Catalyst activity, stability, and transformations during oxidation in supercritical water

We used three different catalysts (bulk MnO₂, bulk TiO₂, and CuO/Al₂O₃) to oxidize phenol in supercritical water in a tubular flow reactor. CuO/Al₂O₃ was the most active of the three on a mass of catalyst basis whereas MnO2 was the most active on an areal basis. All three catalysts largely maintained their activities for phenol disappearance and for CO₂ formation throughout more than 100 h of continuous use. MnO₂ and TiO₂ were stable in the sense that no Mn or Ti was detected in the reactor effluent. The CuO/Al₂O₃ catalyst, on the other hand, was not stable. Both Cu and Al were detected in the reactor effluent. The bulk transition metal oxide materials experienced a 3-4-fold reduction in specific surface area after exposure to supercritical water oxidation (SCWO) conditions, whereas the supported CuO/Al₂O₃ catalyst experienced a 20-fold reduction. Being used as an oxidation catalyst in supercritical water transformed the bulk MnO₂ into Mn₂O₃, the CuO catalyst into Cu₂O, the Al₂O₃ support into AlO(OH), and anatase TiO₂ into rutile TiO₂. Of the three materials considered, bulk MnO₂ appears to be the best oxidation catalyst for supercritical water conditions. It is stable under reaction conditions, and it provided high activities and good activity maintenance.