Nano-sized Ag rather than single-atom Ag determines CO oxidation activity and stability

Single-atom catalysis recently attracts great attentions, however, whether single atom or their nanoparticle (NP) has the advantage in its intrinsic activity remains under heated debate. Ag/Al₂O₃ is a widely used catalyst for many catalytic reactions, while the effect of Ag particle size on the activity is seldom investigated due to the great difficulty in synthesizing single atom Ag and Ag clusters/ particles with different sizes. Herein, we firstly prepared an atomically dispersed Ag/Al₂O₃ catalyst using a nano-sized γ-Al₂O₃ as the support, subsequently obtained a series of Ag⁰/Al₂O₃ catalysts with different Ag particle sizes by H₂ reducing single-atom Ag/Al₂O₃ catalyst at various temperatures. The Ag⁰/Al₂O₃ treated at 600 ℃ demonstrated superior CO oxidation performance over single-atom Ag/Al₂O₃ and the Ag/Al₂O₃ treated at 400 and 800 ℃. Based on experimental data and density functional theory (DFT) calculation results, we reveal that the larger Ag⁰ particle is beneficial to oxygen activation and improves the valence stability during oxidation reaction, while the aggregation of Ag⁰ particle also accordingly decreases the concentration of surface active sites, hence, there is an optimum Ag⁰ particle size. Our findings clearly confirm that Ag⁰ nanoparticle has the advantage over single-atom Ag species in its intrinsic activity for CO oxidation.