Metal nanoparticles supported on two-dimensional graphenes as heterogeneous catalysts

Abstract

The catalytic activity of metal nanoparticles (MNPs) is highly dependent on the nature of the support. In addition to the role of particle size stabilization in decreasing the spontaneous growth of small MNPs, the main role of the support is to cooperate by providing efficient pathways that lead to the target product. Thus, the necessary requirements for supports include a large surface area, strong metal–support interaction, and the presence of active sites that participate in the reaction mechanism. Active carbons as well as organic polymers and large surface area inorganic metal oxides are typical insoluble solids that are used frequently as supports. Furthermore, the recent availability of suspensions of graphene oxide (GO), reduced GO, and other graphene-based materials (Gs) has provided new opportunities for the development of supported MNPs as catalysts. As supports, Gs combine several useful properties that are not encountered in classical solid supports. Gs comprise sheets that are a single carbon atom in thickness, which approaches the physical limit for a two-dimensional (2D) surface in which MNPs can be deposited. Therefore, Gs are among the solids with the highest possible surface area and due to their single layer morphology, they are readily dispersed in a liquid phase with the appearance of homogeneous catalyst, but they are easily recovered by filtration or centrifugation. In addition, Gs may cooperate with the catalytic cycle involving MNPs in at least four distinctive ways: (i) by strong adsorption of the substrates and reagents near the MNP; (ii) via d–π metal support interaction, which influences the electron density of the MNP; (iii) promoting substrate reactivity by giving or withdrawing the electron density from the substrate; and (iv) by making specific catalytic sites available on the G nanosheet due to defects, oxygenated functional groups, or the presence of dopants. This review highlights the specific features derived from the morphology and characteristics of Gs, as well as the different catalytic behaviors of G-supported MNPs compared with related catalysts. One of the aims of this review is to provide a reference to indicate best practices as well as suggesting benchmark reactions to evaluate the catalytic activity of different materials. Considering the growth in the use of G as supports and the unique features obtained by employing 2D Gs as supports for MNPs, the present review has implications in the fields of catalysis, biocatalysis, and material science.