G as Support of Metal NPs

G as support of metal NPs exhibit at least four unique features that makes this type of 2D materials very suited for this application. The main characteristic 

According to the previous considerations, the large number of reports already existing in the literature using metal NPs supported on G materials 

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G as Support of Metal NPs Used as Catalyst for Oxidation Reactions... 

In the present chapter we will summarize some of the properties of G that have special relevance in the use of these materials in catalysis. We will also present those experimental techniques that can be used to determine the nature and structure of the active sites. The purpose of these introductory sections is to provide enough information to understand the origin of the catalytic activity of G-based materials and how to characterize G for its use as catalyst. The second part of this chapter will describe some of the most important catalytic reactions that have been reported so far using G in catalysis. These sections include not only the use of G in catalysis but also the use of G as support of metal or metal oxide nanoparticles (NPs). The purpose of this part is not to cover all the existing literature, but rather to present some of the most relevant achievements and also to compare the activity of G-based materials with other catalysts. In the last section we will comment on our view on the future challenges and trends in the field. 

As described above, ILs for functionalization of CNTs are generally low molecular organic salts. Recently, Chen et al. [146] have employed PIL-functionalized MWCNTs to serve as the templates for fabrication of metal NP hybrids. In this method, the IL of 3-ethyl-l-vinylimidazolium tetrafluoroborate (EVimBF ) was in situ polymerized to form a poly(EVunBF ) shell on the surface of MWCNTs (Fig. 15.11). This not only makes MWCNTs uniform dispersion in water but also serves as the medium to stabilize and grow metal NPs. It is noted that metal NPs (e.g., Pt, PtRu) are uniformly dispersed on the supported surface of poly(EVimBF )/... 

In those cases in where G is acting as support of active sites such as metal NPs, then the typical techniques for this type of NPs, as for instance... 

In some of the above precedents on oxidation reactions using metal NPs supported on G it can be concluded that very frequently the use of G as support has considerable advantages in terms of catalytic activity with respect to... 

Considering the featmes of G as one atom thick surface combining the possibility to imprint the active site or support the metal NPs, the use of G could lead in a few years to a drastic change in the panorama of catalysis optimizing the use of noble and critical metals and reducing the dependency of catalysis on these inorganic elements. 

The use of organometallic chemistry concepts already gave rise to relevant results in the synthesis of precisely defined metal NPs employing polymers, ligands, ILs, or supports as stabilizing environments. A selection of tools for the synthesis of metal NPs of diverse nature, the identification and quantification of surface species (e.g., the reactivity of surface hydrides), the determination of coordination mode of Ugands and small reactive molecules (e.g., reactivity of surface carbon monoxide (CO)), and selected examples of applications of the so-obtained nanomaterials in catalysis will be described in the following sections of this chapter. 

As we have commented earlier, there are some reports in where polymers are used to act as interface between the metal NPs and the G support increasing the adherence of the metal NPs to Gs. In one of these examples polypropylene imine dendrimer has been used to increase the stability of Pd-Co alloy NPs on r-GO (Scheme 3.42). This dendrimer modified r-GO catalyst is able to promote the Sonogashira coupling of alkynes and aryl halides using K COj at 25 °C. Importantly metal leaching was not observed and as a reflection of the catalyst stability, the material could be reused six times with the catalytic activity decreasing only from 99 to 93 % after the sixth cycles. 

With regard to the use of G materials as supports, also design and modification of G support to increase the interaction with the metal are key concepts to fully exploit the promises hold by the 2D morphology of Gs and their interaction with the metal NPs. The target in this area is to show the advantages in terms of optimal use of support, fine tuning of the catalytic activity of the metal and stability of G-based supported catalyst with respect to any other support including metal oxides. 

For Au NPs supported on the MgO(lOO) surface TEM has shown that even very small clusters are well faceted and fee ordered (36). This is explained by the strong adhesion and small lattice mismatch ( 3%) between Au and MgO, which favors epitaxial Wulf-Kaischew-Iike morphologies. For larger lattice mismatch (e.g. Pd/MgO) NPs may be significantly strained and contain dislocations. To model these effects theoretically requires carefiil treatment of the metal-substrate interaction (37). In contrast for weakly interacting supports, such as graphite, strained decahedral and icosahedral clusters have been observed by TEM (33). 

In the last 15 years, the application of supported metal nanoparticles as catalysts in organic synthesis has received a renewed interest. The association between a metal and a support could result in synergistic effects which would precisely drive the reactivity of these nanocatalysts, e.g. supported-gold NPs for the oxidation of carbon monoxide [127]. The recyclability and recovery from the reaction medium still remains one of the major drawbacks to a widespread use of NPs in catalysis. To overcome these problems, the immobilization of MNPs on solid supports appears as a promising alternative. Synthetic methods are also developed to achieve the direct synthesis of MNPs in the presence of a support in a controlled manner. Intensive work is made on the functionalization of the support to increase the anchorage of the particles, inspired by the ligands which are used to stabilize MNPs in solution. 

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