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Chemistry

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

Published: April 24th, 2014

DOI:

10.3791/51548

1Chemistry Institute, The Hebrew University of Jerusalem, 2Biological Chemistry Department, Ariel University, 3Chemistry Department, Faculty of Exact Science, Bar Ilan University, 4Chemistry Department, Ben-Gurion University

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.

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 signal4-6. 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.

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

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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 s.......

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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 sampl.......

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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.......

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SR acknowledges the support of the Israel science foundation, grant no. 280/12.

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NameCompanyCatalog NumberComments
EPR spectrometerBrukerElexsys E500
EPR quartz tubeWilmad-Lab Glass
vacuum oven Heraeus VT6060
BalanceDenver Instrument100A
High Vacuum Silicone GreaseVWR international59344-055
Teflon putty 
Laboratory (Rubber) StoppersSigma-AldrichZ114111
Aluminum Crimp seals Sigma-AldrichZ114146
Hand CrimperSigma-AldrichZ114243
Borosilicate vials Sigma-AldrichZ11938
Rubber tubing 
Aluminum hose clamps
Screwdriver 
Custom made vacuum system 
glass storage cylinders 
BD Regular Bevel NeedlesBD 305122
Helium  oxar LTD 
Argon    oxar LTD 
CO2       99.99%Maxima
N2       99.999%oxar LTD 
O2       Maxima
AirMaxima

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