Single‐Atom Catalysts

In the arena of catalysis, single-atom catalysts (SACs) have been developed as the best alternative to not only homogeneous but also heterogeneous catalysts used in energy, and environmental applications because of their interesting properties. In a quest for advanced, efficient, and sustainable catalysts, atomically dispersed single-atom materials have been growing in importance in the area of heterogeneous catalysis. SACs have not only maximized the utilization of active sites but have also boosted efficiency, and enhanced selectivity. SACs are important candidates for active atomic sites, which are made accessible and provide the maximum atom utilization competence with high catalytic performance. Single-atom catalysis, which includes isolated metal atoms stabilized on suitable carriers, is one of the most advanced and rapidly growing research fields in catalyst science today. The importance of this special issue, "Single-Atom Catalysts" has led to contributions by very eminent researchers in the respective fields. This issue comprises of advanced synthesis, and applications of SACs in which two reviews, five full papers, one research article, two communications, and two progress reports have been published. This special issue certainly will help readers in the field and create interest and inspiration for their future research. SACs is a highly interdisciplinary field and the products of these studies are impactful, attracting interest from a broad scientific readership of Advanced Materials Interfaces. The various contributions to this special issue represent a variety of SACs applications. The result is an exciting collection of original contributions and review articles on SACs, reflecting the field's inherent interdisciplinary nature. In this issue, we have collected the latest advances concerning the applications of SACs in the oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), direct alcohol fuel cells, lithium-sulfur batteries, and organic reactions such as reduction, and hydrogenation (Figure 1). In this special issue, Zhou and co-workers reviewed the current progress of carbon-based SAC in modulating activity and selectivity toward two-electron transfer ORR (https://doi.org/10.1002/admi.202001360). This review described the characterizing methods and reaction mechanisms of ORR via two-electron and four-electron pathways. The three aspects regarding the regulation of the central metal atoms, the coordinated atoms, and the environmental atoms, were discussed in detail. Carbon-based SACs have proven the ability to serve as a promising, alternative electrocatalyst for efficient H2O2 production via the two-electron transfer ORR pathway. The role of binding strength between intermediate OOH and active sites in influencing the H2O2 activity and selectivity is also illustrated. In a similar line of applications, Liu et al. developed isolated single-atom palladium (Pd) doped MnO2 nanofiber for ORR by rationally designing a synthesis based on DFT calculations (https://doi.org/10.1002/admi.202002060). The substituted Pd can work synergistically with vicinal Mn sites toward cleavage of O-O bonds in electrochemical oxygen reduction. The electrochemical measurements confirmed the advantageous performance of Hyd-Pd/MnO2 (hydrothermal method) with better conductivity and electrocatalytic activity for ORR, stronger tolerance to methanol than Imp-Pd/MnO2 (impregnation method), and Dep-Pd/MnO2 (liquid-phase deposition). This work highlighted the great importance of controlling the atomistic doping site of foreign atoms in metal oxides for electrocatalytic reactions. Catalysts are employed to facilitate effective and specific selective conversions in the majority of cases, and their nanostructural analyses and regulation are important subjects in advanced catalyst study. In this context, another progress report addresses catalyst fabrication and functions using an atomic- and nanoscale-level nanoarchitectonics approach by Ariga and co-workers (https://doi.org/10.1002/admi.202001395). This progress report discussed atomically controlled catalyst designs and the classification of nanoarchitectonics. The development of catalytic nanoarchitectonics would lead to precise conjugations between biological conversion systems and artificial catalytic mechanisms. Nanoarchitected catalytic systems are potentially capable of generating sophisticated multiple conversions with much higher structural regulations that have atom-level arrangements. In another progress report, the thiolate-protected single-atom alloy (SAA) nanoclusters are summarized well. The SAA has interesting optical, chemical, and electronic properties over monometallic nanoclusters. Walsh et al. briefly described the electronic properties and catalytic activities of Ag-based and Au-based thiolate-protected SAA nanoclusters (https://doi.org/10.1002/admi.202001342). To compare the changes in charge states of thiolate-protected SAA nanoclusters with their superior catalytic activity versus monometallic nanoclusters, the SAA was characterized by density functional theory (DFT), X-ray absorption spectroscopy, and X-ray photoelectron spectroscopy. Doping nanoclusters with another metal allows charge transfer between the surface atoms and the dopant in the core, resulting in either the core atom or the surface atoms being more negatively charged. This significantly enhanced the electronic conductivity and catalytic efficiency to drive the chemical transformation in positive directions. Therefore, the SAA nanoclusters were more stable, and thus had higher catalytic activities in several chemical conversions over traditional catalysts. Among the several energy storage devices and fuel cells, because of high-potential energy density and low cost, lithium-sulfur (Li-S) batteries have piqued interest as promising next-generation energy storage devices. In this regard, Yamauchi and co-workers summarized the recent advances in utilizing SACs for Li−S batteries, which involve catalyst preparation, characterization technologies, battery performance, and mechanistic behavior (https://doi.org/10.1002/admi.202002159). The distinctive electron structures and ultra-high atom utilization efficiency of SACs display excellent catalytic performance in improving Li–S battery performance. Notably, the utilization of sulfur during cycling was improved, and the lithium polysulfide "shuttle effect" was suppressed. The SAC-based materials help to develop Li–S batteries with high rates and long cycle stability. In another original report, Jiang and co-workers demonstrated a feasible and scalable atomic isolation technique for synthesizing single Fe atoms supported on N-doped graphene (SAFe@ NG) with a high atomic loading of 4.6 wt% (https://doi.org/10.1002/admi.202001788). The SAFe@NG was employed for electrochemical ORR in an alkaline and acidic environment. The Fe–N–G catalyst displays high intrinsic ORR performance, with excellent microstructure stability. The DFT study verified that the Fe atoms in coordination with four nitrogen atoms i.e., FeN4, in graphene are the active center for the four-electron pathway for the ORR process. A very interesting protocol based on mixed-valence single atom copper catalyst was reported by Gawande and co-workers (https://doi.org/10.1002/admi.202001822). They synthesized isolated Cu immobilized on cyanographene i.e., graphene functionalized with nitrile groups (G(CN)-Cu) by a simple complexation reaction. Herein, Cu metal atoms could strongly couple with the N atoms of G-CN and exhibited excellent electrical conductivity and superior catalytic activity toward methanol oxidation reaction and CO2RR in alkaline solution. Apart from electrocatalytic CO2 reduction, catalytic hydrogenation is also another way to convert CO2 to liquid fuel and hydrocarbons. In this regard, Rivera-Cárcamo et al. reported hydrogenation of CO2 catalyzed by single atoms of nickel (Ni) and ruthenium (Ru) supported by a carbon nanotube (CNT) and TiO2 catalyst (https://doi.org/10.1002/admi.202001777). The synergistic effect of atomically dispersed Ni and Ru and vacancy-like defects in carbon, as well as TiO2 supports enhanced the hydrogenation efficiency of the catalyst. Notably, the nickel SACs are very selective in the production of CO, and ruthenium SACs are more selective in the production of methane from CO2. Atomically dispersed SACs are also employed for several organic transformations such as oxidation, reduction, and multicomponent reactions. In this special issue, Yang et al. reported the guest molecular incorporation i.e., Au single atoms encapsulated in metal-organic framework-derived nitrogen-containing nanoporous carbon (Au@NC) by the water-based method. The prepared single Au atom catalysts have been employed for the reduction of 4-nitrophenol (https://doi.org/10.1002/admi.202001638). They demonstrated the significance of this water-based synthesis in controlling the size of a metal atom either in nanoparticles or in single atom by simply changing Au/Zn molar ratios and carbonization temperatures. The aim of catalysis research has long been to stabilize ever-smaller metallic clusters on low-cost metal oxide supports. SACs reflect the ultimate target, but stabilizing single atoms against agglomeration under reaction conditions is difficult. For stability, metal atoms must form chemical bonds with the support in order to be stable, which affects their electronic structure and catalytic properties. In this context, Kraushofer et al. studied the rhodium (Rh) adsorption and incorporation on the (1102) surface of hematite (α-Fe2O3) (https://doi.org/10.1002/admi.202001908). Rh forms small clusters on both surface terminations (stoichiometric 1 × 1 and reduced 2 × 1) upon vapor-deposition at room temperature, but Rh atoms incorporated and stabilized into the support lattice as single atoms upon annealing above 400 °C. Based on advanced characterization techniques and computational studies, they reported that the Rh atoms are stabilized in the immediate subsurface, rather than the surface layer. In another report, Otyepka and co-workers investigated the nature of chemical bonding and oxidation states of single metal atoms and ions anchored on CG (cyanographene) and GA (graphene acid) substrates through DFT (https://doi.org/10.1002/admi.202001392). This includes the iron triad, light platinum group elements, and coinage metals (Fe, Co, Ni, Ru, Rh, Pd, Cu, Ag, and Au), in differing states of oxidation (from 0 to +III) bonded to either CG or GA was analyzed in terms of bond dissociation energy and charge exchange. The ability of CG and GA to reduce metal cations and oxidize metal atoms is attributed to the π-conjugated lattice of the graphene derivatives. In the case of zero-valent metals, either negligible charge transfer or oxidation can occur. The level of oxidation/reduction of a metal atom/cation can be explained in terms of the HOMO and LUMO energies of the respective metal atom/cation and the substrate. In a similar line of study based on DFT calculations, Jovanović et al. described the model SACs consisting of nine transition metal atoms i.e., Ni, Cu, Ru, Rh, Pd, Ag, Ir, Pt, and Au on four separate supports: pristine graphene, N- and B-doped graphene and graphene with a single vacancy (https://doi.org/10.1002/admi.202001814). Among them, SACs can only be formed in graphene with a single vacancy, which is stable in terms of aggregation and dissolution under electrochemical conditions. Graphene-based surfaces were selected due to their high conductivity and large surface area, making them attractive for building SACs for electrocatalysis. It is predicted that only M@graphene vacancy systems (excluding Ag and Au) are stable under hydrogen evolution conditions in highly acidic solutions. It is an honor for us to serve as guest editors for this special issue. We would like to thank all of the authors and contributors to this special issue of "Single-Atom Catalysts." We are optimistic that the developments, observations, and ideas provided here will captivate and encourage researchers all over the world. We also express our sincere thanks to the editorial team of Advanced Materials Interfaces. Manoj B. Gawande received his Ph.D. in 2008 from the Institute of Chemical Technology, Mumbai, India, and then undertook several research stints in Germany, South Korea, Portugal, Czech Republic, USA, and UK. He also worked as a visiting professor at CBC-SPMS, Nanyang Technological University, Singapore in 2013. Presently, he is an associate professor at the Institute of Chemical Technology, Mumbai-Marathwada Campus, Jalna, India. Concurrently, he is invited as visiting professor in chemistry at RCPTM-CATRIN, Olomouc, Palacky University, Czech Republic. His research interests focus on single-atom catalysts, advanced nanomaterials, and sustainable technologies, as well as cutting-edge catalysis and energy applications. Currently, he is supervising several doctoral students and postdoctoral co-workers. Katsuhiko Ariga received his Ph.D. degree from Tokyo Institute of Technology in 1990. He joined to the National Institute for Materials Science (NIMS) in 2004 and is currently the leader of the Supermolecules Group and principal investigator of the World Premier International (WPI) Research Centre for Materials Nanoarchitectonics (MANA), NIMS. He is also appointed as a professor of The University of Tokyo. He acts as Executive Advisory Board of Advanced Materials, International Advisory Board of Angewandte Chemie International Edition, Chemistry An Asian Journal, and ChemNanoMat, and Executive Board Member of Small Methods. Yusuke Yamauchi received his Bachelor degree (2003), Master degree (2004), and Ph.D. (2007) from the Waseda University, Japan. After receiving his Ph.D., he joined the National Institute of Materials Science (NIMS), Japan, to start his own research group. In 2017, he moved to The University of Queensland (UQ). Presently, he is a senior group leader at AIBN and a full professor at School of Chemical Engineering. He concurrently serves as an honorary group leader at NIMS, an associate editor of the Journal of Materials Chemistry.