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Mechanistic and kinetic aspects of the direct decomposition of N2O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N2O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848K. The transient responses of N2O, N2, and O2 obtained upon N2O 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 N2O decomposition. Oxygen formation occurs via decomposition of bi-atomic (*O2) 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 N2O with iron site possessing mono-atomic oxygen species (*O). Based on our modeling, the recombination of two mono-atomic oxygen (*O) species or direct O2 formation via reaction of N2O with *O can be excluded as potential reaction pathways yielding gas-phase O2. The simulation results predict that the overall rate of N2O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700K. [Copyright &y& Elsevier]

Abstract: Various commercially available zeolite frameworks (MFI, FER, BEA, MOR, FAU) with iron impurities ranging from 85 to 780ppm and nominal Si/Al ratios of 10–15 have been tested in direct N2O decomposition after calcination and steam treatment. Our results show a significant impact of the framework type on the catalytic activity at these high degrees of iron dilution (Fe/Al<0.01). H-beta was the most effective material in the reaction, followed by the other pentasyl-type H-ferrierite and H-ZSM-5 zeolites. [Copyright &y& Elsevier]

Abstract: Transient isotopic studies in the Temporal Analysis of Products (TAP) reactor evidenced a clear mechanistic regularity between methane and propane for N2O reduction over Fe-silicalite in the temperature range of 623–773K. The lifetime of the oxygen species generated by N2O decomposition on extraframework iron sites was found to be crucial for alkane oxidation. When N2O and CH4 or C3H8 were pulsed together over Fe-silicalite, oxygen species deposited on iron sites effectively activated the hydrocarbon. However, these oxygen species become inactive for alkane oxidation if the pulses of N2O and the alkane were separated by more than 0.1s. Transient studies have indicated that C3H8 is more efficient for N2O reduction than CH4, as expected from the facilitated activation of the Con increasing the carbon number in alkanes. The performance derived from the TAP reactor agrees well with experiments under flow conditions at ambient pressure. [Copyright &y& Elsevier]

Abstract: The mechanism of the reaction of N2O and 13CO over Fe-silicalite was investigated with the use of the temporal analysis of products (TAP) reactor and compared with that of the reaction of N2O and C3H8 previously reported (Appl. Catal. A 267 (2004) 181). Upon direct N2O decomposition at 523–573 K, Fe-silicalite stored ca. 1018 atoms of oxygen per gram, with a ratio of 1 O atom per each 30–60 Fe atoms in the sample. Only a small fraction of the deposited oxygen was reactive for CO oxidation. Pump-probe experiments at different time delays (0–2 s) between the pulses of nitrous oxide and the reducing agent indicated the markedly different mechanisms of the N2O–13CO and N2O–C3H8 reactions in the temperature range of 623–673 K. Fe-silicalite is active for propane oxidation in the presence of short-lived oxygen species, that are produced when N2O and C3H8 are pulsed simultaneously. Time delays between the N2O and C3H8 pulses greater than 0.1 s are sufficient to transform these active oxygen species for hydrocarbon conversion into inactive ones. In contrast, the oxidation of CO by N2O does not depend on the lifetime of the oxygen species in the range of time delays investigated. The mechanisms of the N2O-mediated 13CO and C3H8 oxidations differ as a consequence of the different interactions of the two reducing agents with iron species in the zeolite. Pulse experiments support the occurrence of the scavenging mechanism with both propane and carbon monoxide. In this mechanism, short-lived oxygen deposited by N2O is efficiently eliminated by the reductant. Distinctive to propane, the strikingly high affinity of carbon monoxide for isolated Fe3+ ions in the zeolite gives rise to an additional pathway for N2O reduction in the presence of chemisorbed CO species. These particular Fe3+–CO species were identified by in situ UV/vis and EPR spectroscopies (J. Catal. 223 (2004) 13). [Copyright &y& Elsevier]

The relation between the intrinsic mechanism of various N2O conversions over FeMFI catalysts and the nature of the active iron site(s) has been analyzed. To this end, direct N2O decomposition and N2O reduction with CO in the absence or presence of NO were investigated using a combination of transient pulse and steady-state techniques over steam-activated FeMFI zeolites with a similar iron content (0.6–0.7 wt% Fe) and different framework compositions (Si–Al, Si–Ga, Si–Ge, and Si). The forms of iron in the catalysts were characterized by UV/vis and HRTEM. The intrinsic reaction mechanism determines the optimal iron site distribution, which can be modulated by tuning the steaming temperature during activation. Oligonuclear iron oxo clusters in the zeolite channels are essential in direct N2O decomposition due to a faster desorption of O2 as compared to isolated ions. Such forms of active iron can be achieved at a lower steam-activation temperature over FeAlMFI and FeGaMFI (900 K) than over FeGeMFI and FeMFI (1150 K). Contrarily, zeolites with a more uniform distribution of isolated iron species lead to higher activities in N2O reduction with CO as compared to highly clustered catalysts. In this case, O-removal as CO2 is strongly accelerated vis-à-vis O2 desorption in direct N2O decomposition. The dual role of NO as a promotor in N2O decomposition and as an inhibitor in N2O reduction also supports the participation of different sites in both types of conversions. NO selectively inhibits N2O reduction over isolated iron ions, further evidencing the essential role of oligonuclear iron clusters in the NO-assisted N2O decomposition. [Copyright &y& Elsevier]

The effect of the iron distribution in steam-activated FeZSM-5 and Fe-silicalite on their catalytic performance for direct N2O decomposition and for the oxidative dehydrogenation of propane (ODHP) with N2O and O2 was investigated by means of steady-state and transient pulse experiments. Steam-activated FeZSM-5, containing a large fraction of iron as oligonuclear FexOy clusters, is more active and selective in ODHP and direct N2O decomposition than steam-activated Fe-silicalite, where isolated iron species predominate. This suggests that the same type of iron centers is involved in both conversions. The higher activity of steam-activated FeZSM-5 is associated to the higher mobility of atomic oxygen species attached to oligonuclear iron species, which lead to a faster oxygen desorption in direct N2O decomposition as well as to a faster transfer to propane in the ODHP reaction. Steam-activated FeMFI zeolites were found to be highly effective catalysts for ODHP with N2O, with initial propene yields up to 24%. Relatively high yields of propionaldehyde are also obtained, which mainly originates from the selective oxidation of propene. The unique ODHP performance of iron zeolites requires the use of N2O as monooxygen donor. Such species cannot be created by O2. Only a fraction of oxygen species deposited over iron sites of FeMFI upon N2O activation was found to be active towards propane oxidation under transient vacuum conditions. This is explained by a fast transformation of short-living highly reactive atomic oxygen species into less reactive ones. [Copyright &y& Elsevier]

The reduction of N2O by CO was investigated over FeMFI zeolites prepared by different methods including sublimation, liquid ion exchange, and hydrothermal synthesis followed by steam activation. This leads to catalysts with different nature and distribution of iron species, as characterized by HRTEM, UV/vis, and EPR techniques. A common denominator in the FeZSM-5 samples is the heterogeneous iron constitution, with a significant degree of clustering in the form of iron oxide particles. Iron clustering was suppressed in steam-activated Fe-silicalite, presenting a remarkable uniform distribution of isolated iron species. In the presence of CO, the conversion of N2O over the catalysts is strongly accelerated with respect to direct N2O decomposition. The reaction rate increases linearly with the molar CO/N2O feed ratio and strongly depends on the preparation method applied. A correlation was found between the fraction of isolated Fe(III) species in the as-prepared catalysts and the activity for N2O reduction with CO. Steam-activated Fe-silicalite, containing mostly isolated iron ions in extraframework positions, shows the highest activity per mole of iron, while the highly clustered liquid-ion-exchanged catalyst presents the lowest activity. In situ UV/vis and EPR studies evidence the participation of mononuclear iron ions in the N2O–CO reaction, and also support the involvement of oligonuclear FexOy species. The reaction mechanism is iron site dependent. Over isolated sites, the reduction of N2O with CO occurs via coordinated CO species on Fe3+ ions. The reaction over oligonuclear sites proceeds via a redox Fe3+/Fe2+ process with intermediate formation of O− radicals. [Copyright &y& Elsevier]

Preparation of steam-activated Fe-silicalite containing mainly isolated iron species in extraframework positions was essential for deriving structure-activity relationships in various N2O conversion reactions over iron zeolite catalysts. Characterization by UV–Vis/DRS revealed that any significant clustering of iron did not occur in this sample. Other steam-activated FeMFI zeolites, with different framework compositions or treated at higher temperatures, showed various degrees of clustering. The activity of the cluster-free Fe-silicalite was significantly higher in N2O reduction with C3H8 and CO. However, some level of association of iron species leads to higher activities in direct N2O decomposition. Due to the intrinsic reaction mechanism, this result demonstrates the sensitivity of reactions for the form of the iron species in Fe-zeolites, rather than the existence of a unique active site. [Copyright &y& Elsevier]

Carbon templating was used to synthesize high-surface area LaFeAl11O19 hexaaluminates leading to improved catalytic performance. The oxide precursor–template composites were prepared by impregnation, complexation with citric acid, and coprecipitation routes varying the amounts of acetylene black incorporated. Calcination in static air at 1473 K led to single-phased hexaaluminates. The samples were characterized at different stages of the synthetic protocol by ICP–OES, in situ and ex situ XRD, TEM, N2 adsorption, He pycnometry, TGA, H2-TPR, and XPS. The templating effect of carbon originates fine oxide particles. Consequently, the specific surface area of the non-templated hexaaluminates (1–11 m2 g−1 depending on the preparation method) increased up to a factor of 25 upon incorporation of carbon in the synthesis. The stabilization of small particles occurs despite the temperature difference between the elimination of carbon (973 K) and the attainment of the hexaaluminate phase above 1373 K. The morphology of the oxide obtained upon carbon removal determines the textural properties of the hexaaluminate particles. Our results demonstrate that the confined space synthesis is not exclusively responsible for small oxide particles. The stabilization of small crystallites on the carbon surface in the composite and breakage of the oxide layer caused by template combustion are key aspects for attaining finely dispersed particles when the carbon is macroporous. The enhanced catalytic activity and time-on-stream stability of the templated hexaaluminates was demonstrated in the direct N2O decomposition using model and simulated feed mixtures. In this application, a quasi-linear relation between the reaction rate and the specific surface area of the samples was obtained. Characterization by H2-TRP and XPS indicated that the nature of iron in LaFeAl11O19 was not altered by carbon incorporation in the synthesis. [Copyright &y& Elsevier]

Abstract: The mechanisms of direct N2O decomposition and N2O reduction by H2 or NH3 on RuO2 has been studied by means of steady-state catalytic tests at ambient pressure, transient experiments in a temporal analysis of products (TAP) reactor, and density functional theory (DFT) simulations. Bulk RuO2 is very active for N2O decomposition into N2 and O2, achieving full conversion at 723K. According to the DFT calculations, coordinatively unsaturated ruthenium (Rucus) sites are active for the decomposition and oxygen elimination from the surface is rate-limiting step. When pre-reducing RuO2 with H2, the de-N2O activity in the low-temperature region increases. This is attributed to the higher reactivity of surface oxygen vacancies created by H2 at bridge sites compared to Rucus sites. Co-feeding of hydrogen or ammonia with nitrous oxide strongly accelerates the rate of N2O elimination. TAP studies show that oxygen species formed upon N2O decomposition over RuO2 react with NH3 and H2 yielding N2, NO, N2O, and H2O. Both TAP experiments and DFT simulations show that the N-containing intermediates coming from ammonia adsorb much stronger on the catalyst surface than H-containing intermediates coming from hydrogen. This causes blockage of active sites for N2O decomposition by the former species, accounting for the higher efficiency of H2 as reductant for N2O. The presence of O2 in the feed cancels the reducing effect of H2 or NH3 due to the more favorable re-oxidation of reduced RuO2 by O2 compared to N2O. [Copyright &y& Elsevier]

Abstract: Mechanistic and kinetic aspects of direct N2O decomposition over BaFeAl11O19 hexaaluminate were investigated in the Temporal Analysis of Products (TAP) reactor and compared with those previously determined for Fe-MFI zeolites. The catalysts were chosen due to their de-N2O operation in significantly different temperature regimes. Several micro-kinetic models were evaluated for describing the transient responses of N2O, N2, and O2 obtained in N2O pulse experiments at 823–973K. Thorough discrimination between these models enabled us to conclude that the preferred models of N2O decomposition over BaFeAl11O19 and Fe-MFI zeolites differ in the reaction pathways leading to O2 and N2. Gas-phase N2 and O2 are simultaneously formed over BaFeAl11O19 upon interaction of gas-phase N2O with a bi-atomic surface oxygen (*–O2) species. Contrarily, the formation of O2 over Fe-MFI occurs via a sequence of three elementary heterogeneous steps and limits the overall rate of N2O decomposition. Despite the easy O2 formation, BaFeAl11O19 is less active for N2O decomposition below 973K than the Fe-MFI zeolites due to the low coverage by *–O2. According to our quantitative micro-kinetic analysis, this species is formed when gas-phase N2O reacts with a mono-atomic oxygen (*–O) species. This reaction pathway is strongly influenced by the degree of isolation of iron species. The higher the degree of iron isolation in the catalyst, the lower the de-N2O activity. [ABSTRACT FROM AUTHOR]

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low (<550 K) and high (>650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N2O, N2, and O2. Classical reaction schemes failed to describe the experimental data. The preferred models of N2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N2O participate in the decomposition of gas-phase N2O. Gas-phase O2 is formed directly on N2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N2O adsorption and, most importantly, the faster desorption of O2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N2O activity. [Copyright &y& Elsevier]

Abstract: The effect of reaction conditions and rhodium content in H2-pretreated Pt–Rh materials on their initial activity and time-on-stream behavior in direct N2O decomposition at 1023K was investigated by means of continuous-flow catalytic tests at ambient pressure and X-ray photoelectron spectroscopy (XPS) analysis before and after catalytic tests. Reduced pure Pt (Pt100) gauze shows a slightly higher initial activity (after 10min on stream) in the reaction than reduced pure Rh (Rh100) wire. The initial N2O conversion over various Pt–Rh alloys (95–5, 90–10, and 80–20) was considerably lower than that of the pure metals. The following order of initial activity was obtained: Pt100 >Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20. However, Pt100 gauze rapidly deactivates with time-on-stream. This deactivation is tentatively attributed to the penetration of oxygen (generated upon N2O decomposition) into the bulk of the metal. In contrast to Pt100 gauze, Rh100 wire activates with time-on-stream leading to a significantly enhanced and stable N2O conversion. The activity trend after 900min on stream is changed to: Rh100 >Pt95–Rh5 >Pt90–Rh10 >Pt80–Rh20 >Pt100. The different catalytic performance of Rh is related to the gradual transformation of metallic Rh to Rh2O3 under reaction conditions as concluded from surface composition obtained by XPS analyses. Our results indicate that the Pt–Rh alloy gauzes are inactive for direct N2O decomposition in the presence of excess oxygen, as typically found in ammonia burners. [Copyright &y& Elsevier]