Heterogeneous Catalysts. Группа авторов

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 Catalyst on Carbon Material from Hydrothermal Carbonization of Biomolecules

      Hydrothermal carbon is synthesized by the treatment of biomass or carbohydrate molecules under high pressure hydrothermal conditions [36]. The carbonaceous material produced has a spherical shape and diameters of a few hundreds of nanometers and contains high oxygen content (30–40%). It is characterized by hydrophilic external layers, while being more hydrophobic (bearing less oxygen) at the core.

Metal catalysts have been prepared using the one‐pot approach by introducing a metal precursor during the hydrothermal treatment. Depending on the metal precursor, the metal is deposited on the external hydrophilic surface or on the internal hydrophobic surface. Hydrophobic metal precursors such as Pd acetylacetonate tend to be reduced on the internal hydrophobic core of the carbon spheres, leading to the metal core–carbon–shell structure (Figure 4.4) [37]. The catalyst shows higher selectivity in the hydrogenation of phenol to cyclohexanone compared to the charcoal‐supported catalyst. The enhanced performance of the former was attributed to hydrophilicity of the carbon shell. On the other hand, transition‐metal ions (Fe³⁺, Ni²⁺, Co²⁺, Ce⁴⁺, Mg²⁺, and Cu²⁺), which are less reducible, tend to bound to the hydrophilic shell of the carbon particles, leading to carbon core–metal shell structures upon calcination [38].

Figure 4.4 Hydrothermal carbon spheres (a) and noble metal@carbon core–shell structures (b).

      Source: Makowski et al. 2008 [37]. Reprinted with permission of Royal Society of Chemistry.

To the best of our knowledge, there are only a handful of examples in the literature on the use of hydrothermal carbon for catalytic applications, either as metal‐free or hybrid catalysts with supported metals or metal oxides. For instance, hybrid inorganic–organic niobia–carbon catalyst has been prepared in one pot by hydrothermal carbonization of glucose, ammonium niobium oxalate, and urea [39]. Improved hydrothermal stability for aqueous‐phase reactions was demonstrated for the highly dispersed niobia particles embedded within carbon. Besides the addition of active phase in one pot, metal active phase has been supported on previously prepared hydrothermal carbons in a second step. In some cases, the hydrothermal carbon materials are further carbonized in an inert atmosphere at high temperatures. In a recent work, hydrothermal carbon spheres were graphitized at 1900 °C, on which cobalt Fischer–Tropsch catalyst was subsequently dispersed by both chemical vapor deposition (CVD) and wet impregnation [40]. In both cases, the primary particles had a mean size of 5 nm, whereas the catalyst particles prepared by wet impregnation were aggregated up to 100 nm. The former produced 5 times more oxygenates than conventional Co on alumina catalyst.

      The use of hydrothermal carbon in catalysis is still in its infancy. The hydrothermal carbon material has the potential as a catalyst support due to its natural origin, sustainable production process, and the ability to tune the carbon material properties (porosity, wetting properties, amphiphilicity, doping) for different reactions.

4.3 Emerging Techniques for Carbon‐Based Catalyst Synthesis

Figure 4.5 displays some of the most relevant techniques for the preparation of metal catalyst on carbon supports. One of the core objectives is to produce metal catalysts of uniform and defined particle size, whether as single atoms, clusters, or nanoparticles. It has been recognized recently that SACs provide higher activity and selectivity in several catalytic reactions that involve small molecules such as oxygen reduction, hydrogen evolution, methane activation, CO₂ reduction, CO oxidation, or organic synthesis among others (see Chapter 6). Likewise, there are reactions that require large ensembles of atoms or nanoparticles. For example, cobalt catalyst size 5–6 nm is required for Fischer–Tropsch synthesis (FTS) to achieve optimum activity and selectivity toward large‐chain hydrocarbons [41]. The deposition of presynthesized colloidal nanoparticles of well‐defined size and compositions on carbon supports with controlled interparticle distances is a major challenge that will be addressed below. The goal is to enable catalyst engineering with close to atomic‐scale precision to enable efficient heterogeneous reactions.

Figure 4.5 Relevant techniques optimized to prepare engineered catalysts on carbon materials.

      4.3.1 Deposition of Colloidal Nanoparticles

Recent advances in colloidal synthesis have enabled preparing monometallic or bimetallic nanoparticles (e.g. alloys, core–shell structures) with well‐defined structures and sizes. These nanocrystals have ligands or capping agents that need to be removed prior to catalysis to increase the activity, although in some cases the ligands can be reaction promoters when chosen judiciously. In general, the two main challenges to prepare an efficient and practical catalyst from colloidal nanoparticles