Thermal Oxidation of Carbonaceous Nanomaterials Revisited: Evidence of Mechanism Changes

Abstract

Kinetic data, for example, activation energy and reaction order, are crucial for the understanding of chemical reactions and processes. Here, we describe a novel method for obtaining kinetic data based on thermogravimetric measurements (TGA) that exploits in each measurement multiple successive isothermal steps (SIS). We applied this method to the notoriously challenging carbon combustion process for vastly different carbons for oxygen molar fractions between 1.4 % and 90 %. Our obtained apparent EA values are within the wide range of results in the literature and vary in a systematic way with the oxygen partial pressure. The improved accuracy and large amount of obtainable data allowed us to show that the majority of experimentally obtained apparent data for apparent EA are neither in a kinetic regime nor in a diffusion‐controlled one but rather in a transition regime.

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

“…Carbon combustion reaction is a well‐known illustration, being of a solid–gas type. It is recognized for its complexity and diffusion limitation is a major concern [8,9] . To describe the impact of different diffusion processes on the apparent reaction rate, Walker and co‐workers summarized in their 1959 review paper, a model based on Wicke and Rossberg works, [10,11] to distinguish 5 different zones for the carbon combustion reaction (kinetic (I), internal (II) and external (III) diffusion and 2 transition regimes) which is still today widely used [12] .…”

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

“…To describe the impact of different diffusion processes on the apparent reaction rate, Walker and co‐workers summarized in their 1959 review paper, a model based on Wicke and Rossberg works, [10,11] to distinguish 5 different zones for the carbon combustion reaction (kinetic (I), internal (II) and external (III) diffusion and 2 transition regimes) which is still today widely used [12] . In the carbon combustion case it is exceedingly difficult to obtain a purely kinetic reaction, and this can be only achieved by fine tuning of reaction parameters, temperature, particle size, flow rate, oxygen partial pressure, among others [8] . Experimentally, only the global reaction rate can be measured and corresponds to the rate of the slowest chemical step (kinetic regime) or physical process (other regimes).…”

“…A second experiment is performed in a fully diffusion‐limited regime (zone III, also called external diffusion regime or TD−L) [12] . The selection of parameters to reach zone III or not is based on a previous study and discussed in ESI (section III) [8] . All experimental details are available in ESI (section II).…”

Carbon Surface Chemistry: New Insight into the Old Story

“…Despite the long tradition of fueling the mankind since the age of homo sapiens, the science of carbon materials nowadays is still developing and progressing. [ 1 ] The elemental carbon can form a great variety of crystalline and disordered structures as it exists in three different hybridizations: sp 3 , sp 2 , and sp 1 . [ 2 ] This brings infinite possibilities of combining different hybridizations for the development of new carbon structures as material solutions to meet diverse requirements.…”

“…Besides the abovementioned structural defects, amorphous carbon attached on the graphitic surface also inevitably exists, which could have significant effects on the carbon surface chemistry. [ 1b,12 ] It is worth mentioning that the removal process of the amorphous part by a washing method has been used to synthesize carbon nanodots, [ 13 ] but these nanodots do not represent the surface of the carbon backbone. Some amorphous fragments could form chemical bonds with the structural defects on the carbon surface, and thus are removed with difficulty.…”