Single‐Atom Catalysts

Single-atom catalysis, which involves isolated metal atoms stabilized on suitable carriers/supports, is one of the most recent, revolutionary and rapidly expanding research fields in catalyst science. Lately, single-atom catalysts (SACs) have emerged as one of the finest alternatives for not only homogeneous but also heterogeneous catalysis when employed in several kinds of catalytic applications. The SACs with isolated metal sites have attracted extensive attention in the field of catalysis because of their remarkable catalytic actions and maximum atom utilization. In search of advanced, effective, and sustainable catalysts, SACs have been contributing immensely as a bridge between homo- and heterogeneous catalysts. Atomically dispersed, supported metal catalysts can provide efficient and highly active surface sites for sustainable and benign catalytic transformations. The SACs not only maximize the utilization of the active sites but also increase efficiency, reduce metal use, and enhance the catalytic selectivity during transformations. Nowadays, advances in characterization techniques have made it easier to confirm the existence of atomically dispersed metal species, allowing a better understanding of their structure, stability, and catalytic properties. With the advanced knowledge of surface phenomenon regarding SACs, several research groups explore atomically isolated single-site catalysts for a variety of energy applications including thermochemical, electrochemical, and photochemical transformations, ranging from small-molecule activation to the production of fine chemicals to biocatalysis (Figure 1). Since the term, “single-atom catalyst” was coined by Zhang and co-workers in 2011, incredible research progress has been achieved in this specialized field of catalysis. The information in Figure 2 is clearly indicative of the growing interest among the research community related to single-atom catalysts. In the last decade, there has been an exponential increase in the publication of papers in the field of “single-atom catalysts”. Considering the broad scope of SACs in various advanced applications from environmental to energy, this special issue on “Single-Atom Catalysts” is made available to a larger research community. This special issue contains nine original research articles as well as eight review articles, all of which cater to the broad readership of Small. This issue helps readers to find something interesting and inspirational for their future research targets as the different contributions to this special issue represent selected aspects of SACs, reflecting the field's inherent interdisciplinary nature. In this issue, we have collected the latest advances in the applications of transition metal-based SACs for organic catalysis, photocatalysis, electrocatalysis, and biocatalysis. Moreover, we've collected leading researchers working in the field of single-atom catalysis to include a complementary set of original articles and review papers that offer a broad overview of the field's current directions and challenges. The downsizing of metal nanoparticles into isolated single-atom catalyst efficiently enhances the catalytic activity and selectivity. When metal nanoparticles on supports are shrunk to the point of atomic dispersion, they become cationic and acquire new catalytic properties that were only recently discovered for the several catalytic applications. In this context, atomically dispersed supported platinum and iridium catalysts typically consist of cations bonded to support oxygen, carbon, or nitrogen atoms significantly increases the chemical and structural stability of catalyst by metal-support interactions. In this special issue, Chen et al. broadly reviewed the methods for the synthesis of atomically dispersed platinum and iridium catalysts supported on various suitable supporting mediums. Additionally, the synthesis of Pt- and Ir- based SACs, structural characterization, reactivity, and their catalytic performance for several kinds of organic reactions and electrocatalytic applications are briefly described (https://doi.org/10.1002/smll.202004665). Energy demand is continuously increasing due to the increasing population and globalization. To fulfill energy world demands, SACs have received tremendous attention due to their extraordinary catalytic performances for the production of fuels and chemicals. As a new frontier in the electrocatalytic field, SACs provide a feasible solution to the excessive consumption of noble metals. In recent years, the fuel cell has been continuously gaining popularity and has become the leading research topic in the context of developing alternative energy production devices. In this regard, Gawande and co-workers systematically summarized the recent progress on SACs for electrocatalytic applications such as the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), the hydrogen evolution reaction (HER), metal-air batteries, and electrochemical organic transformations (https://doi.org/10.1002/smll.202006473). The development of single atom-based electrocatalysts using noble and non-noble metals and their applications over the last few years are briefly discussed. Similarly, Han et al. described the recent development towards metal-based SACs anchored on typical carbon-based supports for the formic acid oxidation reaction (FAOR) and ORR (https://doi.org/10.1002/smll.202004500). They categorize and analyze the various active metal sites and the corresponding FAOR/ORR mechanisms on the surface of carbon supports. Among the reported carbon-supported SACs, Rh SACs/CN and Ir SACs/CN, as the efficient active centers, showed remarkable performance for the FAOR. Surprisingly, Pt SACs and Pd SACs dispersed on CN supports showed no catalytic activity, which indicates that the potential of single metal active centers on noncarbon supports still needs to be explored in detail for both FAOR and ORR catalysis. In similar lines of electrocatalytic applications of SACs, Zou et al. (https://doi.org/10.1002/smll.202004809) thoroughly reviewed the syntheses and characterizations of metal-organic frameworks (MOFs) derived SACs and their electrochemical applications such as the carbon dioxide reduction reaction (CO2RR), the nitrogen reduction reaction (NRR), ORR, OER, HER, etc. They methodically described the synthesis strategies based on the carbonization of MOFs. In a similar context, the advancement on pristine MOFs, MOF hybrids, and MOF-derived carbon-based SACs for the electrochemical reduction of CO2 is systematically discussed by Tang and co-workers (https://doi.org/10.1002/smll.202006590). Electrocatalytic carbon dioxide reduction to value-added chemicals is a sustainable technology that can help the ecosystem to achieve a carbon-neutral energy cycle and also combat climate change. In this review, several MOF-derived carbon-based SACs are reported for the production of liquid fuels and hydrocarbons from CO2 reduction electrocatalytically. Lastly, the limitations and future improvement paths for MOF-related materials advancement in the field of research are also nicely highlighted. In another review, Wu and co-workers summarized the recent advances in the synthesis of carbon-supported SACs and their application toward electrocatalytic CO2 reduction to CO and other C1/C2 products (https://doi.org/10.1002/smll.202005148). This review begins with the primary classification of SACs and the theoretical support to form CO and other products on single atomic active sites. The authors mostly emphasize the fine-tuning of SAC coordination structures via proper designing of experimental conditions, which helps the rational design of active and selective SACs for the reduction of CO2. The identification of a single atom on the catalyst surface is as much important as the performance of a catalyst. Tieu et al. nicely summarized the recent development and applications of transmission electron microscopy (TEM) techniques to garner information about the location, bonding characteristics, homogeneity, and stability of SACs (https://doi.org/10.1002/smll.202006482). In another review on nanocatalytic medicine, Shi and co-workers provided a concise summary of SACs applied in various nanocatalytic tumor therapy-based modalities, such as chemodynamic therapy by tumor microenvironment responsive catalytic reactions, photodynamic therapy by photocatalytic reactions, sonodynamic therapy by sonocatalytic reactions, and parallel catalytic therapy by parallel catalytic reactions, etc. (https://doi.org/10.1002/smll.202004467). By taking advantage of the extraordinarily high activity and stability of metal atoms in a solid matrix, SACs can be viewed as the most plausible solution to achieve efficient cancer therapy with minimized ion toxicity. In this special issue, some exciting original findings are also reported for the electrochemical ORR. In this regard, Ding et al. reported the preparation of single-atom Fe–N–C catalyst (Fe-IICSAC) by ion-imprinting technology for electrochemical ORR (https://doi.org/10.1002/smll.202004454). This synthesis strategy involves the preparation of ion-imprinted mesoporous MCM-41-based precursor and the subsequent high-temperature pyrolysis. These carbon-supported Fe SACs demonstrate high ORR performance with a half-wave potential of 0.908 V in alkaline conditions. Hydrogen is one of the most promising energy sources for modern society that is facing severe fossil fuel exhaustion and environmental issues. The SACs with the maximum atom utilization and exceptional activities toward HER have attracted considerable research interests. The downsizing platinum nanoparticles to atomically dispersed atoms is an effective strategy to achieve a comparable electrocatalytic hydrogen evolution performance to that of the commercial Pt/C catalyst and greatly reduces the cost of catalyst preparation. In this context, Wang et al. reported single atoms Pt immobilized on the nitrogen–carbon substrate (PtSA/N–C) for HER (https://doi.org/10.1002/smll.202005713). The enhanced electrochemical hydrogen evolution performance is proven by repetitive linear sweep voltammetry and cyclic voltammetry scanning experiments. Similarly, Zhang et al. recently demonstrated the electrochemical HER via Pt single atom electrocatalyst supported on carbon nanotubes (https://doi.org/10.1002/smll.202004453). The carbon nanotubes with varying types of nitrogen such as pyridine-like nitrogen, pyrrole-like nitrogen, and quaternary nitrogen are used as a support to immobilized Pt atoms by the atomic layer deposition. From the as-prepared catalysts pyrrole-like nitrogen in the carbon nanotube support significantly enhanced the HER activity and stability of Pt single atoms. In another protocol, Asefa and co-workers have synthesized Co-based SACs, with well-defined Co(II) sites by a simple synthetic method (https://doi.org/10.1002/smll.202006477). Co-based single atom electrocatalyst (G(CN)-Co SACs) was employed for hydrazine oxidation reaction (HzOR). With a low onset potential, a high current density, and good stability, the material demonstrated effective and selective electrocatalytic activity against HzOR. The computational study verified that the single Co-active sites can readily interact with the hydrazine molecules, significantly promoting the N–H bond dissociation steps. The solar-driven photocatalytic reduction of the most abundant industrial exhaust gas, i.e., CO2 has garnered extensive attention. Photocatalytic conversion of CO2 using solar energy is an appealing route for the transformation of CO2; strategy enabling to produce of energy-rich fuels and value-added chemicals, but also to mitigate the CO2 emission. In line with this, Zbořil and co-workers recently reported photoactive single-site ruthenium atoms embedded in the mesoporous carbon nitride (RuSA-mC3N4) catalyst for the conversion of CO2 into methanol under visible light irradiation (https://doi.org/10.1002/smll.202006478). The methanol production was achieved with a high yield of 1500 μmol g−1 (6 h of reaction time period) under photocatalytic conditions, with low metal loading (0.4 wt%) of a single ruthenium atom using water as an electron donor. The as-prepared RuSA-mC3N4 demonstrated superior catalytic activity and long-term operational stability along with exceptional reusability. The development of a novel single-atom catalyst (SAC) is highly desirable in organic synthesis to achieve the maximized atomic efficiency. In their original research, Liu and co-workers reported the synthesis of Co-based SACs on nitrogen-doped graphene support (SACo@NG) with 4.1 wt% Co loading which is subsequently applied to the activation of peroxymonosulfate (PMS) for the aqueous oxidation reaction under mild conditions (https://doi.org/10.1002/smll.202004579). The SACo@NG catalyst is successfully applied to the selective oxidation of benzyl alcohol (BzOH) into benzaldehyde (BzH) by the activation of the environmentally benign PMS oxidant and shows superior catalytic performance compared to nitrogen-doped graphene and Co nanoparticle-supported nitrogen-doped graphene. Pérez-Ramírez and co-workers also reported an N-doped and activated carbon (NC and AC) supported Pt nanostructures, ranging from single atoms to nanoparticles for dibromomethane hydrodebromination (https://doi.org/10.1002/smll.202005234). Notably, single atom-based Pt catalysts exhibit higher selectivity up to 98% for methyl bromide production as well as 60% selectivity towards the formation of methane over the larger nanoparticles. The findings presented in this study highlight the potential of SACs in dibromomethane semi-hydrogenation, opening up options for the further development of improved catalytic systems. In similar line of study, Kaiser et al. developed a sustainable and potentially scalable route for the synthesis of carbon-supported gold nanostructures as a bimetallic catalyst by employing metal salts (i.e., H2PtCl6) as an alternative chlorine source to replace corrosive aqua regia (https://doi.org/10.1002/smll.202004599). The nanostructured gold and platinum bimetallic SACs were employed for the acetylene hydrochlorination. They proved that the stability of gold-based analogs tremendously increases due to the vicinity of a single Pt atom. The lifetime of acetylene hydrochlorination is enhanced by two folds because of the high catalytic activity of Au(I)–Cl active sites. Exploiting the co-operativity effects of a second metal is a promising strategy towards the practical applicability of gold SACs, opening up exciting opportunities for multifunctional single-atom catalysis. In another protocol, Dawson et al. investigated the use of a range of sulfur-containing compounds as promoters for the production of highly active Au/C catalysts (https://doi.org/10.1002/smll.202007221). The Au-based bimetallic catalysts prepared with chloroauric acid and metal sulfates are found to be active catalysts for the acetylene hydrochlorination reaction. It is our honor to serve as the Guest Editors of this special issue. We would like to again take this opportunity to thank all the authors and contributors of this special issue on “Single-Atom Catalysts”. We are confident that the advances, insights, and recommendations included herein will captivate and inspire the imagination of researchers working on SACs across the globe. We also express our sincere thanks to the excellent editorial team of Small whose timely help made this possible. 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, 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 A published by the Royal Society of Chemistry (RSC). He has been selected as one of the Highly-Cited Researchers (Chemistry in 2016-2020 and Materials Science in 2020).