Metal-Free Carbon Nanomaterials Become More Active than Metal Catalysts and Last Longer

Many reactions involve metals, especially noble metals or metal oxides as catalysts. Although metal-based catalysts have been playing a major role in various industrial processes, they still suffer from multiple competitive disadvantages, including their high cost, susceptibility to gas poisoning, and detrimental effects on the environment. Owing to their wide availability, environmental acceptability, corrosion resistance, and unique surface properties, certain carbon nanomaterials have recently been demonstrated to be promising metal-free alternatives for low-cost catalytic processes. This perspective highlights recent progresses in the development of carbon-based metal-free catalysts. G enerally speaking, a working catalyst must possess surface active sites necessary for adsorption of the reactants, bond-breaking and bond-formation, and desorption of the products. Additionally, an excellent structural stability is also essential to ensure that the catalytic activity is effective and efficient over a long period. Metals and metal oxides are undoubtedly the most widely used catalysts in many industrialized catalytic processes. Pt, Au, and Ru are a few examples of the noblemetal catalysts used in fuel cells to accelerate the oxygen reduction reaction (ORR) at the cathode,whilemanyhydrogenation anddehydrogenation reactions involve metal oxides as catalysts. However, these metal-based catalysts often suffer from multiple competitive disadvantages, including their high cost, low selectivity, poor durability, and detrimental environmental effects caused by catalyst residues and/or undesirable side-products. Therefore, it is highly desirable to develop inexpensive, metal-free catalysts of high performance. Owing to their wide availability, environmental acceptability, corrosion resistance, and unique surface properties, carbon nanomaterials are ideal candidates for metal-free catalysts. While activated carbon and glassy carbon (GC) have been long used as catalysts for certain chemical and electrochemical processes, the recent availability of carbon nanomaterials of various peculiarmolecular structures andoptoelectronic properties, including fullerenes, carbon nanotubes (CNTs), nanodiamonds, and graphene sheets, offer new opportunities for the development of advanced carbon-based catalysts with much improved catalytic performance. The introduction of surface heteroatoms (e.g., nitrogen) into these carbon nanomaterials could further cause electronmodulation to provide desirable electronic structures for many catalytic processes of practical significance. Consequently, considerable effort has recently been directed toward the development of metal-free carbon nanomaterials for various catalytic processes, involving either oxidation or reduction reactions. This perspective highlights recent progresses in the development of carbon-based metal-free catalysts with an emphasis on the use of CNTs for oxidative dehydrogenation (ODH) of aromatic hydrocarbons and alkanes as well as ORRs, particularly in alkaline medium (Scheme 1). Metal-Free Carbon Catalysts for ODH of Aromatic Hydrocarbons and Alkanes. Conventional heterogeneous catalysts often containmany active sites with a low free energy for chemisorptions, which also act as a matrix for a relatively small Scheme 1. ODH of (1) Ethylbenzene and (2) Alkanes (a) and ORR in Alkaline (b) and Acidic (c) Media Received Date: April 26, 2010 Accepted Date: June 23, 2010