Reactions of evolved gases with chars and cokes: A study of electron spin resonance

Abstract

A simple technique has been developed to observe the changes in the ESR absorption of chars and cokes resulting from chemical attack of various gases at pressures below atmospheric and temperatures below 1100°C. A gas evolved in the process of heattreatment of a polyvinylidene chloride polymer-300°C char (the gas being composed mostly of HCl and Cl 2) has been found to be at pressures of a few millimeters of Hg and temperatures exceeding 700°, a most effective agent for formation of artificial localized spin centers ( A-centers) on the accessible surface of chars. Some of such A-centers are highly resistant to reheat in vacuum and to oxidation in air. Before the new spin centers are formed, however, in some cases a partial destruction of the originally present spin centers (centers formed in carbonization process) by the PVDC-gas occurs at lower temperature (~600°C). Sugar-char gas, phenolbenzaldehyde resin char-gas and pure hydrogen have been found also to create A-centers on chars, the spin concentration being controlled at a given temperature by a dynamic equilibrium (the final concentration increasing with pressure and decreasing with temperature). Neither C 3O 2 polymer-gas, nor water vapor, nor their mixture produce A-centers under the same pressure-temperature conditions. Presence of nitrogen was found to have a deleterious effect on some of the spin formation reactions. Observations concerning the dependence of the width of the ESR line on various factors are also reported.