Name: exploring the radical nature of a carbon surface by electron paramagnetic resonance and a calibrated gas flow
Uploaded: 2014-04-24T14:15:00
Description: Watch this Scientific Journal Video about Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow at JoVE.com

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

1. Preparing Carbon Samples
<ol>
	<li>Grind the carbon samples to the desired fraction size (here, coal samples were ground to a fraction size of between 74-250 mm).</li>
	<li>During the grinding process the grinder should be held in a regulated environment (AC cooled to 20 &#176;C). Additionally, purging the grinder chamber with a flow of nitrogen gas prior to grinding minimizes oxidation at this stage.</li>
	<li>Transfer the carbon samples to sealable canisters and replace the air atmosphere with nitrogen. Keep the s...

<ol>
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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...

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 <img alt="Equation 1" fo:content-width="0.8in" src="/files/ftp_upload/51548/51548eq1.jpg" /> defines the g-value, where h is the Planck constant, v is the constant mw frequency applied in the experiment, B0 is the resonance magnetic field and &#946;e is the Bohr magneton. For free electrons the g-value is 2.00232. Variations in the g-value from the 2.00232 are related to magnetic interactions involving the orbital angular momentum of the unpaired electron and its chemical environment. Organic radicals usually have g-values close to the free electron g, which depends on the location of the free radical in the organic matrix3,8-10. Carbon-centered radicals have g-values that are close to the free electron g-value 2.0023. Carbon-centered radicals with an adjacent oxygen atom have higher g-values in the range of 2.003-2.004, while oxygen centered radicals have g-values that are &#62;2.004. The g-value of 2.0034-2.0039 is characteristic for carbon-centered radicals in a nearby oxygen heteroatom that results in increased g-values over that of purely carbon-centered radicals11-15. Line-width is governed by the spin-lattice relaxation process. Therefore, an interaction between adjacent radicals or between a radical and paramagnetic oxygen results in a decrease in the spin lattice relaxation time, and hence, an increase in the line-width4-6.
Stopped flow experiments with EPR detection allow the observation of time-dependent changes in the amplitude of an EPR signal at a distinct field value during the interaction of two phases by time sweep acquisition (kinetic display). The result of such a measurement is a rate constant for the formation, decay or conversion of a paramagnetic species. The procedure is analogous to the well-established case of stopped flow operation with optical detection in which a time-dependence of the optical absorption at a distinct wavelength is observed. Typically stopped flow experiments are conducted in a liquid state as radicals that are not EPR detected in liquid state due to short relaxation time T1, as e.g.&#160;hydroxyl (OH&#215;) or superoxide (O2-) cannot be studied directly by EPR-stopped flow techniques. It is, however, possible to study the spin-adducts of these radicals with nitrones, yielding nitroxide-type radicals (spin-traps), as they are EPR-active and their kinetics can be monitored also by stopped flow EPR16-18.
The method of measurement of rates of chemical reactions using fast-flow gaseous techniques with EPR detection has also previously been established19-22. In essence, the method depends on the measurement, by EPR, of the concentration of a reactant as a function of distance (and thus at a constant velocity, the time) over which the reactant has been in contact with a reactive gas in the flow tube. Conditions whereby the concentration of the reactive gas is approximately constant are usually employed so that the measured decay is pseudo first order.
In the current work, a simple gas flow setup was implemented and a constant flow of gas was introduced to the surface of the solid carbon substrate.
With the method detailed in the current work we succeeded in achieving 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 STP. As a result of this method the removal of the interacting paramagnetic gas uncovers a new radical surface with a g value, which is closer to that of a free electron.

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exploring the radical nature of a carbon surface by electron paramagnetic resonance and a calibrated gas flow

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.

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 (&#177;10%). Figure 3A presents two scans: 0 sec and 1,900 sec after the coal sampl...

Watch this Scientific Journal Video about Exploring the Radical Nature of a Carbon Surface by Electron Paramagnetic Resonance and a Calibrated Gas Flow at JoVE.com

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

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 ...

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