Strikingly Facile Cleavage of N-H/N-O Bonds Induced by Surface Frustrated Lewis Pair on CeO2(110) to Boost NO Reduction by NH3

Ceria with surface solid frustrated Lewis pairs (FLPs), formed by regulating oxygen vacancies, demonstrate remarkable ability in activating small molecules. In this work, we extended the application of FLPs on CeO2(110) to the selective catalytic reduction of NO by NH3 (NH3-SCR), finding a notable enhancement in performance compared to ordinary CeO2(110). Additionally, an innovative approach involving H2 treatment was discovered to increase the number of FLPs, thereby further boosting the NH3-SCR efficiency. Typically, NH3-SCR on regular CeO2 follows the Eley-Rideal (E-R) mechanism. However, density functional theory (DFT) calculations revealed a significant reduction in the energy barriers for the activation of N-O and N-H bonds under the Langmuir-Hinshelwood (L-H) mechanism with FLPs present. This transition shifted the reaction mechanism from the E-R pathway on regular R-CeO2 to the L-H pathway on FLP-rich FR-CeO2, as corroborated by the experimental findings. The practical application of FLPs was realized by loading MoO3 onto FLP-rich FR-CeO2, leveraging the synergistic effects of acidic sites and FLPs. This study provides profound insights into how FLPs facilitate N-H/N-O bond activation in small molecules, such as NH3 and NO, offering a new paradigm for catalyst design based on catalytic mechanism research.

Unveiling alkali metal poisoning of CrMn catalyst for selective catalytic reduction of NOx with NH3: An experimental and theoretical study

Alkali metal poisoning has been an intricate and unsolved issue confining the catalytic activity of NH₃-SCR catalysts up to now. Herein, the effect of NaCl and KCl on catalytic activity of CrMn catalyst for NH₃-SCR of NO_x was systematically investigated to clarify the alkali metal poisoning by combined experiments and theoretical calculations. It unveiled that NaCl/KCl could deactivate CrMn catalyst due to the decrease in specific surface area, electron transfer (Cr⁵⁺+Mn³⁺↔Cr³⁺+Mn⁴⁺), redox ability and oxygen vacancy and NH₃/NO adsorption. In addition, NaCl cut off E-R mechanism reactions by inactivating surface Brønsted/Lewis acid sites. DFT calculations revealed that (1) Na and K could weaken MnO bond, (2) competitive adsorption between Cl and NH₃ was a main reason weakening Lewis acid, (3) Cl adsorption was also a major cause diminishing Brønsted acid and oxygen vacancy, (4) Both Na and K seriously impeded NO adsorption/activation, (5) NaCl/KCl increased the reaction heat of H₂O desorption (rate-determining step) in E-R mechanism reactions and KCl elevated its energy barrier in L-H mechanism reactions. Thus, this study provides the deep understanding of alkali metal poisoning and a well strategy to synthesize NH₃-SCR catalysts with outstanding alkali metal resistance.

Elucidating the facet-dependent reactivity of CrMn catalyst for selective catalytic reduction of NOx with NH3

The facet-dependent reactivity of CrMn catalysts was still unclear, hindering the further enhancement of their low-temperature SCR performance. Herein, the facet-dependent reactivity of CrMn_1.5O₄ catalyst for NH₃-SCR of NO_x was innovatively illustrated by numerous characterizations and density functional theory (DFT) calculations. Exposed (100) facet of CrMn_1.5O₄ catalyst exhibited best low-temperature SCR activity with ≥90 % NO conversion within 148-296 °C and 2.86 × 10^-3 mol/(g·s) reaction rate within 160-240 °C. The characterizations revealed that (100) facet could induce the increase of BET specific area, electron transfer, concentration of Mn⁴⁺ and O_α, surface acidity, redox ability, NH₃ and NO_x adsorption/activation capacity. Subsequently, DFT calculations demonstrated that (100) facet exhibited the strongest affinity for NH₃ and NO due to its unique 3O_3c-Mn_5c-2O_4c bond and abundant charges transfer near the active adsorption sites, and Brønsted acid and oxygen vacancies were most easily formed on (100) facet. Furthermore, H₂O formation as the rate determining step easily occurred on (100) facet. Eventually, we successfully improved the low-temperature SCR activity of CrMn_1.5O₄ catalyst by further tailoring highly active (100) facet from 0.754 to 0.865. This work provides the deeper understanding of facet-dependent reactivity and a good strategy to improve the catalytic activity of the catalysts.

Calcium poisoning mechanism on the selective catalytic reduction of NOx by ammonia over the γ-Fe2O3 (001) surface

γ-Fe₂O₃ has an excellent low-temperature selective catalytic reduction (SCR) deNO_x performance, but its resistance to alkaline earth metal calcium (Ca) is poor. In particular, the detailed mechanism of Ca poisoning on the γ-Fe₂O₃ catalyst at the atomic level is not clear. Hence, the density functional theory method was used in this research to investigate the influence mechanism of Ca poisoning on the NH₃-SCR over the γ-Fe₂O₃ catalyst surface. The findings reveal that NH₃, NO, and O₂ molecules can bind to the γ-Fe₂O₃ (001) surface to generate coordinated ammonia, monodentate nitroso, and adsorption oxygen species, respectively. The main active site is Fe₁-top. For the γ-Fe₂O₃ with Ca poisoning, the Ca atom has a high adsorption energy on the surface of γ-Fe₂O₃ (001), which covers the catalyst surface and reduces the active sites. The presence of Ca atom decreases the adsorption performance of NH₃, while slightly improving the NO and O₂ adsorption. In particular, the Ca atom restrains the NH₃ activation and NH₂ formation, which is detrimental to the NH₃-SCR process.

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.

Single-Atom Ce-Modified α-Fe2O3 for Selective Catalytic Reduction of NO with NH3

A single-atom Ce-modified α-Fe₂O₃ catalyst (Fe_0.93Ce_0.07O_x catalyst with 7% atomic percentage of Ce) was synthesized by a citric acid-assisted sol-gel method, which exhibited excellent performance for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) over a wide operating temperature window. Remarkably, it maintained ∼93% NO conversion efficiency for 168 h in the presence of 200 ppm SO₂ and 5 vol % H₂O at 250 °C. The structural characterizations suggested that the introduction of Ce leads to the generation of local Fe-O-Ce sites in the FeO_x matrix. Furthermore, it is critical to maintain the atomic dispersion of the Ce species to maximize the amounts of Fe-O-Ce sites in the Ce-doped FeO_x catalyst. The formation of CeO₂ nanoparticles due to a high doping amount of Ce species leads to a decline in catalytic performance, indicating a size-dependent catalytic behavior. Density functional theory (DFT) calculation results indicate that the formation of oxygen vacancies in the Fe-O-Ce sites is more favorable than that in the Fe-O-Fe sites in the Ce-free α-Fe₂O₃ catalyst. The Fe-O-Ce sites can promote the oxidation of NO to NO₂ on the Fe_0.93Ce_0.07O_x catalyst and further facilitate the reduction of NO_x by NH₃. In addition, the decomposition of NH₄HSO₄ can occur at lower temperatures on the Fe_0.93Ce_0.07O_x catalyst containing atomically dispersed Ce species than on the α-Fe₂O₃ reference catalyst, resulting in the good SO₂/H₂O resistance ability in the NH₃-SCR reaction.

Understanding the roles of copper dopant and oxygen vacancy in promoting nitrogen oxides removal over iron-based catalyst surface: A collaborative experimental and first-principles study

In this work, we proposed a novel strategy of copper (Cu) doping to enhance the nitrogen oxides (NO_x) removal efficiency of iron (Fe)-based catalysts at low temperature through a simple citric acid mixing method, which is critical for its practical application. The doping of Cu significantly improves the deNO_x performance of Fe-based catalysts below 200 °C, and the optimal catalyst is (Cu_0.22Fe_1.78)_1-δO₃, which deNO_x efficiency can reach 100% at 160-240 °C. From the macro aspects, the main reasons for the excellent catalytic activity of the (Cu_0.22Fe_1.78)_1-δO₃ catalyst are the large number of oxygen vacancies (O_vac), appropriate Fe³⁺ and Cu²⁺ contents, stronger surface acidity and redox ability. From the micro aspects, the O_vac plays a key role in enhancing molecular adsorption, oxidation, and the deNO_x reaction over the Fe-based catalyst surface, which promoting order is CuO_vac > O_vac > Cu. This work provides a new insight for the mechanism study of oxygen vacancy engineering and also accelerates the development of CuFe bimetal composite catalysts at low temperature.

Effect of doping Cr on NH3 adsorption and NO oxidation over the FexOy/AC surface: A DFT-D study

Activated carbon supported iron-based catalysts (Fe_xO_y/AC) show good deNO_x efficiency at low temperature. The doping of chromium (Cr) greatly improves the catalyst activity. However, the detailed effect of doping Cr over Fe_xO_y/AC surface at molecular level is still a grey area. In this study, the roles of Cr dopant on gas adsorption and NO oxidation were deeply investigated by a DFT-D3 method. Results show that the synergy of Cr-Fe bimetal improves the binding capacity of Fe₂O₃/AC and Fe₃O₄/AC surfaces after doping Cr. NH₃ can be adsorbed on Cr and Fe sites to form coordinated NH₃. Doping Cr greatly improves the NH₃ adsorption property on the Fe₃O₄/AC surface. NO molecule can combine with Cr, Fe, and O sites to form nitrosyl and nitrite. The doping of Cr increases the adsorption performance of NO on the Fe₂O₃/AC and Fe₃O₄/AC surfaces, especially for Fe₃O₄/AC surface. Furthermore, NO can be oxidized to NO₂ by adsorption oxygen or active O sites of Fe_xO_y clusters. The doping of Cr restrains the formation of insoluble chelating bidentate nitrates and greatly reduces the reaction energy barrier of NO oxidation on the Fe_xO_y/AC surface, which can promote the deNO_x reaction.

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.

Superior Oxidative Dehydrogenation Performance toward NH3 Determines the Excellent Low-Temperature NH3-SCR Activity of Mn-Based Catalysts

Mn-based oxides exhibit outstanding low-temperature activity for the selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) compared with other catalysts. However, the underlying principle responsible for the excellent low-temperature activity is not yet clear. Here, the atomic-level mechanism and activity-limiting factor in the NH₃-SCR process over Mn-, Fe-, and Ce-based oxide catalysts are elucidated by a combination of first-principles calculations and experimental measurements. We found that the superior oxidative dehydrogenation performance toward NH₃ of Mn-based catalysts reduces the energy barriers for the activation of NH₃ and the formation of the key intermediate NH₂NO, which is the rate-determining step in NH₃-SCR over these oxide catalysts. The findings of this study advance the understanding of the working principle of Mn-based SCR catalysts and provide a fundamental basis for the development of future generation SCR catalysts with excellent low-temperature activity.

A DFT-D study on the reaction mechanism of selective catalytic reduction of NO by NH3 over the Fe2O3/Ni(111) surface

The adsorption and SCR reaction mechanism of NH₃, NO, and O₂ molecules on the Fe₂O₃/Ni(111) catalyst surface was revealed.

Adsorption mechanism of NH3, NO, and O2 molecules over the FexOy/AC catalyst surface: a DFT-D3 study

The surface model of the Fe_xO_y/AC catalyst was constructed and the adsorption mechanism of gas molecules on its surface was revealed.