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 are: (i) to uniformly disperse the nanoparticles on a support and (ii) the subsequent activation by the removal of the ligand using thermal treatment or mild oxidation. Carbon supports such as CNT or graphene are ideal supports to deposit preformed nanoparticles since the absence of microporosity and open porosity favors the infiltration of the nanoparticles to all the deep surfaces of the carbon material. To disperse the nanocrystals uniformly on a carbon support, the particle–support and particle–particle interactions should be balanced. If the interaction of the particles between themselves is stronger than with the support, the particles will tend to aggregate. The amount of ligand and the temperature can tune the strength of interactions of the nanoparticles with the support as demonstrated in the assembly of Fe nanocrystals on CNT [42]. To develop an active catalyst and make the metal accessible to reactants, the capping agent must be removed by thermal treatment or mild oxidation. The problem is that in this step, the nanoparticles may begin to coalesce. Therefore, the particles can change their size and shape if the oxidation step is severe. Therefore, the ligand should be removed under milder conditions as possible. In principle, stabilization should be achieved more easily on a carbon material with a higher amount of defects and oxygenated surface groups than on undefective graphitic carbon materials because the former has more anchoring sites for the nanoparticles after removing the capping agent. The removal of ligands is even more difficult for transition metals than for noble metal nanocrystals since the ligands usually bind stronger to the former.
4.3.2 Single‐Metal Atom Deposition by Wet Chemistry
Carbons doped with heteroatoms (e.g. N, B, P, S) have the ability to coordinate transition‐metal atoms and thereby stabilizing them as single‐site catalyst. To prepare single‐atom sites, loading of metal deposits on the heteroatom‐doped carbon substrate can be carried out using traditional wet impregnation, followed by acid leaching to remove nanoparticles and noncoordinated metal atoms. This leaves behind the strongly bound single metal atoms, each coordinated by the surrounding heteroatom dopants. In particular, the strong interaction between N‐dopant and metals has been verified theoretically and experimentally using techniques X‐ray photoelectron spectroscopy (XPS), near edge X‐ray absorption fine structure (NEXAFS) [43], and transmission electron microscopy (TEM)–electron energy loss spectroscopy (EELS) [44]. The interaction is enabled by the delocalized free electrons of N on carbon. Their origins in turn can be explained by the electron back‐donation, where the σ‐type electron from
