Possible role of nitrite/nitrate redox cycles in N2O decomposition and light-off over Fe-ZSM-5

Surface nitrite/nitrate redox cycles were proposed to explain light-off behavior that was observed during the decomposition of N₂O over Fe-ZSM-5. Further study has demonstrated that while the nitrite/nitrate model can explain the original observations as an isothermal, mechanistic phenomenon, the light-off behavior is thermal, and not a mechanistic effect. Nonetheless, a pathway involving nitrite/nitrate redox cycles appears to be more consistent with experimental observation than the simple two-step pathway involving cation redox cycles. In particular, the nitrite/nitrate pathway can explain the effect of added NO upon the reaction kinetics and the reported isotopic product composition when unlabeled N₂O reacts over an oxygen-labeled catalyst. Further, a nitrite/nitrate pathway is consistent with the steady-state kinetics as well as published thermal desorption and infrared spectroscopic results.     

Preparation of acidic or bifunctional catalysts by means of straightforward calcination of as-synthesized [Ga]-ZSM-5 zeolites obtained from alkali-free media. Propane aromatization

Propane aromatization (530°C, 1 atm) was used as a reaction model to evaluate the effect of the calcination temperature on the catalytic properties of an as-synthesized [Ga_1.3]-ZSM-5 zeolite obtained from alkali-free media and calcined at two different temperatures: 530°C (C-530) and 750°C (C-750). Results show that in spite of its lower acidity, C-750 is more active and selective toward aromatics than C-530. This is probably due to the fact that at higher temperature the decomposition of organic compounds used during the zeolite synthesis is accompanied by a partial degalliation of the zeolitic support leading to the production of a bifunctional xGa₂O₃ /H-[Ga_y-ZSM-5(2x+y=1.3)catalyst.     

碱土金属修饰的Co/RPSA催化剂催化分解N2O

以RPSA分子筛为载体,采用浸渍法制备了一系列以Co为主活性组分和碱土金属为助剂的CoM/RPSA（M=Mg、Ca、Sr和Ba）催化剂,考察了含硫和含氧条件下直接催化分解N2O的性能。采用X射线衍射、热重-质谱联用系统、NH3-TPD和N2O-TPD等方法对催化剂进行了表征。XRD结果表明,Co物种主要以Co3O4尖晶石形态存在。NH3-TPD和N2O-TPD结果表明,碱土金属的种类影响催化剂酸性和对反应物N2O的吸附-脱附性能,催化剂活性与催化剂酸性及其对反应物N2O的吸附-脱附性能有关。催化剂活性评价结果显示,CoSr/RPSA催化剂的N2O分解活性较好,N2O转化率达到95%时的反应温度为471 ℃。     

Synergy between metals in bimetallic zeolite supported catalyst for NO-promoted N2O decomposition

The detrimental effect of NO on N₂O decomposition over zeolite supported noble metal catalysts can be (partly) eliminated by combining noble metal with iron or cobalt. In the presence of NO, the total conversion of N₂O over these bimetallic-zeolites exceeds the sum of conversions over the monometallic analogues of the individual components. A synergistic effect between the metals is proposed to be responsible for this phenomenon. This synergistic effect gives superior N₂O decomposition activity under realistic conditions, i.e. in the presence of water and NO, at temperatures as low as 350°C. The nature of the synergy is discussed in the light of some preliminary observations.     

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.     

Catalytic Conversion of N2O to N2 over Metal-Based Catalysts in the Presence of Hydrocarbons and Oxygen

The catalytic conversion of N₂O to N₂ in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al₂O₃, SiO₂, TiO₂, ZrO₂ and calcined hydrotalcite MgAl₂(OH)₈·H₂O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on -alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH₄, C₃H₆, C₃H₈) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N₂O to N₂ was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO₂ and H₂O in the feed. A shift of the N₂O conversion versus temperature curve to higher temperatures was observed when SO₂ and H₂O were added, separately or simultaneously, to the feed. The inhibition caused by SO₂ was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H₂O and N₂O on the same active sites.     

甲烷催化还原N_2O杂原子磷酸铝分子筛的制备、修饰及其催化性能

采用水热法合成了杂原子质量分数均为1.8%的FeAlPO-5和CuAlPO-5分子筛,比较了两者对甲烷催化还原氧化亚氮的催化性能,结果表明,FeAlPO-5分子筛的催化性能优于CuAlPO-5分子筛。通过添加少量贵金属(Ag和Pd)可进一步提高FeAlPO-5分子筛的低温活性,其中,Pd/FeAlPO-5分子筛的低温活性优于Ag/FeAlPO-5分子筛。     

AgCl/H-ZSM-5催化分解NO反应及失活机理

采用水热分散法制备了AgCl/H-ZSM-5催化剂，并对其催化分解NO的反应进行了研究。运用X-射线衍射（XRD）、固体魔角旋转核磁共振（MASNMR）和能量色散X-射线微区分析（EDAX）对Ag-Cl/H-ZSM-5催化剂进行表征，着重阐明AgCl/H-ZSM-5催化剂在NO分解反应过程中活性相、载体的结构变化与催化分解NO反应活性的关系。     

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N₂O with C₂H₆ took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N₂O with C₂H₆ over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C₂H₆ by O₂ took place below 350 °C (in C₂H₆/O₂). In the N₂O/C₂H₆/O₂ system, however, it was found that the reaction of C₂H₆ with O₂ was drastically enhanced by the presence of N₂O even at low temperatures (200-300 °C). Therefore, it was concluded that N₂O played an important role in the oxidation of C₂H₆ (i.e., activation of C₂H₆ at an initial step). On the basis of these findings, the mechanism in the SCR of N₂O with C₂H₆ is discussed.     

负载型NiO和CoO催化剂上N2O分解研究

本研究在对多种金属氧化物催化剂进行初步筛选的基础上，研制出以莫来石为载体的负载型NiO、CoO催化剂，考察了分解温度、催化剂组成和负载量对N2O分解率的影响，并对其分解反应动力学进行了研究。结果表明莫来石负载NiO、CoO催化剂对N2O分解有良好的催化性能；其反应速度对N2O均为一级反应；同样负载量下NiO有更好的催化分解活性；这一研究为开发阻力低、催化性能好的工业用蜂窝型规整填料奠定了基础。     

Preparation and Pretreatment Temperature Influence on Iron Species Distribution and N2O Decomposition in Fe–ZSM-5

Fe–ZSM-5 catalysts are prepared by FeCl₃ sublimation between 320 and 850 °C. The catalysts are characterised by XRD, H₂–TPR, NH₃–TPD, NO adsorption by DRIFTs, and catalytic activity is evaluated for N₂O decomposition. The influence of high temperature (850 °C) and pretreatment environment (air, He, He+H₂O and H₂) on the nature of iron species in Fe–ZSM-5 is further investigated by DRIFTs. High temperature FeCl₃ sublimation results in decreased FeO_x formation, easily reducible and narrow distribution of iron species in close proximity to alumina in Fe–ZSM-5. High temperature FeCl₃ sublimation or pretreatment results in isolated hydroxylated iron species, –Fe(OH)₂, which are not significant in Fe–ZSM-5 prepared by 320 °C FeCl₃ sublimation followed by calcination below 600 °C. Fe–ZSM-5 prepared by high temperature FeCl₃ sublimation show high N₂O decomposition activity and the improved performance can be correlated to –Fe(OH)₂ species in close proximity to alumina.     

An investigation into the effect of Cs promotion on the catalytic activity of NiO in the direct decomposition of N2O

A series of Cs promoted NiO catalysts have been prepared and tested for direct decomposition of N₂O. These catalysts are characterized by BET surface area, X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of N₂O (TPD-N₂O) and X-ray photo electron spectroscopy (XPS). The Cs promoted NiO catalysts exhibit higher activity for the decomposition of N₂O compared to bulk NiO. The catalyst with Cs/Ni ratio of 0.1 showed highest activity. The enhancement in catalytic activity of the Cs promoted catalysts is attributed to the change in the electronic properties of NiO. The characterization techniques suggest weakening of Ni–O bond thereby the desorption of oxygen becomes more facile during the reaction. The Cs promoted NiO catalyst is effective at low reaction temperature and also in the presence of oxygen and steam in the feed stream. IICT Communication No: 070523.     

Infrared Evidence for the Existence of Nitrate Species on Cu-ZSM5 During Isothermal Rate Oscillations in the Decomposition of N2O

Isothermal oscillations in the rate of decomposition of N₂O were studied on an over-exchanged Cu-ZSM-5 catalyst by mass spectroscopy and in situ transient FTIR. Oscillations in the production of O₂ and N₂ were observed to occur in a temperature range of 410–490°C at a total pressure of 1.0 Torr pure N₂O. FTIR has provided the first spectroscopic evidence that surface nitrate species are present under oscillatory conditions. This study confirmed a previously proposed model that predicts a slow build-up of surface nitrates, followed by a rapid nitrate decomposition coupled with an increase in the rate of N₂O decomposition. The IR signature of the surface nitrates suggests they are monodentate nitrate species bound to Cu²⁺ ions. Temperature-programmed desorption studies reveal a strong correlation between the stability of the surface nitrate species and the temperature range in which oscillations occur.     

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.     

The desorption of O2 during NO and N2O decomposition on Cu- and Fe-zeolites

The decomposition of NO and of N₂O over a CuZSM-5 zeolite and a Fe-mordenite, respectively, has been studied using tracer techniques. The results demonstrate the high mobility of the lattice oxygen ions in self-diffusion. They afford a possible explanation for the problem of how two extralattice oxygens located at positions remote from each other may combine to form the O₂ molecules which are spontaneously desorbed in these redox reactions. They show that a portion of the lattice oxygen mixes into the O₂ released on decomposition. The data also show that N¹⁸O and N₂ ¹⁸O undergo exchange with the catalyst oxygen under reaction conditions.On leave from Central Research Institute for Chemistry, Hungarian Academy of Sciences, H-1525 Budapest, Hungary.     

NO-Assisted N2O Decomposition over ex-Framework FeZSM-5: Mechanistic Aspects

The decomposition of N₂O over an ex-framework FeZSM-5 catalyst is strongly promoted by NO. Activity data show that the promoting effect of NO is catalytic, and that besides NO₂, O₂ is formed much more extensively in the presence, than in the absence of NO. Transient in situ FT-IR/MS measurements indicate that NO is strongly adsorbed on the catalyst surface up to at least 650 K, showing absorption frequencies at 1884 and 1876 cm^–1. A change in gas phase composition from NO to N₂O results in the formation of adsorbed NO₂, identified by a sharp IR band at 1635 cm^–1. Switching back to the original NO gas phase induces a rapid desorption of NO₂, restoring the original NO absorption frequencies. During the IR measurements, bands typical of nitro- or nitrate groups were not observed. Multi-Track (a TAP-like technique) experiments show that the presence of NO or NO₂ on the catalyst surface significantly enhances the rate of oxygen desorption at the time of N₂O exposure to the catalyst. The spectral changes and transient experiments are discussed and catalytic cycles are proposed, to explain the formation of NO₂ and the (enhanced) formation of oxygen. The latter can be either explained by an indirect effect (electronic, steric) of NO adsorbed on sites neighboring the active sites, or by a direct effect involving reaction of adsorbed NO₂ groups with neighboring oxidized sites yielding O₂.     

The state of adsorbed oxygen species formed in the decomposition of N2O on CaO

Temperature programmed desorption, FT-IR spectra and hydrolysis of adsorbed oxygen species revealed that a considerable amount of adsorbed peroxide species were formed on CaO by decomposition of N₂O, whereas no adsorbed species were formed by molecular oxygen.     

The Synthesis of Cresol from Toluene and N2O on H[Al]ZSM-5: Minimizing the Product Diffusion Limitation by the Use of Small Crystals

The direct hydroxylation of toluene with nitrous oxide to cresol has been studied on two different H[Al]ZSM-5 zeolites with an Si/Al ratio of around 25 and different crystal sizes (30-70 nm and 1-3 m). The samples were activated by calcination and characterized by X-ray diffraction, temperature programmed desorption of ammonia, adsorption of nitrogen and transmission electron microscopy. For the two different crystal sizes, different macroscopic cresol yields and time on stream behaviours are observed. The sample having larger crystals shows a decrease in toluene conversion with increasing reaction temperature. For the smaller crystals an increase in toluene conversion, selectivity to cresol and amount of para-cresol in the cresol fraction with increasing reaction temperature is observed. The para-cresol selectivity is lower on the sample with the longer diffusion path. The findings are explained by product diffusion limitation caused by high reactivity and strong adsorption of the polar product cresol on H[Al]ZSM-5, resulting in a rapid deactivation of the larger crystals.     

Benzene Selective Oxidation with N2O on Fe/MFI Catalysts: Role of Zeolite and Iron Sites on the Deactivation Mechanism

The catalytic performances of Fe-zeolites having MFI structures and in which the Fe introduced either by ion exchange or during the hydrothermal synthesis has undergone partial framework to extra-framework migration induced by controlled heat treatment are reported. In particular, the catalytic behavior as function of time-on-stream and the formation of carbonaceous species were studied. The results suggest that only a small fraction of the iron is active in the selective oxidation of benzene to phenol in the presence of N₂O. It is suggested that the active fraction is formed by isolated iron ions in a pseudo-octahedral configuration with the sites positioned in hydroxyl nests (defects) of the zeolite and is selective in phenol formation as a result of in situ reduction during the catalytic tests. Two possible pathways of carbonaceous species were identified, the first through the intermediate further hydroxylation of phenol and the second through the coupling of phenol with benzene or another phenol molecule. This second pathway is the dominant mechanism of formation of carbonaceous species, although the relative rate of the two pathways depends on the zeolite characteristics and iron loading. It is also suggested that the second pathway depends on the strong chemisorption of phenol, probably on Lewis acid sites, which hinders the fast back-desorption of phenol out from the zeolite channels and thus favors the formation of carbonaceous species. Catalysts prepared by hydrothermal treatment show a lower rate of deactivation than those prepared by ion exchange, although the latter show a comparable productivity to phenol for amounts of iron in extra-framework positions around 20 to 30 times lower. The results also indicate that the presence of Al in the zeolite framework is beneficial for reducing the rate of deactivation as compared to that of Fe-silicalite samples.     

[(C_2H_5)_4N]_2｛Fe_4S_4[S_2CN(C_2H_5)_2]_4｝的晶体和分子结构

本文报导含类立方烷型簇核Ｆｅ４ｓ４的金硫配位化合物［（Ｃ２Ｈ５）４Ｎ］２｛Ｆｅ４Ｓ４［Ｓ２ＣＮ（Ｃ２Ｈ５）２］４｝的合成、晶体和分子结构测定的结果。