Allotropic Carbon Nanoforms as Advanced Metal-Free Catalysts or as Supports

This perspective paper summarizes the use of three nanostructured carbon allotropes as metal-free catalysts (“carbocatalysts”) or as supports of metal nanoparticles. After an introductory section commenting the interest of developing metal-free catalysts and main features of carbon nanoforms, the main body of this paper is focused on exemplifying the opportunities that carbon nanotubes, graphene, and diamond nanoparticles offer to develop advanced catalysts having active sites based on carbon in the absence of transition metals or as large area supports with special morphology and unique properties. The final section provides my personal view on future developments in this field. 1. Introduction: From Active Carbons to Carbon Allotropes In classical heterogeneous catalysis, active carbons (ACs) have been widely used as supports for noble metal and metal oxides [1–3]. ACs are high surface area materials having carbon as predominant element in their composition that are obtained by pyrolysis of available biomass wastes, upon addition of inorganic reagents to promote the carbonisation process. For instance, one popular active carbon comes from coconut shells adequately powdered, pyrolyzed at 600°C under N2, mixed with phosphoric acid for activation, and then baked at temperatures below 300°C [4, 5]. In other recipes, olive seeds or almond shells are used as AC precursors and other mineral acids or oxidizing chemicals are employed as additives [6–8]. The structure of ACs is poorly defined with domains of amorphous carbon and the presence of condensed polycyclic aromatic compounds forming platelets of nanometric dimensions that are interconnected by bridges that can be CH2 and heteroatoms such as O, NH, and S. In certain regions, ACs have graphitic domains when the platelets are large enough and stacking of the imperfect graphene (G) sheets can take place. Besides oxygen and other elements such as nitrogen or sulphur, metal traces such as iron, zinc, and copper, are very frequently present in the final composition of the material because these transition metals have been introduced as additives in the pyrolysis process and they remain in residual, sometimes not negligible, percentages. Understanding the mechanism in heterogeneous catalysis largely depends on the exhaustive characterization of the solid catalyst and on the knowledge on the architecture of the active sites [9]. In this sense, while ACs are available and affordable materials exhibiting high adsorption capacity, this property being suitable for their use as support, they are too complex and