Nanostructured Carbon Materials for Catalysis

Philippe Serp, Bruno Machado

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782624578
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

"We heartily recommend this book to all readers who wish to gain a better understanding of nanostructured carbon materials surface properties and used in catalysis." An-Hui Lu, ChemCatChem
There is great interest in using nanostructured carbon materials in catalysis, either as supports for immobilizing active species or as metal-free catalysts due to their unique structural, thermal, chemical, electronic and mechanical properties, and tailorable surface chemistry. This book looks at the structure and properties of different doped and undoped nanocarbons including graphene; fullerenes; nanodiamonds; carbon nanotubes and nanofibers; their synthesis and modification to produce catalysts. Special attention is paid to adsorption, as it impacts the application of these materials in various industrially relevant catalytic reactions discussed herein, in addition to photocatalysis and electrocatalysis. Written by leading experts in the area, this is the first book to provide a comprehensive view of the subject for the catalysis community.

CHAPTER 1

Carbon (Nano)materials for Catalysis

1.1 Introduction

Both organic and inorganic carbons play a key role in catalysis. Organic molecules form the huge and very complex discipline of organic chemistry, and they are, in most catalytic applications, the substrates and the products of the process under consideration. In homogeneous catalysis, carbon is often the main constituent of the organic ligands surrounding the metallic center. In enzymatic catalysis it constitutes the backbone of the active species. In heterogeneous catalysis, carbon materials are unique catalyst supports, allowing the anchoring of the active phase, and can also act as catalysts or catalyst poisons (carbon deposits) by themselves.

The physical and chemical properties of carbon materials, such as their tunable porosity and surface chemistry, make them suitable for application in many catalytic processes. Traditionally, carbon materials have been used as supports for catalysts in heterogeneous catalytic processes, although their use as catalysts on their own is becoming more and more common.¹ Although several kinds of carbon materials have been studied, activated carbon (AC) and carbon black (CB) are the most commonly used carbon supports. The typically large surface area and high porosity of activated carbon catalysts favor the dispersion of the active phase over the support and increase its resistance to sintering at high metal loadings. The pore size distribution can be adjusted to suit the requirements of several reactions. The surface chemistry of carbon catalysts influences their performance as catalysts and catalyst supports. Carbon materials are normally hydrophobic and they usually show a low affinity towards polar solvents, such as water, and a high affinity towards solvents such as acetone. Although their hydrophobic nature may affect the dispersion of the active phase over the carbon support, the surface chemistry of carbon materials can easily be modified, for example by oxidation, to increase their hydrophilicity and favor ionic exchange. Apart from an easily tailorable porous structure and surface chemistry, carbon materials present other advantages: (i) metals on the support can be easily reduced; (ii) the carbon structure is resistant to acids and bases; (iii) the structure is stable at high temperatures (even above 1023 K under inert atmosphere); (iv) porous carbon catalysts can be prepared in different physical forms, such as granules, cloth, fibers, pellets, etc.; (v) the active phase can be easily recovered; and (vi) the cost of conventional carbon supports is usually lower than that of other conventional supports, such as alumina and silica. Nevertheless, carbon supports also present some disadvantages: they can be easily gasified, which makes them difficult to use in high temperature hydrogenation and oxidation reactions, and their reproducibility can be poor, especially activated carbon-based catalysts, since different batches of the same material can contain varying ash amounts. In this introductory chapter, we will: (i) briefly introduce the main carbon and graphite (nano)materials relevant to catalysis, (ii) present the main application of carbon and graphite materials in catalysis, and (iii) highlight the possible perspectives of using nanocarbons in catalysis.

1.2 Carbon (Nano)materials

The capability of a chemical element to combine its atoms to form such polymorphs is not unique to carbon. Other elements in the fourth column of the periodic table (silicon, germanium, and tin) also have this characteristic. However carbon is unique in the number and the variety of its polymorphs. These allotropes are composed entirely of carbon but have different physical structures and, exclusively for carbon, have different names: graphite, diamond, lonsdaleite, and fullerene, among others. Additionally, carbon as a solid denotes all natural and synthetic substances consisting mainly of atoms of the element carbon, such as single crystals of diamond and graphite, as well as the full variety of carbon and graphite materials. A result of this diversity is that the carbon terminology can be confusing for the non-specialist. The terminology used so far is mainly based on technological tradition and on the standardized characterization methods derived from decades of industrial experience. As a consequence, for many years, carbon science was a very specialized field, considered by many to be too complicated. More recently, carbon science has gained high visibility with the discovery of fullerenes in 1985 and the first HR-TEM observations of carbon nanotubes (CNTs) in 1991. This visibility has been further heightened by the 1996 Nobel Prize in Chemistry awarded to R. F. Curl, H. Kroto and R. E. Smalley for their discovery of fullerenes and the 2010 Nobel Prize in Physics awarded to A. Geim and K. Novoselov for ground-breaking experiments regarding the two-dimensional material graphene (denoted SG for single layer graphene). Because of the increasing interdisciplinary importance of this group of materials in science and technology, it is obvious that clear definitions of the corresponding terms are required. In order to clarify the terminology, we will refer to the recommended terminology for the description of carbon as a solid (IUPAC Recommendations 1995). In the following part of this chapter we will briefly review the main carbon and graphite (nano)materials relevant to catalysis and their production processes.

1.2.1 Activated Carbons

The term activated carbon (also known as activated charcoal) defines a group of materials with highly developed internal surface area and porosity, and hence a large capacity for adsorbing chemicals from gases and liquids.² The adsorption on the surface is essentially due to Van der Waals or London dispersion forces. This force is strong over short distances, equal between all carbon atoms and not dependent on external parameters such as pressure or temperature. Thus, adsorbed molecules will be held most strongly where they are surrounded by the most carbon atoms. The area presenting a high density of graphitic basal structural units will favor a high adsorption. High temperature treatment (>1500 K) of AC can favor the adsorption sites by increasing the density of “π-sites” present on partly graphitized structure.^3,4 Almost all precursors containing a high fixed carbon content can potentially be activated. The most commonly used raw materials are coal, coconut shells, wood (both soft and hard),⁵ peat and petroleum based residues^6,7 or agricultural residues.^8,9 Most carbonaceous materials do have a certain degree of porosity and an internal surface area in the range of 10–15 m² g⁻¹. There are many activation methods to produce an AC that can basically be categorized into two: physical and chemical activation. In the same way, the manufacture of AC involves two main stages, the carbonization of the starting material and the activation of the resulting char. The choice of the activation method is dependent upon the starting material and whether a low or high density, powdered or granular carbon is desired.

Carbonization. The first step is a thermal treatment of the raw material that implies dehydration (eqn (1.1)) and where most of the non-carbon elements, such as dust and volatile substances, are eliminated by heating the source under anaerobic conditions. The aim of the carbonization stage is to conserve the carbonaceous structure of the material, which is achieved by burning off the material at a range of temperatures from 673 to 1123 K. The char is constituted when carbon atoms regroup themselves into sheets forming rigid and dense clusters of microcrystals, each one consisting of several layers of graphitic planes. Each atom inside one stack is bonded to four adjacent carbon atoms. Thus, the carbon atoms on the edges of the planes have a high adsorption potential available. The internal structure is neither homogeneous nor regular; leaving free interstices that constitute the porosity of the char. These interstices may be filled or blocked by disorganized carbon resulting from deposition and decomposition of tars making them not always accessible and reducing the porosity.

C_x(H₂O)_y → C(s) + yH₂O Carbonization (1.1)

There are three stages in the carbonization process. First of all, is the loss of water in the 373–473 K range. The second stage is the primary pyrolysis, which takes place in the 473–773 K temperature range and is characterized by a large gene...

Table of contents

• Cover image
• Title page
• Copyright
• Foreword
• Preface
• Contents
• Chapter 1 Carbon (Nano)materials for Catalysis
• Chapter 2 Classification, Structure and Bulk Properties of Nanostructured Carbon Materials
• Chapter 3 A Molecular View of Adsorption on Nanostructured Carbon Materials
• Chapter 4 Surface Chemistry of Nanostructured Carbon Materials and Preparation of Nanocarbon Supported Catalysts
• Chapter 5 Nanostructured Carbon Materials as Catalysts
• Chapter 6 Doped Nanostructured Carbon Materials as Catalysts
• Chapter 7 Heterogeneous Catalysis on Nanostructured Carbon Material Supported Catalysts
• Chapter 8 Photocatalysis on Nanostructured Carbon Supported Catalysts
• Chapter 9 Nanostructured Carbon Materials for Energy Conversion and Storage
• Chapter 10 Engineering and Safety Issues
• Subject Index