CHROMIA SUPPORTED ON TITANIA .2. MORPHOLOGICAL PROPERTIES AND CATALYTIC BEHAVIOR IN THE SELECTIVE REDUCTION OF NITRIC-OXIDE BY AMMONIA

The chemical and structural properties of chromia on titania have been investigated after different pretreatments and after use for the low temperature selective catalytic reduction (SCR) of NO by NH3. A series of catalysts containing 0.5 to 30 wt% chromia was prepared by impregnation with aqueous chromium nitrate solution. The catalysts exhibited high activity for SCR in the low temperature range (T < 470 K). The NO conversion rate was proportional to the chromia loading up to 5 wt%. Higher loadings resulted in a less efficient use of the chromia species. Selectivities to N2 and N2O were not influenced by the amount of deposited chromia at higher loadings (≥5 wt%), but depended markedly on the pretreatment of the catalysts. Oxidative pretreatment at 573 K for 3 h led to enhanced formation of undesired N2O, whereas previous exposure to a H2-containing atmosphere at T < 720 K yielded selectivities of N2 exceeding 90%. The overall conversion of NO was comparatively little influenced by these pretreatments. Thermal analysis (TG, DTA) and temperature-programmed reduction indicated the formation of Cr(III) species upon reduction in the H2-containing atmosphere at 720 K. These species were reoxidizable in air at 670 K and probably highly hydroxylated, as indicated by the substantial formation of water during the thermoanalytical run. Exposure to higher temperatures (>720 K) in an inert or a hydrogen-containing atmosphere led to the formation of less active α-Cr2O3 which was not reoxidizable under similar conditions. Electron paramagnetic resonance (EPR) measurements of catalysts exposed to SCR conditions indicated uniformly sized clusters of Cr(III) together with Cr(V) surface species. After oxidative pretreatment and SCR use, EPR revealed the existence of Cr(I) species, probably present as (CrNO)2+ surface complexes. Such species were not observed after reductive pretreatment of the catalysts. © 1994 Academic Press, Inc.