Catalytic reduction of nitric oxide over amorphous and crystalline chromia: II. Structural dependence of selective and non-selective reactions

Abstract

The reactions of NH 3 with O 2 and NO and NH 3 in the absence of O 2 have been investigated over amorphous and crystalline chromia (α-Cr 2O 3) and compared to the reaction of NO and NH 3 in the presence of excess O 2. NH 3 oxidation results in N 2 and N 2O in equimolar proportions over amorphous chromia and predominantly N 2 over α-Cr 2O 3. A comparison of rates and apparent activation energies shows the N 2 forming reaction to be more structure sensitive. The reaction of NO and NH 3 without oxygen produces predominantly N 2O with some N 2 on both morphologies. The rates and apparent activation energies indicate that neither reactions appear to be significantly affected by the morphology of the chromia. It is also shown that the N 2 formation reaction from NO and NH 3 in the presence of excess O 2 is strongly affected by the chromia morphology whereas the reactions producing N 2O are not. The results suggest that the sites created by oxygen adsorption are the most important for promoting the rapid selective reduction of NO by NH 3 in excess O 2 and that these sites are more readily formed on amorphous chromia than α-Cr 2O 3.