Development of a new Rh/TiO2–sepiolite monolithic catalyst for N2O decomposition

Monolithic catalysts based on Rh/TiO₂–sepiolite were developed and tested in the decomposition of N₂O traces. Several effects such as the presence of NO, O₂ and NO + O₂ 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 O₂ compete for the same adsorption sites as N₂O; however, this effect was not as severe as for other previously reported Rh systems. For NO + O₂ gas mixtures the inhibition effect was stronger than when only NO or O₂ was present. Analysis of the pre-reduced sample by XPS showed Rh mainly in the metal state, even after treatment with N₂O + O₂ 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 TiO₂ particles were distributed, seems to play an important effect in the migration process of oxygen species through the support vacancies. The Rh/TiO₂ monolithic system is an attractive alternative for the elimination of N₂O traces from stationary sources due to the combination of high catalytic activity with a low pressure drop and optimum textural/mechanical properties.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ 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 H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

Mechanistic insights into the formation of N2O and N2 in NO reduction by NH3 over a polycrystalline platinum catalyst

The reaction pathways of N₂ and N₂O formation in the direct decomposition and reduction of NO by NH₃ 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 NH₃–NO interaction, N₂O formation was most marked between 573 and 673 K, whereas N₂ 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. N₂O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide.     

Potassium-doped Co3O4 catalyst for direct decomposition of N2O

Direct decomposition of nitrous oxide (N₂O) on K-doped Co₃O₄ catalysts was examined. The K-doped Co₃O₄ catalyst showed a high activity even in the presence of water. In the durability test of the K-doped Co₃O₄ catalyst, the activity was maintained at least for 12 h. It was found that the activity of the K-doped Co₃O₄ 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 Co²⁺ species from the Co³⁺ species formed by oxidation of Co²⁺ with the oxygen atoms formed by N₂O decomposition was promoted by the addition of K to the Co₃O₄ catalyst.     

Catalytic reduction of N2O over steam-activated FeZSM-5 zeolite: Comparison of CH4, CO, and their mixtures as reductants with or without excess O2

The catalytic reduction of N₂O by CH₄, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N₂O and the reducing agents, the gas-hourly space velocity, and the presence of O₂ on the catalytic performance were studied in the temperature range of 475–850 K. The CH₄ is more efficient than CO for N₂O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N₂O reduction by CH₄ was very similar to that of direct N₂O decomposition (140 kJ mol⁻¹), being much lower for the N₂O 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 N₂O + CH₄ + CO system did not affect the N₂O conversion with respect to the binary N₂O + CH₄ system, indicating that CO does not interfere in the low-temperature reduction of N₂O by CH₄. In the ternary system, CO contributed to N₂O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N₂O with CH₄, CO, and their mixtures were not altered upon adding excess O₂ in the feed.     

Partial oxidation of methane by O2 and N2O to syngas over yttrium-stabilized ZrO2

Catalytic partial oxidation of methane to synthesis gas over ZrO₂ and yttrium-stabilized zirconia (YSZ) is studied using O₂ and N₂O as oxidants. ZrO₂ is much more active than YSZ in oxidation of methane with N₂O. In contrast, YSZ is significantly more active than ZrO₂ when O₂ is used as an oxidant. The presence of O₂ does not influence the rate of N₂O decomposition over ZrO₂ and YSZ, while the presence of H₂O in the system decreases N₂O conversion significantly. O₂ and N₂O are activated at different active sites. Y-induced oxygen vacancies are active for O₂ activation, whereas oxygen co-ordinatively unsaturated Zr cations (Zr-CUS) located at corners, edges, steps and kinks are responsible for N₂O activation. These sites are also capable of dissociating H₂O, resulting in competition between H₂O and N₂O. As compared with N₂O, molecular O₂ is easier to be activated over YSZ and ZrO₂.     

Mechanistic peculiarities of the N2O reduction by CH4 over Fe-silicalite

Transient isotopic studies in the temporal analysis of products (TAP) reactor evidenced the importance of the lifetime of oxygen species generated upon N₂O decomposition on extraframework iron sites of Fe-silicalite for methane oxidation at 723 K. Fe-silicalite effectively activates CH₄ when N₂O and CH₄ are pulsed together in the reactor. However, these oxygen species gradually become inactive for methane oxidation as the time delay between the N₂O and CH₄ pulses is increased from 0 to 2 s.     

N2O catalytic decomposition over various spinel-type oxides

Various spinel-type catalysts AB₂O₄ (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 N₂O to N₂ and O₂ 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 MgCo₂O₄ 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.     

Water vapor sensitivity of nanosized La(Co, Mn, Fe)1−x(Cu, Pd)xO3 perovskites during NO reduction by C3H6 in the presence of oxygen

A series of La(Co, Mn, Fe)_1−x(Cu, Pd)_xO₃ perovskites having high specific surface areas and nanosized crystal domains was prepared by reactive grinding. The solids were characterized by N₂ adsorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature programmed desorption (TPD) of O₂, NO + O₂, C₃H₆, in the absence or presence of 5% H₂O, Fourier transform infrared (FTIR) spectroscopy, as well as activity tests towards NO reduction by propene under the conditions of 3000 ppm NO, 3000 ppm C₃H₆, 1% O₂, 0 or 10% H₂O, and 50,000 h⁻¹ space velocity. The objective was to investigate the influence of H₂O addition on catalytic behavior. A good performance (100% NO conversion, 77% N₂ yield, and 90% C₃H₆ conversion) was achieved at 600 °C over LaFe_0.8Cu_0.2O₃ under a dry feed stream. With the exposure of LaFe_0.8Cu_0.2O₃ to a humid atmosphere containing 10% water vapor, the catalytic activity was slightly decreased yielding 91% NO conversion, 51% N₂ yield, and 86% C₃H₆ conversion. A competitive adsorption between H₂O vapor with O₂ and NO molecules at anion vacancies over LaFe_0.8Cu_0.2O₃ 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 C₃H₆ + NO + O₂ reaction. This H₂O deactivation was also verified to be strictly reversible by removing steam from the feed.     

The importance of Fe loading on the N2O reduction with NH3 over Fe-MFI: Effect of acid site formation on Fe species

Effect of the loading amount of Fe over ion-exchanged Fe-MFI catalysts on the catalytic performance of N₂O reduction with NH₃ 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 N₂O + NH₃ reaction was much lower than that of N₂O + CH₄ reaction over Fe-MFI (Fe/Al = 0.40), and the preadsorption of NH₃ decreased drastically the activity of N₂O + CH₄ reaction. The temperature-programmed desorption (TPD) of NH₃ 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 O₂ in O₂-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 NH₃ on the strong acid sites inhibited N₂O dissociation, which can be related to the low activity of N₂O + NH₃ reaction over Fe-MFI with high Fe loading.     

Oxidation of NH3 on polycrystalline copper and Cu(1 1 0): a combined FT-IRAS and kinetics investigation

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, P_NH3=P_O2, and up to 650 K, dinitrogen is the only product of the reaction; N₂O being formed when T or P_O2 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.     

DeNOx reactions on Cu-zeolites Decomposition of NO, N2O and SCR of NO by C3H8 and CH4 on Cu-ZSM-5 and Cu-AlTS-1 catalysts

Cu-ZSM-5 and Cu-AlTS-1 catalysts were prepared by solid state ion exchange and studied in DeNO_x reactions. A NO₃ type surface complex was found to be an active intermediate in the decomposition of NO and N₂O. Copper was oxidized to Cu²⁺ in the decomposition reactions. Oscillations at full N₂O conversion were observed in the gas phase O₂ concentration, without any change in the N₂ concentration. The oscillation was synchronized by gas phase NO formed from the NO₃ 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.     

High thermal conductive β-SiC for selective oxidation of H2S: A new support for exothermal reactions

This study aims at synthesizing a new by substituting 1 atom% Pd²⁺ in ionic state in TiO₂ 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 O₂ and NO reduction by CO was proposed. The model based on CO adsorption on Pd²⁺ and dissociative chemisorption of O₂ 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 Pd²⁺, 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 N₂, defined as the ratio of the formation of N₂ and formation of N₂ and N₂O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N₂O + 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.     

Mechanism of N2O decomposition over a Rh black catalyst studied by a tracer method: The reaction of N2O with O(a)

N₂O decomposition on an unsupported Rh catalyst has been studied using tracer technique in order to reveal the reaction mechanism. N₂¹⁶O was pulsed onto ¹⁸O/oxidized Rh catalyst at 220°C and desorbed O₂ 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 O₂ 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 N₂O decomposition reaction at the low temperature was proposed.     

The influence of surface defects on ethanol dehydrogenation versus dehydration on the UO2(1 1 1) surface

The decomposition of ethanol has been investigated on Ar⁺-sputtered surfaces of a UO₂(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: U⁴⁺ (U 4f_7/2 at 380.2 eV) and U⁰ (U 4f_7/2 at 377.4 eV). Upon ethanol exposure at room temperature, the peak at 377.4 eV was attenuated, indicating that U⁰ 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 UO₂(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.     

The CO–H2 and CO–H2O reactions over TiO2 nanotubes filled with Pt metal nanoparticles

We have investigated the catalytic behavior of Pt encapsulated TiO₂ nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO₂ nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH₃)₄](HCO₃)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/TiO₂ catalysts, and the selectivity for methanol in CO–H₂ reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO₂ (Ti³⁺ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti³⁺ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H₂ reaction, similar promotion effect of Ti³⁺ species may be operating for selective methanol formation by supplying active OH(a).     

The origin of N2O formation in the selective catalytic reduction of NOx by NH3 in O2 rich atmosphere on Cu-faujasite catalysts

The selective catalytic reduction (SCR) of NO_x (NO + NO₂) by NH₃ in O₂ rich atmosphere has been studied on Cu-FAU catalysts with Cu nominal exchange degree from 25 to 195%. NO₂ promotes the NO conversion at NO/NO₂ = 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 NO₂ as a key intermediate. Special attention was paid to the origin of N₂O formation. CuO aggregates form 40–50% of N₂O at ca. 550 K and become inactive for the SCR above 650 K. NNN Cu ions located within the sodalite cages are active for N₂O formation above 600 K. This formation is greatly enhanced when NO₂ is present in the feed, and originated from the interaction between NO (or NO₂) and NH₃. The introduction of selected co-cations, e.g. Ba, reduces very significantly this N₂O formation.     

Temperature programmed desorption study of adsorbed species formed by the decomposition of N2O on ion-exchanged copper zeolite catalysts

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 (N₂O) came into contact with Cu-zeolites above 573 K, the decomposition of N₂O occurred accompanied by the formation of adsorbed oxygen species and adsorbed nitrogen oxide species. In the TPD runs, three O₂ desorption peaks appeared at temperatures of 623, 673, and 753 K and were named -, β-, and γ-peaks, respectively. The O₂ desorption at the - and γ-peaks became quickly saturated after contacting N₂O at 598 K, while the amount of O₂ desorbed at the β-peak increased with time, not reaching a constant level until 120 min of exposure. The activity for the decomposition of N₂O decreased with the accumulation of β-oxygen over the catalyst. The rate of N₂O decomposition depended upon the nature and amount of the copper zeolite catalysts available, as determined by the formation of - and/or β-oxygen.     

Micro-kinetic analysis of direct N2O decomposition over steam-activated Fe-silicalite from transient experiments in the TAP reactor

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 773–848 K. 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 700 K.     

Catalytic decomposition of N2O over CeO2 promoted Co3O4 spinel catalyst

A series of CeO₂ promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N₂O). Addition of CeO₂ to Co₃O₄ led to an improvement in the catalytic activity for N₂O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N₂O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O₂, H₂O or NO. Methods of XRD, FE-SEM, BET, XPS, H₂-TPR and O₂-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO₂ could increase the surface area of Co₃O₄, and then improve the reduction of Co³⁺ to Co²⁺ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N₂O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO₂, are responsible for the enhancement of catalytic activity of Co₃O₄.