Understanding the Density-Dependent Activity of Cu Single-Atom Catalyst in the Benzene Hydroxylation Reaction

Regulating the density of metal single atoms and exploring the interaction among them are showing great potential to further raise the performance of single-atom catalysts (SACs). Herein, we produce a series of Cu SACs with densities ranging from 0.1 to 2.4 atoms/nm2 and find that the catalytic activity is proportional to Cu single-atom density in the benzene hydroxylation reaction. Mechanistic studies reveal that the interactions among neighboring single-atom moieties in ultra-high-density Cu SAC alter the electronic structures of Cu single atoms, resulting in stronger •OH adsorption, which is beneficial for the benzene hydroxylation reaction by suppressing the O2 formation side reaction. The adsorption energy of hydroxyl radicals is further proposed and verified as a key descriptor to explain the reaction differences of Cu SACs with various densities. As a result, the ultra-high-density Cu SAC of 2.4 atoms/nm2 (21.3 wt %) exhibits maximum mass specific activity and H2O2 utilization efficiency, which are both significantly higher than the reported results in the literature and solved the key challenges of SACs in the benzene hydroxylation reaction. This study sheds light on how the SAC density affects active sites and offers a practical catalyst for phenol production by the H2O2 route.