Modeling the Adsorption of NO and NH₃ on Fe-SSZ-13 from First-Principles: A DFT Study
2018
Zhang, Renqin | Anderst, Emily | Groden, Kyle | McEwen, Jean-Sabin
As emission standards continue to tighten, there is a critical need for more selective and efficient methods to remove NOₓ from the exhaust of lean-burn engines. Currently, the predominant method employed for NOₓ removal is through selective catalytic reduction (SCR) using ammonia as a reducing agent. Typically, Fe- and Cu-exchanged CHA catalysts (Fe-SSZ-13 and Cu-SSZ-13) are used, but much less is known about Fe-SSZ-13 as compared to Cu-SSZ-13, which has been extensively characterized via experimental and computational techniques. In this work, density functional theory (DFT) calculations were performed to determine the most likely adsorption complexes that form within the six- or eight-membered rings (6MR or 8MR) of Fe-SSZ-13. We examine both Fe²⁺ (as [Fe(OH)]⁺) and Fe³⁺ (as [Fe(OH)]²⁺) as the possible active sites. Based on our results, the most energetically favorable site lies initially within the 6MR. However, with the addition of adsorbates, we find that the 8MR sites become more energetically favorable. Our results show, for an Fe²⁺ cation, that the most stable configuration occurs within the 8MR, where NO is coadsorbed on Fe with an OH ligand. For an Fe³⁺ cation, the lowest energy configuration occurs when NO adsorbs directly onto Fe within the 8MR without interacting with the OH ligand. The vibrational frequencies of chemisorbed NO were also calculated and compared with the experimental IR spectra. The experimental values for such frequencies agree well with the DFT-calculated values, except for Fe²⁺(NO)₂. We anticipate that this improved understanding of the structural and electronic features of Fe-SSZ-13 will be useful for the future determination of the underlying SCR reaction mechanism on Fe-SSZ-13.
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