CATALYTIC REDUCTION OF NITRIC-OXIDE OVER AMORPHOUS AND CRYSTALLINE CHROMIA .2. STRUCTURAL DEPENDENCE OF SELECTIVE AND NONSELECTIVE REACTIONS

The reactions of NH3 with O2 and NO and NH3 in the absence of O2 have been investigated over amorphous and crystalline chromia (alpha-Cr2O3) and compared to the reaction of NO and NH3 in the presence of excess O2. NH3 oxidation results in N2 and N2O in equimolar proportions over amorphous chromia and predominantly N2 over alpha-Cr2O3. A comparison of rates and apparent activation energies shows the N2 forming reaction to be more structure sensitive. The reaction of NO and NH3 without oxygen produces predominantly N2O with some N2 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 N2 formation reaction from NO and NH3 in the presence of excess O2 is strongly affected by the chromia morphology whereas the reactions producing N2O 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 NH3 in excess O2 and that these sites are more readily formed on amorphous chromia than alpha-Cr2O3.