Heterogeneous Catalysts. Группа авторов

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and (b) PCTF‐1 synthesized by phosphorous pentoxide catalyst (P2O5) catalyzed condensation method.

      4.2.3 Catalyst on Nanodiamonds and Onion‐Like Carbon

      Source: Zeiger et al. 2016 [24]. Reprinted with permission of Royal Society of Chemistry.

      4.2.4 SACs on Carbon Nitrides and Covalent Triazine Frameworks

      In graphene doped with heteroatoms (N, O, S, P, etc.), the doped element can function as an anchoring site for metal or metal oxide, leading to the synthesis of hybrid organic–inorganic materials with high stability as a result of the strong binding between the metal species and dopant atoms. Graphitic carbon nitride (g‐C3N4) belongs to the family of carbon nitride compounds with a general formula near to C3N4 (albeit typically with nonzero amounts of hydrogen) and two major substructures based on heptazine and poly(triazine imide) units (Figure 4.2D). Graphitic carbon nitride is usually prepared by polymerization of cyanamide, dicyandiamide, or melamine. Depending on reaction conditions, carbon nitride exhibits different degrees of condensation, properties, and reactivities. On the other hand, covalent triazine frameworks (CTFs) are structurally related to polymeric carbon nitride (Figure 4.2E). CTF is a high‐performance polymer framework based on triazine with regular porosity and high surface area. It can be obtained by dynamic trimerization reaction of simple, economical, and abundant aromatic nitriles in ionothermal conditions. Primarily, these materials are large bandgap semiconductors, but their bandgaps are tailorable. For this reason, they are being investigated for photocatalysis. In addition, the loading of metal catalyst should be feasible, due to the presence of the abundant nitrogen atoms and voids within the structure. In fact, they are outstanding supports for SACs because they can stabilize metal ions or small metal nanoparticles even under harsh reaction conditions.

      Graphitic carbon nitride (g‐C3N4) has been proposed as support to coordinate metal (M–N2). This single‐site catalyst has shown excellent performance for the oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) electrocatalytic reaction and hydrogenation reactions. While the atoms on the alumina support are unstable and tend to aggregate, forming a Pd cluster, this does not occur for Pd SACs on mpg‐C3N4, which exhibited more stable performance. Besides SACs, atomically precise clusters with two Fe atoms (Fe2) have been stabilized on g‐C3N4 [27]. The preselected metal precursor bis(dicarbonylcyclopentadienyliron) (Fe2O4C14H10), containing two Fe atoms, ensures the formation of diatomic clusters, whereas mpg‐C3N4 provides abundant anchoring sites to stabilize the metallic species. A mild reduction process was selected (300 °C in 5% H2), leading to a complete removal of organic ligands from the precursors and, at the same time, prevent agglomeration of the Fe2 clusters. For the sake of comparison, Fe SACs from iron porphyrin precursor and Fe nanoparticles were supported on g‐C3N4 following the same methodology but with different precursors. Biatomic Fe2 species exhibit highest activity in epoxidation, possibly promoted by the formation of reactive oxygen species.

      CTFs can be used as support for SACs in electrocatalysis despite their modest conductivity. To increase their conductivity, CTFs have been hybridized with carbon nanoparticles [33]. SACs on CTFs are more stable for electrooxidation and electroreduction than homogeneous catalysts and immobilized organometallic catalysts, respectively, due to the rigid cross‐linked structure of covalent bonds in CTFs [34]. Pt SACs have been also supported on CTFs leading to a performance comparable to commercial catalysts but with a reduction of Pt loading by one order of magnitude [35].

      In summary, g‐C3N4 and CTFs are ideal to disperse single‐atom catalysts in a stable manner for catalysis and electrocatalysis combining the ligand effect found in homogeneous catalysis and the robustness typical of heterogeneous catalysis.

      4.2.5

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