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.

Composition-dependent morphologies of CeO2 nanoparticles in the presence of Co-adsorbed H2O and CO2: a density functional theory study

Catalytic activity is affected by surface morphology, and specific surfaces display greater activity than others. A key challenge is to define synthetic strategies to enhance the expression of more active surfaces and to maintain their stability during the lifespan of the catalyst. In this work, we outline an ab initio approach, based on density functional theory, to predict surface composition and particle morphology as a function of environmental conditions, and we apply this to CeO2 nanoparticles in the presence of co-adsorbed H2O and CO2 as an industrially relevant test case. We find that dissociative adsorption of both molecules is generally the most favourable, and that the presence of H2O can stabilise co-adsorbed CO2. We show that changes in adsorption strength with temperature and adsorbate partial pressure lead to significant changes in surface stability, and in particular that co-adsorption of H2O and CO2 stabilizes the {100} and {110} surfaces over the {111} surface. Based on the changes in surface free energy induced by the adsorbed species, we predict that cuboidal nanoparticles are favoured in the presence of co-adsorbed H2O and CO2, suggesting that cuboidal particles should experience a lower thermodynamic driving force to reconstruct and thus be more stable as catalysts for processes involving these species.

Demonstration of the Influence of Specific Surface Area on Reaction Rate in Heterogeneous Catalysis

Heterogeneous catalysis plays an important role in many chemical reactions, especially those applied in industrial processes, and therefore, its theoretical foundations are introduced not only to students majoring in chemical engineering or catalysis but also as part of general chemistry courses. The consideration of catalytic activity of various solids and mechanisms of catalytic reactions requires the introduction of the concept of an active site, which together with the catalyst specific surface area are discussed as key parameters controlling the reaction rate. There are many known demonstrations of heterogeneous catalysis phenomena that can be performed live in a lecture hall, but all of them focus only on the general idea of catalytic processes and are not suitable for quantitative analysis. Therefore, herein we present a simple demonstration of the influence of the specific surface area of a catalyst on the rate of a catalytic reaction. This demonstration is based on a model reaction of hydrogen peroxide decomposition catalyzed by cobalt spinel (Co₃O₄) calcined at various temperatures. The differences in reaction rates can be monitored visually, and the obtained data can be used directly for a simple kinetic analysis, including comparison of numerical values of the reaction rate constants.

Cobalt Based Catalysts on Alkali-Activated Zeolite Foams for N2O Decomposition

In this work, we studied the effect of alkali-activated zeolite foams modifications on properties and catalytic activity of cobalt phases in the process of catalytic decomposition of N2O. The zeolite foam supports were prepared by alkali activation of natural zeolite followed by acid leaching and ion exchange. The cobalt catalysts were synthesised by a different deposition technique (direct ion exchange (DIE) and incipient wetness impregnation (IWI) method of cobalt on zeolite foams. For comparison, catalysts on selected supports were prepared and the properties of all were compared in catalytic tests in the pellet form and as crushed catalysts to determine the effect of internal diffusion. The catalysts and supports were in detail characterized by a variety of techniques. The catalyst activity strongly depended on the structure of support and synthesis procedure of a cobalt catalyst. Ion exchange method provided active phase with higher surface areas and sites with better reducibility, both of these factors contributed to higher N2O conversions of more than 80% at 450 °C. A large influence can also be attributed to the presence of alkali metals, in particular, potassium, which resulted in a modification of electronic and acid base properties of the cobalt oxide phase on the catalyst surface. The promotional effect of potassium is better reducibility of cobalt species.

K-Modified Co–Mn–Al Mixed Oxide—Effect of Calcination Temperature on N2O Conversion in the Presence of H2O and NOx

The effect of calcination temperature (500–700 °C) on physico-chemical properties and catalytic activity of 2 wt. % K/Co-Mn-Al mixed oxide for N2O decomposition was investigated. Catalysts were characterized by inductively coupled plasma spectroscopy (ICP), X-ray powder diffraction (XRD), temperature-programmed reduction by hydrogen (TPR-H2), temperature-programmed desorption of CO2 (TPD-CO2), temperature-programmed desorption of NO (TPD-NO), X-ray photoelectron spectrometry (XPS) and N2 physisorption. It was found that the increase in calcination temperature caused gradual crystallization of Co-Mn-Al mixed oxide, which manifested itself in the decrease in Co2+/Co3+ and Mn3+/Mn4+ surface molar ratio, the increase in mean crystallite size leading to lowering of specific surface area and poorer reducibility. Higher surface K content normalized per unit surface led to the increase in surface basicity and adsorbed NO per unit surface. The effect of calcination temperature on catalytic activity was significant mainly in the presence of NOx, as the optimal calcination temperature of 500 °C is necessary to ensure sufficient low surface basicity, leading to the highest catalytic activity. Observed NO inhibition was caused by the formation of surface mononitrosyl species bonded to tetrahedral metal sites or nitrite species, which are stable at reaction temperatures up to 450 °C and block active sites for N2O decomposition.

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.

Oxygen Vacancy-rich Porous Co3O4 Nanosheets toward Boosted NO Reduction by CO and CO Oxidation: Insights into the Structure-Activity Relationship and Performance Enhancement Mechanism

Oxygen vacancy-rich porous Co₃O₄ nanosheets (OV-Co₃O₄) with diverse surface oxygen vacancy contents were synthesized via facile surface reduction and applied to NO reduction by CO and CO oxidation. The structure-activity relationship between surface oxygen vacancies and catalytic performance was systematically investigated. By combining Raman, X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and O₂-temperature programmed desorption, it was found that the efficient surface reduction leads to the presence of more surface oxygen vacancies and thus distinctly enhance the surface oxygen amount and mobility of OV-Co₃O₄. The electron transfer towards Co sites was promoted by surface oxygen vacancies with higher content. Compared with the pristine porous Co₃O₄ nanosheets, the presence of more surface oxygen vacancies is beneficial for the catalytic performance enhancement for NO reduction by CO and CO oxidation. The OV-Co₃O₄ obtained in 0.05 mol L^-1 NaBH₄ solution (Co₃O₄-0.05) exhibited the best catalytic activity, achieving 100% NO conversion at 175 °C in NO reduction by CO and 100% CO conversion at 100 °C in CO oxidation, respectively. Co₃O₄-0.05 exhibited outstanding catalytic stability and resistance to high gas hour space velocity in both reactions. Combining in situ DRIFTS results, the enhanced performance of OV-Co₃O₄ for NO reduction by CO should be attributed to the promoted formation and transformation of dinitrosyl species and -NCO species at lower and higher temperatures. The enhanced performance of OV-Co₃O₄ for CO oxidation is due to the promotion of oxygen activation ability, surface oxygen mobility, as well as the enhanced CO₂ desorption ability. The results indicate that the direct regulation of surface oxygen vacancies could be an efficient way to evidently enhance the catalytic performance for NO reduction by CO and CO oxidation.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

Whereas noble metal compounds have long been central in catalysis, Earth-abundant metal-based catalysts have in the same time remained undeveloped. Yet the efficacy of Earth-abundant metal catalysts was already shown at the very beginning of the 20th century with the Fe-catalyzed Haber-Bosch process of ammonia synthesis and later in the Fischer-Tropsch reaction. Nanoscience has revolutionized the world of catalysis since it was observed that very small Au nanoparticles (NPs) and other noble metal NPs are extraordinarily efficient. Therefore the development of Earth-abundant metals NPs is more recent, but it has appeared necessary due to their "greenness". This review highlights catalysis by NPs of Earth-abundant transition metals that include Mn, Fe, Co, Ni, Cu, early transition metals (Ti, V, Cr, Zr, Nb and W) and their nanocomposites with emphasis on basic principles and literature reported during the last 5 years. A very large spectrum of catalytic reactions has been successfully disclosed, and catalysis has been examined for each metal starting with zero-valent metal NPs followed by oxides and other nanocomposites. The last section highlights the catalytic activities of bi- and trimetallic NPs. Indeed this later family is very promising and simultaneously benefits from increased stability, efficiency and selectivity, compared to monometallic NPs, due to synergistic substrate activation.

Improved durability of Co3O4 particles supported on SmMn2O5 for methane combustion

To eliminate the aggregation of Co₃O₄ in the methane combustion process at high temperature, a thermally stable mullite structure, SmMn₂O₅ (SMO), was utilized as a support to improve the catalytic durability of Co₃O₄ particles.

The recent development of efficient Earth-abundant transition-metal nanocatalysts

This review presents the recent remarkable developments of efficient Earth-abundant transition-metal nanocatalysts.