Читать книгу Heterogeneous Catalysts - Группа авторов - Страница 52

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.

On the other hand, CTF also coordinated noble metal atoms by simply hydrothermally treating (60 °C) the mixture of precursors. The 2 N atoms of bipyridinic regions can coordinate the molecular catalyst [28] and also metal atoms after the corresponding reduction [29]. Pt atoms coordinated to CTF have close similarities to the molecular Periana catalyst Pt(bpym)Cl₂, which is active and selective for the partial oxidation of methane via C–H activation in fuming sulfuric acid [30]. Therefore, Pt/CTF combines the advantages of homogeneous catalyst, CTFs acting as a ligand, and the robustness of heterogeneous catalysts supplied by the rigid CTF network. The porous structure of CTFs can be also tailored. CTFs with varying pore size, specific surface area, and N content could be prepared varying the monomers, the linker, and the synthesis time [31]. Ru clusters on CTFs with a mesoporous structure provides highest conversion in the selective oxidation of hydroxylmethylfurfural (HMF) compared to other support materials such as activated carbon, g‐Al₂O₃, hydrotalcite, or MgO. Moreover, CTFs have been shaped into spheres of a few hundreds of microns in diameter to increase their robustness [32].

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.