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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 ...

The Hebrew University of Jerusalem

Ariel University

Once coal is excavated it comes into contact with atmospheric oxygen and begins to undergo low temperature oxidation. The mechanism by which the molecular oxygen interacts with the coal macromolecule is suggested to occur in several steps. These steps primarily involve O(2) diffusion to the surface where physical adsorption followed by chemical adsorption takes place. The chemical adsorption forms several types of oxides that can subsequently react to form several products, primarily CO(2). It has also been suggested that some of these oxidation mechanisms might involve radical reactions. As the previous studies were conducted under conditions where significant structural changes occur it is possible that in the low temperature range (T < 100 °C) the oxidation mechanism is different. Several different rank (lignite-subbituminous-bituminous) coals were isothermally heated at 95 °C in an air atmosphere for a period of up to 6 months and samples were collected at two week intervals. The radical concentration of each sample was measured by Continuous Wave Electron Paramagnetic Resonance (CW-EPR). It is apparent that there are distinct differences between the lower rank (lignite) and the higher rank (subbituminous, bituminous) coals. The lower rank coals exhibited only carbon centered radicals with an adjacent oxygen atom and the higher rank coals exhibited only carbon centered radicals. Interestingly, the lower rank coals exhibited no change in radical concentration due to the long term oxidation treatment while the higher rank coals showed a distinct increase in the radical concentration. These findings shed new light on the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

Stable radicals formation in coals undergoing weathering: effect of coal rank.

We report the discovery that a flow of CO2, N2 or He can sufficiently reduce the spin-spin interactions of specific stable carbon centered radicals by displacing the molecular oxygen in the atmosphere enabling their detection via electron paramagnetic resonance (EPR). This finding unlike other reported effects on carbon radicals occurs under STP conditions and is reversible.

Reducing the spin-spin interaction of stable carbon radicals.

Recently, the nature of the carbon radicals stabilized in various coals was characterized using Electron Paramagnetic Resonance (EPR) spectroscopy. It was demonstrated that introducing diamagnetic gases, such as He, CO2, or N2, under STP conditions to the coal surface induces the appearance of a new type of carbon surface radical. This interesting phenomenon was not observed for all coal types, which suggests that the use of EPR measurements can provide information on functional groups that exist on the carbon surface. In the current study coupling Nuclear Magnetic Resonance (NMR) with gas flow in situ EPR measurements significantly enhances the ability to characterize the nature of these radicals and the surface functional groups of coal samples. It was observed that the oxidative reaction with aliphatic groups leads to the increase in stable carbon centered radicals. In addition, there are some species of carbon centered radicals that show reversible binding to O2. This phenomena, however, is dependent on the coal rank, sample porosity and the degree of the coal sample to undergo structural changes under the LTO process. These findings shed new light onto the complex heterogeneous low temperature oxidation reactions occurring at the coal surface.

Elucidating the role of stable carbon radicals in the low temperature oxidation of coals by coupled EPR-NMR spectroscopy - a method to characterize surfaces of porous carbon materials.

Uri Green

Chemistry Institute

The Hebrew University of Jerusalem

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