Role of Zeolitic Oxygens During the Decomposition of 15N 2 18 O Over Fe-ferrierite

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.     

Isotopic Study of N2O Decomposition on an Ion-Exchanged Fe-Zeolite Catalyst: Mechanism of O2 Formation

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).     

Transient and isotopic studies of the oxygen transport and exchange during oxidative coupling of methane on Sr promoted La2O3

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.     

Kinetic evaluation of mechanistic models for O2 release from ZSM-5-supported [Cu2+ –O–Cu2+] ions by thermal reduction or chemical interaction with impinging N2O molecules

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.     

Decomposition and oxidation of CH3 13CH2OH on A12O3, Pd/Al2O3, and PdO/Al2O3 catalysts

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.     

Oxygen isotope equilibration and exchange as probes for differing dioxygen interactions with pure and rhodia-promoted CeO2 and Al2O3

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.     

Time-of-flight SIMS and in-situ XPS study of O2 and O2-N2 post-discharge microwave plasma-modified high-density polyethylene and hexatriacontane surfaces

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).     

Transient Studies of Direct N2O Decomposition over Pt–Rh Gauze Catalyst. Mechanistic and Kinetic Aspects of Oxygen Formation

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.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

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 VO_x 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 CO_x 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 CO_x and in O₂ as well as on the increased CO_x 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 CO_x formation. These species are not formed from N₂O.     

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.     

N2O decomposition over [Fe]-ZSM-5 and Fe-HZSM-5 zeolites

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.     

Homogeneous reduction of nitrobenzene to aniline under CO/H2O, catalyzed by cis-[Rh(CO)2(amine)2]PF6. The role of the amine effect

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.     

Comparison of 18O Isotopic Exchange After and During the Interaction of N2O, NO, and NO2 with Fe Ions in Ferrierites

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.     

Electron Beam Induced Reduction of V2O5 Studied by Analytical Electron Microscopy

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 2p_3/2 and V 2p_1/2 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.     

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.     

Photocatalytic decomposition of N2O on Cu+/Y-zeolite catalysts prepared by ion-exchange

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.     

Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2

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 AgO_x 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.     

Decomposition of Nitrous Oxide Over Fe-Ferrierites. Effect of Deposited Oxygen on the Framework Oxygens Studied by 18O Isotopic Exchange

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.     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

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 kg_cat⁻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.     

Transient measurements of lattice oxygen in photocatalytic decomposition of formic acid on TiO2

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.