Burn Them Right! Determining the Optimal Temperature for the Purification of Carbon Materials by Combustion

Abstract

A new purification procedure for carbon nanoforms is proposed. It was tested on multiwall carbon nanotubes (MWCNTs) prepared by arc discharge, which is among the most challenging of cases due to the chemical and structural similarity between the MWCNTs and most of the impurities to be removed. Indeed, the various methods for synthesizing carbon nanoforms lead to a distribution of carbonaceous products, such as carbon shells, carbon spheres, fullerenes, and a variety of other species. Thus, many strategies to purify the desired products have been developed. Among the most successful ones, thermal oxidation (combustion) seems particularly efficient. To be successful while preserving a reasonable amount of MWCNTs, the combustion temperature has to be carefully selected. Moreover, the ideal combustion temperature does not only depend on the material to be treated but also on the overall system used to perform the reaction, including the reactor type and the parameters of the gaseous reactant. Typically, the optimization of the purification relies on multiple experiments and analysis of the products. However, to the best of our knowledge, a strategy to determine a priori the most suitable temperature has not been reported yet. We demonstrate here that a thermogravimetric method, namely the constant decomposition rate thermal analysis (CRTA), is particularly well adapted to answer this question. An isothermal treatment based on the results obtained from a CRTA program allowed arc-MWCNTs to be successfully purified from graphenic shells while optimizing the yield of the MWCNTs. This strategy is believed to be valuable not only for purifying MWCNTs but also for the purification of other carbonaceous forms, including new carbon nanoforms.

Burning Graphite Faster than Carbon Black: A Case of Diffusion Control

“…carbon nanotubes opening) [21,22] . Advanced methodology have been developed, such as the controlled transformation rate thermal analysis programs, that automatically carries out such kind of TGA experiments [23–25] …”

“…Combustion reaction is commonly used in carbon (nano)material science as a purification tool. It is for example possible to highly increase the ratio of carbon nanotubes (CNTs) after their production, while electric arc methods allow to reach no more than 30 % in mass of CNTs [21,24] . The impurities being slightly more defective in that case, the natural selectivity of the combustion becomes a powerful tool.…”

“…[21,22] Advanced methodology have been developed, such as the controlled transformation rate thermal analysis programs, that automatically carries out such kind of TGA experiments. [23][24][25] While removal of less crystalline or less temperature stable compounds in a mixture is simple, the reverse is a challenge. Doing it by combustion was thought impossible.…”

Thermoporometry and Oxyreactive Thermal Analysis for Study of Carbon Matrices

“…В общем случае реакция взаимодействия кислорода с углеродом может быть выражена следующими схематическими реакциями [22]:…”

“…Стоит отметить, что в статической атмосфере воздуха в процессе сгорания углеродного материала непосредственно около углеродной поверхности происходит уменьшение содержания кислорода, вследствие его быстрого взаимодействия с углеродом, что способствует переходу реакции окисления в диффузионно-контролируемый режим [22] и реакция становится «неструктурнозависимой». Подобное поведение может наблюдаться и при динамическом нагреве в окислительной среде без должной пробоподготовки.…”