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.

Principle on Selecting the Coordination Ligands of Palladium Precursors Encapsulated by Zeolite for an Efficient Purification of Formaldehyde at Ambient Temperature.

High-performance zeolite-supported noble metal catalysts with low loading and high dispersion of active components are the most promising materials for achieving the complete oxidation of formaldehyde (HCHO) at room temperature. In this work, palladium nanoparticles (Pd NPs) with different sizes were successfully encapsulated inside the silicalite-1 (S-1) zeolite framework by using diverse stabling ligands via the one-pot method. Thereafter, the rule on selecting the coordinative ligands for palladium was clarified: more N atoms, a short carbon chain, a smaller branch chain, and bidentate coordination are characteristics of an ideal ligand. Accordingly, the best-performing 0.2Pd@S-1(Ethylenediamine) catalyst exhibited outstanding performance for HCHO oxidation, achieving 100% conversion even at room temperature. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HR HAADF-STEM) and density functional theory (DFT) calculations indicate that the chelate is formed by complexation of Pd2+ ions with ethylenediamine, displaying the smallest spatial site resistance simultaneously with the zeolite synthesis, resulting in Pd located mostly within the 5-membered ring (5-MR) channels of S-1 after calcination, thus limiting the growth of Pd clusters and promoting their dispersion.

Fabrication of highly dispersed Pd-Mn3O4 catalyst for efficient catalytic propane total oxidation.

Adjusting the interaction between dual active components for enhancing volatile organic compounds (VOCs) degradation is an effective but still challenging means of air pollution control. Herein, a limited pyrolysis oxidation strategy was adopted to prepare Pd-Mn3O4 spinel catalysts with uniform morphology and active component dispersion. Among these, 1.08Pd-Mn3O4 presented the highest catalytic efficiency with a T90 value of 240 °C, which was 94 °C lower than that of Mn3O4. Characterization and density functional theory (DFT) calculation results revealed that the strong metal-support interaction (SMSI) effect between Pd and Mn3O4 promoted the redistribution of surface charges, thus strengthening the oxidation-reduction ability of the active sites. Moreover, the SMSI effect led to a better migration of surface oxygen species, and boosted the generation of active surface oxygen species. Simultaneously, the Pd catalyst further reduced the energy barrier in the initial stage of the dehydrogenation of propane. Overall, this study provided a novel design strategy for dual active components catalysts with SMSI effect and extended the application of these catalysts in the important field of VOCs elimination.

Urea-H2O2 defect engineering of δ-MnO2 for propane photothermal oxidation: Structure-activity relationship and synergetic mechanism determination.

Photothermal catalysis has an advantage in effective and economical elimination technology of volatile organic compounds (VOCs) in the ascendant. Herein, various surface defect engineering routes were adopted to enhance the low-temperature propane oxidation of δ-MnO2. Compared to reducing etchants urea and vitamin C, δ-MnO2 treated with urea - H2O2 exhibited an excellent thermal (T90 = 240 ℃) and photothermal (T90 = 196 ℃) activities of propane oxidation. Urea - H2O2 treatment provided high concentration of Mn4+ and surface-active oxygen (Mn4+-Osur) species as surface-active sites, and produced numerous oxygen vacancies to improve charge separation and superoxide species generation capacity. Thus, the photothermal conversion efficiency and low-temperature reducibility were remarkably enhanced. Furthermore, the photothermal synergistic catalytic mechanism was proposed based on in-situ diffuse reflectance infrared Fourier transform spectroscopy and control experiments. The strategy here offered insight into the rational design of efficient transition catalysts, and in-depth understanding of the photothermal catalytic VOCs removal mechanism.

N/Ce doped graphene supported Pt nanoparticles for the catalytic oxidation of formaldehyde at room temperature.

Pt catalysts with nitrogen-doped graphene oxide (GO) as support and CeO2 as promoter were prepared by impregnation method, and their catalytic oxidation of formaldehyde (HCHO) at room temperature was tested. The Pt-CeO2/N-rGO (reduced GO) with a mass fraction of 0.7% Pt and 0.8% CeO2 exhibited an excellent catalytic performance with the 100% conversion of HCHO at room temperature. Physicochemical characterization demonstrated that nitrogen-doping greatly increased the defect degree and the specific surface area of GO, enhanced the dispersion of Pt and promoted more zero-valent Pt. The synergistic effect between CeO2 and Pt was also beneficial to the dispersion of Pt. Nitrogen-doping promoted the production of more Ce3+ ions, generating more oxygen vacancies, which was conducive to O2 adsorption. As a result, the catalyst exhibited enhanced redox properties, leading to the best catalytic activity. Finally, an attempt to propose the reaction mechanism of HCHO oxidation has been made.

Solvent-Free Thermal Synthesis of Extra-Large-Pore Aluminophosphate Zeotype via Self-Assembly of Double-Four-Ring Unit.

The use of Ge as framework atom to direct the formation of double-four-ring (d4r) unit is an effective way to prepare zeolite with extra-large pores, which however also brings about serious problems with regard to production cost and structure stability for their practical application. A new solvent-free thermal synthesis strategy is presented here for facile preparation and molecular-level mechanism study of extra-large-pore aluminophosphate zeotype DNL-1 with -CLO structure. For the first time, the formation of intermediate d4r in the induction period was successfully confirmed, and was correlated with the synthesis condition of low water content and quaternary ammonium template with suitable alkyl chain length. The self-assembly pathway via d4r to form lta, afterwards clo and super cages was further revealed. This work shed light on the rational synthesis of Ge-free extra-large-pore zeotypes based on d4r route.

Selective catalytic reduction of NO x with NH3: opportunities and challenges of Cu-based small-pore zeolites.

Zeolites, as efficient and stable catalysts, are widely used in the environmental catalysis field. Typically, Cu-SSZ-13 with small-pore structure shows excellent catalytic activity for selective catalytic reduction of NO x with ammonia (NH3-SCR) as well as high hydrothermal stability. This review summarizes major advances in Cu-SSZ-13 applied to the NH3-SCR reaction, including the state of copper species, standard and fast SCR reaction mechanism, hydrothermal deactivation mechanism, poisoning resistance and synthetic methodology. The review gives a valuable summary of new insights into the matching between SCR catalyst design principles and the characteristics of Cu2+-exchanged zeolitic catalysts, highlighting the significant opportunity presented by zeolite-based catalysts. Principles for designing zeolites with excellent NH3-SCR performance and hydrothermal stability are proposed. On the basis of these principles, more hydrothermally stable Cu-AEI and Cu-LTA zeolites are elaborated as well as other alternative zeolites applied to NH3-SCR. Finally, we call attention to the challenges facing Cu-based small-pore zeolites that still need to be addressed.

Terminal Hydroxyl Groups on Al2O3 Supports Influence the Valence State and Dispersity of Ag Nanoparticles: Implications for Ozone Decomposition.

Ozone is a poisonous gas, so it is necessary to remove excessive ozone in the environment. Catalytic decomposition is an effective way to remove ozone at room temperature. In this work, 10%Ag/nano-Al2O3 and 10%Ag/AlOOH-900 catalysts were synthesized by the impregnation method. The 10%Ag/nano-Al2O3 catalyst showed 89% ozone conversion for 40 ppm O3 for 6 h under a space velocity of 840 000 h-1 and a relative humidity of 65%, which is superior to 10%Ag/AlOOH-900 (45% conversion). The characterization results showed Ag nanoparticles to be the active sites for ozone decomposition, which were more highly dispersed on nano-Al2O3 as a result of the greater density of terminal hydroxyl groups. The understanding of the dispersion and valence of silver species gained in this study will be beneficial to the design of more efficient supported silver catalysts for ozone decomposition in the future.

Ce-promoted Mn/ZSM-5 catalysts for highly efficient decomposition of ozone.

Manganese oxides supported by ZSM-5 zeolite (Mn/ZSM-5) as well as their further modified by Ce promoter were achieved by simple impregnation method for ozone catalytic decomposition. The yCe20Mn/ZSM-5-81 catalyst with 8% Ce loading showed the highest catalytic activity at relative humidity of 50% and a space velocity of 360 L/(g × hr), giving 93% conversion of 600 ppm O3 after 5 hr. Moreover, this sample still maintained highly activity and stability in humid air with 50%-70% relative humidity. Series of physicochemical characterization including X-ray diffraction, temperature-programmed technology (NH3-TPD and H2-TPR), X-ray photoelectron spectroscopy and oxygen isotopic exchange were introduced to disclose the structure-performance relationship. The results indicated that moderate Si/Al ratio (81) of zeolite support was beneficial for ozone decomposition owing to the synergies of acidity and hydrophobicity. Furthermore, compared with 20Mn/ZSM-5-81, Ce doping could enhance the amount of low valance manganese (such as Mn2+ and Mn3+). Besides, the Ce3+/Ce4+ ratio of 8Ce20Mn/ZSM-5-81 sample was higher than that of 4Ce20Mn/ZSM-5-81. Additionally, the synergy between the MnOx and CeO2 could easily transfer electron via the redox cycle, thus resulting in an increased reducibility at low temperatures and high concentration of surface oxygen. This study provides important insights to the utilization of porous zeolite with high surface area to disperse active component of manganese for ozone decomposition.

Highly Efficient NO Abatement over Cu-ZSM-5 with Special Nanosheet Features.

Conventional Cu-ZSM-5 and special Cu-ZSM-5 catalysts with diverse morphologies (nanoparticles, nanosheets, hollow spheres) were synthesized and comparatively investigated for their performances in the selective catalytic reduction (SCR) of NO to N2 with ammonia. Significant differences in SCR behavior were observed, and nanosheet-like Cu-ZSM-5 showed the best SCR performance with the lowest T50 of 130 °C and nearly complete conversion in the temperature range of 200-400 °C. It was found that Cu-ZSM-5 nanosheets [mainly exposed (0 1 0) crystal plane] with abundant mesopores and framework Al species were favorable for the formation of high external surface areas and Al pairs, which influenced the local environment of Cu. This motivated the preferential formation of active copper species and the rapid switch between Cu2+ and Cu+ species during NH3-SCR, thus exhibiting the highest NO conversion. In situ diffused reflectance infrared Fourier transform spectroscopy (DRIFTS) results indicated that the Cu-ZSM-5 nanosheets were dominated by the Eley-Rideal (E-R) mechanism and the labile nitrite species (NH4NO2) were the crucial intermediates during the NH3-SCR process, while the inert nitrates were more prone to generate on Cu-ZSM-5 nanoparticles and conventional one. The combined density functional theory (DFT) calculations revealed that the decomposition energy barrier of nitrosamide species (NH2NO) on the (0 1 0) crystal plane of Cu-ZSM-5 was lower than those on (0 0 1) and (1 0 0) crystal planes. This study provides a strategy for the design of NH3-SCR zeolite catalysts.

Detrimental role of residual surface acid ions on ozone decomposition over Ce-modified γ-MnO2 under humid conditions.

In the study, the catalyst precursors of Ce-modified γ-MnO2 were washed with deionized water until the pH value of the supernatant was 1, 2, 4 and 7, and the obtained catalysts were named accordingly. Under space velocity of 300,000 hr-1, the ozone conversion over the pH = 7 catalyst under dry conditions and relative humidity of 65% over a period of 6 hr was 100% and 96%, respectively. However, the ozone decomposition activity of the pH = 2 and 4 catalysts distinctly decreased under relative humidity of 65% compared to that under dry conditions. Detailed physical and chemical characterization demonstrated that the residual sulfate ions on the pH = 2 and 4 catalysts decreased their hydrophobicity and then restrained humid ozone decomposition activity. The pH = 2 and 4 catalysts had inferior resistance to high space velocity under dry conditions, because the residual sulfate ion on their surface reduced their adsorption capacity for ozone molecules and increased their apparent activation energies, which was proved by temperature programmed desorption of O2 and kinetic experiments. Long-term activity testing, X-ray photoelectron spectroscopy and density functional theory calculations revealed that there were two kinds of oxygen vacancies on the manganese dioxide catalysts, one of which more easily adsorbed oxygen species and then became deactivated. This study revealed the detrimental effect of surface acid ions on the activity of catalysts under humid and dry atmospheres, and provided guidance for the development of highly efficient catalysts for ozone decomposition.

Morphology-Oriented ZrO2-Supported Vanadium Oxide for the NH3-SCR Process: Importance of Structural and Textural Properties.

ZrO2 supports, with diverse morphologies (hollow sphere, star, rod, mesoporous), were produced using hydrothermal and evaporation-induced self-assembly methods. Zirconia-supported vanadium oxide catalysts were prepared by wet impregnation and used for the low-temperature selective catalytic reduction (SCR) of NO with ammonia. Characterization of catalysts includes N2 physisorption, elementary analysis, X-ray diffraction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction by H2, and temperature-programmed desorption of NH3. Significant differences in terms of activity are measured. 3 wt % V2O5 supported on mesoporous ZrO2 (V/MZ) presents excellent N2 yields (>90%, in the 200-400 °C interval), with a wide operating temperature window (NO conversion > 95%, in the 225-425 °C interval), and less interesting performances were obtained when vanadium oxide is supported over stars, hollow spheres, and rods. Surface characterization showed a content of tetravalent vanadium ion, when supported, decreasing in the order of mesoporous > hollow sphere > star > rod. This order is in perfect agreement with the order of performance of the catalyst in the NH3-SCR reaction. The impact of tetravalent ion's presence on the surface is confirmed by diffuse reflectance infrared Fourier transform spectroscopy analysis, Brønsted acid sites generated on the surface, and the V4+-OH species involved in the reaction. The production of more important nitrite species over the tetragonal supported vanadium oxide catalyst could be another reason for the excellent NH3-SCR performance displayed by the V/MZ catalyst. When supported over monoclinic zirconia, like vanadium oxide over star-type morphology, the adsorbed NH3 species (NH4+ and coordinated NH3) reacted with NO x adsorption species (nitrate) to form ammonium nitrate. Ammonium nitrate can be decomposed to N2 and N2O (or NO2). Thus, NO conversion curves and N2 yield curves over tetragonal zirconia (MZ) at lower temperature were ahead of those over V/star ZrO2 because of the higher V4+ surface content and more active B acid sites associated with an easy formation of the nitrito intermediate.

Facile synthesis of Ag-modified manganese oxide for effective catalytic ozone decomposition.

O3 decomposition catalysts with excellent performance still need to be developed. In this study, Ag-modified manganese oxides (AgMnOx) were synthesized by a simple co-precipitation method. The effect of calcination temperature on the activity of MnOx and AgMnOx catalysts was investigated. The effect of the amount of Ag addition on the activity and structure of the catalysts was further studied by activity testing and characterization by a variety of techniques. The activity of 8%AgMnOx for ozone decomposition was significantly enhanced due to the formation of the Ag1.8Mn8O16 structure, indicating that this phase has excellent performance for ozone decomposition. The weight content of Ag1.8Mn8O16 in the 8%AgMnOx catalyst was only about 33.76%, which further indicates the excellent performance of the Ag1.8Mn8O16 phase for ozone decomposition. The H2 temperature programmed reduction (H2-TPR) results indicated that the reducibility of the catalysts increased due to the formation of the Ag1.8Mn8O16 structure. This study provides guidance for a follow-up study on Ag-modified manganese oxide catalysts for ozone decomposition.

Ionothermal Synthesis of Germanosilicate Zeolites Constructed with Double-Four-Ring Structure-Building Units in the Presence of Organic Base.

The crystallization chemistry of silica-based zeolites in ionic liquids remains highly puzzling and interesting in the field of zeolite science. Herein, we report the successful ionothermal synthesis of germanosilicate zeolites. The ionothermal templating effect with respect to the structure, alkalinity and concentration of organic additives was comparatively studied. The results showed that a small amount of organic base could effectively aid the dissolution of silica source and facilitate the crystallization of germanosilicate zeolites with ionic liquid as template. Remarkably, STW and IRR structures constructed with double-four-ring (D4R) structure-building units were ionothermally prepared using 1-ethyl/butyl-3-methyl imidazolium and 1-benzyl-3-methyl imidazolium ionic liquids as template, respectively. Ionothermal synthesis tailored with organic base showed suitable chemistry for the formation of germanium-containing siliceous D4R units. This finding will be helpful for the rational exploration of novel extra-large-pore zeolitic structures.

Oxygen Vacancies Induced by Transition Metal Doping in γ-MnO2 for Highly Efficient Ozone Decomposition.

Transition metal (cerium and cobalt) doped γ-MnO2 (M-γ-MnO2, where M represents Ce, Co) catalysts were successfully synthesized and characterized. Cerium-doped γ-MnO2 materials showed ozone (O3) conversion of 96% for 40 ppm of O3 under relative humidity (RH) of 65% and space velocity of 840 L g-1 h-1 after 6 h at room temperature, which is far superior to the performance of the Co-γ-MnO2 (55%) and γ-MnO2 (38%) catalysts. Under space velocity of 840 L g-1 h-1, the conversion of ozone over the Ce-γ-MnO2 catalyst under RH = 65% and dry conditions within 96 h was 60% and 100%, respectively, indicating that it is a promising material for ozone decomposition. XRD and HRTEM data suggested that Ce-γ-MnO2 formed mixed crystals consisting of α-MnO2 and γ-MnO2 with specific surface area increased from 74 m2/g to 120 m2/g compared to undoped γ-MnO2, thus more surface defects were introduced. H2-TPR, O2-TPD, XPS, Raman, and EXAFS confirmed that Ce-γ-MnO2 exhibited more surface oxygen vacancies and surface defects, which play a key role during the decomposition of ozone. This study provides important insights for developing improved catalysts for gaseous ozone decomposition and promoting the performance of manganese oxide for practical ozone elimination.

Mechanistic insight into selective catalytic combustion of HCN over Cu-BEA: influence of different active center structures.

HCN being a highly toxic N-containing volatile organic compound (VOCs) poses great threat to human living environment. Selective catalytic combustion of HCN (HCN-SCC) over metal modified zeolite catalysts has attracted great attention due to related high efficiency and excellent N2 selectivity. In the present work, three types of 24T-Cu-BEA models with different active centers of single [Cu]+, double [Cu]+, and [Cu-O-Cu]2+ were constructed for HCN-SCC mechanism simulations based on density functional theory (DFT). DFT simulation results revealed that HCN-SCC followed an oxidation mechanism over double [Cu]+ through an intermediate of NCO, wherein the synergistic effects of double [Cu]+ active centers were clearly observed, resulting in a significantly lowered energy barrier (1.6 kcal mol-1) during HCN oxidation into NCO. However, an oxidation mechanism (HCN oxidized into NH radical and CO2 through intermediate of HNCO) combining with a hydrolysis mechanism (NH radical hydrolyses into NH3) occurred over single [Cu]+ and [Cu-O-Cu]2+, wherein the NH2 hydrolysis to NH3 step was regarded as the rate determining step with an energy barrier of 72.3 and 74.3 kcal mol-1, respectively. Finally, Mulliken charge transfer (CT) analysis was conducted, based on which the electric properties of different active centers were well illustrated.

Mechanistic insight into selective catalytic combustion of acrylonitrile (C2H3CN): NCO formation and its further transformation towards N2.

A series of zeolite catalysts, M(Cu, Fe, Co)-ZSM-5, was prepared by an impregnation method and evaluated for the selective catalytic combustion of acrylonitrile (AN-SCC). Cu-ZSM-5, exhibiting the highest AN conversion activity and best N2 yield, was further selected for an AN-SCC mechanism investigation, wherein both experimental [in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)] and theoretical [density functional theory (DFT)] approaches were employed. The in situ DRIFTS revealed that AN-SCC followed a hydrolysis mechanism at T < 300 °C via intermediates of acylamino species (-CONH2) and NH3, while it followed an oxidation mechanism at T > 300 °C via an intermediate of NCO. The DFT simulations gave much deeper insights suggesting that: (i) the NCO could be generated by oxidation of AN over [Cu]+ active sites, with an assistance of dissociated atomic O from gaseous O2; (ii) three types of reaction routes could be proposed for the further reaction of NCO to produce N2, namely NCO direct dissociation, NCO coupling, and NO + NCO reaction; and (iii) the last route (NO + NCO), possessing the lowest energy barrier, was the most probable reaction pathway, wherein the NO could be produced by oxidation of NCO. The DFT energy calculation results and microkinetic analyses revealed that the NCO generation step, possessing an energy barrier of 17.0 kcal mol-1 and a forward reaction rate constant of 2.20 × 107 s-1, was the rate-determining step of the whole catalytic cycle.

Selective Transformation of Various Nitrogen-Containing Exhaust Gases toward N2 over Zeolite Catalysts.

In this review we focus on the catalytic removal of a series of N-containing exhaust gases with various valences, including nitriles (HCN, CH3CN, and C2H3CN), ammonia (NH3), nitrous oxide (N2O), and nitric oxides (NO(x)), which can cause some serious environmental problems, such as acid rain, haze weather, global warming, and even death. The zeolite catalysts with high internal surface areas, uniform pore systems, considerable ion-exchange capabilities, and satisfactory thermal stabilities are herein addressed for the corresponding depollution processes. The sources and toxicities of these pollutants are introduced. The important physicochemical properties of zeolite catalysts, including shape selectivity, surface area, acidity, and redox ability, are described in detail. The catalytic combustion of nitriles and ammonia, the direct catalytic decomposition of N2O, and the selective catalytic reduction and direct catalytic decomposition of NO are systematically discussed, involving the catalytic behaviors as well as mechanism studies based on spectroscopic and kinetic approaches and molecular simulations. Finally, concluding remarks and perspectives are given. In the present work, emphasis is placed on the structure-performance relationship with an aim to design an ideal zeolite-based catalyst for the effective elimination of harmful N-containing compounds.

Economical way to synthesize SSZ-13 with abundant ion-exchanged Cu+ for an extraordinary performance in selective catalytic reduction (SCR) of NOx by ammonia.

In this study, an economical way for SSZ-13 preparation with the essentially cheap choline chloride as template has been attempted. The as-synthesized SSZ-13 zeolite after ion exchange by copper nitrate solution exhibited a superior SCR performance (over 95% NOx conversion across a broad range from 150 to 400 °C) to the traditional zeolite-based catalysts of Cu-Beta and Cu-ZSM-5. Furthermore, the opportune size of pore opening (∼3.8 Å) made Cu-SSZ-13 exhibiting the best selectivity to N2 as well as satisfactory tolerance toward SO2 and C3H6 poisonings. The characterization (XRD, XPS, XRF, and H2-TPR) of samples confirmed that Cu-SSZ-13 possessed the most abundant Cu cations among three investigated Cu-zeolites; furthermore, either on the surface or in the bulk the ratio of Cu(+)/Cu(2+) ions for Cu-SSZ-13 is also the highest. New finding was announced that CHA-type topology is in favor of the formation of copper cations, especially generating much more Cu(+) ions than the others, rather than CuO. The activity test of Cu(CuCl)-ZSM-5 (prepared by a solid-state ion-exchange method) clearly indicated that Cu(+) ions could make a major contribution to the low-temperature deNOx activity. The activity of protonic zeolites (H-SSZ-13, H-Beta, H-ZSM-5) revealed the topology effect on SCR performances.

Design of nanocrystalline mixed oxides with improved oxygen mobility: a simple non-aqueous route to nano-LaFeO3 and the consequences on the catalytic oxidation performances.

LaFeO3 nanoparticles, with improved oxygen mobility, leading to enhanced catalytic oxidation activities, are obtained through a simple nonaqueous route.