Carbons

Abstract

The element carbon plays a multiple role in heterogeneous catalysis. In homogeneous catalysis it occurs of course as the most prominent constituent of ligand systems and will be treated as such there. Carbon-containing molecules are, in most catalytic applications, the substrates of the process under consideration. Deposits and polymers of carbon often occur as poisons on catalysts. Carbon deposition is the most severe problem in certain zeolite applications. In hydrogenation reactions carbon deposits are thought to act as modifiers for the activity of the catalyst and to provide selectivity by controlling the hydrogenationdehydrogenation activity of the metal part of the catalyst. Carbon deposits are even thought to constitute part of the active sites in some cases. Carbon is a prominent catalyst support material as it allows the anchoring of metal particles on a substrate which does not exhibit solid acid-base properties. Carbon is finally a catalyst in its own right allowing the activation of oxygen and chlorine for selective oxidation, chlorination and de-chlorination reactions. These multiple roles of the element carbon are in parallel with a complex structural chemistry giving rise to families of chemically vastly different modifications of the element. It is a special characteristic of carbon chemistry that many of these modifications cannot be obtained as phase-pure materials. This limits the exact knowledge of physical and chemical properties to a few archetype modifications, namely graphite and diamond. The reason for this poor definition of materials is found in the process of its formation. This is comprised of difficult to control polymerisation reactions. Such reactions will occur also in catalytic reactions with small organic molecules. The nature of carbon deposits will therefore reflect all the complexity of the bulk carbon materials. It is one aim of this article to describe the structural and chemical complexity of "carbon" or "soot" in order to provide an understanding of the frequently observed complexity of the chemical reactivity (e.g. in re-activation processes aiming at an oxidative removal of deposits). The surface chemistry of carbon which determines its interfacial properties is a rich field as a wide variety of surface functional groups are known to exist on carbon. The by far most important family of surface functional groups are those involving oxygen. Other prominent heteroatoms on the surface are hydrogen and nitrogen. This article focusses on the description and characterisation of oxygen surface functional groups. In the final section, selected case studies of carbon chemistry in catalysis will be discussed in order to illustrate characteristic properties of carbon materials as wanted or unwanted components in catalytic systems. Regarding the literature on basic properties of carbon materials, the reader is referred to the extensive collection of review papers published in the series Chemistry and Physics of Carbon. Editors P.L.

Activated carbons for applications in catalysis: the point of view of a physical-chemist

“…Despite the large amount of literature devoted to the structural and surface characterization of ACs for catalytic applications (Henning and von Kienle 2010;Marsh and Rodríguez-Reinoso 2006;Schlögl 2008), conflicting information and highly fragmented results are often found. Due to the intrinsic complexity of ACs, a single technique is not able to render a complete picture of the overall properties of these materials.…”

“…It is dominated by 2 intense bands, attributed to vibrational modes involving sp 2 carbon species of disordered microcrystalline domains. The signal around 1600 cm −1 (G band) is due to the stretching of sp 2 C-C bonds (either in aromatic rings or chains) (Henning and von Kienle 2010;Marsh and Rodríguez-Reinoso 2006;Schlögl 2008). For crystalline graphite, this mode has E 2g symmetry (Fig.…”

“…1a) with an average particle size in the order of tens of micrometers. Each particle is constituted by small-sandwiched graphitic platelets, each of those is decorated by a different amount and type of functional groups (Henning and von Kienle 2010;Marsh and Rodríguez-Reinoso 2006;Schlögl 2008). (1), carboxylates (2), lactones (3), lactols (4) and CH groups, present at the edges of a sp 2 domain, as well as a possible defect (H-terminated hole).…”