Insight into the Nature of Active Species of Pt/Al 2 O 3 Catalysts for low Temperature NH 3 Oxidation

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.

Design Strategies for High-Performance NH3-SCO Catalysts: Identifying and Modulating Direct Anchoring Sites for Ag on TiO2

Ammonia (NH3) slip from diesel vehicle aftertreatment systems and internal combustion engines fueled by NH3 or NH3/H2 poses serious environmental problems. Ag-based catalysts are widely used for the selective catalytic oxidation of NH3 to N2 (NH3-SCO), and their performance is greatly dependent on the state of Ag, which is influenced by the anchoring sites on the support. Despite efforts to identify the direct anchoring sites of metal atoms on TiO2, conflicting views persist. Here, we compared the correlation between Ag dispersion and the content of hydroxyl (OH) groups or defects on TiO2 and conducted density functional theory (DFT) calculations, and the results confirmed that the surface OH groups of TiO2 serve as the direct anchoring sites for Ag. By modulating the OH group content through thermal induction, the optimal OH group content on TiO2-800 resulted in more metallic Ag nanoparticles (Ag0 NPs) in larger sizes, leading to the development of an excellent NH3-SCO catalyst. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), kinetic studies, and DFT calculations suggested that more Ag0 NPs in larger sizes on 10Ag/TiO2-800 were conducive to O2 activation and NH3 dissociation. Our findings provide new insights for designing efficient NH3-SCO catalysts, and OH groups as direct anchoring sites could be extended to other metals and supports for the rational design of catalysts.

Catalytic NH3 oxidation affected by the nanometric roughness of the platinum overlayer

Pulsed cathodic arc-plasma deposition was employed to create a few nanometre-thick Pt overlayer on a 50 μm-thick Fe-Cr-Al metal (SUS) foil, resulting in an effective NH3 oxidation catalyst fabrication. This catalyst exhibited a turnover frequency (TOF) exceeding 100 times that of Pt nanoparticles. In this study, Pt overlayer catalysts with varying degrees of surface roughness were fabricated using different metal foil substrates: mirror-polished (Pt/p-SUS), unpolished (Pt/SUS) and roughened by the formation of a surface oxide layer (Pt/Al2O3/SUS). The nanoscale roughness was comprehensively analysed using electron microscopy, laser scanning confocal microscopy and chemisorption techniques. NH3 oxidation activity, measured at 200 °C, followed an increasing trend in the order of Pt/Al2O3/SUS < Pt/SUS < Pt/p-SUS, despite a decrease in the apparent Pt surface area in the same order. Consequently, the calculated TOF was markedly higher for Pt/p-SUS (267 min-1) compared to Pt/SUS (107 min-1) and Pt/Al2O3/SUS (≤22 min-1). The smooth Pt overlayer surface also favoured N2 yield over N2O at this temperature. This discovery enhances our fundamental understanding of high-TOF NH3 oxidation over Pt overlayer catalysts, which holds significance for the advancement and industrial implementation of selective NH3 oxidation processes.

Cascade NH3 Oxidation and N2O Decomposition via Bifunctional Co and Cu Catalysts

The selective catalytic oxidation of NH3 (NH3-SCO) to N2 is an important reaction for the treatment of diesel engine exhaust. Co3O4 has the highest activity among non-noble metals but suffers from N2O release. Such N2O emissions have recently been regulated due to having a 300× higher greenhouse gas effect than CO2. Here, we design CuO-supported Co3O4 as a cascade catalyst for the selective oxidation of NH3 to N2. The NH3-SCO reaction on CuO-Co3O4 follows a de-N2O pathway. Co3O4 activates gaseous oxygen to form N2O. The high redox property of the CuO-Co3O4 interface promotes the breaking of the N-O bond in N2O to form N2. The addition of CuO-Co3O4 to the Pt-Al2O3 catalyst reduces the full NH3 conversion temperature by 50 K and improves the N2 selectivity by 20%. These findings provide a promising strategy for reducing N2O emissions and will contribute to the rational design and development of non-noble metal catalysts.

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.

Origin of Ammonia Selective Oxidation Activity of SmMn2O5 Mullite: A First-Principles-Based Microkinetic Study

Based on first-principles calculations and microkinetic analysis, the reaction routes and origin of the activity of SmMn₂O₅ mullite for the selective catalytic oxidation of ammonia (NH₃-SCO) are systematically investigated on three low-index surfaces under experimentally operating conditions. Key influencing factors and contributions of different iconic intermediate species (NH*, N₂H₄*, and HNO*) to the overall reaction process have been identified. In detail, Mn⁴⁺ serves as the primary active site for NH₃ adsorption, while lattice oxygen participates in the dehydrogenation of NH₃ on (010)⁴⁺ and (001)⁴⁺ surfaces. Furthermore, the (010)⁴⁺ surface shows both the best activity and the highest N₂ selectivity at low temperatures via the synergy effect of exposed Mn-Mn dimers and the most labile O² atoms. We further evaluate the potential catalytic performances of six A-site doped (010)⁴⁺ facets, among which La, Pr, and Nd dopings are predicted to possess better catalytic performances. Our study provides deep insights into the microscope reaction mechanisms and provides the specific optimization strategy for NH₃-SCO on mullite oxides.

Single-atom Ir1 supported on rutile TiO2 for excellent selective catalytic oxidation of ammonia

Gaseous ammonia (NH₃) in the atmosphere is potentially harmful to both human health and the environment. The selective catalytic oxidation of NH₃ (termed as NH₃-SCO) into N₂ and H₂O is a promising method for decreasing NH₃ emissions. A highly efficient catalyst is required for controlling NH₃ emissions by this method in practice. In this study, we prepared Ir/TiO₂ catalysts using different crystal structures of TiO₂ (rutile, P25 or anatase) as supports by a simple impregnation method and evaluated their performance in the NH₃-SCO. We found that the Ir/TiO₂-R (rutile) catalyst performed better than the Ir/TiO₂-P25 (mixed-phase) and Ir/TiO₂-A (anatase) catalyst. High-angle annular dark-field images of the aberration-corrected scanning transmission electron microscopy revealed that the Ir species were mainly atomically dispersed on the TiO₂ support in Ir/TiO₂-R with 1 wt% Ir loading, whereas the Ir species agglomerated to form clusters or nanoparticles in Ir/TiO₂-P25 and Ir/TiO₂-A. The combined results of X-ray absorption fine structure, H₂-temperature-programmed reduction, and in situ diffuse reflectance for infrared Fourier Transform spectroscopy studies suggested that atomically dispersed Ir species had stronger electronic metal-support interaction with rutile TiO₂, which resulted in easier to adsorb and activate O₂ at the interface and thus, better low-temperature activity of the Ir/TiO₂-R catalyst.

Catalytic activity of Cu/η-Al2O3 catalysts prepared from aluminum scraps in the NH3-SCO and in the NH3-SCR of NO

Copper-loaded η-alumina catalysts with different copper contents were prepared by impregnation/evaporation method. The catalysts were characterized by XRD, FTIR, BET, UV-Vis, and H₂-TPR and evaluated, for the first time, in the selective catalytic reduction of NO by NH₃ and in the selective catalytic oxidation of NH₃. The characterization techniques showed that the impregnation/evaporation method permits to obtain highly dispersed copper oxide species on the η-alumina surface when a low amount of copper is used (1wt. % and 2 wt.%). The wet impregnation method made it possible to reach a well dispersion of the copper species on the surface of the alumina for the low copper contents Cu(1)-Al₂O₃ and Cu(2)-Al₂O₃. The latter justifies the similar behavior of Cu(1)-Al₂O₃) and Cu(2)-Al₂O₃ in the selective catalytic oxidation of NH₃ where these catalysts exhibit a conversion of NH₃ to N₂ of the order of 100% at T> 500 °C.

Ordered mesoporous TiO2/SBA-15 confined CexWy catalysts for selective catalytic reduction of NO using NH3

The ordered mesoporous structure could improve the dispersion of nanoparticles, promote effective collision, and enhance redox capacity and surface acidity.

A mixed catalyst prepared by mechanically milling VW/TiO2 and low content of Pt/Al2O3 for SCO of high-concentration NH3

The combination of NH3 oxidation and NH3-SCO reaction made Pt0.01@VW catalyst exhibited excellent NH3-SCO performance and very low Pt content.

Progress on Noble Metal-Based Catalysts Dedicated to the Selective Catalytic Ammonia Oxidation into Nitrogen and Water Vapor (NH3-SCO)

A recent development for selective ammonia oxidation into nitrogen and water vapor (NH3-SCO) over noble metal-based catalysts is covered in the mini-review. As ammonia (NH3) can harm human health and the environment, it led to stringent regulations by environmental agencies around the world. With the enforcement of the Euro VI emission standards, in which a limitation for NH3 emissions is proposed, NH3 emissions are becoming more and more of a concern. Noble metal-based catalysts (i.e., in the metallic form, noble metals supported on metal oxides or ion-exchanged zeolites, etc.) were rapidly found to possess high catalytic activity for NH3 oxidation at low temperatures. Thus, a comprehensive discussion of property-activity correlations of the noble-based catalysts, including Pt-, Pd-, Ag- and Au-, Ru-based catalysts is given. Furthermore, due to the relatively narrow operating temperature window of full NH3 conversion, high selectivity to N2O and NOx as well as high costs of noble metal-based catalysts, recent developments are aimed at combining the advantages of noble metals and transition metals. Thus, also a brief overview is provided about the design of the bifunctional catalysts (i.e., as dual-layer catalysts, mixed form (mechanical mixture), hybrid catalysts having dual-layer and mixed catalysts, core-shell structure, etc.). Finally, the general conclusions together with a discussion of promising research directions are provided.

In situ probing of Pt/TiO2 activity in low-temperature ammonia oxidation

NAP-XPS and operando XAS study of Pt/TiO₂ catalysts shows that Pt⁰ species stabilized on TiO₂ surface have the highest activity at low-temperature NH₃ oxidation.

Influence of Titania Synthesized by Pulsed Laser Ablation on the State of Platinum during Ammonia Oxidation

A set of physicochemical methods, including X-ray photoelectron spectroscopy (XPS), X-ray diraction, electron microscopy and X-ray absorption spectroscopy, was applied to study Pt/TiO2 catalysts prepared by impregnation using a commercial TiO2-P25 support and a support produced by pulsed laser ablation in liquid (PLA). The Pt/TiO2-PLA catalysts showed increased thermal stability due to the localization of the highly dispersed platinum species at the intercrystalline boundaries of the support particles. In contrast, the Pt/TiO2-P25 catalysts were characterized by uniform distributionof the Pt species over the support. Analysis of Pt4f XP spectra shows that oxidized Pt2+ and Pt4+ species are formed in the Pt/TiO2-P25 catalysts, while the platinum oxidation state in the Pt/TiO2-PLA catalysts is lower due to stronger interaction of the active component with the support due to stronginteraction via Pt-O-Ti bonds. The Pt4f XP spectra of the samples after reaction show Pt2+ and metallic platinum, which is the catalytically active species. The study of the catalytic properties in ammonia oxidation showed that, unlike the catalysts prepared with a commercial support, the Pt/TiO2-PLA samples show higher stability during catalysis and significantly higher selectivity to N2 in a wide temperature range of 200–400 C.

Selective catalytic oxidation of NH3 over noble metal-based catalysts: state of the art and future prospects

The state of the art and future prospects for selective catalytic oxidation of NH₃ over noble metal-based catalysts are presented.