Selective catalytic reduction of nitric oxide with ammonia overV 2O 5/TiO 2 catalyst: A steady-state and transient kinetic study

Abstract

The kinetics of the selective catalytic reduction (SCR) of nitric oxide with ammonia over an 8 mol-% V 2O 5/TiO 2 catalyst was studied in the temperature range 180–380°C, nitric oxide and ammonia feed concentrations in the range 500–2500 ppm with excess of oxygen. It was found that the reaction order with respect to ammonia strongly depends on reaction temperature, in contrast to the case of the reaction order with respect to nitric oxide. The apparent activation energy of the reaction for nitrogen formation depends more on the feed concentration of ammonia than of nitric oxide. This activation energy varies between 12 and 9 kcal mol −1 for ammonia concentrations in the range 500–2000 ppm. Temperature-programmed desorption (TPD) studies revealed the presence of three well-resolved ammonia peaks corresponding to desorption energies in the range 22–28 kcal mol −1. Transient isotopic experiments with 18O 2 showed that at 400°C only small amounts of lattice oxygen of V 2O 5 can be exchanged with gaseous oxygen. Similar experiments with 15NO showed also that only very small quantities of nitric oxide adsorbed on the catalyst surface from a mixture containing 15NO/O 2/He. The partial oxidation reaction of ammonia to nitrogen and nitrous oxide at 350°C was studied by steady-state tracing techniques. The results obtained suggest that at the level of ammonia conversion of 75% there is an appreciable amount of NH x intermediate species (8.3 μmol/g) which are found in the reaction pathway of nitrogen formation, but a small amount (0.4 μmol/g) is found in the reaction pathway of nitrous oxide formation. In addition, adsorption and desorption steps of ammonia must be considered as faster steps than those involved in reaction between adjacent adsorbed NH x species to form nitrogen and nitrous oxide.