Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow

Summary

Stable radicals that are present in carbon substrates interact with paramagnetic oxygen through a Heisenberg spin exchange. This interaction can be significantly reduced under STP conditions by flowing a diamagnetic gas over the carbon system. This manuscript describes a simple method to characterize the nature of those radicals.

Abstract

While the first Electron Paramagnetic Resonance (EPR) studies regarding the effects of oxidation on the structure and stability of carbon radicals date back to the early 1980s the focus of these early papers primarily characterized the changes to the structures under extremely harsh conditions (pH or temperature)^1-3. It is also known that paramagnetic molecular oxygen undergoes a Heisenberg spin exchange interaction with stable radicals that extremely broadens the EPR signal^4-6. Recently, we reported interesting results where this interaction of molecular oxygen with a certain part of the existing stable radical structure can be reversibly affected simply by flowing a diamagnetic gas through the carbon samples at STP⁷. As flows of He, CO₂, and N₂ had a similar effect these interactions occur at the surface area of the macropore system.

This manuscript highlights the experimental techniques, work-up, and analysis towards affecting the existing stable radical nature in the carbon structures. It is hoped that it will help towards further development and understanding of these interactions in the community at large.

Introduction

Substrates of varying (wt%) ratios of C/H/O atoms present different types and concentrations of stable radicals that are detectable via Electron Paramagnetic Resonance (EPR)⁸. These radicals depend on the structure of the macromolecules and are highly influenced by their aromatic nature. The EPR spectrum of coal radicals is characterized by a single broad resonance. In such cases, only the g-value, the line width and the spin concentration can be obtained. The g-values of EPR spectra can be used to determine whether a radical is carbon-centered or oxygen-centered. The basic equation for the electron Zeeman interaction

Protocol

1. Preparing Carbon Samples

1. Grind the carbon samples to the desired fraction size (here, coal samples were ground to a fraction size of between 74-250 mm).
2. During the grinding process the grinder should be held in a regulated environment (AC cooled to 20 °C). Additionally, purging the grinder chamber with a flow of nitrogen gas prior to grinding minimizes oxidation at this stage.
3. Transfer the carbon samples to sealable canisters and replace the air atmosphere with nitrogen. Keep the samples in a temperature regulated room (AC cooled to 20 °C). 
4. Prepare the carbon samples for EPR measurements by heating the carbo....

Results

When preforming the EPR experiments on various coal samples, as a function of the exposure time to a diamagnetic gas flow it was noted that during the gas flow, a second species at g~2.0028 appeared. This g-value is close to the value of a free electron and consistent with unsubstituted aliphatic carbon centered radicals. However, the total spin concentration for each sample remained constant within our experimental error (±10%). Figure 3A presents two scans: 0 sec and 1,900 sec after the coal sample had.......

Discussion

Surface oxidation of carbon materials is of significant industrial and academic interest. The effects of carbon substrate oxidation have been characterized with a wide range of analytical techniques including EPR. When investigating the interaction of molecular oxygen with carbon substrate such as coal which has a propensity to undergo oxidation (hence its main utilization as an energy resource) sample preparation and storage is extremely important.

Our samples are coal substrates that have be.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
EPR spectrometer	Bruker	Elexsys E500
EPR quartz tube	Wilmad-Lab Glass
Vacuum oven	Heraeus	VT6060
Balance	Denver Instrument	100A
High Vacuum Silicone Grease	VWR International	59344-055
Teflon putty
Laboratory (Rubber) Stoppers	Sigma-Aldrich	Z114111
Aluminum Crimp seals	Sigma-Aldrich	Z114146
Hand Crimper	Sigma-Aldrich	Z114243
Borosilicate vials	Sigma-Aldrich	Z11938
Rubber tubing
Aluminum hose clamps
Screwdriver
Custom made vacuum system
Glass storage cylinders
BD Regular Bevel Needles	BD	305122
Helium	Oxar Ltd
Argon	Oxar Ltd
CO2 99.99%	Maxima
N2 99.999%	Oxar Ltd
O2	Maxima
Air	Maxima