Heterogeneous Catalysts

Figure 4.1 Structure of graphene oxide based on the Lerf–Klinowski model.

Source: Dreyer et al. 2010 [6]. Reproduced with permission of Royal Society of Chemistry.

To prepare supported metal catalysts on GO while preserving properties of the latter (i.e. high oxygenated group content), one should ensure the reduction of the metal precursor but avoid the overreduction of GO. Overreduction of GO leads to reduced graphene oxide (rGO) support. Therefore, mild reduction methods should be used to preserve GO surface chemistry. The nucleation of noble metal nanoparticles can be carried out by hydrothermal treatment without any reductant, microwave heating, or using mild reductants such as glucose. Depending on the reduction temperature, the resulting substrate may be retained as GO (at 25 °C) or converted to rGO (at 60 °C) [7]. Alternatively, the reduction of pre‐impregnated metal precursors can be carried out in the gas phase under flowing H 2at low temperatures (250 °C).

The main advantage of using GO as metal catalyst supports is that the oxygenated surface groups of GO naturally act as nucleation sites for nanoparticles, thus minimizing the aggregation. In that sense, the use of a capping agent that stabilizes bare metal nanoparticles can be mitigated. Moreover, GO can be functionalized with specific ligands such as dopamine that allow subsequent binding with metal nanoparticles [8]. The rich surface chemistry of GO makes it hydrophilic, and it can even catalyze side reactions. These reactions can be beneficial as a bifunctional catalyst in tandem reactions such as coupling followed by hydrogenation [9]. In the event that the reduction potential of the reductant is greater than that of pristine GO, another type of support, i.e. rGO, results, as will be explained in Section 4.2.2.1. To the best of our knowledge, the formation of single‐metal sites on GO has not been described yet, probably because the excessive density of nucleation sites prevents the isolation of single sites and favors the aggregation to metal nanoparticles.

Graphene has a number of favorable properties as catalyst support compared to other carbon materials. Graphene has a theoretical specific surface area as high as ∼2600 m 2/g, which is twice that of single‐walled CNTs and much higher than those of most carbon blacks and activated carbons. This structure makes graphene highly desirable for potential applications as a 2D support for loading metal catalysts. The absence of micropores favors accessibility of reactants and desorption of products. Moreover, the locally conjugated structure endows graphene with enhanced adsorption capacities toward nonpolar aromatic substances, which can be beneficial for catalytic reactions involving such compounds. In contrast, graphene has low wettability in polar solvents, rendering aqueous‐phase impregnation forbidden. Graphene materials can be obtained at a relatively low cost on a large scale by using graphite or graphite oxide and its derivatives as starting materials. The catalyst preparation can start from graphene or from GO that is subsequently reduced to rGO. The graphene materials are free from the metallic impurities that are almost inevitably present in CNTs, which is one of the drawbacks of the latter as catalyst supports. The superior electron mobility of graphene can facilitate efficient electron transfer during the catalytic reactions, thereby improving its catalytic activity. Finally, graphene has also high chemical, thermal, optical, and electrochemical stabilities, which can possibly improve the durability of the catalysts.

A wide variety of methods, such as hydrothermal procedure and microwave‐assisted heating, have been developed for the synthesis of metal nanoparticles supported on graphene sheets. To prepare the supported noble metal nanoparticles, hydrazine, NaBH 4, and ethylene glycol are generally used as reductants. For transition‐metal nanoparticles (Fe, Co, Mn), NaOH or aqueous ammonia is often used to hydrolyze metal salts (to metal hydroxide precipitates). Sometimes, reduction is carried out under high pressures using supercritical water or CO 2where the graphene support would act as a reductant (carbothermal reduction). Although small noble metal clusters (< 2 nm) could be formed directly on rGO without adding any reductant or capping agent [10], the alternative is to presynthesize the fine nanoparticles using the organometallic approach, and followed by the deposition on rGO and removal of capping agents.

TrGO tends to have higher concentration of defective (heteronuclear) sites, which turns to be beneficial as the adsorption sites for metal cations. With its mildly reducing behavior, the rGO sheets subsequently act as electron donors for metal clusters to grow on its surface. The formation of small clusters only occurs when using rGO with low density of heteronuclear sites, where nucleation sites are distanced adequately. If the GO substrate is not sufficiently reduced, the rGO contains excessive nucleation sites and results in large nanoparticles. In a recent work [11], the reducing potential of graphite intercalation compounds (GICs) as precursors for graphenide solutions is used to deposit transition‐metal nanoparticles and metal oxides on graphene under mild conditions and without the use of other reductants. Small Fe nanoparticles (2–5 nm) were prepared by this method.

In general, the main problem encountered when using graphene as a catalyst support is that the basal planes lack the anchoring points for the metal and the metal sinters in the subsequent treatments of removal of capping agents or reduction. A nonpolar solvent or a mildly polar solvent is preferred for impregnation of the metal precursor. The advantage is that graphene is intrinsically reductive (albeit mildly) to circumvent the use of reductants. Sometimes, sacrificial stabilizing agents such as metal oxides are used to prevent the sintering of nanoparticles during thermal treatment [12]. Another option is to use GO to anchor a metal precursor and reduce both GO and the metal precursor simultaneously, i.e. GO reduced to rGO and metal precursor to metal nanoparticles. This approach is explained in Section 4.2.2.2.

This method is derived from the preparation described in Section 4.2.1, but the reduction is stronger and leads to the simultaneous reduction of both the metal precursor and GO. This is the most commonly used method to prepare metal/graphene catalysts due to the strong interactions of GO with the metal precursor and the possibility of carrying out impregnation in aqueous phase. A composite based on metal ions and GO is prepared first, followed by a harsh reduction treatment using a reducing agent either in liquid or gas phase. The final product consists of metal nanoparticles on rGO. When the reduction is carried out in liquid phase, nucleation and reduction of the metal take place simultaneously. This is the case of solvothermal reduction in liquid phase at moderate temperatures. Generally, the mixture of GO, metal precursor, and reducing agent (e.g., hydrazine, ethylene glycol, NaBH 4) is heated (100–150 °C) by conventional heating or microwave, leading to the simultaneous reduction of GO that becomes rGO, resulting in a metal supported on graphene material. The simultaneous reduction of GO and metal in solution usually leads to a broad metal distribution or to relatively large nanoparticles (50–100 nm).

The reduction can also be carried out in gas phase (e.g., H 2gas) after drying the pre‐impregnated metal precursor on GO. The method involves several steps of impregnation, drying, and reduction, leading to nanometric particles, i.e. Pd and Pt nanoparticles of around 2 nm, whereas the size of Ni and Mn nanoparticles is between 4 and 6 nm [13]. The first step is the intercalation of a chosen metal salt into GO via impregnation, forming the metal salt anchored on GO oxygenated groups. In the second step, this composite undergoes an explosive reaction or “popping” at around 200 °C, leading to the formation of large‐surface‐area graphene with well‐dispersed, partially decomposed metal precursors strongly anchored on the graphene sheets.