Evolution of reactivity of highly porous chars from Raman microscopy

The effect of heat treatment on the evolution of reactivity of a highly porous pure synthetic char was studied with the aid of Raman scattering measurements. Two sets of experiments were carried out: A) Samples were oxidized in air at 600°C without heat treatment and Raman spectra were measured as a function of conversion. B) Samples were heat-treated in the range 900-1400°C, in nitrogen, then characterized by micro-Raman spectroscopy, in the range 800-1800 cm^-1, and then oxidized by air at 500°C and by CO₂ at 900°C in a thermogravimetric balance. In all Raman scattering measurements the G and D bands were dominant and a weak '1180 cm^-1' band was apparent. The following conclusions can be drawn from results 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 thermal resistivity and intrinsic reaction rate. B) Heat treatment up to 900°C at time duration up to 300 minutes did not show any effect on reactivity. Decrease in reactivity was apparent at and beyond 1200°C. The characteristics of each of the three Raman bands showed clear correlation with reactivity changes both in air and CO₂. These were attributed to morphological (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.