¹⁵N-Labeling Studies of the Effect of Water on the Reduction of NO with NH₃ over Chromia SCR Catalysts in the Absence and Presence of O₂

Activity studies and isotopic labelling experiments have been performed to determine the effect of water on the reaction of NO and NH₃, in the absence and presence of oxygen, for both amorphous and crystalline (α-Cr₂O₃) chromia catalysts. The products of the reaction of ¹⁵NO and ¹⁴NH₃ were determined using mass spectrometry, Fourier transform infrared spectroscopy, and gas chromatography. In the presence of excess (1.8%) O₂, the addition of 1.5% H₂O decreases NO and NH₃ conversions over amorphous chromia for all temperatures below 275°C. For α-Cr₂O₃, the effect of water varies with reaction temperature: at temperatures less than 225°C, NO conversion is lower with water present, whereas above 225°C, higher NO conversions are observed under wet gas conditions. The effect of water on activity and selectivity is reversible. Labelling experiments show that water addition significantly decreases the formation of products arising from ammonia oxidation reactions. The formation of ¹⁴N₂O and ¹⁴NO is completely inhibited for both forms of chromia. For α-Cr₂O₃, ¹⁴N₂ is the dominant form of nitrogen under dry feed conditions. Increasing water vapour pressures suppress the formation of ¹⁴N₂, and ¹⁴N¹⁵N becomes the dominant nitrogen species in the presence of 1.5% H₂O. Increasing amounts of water favour the formation of ¹⁴N¹⁵N relative to that of ⁴N¹⁵NO for both amorphous chromia and α-Cr₂O₃. In the absence of O₂, the activity and selectivity of both amorphous and α-Cr₂O₃ is markedly affected by the presence of small concentrations (0.5%) of added H₂O, the effect being much more significant than in excess O₂. Unlike reaction in the presence of oxygen, the selectivity to nitrous oxide increases with added water; however, the relative proportions of ¹⁴N¹⁵NO and ¹⁵N₂O (the main nitrous oxide species) remain constant. A tentative mechanism, which can account for the observations, is given.