Activity Self-Optimization Steered by Dynamically Evolved Fe3+@Fe2+ Double-Center on Fe2O3 Catalyst for NH3-SCR

Identification of the active centers dynamically stable under the reaction condition is of paramount importance but challenging because of the limited knowledge of steady-state chemistry on catalysts at the atomic level. Herein, focusing on the Fe₂O₃ catalyst for the selective catalytic reduction of NO with NH₃ (NH₃-SCR) as a model system, we reveal quantitatively the self-evolving Fe³⁺@Fe²⁺ (∼1:1) double-centers under the in-situ condition by the first-principles microkinetic simulations, which enables the accurate prediction of the optimal industry operating temperature (590 K). The cooperation of this double-center achieves the self-optimization of catalytic activity and rationalizes the intrinsic origin of Fe₂O₃ catalyzing NH₃-SCR at middle-high temperatures instead of high temperatures. Our findings demonstrate the atomic-level self-evolution of active sites and the dynamically adjusted activity variation of the catalyst under the in-situ condition during the reaction process and provide insights into the reaction mechanism and catalyst optimization.

Reaction Pathways of the Selective Catalytic Reduction of NO with NH3 on the α-Fe2O3(012) Surface: a Combined Experimental and DFT Study

Fe2O3-based catalysts have promising potential in the selective catalytic reduction (SCR) of NO with NH3 with the advantages of environmental friendliness, excellent medium-high SCR activity, good N2 selectivity, and high SO2 tolerance. However, the NH3-SCR mechanism over Fe2O3-based catalysts remains highly uncertain and controversial due to the complex nature of the SCR reaction. Herein, the NH3-SCR reaction pathways over the α-Fe2O3(012) surface are elucidated at the atomic level by density functional theory calculations and experimental measurements. We demonstrate that, different from the NH3 activation mechanism in numerous SCR catalytic systems, the reaction tends to follow the NO activation mechanism, in which NO activated at Fe sites reacts with NH3 to form a NH2NO intermediate and further decomposes into N2 and H2O, in synchronization with the formation of a surface OH group. Subsequently, the catalyst is regenerated by an O2-assisted surface-dehydrogenation process. The activation of NO as well as the formation of the NH2NO intermediate is the rate-determining step of the complete SCR cycle. This study enhances the atomic-level understanding toward the NH3-SCR reaction and provides insights for the development of Fe2O3-based SCR catalysts.