SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE WITH AMMONIA OVER V2O5/TIO2 CATALYST - A STEADY-STATE AND TRANSIENT KINETIC-STUDY

The kinetics of the selective catalytic reduction (SCR) of nitric oxide with ammonia over an 8 mol-% V2O5/TiO2 catalyst was studied in the temperature range 180-380 degrees 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, showed that at 400 degrees C only small amounts of lattice oxygen of V2O5 can be exchanged with gaseous oxygen. Similar experiments with (NO)-N-15 showed also that only very small quantities of nitric oxide adsorbed on the catalyst surface from a mixture containing (NO)-N-15/O-2/He. The partial oxidation reaction of ammonia to nitrogen and nitrous oxide at 350 degrees 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 NHx intermediate species (8.3 mu mol/g) which are found in the reaction pathway of nitrogen formation, but a small amount (0.4 mu mol/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 NHx species to form nitrogen and nitrous oxide.