(69f) NOx Reduction by CO Over Noble Metal-Based FCC CO Emission Control Additives

Fluid Catalytic Cracking (FCC) is a primary process used to convert crude oil into a variety of higher-value products in modern petroleum refineries.  Since crude oil feedstocks incoprorate various compounds containing nitrogen, a portion of nitrogen ends up in the coke formed during the cracking process.  While 90 % of the coke-bound nitrogen is converted to N₂ in the FCC regenerator, the remaining part is released in the form of NO_x, HCN, and NH₃.  Specific levels of these pollutants depend primarily on the content and the type of the nitrogen compounds present in the feedstock, as well as on the combustion mode used in the FCC regenerator.  Since current environmental regulations enforce CO and NO_x emissions from refineries, several new technologies were examined in efforts to control these emissions.  Among them, the use of catalytic additives is the most attractive one because it is simple, cost-effective, and applicable to existing FCC units.  

In this work, results of in-situ FTIR, TPD, and kinetic measurments were combined to clarify the chemistry of NO_x reduction by CO over FCC CO combustion promoters with a general composition M/Ceⁿ⁺/Na⁺/γ-Al₂O₃, where M is Pd, Pt, Rh.  Our goal was to understand the structure of these additives under reducing and oxidizing conditions and to examine the role of each component in the NO_x reduction process.

The STEM results indicate the presence of Ceⁿ⁺ cations along with amorphous and crystalline CeO₂ phases on the surface of these additives.  XRD and STEM results further suggest that both Ce and Pd can aggregate to a some degree at elevated temperatures in the presence of the reducing environment.  This result is consistent with XPS data demonstrating that Pd/Al, Pd/Ce, and Pd/Na ratios decrease under reducing conditions at elevated temperatures.  All M/Ceⁿ⁺/Na⁺/γ-Al₂O₃ samples were tested in the NO+CO reaction in the presence and absence of O₂ in the 500–700^oC range of temperatures.  When O₂ is absent in the feed or present in less than stoichiometric amounts, the complete reduction of NO by CO takes place in this range of temperatures.  However, when O₂ is present in large amounts, the reaction of CO oxidation dominates the reaction network.  The exposure of all samples to the NO/CO mixture in the 400–600^oC temperature range leads to the formation of isocyanate species on the surface.  FTIR and kinetic data further suggest that isocyanate species thus formed are reactive towards O₂ and NO, yielding CO₂ and N₂ as the major products.  These results suggest that isocyanate species could be intermediates in the CO–NO reaction network that takes place on surfaces of catalytic additives examined.