15. Nanocarbon materials for heterogeneous catalysis

Abstract

Heterogeneous catalysis is of paramount importance in many areas of the chemical and energy industries as more than 90 % [1] of the chemical manufacturing processes in use throughout the world utilize catalysis and primarily heterogeneous catalysis. Catalytically active metal (oxide) nanoparticles are often dispersed on a mechanically stable high surface area support to maximize and stabilize their specific surface area as well as to fine-tune their properties and minimize the materials usage, e.g., of expensive noble metals. Among the different types of supports used in heterogeneous catalysis the carbon materials attract a permanent interest due to their specific characteristics, which are mainly: (i) resistance to acidic/basic media and thermal sintering, (ii) control over macroscopic shape, porosity, and surface chemistry, and (iii) disposal or recovery of precious metals by support burning, which results in a low environmental impact. The use of, e.g., graphite, carbon black, activated carbon, activated carbon fibers, glassy carbon, pyrolytic carbon, or polymer-derived carbon has a long history in this field of research [2][3][4]. Carbon materials providing a distinct structure on the nanometer scale, hereinafter referred to as nanocarbons, are equipped with further exceptional physical and chemical properties. Thus, the discovery of lowdimensional nanostructured carbon allotropes, such as carbon nanotubes (CNTs), nanofibers (CNFs), graphene, or fullerenes, ushered in a new era of cutting-edge applications for elemental carbon with a wide-ranging impact also on heterogeneous catalysis. The development of large-scale synthesis processes for multi-walled CNTs [5, 6] stimulated a vital research activity in academia and industry. It is recognized that carbon as a catalyst support as well as a catalyst on its own offers unparalleled flexibility in tailoring catalyst properties to specific needs. Carbon-supported noble metals for fine chemicals production are well established, but only a few large-volume processes currently use these systems.Besides the practical application, the diversity of nanostructured carbon allotropes makes nanocarbon also an ideal model system for the investigation of structure-function correlations in heterogeneous catalysis. Nanocarbons can be tailored in terms of their hybridization state, curvature, and aspect ratio, i.e., dimensions of stacks of basic structural units (BSU), Chapters 1 and 2. The preferred exposition of two types of surfaces, which strongly differ in their physico-chemical behavior, i.e., the basal plane and prismatic edges, can be controlled. Such controlled diversity is seldom found for other materials giving carbon a unique role in this field of basic research. The focus of this chapter is set on the most prominent representatives of the