atom, leading to a π back‐donation from filled metal atomic orbital to the antibonding orbital of the N atom.
4.3.3 Immobilization of Metal Clusters and SACs by Organometallic Approach
Mixed‐metal clusters modified with appropriate ligands can be immobilized onto CNTs and graphene using covalent or noncovalent π–π interactions [45] (see Chapter 5 on metal clusters synthesis). Subsequent removal of the stabilizing ligand by thermal treatment may lead to a different extent of coalescence, which depending on the exposed temperatures can result in carbon‐supported nanoparticles.
When the preparation of SACs on carbon is sought, an interesting approach is to use organometallic precursors containing both the dopant element such as nitrogen and the metal. Macrocyclic complexes bearing transition‐metal atoms similar to porphyrins, which contain both the dopant element and the metal, are excellent precursors to CUS or SACs. These macrocycles are noncovalently bound on graphitic materials by π–π stacking. The covalent anchoring of these macrocycles on carbon lattice upon pyrolysis is favored by the affinity of nitrogen atom for carbon materials. This will pave the way for stabilizing the catalyst under different solvents and under severe reaction conditions. Several macrocyclic molecules containing both metal and nitrogen atoms such as iron porphyrin or macrocycles formed by mixing phenanthroline or 2,2‐bipyridine and Fe salt have been used to prepare atomically dispersed Fe–Nx species on different carbon materials. Figure 4.6 shows some macrocycles used as precursors of single‐atom metal–Nx species on carbon materials. The special ligand endows the metal with the electronic density necessary to perform a reaction selectively suppressing undesired collateral reactions. The catalysts prepared have shown high activity in ORR comparable to Pt catalyst with high selectivity in the four‐electron reduction.
Figure 4.7 Different precursors to prepare nitrogen‐coordinated SACs. (a) Iron porphyrin, (b) iron phthalocyanine, (c) phenanthroline + Fe salt, and (d) 2,2‐bipyridine + Fe salt.
4.3.4 Chemical Vapor Deposition Techniques on Carbon Supports
CVD of organometallic precursors under oxidative conditions is a conventional technique for the deposition of nanoparticles on supports. Recently, atomic‐layer deposition (ALD) has emerged as a particular case of CVD. In contrast to conventional CVD, precursors are introduced as a series of sequential and nonoverlapping pulses. In each of these pulses, the precursor molecules react with the surface and the reaction ends once all the reactive sites on the surface are utilized. Therefore, the amount of material deposited depends on the precursor–surface interaction and the number of ALD cycles.
The ALD on pristine carbon supports such as graphene can be hampered by the limited interactions associated with basal planes, and ALD is only able to deposit material on defects and edges. Moreover, noble metal ALD on graphene is reported to lead to the formation of a mixture of atoms, clusters, and nanoparticles of Pt. Accordingly, in order to deposit single‐atom metal sites on graphene by ALD, a previous step of functionalization with oxygenated groups is necessary. Only recently, ALD has been used to prepare single‐atom Pt site on graphene functionalized with oxygen reactive sites [46]. By repeating the number of ALD cycles (exposure to metal precursor – O2), it is possible to go from single atoms to clusters of controlled size.
4.3.5 Simultaneous Formation of Metallic Catalyst and Porous Carbon Support by Pyrolysis
The loading of SACs by wet chemistry methods depends greatly on the density of anchoring sites on the support, and it is usually low. To circumvent the limitation, pyrolysis (i.e. treatment at high temperatures under inert gas) of molecularly defined porous structures containing both the source of metal and carbon can lead to higher loadings. In fact, it would be interesting if the porous structure is retained in the final carbon material. Otherwise, a sacrificial template can be used to preserve the porosity upon pyrolysis. Metal–organic frameworks (MOFs; see Chapter 8 for more details), composed of periodic and porous networks of organic linkers with metal atom centers, are an excellent platform to prepare single‐site catalysts on porous carbon in one‐step pyrolysis. This approach has been used to attain transition‐metal single sites on nitrogen‐doped carbon [47].
4.3.6 Dry Mechanical Methods
There is direct formation of noble metal nanoparticles (<5 nm size) on CNT by high‐energy ball milling of a mixture of CNT and acetate precursor [48]. In fact, transition metal CUS on graphene can be achieved, for example, through the mechanochemical ball milling of graphene and phthalocyanine precursors [49]. Ball milling supplies the activation energy necessary to form bonds between the nitrogen atoms of phthalocyanine with dangling carbon atoms at the defects of graphene. This method is aggressive to the support, forming defects on the materials or decreasing the particle size. Accordingly, it is only applicable to carbon supports in powder form, which could be subsequently pelletized or structured macroscopically, if necessary.
4.3.7 Electrodeposition
Electrically conductive supports such as carbon are amenable to the deposition of metal nanoparticles by electrodeposition. This method allows the use of bulk metal, which is electrochemically stripped and subsequently electrochemically or electroless deposited on the carbon. It is a waste‐free method, comparing advantageously to other methods using metal precursors based on salts such as nitrates. A ligand is used, but this is recycled upon metal deposition. Generally, smaller particles are prepared by decreasing the concentration of the metal solution, the deposition time, or pH. The method must be optimized to prepare small particles of typically 7–20 nm [50]. Since the effects of loading and particle size are coupled, there is a trade‐off between achieving higher metal loadings and having smaller particle sizes.
4.3.8 Photodeposition
This method is mainly used for the deposition of metals on semiconductor materials (see for examples Chapters 11, 31, and 37 on the principles of photocatalysis). Carbon nitride is a carbon‐based semiconductor material. Pd nanoparticles have been deposited on carbon nitride, by photoassisted reduction of Pd(NO3)2 using ultraviolet (UV) illumination [51]. This method led to uniform distribution of Pd nanoparticles with the size of 6 ± 0.7 nm, and exhibited higher stability in ORR than the commercial Pt on carbon catalyst. A variant of photodeposition is the photo‐Fenton process, which capitalizes on the homogeneous photoreaction between dissolved Fe2+ and H2O2. On CNTs, Co and Fe nanoparticles (2–5 nm) have been deposited under UV illumination through a photo‐Fenton process [52]. In this process, UV‐induced hydroxyl radicals (•OH) oxidize the CNT surface in the presence of Fe or Co ions that are reduced and subsequently precipitated on the CNT surface in the form of nanoparticles.
4.4 Conclusions and Outlook
The optimization of catalyst prepared on activated
