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

Abstract

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.

Adsorption of NOx Molecules on the Hematite Surface: Isolated Versus Paired

“…Moreover, the HOMO of NO becomes unoccupied, with its electron hopping into an empty d orbital of the neighboring iron atom, as shown in Figure 2i. The overall reaction can be expressed as A recent work 13 shows that NH 3 adsorption on the hematite (0001) surface can lead to the reduction of Fe 3+ into Fe 2+ , which plays a major role in the overall SCR reaction. Here, we see that NO adsorption at the surface O site can also reduce Fe 3+ into Fe 2+ .…”

“…37,38 As such, it is widely used as a prototype to study the surface reactions of hematite. 12,13,33,39,40 Here, we model the (0001) surface with a periodic 13 atomiclayer slab separated along the perpendicular c-axis by 24 Å of vacuum. Atoms in the top and bottom 5 layers were free to move during structural relaxation, whereas those in the middle 3 layers had their positions fixed to those in bulk hematite.…”

“…Hematite (α-Fe 2 O 3 ) exhibits a corundum-type crystal structure and is antiferromagnetic along its c -axis . Its (0001) surface with Fe–O 3 –Fe termination is thermodynamically most stable and catalytically active. , As such, it is widely used as a prototype to study the surface reactions of hematite. ,,,, Here, we model the (0001) surface with a periodic 13 atomic-layer slab separated along the perpendicular c -axis by 24 Å of vacuum. Atoms in the top and bottom 5 layers were free to move during structural relaxation, whereas those in the middle 3 layers had their positions fixed to those in bulk hematite .…”

“…Despite this importance, a comprehensive investigation of the topic is lacking. Existing computational studies based on density functional theory (DFT) are mainly focused on NO, ,− but the adsorption of NO 2 on the hematite surface is less explored. Moreover, these studies predicted that NO only binds to surface iron atoms and forms isolated adsorbates, whereas infrared absorption spectra measured experimentally exhibited multiple peaks, , indicating there may be more than one type of NO adsorbate.…”

Structural and Composition Evolution of Palladium Catalyst for CO Oxidation under Steady-State Reaction Conditions

“…However, less is known about how the active phase is formed under reaction conditions and what the specific structure of the active phase is to catalyze the reaction. The structural evolution of catalysts has received much attention for the past decade, since complicated phase transitions may occur, depending crucially on the reacting atmosphere and temperature/pressure. − For instance, the transition metal catalysts may form O–metal bonds in the presence of oxygen, resulting in the formation of O-chemisorbed structure and surface oxides . A broad range of local configurations could exist under reaction conditions, some of which may serve as the real active phase to catalyze the reactions.…”