A single-atom catalyst (SAC) that was first proposed by us in 2011 has aroused significant recent interest. Among the various SACs, FeO_x-based ones including Pt₁/FeO_x, Ir₁/FeO_x, Au₁/FeO_x, Ni₁/FeO_x, and Fe₁/FeO_x have been investigated either experimentally or theoretically for CO oxidation. However, a systematic study of FeOx-based SACs has not been conducted. For a comprehensive understanding of FeO_x-supported single-metal-atom catalysts, extensive density functional theory calculations were carried out on the activities and catalytic mechanisms of SACs with the 3d, 4d, and 5d metals of group VIII to IB, i.e., M₁/FeO_x (M = Fe, Co, Ni, Cu; Ru, Rh, Pd, Ag; Os, Ir, Pt, Au) for CO oxidation. Remarkably, a new noble metal SAC, Pd₁/FeO_x, with high activity in CO oxidation was found and is predicted to be even better than the previously reported Pt₁/FeO_x and Ni₁/FeO_x. In comparison, other M₁/FeO_x SACs (M = Fe, Co, Cu; Ru, Rh, Ag; Os, Ir, Au) showed only low activities in CO oxidation. Moreover, the adsorption strength of CO on the single-atom active sites was found to be the key in determining the catalytic activity of these SACs for CO oxidation, because it governs the recoverability of oxygen vacancies on their surfaces in the formation of a second CO₂ during CO oxidation. Our systematic studies of FeO_x-supported SACs will help in understanding the fundamental mechanisms of the interactions between singly dispersed surface metal atoms and FeOx substrate and in designing highly active FeO_x-supported SACs.

In this article, we introduce Tsinghua Global Minimum (TGMin) as a new program for the global minimum searching of geometric structures of gas-phase or surface-supported atomic clusters, and the constrained basin-hopping (BH) algorithm implemented in this program. To improve the efficiency of the BH algorithm, several types of constraints are introduced to reduce the vast search space, including constraints on the random displacement step size, displacement of low-coordination atoms, and geometrical structure adjustment after displacement. The ultrafast shape-recognition (USR) algorithm and its variants are implemented to identify duplicate structures during the global minimum search. In addition to the Metropolis acceptance criterion, we also implemented a morphology-based constraint that confines the global minimum search to a specific type of morphology, such as planar or non-planar structures, which offers a strict divide-and-conquer strategy for the BH algorithm. These improvements are implemented in the TGMin program, which was developed over the past decade and has been used in a number of publications. We tested our TGMin program on global minimum structural searches for a number of metal and main-group clusters including C₆₀, Au₂₀ and B₂₀ clusters. Over the past five years, the TGMin program has been used to determine the global minimum structures of a series of boron atomic clusters (such as [B₂₆]^–, [B₂₈]^–, [B₃₀]^–, [B₃₅]^–, [B₃₆]^–, [B₃₉]^–, [B₄₀]^–, [MnB₁₆]^–, [CoB₁₈]^–, [RhB₁₈]^–, and [TaB₂₀]^–), metal-containing clusters Li_n (n = 3–20), Au₉(CO)₈⁺ and [Cr₆O₁₉]^2–, and the oxide-supported metal catalyst Au₇/γ-Al₂O₃, as well as other isolated and surface-supported atomic clusters. In this article we present the major features of TGMin program and show that it is highly efficient at searching for global-minimum structures of atomic clusters in the gas phase and on various surface supports.

Single-atom catalysts are of great interest and importance for designing new high-performance low-cost catalysts. We investigated CO oxidation catalyzed by single gold atoms supported on thoria (Au/ThO₂) and doped ThO₂ using density functional theory with Hubbard-type on-site Coulomb interaction (DFT + U). The calculation results show that the Au-doped ThO₂(111) catalyst exhibits remarkable catalytic activity for CO oxidation via the Eley–Rideal mechanism in three steps, where the rate-determining step is decomposition of the OCOO* intermediate with an energy barrier of 0.58 eV. Moreover, our results also reveal a new mechanism of CO oxidation on a gold adatom supported by ThO₂(111), where O₂ is adsorbed only at the Th site on the surface, and the gas-phase CO then reacts directly with the activated O₂^* to form CO₂, which is the rate-limiting step, with a barrier of 0.46 eV. It is found that CO oxidation can occur without CO and O₂ coadsorption on Au, which was previously considered a key intermediate. Therefore, these results provide new insights into CO oxidation on isolated gold atoms supported by the 5f-element compound ThO₂(111). This mechanism can help clarify the catalytic cycle of CO oxidation, support the design of high-performance low-cost catalysts, and elucidate the redox properties of actinide oxides.

Activation of molecular O₂ is the most critical step in gold-catalyzed oxidation reactions; however, the underlying mechanisms of this process remain under debate. In this study, we propose an alternative O₂ activation pathway with the assistance of hydrogen-containing substrates using density functional theory. It is demonstrated that the co-adsorbed H-containing substrates (R–H) not only enhance the adsorption of O₂, but also transfer a hydrogen atom to the adjacent O₂, leading to O₂ activation by its transformation to a hydroperoxyl (OOH) radical species. The activation barriers of the H-transfer from 16 selected R–H compounds (H₂O, CH₃OH, NH₂CHCOOH, CH₃CH=CH₂, (CH₃)₂SiH₂, etc.) to the co-adsorbed O₂ are lower than 0.50 eV in most cases, indicating the feasibility of the activation of O₂ via OOH under mild conditions. The formed OOH oxidant, with an increased O–O bond length of ~1.45 Å, either participates directly in oxidation reactions through the end-on oxygen atom, or dissociates into atomic oxygen and hydroxyl (OH) by crossing a fairly low energy barrier of 0.24 eV. Using CO oxidation as a probe, we have found that OOH has superior activity than activated O₂ and atomic oxygen. This study reveals a new pathway for the activation of O₂, and may provide insight into the oxidation catalysis of nanosized gold.

We report a comprehensive theoretical investigation of the catalytic reaction mechanisms of propene epoxidation on gold nanoclusters using density functional theory (DFT). We have shown that water acts as a catalytic promoter for propene epoxidation on gold catalysts. Even without reducible supports, hydroperoxyl (OOH) and hydroxyl (OH) radicals are readily formed on small-size gold clusters from co-adsorbed H₂O and O₂, with energy barriers as low as 4-6 kcal/mol (1 cal = 4.186 J). Propene epoxidation occurs easily through reactions between C₃H₆ and the weakened O-O bond of the OOH radicals on the surfaces of gold clusters.