EFFECT OF PREPARATION PROCEDURE, OXYGEN CONCENTRATION AND WATER ON THE REDUCTION OF NITRIC-OXIDE BY AMMONIA OVER CHROMIA SELECTIVE CATALYTIC REDUCTION CATALYSTS

The effects of preparation procedure, oxygen concentration and water o n the reaction of nitric oxide and ammonia over chromia catalysts prep ared by thermal treatment of chromium hydroxide have been determined. Analyses were performed using a combination of mass spectrometry, Four ier transform infrared spectroscopy and gas chromatography. Thermograv imetric and differential thermal analysis techniques show that the tem perature at which transformation of amorphous chromia to the alpha-Cr2 O3 phase occurs is dependent on the gaseous atmosphere during the heat -treatment process, and increases in the order oxygen, nitrogen and hy drogen. When heating the catalyst precursor in hydrogen, the chromia m aterial remains amorphous for temperatures below about 420 degrees C. Samples heated above ca. 470 degrees C are highly crystalline. The cry stallinity of the chromia materials is the most significant factor in determining the activity, and selectivity, of the resulting catalysts for the NO/NH3 reaction. Amorphous chromia catalysts prepared with a t wo order-of-magnitude difference in ammonia addition rate show similar performances. Kinetic orders in oxygen to produce both nitrogen and n itrous oxide have been determined. Amorphous chromia shows a much high er order in oxygen for nitrogen formation than does alpha-Cr2O3. Howev er, for nitrous oxide formation, the orders are approximately the same for both morphologies. Addition of 1.5% H2O vapour decreases nitric o xide and ammonia conversions over amorphous chromia for all temperatur es below 275 degrees C, whereas for alpha-Cr2O3, the effect varies wit h reaction temperature. For amorphous chromia the addition of water su bstantially decreases the formation of nitrous oxide.