Unraveling the Promotion Effects of Dynamically Constructed CuOx-OH Interfacial Sites in the Selective Catalytic Oxidation of Ammonia

Surface ammonification regulates oxygen vacancies and acidic sites in δ-MnO2 to enhance low-temperature selective catalytic oxidation activity of ammonia

Selective catalytic oxidation (NH3-SCO) is presently among the foremost technologies for eliminating the malodorous gaseous NH3. Simultaneously achieving high NH3 conversion and high selectivity towards N2 at low temperatures poses a considerable hurdle. Surface oxygen vacancies and acidic sites, acting as adsorption and active sites in NH3-SCO, hold the key to realizing efficient catalytic activity. In this paper, a facile surface ammoniation was implemented in layered birnessite-type MnO2 (δ-MnO2) to enhance its NH3-SCO activity. Surface ammoniation is conducive to the generation of oxygen vacancies in δ-MnO2, thereby promoting the activation of molecular oxygen and further oxidation of NH3; Moreover, ammoniation can also regulate the surface Lewis and Brønsted acidic sites to a reasonable ratio and strength, thereby enhancing the adsorption and activation of NH3. Compared with pristine δ-MnO2, both the activity and selectivity of NH4+-modified layered δ-MnO2 has been greatly enhanced, lowering the complete NH3 elimination temperature from 150 °C to 130 °C. The results of in situ DRIFTS indicate the layered δ-MnO2 followed the i-SCR mechanism, and the pristine δ-MnO2 were dominated by monodentate nitrate, while the NH4+-modified layered δ-MnO2 were dominated by bidentate nitrate and supplemented by monodentate nitrate. The further reaction of in situ generated nitrate with NH3 is the controlling step of the reaction, and the coexistence of monodentate nitrate and bidentate nitrate not only enhances the catalytic oxidation activity of NH3 but also improves the selectivity of N2. This study provides a facile surface engineering approach for designing MnO2 catalysts for low-temperature NH3-SCO.

Synthesis of Ru-W/CeZrOx catalyst with superior NH3-SCO performance: Synergy between Ru and W species

Ammonia selective catalytic oxidation (NH3-SCO) is an effective method for NH3 removal. However, it is still a great challenge to develop catalysts with a wide operating temperature window, high catalytic activity and N2 selectivity, particularly for the removal of high-concentration NH3 from NH3-fueled engine exhaust gas. Herein, a small amount of Ru (0.5 wt%) together with W were used to co-modify CeZrOx through impregnation method to synthesize a novel NH3-SCO catalyst. The as-prepared Ru-W/CeZrOx catalyst could achieve a complete NH3 conversion at 300 °C, and a superior N2 selectivity, which exceeded 95.7 % over a wide temperature range of 225-400 °C. The physicochemical properties of the prepared catalysts were compared using various characterization techniques to reveal the possible roles of Ru and W species in NH3-SCO reaction. XPS results showed that the introduction of Ru species was in favor of forming abundant surface adsorbed oxygen species on the surface of Ru-W/CeZrOx catalyst, significantly enhancing the low-temperature activity. Besides, the co-existing W species could suppress the excessive oxidation of NH3 on catalyst surface, which was crucial for improving N2 selectivity. In-situ DRIFTS results suggested that Ru-W/CeZrOx catalyst followed both the internal selective catalytic reduction (i-SCR) and amide (-NH) mechanisms during NH3-SCO reaction. More importantly, the NH3 species adsorbed on Ru-W/CeZrOx catalyst surface reacted more rapidly than that of W/CeZrOx catalyst, and were mainly converted to N2 rather than NOx or N2O under the synergy of Ru and W species.

Tandem Reaction on Ru/Cu-CHA Catalysts for Ammonia Elimination with Enhanced Activity and Selectivity

Ammonia emissions from vehicles and power plants cause severe environmental issues, including haze pollution and nitrogen deposition. Selective catalytic oxidation (SCO) is a promising technology for ammonia abatement, but current catalysts often struggle with insufficient activity and poor nitrogen selectivity, leading to the formation of secondary pollutants. In this study, we developed a bifunctional Ru/Cu-CHA zeolite catalyst for ammonia oxidation, incorporating both SCO sites (Ru) and selective catalytic reduction sites (SCR, Cu). Various characterizations, including HAADF-STEM, XAFS, and H2-TPR, revealed that Cu2+ cations are dispersed within the CHA zeolite, while RuOx clusters and nanoparticles are present both inside and on the surface of the zeolite. Operando DRIFTS-MS, in situ Raman spectroscopy, and DFT calculations confirmed that NH3 adsorbed on Cu2+ Lewis acid sites efficiently reduced RuO2 with a lower energy barrier, significantly enhancing the low-temperature activity of the Ru/Cu-CHA catalyst. Additionally, Cu2+ cations further facilitated the elimination of byproducts (NOx) via the tandem SCR reaction, thus greatly improving the nitrogen selectivity. This synergistic effect contributed to high catalytic activity (>94% at 200 °C) and excellent nitrogen selectivity (>90% even at high temperatures above 325 °C) for Ru2.5/Cu-CHA during practical ammonia elimination in the presence of NOx and water vapor.

Subsurface Single-Atom Catalyst Enabled by Mechanochemical Synthesis for Oxidation Chemistry

Single-atom catalysts have garnered significant attention due to their exceptional atom utilization and unique properties. However, the practical application of these catalysts is often impeded by challenges such as sintering-induced instability and poisoning of isolated atoms due to strong gas adsorption. In this study, we employed the mechanochemical method to insert single Cu atoms into the subsurface of Fe2O3 support. By manipulating the location of single atoms at the surface or subsurface, catalysts with distinct adsorption properties and reaction mechanisms can be achieved. It was observed that the subsurface Cu single atoms in Fe2O3 remained isolated under both oxidation and reduction environments, whereas surface Cu single atoms on Fe2O3 experienced sintering under reduction conditions. The unique properties of these subsurface single-atom catalysts call for innovations and new understandings in catalyst design.

Low-temperature NH3 abatement via selective oxidation over a supported copper catalyst with high Cu+ abundance

Selective catalytic NH3-to-N2 oxidation (NH3-SCO) is highly promising for abating NH3 emissions slipped from stationary flue gas after-treatment devices. Its practical application, however, is limited by the non-availability of low-cost catalysts with high activity and N2 selectivity. Here, using defect-rich nitrogen-doped carbon nanotubes (NCNT-AW) as the support, we developed a highly active and durable copper-based NH3-SCO catalyst with a high abundance of cuprous (Cu+) sites. The obtained Cu/NCNT-AW catalyst demonstrated outstanding activity with a T50 (i.e. the temperature to reach 50% NH3 conversion) of 174°C in the NH3-SCO reaction, which outperformed not only the Cu catalyst supported on N-free O-functionalized CNTs (OCNTs) or NCNT with less surface defects, but also those most active Cu catalysts in open literature. Reaction kinetics measurements and temperature-programmed surface reactions using NH3 as a probe molecule revealed that the NH3-SCO reaction on Cu/NCNT-AW follows an internal selective catalytic reaction (i-SCR) route involving nitric oxide (NO) as a key intermediate. According to mechanistic investigations by X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray absorption spectroscopy, the superior NH3-SCO performance of Cu/NCNT-AW originated from a synergy of surface defects and N-dopants. Specifically, surface defects promoted the anchoring of CuO nanoparticles on N-containing sites and, thereby, enabled efficient electron transfer from N to CuO, increasing significantly the fraction of SCR-active Cu+ sites in the catalyst. This study puts forward a new idea for manipulating and utilizing the interplay of defects and N-dopants on carbon surfaces to fabricate Cu+-rich Cu catalysts for efficient abatement of slip NH3 emissions via selective oxidation.

Unraveling the Mechanistic Origin of High N2 Selectivity in Ammonia Selective Catalytic Oxidation on CuO-Based Catalyst

NH3 emissions from industrial sources and possibly future energy production constitute a threat to human health because of their toxicity and participation in PM2.5 formation. Ammonia selective catalytic oxidation to N2 (NH3-SCO) is a promising route for NH3 emission control, but the mechanistic origin of achieving high N2 selectivity remains elusive. Here we constructed a highly N2-selective CuO/TiO2 catalyst and proposed a CuOx dimer active site based on the observation of a quadratic dependence of NH3-SCO reaction rate on CuOx loading, ac-STEM, and ab initio thermodynamic analysis. Combining this with the identification of a critical N2H4 intermediate by in situ DRIFTS characterization, a comprehensive N2H4-mediated reaction pathway was proposed by DFT calculations. The high N2 selectivity originated from the preference for NH2 coupling to generate N2H4 over NH2 dehydrogenation on the CuOx dimer active site. This work could pave the way for the rational design of efficient NH3-SCO catalysts.

Boosting Ozone Catalytic Oxidation of Toluene at Room Temperature by Using Hydroxyl-Mediated MnOx/Al2O3 Catalysts

Ozone catalytic oxidation (OZCO) has gained great interest in environmental remediation while it still faces a big challenge during the deep degradation of refractory volatile organic compounds (VOCs) at room temperature. Hydroxylation of the catalytic surface provides a new strategy for regulating the catalytic activity to boost VOC degradation. Herein, OZCO of toluene at room temperature over hydroxyl-mediated MnO_x/Al₂O₃ catalysts was originally demonstrated. Specifically, a novel hydroxyl-mediated MnO_x/Al₂O₃ catalyst was developed via the in situ AlOOH reconstruction method and used for toluene OZCO. The toluene degradation performance of MnO_x/Al₂O₃ was significantly superior to those of most of the state-of-the-art catalysts, and 100% toluene was removed with an excellent mineralization rate (82.3%) and catalytic stability during OZCO. ESR and in situ DRIFTs results demonstrated that surface hydroxyl groups (HGs) greatly improved the reactive oxygen species generation, thus dramatically accelerating the benzene ring breakage and deep mineralization. Furthermore, HGs provided anchoring sites for uniformly dispersing MnO_x and greatly enhanced toluene adsorption and ozone activation. This work paves a way for deep decomposition of aromatic VOCs at room temperature.

Heat Treatment Improves the Activity and Water Tolerance of Pt/Al2O3 Catalysts in Ammonia Catalytic Oxidation

Ammonia selective catalytic oxidation (NH₃-SCO) is a commercial technology applied to diesel vehicles to eliminate ammonia leakage. In this study, a series of Pt/Al₂O₃ catalysts were synthesized by an impregnation method, and the state of Pt species was carefully adjusted by heat treatment. These Pt/Al₂O₃ catalysts were further systematically characterized by Brunauer-Emmett-Teller, X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption fine structure, UV-vis, H₂-tempertaure-programmed reduction, and NH₃-temperature-programmed desorption. The characterization results showed that dispersed oxidized Pt species were present on conventional Pt/Al₂O₃ samples, while high-temperature treatment induced the aggregation of platinum species to form metallic Pt nanoparticles. The Pt/Al₂O₃ catalysts treated at high temperatures showed superior activity and water tolerance in the NH₃-SCO reaction. Diffuse reflectance infrared Fourier-transform spectroscopy combined with mass spectrometry experiments revealed that the Lewis acid sites were more reactive than the Brønsted acid sites. Moreover, compared to oxidized Pt species, metallic Pt nanoparticles were beneficial for oxygen activation and were less affected by water vapor, thus contributing to the superior activity and water tolerance of Pt/Al-800.

Ammonia Abatement via Selective Oxidation over Electron-Deficient Copper Catalysts

Selective catalytic ammonia-to-dinitrogen oxidation (NH₃-SCO) is highly promising for the abatement of NH₃ emissions from flue gas purification devices. However, there is still a lack of high-performance and cost-effective NH₃-SCO catalysts for real applications. Here, highly dispersed, electron-deficient Cu-based catalysts were fabricated using nitrogen-doped carbon nanotubes (NCNT) as support. In NH₃-SCO catalysis, the Cu/NCNT outperformed Cu supported on N-free CNTs (Cu/OCNT) and on other types of supports (i.e., activated carbon, Al₂O₃, and zeolite) in terms of activity, selectivity to the desired product N₂, and H₂O resistance. Besides, Cu/NCNT demonstrated a better structural stability against oxidation and a higher NH₃ storage capacity (in the presence of H₂O vapor) than Cu/OCNT. Quasi in situ X-ray photoelectron spectroscopy revealed that the surface N species facilitated electron transfer from Cu to the NCNT support, resulting in electron-deficient Cu catalysts with superior redox properties, which are essential for NH₃-SCO catalysis. By temperature-programmed surface reaction studies and systematic kinetic measurements, we unveiled that the NH₃-SCO reaction over Cu/NCNT proceeded via the internal selective catalytic reaction (i-SCR) route; i.e., NH₃ was oxidized first to NO, which then reacted with NH₃ and O₂ to form N₂ and H₂O. This study paves a new route for the design of highly active, H₂O-tolerant, and low-cost Cu catalysts for the abatement of slip NH₃ from stationary emissions via selective oxidation to N₂.

Compensation or Aggravation: Pb and SO2 Copoisoning Effects over Ceria-Based Catalysts for NOx Reduction

Severe catalyst deactivation caused by multiple poisons, including heavy metals and SO₂, remains an obstinate issue for the selective catalytic reduction (SCR) of NO_x by NH₃. The copoisoning effects of heavy metals and SO₂ are still unclear and irreconcilable. Herein, the unanticipated differential compensated or aggravated Pb and SO₂ copoisoning effects over ceria-based catalysts for NO_x reduction was originally unraveled. It was demonstrated that Pb and SO₂ exhibited a compensated copoisoning effect over the CeO₂/TiO₂ (CT) catalyst with sole active CeO₂ sites but an aggravated copoisoning effect over the CeO₂-WO₃/TiO₂ (CWT) catalyst with dual active CeO₂ sites and acidic WO₃ sites. Furthermore, it was uniquely revealed that Pb preferred bonding with CeO₂ among CT while further being combined with SO₂ to form PbSO₄ after copoisoning, which released the poisoned active CeO₂ sites and rendered the copoisoned CT catalyst a recovered reactivity. In comparison, Pb and SO₂ would poison acidic WO₃ sites and active CeO₂ sites, respectively, resulting in a seriously degraded reactivity of the copoisoned CWT catalyst. Therefore, this work thoroughly illustrates the internal mechanism of differential compensated or aggravated deactivation effects for Pb and SO₂ copoisoning over CT and CWT catalysts and provides effective solutions to design ceria-based SCR catalysts with remarkable copoisoning resistance for the coexistence of heavy metals and SO₂.

A Comparative Mini-Review on Transition Metal Oxides Applied for the Selective Catalytic Ammonia Oxidation (NH3-SCO)

The selective catalytic oxidation of NH₃ (NH₃-SCO) into N₂ and H₂O is an efficient technology for NH₃ abatement in diesel vehicles. However, the catalysts dedicated to NH₃-SCO are still under development. One of the groups of such catalysts constituted transition metal-based catalysts, including hydrotalcite-derived mixed metal oxides. This class of materials is characterized by tailored composition, homogenously dispersed mixed metal oxides, exhibiting high specific surface area and thermal stability. Thus, firstly, we give a short introduction to the structure and composition of hydrotalcite-like materials and their applications in NH₃-SCO. Secondly, an overview of other transition metal-based catalysts reported in the literature is given, following a comparison of both groups. The challenges in NH₃-SCO applications are provided, while the reaction mechanisms are discussed for particular systems.