and (b) PCTF‐1 synthesized by phosphorous pentoxide catalyst (P2O5) catalyzed condensation method.
4.2.3 Catalyst on Nanodiamonds and Onion‐Like Carbon
Nanodiamonds are diamonds (carbon allotrope with sp3 hybridization) of 2–5 nm found in meteorites and interstellar dust [23]. Recently, nanodiamonds have been produced synthetically in the form of films and powders. A large quantity of nanodiamond powder has been successfully synthesized by the detonation of explosive carbonaceous mixture. In fact, carbon atoms in nanodiamonds do not have a purely diamond structure, rather they have an intermediary structure with sp2 and sp3 character, with a diamond‐like core covered by an outer shell of graphitic/amorphous carbon (Figure 4.3). High‐temperature annealing (>1750 °C) of nanodiamonds could transform them into “carbon nano‐onions” (Figure 4.3a). Annealing at lower temperatures gives rise to intermediate sp3@sp2 core–shell structures. The outer graphitic layer is amenable to functionalization and doping, which has been exploited to prepare metal‐free catalysts in several reactions. Moreover, nanodiamonds have been used as metal catalyst support, and the following paragraphs describe more in detail some of the few preparations of catalysts supported on nanodiamonds. As explained, annealing of nanodiamonds at increasing temperatures increases the thickness of the graphitic shell, ultimately transforming them into onion‐like carbon (OLC). By exploiting this concept, some researchers prepared intermediate nanodiamond core/graphitic shells (ND@G) and OLC. Pt nanoparticles (<2 nm) were deposited by incipient wetness impregnation on both supports, and Pt/ND@G exhibited higher turnover rate in CO catalytic oxidation compared to Pt/OLC and Pt/Al2O3 [25]. Moreover, they were tested in propane dehydrogenation at 600 °C. Under this demanding reaction condition, Pt/ND@G was shown to be significantly more stable than Pt/Al2O3, and its stability was attributed to the donation of electron density from the support to the metal (strong metal–support interaction [SMSI]), leading to less crystalline nanoparticles. Due to SMSI, sintering of the metal is prevented and formation of coke is decreased due to the enhanced desorption of propylene. One of the disadvantages of nanodiamonds is that they tend to form aggregates (up to 200 nm). Aggregation can be overcome by dispersing and stabilizing nanodiamonds on a support such as the formation of hybrids with graphene [26].
Figure 4.3 (a) Gradual transformation of nanodiamond to onion‐like carbon at increasing annealing temperatures. (b) Schematic representation of nanodiamond.
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.
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.
4.2.5
