Supercritical Water Oxidation of NH₃ over a MnO₂/CeO₂ Catalyst

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 MnO 2 /CeO 2 for oxidation of ammonia in SCW was evaluated. In this reaction environment, ammonia was predominantly converted into molecular nitrogen (N 2 ), and the rate of ammonia conversion was enhanced by MnO 2 /CeO 2 . For example, 40% of the ammonia was converted when using the MnO 2 /CeO 2 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 MnO 2 /CeO 2 was obtained as follows: r = 1.14 × 10 14 exp(−189 kJ/mol/ RT ) [NH 3 ] 0.63 [O 2 ] 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.