Effect of noble metals in the decomposition of nitrous oxide over Fe-ferrierites

The decomposition of nitrous oxide was studied over Fe-ferrierite, Me-ferrierites and Fe/Me-ferrierites (Me: Pt, Rh and Ru). Flow as well as batch experiments were carried out and showed a synergy between Fe and Me ions. Ions of noble metals in Fe-ferrierite increased the catalytic activity in the sequence Pt < Rh ≅ Ru. Addition of NO substantially decreased the decomposition of N₂O over Rh/ferrierite and Ru/ferrierite, but not over bimetallic ferrierites. NO _x species created during the decomposition of nitrous oxide alone as well as with addition of NO, and employment of nitrous oxide labeled with ¹⁸O allowed us to assume a changing decomposition mechanism in the presence of Me ions in Fe-ferrierites.     

NO – oxygen scavenger or reaction intermediate in the decomposition of N2O?

Nitric oxide and nitrogen dioxide were found during the thermal desorption of surface species left on Fe-ferrierites after the decomposition of nitrous oxide. This demonstrates the formation of surface NOx species during N₂O decomposition. Repeated decomposition and subsequent desorption of surface species confirm the active role of surface NOx species. Addition of NO up to a fraction of 0.1 times the amount of N₂O increased the decomposition of nitrous oxide as well as the amount of surface NOx species. The use of nitrous oxide labeled with ¹⁸O demonstrated that the zeolite oxygens participate in the reaction and that the presence of NO enhances this participation.     

Selective N2O Removal from the Process Gas of Nitric Acid Plants Over Ceramic 12CaO · 7Al2O3 Catalyst

Catalytic high temperature decomposition (secondary abatement) of nitrous oxide over calcium aluminate 12CaO · 7Al₂O₃ (mayenite) was studied in the model laboratory tests (TPSR) and pilot units (steady-state) using the real feed. X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂-sorption (BET), electron paramagnetic resonance (EPR) and Raman spectroscopies were used to characterize the synthesized material. The catalyst exhibited high efficiency and selectivity in N₂O removal, reaching practically 100% conversion at 1150 K without appreciable total losses of NO _x . Owing to its high thermal stability and resistivity to sintering and low cost of production raw materials, mayenite was found to be a promising catalyst for economically appealing secondary abatement of nitrous oxide in nitric acid plants.     

Mechanism of the Reduction by Ammonia of Nitrates Stored onto a Pt–Ba/Al2O3 LNT Catalyst

The mechanism involved in the formation of N₂ and of N₂O during the reduction of nitrates stored onto a Pt–Ba/Al₂O₃ LNT catalyst is investigated using labeled NO and unlabeled ammonia, in the presence and in the absence of NO in the gas phase. The reduction of the stored NO _x species (labeled nitrates) with NH₃ leads to the selective formation of N₂. Based on the isotopic distribution, it appears that N₂ formation occurs primarily through the statistical coupling of N-atoms formed by dissociation of NO and NH₃ at metal Pt sites. When the reduction of the stored nitrates is carried out in the presence of NO in the gas phase, NO is preferentially reduced. This implies that the rate determining step of the reduction of nitrates by ammonia is likely associated with the release of stored NO _x . Negligible amounts of nitrous oxide have been observed during the NH₃-TPSR with adsorbed nitrates, whereas relevant quantities of N₂O have been detected at low temperatures (below 180 °C) in the runs performed in the presence of NO in the gas phase. The data converge to indicate that N₂O formation involves the presence of gaseous NO and this suggests that the formation of nitrous oxide occurs either through the coupling of two adsorbed NO molecules or the recombination of an adsorbed NO molecule with an adsorbed NH _x species.     

Reduction of (NO x + N2O) on different catalysts in their joint presence in the nitrous gas

To select a catalyst for purification of the tail gas from (NO _x + N₂O) present in the nitrous gas in the production of nitric acid, we (1) studied the composition of the tail gas in UKL-7 units, as well as in the combined schemes 1/3.5 at and AK-72 and AK-72M on large-scale selective (SCP) and nonselective (NSCP) catalytic purification plants, and (2) carried out pilot tests of different catalysts supported on γ-Al₂O₃, calcium aluminate, and CaO using industrial gas. Tests were carried out in a reactor 1 l in volume at flow rates of 12000 h^?1 and, for the ruthenium catalyst, at 25500 h^?1. NSCP was shown to decrease the N₂O content in the tail gas to 0.002 vol %. In SCP processes, NO _x is completely reduced but N₂O is formed. Over the Zr catalyst, NO _x can be reduced without the production of N₂O. Over the ruthenium catalyst, approximately 62% N₂O is decomposed at 410–540°C.     

Kinetic model for the NOx reduction process by potassium containing coal char pellets at moderate temperature (350–450 °C) in the presence of O2 and H2O

A four-step mechanism is proposed to describe the reduction of NO_x by potassium containing coal char pellets under O₂-rich atmospheres. The four-step mechanism includes the chemisorption of both O₂ (step 1) and NO_x (step 3) on the so-called free sites (C_f) of carbon, which generates surface oxygen complexes (denoted by (CO)^#). The step 2 considers the direct decomposition of (CO)^# to yield CO₂ and the step 4 the reaction between NO_x and (CO)^# to yield CO₂ and C_f. NO_x reduction isothermal reactions between 350 and 450 °C have been carried out with potassium containing coal char pellets (16.8% w/w of catalyst) under 0.2%NO_x/5%O₂/N₂ and 0.2%NO_x/2%H₂O/5%O₂/N₂. The NO_x reduction and sample conversion experimental profiles have been successfully simulated by the system of algebraic and differential equations deduced from the four-step mechanism, which indicates that the mechanism seems to be feasible. Experimental results pointed out that the selectivity of pellets towards NO_x reduction against O₂ combustion decreases with temperature. This is in agreement with the elemental step rate constants (k_{step number = 1, 2, 3, or 4}) predicted by the model, that is, k₁ and k₂ increased with temperature in a major extent than k₃ and k₄, which are the steps in which NO_x are involved. The selectivity also decreases when H₂O is present in the reactive mixture. This is due to the destabilisation of (CO)^# in the presence of H₂O, thus creating C_f through step 2 that react with O₂ (step 1) in a major extent than with NO_x (step 3), as it is also deduced from the elemental step rate constants predicted by the model.     

Selective catalytic reduction of nitrogen oxides by ammonia on iron oxide catalysts

In this study, new Fe₂O₃ based materials are developed for the selective catalytic reduction (SCR) of NO_x by NH₃ in diesel exhaust. As a result of the catalyst screening, performed in a synthetic model exhaust, ZrO₂ is considered to be the most effective carrier for Fe₂O₃. The modification of the Fe₂O₃/ZrO₂ system with tungsten leads to drastic increase of SCR performance as well as pronounced thermal stability. These results show that tungsten acts as bifunctional component. The highest catalytic activity is observed for ZrO₂ that is coated with 1.4 mol% Fe₂O₃ and 7.0 mol% WO₃ (1.4Fe/7.0W/Zr). By the use of this catalyst quantitative conversion of NO_x is obtained between 285 and 430 °C with selective formation of N₂. Here, the turnover frequency of NO_x per Fe atom is found to be 35 × 10⁻⁵ s⁻¹ that indicates a high catalytic performance. The SCR activity of the 1.4Fe/7.0W/Zr material is decreased in the presence of H₂O and CO₂, whereas it is increased by NO₂.Temperature programmed reduction by H₂ (HTPR) analyses show that the Fe sites of the 1.4Fe/7.0W/Zr catalyst are mainly in the form of crystalline Fe₂O₃, whereby relatively small oxide entities are also present. The strongly aggregated Fe₂O₃ species are associated with the presence of the promoter tungsten. Based upon stationary catalytic examinations as well as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) studies we postulate an Eley Rideal type mechanism for SCR on 1.4Fe/7.0W/Zr catalyst. The mechanistic model includes a redox cycle of the active Fe sites. As first reaction step, we assume dissociative adsorption of NH₃ that leads to partial reduction of the iron as well as to production of very reactive amide surface species. These amide intermediates are supposed to react with gaseous NO to form N₂ and H₂O. In the final step, the reduced Fe sites be regenerated by oxidation with O₂. As a side reaction of SCR, imide species, originated from decomposition of amide, are oxidized by NO₂ or O₂ into NO.     

On the Characterisation of Silver Species for SCR of NO x with Ethanol

Reducing of nitrogen oxides (NO _x ) in a lean exhaust gases has become one of the most important environmental concerns. Among the different active phases studied for NO _x reduction reaction, silver-based catalysts supported over alumina show good performances using, as reducing agents, either hydrocarbons or oxygenated compounds. Nevertheless, a good understanding of the mechanism reaction has not been reached yet. This comprehension requires a better characterisation of the silver-based catalysts system. In our study, Ag/Al₂O₃ catalysts showed high efficiency in NO _x reduction using ethanol as reducing agent. The conversion plots, in steady state conditions for the different samples Ag/Al₂O₃ (0.8–3.5% Ag wt), show a great dependance of the activity with the metal loading. The optimal silver loading has been established around 2 wt.% Increasing the silver loading, the temperature of maximal NO _x conversion shifted toward the lower temperatures. According to the literature, a reduced and an oxide phase of silver have been observed by UV–Vis spectroscopy. The ratio between the two phases is changing with the silver loading. However, temperature programmed reduction (TPR) measurements reveal the presence of two types of oxide phases. TPR reveal the coexistence of a silver oxide phase (Ag₂O), according to a production of water in the course of the reaction, and a non-oxygenated phase attributed to isolated Ag⁺ cation. Thus, an original way using TPR measurements has been developed to differentiate the various oxidized phases. The aim of this characterisation is to correlate the catalyst’s activity with the observed silver phases, in order to understand the nature of phase active for NO _x reduction at low temperatures.     

A study of the NOx storage catalyst of Ba–Fe–O complex oxide

The complex oxide Ba–Fe–O catalysts were prepared by sol–gel method. The XRD, DTA, NO-TPD, XPS and NSC measurements were used to characterize the structures, NO_x storage property and sulfur resistance ability. It is concluded that when coadsorption of NO and O₂ at 400 °C, the sample calcined at 750 °C possesses high NO_x storage capacity and sulfur resistance. The perovskite type BaFeO₃ and BaFeO_3−x phases are the active centers in the catalyst for NO_x storage.     

Organo-NOx formation–decomposition as the origin of the changes in the low temperature NO-TPD profile in the presence of n-decane on Ag/γ-Al2O3

The NO_x desorption profiles obtained in O₂–He and n-C₁₀–O₂–He were compared on γ-Al₂O₃ and Ag/γ-Al₂O₃. On Ag/γ-Al₂O₃, the low-temperature NO_x desorption profiles obtained in n-C₁₀–O₂–He were significantly different from those obtained in O₂–He. In particular, at 190–220 °C excess release of NO, instead of NO₂, was observed concomitantly with n-C₁₀ consumption. This is interpreted as the result of the formation–decomposition of organo-NO_x species issued from the interaction of NO₂ and hydrocarbons initially chemisorbed on Al₂O₃ and activated on Ag species, respectively. The occurrence of such a phenomenon at temperatures close to those at which the n-C₁₀-SCR reaction starts provides support for the involvement of the organo-NO_x species as intermediates.     

NOx Adsorption/Desorption Processes Over Ce0.76Zr0.24O2 and Their Influence on DeSoot Activity: Effect of the Catalyst Calcination Temperature

The catalytic activity of a Ce_0.76Zr_0.24O₂ mixed oxide, calcined at different temperatures, for soot oxidation under NO _x /O₂ was correlated with the catalytic activity for NO₂ production. The Ce_0.76Zr_0.24O₂ mixed oxide samples were prepared by co-precipitation and calcination at different temperatures: 500–1000 °C. A satisfactory correlation between the total amount of NO₂ desorbed after NO + O₂ adsorption at 60 °C and the T50% for soot combustion was found. The NO _x adsorption process was also explored by in situ DRIFTS.     

Direct catalytic decomposition of nitrous oxide gas over rhodium supported on lanthanum silicate

The environmental impacts of nitrous oxide (N₂O) have received much attention, including contributions to the greenhouse effect and ozone depletion. Currently, the direct catalytic decomposition of N₂O is considered to be the simplest and most promising method for N₂O abatement. In this study, we focused on the high activity of rhodium and the oxide-ion conducting property of lanthanum silicate and prepared novel Rh/La₁₀Si_6 − xFe_xO_27 − δ catalysts. From the results of catalytic N₂O decomposition activities, Rh/La₁₀Si_6 − xFe_xO_27 − δ (x = 1.0) exhibited the highest catalytic activity and N₂O was completely decomposed at 600 °C.     

Formation Mechanism of Alkyl Nitrites,Valuable Intermediates in C1-Upgrading Chemistry and Oxidation Processes

In this contribution we use computational tools to investigate the reaction of alcohol substrates with reactive nitrogen oxide species such as N₂O₃ and N₂O₄, leading to the formation of alkyl nitrites. These nitrites are interesting intermediates which can be processed to various valuable chemicals such as ketones/aldehydes and dimethyl oxalate while regenerating NO _x . As such, NO _x is used as an oxidation mediator, converting alcohol substrates to more reactive nitrites which can be selectively converted to more desired compounds, closing a catalytic cycle in NO _x species.     

Diesel Soot Catalyzes the Selective Catalytic Reduction of NOx with NH3

The catalytic activity of soot samples for the selective catalytic reduction (SCR) of NO_x with NH₃ was investigated in dependence of the NO₂, NO and NH₃ concentration in the temperature range between 200 and 350 °C. The highest NO_x reduction of up to 25 % was measured in the presence of both NO₂ and NO at a GHSV of 35,000 h^?1. Decreasing space velocities resulted in an increase of the SCR activity. In the absence of NO₂, NO_x reduction was not observed. Carbon oxidation and SCR reaction occurred in parallel due to the presence of NO₂ and O₂, but hardly influenced each other, which suggested that in the NO_x reduction on soot most probably physisorbed species were involved. The observed stoichiometries indicated the action of the fast SCR reaction in the presence of NO and the NO ₂ SCR reaction in the absence of NO, while the observed gas phase and surface species pointed at reaction steps similar to those on classical SCR catalysts.     

Low temperature selective catalytic reduction of NOx with NH3 over amorphous MnOx catalysts prepared by three methods

Unsupported manganese oxide catalysts with amorphous phase were prepared by three methods, and their activities for SCR of NO_x with ammonia were investigated in the presence of O₂. The results showed the catalysts have superior low temperature activity, and the NO_x conversion is about 98% at 80 °C, and nearly 100% NO_x conversion between 100 and 150 °C. Due to competing adsorption with the reactant, H₂O has slight impact on the activity. The activity was suppressed with coexisting of SO₂, however the deactivation of SO₂ is reversible. The excellent low temperature catalytic activity of amorphous MnO_x catalysts is mainly due to their amorphous phase and high specific areas.     

Role of Zeolitic Oxygens During the Decomposition of 15N 2 18 O Over Fe-ferrierite

Isotopic species of dioxygen released during the decomposition of ¹⁵N₂¹⁸O over Fe-ferrierite show that the zeolite oxygens participate in the reaction. While Fe-ferrierite alone does not exchange its oxygens with ¹⁸O₂ below 400 °C, this exchange is very rapid in the mixture of ¹⁸O₂+N₂O. The amount of participating zeolite oxygen (ca. 1–6 per iron atom) is practically the same in the latter case as in the decomposition of ¹⁵N₂¹⁸O. The time dependence of individual dioxygen isotope species released during the ¹⁵N₂¹⁸O decomposition points to the primary release of ¹⁸O₂ which is very rapidly exchanged for the zeolite oxygen by a single-step mechanism.     

Oxygen evolution over Ag/FexAl2−xO3 (0.0 ≤ x ≤ 2.0) catalysts via N2O and H2O2 decomposition

In this paper, a comparative study between nitrous oxide and hydrogen peroxide decomposition over a series of catalysts prepared via the combustion of silver, aluminum, and iron nitrates (with different aluminum: iron ratios). Urea was used as a combustion fuel. The calcinations were affected at the 400–700 °C temperature range. The produced catalysts were characterized by using XRD and SEM analyses. The obtained results revealed that silver metal supported on Al₂O₃ and/or Fe₂O₃ represent the major constituents of all the calcinations products, i.e. Ag/Fe_xAl_2−xO₃. However, two different interfaces are involved in the two test reactions, all the catalysts were able to decompose both reactants yielding oxygen as a joint product. Meanwhile, it was found nitrous oxide destruction activity increases with decreasing both silver particles size and iron content in the catalysts substrate. On the contrary, increasing iron content in the different catalyst was found to enhance hydrogen peroxide decomposition activity. Moreover, a synergic effect was observed for the catalysts having Al:Fe ratio of 0.5:1.5.     

TiO2 Supported Nano-Au Catalysts Prepared Via Solvated Metal Atom Impregnation for Low–Temperature CO Oxidation

The Ce _x Ti_1?x O₂ mixed oxides at different mole ratios (x=0.1–1.0) were prepared by co-precipitation of TiCl₄ and Ce(NO₃)₃. The structural and reductive properties of the Ce _x Ti_{1? x} O₂ were affected by calcination temperature. At x=0.1–0.3, CeTi₂O₆ phase was formed and mainly as amorphous after calcination at 650°C. At x=0.3, only CeTi₂O₆ was formed after calcination at 750°C and CeTi₂O₆ crystallized completely after calcination at 800°C. TPR analyses showed that the amount of H₂ consumption by Ce _x Ti_1?xO₂ (650°C) (except x=0.1) was greater than that by single CeO₂, and the valence of CeO₂was the lowest (+3.18) at x=0.3. CuO/Ce_0.3Ti_0.7O₂ was prepared by the impregnation method and catalytic properties were examined by means of a GC micro-reactor NO+CO reaction system, BET, TPR, XRD, XPS and NO-TPD. It was found that CuO/Ce_0.3Ti_0.7O₂ calcined at 650°C had the highest activity in NO+CO reaction with 100% NO conversion at reaction temperature of 300°C, and at 650°C Ce_0.3Ti_0.7O₂just began to crystallize. The catalytic activities were largely affected by the pre-treatment conditions. At low reduction temperature (100°C), CuO species was difficult to reduce. When high degree of reductions took place, both CuO species and Ce_0.3Ti_0.7O₂ reduced and thus a part of CuO species on the support surface would be covered. The XPS and NO-TPD analyses showed that CuO/Ce_0.3Ti_0.7O₂ had four NO absorption centers (Cu⁺, Cu²⁺(I), Cu²⁺(II) and Ce³⁺). The CuO species involving in NO+CO reaction included Cu²⁺(I) and Cu⁺, and CeO₂ species (Ce³⁺ and Ce⁴⁺).     

Synthesis of nano-sized indium oxide (In2O3) powder by a polymer solution route

Indium oxide (In₂O₃) is a n-type semiconductor with various applications in thin film coatings, on the basis of its optical properties, and in gas sensing equipment, due to its high sensitivity to various oxides such as CO_x and NO_x. In this study, a synthesis process for obtaining In₂O₃ nanoparticles is examined. The precursor used is indium nitrate hydrate (InN₃O₉·H₂O) because of its high solubility in water. By dissolving the nitrate salt in a PVA (polyvinyl alcohol) solution, the precursor is dispersed homogeneously, which reduces the agglomeration of the resulting powder. Calcination at a low temperature of 200–250 °C burns out the organic materials of the PVA with NO_x gas emission and allows the oxidation of the indium, resulting in indium oxide nanoparticles. The influence of the PVA solution characteristics and the heat treatment temperature on the powder morphology and size was analyzed by using SEM, TEM, XRD, TGA/DSC, and four point BET for a specific surface area analysis. The measured specific surface area varies from 3 m²/g to 76 m²/g depending on the calcination temperature, and the particle size of the synthesized powders is under 10 nm for the samples heat treated at 300 °C.