共查询到20条相似文献,搜索用时 15 毫秒
1.
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. 相似文献
2.
Takeshi Nobukawa Shin-ichi Tanaka Shin-ichi Ito Keiichi Tomishige Satoshi Kameoka Kimio Kunimori 《Catalysis Letters》2002,83(1-2):5-8
N2O decomposition on an ion-exchanged Fe-MFI catalyst has been studied using an 18O-tracer technique in order to reveal the reaction mechanism. N2
16O was pulsed onto an 18O2-treated Fe-MFI catalyst at 693 K, and the O2 molecules produced were monitored by means of mass spectrometry. The 18O fraction in the produced oxygen had almost half the value of that on the surface oxygen, and 18O18O was not detected. The result shows that O2 formation proceeds via the Eley–Rideal mechanism (N2
16O + 18O(a) N2 + 16O18O). 相似文献
3.
Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La2O3 catalysts. Results of the18O2/16O2 isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La2O3. Steps of18O2 in the presence of a steady flow of methane over Sr/La2O3 catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase18O2. Steps of CO2 over catalysts in which lattice oxygen has been exchanged with18O2, show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen. 相似文献
4.
Deuk Ki Lee 《Catalysis Letters》2005,99(3-4):215-219
For a series of oxidized Cu-ZSM-5 catalysts which were characterized in the catalytic amounts of the oxygen-bridged Cu2+-dimers, [Cu2+–O–Cu2+], activation energies required for the reduction of the Cu2+-dimer species by O2 release were determined using the temperature-programmed experiments of thermal O2 desorption (TPD) and N2O decomposition reaction. The activation energy for the thermal reduction of the Cu2+-dimers during the TPD decreased linearly with increasing molar number of the Cu2+-dimers available on the ZSM-5, suggesting that the energy barrier of the O2 formation via a Langmuir-Hinshelwood (LH) mechanism increased in proportion to the distance between the two Cu2+-dimers in the nearest neighbor. Activation energies of thermal O2 release were comparable to the literature-reported binding energies of the differently spaced Cu2+-dimers. It was also revealed that the activation energy of O2 release during the temperature programmed N2O decomposition reaction over an oxidized catalyst was generally low as compared to that in the TPD, and that the degree of reduction of the Cu2+-dimers was much greater in the N2O decomposition reaction than in the TPD at the same temperatures. These beneficial effects N2O decomposition on the reduction of the Cu2+-dimers were discussed in respect of the removal mechanism of the Cu2+-dimer bridged oxygen. 相似文献
5.
Temperature-programmed desorption (TPD) and oxidation (TPO) were used to investigate the decomposition and oxidation of ethanol on Al2O3, Pd/Al2O3, and PdO/Al2O3. Ethyl--13C alcohol (CH3
13CH2OH) was adsorbed on the catalysts so that reaction pathways of the two carbons could be distinguished. Alumina was mainly a dehydration catalyst, but dehydrogenation was also observed and some carbon remained on the surface. In the presence of O2, A12O3 oxidized the decomposition products and the-carbon was oxidized faster. Ethanol, which was adsorbed on A12O3, decomposed much faster on Pd/A12O3 by diffusing to Pd and undergoing CO elimination to form CH4,13CO, H2, and surface carbon. On PdO/A12O3, the decomposition was slower than on Pd/A12O3 until lattice oxygen was extracted above 450 K; the decomposition products were oxidized by lattice oxygen. In the presence of gas phase O2, Pd/Al2O3 was an active oxidation catalyst at low temperature, but lattice oxygen had to be extracted from PdO/A12O3 before it had significant oxidation activity. 相似文献
6.
Cunningham Joseph Cullinane Dennis Farrell Frank Gibson Colum 《Topics in Catalysis》1999,8(3-4):179-187
Comparisons are made between the catalytic activities of CeO2, Al2O3 and Rh2O3 when pure, or in the case of CeO2 and Al2O3 when promoted by rhodia dispersed thereon, in respect of: (a) activity at 290 K for homomolecular oxygen isotope equilibration
of an equimolar (18O2 + 16O2) probe gas, the so-called R0 process, (b) activity under T-ramp for heterophase oxygen isotope exchange between surface lattice
and O2 highly enriched in 18O, which is shown to occur predominantly by single-stay exchange of both oxygen atoms of dioxygen (the so-called R2 process).
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
7.
《Journal of Adhesion Science and Technology》2013,27(11):1165-1197
The O2 and O2-N2 ([N2] < 15%) post-discharge microwave plasma modifications of high-density polyethylene (HDPE) and hexatriacontane (HTC) surfaces have been studied as a function of the distance from the discharge and the gas composition. They are compared in terms of the in-situ XPS O/C ratios and C 1s components, and the ex-situ ToF-SIMS O-/CH- ratios and relative intensities of series of peaks. The results on the effect of the distance from the discharge showed a clear correlation between the in-situ XPS results and the O2 post-discharge modeling, exhibiting the double role of oxygen atoms: functionalization initialization by creating radicals (which react with molecular oxygen) and degradation due to the energy released by the oxygen atom recombination process. Specific in-situ XPS and ex-situ ToF-SIMS signatures of this in-situ degradation related to the oxygen atom recombination process were exhibited. When N2 was introduced in the plasma gas, the in-situ XPS results and the ex-situ ToF-SIMS results were very different. The in-situ functionalization decreased as a function of the N2 addition and the ex-situ functionalization exhibited a high maximum for the 5% N2-95% O2 post-discharge plasma and then decreased. Despite the absence of a complete O2-N2 post-discharge modeling, it can be assumed that there is also a maximum of the oxygen atom content for the 5% N2-95% O2 post-discharge. Thanks to the in-situ XPS and ex-situ ToF-SIMS specific signatures, it was also shown that this maximum corresponds to a low in-situ degradation effect. Nitrogen introduction could influence the role of oxygen atoms in such a way that there is a decrease in oxygen atom recombination processes (thus in degradation) for small N2 addition and even a decrease in oxygen functionalization initialization for further N2 incorporation in the plasma gas. No nitrogen was observed in the in-situ XPS results, whereas some ex-situ ToF-SIMS nitrogen-containing ions were observed for the O2 and O2-N2 post-discharge. However, their relative intensities followed the variation in oxidation and not the variation in N2 concentration in the plasma gas. They could be related to a post-treatment functionalization effect. Differences observed between HDPE and HTC are explained in terms of their structural differences (desorption of low molecular weight oxygen-containing fragments for HTC). 相似文献
8.
For elucidating the mechanistic aspects of oxygen formation during N2O decomposition over commercial woven Pt–Rh gauze, transient experiments were carried out in the temporal analysis of products (TAP) reactor by pulsing N2 16O over 18O-pretreated gauze catalyst at temperatures typical of industrial ammonia burners (1073–1273 K). The transient responses of N2O and the products of its decomposition (O2 and N2) were fitted to two different mechanistic models. From the isotopic studies and the fitting of transient experiments, two separate routes of oxygen formation during catalytic N2O decomposition have been identified. Oxygen is produced via both (i) interaction of N2O with adsorbed oxygen species formed from N2O and (ii) recombination of adsorbed oxygen species on the catalyst surface. The relative contribution of these reaction pathways depends on the reaction temperature. 相似文献
9.
The effect of O2 and N2O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VOx species (0.79 V/nm2) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O2 with N2O. This is due to suppressing COx formation in the presence of N2O. The most significant improving effect of N2O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N2O were derived from transient experiments using 18O2 traces. 18O16O species were detected in 18O2 and 18O2–C3H8 transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O2 strongly increased. Based on the distribution of labeled oxygen in COx and in O2 as well as on the increased COx formation in sequential O2–C3H8 experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O2 are non-selective ones and responsible for COx formation. These species are not formed from N2O. 相似文献
10.
H. Uetsuka K. Aoyagi S. Tanaka K. Yuzaki S. Ito S. Kameoka K. Kunimori 《Catalysis Letters》2000,66(1-2):87-90
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. 相似文献
11.
The N2O decomposition over an [Fe]-ZSM-5 and an Fe-HZSM-5 zeolite was studied. We found that framework incorporated iron species were much more active than Fe(III) introduced as framework charge countercations by ion exchange (TOF at 0.1 vol% N2O:1.47 × 10–4 at 280°C for [Fe]-ZSM-5 vs. 2.58 × 10–4 at 468°C for Fe-HZSM-5). The higher activity of [Fe]-ZSM-5 was attributed to the uniqueness of framework iron species. Both [Fe]-ZSM-5 and Fe-HZSM-5 zeolites showed enhanced activity in the presence of excess oxygen. This is in sharp contrast to ruthenium exchanged zeolites which showed strong oxygen inhibiting effect on the rate of N2O decomposition. 相似文献
12.
C. Linares M. Mediavilla A.J. Pardey P. Baricelli C. Longo- Pardey S.A. Moya 《Catalysis Letters》1998,51(3-4):183-185
The CO2 reforming of methane over reduced NiO/MgO solid solution catalysts was studied at 800°C by a novel transient method, which
couples a broadened pulse of CH4/CO2 with a step change to the carrier gas and/or with a sharp isotopic pulse of either 18O4, CO18
2 or 13CO16
2. The response curves indicated that two kinds of oxygen were formed over the catalysts during reaction: adsorbed oxygen which
reacts fast with C species and lattice oxygen which reacts more slowly with C species. One concludes that a redox cycle of
lattice oxygen formation through the oxidation of Ni and its reaction with C species takes place on the catalyst surface.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
13.
Nitrous oxide deposits its oxygens on Fe-ferrierites at 200–250 °C in contrast with nitric oxide and nitrogen dioxide. This oxygen is readily exchangeable for 18O2 at room temperature and the reaction proceeds via a single-step exchange mechanism. All three nitrogen oxides in a mixture with 18O labeled dioxygen undergo isotopic exchange (IE) at 200–250 °C, N2O via the same single-step mechanism, while NO and NO2 react via a multiple-step mechanism. Zeolitic oxygens participate in IE above 250 °C during temperature-programmed desorption of surface species formed in the reaction of nitrogen oxides with 18O2. 相似文献
14.
The reduction of V2O5 under electron irradiation was studied by means of electron energy-loss spectroscopy, electron diffraction, and high-resolution imaging. The decrease of spectral intensity of O 1s excitations indicates a preferential removal of oxygen. The observed chemical shifts of the V 2p3/2 and V 2p1/2 peaks reveal that V5+ is reduced to V2+. Electron diffraction and high-resolution imaging show a structural change from the orthorhombic V2O5 to cubic VO. The beam induced reduction is compared with thermal decomposition of V2O5. 相似文献
15.
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. 相似文献
16.
Masakazu Anpo Masaya Matsuoka Kanami Hano Hirotsugu Mishima Takehiko Ono Hiromi Yamashita 《Korean Journal of Chemical Engineering》1997,14(6):498-501
Insitu characterization of Cu+/Y-zeolite catalysts and their photocatalytic reactivities for the decomposition of N2O into N2 and O2 have been investigated by means ofin situ photoluminescence, XAFS, and ESR techniques along with an analysis of the reaction products. It was found that Cu(I) ions
included within the nanopores of Y-zeolite exist as the [Cu(I)--Cu(I)] dimer species as well as the isolated Cu(I) monomer
species, their ratio being much dependent on the SiO2/Al2O3 ratio of Y-zeolite. UV irradiation of these Cu+/Y-zeolite catalysts in the presence of N2O led to the photocatalytic decomposition of N2O into N2 and O2 at temperatures as low as 275 K. The electronically excited state of Cu(I) ion (3d94s1 state) plays a vital role in the photocatalytic decomposition of N2O into N2 and O2. The photocatalytic reactivity of these Cu+/zeolite catalysts was found to be strongly affected by the local structure of the Cu(I) ions which could easily be modified
by changing the SiO2/Al2O3 ratio of Y-zeolite. The isolated linear 2 coordinated Cu(I) monomer species formed on Y-zeolite having a moderate SiO2/ A12O3 ratio exhibited a high photocatalytic reactivity for the direct decomposition of N2O into N2 and O2, clearly showing the importance of the coordinative unsaturation of the active sites. 相似文献
17.
V.A. Kondratenko U. Bentrup M. Richter T.W. Hansen E.V. Kondratenko 《Applied catalysis. B, Environmental》2008,84(3-4):497-504
A mechanistic scheme of N2O and N2 formation in the selective catalytic reduction of NO with NH3 over a Ag/Al2O3 catalyst in the presence and absence of H2 and O2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N2 and N2O are formed via direct or oxygen-induced decomposition of surface NH2NO species. These intermediates originate from NO and surface NH2 fragments. The latter NH2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N2O and O2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgOx particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N2O production in the presence of H2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N2O decomposition. 相似文献
18.
About 50-75% of oxygen captured during decomposition of N2O at 200 °C over Fe-FER (Fe/Al 0.03-0.6) was exchanged by 18O at room temperature. Complete desorption of captured oxygen containing mostly 16O isotope was reached at higher temperature. The 18O deficiency was rationalized by assuming labilization of the framework oxygen in Fe-FER. 相似文献
19.
A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields
of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N2O or O2 as the oxidant. Co-feeding N2O and O2 boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields
of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O2 on the propane ammoxidation with N2O contrasts with other N2O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation
of propane), where a small amount of O2 in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially
AcCN, expressed as mol product h−1 kgcat−1, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our
results open new perspectives to improve the performance of alkane ammoxidation catalysts. 相似文献
20.
In the absence of gas-phase O2, formic acid extracted lattice oxygen from TiO2 during photocatalytic decomposition (PCD) at room temperature. The amount of oxygen extracted was determined by interrupting PCD of a monolayer of formic acid after various reaction times and measuring O2 uptake in the dark. After surface oxygen was depleted by PCD, oxygen diffused from the bulk to replenish the surface oxygen vacancies. The rate of oxygen diffusion to the surface was determined by measuring O2 uptake after various dark times. A small fraction of the CO2 that formed during PCD remained on the reduced sites of the TiO2 surface, but this CO2 was displaced by O2 adsorption at room temperature. 相似文献