Abstract
The application of carbon nanotubes (CNTs) as metal-free catalysts is a novel approach for heterogeneous liquid phase catalytic systems. Textural and chemical modifications by liquid/gas phase or mechanical treatments, as well as solid state reactions, were successfully applied to obtain carbon nanotubes with different surface functionalities. Oxygen, nitrogen, and sulfur are the most common heteroatoms introduced on the carbon surface. This short-review highlights different routes used to develop metal-free carbon nanotube catalysts with enhanced properties for Advanced Oxidation Processes.
Highlights
The work published in 1991 by Ijima [1] boosted the interest by carbon nanotubes (CNTs) and since they have been intensively studied due to their unusual mechanical, electronic, and thermal properties [2], allowing them to be currently used in a wide range of technological applications [3,4], including as catalysts or catalyst supports in the fields of renewable energy and environmental technologies
In addition addition to to the the surface surface chemistry chemistry characterization, characterization, textural properties, such as the specific surface area and and pore poresize sizedistribution, distribution,are arerequired required evaluate modifications promoted by to to evaluate thethe modifications promoted by the the chemical treatments and to compare different carbon nanotubes, which will present significant chemical treatments and to compare different carbon nanotubes, which will present significant differences differences depending on their provenience synthesis method
When other N-doped carbon materials were tested in Catalytic Wet Air Oxidation (CWAO) similar evidences were observed, suggesting again that hydroxyl radicals in solution are not the main species involved in the oxidation mechanism, and that oxalic acid conversion by CWAO can occur by means of surface active species produced from the decomposition of oxygen
Summary
The work published in 1991 by Ijima [1] boosted the interest by carbon nanotubes (CNTs) and since they have been intensively studied due to their unusual mechanical, electronic, and thermal properties [2], allowing them to be currently used in a wide range of technological applications [3,4], including as catalysts or catalyst supports in the fields of renewable energy (storage and generation) and environmental technologies (oxidation/remediation/adsorption). CNTs have been intensively investigated as catalyst supports for those technologies, and less often as catalysts on their own, but recent advances in the development of reliable methods to tune their physicochemical properties by suitable thermal or chemical post-treatments provide a major asset for their use as catalysts in these applications [5,6]. In this review relevant methods for the chemical functionalization of CNTs will be presented, and their properties discussed, with emphasis on their use as catalysts in environmental metal-free processes. Environmental compatibility, low energy consumption, and corrosion resistance are some of the advantages that nanocarbons offer compared with metal-based catalysts, in addition to high selectivity and long term stability under mild conditions in many catalytic processes [7]
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