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.

In single-atom catalysts (SACs), the single atoms are often exposed as protrusions above the substrate. The solvent molecules in the electrocatalytic environment can interact or even bind to these coordination-unsaturated single atoms and thus influence the reaction process, but this has not been studied in depth. In this work, we systematically investigate the thermodynamics of CO₂ reduction reaction (CO₂RR) to CO over MoS₂-supported single metal atom catalysts (TM@MoS₂, TM = transition metal) under vacuum and explicit solvent environments using density functional theory. In addition, the ab initio molecular dynamics results show that explicit H₂O molecules can coordinate to the TM site and undergo competitive adsorption with the CO₂RR intermediates, which significantly affects the energy and conformation of the CO₂RR pathway. Electronic structure analysis reveals that the occupying H₂O molecules change the electronic state of single atom and further influence the adsorption strength of different CO₂RR intermediates. Our work shows that water molecules can not only act as ligands to influence the electronic state of TM, but also affect the energy and conformation of CO₂RR intermediates, which highlights the important role of occupying H₂O molecules at the single-atom sites in CO₂RR and provides useful insights for the design of SACs for efficient CO₂RR.

Single-atom catalysts (SACs) have recently emerged as stars in boosting the synthesis of NH₃ from N₂, as the catalytic performance of the supported single atoms can be modulated by their coordination environment. In this work, we propose a new strategy, based on comprehensive density functional theory calculations, whereby the coordination environment of a single Mo atom can be tuned by a central heteroatom (X = Fe, Co, Ni, Cu, Zn, Ga, Ge, and As) in the Kegging-type polyoxometalate (POM, (XW₁₂O₄₀)ⁿ⁻) substrate to catalyze the electrochemical nitrogen reduction reactions (NRR). Firstly, we demonstrate that the single Mo atom binds strongly to the POM surface oxygen hollow sites without aggregation. Secondly, the adsorption of *N₂ on the POM-supported Mo atom is investigated and the reactivity is assessed by calculating the thermodynamics of the NRR. The results show that the POM (X = Co and As) supported Mo atom has high NRR activity with low limiting potentials. Finally, we reveal the origin of the NRR activity by analyzing the electronic structure. The results show that the charge on the O atoms of oxygen hollow sites is affected by the central heteroatom. Due to such effect, it can be found that more d electrons are transferred from Mo supported by POM (X = Co and As) to *N₂, thus the N≡N triple bond is activated. This strategy of coordination environment tuning proposed in this work provides a useful guide for the design of efficient catalysts for electrocatalysis.