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

Abstract

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.

Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon Atoms Growth via Energetic Material Deflagration

“…As shown in Figure 5c, signals between 100-160 ppm represent aromatic carbons (B1 area), signals between 60-100 ppm represent aliphatic carbon with an adjacent oxygen atom (B2 area), whereas signals at 0-60 ppm represent aliphatic carbons (B3 area). 35 The aromatic carbon signals in B1 area demonstrate the formation of 6-membered rings in the small carbon molecules, which corroborates our growth mechanism of GQDs. The low signal intensity of aliphatic carbons in B3 area reveals their low concentration in the small carbon molecules, indicating the transformation of aliphatic carbon to aromatic carbon in the deflagration flame.…”

Endogenous Stable Radicals for Characterization of Thermally Carbonized Porous Silicon by Solid-State Dynamic Nuclear Polarization 13C NMR

“…Furthermore, no signal is detected between 60 and 100 ppm, indicating the absence of aliphatic carbons bonded to oxygen. 39 Further measurements were performed to confirm the observations in the 1 H− 13 C DNP CPMAS NMR experiment as well as to get a further insight on the chemical structure of TCPSi. The 29 Si MAS NMR spectrum (Figure 4, inset) shows a narrow signal at −81.5 ppm corresponding to crystalline bulk silicon.…”

Thermal Stability of Carbon-Centered Radicals Involved in Low-Temperature Oxidation of Bituminous and Lignite Coals as a Function of Temperature

“…Recently, we have studied 1 , 16 , 20 the coal LTO process using two types of coal, namely bituminous coal (Baily mine, denoted as BA) and lignite coal (Hambach mine, denoted as HA), as these two coals are used for power production. 2 , 12 , 21 , 22 These coals can be differentiated by the chemical composition of the coal macromolecule, a property which affects many aspects of the oxidation process.…”

“…The structure of these radicals is dependent on the coal rank, as the type of radical formed is dependent on the chemical environment and changes according to the chemical content of the coal. 16 , 21 , 24 , 25 The radicals were studied using electron paramagnetic resonance (EPR) spectroscopy. 16 , 20 Such studies also found that the precursors of the radicals are hydroperoxide groups, coal–OOH, formed at the coal surface when decomposed chemisorbed oxygen reacts with nearby active carbon–hydrogen groups in the coal macromolecule.…”

“… 16 , 21 , 24 , 25 The radicals were studied using electron paramagnetic resonance (EPR) spectroscopy. 16 , 20 Such studies also found that the precursors of the radicals are hydroperoxide groups, coal–OOH, formed at the coal surface when decomposed chemisorbed oxygen reacts with nearby active carbon–hydrogen groups in the coal macromolecule. 1 Carbon-centered radicals of aromatic character are of higher concentration than are aliphatic-based radicals.…”