Ceria–zirconia-supported rhodium catalyst for NOx reduction from coal combustion flue gases

The catalytic activities of ceria–zirconia mixed oxides CexZr1−xO2 (x = 0.17, 0.62 and 0.8) rhodium catalysts were determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNOx reaction in the presence of the mentioned gas mixture. However, their catalytic activity varies with the content of cerium and goes through a maximum for x = 0.62, leading to 27% NOx consumption. The effect of rhodium on Ce0.62Zr0.38O2 considerably improves the catalytic activity during the deNOx process assisted by hydrocarbons. The rhodium addition decreases by about 34 °C the temperature of NOx consumption, which goes up to 57%. A mechanism of hydrocarbon (HC) assisted reduction of NO is proposed on ceria–zirconia-supported rhodium catalysts. This mechanism is divided in three catalytic cycles involving (i) the oxidation of NO into NO2, (ii) the reaction of NO2 and the hydrocarbons leading to RNOx species and CxHyOz, and finally (iii) the decomposition of NO assisted by these latter CxHyOz species.