# Char from Pyrolysis of Waste Tires to Increase Bitumen Performances
## 3 Results and Discussion
### Char Characterization
**Lower panel**: particle size distributions for CO\({}_{2}\)-char and N\({}_{2}\)-char dispersed in silicone oil. slower for the other chars. This confirms that the more oxidizing/reactive environment in which CO\({}_{2}\)-char is prepared leads to a more reactive char particle surface. Moreover, investigating more deeply, a slight weight loss is seen for the N\({}_{2}\)- and N\({}_{2}\)+CO\({}_{2}\)-chars at a temperature just above 100 \({}^{\circ}\)C, attributable to some residual humidity tightly bonded to the char. Interestingly, this behavior is not present in CO\({}_{2}\)-char, which maintained its initial weight up to the degradation temperature of 500 \({}^{\circ}\)C. Further char characterization was carried out using FT-IR. The spectra are reported in Figure S5 of Supporting Information. The spectra are quite similar, due to the common carbonaceous nature of the three materials. The typical C=C and C=O stretching modes are visible around 1600 cm\({}^{-1}\). This band also reasonably contains the ring stretch of the benzene ring in aromatic compounds. At higher frequencies, the C-H stretching modes are present in the region 2800-3000 cm\({}^{-1}\), as well as the broad band due to O-H and/or N-H stretching around 3400 cm\({}^{-1}\). On the other hand, at lower frequencies, a peak centered at 1120 cm\({}^{-1}\) can be due to the vibrations of sulfur-containing functional groups, whose presence is expected since rubber sulfur compounds are used in the vulcanization process, making the tire rubbers. This information is confirmed through an elemental analysis performed through Scanning Electron Microscopy-Wavelength Dispersive Spectroscopy (SEM-WDS) on selected areas (see complete report in Supporting Information), which showed that the char contains small domains of ZnS. In fact, to facilitate rubber vulcanization for tire preparation, ZnO is typically used as an additive. Then, during the waste tire pyrolysis process, ZnO retains sulfur forming ZnS as a consequence of the reaction with H\({}_{2}\)S evolving from the decomposition of the sulfur rubber compounds resulting from the vulcanization process. Apart from these isolated inorganic domains, the matrix is made up of carbonaceous materials, with an oxygen content generally increasing when passing from N\({}_{2}\)-char to CO\({}_{2}\)-char as a consequence of the presence of CO\({}_{2}\) during pyrolysis. Figure 2 shows the nitrogen adsorption/desorption isotherms acquired on the samples labelled as N\({}_{2}\)-char, N\({}_{2}\)+CO\({}_{2}\)-char and CO\({}_{2}\)-char at liquid nitrogen temperature. The curve relative to an unpyrolysed waste tire is not shown since, due to its almost zero specific surface area, it was not possible to acquire the isotherm. According to the IUPAC classification [31], the shapes of the isotherms are predominantly of Type II with a very small contribution of Type I (P/P\({}_{0}\) < 0.05) and a more meaningful contribution of Type IV (0.85 \(\leq\) P/P\({}_{0}\)\(\leq\) 1.0), with a presence of a slight H1-type hysteresis. For these reasons, we can assert that synthetized materials have a slightly porous structure, which are composed of a very small part of micropores (<2 nm) and a preponderant part of mesopores (>2 nm). The Type II isotherm exhibits unrestricted monolayer-multilayer adsorption. The beginning of the knee on the isotherms, which occurs almost immediately (micropores have  
Figure 2: Nitrogen adsorption (full circles) and desorption (open circles) isotherms. been filled), indicates the stage at which monolayer coverage is complete and multilayer adsorption is about to begin. The completion of the multilayer followed by capillary condensation with its hysteresis loop, typical of Type IV isotherms, highlights adsorption on the mesoporous part over a range of a high \(\mathrm{P/P_{0}}\). Table 3 summarizes the structural differences. It reports the parameters calculated by analyzing adsorption isotherms.