 54 (B) structures. Both NPs on γ-Al 2 O 3 (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 Ag 54 (B) disappeared due to loss of the high symmetry. In turn, the moderate size dependence of neutral isolated NPs can be applied.