Single-atom catalysts (SACs) have emerged as a focal point of research in the field of heterogeneous catalysis. This paper reviews the progress in the studies of single atoms as promoters in various catalytic reactions, elucidating their distinctive role in comparison to the dominant active sites. We provide a discussion on the application of single-atom promoters (SAP) within host-guest systems in various catalysts, including metal oxide supported catalysts, molybdenum carbide-based catalysts, bimetallic catalysts, and others. The behavior of SAP is diverse. They often promote the formation of oxygen vacancies for oxide support, leading to local site reconstruction that creates specific reaction route. Moreover, they can also precisely modify the electronic structure of hetero-metal atomic or nanoparticle sites, then regulating the adsorption of reactants or intermediates and catalytic performance. Finally, the potential for the development of SAP is outlined, proposing novel approach for the design of SACs with enhanced activity and stability.

Water gas shift (WGS) reaction is an important process to remove CO contaminant in industrial hydrogen resources. The efficient catalyst that can work at low temperature is keenly desired to achieve high CO conversion from the viewpoint of thermodynamic equilibrium. Here, we develop a noble-metal-free catalyst of MoC nanorod (MoC NR) with the dominant (2 0 0) facet for the low-temperature WGS reaction. It exhibits a high performance with similar to 100 % CO conversion at 175 degrees C, better than conventional MoC with the dominant (1 1 1) facet or the commercial Cu-based catalyst. Moreover, it shows rather better stability due to the presence of the dominant (2 0 0) facet. Various characterizations demonstrate that this facet facilitates MoC NR possessing higher removal capability of surface oxygen species to enhance the stability and lower activation energy to endow higher performance compared with conventional MoC. This study provides important implications for the rational design of cost-effective catalysts for the low-temperature WGS reaction.

For single-atom catalysts (SACs), the dopants situated near the metal site have demonstrated a significant impact on the catalytic properties. However, the effect of dopants situated further away from the metal centers and their working mechanisms remain to be elucidated. Herein, we conduct density functional theory-driven studies on regulating the peripheral nitrogen dopants in graphene-based SACs, with a particular focus on Ir1 SAC, for propane dehydrogenation (PDH). It is found that increasing the distance between the N dopant and the Ir1 site results in a different energy change for the reaction process compared to the dense doping models with only first and second-shell N species. The proposed stochastic doping models demonstrate statistically that increasing the N dopant in farther shells not only enhances the activity of Ir1 but also maintains a high selectivity for propene, which is verified by experimental tests. The modulation of the d-band center of Ir1 by stochastic N dopants effectively modifies the binding strength of reaction intermediates, thereby enabling the optimization of the potential energy surface of PDH. These results deepen the understanding of dopant states around metal sites and provide an important implication for the doping engineering in heterogeneous catalysis.

Single-atom catalysts with uniform metal active sites show potential for selectivity control. However, their application to high-temperature propane dehydrogenation remains challenging. Here we develop a highly stable and efficient single-atom catalyst for propane dehydrogenation that is based on Ru single atoms on nitrogen-doped carbon (Ru 1 /NC). The turnover frequency of Ru 1 /NC is at least three times higher than that of nanoparticle counterparts, resulting in propylene selectivity of around 92% with a lower deactivation rate at 560 °C. Experimental and density functional theory studies reveal the important role of peripheral N species around the Ru 1 centre. The inner-shell N stabilizes the atomically dispersed Ru to inhibit structure-sensitive propane cracking, while the outer-shell N promotes electron accumulation at the Ru 1 centre, inducing a significant charge repulsion between Ru 1 and propylene to facilitate its desorption. The combined functions of inner-shell and outer-shell N species at single-atom Ru sites contribute to the high efficiency of Ru 1 /NC.

Dual-atom catalysts (DACs) with paired active sites can provide unique intrinsic properties for heterogeneous catalysis, but the synergy of the active centers remains to be elucidated. Here, we develop a high-performance DAC with Zn1Co1 species anchored on nitrogen-doped carbon (Zn1Co1/NC) as the dominant active site for the propane dehydrogenation (PDH) reaction. It exhibits several times higher turnover frequency (TOF) of C3H8 conversion and enhanced C3H6 selectivity compared to Zn1/NC or Co1/NC with only a single-atom site. Various experimental and theoretical studies suggest that the enhanced PDH performance stems from the promoted activation of the C-H bond of C3H8 triggered by the electronic interaction between Zn1 and Co1 colligated by N species. Moreover, the dynamic sinking of the Zn1 site and rising of the Co1 site, together with the steric effect of the dissociated H species at the bridged N during the PDH reaction, provides a feasible channel for C3H6 desorption through the more exposed Co1 site, thereby boosting the selectivity. This work provides a promising strategy for designing robust hetero DACs to simultaneously increase activity and selectivity in the PDH reaction.

Chemoselective hydrogenation of quinoline and its derivatives under mild reaction conditions still remains a challenging topic, which requires a suitable interaction between reactants and a catalyst to achieve high performance and stability. Herein, FePO4-supported Rh single atoms, subnano clusters and nanoparticle catalysts were synthesized and evaluated in the chemoselective hydrogenation of quinoline. The results show that the Rh subnano cluster catalyst with a size of ∼1 nm gives a specific reaction rate of 353 molquinoline molRh-1 h-1 and a selectivity of >99% for 1,2,3,4-tetrahydroquinoline under mild conditions of 50 °C and 5 bar H2, presenting better performance compared with the Rh single atoms and nanoparticle counterparts. Moreover, the Rh subnano cluster catalyst exhibits good stability and substrate universality for the hydrogenation of various functionalized quinolines. A series of characterization studies demonstrate that the acidic properties of the FePO4 support favors the adsorption of quinoline while the Rh subnano clusters promote the dissociation of H2 molecules, and then contribute to the enhanced hydrogenation performance. This work provides an important implication to design efficient Rh-based catalysts for chemoselective hydrogenation under mild conditions.

Ru-based catalysts are rarely considered for water gas shift (WGS) reaction due to either inferior activity or methanation side reaction. Here, Ru-1/FeOx single-atom catalyst is synthesized via a co-precipitation method, which can realize high CO conversion with low loading of 0.18 wt. % and stable specific rates with-3 times higher than Ru nanoparticles (NPs) with loading of 2.00 wt. % at 300 ?C. Moreover, it shows no formation of methanation byproducts even under CO2- and H-2-rich WGS stream which can occur on Ru NPs. Detailed characterizations demonstrate that Ru single atoms have stronger interaction with FeOx to form dual active sites for WGS reaction. The positive Ru1 species own rather weaker bonded CO* and the neighbored FeOx can help activate H2O to generate OH*, which then react facilely via an associative process. Meanwhile, the single atom Ru prohibits the dissociation of H-2 with low adsorption strength to avoid the methanation.

Tuning the electronic state and local structure of single-atom dispersed metals is important to improve the performance of single-atom catalysts. Here, we find that the electronic structure of Pt single atoms (Pt1) depends on the facets of CeO2. Various characterizations and density functional theory calculations demonstrate that Pt1 anchored on the (110) facet of CeO2 nanorods exhibits a positively charged state, while it approaches the metallic state on the (100) facet of CeO2 nanocubes. The different Pt1 states on these facets are attributed to the Pt-O-Ce microstructures with different electron transfer patterns. The positively charged Pt1 shows much weaker adsorption of CO which is favorable for its complete oxidation at lower temperatures. In contrast, the metallic Pt1 can facilitate the activation of H2 in the form of dissociation. The activated H species on metallic Pt1 then promote the preferential oxidation of CO to a greater extent and significantly reduce the reaction barrier. This study provides important implications for tailoring the electronic structure of single-atom centers for heterogeneous catalysis by bonding to specific facet supports.

Catalytic ethane dehydrogenation (EDH) to ethylene over Pt-based catalysts has received increasing interests in recent years as it is a potential alternative route to conventional steam cracking. However, the catalysts used in this reaction often suffer from rapid deactivation due to serious coke deposition and metal sintering. Herein, we reported the effects of Zn modification on the stability of Pt/Al2O3 for EDH. The Zn-modified sample (PtZn2/Al2O3) exhibits stable ethane conversion (20%) with over 95% ethylene selectivity. More importantly, it exhibits a significantly low deactivation rate of only 0.0 03 h(-1) at 60 0 degrees C for 70 h, which surpasses most of previously reported catalysts. Detailed characterizations including in situ FT-IR, ethylene adsorption microcalorimetry, and HAADF-STEM etc. reveal that Zn modifier reduces the number of Lewis acid sites on the catalyst surface. Moreover, it could modify Pt sites and preferentially cover the step sites, which decrease surface energy and retard the sintering of Pt particle, then prohibiting the further dehydrogenation of ethylene to ethylidyne. Consequently, the good stability is realized due to anti-sintering and the decrease of coke formation on the PtZn2/Al2O3 catalyst. (c) 2020 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved.