*3.3. FTIR*

Figure 4 reports the FTIR spectra of the samples. The deflection at 3300 cm−<sup>1</sup> indicates the stretching of the hydroxyl group, occurring generally in the range 3700–3200 cm−<sup>1</sup> [47]. Increasing HTC temperature results in a decreasing intensity in O-H stretching, because dehydration phenomena occur during HTC treatment. The peak at 3010 cm−<sup>1</sup> represents =C-H group stretch, which tends to gradually disappear in the hydrochars. This fact can be related to double bonds present in the oil, which degrades, and it is even not present in HC 250 ◦C. Purnomo et al. [48] identified hydrogen bonded to the unsaturated carbon chain (C-H), whereas peaks at lower wavelength represent attachment to the saturated carbon chain. The peak at 1744 cm−<sup>1</sup> indicates a typical bending of carbonyl group >C=O, possibly in the form of aldehyde [5], and/or the ester group in glyceride compounds. It tends to attenuate by increasing HTC treatment severity. Whereas grape seeds and HC 180 ◦C still present a peak, the HC 220 ◦C spectrum moves toward a lower wavelength. In the HC 250 ◦C spectrum the peak disappeared, confirming degradation and formation of new compounds during the hydrothermal treatment. Furthermore, Diaz et al. [41] state that peaks around 1030 cm−<sup>1</sup> indicate C-O and C-O-C stretching, confirming that both decarboxylation and dehydration occurred. The grape seed spectrum exhibits a shoulder at a wavelength equal to 1655 cm−1, attributed to stretching of double bond >C=C< [47], that tends to flatten out in hydrochars. Unsaturated double-bound >C=C< could be related to the presence of oil [6]—grape seed oil is mainly composed of linoleic acid [48,49], having two double bonds C=C in position 9 and 12. In general, functional groups act a stretch toward the region at a shorter wavelength.

**Figure 4.** Fourier transform infrared spectroscopy (FTIR) curves of grape seeds and hydrochars (HC).
