Ag-based catalytic additives for the simultaneous reduction of NO and CO emissions from the regenerator of a FCC unit

Spent FCC catalyst was regenerated in a fluidized-bed reactor at 700 degreesC using a 2% O-2 in N-2 feed. The NO and CO emissions during regeneration were measured when Ag-based additives were used and compared with those when a commercial Pt-based CO promoter (1% CP-3) was added. All Ag/alumina additives exhibited high NO reduction ability (up to 70%), with the highest deNOx efficiency achieved with moderate Ag loading (3%). A different trend was measured for the CO emissions: the higher the Ag loading (1-5%) on alumina, the higher was the CO oxidation. However, higher CO emissions were always measured when Ag-based additives were used as compared to those when CP-3 was used. The Ag-based additives exhibited stability problems, as disaggregation of Ag particles from the alumina support was observed. When the support was promoted with 10% Ce, stable Ag-based additives were synthesized and the CO oxidation ability was enhanced. When we increased the metal loading (up to 10% Ag) and/or the Ce percentage (up to 20%) on the catalytic support, the CO oxidation over Ag/Ce-based additives was further enhanced and the NO reduction was maintained to adequately high levels (greater than or equal to36%). The NO and CO emissions were simultaneously reduced to the desired level, when we increased the amount of the most promising additive (Ag(5%)/Ce(20%)CPBase) in the reactor or when we used this additive in combination with small amounts of CP-3. The performance of Ag/Ce-based additives was linked with formation of different crystal phases on the additive. The presence of the Ag2O crystal phase on the most promising additive resulted in the reduction of the NO and CO emissions. When the Ag loading increased, metallic silver was formed that enhanced the CO oxidation.