The proximity between hydroxyl and single atom determines the catalytic reactivity of Rh₁/CeO₂ single-atom catalysts

The local structure of the metal single-atom site is closely related to the catalytic activity of metal single-atom catalysts (SACs). However, constructing SACs with homogeneous metal active sites is a challenge due to the surface heterogeneity of the conventional support. Herein, we prepared two Rh₁/CeO₂ SACs (0.5Rh₁/r-CeO₂ and 0.5Rh₁/c-CeO₂, respectively) using two shaped CeO₂ (rod and cube) exposing different facets, i.e., CeO₂ (111) and CeO₂ (100). In CO oxidation reaction, the T₁₀₀ of 0.5Rh₁/r-CeO₂ SACs is 120 °C, while the T₁₀₀ of 0.5Rh₁/c-CeO₂ SACs is as high as 200 °C. Via in-situ CO diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), we found that the proximity between OH group and Rh single atom on the plane surface plays an important role in the catalytic activity of Rh₁/CeO₂ SAC system in CO oxidation. The Rh single atom trapped at the CeO₂ (111) crystal surface forms the Rh₁(OH)_adjacent species, which is not found on the CeO₂ (100) crystal surface at room temperature. Furthermore, during CO oxidation, the OH group far from Rh single atom on the 0.5Rh₁/c-CeO₂ disappears and forms Rh₁(OH)_adjacent species when the temperature is above 150 °C. The formation of Rh₁(OH)_adjacentCO intermediate in the reaction is pivotal for the excellent catalytic activity, which explains the difference in the catalytic activity of Rh single atoms on two different CeO₂ planes. The formed Rh₁(OH)_adjacent-O-Ce structure exhibits good stability in the reducing atmosphere, maintaining the Rh atomic dispersion after CO oxidation even when pre-reduced at high temperature of 500 °C. Density functional theory (DFT) calculations validate the unique activity and reaction path of the intermediate Rh₁(OH)_adjacentCO species formed. This work demonstrates that the proximity between metal single atom and hydroxyl can determine the formation of active intermediates to affect the catalytic performances in catalysis.