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Supercritical water (SCW) has been studied widely for upgrading complex and low-value carbonaceous materials due to its ability to depress coke formation and improve product quality. Many researchers attributed these effects to SCW’s hydrogen donation ability, including H• and H+, while others to physical effects, such as the cage effect or the solvent effect. To clarify the role of SCW, three probe compounds, naphthalene, p-benzoquinone, and azobenzene, are treated individually in SCW at 400 °C and 25.3 MPa, and their products are analyzed and compared with the products obtained under other conditions, including hot compressed water, SCW with tetralin, and an inert atmosphere with or without tetralin. The products from reaction of the compounds with HO• or HO– are used as the key indicators to identify the hydrogen donation ability of SCW. Results indicate that SCW cannot donate H• radicals and condensation of carbonaceous materials is responsible for generation of H• in the temperature range of SCW. H+ may play a role at a higher extent of water ionization. The different product distribution under the SCW conditions from the inert atmosphere can be attributed mainly to the increased dispersion of radicals due to the solvent effect of SCW.

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