Evolution of reactivity of highly porous chars from raman microscopy

The effect of heat treatment on the evolution of reactivity of a highly por ous pure synthetic char was studied with the aid of Raman scattering measur ements. Two sets of experiments were carried out: A) Samples were oxidized in air at 600 degrees C without heat treatment and Raman spectra were measu red as a function of conversion. B) Samples were heat-treated in the range 960-1400 degrees C, in nitrogen, then characterized by micro-Raman spectros copy, in the range 800-1800 cm(-1), and then oxidized by air at 500 degrees C and by CO2 at 900 degrees C in a thermogravimetric balance. In all Raman scattering measurements the G and D bands were dominant and a weak "1180 c m(-1)" band was apparent. The following conclusions can be drawn from resul ts of experiments A and B respectively: A) Throughout reaction with oxygen the most significant changes in reactivity and Raman features occur in the conversion range 0- 30%. This is consistent with previous measurements of t hermal resistivity and intrinsic reaction rate. B) Heat treatment up to 900 degrees C at time duration up to 300 minutes did not show any effect on re activity. Decrease in reactivity was apparent at and beyond 1200 degrees C. The characteristics of each of the three Raman bands showed clear correlat ion with reactivity changes both in air and CO2. These were attributed to m orphological (dimensions and ordering) changes in the rearrangement of the carbon network structure, as previously suggested. Interpretation of these changes and their effect on reactivity is proposed.