Encyclopedia
Single-atom catalysts (SACs), consisting of metals atomically dispersed on a support, are considered as advanced materials bridging homogeneous and heterogeneous catalysis, representing the catalysis at the limit. The enhanced performance of these catalysts is due to the combination of distinct factors such as well-defined active sites, comprising metal single atoms in different coordination environments also varying its valence state and strongly interacting with the support, in this case porous carbons, maximizing then the metal efficiency in comparison with other metal surfaces consisting of metal clusters and/or metal nanoparticles.
- porous carbons
- atomically dispersed metal catalysts
- fine chemicals
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1. Introduction
Catalysis contributes to enhance our quality of life as a key technology in producing valuable compounds such as plastics, flavors, perfumes, pharmaceuticals, among many other essential chemical products in today’s society. In the frame of environmental sustainability, the scientific community is immersed in the development of new, advanced, and more sophisticated materials with great potential, particularly acting as highly performant catalysts for fine chemical synthesis. In this context, the use of metal supported catalysts is widely extended in fine chemical production, especially metal-supported carbon-based materials [1]. In fact, BASF is a leader in providing catalytic technologies useful in the pharmaceutical and fine chemical industry, for instance heterogeneous precious metals catalysts such as Pd, Pt, Rh, Ru, etc., dispersed on carbon and other supports.
Inertness, surface area, porosity, surface chemistry and purity are crucial parameters conditioning the activity and selectivity of heterogeneous catalysts. The high thermal and chemical stability, in acid or basic media, of carbon materials together with its low corrosion capability and easily tunable surface chemistry, using chemical or thermal methods or even by post-functionalization, make them almost ideal and versatile catalysts or supports for fine chemical synthesis [2,3,4,5].
Other aspects, such as the metal loading, homogeneous distribution, and high dispersion of metal nanoparticles, conditioned by the surface chemistry of the support, notably influence the catalytic performance maximizing the metal efficiency. Therefore, the study of metal-supported, size-limited particles is an issue of capital importance in heterogeneous catalytic processes [6]. Recently, Liu and Corma (2018) summarized the influence of the metal species size on the catalytic performance of different heterogeneously catalyzed reactions [7].
In this context, new advanced highly atomically dispersed metal catalysts, among them the ones known as single-atom catalysts (SACs), emerge as promising catalysts offering high activity and improved selectivity. Metal single atoms can hardly exist by themselves, their presence at the surface being only possible if strong metal–support interactions are stablished. Then, the functions or defects at the support surface plays an important role to prevent the metal cluster aggregation or sintering. In this type of catalysts, the isolated active centers are randomly distributed, generating different heterogenous coordination types, and it is strongly depending on supports nature [8,9,10]. The development of SACs could begin when the catalytic behavior of gold small clusters supported on different oxides for hydrogen and carbon monoxide oxidation was reported. Haruta et al. (1989) reported that the electronic states of the outer surface gold atoms are modified when interacting with supports, thus contributing to the birth of the catalysis at the limit [11,12]. This type of catalysis could be like enzymatic homogeneous catalysis in which transition metal ions are single active sites. Therefore, it can be said that the use of SACs potentially presents the advantages of both homogeneous and heterogeneous catalysis.
Among the supports reported for the development of SACs are certainly other metals, metal oxides and porous carbon materials [13]. Interestingly, some examples of metal single atoms immobilized on Metal–Organic-Frameworks (MOFs) [14] and zeolites [15,16] are also reported.
2. Carbon Materials as Ideal Supports for Development of Highly Dispersed Metal Catalyst
Carbon materials such as biomass and MOF-derived porous carbons, carbon nanotubes, graphene and graphene oxide, and graphitic carbon nitride have been extensively used as catalyst supports due to their well-defined and custom-tailored properties such as surface area, chemical and thermal stability, electronic conductivity, easy heteroatom doping and controllable defect engineering [19,20,21]. In addition, they can be prepared from low cost precursors such as biowastes, which represents a huge advantage.
2.1. Synthetic Strategies and Properties
The synthetic strategies adopted to prepare SACs into carbon materials should guarantee well dispersed active single-atom metals to prevent their further agglomeration and improve the catalytic activity. Defect and doping sites are particularly important to anchor the metal active phases.
2.1.1. Impregnation—Wet and Dry Methods
Impregnation is considered as a post-treatment method of the carbon support since the incorporation of the SACs is performed in the carbon material and not during the synthesis of the carbon support. The wet impregnation method consists of introduction of the liquid solution containing the metal precursor into the carbon porous support providing a strong anchoring of the atoms onto the support. In contrast, a dry method usually provides weaker interactions between the metal and the support [17]. Wet impregnation of carbon support usually comprises the following steps: incorporation of the active metal into the support through a solid–liquid mixing; drying of the mixture to eliminate the solvent which contains the metallic precursor; and finally a calcination and/or a reduction step (under H2 atmosphere) [17,18]. Recent works reporting wet impregnation methods towards SACs over carbon materials have been published: palladium–cobalt phosphide (Pd–Co2P) nanoparticles with Pd single atoms anchored on graphene oxide were used in the ethanol electro–oxidation reaction [22]; single atomic Co supported on phosphorized carbon nitride nanosheets was applied in electrocatalytic alkaline hydrogen evolution reaction (HER) [23]; atomically dispersed Pd onto the surface of nitrogen-doped carbon cages prepared from pyrolysis of hollow ZIF-8 was applied in the electrocatalytic reduction of oxygen [24]; Fe-SACs were incorporated in functionalized carbon cloths with O and N groups [25]; bifunctional Ru/N-doped carbon (activated coal and graphene nanoplatelets) was used as a catalyst of cellobiose hydrogenolysis into sorbitol [26]; Ni single atoms in nitrogen doped ultrathin porous carbon templated from porous g-C3N4 was used for the CO2 reduction [27]; atomic Ru–N sites on boron-doped mesoporous carbon spheres were impregnated followed by mild photo-activation and used as an electrocatalyst for HER [28].
Dry methods consist basically of the physical mixture of the carbon support with the SACs. Recently, strategies such as ball milling are being presented given its simplicity and fastness, providing high efficiency in atomically dispersing metallic catalysts. Jin et al. (2020), for example, synthesized atomically dispersed Pt onto N-doped carbon support by using the ball milling method and successfully applied the developed materials as electrocatalyst for the HER [29]. Using another methodology, Indium–C bonds were formed along the broken edges of graphitic nanoplatelets by dry ball-milling graphite in the presence of Indium beads and the resulting material was tested as an electrocatalyst for oxygen reduction reaction (ORR) [30]. It was also reported that the as-cast AB5 alloy powders were ball milled with carbon nanotubes and the resulting composite was employed as anode catalyst for direct borohydride fuel cell [31].
2.1.2. Organometallic Compounds
A great number of efficient organometallic compounds have been developed and applied as homogeneous catalysts. However, although the majority of these compounds exhibit high catalytic activity and selectivity, they are usually sensitive to moisture and/or air and are difficult to separate from the products [32]. Also, for reactions in aqueous solutions, it is very difficult to get an organometallic compound that can function as a molecular catalyst with high efficiency and durability over a wide pH range [33]. Hybrid materials constituted by supported metal complexes can be designed as heterogeneous catalysts able to overcome the main problems associated with the use of the metal complexes alone, with improved catalytic performance. Graphene and graphene oxide have been applied in recent years for the synthesis of graphene-supported metal complexes [34,35]. According to Kharisov et al. (2016), there are several ligands that can establish interactions with graphene, among them are the N-containing ligands, such as Schiff bases and porphyrins as well as σ- and π- ligands, such as carbonyls, cyclopentadienyls and pyrene [34]. Some of these ligands have been used on the synthesis of hybrid catalysts: zirconocene or titanocene complexes have been attached to reduced graphene oxide via π–π interactions between the cyclopentadienyl rings of the metallocenes and graphitic surface of graphene and applied on the catalysis of polyethylenes with increased molecular weight relative to those produced by free catalysts [36]; in a different approach, cobalt(II), iron(III), or oxovanadium(II) Schiff base metal complexes were covalently grafted on graphene oxide previously functionalized with 3-aminopropyltriethoxysilane and evaluated for the epoxidation of styrene, with higher catalytic performances for the Co- and Fe- graphene oxide hybrids [37]. As seen, the interaction between the metal complex and the graphene can be covalent or established through a π–π interaction. The graphene oxide is more often used as support than graphene due to the wider possibilities for reactions of the O-containing functional groups of graphene oxide with the metal complexes. Beyond graphene nanosheets, other carbon nanoforms, such as fullerene [38], carbon nanotubes [39], nanohorns and nanodiamonds [40,41], among others [35], can also be used on the synthesis of these hybrids for the application into heterogeneous catalysis. Some recent works apply the resulting hybrids of graphene and metal complexes on the catalysis field. Pour and co-authors (2019) immobilized a new macrocyclic Schiff base copper complex on graphene oxide nanosheets and tested its catalytic activity for olefins epoxidation [42]. The catalyst exhibited high activity and selectivity for cyclohexene (100% conversion, without by-products) and for norbornene (100% conversion and 93% selectivity). The catalyst was reused four consecutive times without significant sacrificing activity. In Ren et al. (2020), a novel terpyridine-based hetero-bimetallic Ni/Pd nanosheet supported on graphene oxide was synthesized, exhibiting higher catalytic activity, substrate applicability and recyclability for the Suzuki coupling reaction under mild conditions [43]. On the electrocatalysis field, Sánchez-Page et al. (2020), have determined the influence of graphene sheet properties as supports of iridium-based N-heterocyclic carbene hybrid materials on water oxidation [44].
2.1.3. Deposition
The deposition methods, chemical vapor deposition (CVD) [45,46] and atomic layer deposition (ALD) [46,47] are in situ preparation techniques and, despite being more complex, are considered more promising for the catalyst performance than other in situ methods such as pyrolysis [18,46] and ball-milling [29,48] or than the post-preparation methods such as impregnation [17,46], which with simpler approaches are usually associated with inhomogeneous particle sizes and compositions. In the case of supported metal catalysts produced through conventional methods, the resulting non-uniformity of metal sites can produce a mixture of reaction products instead of a single desired product and often there is a struggle to find a balance between the activity, selectivity, and stability of the catalyst. The deposition methods, CVD and ALD, allow the control of the catalytic materials synthesis on the atomic level, thus being associated with a more effective control over the size and dispersion of metal sites, as well as to the improvement of the catalytic variables referred above.
2.1.4. Pyrolysis
Pyrolysis is considered an in situ or one-pot synthesis method of SACs supported carbons since the deposition of the active phase occurs simultaneously with the support synthesis. Through this method, high loadings of SACs can be achieved, but the metal loading and its dispersion is more difficult to control [18], since at high temperatures metal atoms can be converted to nanoclusters and nanoparticles.
Typical precursors that have been used within this strategy are MOFs that when submitted to high temperatures (>500 °C), under inert atmosphere, are converted into metal/carbon hybrids through the carbonization of the organic coordinating ligand and the deposition/dispersion of the single atoms of metallic species under a strict control of pyrolysis conditions [57]. MOFs are heteroatoms-rich precursors that can be highly dispersed in the resulting carbon materials, providing stable anchoring sites of SACs. Sometimes, it is necessary to perform an acidic leaching of the aggregated metal particles to obtain isolated atoms on the carbon support, which represents a disadvantage. Although some of the high surface area of the original MOFs can be lost during pyrolysis, the resulting carbon still possesses well developed porosity allowing an efficient exposure and accessibility of the generated SACs. Atomically dispersed metal sites in MOF-carbon materials were reviewed by Liang et al. (2018) [57], but recent progressions in the development of these metal/carbon hybrids have been presented: a new Pt electrocatalyst has been produced by pyrolysis of MOFs with Pt2+ cations and phosphomolybdic acid confined in their pores; the resulting nano-sized molybdenum carbide with a Pt loading of only 0.7 wt% presented high electrocatalytic activity for HER [58]. In a recent approach, Ru single atoms distributed in nitrogen-doped porous carbon from ZIF-8 precursor, were synthesized by high-temperature pyrolysis to act as electrocatalysts of Li-O2 batteries [59]; atomically dispersed Ni-N species in carbon nanotubes have been prepared by pyrolysis of ZnO@ZIF-NiZn core–shell nanorods acting as active electrocatalysts for CO2 reduction reaction [60]; cobalt single atoms supported on N-doped carbon were prepared by the direct pyrolysis of Zn/Co bimetallic zeolitic imidazolate framework and used as catalyst in the selective hydrogenation of nitrobenzene [61]; mesoporous carbon nanoframes with hierarchical pore size distribution and atomically dispersed Fe–Nx active sites were synthesized from Zn–Fe bimetallic zeolitic imidazole frameworks and applied as electrocatalyst for ORR [62].
Polymer precursors impregnated with a specific metal precursor have been also carbonized to obtain SACs supported on carbon materials. A fluorine-tuned single-atom catalyst with an ultrathin nanosheet morphology and high Ni content of 5.92 wt% was fabricated by PTFE assisted pyrolysis approach and used for electrocatalytic CO2-to-CO conversion [63]. Also recently, atomically dispersed Mn single atoms on nitrogen doped carbon were produced from pyrolysis of a polypyrrole polymer produced via in situ polymerization with MnO2 that served as a polymerization initiator of the pyrrole monomers, but also as a sacrificial template and metal source. The obtained materials were employed as photocatalyst in CO2 reduction to produce synthesis gas [64].
Also, Fe rich biomass has recently been used to generate single-atomic Fe-N4-contained carbon materials with high-performance for ORR using silica spheres as hard template [65]. Using a different methodology and biomass as the carbon support precursor, Wang et al. (2019) produced oxygen and nitrogen coordinated single copper atom active sites anchored within porous carbon synthesized by direct pyrolysis of Cu2+ saturated aubergine biomass; the authors obtained polymer/aubergine-Cu2+ by immersing Cu2+ saturated aubergine in an aqueous solution of poly(vinyl alcohol) and KHCO3; this mixture was then submitted to pyrolysis treatment at 800 °C under N2; the resultant electrodes proved to possess exposed rich defects and heteroatoms (O, N) doped carbon active sites, delivering bifunctional ORR and oxygen evolution reaction (OER) activities [66].
3. Carbon-Supported Metal Single Atom Catalysts. Application in Fine Chemical Synthesis
The use of carbon-based SACs with application in fine chemical synthesis is getting much attention mainly because it offers the maximum metal efficiency, while providing high activities and selectivities [18]. In this context, graphene [48,67] and carbon nanotubes (CNT) [68] are often the preferred carbon forms for SACs synthesis because they present relatively large surface area and can be synthesized in high yields, by catalytic CVD. Also relevant is the recently great expansion of carbon nitride and carbon-based materials synthetized with assistance of MOFs. New families of porous carbon-based SACs are then summarized in the next sections, classified as different structural carbon forms, involved in the synthesis of valuable compounds mainly through oxidation and hydrogenation reactions, with both types of processes being of utmost importance for chemical industry. These transformations take place in the production of several important compounds such as active pharmaceutical molecules, dyes, rubber chemicals and other chemical upgrade reactions.
This entry is adapted from the peer-reviewed paper 10.3390/catal10121407
