Study on the direct decomposition of nitrous oxide over Fe-beta zeolites: From experiment to theory

Mechanisms for deNOx and deN2O Processes on FAU Zeolite with a Bimetallic Cu-Fe Dimer in the Presence of a Hydroxyl Group-DFT Theoretical Calculations

In this paper, a detailed mechanism is discussed for two processes: deNOx and deN2O. An FAU catalyst was used for the reaction with Cu-Fe bimetallic adsorbates represented by a dimer with bridged oxygen. Partial hydration of the metal centres in the dimer was considered. Ab initio calculations based on the density functional theory were used. The electron parameters of the structures obtained were also analysed. Visualisation of the orbitals of selected structures and their interpretations are presented. The presented research allowed a closer look at the mechanisms of processes that are very common in the automotive and chemical industries. Based on theoretical modelling, it was possible to propose the most efficient catalyst that could find potential application in industry-this is the FAU catalyst with a Cu-O-Fe bimetallic dimer with a hydrated copper centre. The essential result of our research is the improvement in the energetics of the reaction mechanism by the presence of an OH group, which will influence the way NO and NH3 molecules react with each other in the deNOx process depending on the industrial conditions of the process. Our theoretical results suggest also how to proceed with the dosage of NO and N2O during the industrial process to increase the desired reaction effect.

DFT Study on the Combined Catalytic Removal of N2O, NO, and NO2 over Binuclear Cu-ZSM-5

The large amount of nitrogen oxides (N2O, NO, NO2, etc.) contained in the flue gas of industrial adipic acid production will seriously damage the environment. A designed binuclear Cu-ZSM-5 catalyst can be applied to decompose N2O and reduce NO and NO2, purifying the air environment. Using the density functional theory method, the catalytic decomposition mechanisms of N2O, NOX-NH3-SCR, and NOX-assisted N2O decomposition is simulated over the Cu-ZSM-5 model. The results indicate that N2O can be catalytically decomposed over the binuclear Cu active site in the sinusoidal channel. The speed-limiting step is the second N2O molecule activation process. After the decomposition of the first N2O molecule, a stable extra-frame [Cu-O-Cu]2+ structure will generate. The subsequent discussion proved that the NOX-NH3-SCR reaction can be realized over the [Cu-O-Cu]2+ active site. In addition, it proved that the decomposition reaction of NO and NO2 can be carried out over the [Cu-O-Cu]2+ active site, and NO can greatly reduce the energy barrier for the conversion of the active site from [Cu-O-Cu]2+ to the binuclear Cu form, while NO2 can be slightly reduced. Through discussion, it is found that the binuclear Cu-ZSM-5 can realize the combined removal of N2O and NOX from adipic acid flue gas, hoping to provide a theoretical basis for the development of a dual-functional catalyst.

DFT Study on Mechanisms of the N2O Direct Catalytic Decomposition over Cu-ZSM-5: The Detailed Investigation on NO Formation Mechanism

Nitrous oxide (N2O) is an industrial emission that causes the greenhouse effect and damages the ozone layer. Density functional theory study on the N2O direct catalytic decomposition over Cu–ZSM-5 has been performed in this paper. Two possible reaction mechanisms for N2O direct catalytic decomposition over Cu-ZSM-5 were proposed (O2 formation mechanism and Nitric oxide (NO) formation mechanism). The geometrical parameters, vibration frequency and thermodynamic data of the intermediate states in each step have been examined. The results indicate that N2O can be adsorbed on active site Cu in two ways (O-terminal or N-terminal), and N2O decomposition reactions can occur in both cases. The NO formation mechanism exhibits higher N2O dissociation reaction due to lower energy barrier.

Facile fabrication of nanosheet-assembled MnCoOx hollow flower-like microspheres as highly effective catalysts for the low-temperature selective catalytic reduction of NOx by NH3

A series of MnCoO_x flower-like hollow microspheres with various molecular proportions of reactant were prepared through simple solvothermal method for the ammonia selective catalytic reduction (SCR) at low temperatures. The as-prepared samples have been applied by various characterization techniques to explore the formation process of the morphology and physicochemical properties. The Mn(1)Co(1)O_x presented the optimal intrinsic catalytic performance (95% NO_x conversion at 75 °C), favorable thermal stability, and strong SO₂ resistance. The excellent properties mainly related to its higher specific surface area and abundant active sites originated from hollow microsphere special structure consists of abundant nanosheets, robust redox properties beneficial for the strong interaction between the manganese and cobalt, larger number of acidic sites and stronger acid strength, etc., which collaboratively dominate its catalytic properties of NH₃-SCR at low temperatures.

Investigating the Influence of Fe Speciation on N2O Decomposition Over Fe-ZSM-5 Catalysts

The influence of Fe speciation on the decomposition rates of N₂O over Fe-ZSM-5 catalysts prepared by Chemical Vapour Impregnation were investigated. Various weight loadings of Fe-ZSM-5 catalysts were prepared from the parent zeolite H-ZSM-5 with a Si:Al ratio of 23 or 30. The effect of Si:Al ratio and Fe weight loading was initially investigated before focussing on a single weight loading and the effects of acid washing on catalyst activity and iron speciation. UV/Vis spectroscopy, surface area analysis, XPS and ICP-OES of the acid washed catalysts indicated a reduction of ca. 60% of Fe loading when compared to the parent catalyst with a 0.4 wt% Fe loading. The TOF of N₂O decomposition at 600 °C improved to 3.99 × 10³ s^-1 over the acid washed catalyst which had a weight loading of 0.16%, in contrast, the parent catalyst had a TOF of 1.60 × 10³ s^-1. Propane was added to the gas stream to act as a reductant and remove any inhibiting oxygen species that remain on the surface of the catalyst. Comparison of catalysts with relatively high and low Fe loadings achieved comparable levels of N₂O decomposition when propane is present. When only N₂O is present, low metal loading Fe-ZSM-5 catalysts are not capable of achieving high conversions due to the low proximity of active framework Fe³⁺ ions and extra-framework ɑ-Fe species, which limits oxygen desorption. Acid washing extracts Fe from these active sites and deposits it on the surface of the catalyst as Fe_xO_y, leading to a drop in activity. The Fe species present in the catalyst were identified using UV/Vis spectroscopy and speculate on the active species. We consider high loadings of Fe do not lead to an active catalyst when propane is present due to the formation of Fe_xO_y nanoparticles and clusters during catalyst preparation. These are inactive species which lead to a decrease in overall efficiency of the Fe ions and consequentially a lower TOF.

Reductive removal of gaseous nitrous oxide by activated carbon with metal oxide catalysts

N₂O gas can be catalytically reduced to N₂ and CO₂ by Cu-loaded active carbon under mild conditions.

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 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.