SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE OVER AMORPHOUS AND CRYSTALLINE CHROMIA .1. COMPARATIVE-STUDY OF ACTIVITIES

The catalytic reduction of nitric oxide by ammonia in excess oxygen on high surface area amorphous chromia has been compared to the catalytic reduction on α-Cr2O3. At a gas hourly space velocity of 24 000 h-1 and with a feed of 900 ppm NO, 900 ppm NH3 and 1.8% O2, over 90% conversion can be achieved on the amorphous chromia at 140°C and less than 10% conversion occurs over the crystalline chromia. The amorphous chromia was found to be highly selective to nitrogen whilst the crystalline chromia showed higher selectivity to the formation of nitrous oxide. The activity of the amorphous chromia referred to its BET surface area was four times higher than that of the crystalline chromia. Temperature-programmed reduction of the two materials indicated that the amorphous chromia had a higher coverage of labile oxygen species on the surface than the crystalline chromia. The labile oxygen species on the crystalline chromia were more readily removed with hydrogen. The higher specific activity was attributed to the higher density of less labile oxygen species on the amorphous surface. © 1990.