Single‐atom heterogeneous catalysts (SACs) attached to carefully chosen hosts are attracting considerable interest; principally because they offer maximum utilization per metal atom and are usually ...readily recyclable. However, diminution of the atomic population of nanoparticles or nanoclusters to single atoms can significantly alter reactivity because of the consequent changes in the active‐site structure. By examining various diverse applications, we ascertain whether the performance of SACs is enhanced or suppressed. We also note that SACs generally display unique kinds of catalytic cycles. The choice of host is crucial since it influences both the electronic and steric environment of the metal center. Moreover, it may function as a cocatalyst. All these aspects impact upon the design of new SACs, which exhibit similarities to hetero‐ and homogeneous predecessors. Additionally, SACs offer a viable replacement of soluble metal complexes in processes that remain difficult to heterogenize.
Breaking with tradition: The performance of single‐atom heterogeneous catalysts (SACs) is reviewed and discussed in detail. Key factors that influence performance are described and compared to heterogeneous and homogeneous analogues comprising supported nanoparticles/clusters or metal complexes. SACs follow distinct catalytic cycles in many of the examples described.
Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have ...only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom–host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.
Controlling the hybridization of single atoms in suitable host materials opens unique opportunities for catalyst design, but equally faces many challenges. Here, we highlight emerging directions from ...the last, highly productive, decade in single-atom catalysis and identify frontiers for future research.
Single‐atom electrocatalysts (SAECs) have recently attracted tremendous research interest due to their often remarkable catalytic responses, unmatched by conventional catalysts. The electrocatalytic ...performance of SAECs is closely related to the specific metal species and their local atomic environments, including their coordination number, the determined structure of the coordination sites, and the chemical identity of nearest and second nearest neighboring atoms. The wide range of distinct chemical bonding configurations of a single‐metal atom with its surrounding host atoms creates virtually limitless opportunities for the rational design and synthesis of SAECs with tunable local atomic environment for high‐performance electrocatalysis. In this review, the authors first identify fundamental hurdles in electrochemical conversions and highlight the relevance of SAECs. They then critically examine the role of the local atomic structures, encompassing the first and second coordination spheres of the isolated metal atoms, on the design of high‐performance SAECs. The relevance of single‐atom dopants for host activation is also discussed. Insights into the correlation between local structures of SAECs and their catalytic response are analyzed and discussed. Finally, the authors summarize major challenges to be addressed in the field of SAECs and provide some perspectives in the rational construction of superior SAECs for a wide range of electrochemical conversions.
A rich variety of distinct chemical bonding configurations of a single metal atom with its surrounding host atoms creates vast opportunities for the rational design and synthesis of single‐atom electrocatalysts with tunable local atomic environment for high‐performance renewable energy conversion.
Controlling the electronic structure of transition-metal single-atom heterogeneous catalysts (SACs) is crucial to unlocking their full potential. The ability to do this with increasing precision ...offers a rational strategy to optimize processes associated with the adsorption and activation of reactive intermediates, charge transfer dynamics, and light absorption. While several methods have been proposed to alter the electronic characteristics of SACs, such as the oxidation state, band structure, orbital occupancy, and associated spin, the lack of a systematic approach to their application makes it difficult to control their effects. In this Perspective, we examine how the electronic configuration of SACs can be engineered for thermochemical, electrochemical, and photochemical applications, exploring the relationship with their activity, selectivity, and stability. We discuss synthetic and analytical challenges in controlling and discriminating the electronic structure of SACs and possible directions toward closing the gap between computational and experimental efforts. By bringing this topic to the center, we hope to stimulate research to understand, control, and exploit electronic effects in SACs and ultimately spur technological developments.
Halogen chemistry plays a central role in the industrial manufacture of various important chemicals, pharmaceuticals, and polymers. It involves the reaction of halogens or halides with hydrocarbons, ...leading to intermediate compounds which are readily converted to valuable commodities. These transformations, predominantly mediated by heterogeneous catalysts, have long been successfully applied in the production of polymers. Recent discoveries of abundant conventional and unconventional natural gas reserves have revitalized strong interest in these processes as the most cost-effective gas-to-liquid technologies. This review provides an in-depth analysis of the fundamental understanding and applied relevance of halogen chemistry in polymer industries (polyvinyl chloride, polyurethanes, and polycarbonates) and in the activation of light hydrocarbons. The reactions of particular interest include halogenation and oxyhalogenation of alkanes and alkenes, dehydrogenation of alkanes, conversion of alkyl halides, and oxidation of hydrogen halides, with emphasis on the catalyst, reactor, and process design. Perspectives on the challenges and directions for future development in this exciting field are provided.
We report the preparation and hydrogenation performance of a single‐site palladium catalyst that was obtained by the anchoring of Pd atoms into the cavities of mesoporous polymeric graphitic carbon ...nitride. The characterization of the material confirmed the atomic dispersion of the palladium phase throughout the sample. The catalyst was applied for three‐phase hydrogenations of alkynes and nitroarenes in a continuous‐flow reactor, showing its high activity and product selectivity in comparison with benchmark catalysts based on nanoparticles. Density functional theory calculations provided fundamental insights into the material structure and attributed the high catalyst activity and selectivity to the facile hydrogen activation and hydrocarbon adsorption on atomically dispersed Pd sites.
One (sheltered) atom is enough: The unique porous structure of carbon nitride enables the incorporation of isolated palladium atoms in a stable manner. This heterogeneous single‐site catalyst outperforms state‐of‐the‐art catalysts that are based on modified palladium nanoparticles in selective hydrogenation reactions.
The efficient utilization of biomass, the only globally available, renewable and abundant carbon-neutral source, is of high significance in green and sustainable chemistry. Polyoxometalates (POMs) ...and POM-based composites have been widely applied in green catalytic reactions, due to their tunable Brønsted/Lewis-acidity and redox properties enabling high reactivity in a wide range of chemical transformations. This review covers recent advances in the chemocatalytic conversion of biomass into chemicals and fuels over POMs and POM-metal composites. For biomass valorisation over POMs, the advances of acid catalysis including hydrolysis, dehydration, etherification, alcoholysis, transesterification and esterification are summarised. Furthermore, applications in chemical oxidation for the synthesis of organic acids and furan chemicals are discussed. For biomass valorisation over metal-POM composites, an overview of tandem reactions (
e.g.
hydrolysis-hydrogenation, hydrolysis-oxidation, and hydrogenolysis-hydrodeoxygenation) is highlighted. The future prospects of biomass valorisation over POM-based catalysts are finally presented.
POM-based catalysts have wide applications in catalytic biomass refinery.
Methanol synthesis by CO2 hydrogenation is attractive in view of avoiding the environmental implications associated with the production of the traditional syngas feedstock and mitigating global ...warming. However, there still is a lack of efficient catalysts for such alternative processes. Herein, we unveil the high activity, 100 % selectivity, and remarkable stability for 1000 h on stream of In2O3 supported on ZrO2 under industrially relevant conditions. This strongly contrasts to the benchmark Cu‐ZnO‐Al2O3 catalyst, which is unselective and experiences rapid deactivation. In‐depth characterization of the In2O3‐based materials points towards a mechanism rooted in the creation and annihilation of oxygen vacancies as active sites, whose amount can be modulated in situ by co‐feeding CO and boosted through electronic interactions with the zirconia carrier. These results constitute a promising basis for the design of a prospective technology for sustainable methanol production.
Surface oxygen vacancies in indium oxide drive the selective hydrogenation of CO2 to methanol. Strong electronic interactions between this active phase and the ZrO2 carrier and further vacancy formation by CO co‐feeding lead to excellent catalytic activity.
Solar‐driven transfer hydrogenation of unsaturated bonds has received considerable attention in the research area of sustainable organic synthesis; however, water, the ultimate green source of ...hydrogen, has rarely been investigated due to the high barrier associated with splitting of water molecules. We report a carbon‐nitride‐supported palladium single‐atom heterogeneous catalyst with unparalleled performance in photocatalytic water‐donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic‐labeling experiments and operando nuclear magnetic resonance measurements confirm the direct hydrogenation mechanism using in situ‐generated protons from water splitting under visible‐light irradiation. Density functional theory calculations attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the atomic palladium sites.
Photocatalytic water‐donating transfer hydrogenation can be achieved over a palladium single‐atom catalyst supported on mesoporous carbon nitride, in which hydrogen protons generated from photocatalytic water splitting are sequentially added to various unsaturated bonds. In‐depth characterization confirms the direct hydrogenation mechanism and links the outstanding performance to reduced barriers for hydrogenation and accelerated proton supply from water.
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