(12f) Highly Dispersed Fe-Oxide Catalysts for Reduction of NOx By CO: Synthesis, Evaluation, & Operando XAS

The reduction of oxides of nitrogen (NOx) without the use of platinum group metals (PGM) remains a challenge in automotive exhaust catalysis.  Current strategies have employed selective catalytic reduction (SCR) over Cu- or Fe-exchanged zeolites.¹  The structure of the transition metal-oxide active sites in the zeolite framework has yet to be clarified, but it is generally accepted that isolated ions or other low nuclearity species are responsible.^2-4  Dispersion of the metal oxide on a high surface area support represents a potential route to the identification of the NO reduction active site by creating sites that mimic those found in the ion-exchanged zeolites. However, the surface oxide species created by typical synthetic routes to supported metal oxides are largely governed by thermodynamics, and it is difficult to obtain narrow distributions of metal-oxide surface species. Therefore, the current study utilizes a unique synthetic route to highly dispersed Fe-oxides on a non-zeolitic CeO₂support for reduction of NO by CO.

The Fe-oxide catalysts were impregnated onto CeO₂ using Fe coordinated to a bulky organic ligand EDTA and a Na cation (denoted as NaFeEDTA). Surface densities of 0-1.5 Fe/nm² were prepared. These catalysts are compared to the traditional Fe(NO₃)₃ precursor impregnated on Na-promoted CeO₂ over the same surface density range.  Through comprehensive characterization by UV-vis, XRD, Raman, XAS, Mössbauer, and TPR, it was discovered that the NaFeEDTA route was preferred over Fe(NO₃)₃ to produce a narrow distribution of smaller, non-crystalline, surface Fe-oxide species with excellent redox cyclability (Fe³⁺↔Fe²⁺sites).

Evaluation of the steady-state activity for NO reduction by CO demonstrated that the NaFeEDTA catalysts exhibited the highest activity over the studied surface density range. Additionally, a maximum activity was established at ~1.0 Fe/nm² before plateauing at higher surface densities. The plateau in activity is explained by quantifying the redox cyclable Fe sites (Fe³⁺↔Fe²⁺) using TPR/TPO cycles, which correlated with both structure and activity.

This reaction was further probed using operando XANES. The results indicate that removal of NO from the reaction stream begins concurrent with reduction of Fe and Ce. This observation suggests that reduction occurs at the Fe-O-Ce interface and this event is necessary to initiate activity. Further investigation of the role of this site in the NO reduction mechanism will be investigated by in situ FTIR spectroscopy during NO adsorption and temperature programmed surface reaction under flowing CO gas.  The current study is expected to expand the understanding of the necessary redox properties and active site structures of Fe-oxide active sites for NO reduction during automotive exhaust catalysis.

References

1. Kröcher, Stud. Surf. Sci. Catal, vol. 171 (2007) 261.
2. M.S. Kumar, M. Schwidder, W. Grünert, U. Bentrp, A. Brückner J. Catal. 239 (2006) 173.
3. J.-S. McEwen, T. Anggara, W.F. Schneider, V.F. Kispersky, J.T. Miller, W.N. Delgass, F.H. Ribeiro, Catal. Today 184 (2012) 129.
4. J. Szanyi, J.H. Kwak, H. Zhu, C.H.F. Peden, Phys. Chem. Chem. Phys. 15 (2013) 2368.