Kinetic analysis of N2O decomposition over calcined hydrotalcites

Kinetics of N₂O decomposition over catalyst prepared by calcination of Co–Mn hydrotalcite was examined in integral fixed-bed reactor () at various N₂O and O₂ initial partial pressure at temperature range of 330–450 °C. Kinetic data were evaluated by linear and non-linear regression method, 15 kinetic expressions were tested. Based on the obtained results a redox model of N₂O decomposition was proposed. At low pressures of O₂, adsorbed oxygen is formed by the N₂O decomposition; the N₂O chemisorption is considered as the rate-determining step. On the contrary, at high O₂ pressure it could be assumed that adsorbed oxygen species appear as a result of O₂ adsorption and the Eley–Rideal mechanism is the rate determining. N₂O decomposition is well described by the 1st rate law at N₂O and O₂ concentrations typical for waste gases.     

Co-ZSM-5 catalysts for N2O decomposition

The catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. TPR, XRD, N₂-BET, XPS, FTIR and UV–vis spectroscopy were used for characterizing the samples. Well-dispersed cobalt oxide-like species and isolated Co²⁺ ions in charge compensation positions were found in the zeolite. Catalysts prepared using a single step cation exchange method showed high activity for N₂O decomposition in a temperature range 300–550°C, in the presence of 0–5% O₂, and high stability in the presence of 10% H₂O to the feed. UV–vis spectra and TPR experiments indicated the presence of some cobalt oxides, not detected by DRX, in a Co-ZSM-5 catalyst containing 3.76 wt% Co, prepared by a solid-state reaction procedure. The N₂O conversion over this catalyst was strongly affected by addition of both O₂ and H₂O to the feed.     

N2O catalytic decomposition over various spinel-type oxides

Various spinel-type catalysts AB₂O₄ (where A = Mg, Ca, Mn, Co, Ni, Cu, Cr, Fe, Zn and B = Cr, Fe, Co) were prepared and characterized by XRD, BET, TEM and FESEM-EDS. The performance of these catalysts towards the decomposition of N₂O to N₂ and O₂ was evaluated in a temperature programmed reaction (TPR) apparatus in the absence and the presence of oxygen. Spinel-type oxides containing Co at the B site were found to provide the best activity. The half conversion temperature of nitrous oxide over the MgCo₂O₄ catalyst was 440 °C and 470 °C in the absence and presence of oxygen, respectively (GHSV = 80,000 h⁻¹).

On the grounds of temperature programmed oxygen desorption (TPD) analyses as well as of reactive runs, the prevalent activity of the MgCo₂O₄ catalyst could be explained by its higher concentration of suprafacial, weakly chemisorbed oxygen species, whose related vacancies contribute actively to nitrous oxide catalytic decomposition. This indicates the way for the development of new, more active catalysts, possibly capable of delivering at low temperatures amounts of these oxygen species even higher than those characteristic of MgCo₂O₄.     

N2O decomposition on Fe- and Co-ZSM-5: A density functional study

Density functional theory (DFT) calculations are employed to study N₂O decomposition on relaxed [(SiH₃)₄AlO₄M] (where M = Fe, Co) cluster models representing Fe- and Co-ZSM-5 surfaces and Fe-ZSM-5 channel cluster. The catalytic cycle steps are completed both for Fe- and Co-ZSM-5 clusters. It is found that the general trend of the results obtained is in agreement with experimental and theoretical literature: Co-ZSM-5 has a lower activation energy barrier than Fe-ZSM-5 and O₂ desorption step is the rate-limiting step for both clusters. The activation barrier for the decomposition of the first N₂O molecule inside a Fe-ZSM-5 channel cluster increases in comparison with that of the cluster model indicating a channel effect on the activation barrier. The activation barrier reported for the channel cluster is 12.63 kcal/mol. This is also in good agreement with experimental literature.     

Selective catalytic reduction of N2O and NOx in a single reactor in the nitric acid industry

The nitric acid industry is a source of both NO_x and N₂O. The simultaneous selective catalytic reduction of both compounds using propane as a reductant has been investigated. A stacked catalyst bed with first a Co-ZSM-5 catalyst and second a Pd/Fe-ZSM-5 catalyst gives >80% conversion of N₂O and NO_x above 300 °C at atmospheric pressure. At 4 bar absolute pressure (bara) the Co-ZSM-5 DeNO_x catalyst shows higher NO_x and propane conversion. This leaves not enough propane for the Pd/Fe-ZSM-5 DeN₂O catalyst, which causes a ‘dip’ in N₂O conversion. Reducing the space velocity (SV) of the first catalyst bed secures high NO_x and N₂O conversions from 300 °C and up at 4 bara.     

分子筛催化剂上N2O催化分解的研究进展

氧化亚氮(N2O)会造成温室效应和臭氧层破坏,N2O的控制日益引起关注。催化分解技术是控制N2O的有效手段,分别对Fe基、Cu基和Co基分子筛催化剂催化分解N2O的研究现状进行总结和评述,并展望未来发展方向。     

Fe/beta分子筛催化N2O分解

通过液相离子交换法对H-beta分子筛进行改性得到Fe/beta分子筛,并应用于催化N2O直接分解反应,考察加入HNO3后的体系pH值对Fe/beta催化性能的影响。采用N2物理吸附-脱附、XRD、IR、DR UV-Vis、NH3-TPD和ICP-OES等对Fe/beta分子筛进行分析表征。结果表明,溶液pH值降低过程中催化剂的结晶度、比表面积、孔容及Fe3+含量呈现先增加后减少。溶液pH为2.0时所制备的Fe/beta分子筛催化N2O完全分解温度明显低于pH为2.6和1.0时制备的Fe/beta催化剂,显示了较好的催化活性。     

N2O decomposition over wet- and solid-exchanged Fe-ZSM-5 catalysts

The N₂O decomposition activity of Fe-ZSM-5 strongly depends on the iron content and the preparation methods, including wet (WIE) and solid state ion exchanges (SSIE). The state of Fe species formed on the surface of a series of Fe-ZSM-5 catalysts containing a variety of Fe contents with respect to the preparation method and their role for N₂O decomposition activity have been systematically examined. The general trend for the decomposition activity of Fe-ZSM-5-SSIE is higher than that of Fe-ZSM-5-WIE, indicating the formation of a distinctive local structure of Fe on the catalyst surface during the course of the ion-exchange procedure. Based upon the Fourier transformed Fe K-edge EXAFS spectra for the series of Fe-ZSM-5-SSIE and -WIE catalysts, most of the Fe species on the surface of Fe-ZSM-5-SSIE with high Fe loading are well dispersed in the form of oxygen-bridged binuclear Fe species. The turnover frequency (TOF) for N₂O decomposition under dry and wet conditions has been confirmed assuming that Fe-ZSM-5-SSIE samples with Fe/Al = 0.20 and Fe/Al = 0.65 only contain mononuclear and binuclear Fe species, respectively, as active reaction species on their surface. The high performance of Fe-ZSM-5-SSIE may be mainly due to the formation of the binuclear Fe species onto its surface during the preparation of the catalyst.     

Self oscillatory behaviour in N2O decomposition over Co-ZSM-5 catalysts

Isothermal oscillations developed during N₂O decomposition over Co-ZSM-5 catalysts with different Si/Al ratios have been investigated. Spontaneous oscillations were observed between 350 and 450 °C. The maximum amplitude has been obtained for the catalysts having Si/Al of 40 and 50. The activation energies of the obtained oscillations were calculated in respect to cobalt concentration. The results showed that the E_a values increase linearly with an increasing Si/Al ratio of the zeolite. For Co-ZSM-5 catalyst (Si/Al = 25), increasing cobalt content in the catalyst led to a decrease in the frequency as well as the amplitude of the oscillations. Meanwhile, the increase in the E_a values was observed. The calculated reaction rate was found to be first order with respect to nitrous oxide concentration. Moreover, the developed oscillations were found to be sensitive to inlet N₂O concentration, catalyst weight and milling time duration. Decreasing the N₂O inlet concentration as well as the catalyst weight and increasing the milling time would lead to a quenching of the developed oscillations.     

Potassium-doped Co3O4 catalyst for direct decomposition of N2O

Direct decomposition of nitrous oxide (N₂O) on K-doped Co₃O₄ catalysts was examined. The K-doped Co₃O₄ catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co₃O₄ catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co₃O₄ catalyst strongly depended on the amount of K in the catalyst. In order to reveal the role of the K component on the catalytic activity, the catalyst was characterized by XRD, XPS, TPR and TPD. The results suggested that regeneration of the Co²⁺ species from the Co³⁺ species formed by oxidation of Co²⁺ with the oxygen atoms formed by N₂O decomposition was promoted by the addition of K to the Co₃O₄ catalyst.     

Superior performance of Ir-substituted hexaaluminate catalysts for N2O decomposition

Novel Ir-substituted hexaaluminate catalysts were developed for the first time and used for catalytic decomposition of high concentration of N₂O. The catalysts were prepared by one-pot precipitation and characterized by X-ray diffraction (XRD), N₂-adsorption, scanning electronic microscopy (SEM) and temperature-programmed reduction (H₂-TPR). The XRD results showed that only a limited amount of iridium was incorporated into the hexaaluminate lattice by substituting Al³⁺ to form BaIr_xFe_1−xAl₁₁O₁₉ after being calcined at 1200 °C, while the other part of iridium existed as IrO₂ phase. The activity tests for high concentration (30%, v/v) of N₂O decomposition demonstrated that the BaIr_xFe_1−xAl₁₁O₁₉ hexaaluminates exhibited much higher activities and stabilities than the Ir/Al₂O₃-1200, and the pre-reduction with H₂ was essential for activating the catalysts. By comparing BaIr_xFe_1−xAl₁₁O₁₉ with BaIr_xAl_12−xO₁₉ (x = 0–0.8), it was found that iridium was the active component in the N₂O decomposition and the framework iridium was more active than the large IrO₂ particles. On the other hand, Fe facilitated the formation of hexaaluminate as well as the incorporation of iridium into the framework.     

Zn, Al, Rh-mixed oxides derived from hydrotalcite-like compound and their catalytic properties for N2O decomposition

The catalytic decomposition of nitrous oxide was studied over Zn, Al, Rh-mixed oxides derived from hydrotalcite-like compounds (HTlc) as a precursor. The study showed that, when the Zn/Al atomic ratio was 3, the rate of ZnAlRh-HTlc for N₂O decomposition increased with Rh loading up to 1.4 wt% (4.55 × 10⁴ μmol g⁻¹ h⁻¹ at 400°C), and levels off with further increase in Rh loading. Analogous behavior was found in the presence of NO₂ and water. In the presence of 0.5% water, the activity decreased with decreasing Zn/Al ratio, whereas the activity for N₂O decomposition in the presence of 0.1% NO₂ reached a maximum when the Zn/Al ratio was 3. The observed catalytic activities were comparable to those of the reported catalysts in the presence of NO₂ and water.     

Cu-Mn氧化物催化N_2O直接分解性能

分别以Cu(NO_3)_2·3H_2O和50%Mn(NO_3)_2水溶液为铜源和锰源,K_2CO_3为沉淀剂,采用沉淀法和共沉淀法制备单一Cu、Mn氧化物催化剂和Cu-Mn-O复合氧化物催化剂,用于催化N_2O直接分解反应,并利用N_2物理吸附-脱附、XRD、FT-IR和TPR等进行表征。结果表明,单一Cu和Mn氧化物分别以体相CuO和Mn2O_3物相形式存在,Cu-Mn-O复合氧化物中除形成CuMn_2O_4尖晶石物相外,还有一定量小晶粒CuO,较单一氧化物具有更加优异的还原性能,表现出较高的催化N_2O直接分解活性。在空速10 000 h~(-1)和N_2O体积分数0.1%条件下,Cu-Mn-O复合氧化物催化剂可在440℃催化N_2O完全分解,分别较单一Cu和Mn氧化物催化剂降低了40℃和60℃。     

N2O decomposition over K-promoted Co-Al catalysts prepared from hydrotalcite-like precursors

N₂O decomposition was investigated over a series of K-promoted Co-Al catalysts. The activity tests showed that doping with K greatly enhanced the catalytic activity of the Co-Al catalyst, and the enhancement was critically dependent on the amount of K and the calcination temperature. When the catalyst had a K/Co atomic ratio of 0.04 and was calcined at 700–800 °C, a full N₂O conversion could be reached at a reaction temperature of 300 °C. Moreover, even under the simultaneous presence of 4% O₂ and 2.6% water vapor, such high-temperature treated K/Co-Al catalyst exhibited high reactivity and stability, with the N₂O conversion remaining at a constant value of 92% over 40 h run at 360 °C. In contrast, non-doped Co-Al catalyst showed a severe activity loss under such reaction conditions. A combination of characterization techniques was employed to reveal the promoting role of K and the effect of calcination temperature. The results suggest that doping with K increases the electron density of Co and weakens the Co–O bond, thus promoting the activation of N₂O on the Co sites and facilitating the desorption of oxygen from the catalyst surface. High-temperature calcinations made the desorption of O₂ proceed more readily.     

动物骨源羟磷灰石负载Co3O4用于N2O催化分解

通过焙烧猪骨和鸡骨获得羟磷灰石(nHAP)载体,并采用浸渍法制备Co3O4/nHAP催化剂。采用XRD、N2物理吸附-脱附、FT-IR和H2-TPR等对催化剂进行表征,在连续流动微反装置上考察催化剂催化分解N2O的性能。结果表明,相比于鸡骨源Co3O4/nHAP催化剂,以猪骨源HAP为载体的催化剂因其较大的比表面积以及较小的Co3O4粒径尺寸,提供了更多的活性位点。特别是猪骨源Co3O4/nHAP催化剂中适量的K、Na等元素促进了Co^3+到Co^2+的还原,削弱了Co-O键,使催化剂的催化活性显著提高。     

Characteristics of Fe-exchanged natural zeolites for the decomposition of N2O and its selective catalytic reduction with NH3

Natural zeolites obtained from various regions of Turkey and their iron-exchanged forms were characterised by XRD, BET, H₂-TPR and NH₃-TPD methods. Transient experiments with N₂O showed that the iron introduced into natural zeolites have appreciable oxygen deposition capacity due to isolated iron species involved. Atomic surface oxygen species in these zeolites are formed at 250 °C, which is released through increasing the temperature until 900 °C, similar to Fe-containing ZSM-5 zeolite. The steady-state experiments indicate that the iron-containing zeolite of the Yavu-Sivas region, in particular, has high activity in selective catalytic reduction of N₂O with NH₃ as a consequence of isolated cationic and/or dimeric iron content.     

Selective catalytic reduction of N2O in industrial emissions containing O2, H2O and SO2: behavior of Fe/ZSM-5 catalysts

The catalytic behavior in N₂O reduction by propane in the presence of O₂, H₂O and SO₂ of Fe/ZSM-5 catalysts prepared by ion exchange and chemical vapour deposition (CVD) is reported. The catalyst prepared by CVD shows a lower dependence of the rate of selective N₂O reduction on the decrease in C₃H₈ to N₂O ratio in the feed and a higher resistance to deactivation by SO₂ in accelerated durability tests with high SO₂ concentration (500 ppm). This catalyst shows stable catalytic behavior in the presence of SO₂ for more than 600 h of time-on-stream. Characterization of the catalysts by UV–VIS–NIR diffuse reflectance indicates that the poor performances of the sample prepared by ion exchange could be related to the presence of highly clustered Fe³⁺ species, in this catalyst. On the other hand, Fe₂O₃ particles are not present in the sample prepared by CVD while mainly isolated Fe³⁺ ions and iron-oxide nanoclusters are present.     

利用Fluent软件对N_2O催化分解反应器数值模拟

为了设计N_2O催化分解反应器,运用Fluent软件对整体式分子筛催化剂进行数值模拟,考察孔密度和操作条件对整体式分子筛催化剂转化率的影响。结果表明,在相同温度下,N_2O的转化率随着催化剂孔密度的减小而降低。在反应器轴向距离120 mm处,气体反应最快;提高入口温度、浓度或降低空速,均有利于在较短的轴向距离内达到较高的N_2O转化率。在固定床反应器中,比较棒状催化剂和整体式催化剂中床层温度、反应转化率及轴向压降的变化规律,为整体式分子筛催化剂工业化设计提供理论基础。     

Decomposition of NO and N2O on Cu-AlTS-1: oscillatory behaviour at full N2O conversion

Cu-AlTS-1 catalyst was prepared by solid state ion exchange and studied in the NO and N₂O decomposition. Oscillation was observed in a wide range of experimental conditions during the decomposition of N₂O. At full N₂O conversion, oscillations were observed only in the O₂ and NO concentrations the latter being out of phase with respect to O₂ and being originated from the decomposition of an excess oxygen containing nitrito–nitrato-like surface complex. Traces of NO extinguished the oscillations and increased the N₂O conversion if it was below 100%. The NO also plays a key role in the feed back and synchronisation.