Supercritical Water Oxidation of NH₃ over a MnO₂/CeO₂ Catalyst

Abstract

Catalytic oxidation of ammonia in supercritical water (SCW) was studied using a continuous-flow, packed-bed reactor at temperatures ranging from 410 to 470 °C, a nominal pressure of 27.6 MPa, and reactor residence times of less than 1 s. The kinetics and catalyst performance of MnO2/CeO2 for oxidation of ammonia in SCW was evaluated. In this reaction environment, ammonia was predominantly converted into molecular nitrogen (N2), and the rate of ammonia conversion was enhanced by MnO2/CeO2. For example, 40% of the ammonia was converted when using the MnO2/CeO2 catalyst at a temperature of 450 °C and a reactor residence time of 0.8 s. It was reported that, without a catalyst, essentially no ammonia conversion was observed below 525 °C (Helling, R. K.; Tester, J. W. Environ. Sci. Technol. 1988, 22 (11), 1319) and 10% of the ammonia was converted at a temperature of 680 °C, a pressure of 24.6 MPa, and a reactor residence time of 10 s (Webley, P. A.; Tester, J. W.; Holgate, H. R. Ind. Eng. Chem. Res. 1991, 30 (8), 1745). Kinetic models developed for the gas-phase catalytic oxidation of ammonia were adopted and proven to be adequate for catalytic oxidation of ammonia in supercritical water. The best-fit global rate expression for catalytic supercritical water oxidation of ammonia by MnO2/CeO2 was obtained as follows: r = 1.14 × 1014 exp(−189 kJ/mol/RT) [NH3]0.63[O2]0.71. The BET surface area and X-ray diffraction analyses of the exposed catalyst indicated a significant reduction of surface area and changes in the crystalline structure of the catalyst.