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 °C demonstrated superior CO oxidation performance over single-atom Ag/Al2O3 and the Ag/Al2O3 treated at 400 and 800 °C. 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.