Coal and Coal Constituent Studies by Advanced EMR Techniques

Abstract

Advanced electronic magnetic resonance (EMR) methods are used to examine properties of coals, chars, and molecular species related to constituents of coal. We have achieved substantial progress on upgrading the high field (HF) EMR (W-band, 95 GHz) spectrometers that are especially advantageous for such studies. Particularly, we have built a new second W-band instrument (Mark II) in addition to our Mark I. Briefly, Mark II features: (i) an Oxford custombuilt 7 T superconducting magnet which is scannable from 0 to 7 T at up to 0.5 T/min; (ii) watercooled coaxial solenoid with up to ±550 G scan under digital (15 bits resolution) computer control; (iii) custom-engineered precision feed-back circuit, which is used to drive this solenoid, is based on an Ultrastab 860R sensor that has linearity better than 5 ppm and resolution of 0.05 ppm; (iv) an Oxford CF 1200 cryostat for variable temperature studies from 1.8 to 340 K. During this grant period we have completed several key upgrades of both Mark I and II, particularly microwave bridge, W-band probehead, and computer interfaces. We utilize these improved instruments for HF EMR studies of spin-spin interaction and existence of different paramagnetic species in carbonaceous solids.Semiannual Technical Report to DOE -PETC -Grant # DE-FG22-96PC96205 9/01/97-2/28/98Page 3 of EXECUTIVE SUMMARYThe main advanced magnetic resonance method that we report for the third half-year of this project was W-band (95 GHz) electron magnetic resonance (EMR, EPR, or ESR) spectroscopy. High field (HF) EMR, which usually refers to EMR at >90 GHz, among other specific advantages, provides an analytical tool to discriminate among very similar paramagnetic species that are constituents of coals and closely related materials such as chars. These species are difficult to identify from conventional EMR (X-band, 9.5 GHz) because of insufficient spectral resolution for radicals with similar g-factors. In addition to that conditions for fast Heisenberg exchange are often fulfilled at this frequency resulting in a featureless single-line EMR spectrum at 9.5 GHz. At 95 GHz (W-band), the g-factor resolution is ten-fold of that at X-band and the frequency of experiment usually exceeds the exchange frequency. This yields an information-rich spectrum. This report describes some experimental results obtained at 95 GHz and also considerable progress made on enhancing our W-band spectrometers used in such studies.This report focuses on the HF EMR work. The previous two Technical Reports described the use of pulsed proton DNP to study liquid-solid interactions in aqueous slurries of several newly synthesized chars. This report focuses primarily on studies of co-existing paramagnetic species and spin-spin interactions between those in carbonaceous materials and the role of molecular oxygen as a possible "bridge" for those magnetic interactions. Semiannual