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1.
Monolithic catalysts based on Rh/TiO 2–sepiolite were developed and tested in the decomposition of N 2O traces. Several effects such as the presence of NO, O 2 and NO + O 2 in the gas mixture, the catalysts pre-treatment and the metal loading were evaluated. The system was extremely sensitive to the amount of rhodium, passing through a maximum in the catalytic activity at a Rh content of 0.2 wt.%. It has been demonstrated that both NO and O 2 compete for the same adsorption sites as N 2O; however, this effect was not as severe as for other previously reported Rh systems. For NO + O 2 gas mixtures the inhibition effect was stronger than when only NO or O 2 was present. Analysis of the pre-reduced sample by XPS showed Rh mainly in the metal state, even after treatment with N 2O + O 2 mixtures, suggesting that the oxygen consumption observed in the Temperature Programmed Reaction experiments was related to the oxygen uptake by vacancies in the support. The presence of sepiolite in the support preparation and its role as a matrix over which TiO 2 particles were distributed, seems to play an important effect in the migration process of oxygen species through the support vacancies. The Rh/TiO 2 monolithic system is an attractive alternative for the elimination of N 2O traces from stationary sources due to the combination of high catalytic activity with a low pressure drop and optimum textural/mechanical properties. 相似文献
2.
Sharp NO and O 2 desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H 2 treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N 2O treatment. The amounts of O 2 and NO desorption were dependent on the pretreatment pressure of N 2O in the H 2 and N 2O treatment. The adsorbed species could be regenerated by the H 2 and N 2O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N 2O with CH 4. However, the reaction rate of CH 4 activation by the adsorbed species formed after the H 2 and N 2O or the HT and N 2O treatment was not so high as that of the CH 4 + N 2O reaction over the catalyst after O 2 treatment. The simultaneous presence of CH 4 and N 2O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N 2O dissociation can activate CH 4 in the SCR of N 2O with CH 4. 相似文献
3.
The reaction pathways of N 2 and N 2O formation in the direct decomposition and reduction of NO by NH 3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH 3–NO interaction, N 2O formation was most marked between 573 and 673 K, whereas N 2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N 2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide. 相似文献
4.
Direct decomposition of nitrous oxide (N 2O) on K-doped Co 3O 4 catalysts was examined. The K-doped Co 3O 4 catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co 3O 4 catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co 3O 4 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 Co 2+ species from the Co 3+ species formed by oxidation of Co 2+ with the oxygen atoms formed by N 2O decomposition was promoted by the addition of K to the Co 3O 4 catalyst. 相似文献
5.
The catalytic reduction of N 2O by CH 4, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N 2O and the reducing agents, the gas-hourly space velocity, and the presence of O 2 on the catalytic performance were studied in the temperature range of 475–850 K. The CH 4 is more efficient than CO for N 2O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N 2O reduction by CH 4 was very similar to that of direct N 2O decomposition (140 kJ mol −1), being much lower for the N 2O reduction by CO (60 kJ mol −1). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N 2O + CH 4 + CO system did not affect the N 2O conversion with respect to the binary N 2O + CH 4 system, indicating that CO does not interfere in the low-temperature reduction of N 2O by CH 4. In the ternary system, CO contributed to N 2O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N 2O with CH 4, CO, and their mixtures were not altered upon adding excess O 2 in the feed. 相似文献
6.
Catalytic partial oxidation of methane to synthesis gas over ZrO 2 and yttrium-stabilized zirconia (YSZ) is studied using O 2 and N 2O as oxidants. ZrO 2 is much more active than YSZ in oxidation of methane with N 2O. In contrast, YSZ is significantly more active than ZrO 2 when O 2 is used as an oxidant. The presence of O 2 does not influence the rate of N 2O decomposition over ZrO 2 and YSZ, while the presence of H 2O in the system decreases N 2O conversion significantly. O 2 and N 2O are activated at different active sites. Y-induced oxygen vacancies are active for O 2 activation, whereas oxygen co-ordinatively unsaturated Zr cations (Zr-CUS) located at corners, edges, steps and kinks are responsible for N 2O activation. These sites are also capable of dissociating H 2O, resulting in competition between H 2O and N 2O. As compared with N 2O, molecular O 2 is easier to be activated over YSZ and ZrO 2. 相似文献
7.
Transient isotopic studies in the temporal analysis of products (TAP) reactor evidenced the importance of the lifetime of oxygen species generated upon N 2O decomposition on extraframework iron sites of Fe-silicalite for methane oxidation at 723 K. Fe-silicalite effectively activates CH 4 when N 2O and CH 4 are pulsed together in the reactor. However, these oxygen species gradually become inactive for methane oxidation as the time delay between the N 2O and CH 4 pulses is increased from 0 to 2 s. 相似文献
8.
Various spinel-type catalysts AB 2O 4 (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 N 2O to N 2 and O 2 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 MgCo 2O 4 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. 相似文献
9.
A series of La(Co, Mn, Fe) 1−x(Cu, Pd) xO 3 perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N 2 adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O 2, NO + O 2, C 3H 6, in the absence or presence of 5% H 2O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C 3H 6, 1% O 2, 0 or 10% H 2O, and 50,000 h −1 space velocity. The objective was to investigate the influence of H 2O addition on catalytic behavior. A good performance (100% NO conversion, 77% N 2 yield, and 90% C 3H 6 conversion) was achieved at 600 °C over LaFe 0.8Cu 0.2O 3 under a dry feed stream. With the exposure of LaFe 0.8Cu 0.2O 3 to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N 2 yield, and 86% C 3H 6 conversion. A competitive adsorption between H 2O vapor with O 2 and NO molecules at anion vacancies over LaFe 0.8Cu 0.2O 3 was found by means of TPD studies here. A deactivation mechanism was therefore proposed involving the occupation of available active sites by water vapor, resulting in an inhibition of catalytic activity in C 3H 6 + NO + O 2 reaction. This H 2O deactivation was also verified to be strictly reversible by removing steam from the feed. 相似文献
10.
Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N 2O reduction with NH 3 was investigated, and the results indicated that the turnover frequency (TOF) was almost constant in the Fe/Al range between 0.05 and 0.40. The activity of N 2O + NH 3 reaction was much lower than that of N 2O + CH 4 reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH 3 decreased drastically the activity of N 2O + CH 4 reaction. The temperature-programmed desorption (TPD) of NH 3 showed the formation of stronger acid sites on Fe-MFI (Fe/Al = 0.24 and 0.40), and the amount of the acid sites agrees well with the desorption amount O 2 in O 2-TPD in the low temperature range. The acid sites gave a 3610 cm −1 peak (Brønsted acid) in FTIR observation. These results suggest that the acid sites were formed on the bridge oxide ions in binuclear Fe species. Adsorbed NH 3 on the strong acid sites inhibited N 2O dissociation, which can be related to the low activity of N 2O + NH 3 reaction over Fe-MFI with high Fe loading. 相似文献
11.
The kinetics of the reaction of oxidation of ammonia on polycrystalline copper, has been investigated, in a re-circulating reactor, and correlated to a characterisation of the catalyst surface at different extent of conversions. The rapid formation of a nitride or oxynitride phase and its reactivity have been demonstrated. Under oxidising conditions, PNH3=PO2, and up to 650 K, dinitrogen is the only product of the reaction; N2O being formed when T or PO2 increases further. The correlation of these kinetics results to an in situ characterisation of the same reaction at RT by Fourier-transformed infrared reflection-absorption spectroscopy (FT-IRAS), on a well defined Cu(1 1 0) surface, led to the following conclusion: two reaction pathways contribute to the conversion of ammonia: (i) its decomposition on copper; (ii) the reaction between ammonia molecules and oxygen adsorbed from the gas phase. The major adsorbed species is oxygen; intermediate species are NH2, NH and possibly HNO formed when the oxygen surface concentration is sufficient. Increasing the pressure of oxygen induces, at high T, the formation of nitrous oxide; N2O results from an oxidation of the surface copper nitride or from the interaction of two surface HNO intermediates. 相似文献
12.
Cu-ZSM-5 and Cu-AlTS-1 catalysts were prepared by solid state ion exchange and studied in DeNO x reactions. A NO 3 type surface complex was found to be an active intermediate in the decomposition of NO and N 2O. Copper was oxidized to Cu 2+ in the decomposition reactions. Oscillations at full N 2O conversion were observed in the gas phase O 2 concentration, without any change in the N 2 concentration. The oscillation was synchronized by gas phase NO formed from the NO 3 complex. The same complex seems to be an active intermediate also in NO selective catalytic reduction (SCR) by methane, whereas carbonaceous deposits play a role in NO SCR by propane. TPD reveals that only 10–20% of the total copper in the zeolites participates in the catalytic cycles. 相似文献
13.
This study aims at synthesizing a new by substituting 1 atom% Pd 2+ in ionic state in TiO 2 in the form of Ti 0.99Pd 0.01O 1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O 2 and NO reduction by CO was proposed. The model based on CO adsorption on Pd 2+ and dissociative chemisorption of O 2 in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd 2+, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N 2, defined as the ratio of the formation of N 2 and formation of N 2 and N 2O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N 2O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction. 相似文献
14.
N 2O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N 216O was pulsed onto 18O/oxidized Rh catalyst at 220°C and desorbed O 2 molecules ( m/ e=32,34,36) were monitored by means of mass spectrometer. The 18O fraction in the desorbed dioxygen was the same value as that on the surface oxygen. The result shows that the O 2 molecules desorb via Langmuir–Hinshelwood mechanism, i.e., the desorption of dioxygen through the recombination of adsorbed oxygen. On the other hand, TPD measurements in He showed that desorption of oxygen from the Rh black catalyst occurred at the higher temperatures. Therefore, reaction-assisted desorption of oxygen during N 2O decomposition reaction at the low temperature was proposed. 相似文献
15.
The decomposition of ethanol has been investigated on Ar +-sputtered surfaces of a UO 2(1 1 1) single crystal. X-ray photoelectron spectroscopy (XPS) of the U 4f peaks after sputtering for 1 h showed the presence of two distinct oxidation states: U 4+ (U 4f 7/2 at 380.2 eV) and U 0 (U 4f 7/2 at 377.4 eV). Upon ethanol exposure at room temperature, the peak at 377.4 eV was attenuated, indicating that U 0 sites were oxidized to U x+i( x≤4). The presence of a mixture of oxidation states on the surface influenced the reaction products observed during temperature programmed desorption (TPD). While ethylene and acetaldehyde desorbed in one temperature domain (at 560 K) from stoichiometric UO 2(1 1 1), an additional desorption domain (at 475 K) was observed over the substoichiometric surface. The ratio of acetaldehyde to ethylene produced was different in the two temperature domains. While this ratio was near unity for the 560 K domain, it decreased to ca. 0.5 for the 475 K peaks on the substoichiometric surface. The lower temperature reaction channel is likely associated with surface oxygen vacancies, as it leads to greater oxygen abstraction, forming ethylene from surface ethoxide species. 相似文献
16.
We have investigated the catalytic behavior of Pt encapsulated TiO 2 nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO 2 nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH 3) 4](HCO 3) 2 complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO 2 catalysts, and the selectivity for methanol in CO–H 2 reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO 2 (Ti 3+ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti 3+ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H 2 reaction, similar promotion effect of Ti 3+ species may be operating for selective methanol formation by supplying active OH(a). 相似文献
17.
The selective catalytic reduction (SCR) of NO x (NO + NO 2) by NH 3 in O 2 rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO 2 promotes the NO conversion at NO/NO 2 = 1 and low Cu content. This is in agreement with next-nearest-neighbor (NNN) Cu ions as the most active sites and with N xO y adsorbed species formed between NO and NO 2 as a key intermediate. Special attention was paid to the origin of N 2O formation. CuO aggregates form 40–50% of N 2O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N 2O formation above 600 K. This formation is greatly enhanced when NO 2 is present in the feed, and originated from the interaction between NO (or NO 2) and NH 3. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N 2O formation. 相似文献
18.
The nature of the adsorbed species on Cu-ZSM-5 (Cu-Z), Cu-Mordenite (Cu-M), and Cu-Y-zeolite (Cu-Y) was investigated by means of temperature programmed desorption (TPD). When dinitrogen monoxide (N 2O) came into contact with Cu-zeolites above 573 K, the decomposition of N 2O occurred accompanied by the formation of adsorbed oxygen species and adsorbed nitrogen oxide species. In the TPD runs, three O 2 desorption peaks appeared at temperatures of 623, 673, and 753 K and were named -, β-, and γ-peaks, respectively. The O 2 desorption at the - and γ-peaks became quickly saturated after contacting N 2O at 598 K, while the amount of O 2 desorbed at the β-peak increased with time, not reaching a constant level until 120 min of exposure. The activity for the decomposition of N 2O decreased with the accumulation of β-oxygen over the catalyst. The rate of N 2O decomposition depended upon the nature and amount of the copper zeolite catalysts available, as determined by the formation of - and/or β-oxygen. 相似文献
19.
Mechanistic and kinetic aspects of the direct decomposition of N 2O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N 2O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848 K. The transient responses of N 2O, N 2, and O 2 obtained upon N 2O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (*O) species are active for N 2O decomposition. Oxygen formation occurs via decomposition of bi-atomic (*O 2) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O*O), which is initially formed via interaction of N 2O with iron site possessing mono-atomic oxygen species (*O). Based on our modeling, the recombination of two mono-atomic oxygen (*O) species or direct O 2 formation via reaction of N 2O with *O can be excluded as potential reaction pathways yielding gas-phase O 2. The simulation results predict that the overall rate of N 2O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700 K. 相似文献
20.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
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