Spin-mapping of coal structures with ESE and ENDOR

Abstract

_,,-+Advanced electron paramagnetic resonance (EPR) techniques --ENDOR, ESE, and VHF-EPR --areused to probe the molecular structure and surface properties of coals. During a probe of the _+ surface response of coal particles to oxygen, we have also made synthetic chars for comparison. This quarter's report concerns preparation and characterization of these chars. This work will be published. OBJECTIVES and APPROACHThe goals of this prograart include developing a system for the analysis of the chemical tbrms of organic sulfur in coal and for study of coal particle surfaces by multiffequency EPR spectroscopy, ENDOR, and ESE spectroscopy and applying it to coals, to the effects of treatment upon their sulfurcontaining organic components, and to related carbonaceous materials (chars and the like). The approach is to utilize the naturally-occurring unpaired electrons in the organic structures of coals as spies to provide molecular structure information, reading out the intbrrnation with Electron Paramagnetic Resonance (EPR) spectroscopy. Several forms of EPR are employed: Multiffequency contiunuous-wave (CW) EPR, from 1 GHz to 240 GHz source frequency; electron-nuclear double resonance (ENDOR), in which NMR spectra at partmagnetie centers are obtained by EPR detection; and pulsed EPR, including _;E (Electron Spin Echo) spectroscopy. REPORT: EXPERIMENTAL; RFA'ULTS;CONCI,USIONS AND RECOMMENDATIONS P.LC._f_.g:Carbonization results from the heating of a carbonaceous material in an inert atmosphere. This process encourages the evolution of non-carbon elements as gaseous products and tar and the formation of a semipo,'ous solid carbon product. During thermal decomposition, solid graphite-like sheets of aromatic carbon are formed, arising from free radical intermediates, some of which remain in the product carbon [I,2]. Activation processes can increase the porous structure of the product by revealing interstices which would have been obstructed by tar in an non-active carbon. The primary function of the chemical activating agent is one of dehydration, influencing the course of carbonization by reducing the production of volatile by-products and tar, increasing carbon yield and porosity [3]. A second function has been reported by Marsh[4]: potassium and other alkali salts appear to encourage pore formation by catalyzing extensive erosslinking between growing carbon chains formed as carbonization intermediates. Laboratory preparation of active carbon from carbonaceous materials can proceed in two different manners [3]. The first, physical activation, occurs in two steps" an initial carbonization process where the carbonaceous material is heated in an inert atmosphere which is followed by a DISTRIBUTION OF THIS