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.