Balance between Reducibility and N2O Adsorption Capacity for the N2O Decomposition: CuxCoy Catalysts as an Example

Promoting Low-Temperature Toluene Oxidation via Pt-O-Fe Interfacial Sites in a Pt/CuO-Fe3O4 Catalyst

Electronic metal-support interaction (EMSI) has been widely explored in the catalytic degradation of volatile organic compounds (VOCs) owing to the formation of special interfacial sites. Herein, the EMSI effect was engineered by constructing the serial Pt catalysts supported on CuO-Fe3O4 bimetal oxide (Pt/CFO). Among them, the 0.5Pt/CFO catalyst with 0.5 wt % Pt loading exhibited an outstanding catalytic activity, with T90 (the temperature of 90% toluene conversion) lowered to 185 °C, and displayed excellent stability and water resistance. Comprehensive physicochemical characterizations revealed that an evident electron transfer occurred via the interface structure (Pt-O-Fe), producing the positively charged Pt (Ptδ+) and abundant Fe2+ species. Notably, the increased electron density around the Fe species weakened the Fe-O bond and thus activated the surface lattice oxygen (Olatt). Further, temperature-programmed desorption experiments and in situ diffuse reflectance infrared Fourier transform spectroscopy results demonstrated that the electron-deficient Ptδ+ was conducive to the adsorption and activation of toluene at low temperature. Consequently, the deep oxidation of toluene was achieved with the participation of Olatt, benefiting from the Ptδ+-O-Fe2+ interfacial sites with synergistic catalysis for toluene adsorption and oxygen activation. This work provides an interesting idea to explore the relationship between the electron transfer effect and highly efficient VOC abatement.

Enhanced Nitrous Oxide Decomposition on Zirconium-Supported Rhodium Catalysts by Iridium Augmentation

The effective elimination of N2O from automobile exhaust at low temperatures poses significant challenges. Compared to other materials, supported RhOx catalysts exhibit high N2O decomposition activities, even in the presence of O2, CO2, and H2O. Metal additives can enhance the low-temperature N2O decomposition activities over supported RhOx catalysts; however, the enhancement mechanism and active sites require further investigation. In this study, we demonstrate the significant enhancement of the low-temperature N2O decomposition activity of a monoclinic ZrO2-supported Rh catalyst [Rh(1)/ZrO2] with Ir addition in the presence of N2O + O2 + CO2 + H2O. The promotional effect of Ir and the active sites on N2O decomposition in Rh(1)-Ir(1)/ZrO2 (Rh = 1 wt % and Ir = 1 wt %) were investigated by kinetic studies and in situ spectroscopic methods, including X-ray absorption spectroscopy, ambient-pressure X-ray photoelectron spectroscopy, and ultraviolet-visible spectroscopy. These results indicate that both surface Rh and Ir species in Rh(1)-Ir(1)/ZrO2 were active sites for N2O catalytic decomposition at low temperatures, and Ir augmentation promoted the desorption of gaseous O2, which are regarded as key steps in N2O decomposition.

A review of chemical kinetic mechanisms and after-treatment of amino fuel combustion

Ammonia is a highly promising carbon-neutral fuel. The use of ammonia as a fuel for internal combustion engines can reduce fossil energy consumption and greenhouse gas emissions. However, the high ignition energy required for ammonia and the slow flame propagation rate result in low combustion efficiency when ammonia is used directly in internal combustion engines. The combination of ammonia with highly reactive fuels improves combustion quality and increases efficiency. However, the combustion of these combined fuels generates particulate matter, CO, hydrocarbon, and significant amounts of NOx. Therefore, pollutant emissions must be reduced through after-treatment technologies. In this paper, a series of combustion and post-treatment challenges faced by amino fuel combustion in internal combustion engines are extensively discussed and the combustion reaction mechanisms of different amino fuels are also analyzed. The paper then reviews five key technologies applicable to the reprocessing of amino fuels, including selective catalytic reduction, selective catalytic reduction filter technology, electrochemical methods for NOx removal, direct catalytic decomposition of N2O, and ammonia sliding catalysts. An in-depth discussion of the catalytic materials and reaction mechanisms involved in these technologies is also provided in this paper. Finally, the paper summarizes the main technical challenges that must be addressed for the future application of amino fuels in internal combustion engines. These discussions can serve as an essential reference for developing and applying critical technologies for combustion control and pollutant treatment of amino fuels.

Highly active Mn-VO-Co sites by cobalt doping on cryptomethane for enhanced catalytic decomposition of N2O

Direct catalytic decomposition has shown great promise in controlling the greenhouse gas N2O. Herein, we synthesize a series of cobalt-doped cryptomethane (OMS-2) catalysts for N2O catalytic decomposition by a mild one-step sol-gel method. The Co0.1-OMS-2 exhibits superior catalytic performance with 90% N2O conversion at 398 °C, which is attributed to the formation of the Mn-VO-Co structure served as active sites. The increased electron density on oxygen vacancies promotes the electron transfer between oxygen vacancies and N2O molecules, in turn facilitating the adsorption and activation of N2O. Moreover, Co doping reduces the formation energy of oxygen vacancies. However, excessive Co doping results in the decrease of highly active Mn-VO-Co sites and the formation of Co3O4, which makes Co0.3-OMS-2 exhibit poor catalytic activity. The DFT calculation illustrates that Langmuir-Hinshelwood is the primary reaction mechanism over Co0.1-OMS-2. This study broadens the materials applicable for the catalytic decomposition of N2O, offering an effective approach to modulate the electronic structure at oxygen vacancies.

Investigation on the reduction in unburned ammonia and nitrogen oxide emissions from ammonia direct injection SI engine by using SCR after-treatment system

Currently generated nitrogen oxides (NOx) and unburned ammonia (NH3) can be converted into nitrogen and moisture that are harmless to the human body and environment using selective catalytic reduction (SCR). The concentrations of NOx and unburned NH3 emitted from the ammonia combustion engines are significantly higher than those emitted by engines using existing hydrocarbon fuels. In this study, ammonia, a representative carbon-free fuel, was used in spark ignition engines for existing passenger vehicles to identify the trends in exhaust gases emitted from engines and conduct experiments on after-treatment strategies to reduce NOx and unburned NH3. The addition of oxygen significantly maximized the conversion efficiency of the SCR after-treatment system by changing the concentration of both NOx and NH3 in the exhaust gas.

Progress and challenges in nitrous oxide decomposition and valorization

Nitrous oxide (N2O) decomposition is increasingly acknowledged as a viable strategy for mitigating greenhouse gas emissions and addressing ozone depletion, aligning significantly with the UN's sustainable development goals (SDGs) and carbon neutrality objectives. To enhance efficiency in treatment and explore potential valorization, recent developments have introduced novel N2O reduction catalysts and pathways. Despite these advancements, a comprehensive and comparative review is absent. In this review, we undertake a thorough evaluation of N2O treatment technologies from a holistic perspective. First, we summarize and update the recent progress in thermal decomposition, direct catalytic decomposition (deN2O), and selective catalytic reduction of N2O. The scope extends to the catalytic activity of emerging catalysts, including nanostructured materials and single-atom catalysts. Furthermore, we present a detailed account of the mechanisms and applications of room-temperature techniques characterized by low energy consumption and sustainable merits, including photocatalytic and electrocatalytic N2O reduction. This article also underscores the extensive and effective utilization of N2O resources in chemical synthesis scenarios, providing potential avenues for future resource reuse. This review provides an accessible theoretical foundation and a panoramic vision for practical N2O emission controls.

High-Efficiency Electrocatalytic Reduction of N2O with Single-Atom Cu Supported on Nitrogen-Doped Carbon

Nitrous oxide (N2O) is a potent greenhouse gas with a high global warming potential, emphasizing the critical need to develop efficient elimination methods. Electrocatalytic N2O reduction reaction (N2ORR) stands out as a promising approach, offering room temperature conversion of N2O to N2 without the production of NOx byproducts. In this study, we present the synthesis of a copper-based single-atom catalyst featuring atomic Cu on nitrogen-doped carbon black (Cu1-NCB). Attributed to the highly dispersed single-atom Cu sites and the effective suppression of the hydrogen evolution reaction, Cu1-NCB demonstrated an optimal N2 faradaic efficiency (82.1%) and yield rate (3.53 mmol h-1 mgmetal-1) at -0.2 and -0.5 V vs RHE, respectively, outperforming previously reported N2ORR electrocatalysts. Further, a gas diffusion electrode cell was employed to improve mass transfer and achieved a 28.6% conversion rate of 30% N2O with only a 14 s residence time, demonstrating the potential for practical application. Density functional theory calculations identified Cu-N4 as the crucial active site for N2ORR, highlighting the significance of the unsaturated coordination and metal-support electronic structure. O-terminal adsorption of N2O was favored, and the dissociative adsorption (*ON2 → *O + N2) was the rate-determining step. These findings reveal the broad prospects of N2O decomposition via electrocatalysis.

Boosting N2O Catalytic Decomposition by the Synergistic Effect of Multiple Elements in Cobalt-Based High-Entropy Oxides

Nitrous oxide (N2O) has a detrimental impact on the greenhouse effect, and its efficient catalytic decomposition at low temperatures remains challenging. Herein, the cobalt-based high-entropy oxide with a spinel-type structure (Co-HEO) is successfully fabricated via a facile coprecipitation method for N2O catalytic decomposition. The obtained Co-HEO catalyst displays more remarkable catalytic performance and higher thermal stability compared with single and binary Co-based oxides, as the temperature of 90% N2O decomposition (T90) is 356 °C. A series of characterization results reveal that the synergistic effect of multiple elements enhances the reducibility and augments oxygen vacancy in the high-entropy system, thus boosting the activity of the Co-HEO catalyst. Moreover, density functional theory (DFT) calculations and the temperature-programmed surface reaction (TPSR) with isotope labeling demonstrate that N2O decomposition on the Co-HEO catalyst follows the Langmuir-Hinshelwood (L-H) mechanism with the promotion of abundant oxygen vacancies. This work provides a fundamental understanding of the synergistic catalytic effect in N2O decomposition and paves the way for the novel environmental catalytic applications of HEO.

Boosting N2O Decomposition by Fabricating the Cs-O-Co Structure over Co3O4 with Single-Layer Atoms of Cs

Developing effective catalysts for N2O decomposition at low temperatures is challenging. Herein, the Cs-O-Co structure, as the active species fabricated by single-layer atoms of Cs over pure Co3O4, originally exhibited great catalytic activity of N2O decomposition in simulated vehicle exhaust and flue gas from nitric acid plants. A similar catalytic performance was also observed for Na, K, and Rb alkali metals over Co3O4 catalysts for N2O decomposition, illustrating the prevalence of alkali-metal-promotion over Co3O4 in practical applications. The catalytic results indicated that the TOF of Co3O4 catalysts loaded by 4 wt% Cs was nearly 2 orders of magnitude higher than that of pure Co3O4 catalysts at 300 °C. Interestingly, the conversions of N2O decomposition over Co3O4 catalysts doped by the same Cs loadings were significantly inhibited. Characterization results indicated that the primary active Cs-O-Co structure was formed by highly orbital hybridization between the Cs 6s and the O 2p orbital over the supported Co3O4 catalysts, where Cs could donate electrons to Co3+ and produce much more Co2+. In contrast, the doped Co3O4 catalysts were dominated by Cs2O2 species; meanwhile, CsOH species was generated by adsorbed water vapor led to a significant decrease in catalytic activity. In situ DRIFTS, rigorous kinetics, and DFT results elaborated the reaction mechanism of N2O decomposition, where the direct decomposition of adsorbed N2O was the kinetically relevant step over supported catalysts in the absence of O2. Meanwhile, the assistance of adsorbed N2O decomposition by activated oxygen was observed as the kinetically relevant step in the presence of O2. The results may pave a promising path toward developing alkali-metal-promotion catalysts for efficient N2O decomposition.

Strong Electronic Orbit Coupling between Cobalt and Single-Atom Praseodymium for Boosted Nitrous Oxide Decomposition on Co3O4 Catalyst

Nitrous oxide (N₂O) has gained increasing attention as an important noncarbon dioxide greenhouse gas, and catalytic decomposition is an effective method of reducing its emissions. Here, Co₃O₄ was synthesized by the sol-gel method and single-atom Pr was confined in its matrix to improve the N₂O decomposition performance. It was observed that the reaction rate varied in a volcano-like pattern with the amount of doped Pr. A N₂O decomposition reaction rate 5-7.5 times greater than that of pure Co₃O₄ is achieved on the catalyst with a Pr/Co molar ratio of 0.06:1, and further Pr doping reduced the activity due to PrO_x cluster formation. Combined with X-ray photoelectron spectroscopy, X-ray absorption fine structure, density functional theory and in situ near-ambient pressure X-ray photoelectron spectroscopy, it was demonstrated that the single-atom doped Pr in Co₃O₄ generates the "Pr 4f-O 2p-Co 3d" network, which redistributes the electrons in Co₃O₄ lattice and increases the t_2g electrons at the tetracoordinated Co²⁺ sites. This coupling between the Pr 4f orbit and Co²⁺ 3d orbit triggers the formation of a 4f-3d electronic ladder, which accelerates the electron transfer from Co²⁺ to the 3π* antibonding orbital of N₂O, thus contributing to the N-O bond cleavage. Moreover, the energy barrier for each elementary reaction in the decomposition process of N₂O is reduced, especially for O₂ desorption. Our work provides a theoretical grounding and reference for designing atomically modified catalysts for N₂O decomposition.

The Unique CO Activation Effects for Boosting NH3 Selective Catalytic Oxidation over CuOx-CeO2

Slip NH₃ is a priority pollutant of concern to be removed in various flue gases with NO_x and CO after denitrification using NH₃-SCR or NH₃-SNCR, and the simultaneous catalytic removal of NH₃ and CO has become one of the new topics in the deep treatment of such flue gases. Synergistic catalytic oxidation of CO and NH₃ appears to be a promising method but still has many challenges. Due to the competition for active oxidizing species, CO was supposed to hinder the NH₃ selective catalytic oxidation (NH₃-SCO). However, it is first found that CO could significantly promote NH₃-SCO over the CuO_x-CeO₂ catalyst. The NH₃ conversion rates increased linearly with CO concentrations in the range of 180-300 °C. Specifically, it accelerated by 2.8 times with 10,000 ppm CO inflow at 220 °C. Mechanism studies found that the Cu-O-Ce solid solution was more active for CO oxidation, while the CuO_x species facilitated the NH₃ dehydrogenation and mitigated the competition of NH₃ and CO, further stabilizing the promotion effects. Gaseous CO boosted the generation of active isolated oxygen atoms (O_i) by actuating the Cu⁺/Cu²⁺ redox cycle. The enriched O_i facilitated oxidation of NH₃ to NO and was conducive to the NH₃-SCO via the i-SCR approach. This study tapped the potential of CO for promoting simultaneous catalytic oxidation of coexisting pollutants in the flue gas.

Application Prospect of K Used for Catalytic Removal of NOx, COx, and VOCs from Industrial Flue Gas: A Review

NOx, COx, and volatile organic compounds (VOCs) widely exist in motor vehicle exhaust, coke oven flue gas, sintering flue gas, and pelletizing flue gas. Potassium species have an excellent promotion effect on various catalytic reactions for the treatment of these pollutants. This work reviews the promotion effects of potassium species on the reaction processes, including adsorption, desorption, the pathway and selectivity of reaction, recovery of active center, and effects on the properties of catalysts, including basicity, electron donor characteristics, redox property, active center, stability, and strong metal-to support interaction. The suggestions about how to improve the promotion effects of potassium species in various catalytic reactions are put forward, which involve controlling carriers, content, preparation methods and reaction conditions. The promotion effects of different alkali metals are also compared. The article number about commonly used active metals and promotion ways are also analyzed by bibliometric on NOx, COx, and VOCs. The promotion mechanism of potassium species on various reactions is similar; therefore, the application prospect of potassium species for the coupling control of multi-pollutants in industrial flue gas at low-temperature is described.

Surface In Situ Doping Modification over Mn2O3 for Toluene and Propene Catalytic Oxidation: The Effect of Isolated Cuδ+ Insertion into the Mezzanine of Surface MnO2 Cladding

Heterocation insertion and substitution in tunnels and mezzanines of MnO₂ present significant influences on the microchemical environment at the surfaces and interfaces. An innovative surface in situ doping modification method via Cu²⁺-H⁺/KMnO₄ treatment was applied onto the Mn₂O₃ surface to provide Mn₂O₃@MnO₂ nanospheres. Cu was stabilized into resulting MnO₂ cladding substituting original K⁺ during a mild comproportionation reaction between Mn(VII) and Mn(III). The Cu25 (Cu(II): Mn(VII)_atomic = 25%) catalyst shows significant promotion of the catalytic performance compared with bare Mn₂O₃ and Cu0 (without Cu involving). Isolated Cu^δ+ was predominantly inserted into the mezzanine of the [MnO₆]^δ- layers in MnO₂ instead of K⁺, leading to slight electron transfer from Cu^δ+ to outermost Mn^(4-ε)+ and a decrease of interlayer spacing as well as crystallinity. Such a configuration facilitates the formation of additional oxygen vacancies, promoting the redox ability and oxygen mobility at relatively low temperatures. The mechanistic study reveals that Cu^δ+ in MnO₂ cladding boosts the activation of toluene (methyl) to form benzoates and propene (methyl and double bonds) to form carboxylates, enhancing the chemical adsorption of reactants. Moreover, it also inhibits the unfavored accumulation of incomplete oxidized intermediates on the surface at high temperatures.

Bulk, Surface and Interface Promotion of Co3O4 for the Low-Temperature N2O Decomposition Catalysis

Nanocrystalline cobalt spinel has been recognized as a very active catalytic material for N2O decomposition. Its catalytic performance can be substantially modified by proper doping with alien cations with precise control of their loading and location (spinel surface, bulk, and spinel-dopant interface). Various doping scenarios for a rational design of the optimal catalyst for low-temperature N2O decomposition are analyzed in detail and the key reactivity descriptors are identified (content and topological localization of dopants, their redox vs. non-redox nature and catalyst work function). The obtained results are discussed in the broader context of the available literature data to establish general guidelines for the rational design of the N2O decomposition catalyst based on a cobalt spinel platform.