(446g) NH3 Interactions with O2 and NO Over Pd-Based FCC CO Emission Control Additives

Fluid Catalytic Cracking (FCC) is the primary process used to convert crude oil into a variety of higher-value products in modern petroleum refineries. Some portion of the nitrogen present in the oil feed is trapped in the coke formed during the cracking process and is eventually released in the FCC regenerator unit in different forms, including NO_x and NH₃. The levels of these pollutants depend primarily on the content and the type of nitrogen compounds being present in the feed, as well as on the regeneration mode used. Among different emission control technologies, the use of catalytic additives is the most attractive one, as it is simple, cost-effective, and applicable in existing FCC units. Most of the modern refineries for example, use promoters to accelerate the oxidation of CO. The goal of this study is to understand if and how the presence of these additives affects the reactions of NH3 with other gaseous components present (i.e., O₂ and NO) under conditions approaching those existing in FCC regenerators, and to assess their effect on the control of N-containing gaseous species. FTIR and kinetic measurements were used to investigate the possible reactions of NH₃ with O₂ and NO over a Pdⁿ⁺/Ceⁿ⁺/Na⁺/g-Al₂O₃ model additive. The FTIR results show that NH_x intermediates can be formed upon adsorption of NH₃ on the Pdⁿ⁺/Ceⁿ⁺/Na⁺/g-Al₂O₃ surface in a wide range of temperatures. All components of this material (i.e., Pdⁿ⁺, Ceⁿ⁺, and Na⁺) interact with NO_x at elevated temperatures and promote the formation of nitrate/nitrite species to various degrees, essentially "trapping" NO_x species on the surface. Nitrite/nitrate species can also be formed on the surface due to NH₃ oxidation. A combination of FTIR and kinetic measurements indicates that NH₃ can react efficiently with surface nitrites/nitrates to produce N₂. These results suggest that some type of SCR chemistry is taking place on the Pdⁿ⁺/Ceⁿ⁺/Na⁺/g-Al₂O₃ surface, involving the formation of nitrites/nitrates that are capable of interacting with NH_x to form N₂. However, at higher temperatures (i.e., 700^oC) the oxidation of NH₃ becomes dominant and leads to the formation of NO_x, which quickly desorbs from the surface. In the absence of O₂ in the feed, NH₃ continues to interact with NO_x at high rates yielding N₂ and N₂O. Given the dynamic nature and the significant variation of conditions in commercial FCC regenerators, it is possible that both schemes described above contribute to overall NO_x reduction mechanisms.