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

Abstract

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 100h 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.