Quantum chemical study of nitrous oxide adsorption and decomposition on Lewis acid sites

Density functional calculations demonstrate that ordinary Lewis sites containing three‐ and five‐coordinated Al are unlikely to decompose N₂O, since the formation of a weak Al–O bond does not compensate the N–O bond rupture. The ground state of the calculated cluster–oxygen adsorption complexes is triplet. The considered hypothetical site Al(OH)₄AlO can be reactive towards the N₂O decomposition with the heat -17.8 kcal/mol and activation barrier 19.7 kcal/mol. This revised version was published online in July 2006 with corrections to the Cover Date.     

A study of nitrous oxide decomposition over calcium oxide

A study of N₂O decomposition reaction in a fixed bed a reactor over bed of CaO particles has been conducted. Effects of parameters such as concentration of inlet N₂O gas, reacting temperature and content of CO₂/ CO gas present in the reacting materials on the decomposition reaction have been investigated. The results showed that the conversion of N₂O decomposition was accelerated by the increase of reaction temperature, and the existence of CO, while the rate was hindered by the existence of CO₂. Heterogeneous gas solid reaction kinetics was proposed for N₂O decomposition and compared with homogeneous reaction kinetics. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

A study of nitrous oxide decomposition over calcium oxide in combustion condition

A study of N₂O decomposition reaction over a bed of CaO particles in a fixed bed reactor has been conducted. Effects of parameters such as concentration of inlet N₂O, reaction temperature and effects of CO, CO₂, O₂, and NO gas presented in the combustion gas environment have been investigated. The experiment showed that the N₂O decomposition reaction was accelerated by the increase of reaction temperature, and the existence of CO, while the reaction was hindered by the existence of CO₂, NO. O₂ also affected the N₂O decomposition. Heterogeneous gas-solid reaction kinetics were proposed for the reaction conditions and compared with homogeneous reaction kinetics.     

N2O分解催化剂的制备

中国石油辽阳石化分公司尼龙厂己二酸装置年排出N2O约45 kt,本文介绍了该厂N2O排放气的组成、对环境的危害和治理方案,并研究开发出两类N2O分解催化剂——低温高活性含钴-镁-铝类水滑石煅烧催化剂和载Ni蜂窝型规整催化剂。对组成为Co∶Mg∶Al=3∶0.5∶1(摩尔比)的类水滑石催化剂,当反应气中N2O的摩尔分数为0.13%(其余为N2),空速为4000 h-1时,在220℃下N2O开始分解,到400℃时分解完全。负载金属氧化物蜂窝型规整催化剂N2O的分解温度较高:当反应气中N2O的摩尔分数为0.34%,空速为2400 h-1时,对3次浸渍的载Ni催化剂(NiO负载量9.4%),在360℃下N2O开始分解,480℃分解完全,但蜂窝型规整催化剂流体阻力小,对大气量处理有竞争力。     

Heterogeneous catalytic decomposition of nitrous oxide

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N₂O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.     

Kinetics of nitrous oxide decomposition over Cu-ZSM-5

The kinetics of nitrous oxide decomposition on an overexchanged Cu-ZSM-5 catalyst were measured using a gradientless reactor. Isothermal oscillations of nitrous oxide and oxygen concentrations can be observed in a broad range of experimental conditions. A transition of the catalytic activity during oscillations is accompanied by a change in the oxygen content of the catalyst and by the formation of traces of nitric oxide. The presence of excess oxygen does not significantly alter the behaviour of the catalyst whereas NO concentrations as low as 10 ppm quench the oscillations in the whole temperature range studied (375 to 450°C), maintaining steady-state operation at maximum catalytic activity. Reaction rates in this ‘ignited’ state are first order with respect to nitrous oxide concentration and not affected by either oxygen or nitric oxide. At temperatures above 400°C, the observed reaction rates are influenced by pore diffusion effects. In the region of intrinsic kinetics, the temperature dependence of the first order rate constant can be described by an activation energy of ca. 100 kJ/mol.     

Catalytic decomposition of nitrous oxide over Ru-exchanged zeolites

We report that ultrastable faujasite-based ruthenium zeolites are highly active catalysts for N₂O decomposition at low temperature (120–200°C). The faujasite-based ruthenium catalysts showed activity for the decomposition of N₂O per Ru³⁺ cation equivalent to the ZSM-5 based ruthenium catalysts at much lower temperatures (TOF at 0.05 vol.-% N₂O: 5.132 × 10⁻⁴ s⁻¹ Ru⁻¹ of Ru-HNaUSY at 200°C versus 5.609 × 10⁻⁴ s⁻¹ Ru⁻¹ of Ru-NaZSM-5 at 300°C). The kinetics of decomposition of N₂O over a Ru-NaZSM-5 (Ru: 0.99 wt.-%), a Ru-HNaUSY (Ru: 1.45 wt.-%) and a Ru-free, Na-ZSM-5 catalyst were studied over the temperature range from 40 to 700°C using a temperature-programmed micro-reactor system. With partial pressures of N₂O and O₂ up to 0.5 vol.-% and 5 vol.-%, respectively, the decomposition rate data are represented by: −dN₂O/dt=itk(P_N2O) (P_O2)^−0.5 for Ru-HNaUSY, −dN₂O/dt=k(P_N2O) (P_O2)^−0.1 for Ru-NaZSM-5, and −dN₂O/dt=k(P_N2O)^−0.2 (P_O2)^−0.1 for Na-ZSM-5. Oxygen had a stronger inhibition effect on the Ru-HNaUSY catalyst than on Ru-NaZSM-5. The oxygen inhibition effect was more pronounced at low temperature than at high temperature. We propose that the negative effect of oxygen on the rate of N₂O decomposition over Ru-HNaUSY is stronger than Ru-NaZSM-5 because at the lower temperatures (<200°C) the desorption of oxygen is a rate-limiting step over the faujasite-based catalyst. The apparent activation energy for N₂O decomposition in the absence of oxygen is much lower on Ru-HNaUSY (E_a: 46 kJ mol⁻¹) than on Ru-NaZSM-5 (E_a: 220 kJ mol⁻¹).     

Effects of methane and oxygen on decomposition of nitrous oxide over metal oxide catalysts

Catalytic activities of various metal oxides for decomposition of nitrous oxide were compared in the presence and absence of methane and oxygen, and the general rule in the effects of the coexisting gases was discussed. The reaction rates of nitrous oxide were well correlated to the heat of formation of metal oxide, i.e., a V-shaped relationship with a minimum at −ΔH_f⁰ around 450 kJ (O mol)⁻¹ was observed in N₂O decomposition in an inert gas. In the case of metal oxides having the heat of formation lower than 450 kJ (O mol)⁻¹, CuO, Co₃O₄, NiO, Fe₂O₃, SnO₂, In₂O₃, Cr₂O₃, the activities were strongly affected by the presence of methane and oxygen. On the other hand, the activities of TiO₂, Al₂O₃, La₂O₃, MgO and CaO were almost independent. The reaction rate of nitrous oxide was significantly enhanced by methane. The promotion effect of methane was attributed to the reduction of nitrous oxide with methane: 4N₂O+CH₄→2N₂+CO₂+2H₂O. The activity was suppressed in the presence of oxygen on the metal oxides having lower heat of formation. On the basis of Langmuir–Hinshelwood mechanism, the effect of oxygen on nitrous oxide decomposition was rationalized with the strength of metal–oxygen bond.     

Alkali-metal promoted rhodium-on-alumina catalysts for nitrous oxide decomposition

Catalytic activity of γ-alumina supported rhodium catalysts in nitrous oxide decomposition into dinitrogen and dioxygen has been determined, the total conversion being reached at 450 °C. Rhodium is present at alumina surface in three forms: metal particles, Rh₂O₃ and rhodium zero valent atoms interacting with oxygen ions at the metal/support interface. Linear dependence of the catalytic activity on rhodium dispersion has been found. Deposition of alkali metal cations: Li, Na, K and Cs as promoters at the surface of alumina results in a considerable increase of rhodium dispersion and hence catalytic activity. The effect of promoters depends strongly on the speciation of alkali metals and rhodium used in the preparation of the catalyst. Both alkali metal cations and rhodium compete for the same OH groups at the alumina surface. The electronegativity of alkali metal oxides is much greater than that of alumina and their deposition increases the negative charge of surface oxide ions hindering the diffusion of rhodium and preventing the growth of its larger particles.     

Isotopic study of nitrous oxide decomposition on an oxidized Rh catalyst: mechanism of oxygen desorption

N₂O decomposition on an oxidized Rh catalyst (unsupported) has been studied using a tracer technique in order to reveal the reaction mechanism. N₂ ¹⁶O was pulsed onto an ¹⁸O/oxidized Rh catalyst at 493 K and desorbed O₂ molecules were monitored. The ¹⁸O fraction in the desorbed oxygen had the same value as that on the surface oxygen. The result shows that the oxygen molecules do not desorb via the Eley–Rideal mechanism, but via the Langmuir–Hinshelwood mechanism. On the other hand, desorption of oxygen from Rh surfaces (in vacuum or in He) occurs at higher temperatures, which suggests reaction-assisted desorption of oxygen during the N₂O decomposition reaction at low temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic decomposition of nitrous oxide over calcined cobalt aluminum hydrotalcites

Catalytic decomposition of nitrous oxide has been carried out over calcined cobalt aluminum hydrotalcites of general formula [Co_1−xAl_x(OH)₂[CO₃]_x/2 H₂O where x = 0.25–0.33 at 50 Torr (1 Torr = 133 Pa) initial pressure of N₂O in a static glass recirculatory reactor (130 cc) in the temperature range 150–280°C. All catalysts showed a first order dependence in N₂O without significant oxygen inhibition. The activity increased with an increase in cobalt concentration present in the sample. The catalyst precursor synthesized under low supersaturation (LS) exhibited a higher activity than the precursor synthesized by sequential precipitation (SP) method. The observed trend in the activity is explained based on the surface concentration of cobalt, determined by XPS and matrix effects. Prior to catalytic studies, the fresh and calcined samples were characterized by various physicochemical techniques such as XRD, FT–IR, TG–DSC, TEM (with EDAX) and BET surface area measurements.     

Catalytic decomposition of nitrous oxide on strontium-substituted La2CuO4 materials

Catalytic decomposition of nitrous oxide (N₂O) to N₂ and O₂ has been studied on a series of solid oxide solutions of La, Sr and Cu according to the nominal formula La_2–x Sr _x CuO₄ (0x1). The reaction has been carried out in a fixed bed, glass static reactor with gas-recirculation facility. The kinetics of decomposition has been studied in the temperature range 250–480C. Among the catalysts studied,x=0.15 andx=1.0 showed higher catalytic activity (in terms of %conversion). The enhanced activity of the above systems has been explained on the basis of mixed valency of copper (Cu²⁺/Cu³⁺) and anion vacancies respectively.     

Novel cobalt-doped ZrSnO4 catalysts for direct nitrous oxide decomposition

Novel ZrSn_1−xCo_xO_4−δ catalysts for the direct decomposition of nitrous oxide (N₂O) were synthesized using a co-precipitation method. Metastable ZrSnO₄ with an α-PbO₂-type structure was used as the mother solid because of its interstitial open spaces derived from its lattice distortion that may be effective for N₂O adsorption. The doping of Co^2+/3+ into the ZrSnO₄ lattice improved the N₂O decomposition activity, likely owing to the enhancement of the redox properties and the increase in the number of oxygen vacancies. Among the prepared catalysts, Zr_1.17Sn_0.73Co_0.10O_4−δ exhibited the highest activity decomposing N₂O completely decomposed at 550°C. In addition, the Zr_1.17Sn_0.73Co_0.10O_4−δ catalyst showed high durability in the presence of CO₂, O₂, and H₂O vapor.     

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.     

Fe-mordenite/cordierite monolith for the catalytic decomposition of nitrous oxide

Mordenite zeolites were coated on cordierite support by in situ hydrothermal method or dip-coating method. The mordenite/cordierite was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques and their stability was studied. The mordenite/cordierite monolith prepared by in situ synthesis showed much better stability than that prepared by dip-coating. Iron was introduced into mordenite/cordierite by ion exchange and Fe-mordenite/cordierite catalyst was prepared. Fe-mordenite/cordierite prepared from in situ synthesis exhibited good activity and stability for N₂O catalytic decomposition, with great potential for future application.     

Catalytic decomposition of nitrous oxide over perovskite type solid oxide solutions and supported noble metal catalysts

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been investigated over various solid oxide solutions (SOS), La_0.8Sr_0.2MO_3– (M=Cr, Fe, Mn, Co or Y), La_1.8Sr_0.2CuO_4– and supported Pd, Pt catalysts. The reaction was carried out in a gradientless recycle reactor at 1 atm pressure with a feed gas containing about 0.5% N₂O (in helium). Among the various solid solutions, La_0.8Sr_0.2CoO_3– showed a maximum N₂O conversion of 90% at 600C. The order of activity observed for N₂O decomposition was La_0.8Sr_0.2CoO_3–>La_0.8Sr_0.2FeO_3–>La_1.8Sr_0.2CuO_4–> La_0.8Sr_0.2MnO_3–La_0.8Sr_0.2CrO_3–La_0.8Sr_0.2YO_3–. The activity of La_0.8Sr_0.2CoO_3– was compared with supported Pd, Pt and also with unsubstituted LaCoO₃ catalysts under similar reaction conditions. Among all the catalysts tested in this study, Pd/Al₂O₃ showed the lowest light-off temperature for N₂O decomposition. The activity of La_0.8Sr_0.2CoO_3– was found to be comparable to Pd/Al₂O₃ catalyst at temperatures above 500 C. The influence of added oxygen (about 4%) in the feed was examined over La_0.8Sr_0.2CoO_3– and Pd/Al₂O₃ catalysts and only in the case of cobalt catalyst was the conversion of N₂O decreased by 13%. By choosing varied sintering conditions, La_0.8Sr_0.2CoO_3– of different BET surface areas were prepared and the light-off temperature was found to decrease with increase in surface area. The results obtained over solid solutions are discussed on the basis of the cation mixed valency and oxygen properties of the catalyst.     

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.     

Isothermal oscillations in the catalytic decomposition of nitrous oxide over Cu-ZSM-5

The catalytic decomposition of nitrous oxide was studied on a copper-exchanged ZSM-5 catalyst in the temperature range 648–723 K. Using a mixture of 1000 ppm N₂O in nitrogen, isothermal oscillations both in nitrous oxide and oxygen concentrations occurred, accompanied by formation of small amounts of NO. While the addition of excess oxygen did not significantly change frequency and amplitude of the oscillations, even the presence of small amounts of NO immediately quenched the oscillations. The reacting system then remained in the ignited state at high nitrous oxide conversions.     

Effect of hydrogen sulphide on nitric oxide adsorption and decomposition on Cu-containing molecular sieves

NO adsorption and decomposition were studied on sulphided and sulphated copper-exchanged ZSM-5, AlMCM-41 and NbMCM-41 molecular sieves. FTIR, temperature-programmed reduction (H₂-TPR) techniques, adsorption, and catalytic measurements were applied. H₂S adsorption drastically decreases the activity of copper-containing micro- and mesoporous materials studied in NO decomposition due to their sulphidation. This decrease is higher than that observed after SO₂ treatment and is assigned to the formation of sulphide species. H₂S is strongly chemisorbed on all copper active sites. Oxidation of sulphide species leading to the formation of sulphates does not increase the activity in the decomposition of NO, suggesting that some sulphided species are not oxidised.     

A mechanistic study on the simultaneous elimination of soot and nitric oxide from engine exhaust

The non-catalytic interaction between soot and nitric oxide (NO) resulting in their simultaneous elimination was studied on different types of reactive site present on soot. The reaction mechanism proposed previously was extended by including seven new reaction pathways for which the reaction energetics and kinetics were studied using density functional theory and transition state theory. This has led to the calculation of a new rate for the removal of carbon monoxide (CO) from soot. The new pathways have been added to our polycyclic aromatic hydrocarbon (PAH) growth model and used to simulate the NO–soot interaction to form CO, N₂ and N₂O. The simulation results show satisfactory agreement with experiment for the new CO removal rate. The NO–soot reaction was found to depend strongly on the soot site type and temperature. For a set of temperatures, computed PAH structures were analysed to determine the functional groups responsible for the decrease in the reactivity of soot with NO with increasing reaction time. In isothermal conditions, it was found that as temperature is increased, the number of oxygen atoms remaining on the soot surface decreases, while the number of nitrogen atoms increases for a given reaction time.