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 oxid e with ammonia over an 8 mol-% V2O5/TiO2 catalyst was studied in the t emperature range 180-380 degrees C, nitric oxide and ammonia feed conc entrations in the range 500-2500 ppm with excess of oxygen. It was fou nd that the reaction order with respect to ammonia strongly depends on reaction temperature, in contrast to the case of the reaction order w ith respect to nitric oxide. The apparent activation energy of the rea ction for nitrogen formation depends more on the feed concentration of ammonia than of nitric oxide. This activation energy varies between 1 2 and 9 kcal mol(-1) for ammonia concentrations in the range 500-2000 ppm. Temperature-programmed desorption (TPD) studies revealed the pres ence of three well-resolved ammonia peaks corresponding to desorption energies in the range 22-28 kcal mol(-1). Transient isotopic experimen ts with 18O, showed that at 400 degrees C only small amounts of lattic e oxygen of V2O5 can be exchanged with gaseous oxygen. Similar experim ents with (NO)-N-15 showed also that only very small quantities of nit ric 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 tracin g techniques. The results obtained suggest that at the level of ammoni a 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 nit rogen formation, but a small amount (0.4 mu mol/g) is found in the rea ction pathway of nitrous oxide formation. In addition, adsorption and desorption steps of ammonia must be considered as faster steps than th ose involved in reaction between adjacent adsorbed NHx species to form nitrogen and nitrous oxide.