Effect of different oxygen species originating from the dissociation of O2, N2O and NO on the selectivity of the Pt-catalysed NH3 oxidation

An effect of oxygen species formed from O₂, N₂O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O₂, N₂O and NO) and NH₃. In contrast to adsorbed oxygen species formed from NO, those from O₂ and N₂O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O₂, N₂O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O₂ and N₂O rather than from NO. These species may be of bi-atomic nature.     

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.     

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.     

Kinetic analysis of N2O decomposition over calcined hydrotalcites

Kinetics of N₂O decomposition over catalyst prepared by calcination of Co–Mn hydrotalcite was examined in integral fixed-bed reactor () at various N₂O and O₂ initial partial pressure at temperature range of 330–450 °C. Kinetic data were evaluated by linear and non-linear regression method, 15 kinetic expressions were tested. Based on the obtained results a redox model of N₂O decomposition was proposed. At low pressures of O₂, adsorbed oxygen is formed by the N₂O decomposition; the N₂O chemisorption is considered as the rate-determining step. On the contrary, at high O₂ pressure it could be assumed that adsorbed oxygen species appear as a result of O₂ adsorption and the Eley–Rideal mechanism is the rate determining. N₂O decomposition is well described by the 1st rate law at N₂O and O₂ concentrations typical for waste gases.     

Nitrous oxide reduction with ammonia over Co---MgO catalyst and the influence of excess oxygen

N₂O decomposition on Co---MgO was studied under high (6.67 kPa) and low (75 Pa) N₂O pressure and the effect of reductant (C₂H₆, NH₃) was studied. The activity decays because of the strong adsorption of oxygen produced, while the reductant removed the adsorbed oxygen giving a steady catalysis. The reaction between NH₃ and excess amount of O₂ produced N₂O as a main product. Although N₂O gives the same intermediate (O) as O₂ does, the former decomposition seems to proceed faster than the latter on Co---MgO. The reaction mechanism studied here was compared with the SCR (NO---NH₃ in O₂) reaction on V₂O₂-TiO₂. Since NH₃, N₂O and O₂ gives only N₂ and water, Co---MgO can be a possible catalyst used in the boiler exhaust to reduce N₂O concentration by adding ammonia.     

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.     

Conversion of N2O to N2 on TiO2(1 1 0)

In this study, we examine the interaction of N₂O with TiO₂(1 1 0) in an effort to better understand the conversion of NO_x species to N₂ over TiO₂-based catalysts. The TiO₂(1 1 0) surface was chosen as a model system because this material is commonly used as a support and because oxygen vacancies on this surface are perhaps the best available models for the role of electronic defects in catalysis. Annealing TiO₂(1 1 0) in vacuum at high temperature (above about 800 K) generates oxygen vacancy sites that are associated with reduced surface cations (Ti³⁺ sites) and that are easily quantified using temperature programmed desorption (TPD) of water. Using TPD, X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), we found that the majority of N₂O molecules adsorbed at 90 K on TiO₂(1 1 0) are weakly held and desorb from the surface at 130 K. However, a small fraction of the N₂O molecules exposed to TiO₂(1 1 0) at 90 K decompose to N₂ via one of two channels, both of which are vacancy-mediated. One channel occurs at 90 K, and results in N₂ ejection from the surface and vacancy oxidation. We propose that this channel involves N₂O molecules bound at vacancies with the O-end of the molecule in the vacancy. The second channel results from an adsorbed state of N₂O that decomposes at 170 K to liberate N₂ in the gas phase and deposit oxygen adatoms at non-defect Ti⁴⁺ sites. The presence of these O adatoms is clearly evident in subsequent water TPD measurements. We propose that this channel involves N₂O molecules that are bound at vacancies with the N-end of the molecule in the vacancy, which permits the O-end of the molecule to interact with an adjacent Ti⁴⁺ site. The partitioning between these two channels is roughly 1:1 for adsorption at 90 K, but neither is observed to occur for moderate N₂O exposures at temperatures above 200 K. EELS data indicate that vacancies readily transfer charge to N₂O at 90 K, and this charge transfer facilitates N₂O decomposition. Based on these results, it appears that the decomposition of N₂O to N₂ requires trapping of the molecule at vacancies and that the lifetime of the N₂O–vacancy interaction may be key to the conversion of N₂O to N₂.     

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.     

Nitrous oxide formation in low temperature selective catalytic reduction of nitrogen oxides with V2O5/TiO2 catalysts

Nitric oxide and nitric dioxide compounds (NO_x) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N₂O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N₂O formation, using V₂O₅/TiO₂ catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N₂O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O₂ (eventually NO₂). When these active sites are in close proximity they can interact to form an N₂O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading.     

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.     

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.     

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.     

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.     

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.     

Catalytic decomposition of N2O over supported Rh catalysts: effects of supports and Rh dispersion

The study of catalytic decomposition of nitrous oxide to nitrogen and oxygen over Rh catalysts supported on various supports (USY, NaY, Al₂O₃, ZrO₂, FSM-16, CeO₂, La₂O₃) showed that the activities of Rh/Al₂O₃ and Rh/USY (ultrastable Y zeolite) catalysts were comparable to or higher than the other catalysts reported in the literatures. The catalytic activity of N₂O decomposition was sensitive not only to the Rh dispersion but also to the preparation variables such as the Rh precursors and the supports used. A pulsed N₂O experiment over a Rh/USY catalyst suggested that the catalytic N₂O decomposition occurs on oxygen-covered surface and that O₂ may be freed on collision of N₂O molecules with the adsorbed oxygen atoms.     

Low temperature decomposition of nitrous oxide over Fe/ZSM-5: Modelling of the dynamic behaviour

The kinetics of N₂O decomposition to gaseous nitrogen and oxygen over HZSM-5 catalysts with low content of iron (<400 ppm) under transient and steady-state conditions was investigated in the temperature range of 250–380 °C. The catalysts were prepared from the HZSM-5 with Fe in the framework upon steaming at 550 °C followed by thermal activation in He at 1050 °C. The N₂O decomposition began at 280 °C. The reaction kinetics was first order towards N₂O during the transient period, and of zero order under steady-state conditions. The increase of the reaction rate with time (autocatalytic behaviour) was observed up to the steady state. This increase was assigned to the catalysis by adsorbed NO formed slowly on the zeolite surface from N₂O. The formation of NO was confirmed by temperature-programmed desorption at temperatures >360 °C. The amount of surface NO during the transient increases with the reaction temperature, the reaction time, and the N₂O concentration in the gas phase up to a maximum value. The maximum amount of surface NO was found to be independent on the temperature and N₂O concentration in the gas phase. This leads to a first-order N₂O decomposition during the transient period, and to a zero-order under steady state. A kinetic model is proposed for the autocatalytic reaction. The simulated concentration–time profiles were consistent with the experimental data under transient as well as under steady-state conditions giving a proof for the kinetic model suggested in this study.     

Study of the reaction of NOx and soot on Fe2O3 catalyst in excess of O2

This study addresses the catalytic reaction of NO_x and soot into N₂ and CO₂ under O₂-rich conditions. To elucidate the mechanism of the soot/NO_x/O₂ reaction and particularly the role of the catalyst -Fe₂O₃ is used as model sample. Furthermore, a series of examinations is also made with pure soot for reference purposes. Temperature programmed oxidation and transient experiments in which the soot/O₂ and soot/NO reaction are temporally separated show that the NO reduction occurs on the soot surface without direct participation of the Fe₂O₃ catalyst. The first reaction step is the formation of CC(O) groups that is mainly associated with the attack of oxygen on the soot surface. The decomposition of these complexes leads to active carbon sites on which NO is adsorbed. Furthermore, the oxidation of soot by oxygen provides a specific configuration of active carbon sites with suitable atomic orbital orientation that enables the chemisorption and dissociation of NO as well as the recombination of two adjacent N atoms to evolve N₂. Moreover, carbothermal reaction, high resolution transmission electron microscopy and isotopic studies result in a mechanistic model that describes the role of the Fe₂O₃ catalyst. This model includes the dissociative adsorption of O₂ on the iron oxide, surface migration of the oxygen to the contact points of soot and catalyst and then final transfer of O to the soot. Moreover, our experimental data suggest that the contact between both solids is maintained up to high conversion levels thus resulting in continuous oxygen transfer from catalyst to soot. As no coordinative interaction of soot and Fe₂O₃ catalyst is evidenced by diffuse reflectance infrared Fourier transform spectroscopy a van der Waals type interaction is supposed.     

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.     

Contribution to the mechanism of NO reduction by CO over Pt/NaX zeolite

The reaction of NO + CO was studied over Pt/NaX prepared by the decomposition of [Pt(NH₃)₄]²⁺. The decomposition was carried out via calcination followed by reduction, by vacuum decomposition, and by decomposition in hydrogen, by ways which are known to lead to the formation of Pt clusters of different sizes and location. The NO reduction by CO was studied under static conditions for longer (20–30 min) and shorter (100 s) time intervals, and the reaction was followed by temperature programmed decomposition (TPD) of species adsorbed during the preceding isothermal reactions. The effect of various NO/CO ratios and of added oxygen was examined. The reactions of N₂O + CO were compared with those of NO + CO. The increasing size of Pt clusters enhances the reduction of NO by CO, but it is complicated at lower reaction temperatures (below 230°C) by the poisoning of active Pt centres, especially by adsorbed CO. Smaller Pt clusters exhibit higher preference towards NO adsorption from NO + CO mixtures than the larger Pt clusters. The incomplete reduction of NO to N₂O proceeds under our experimental conditions below 230°C, and is accompanied by the formation of adsorbed species. N₂O formation is enhanced by the increased NO/CO ratio and by the addition of oxygen. The reduction of nitrous oxide occurs much slower than that of nitric oxide, and therefore N₂O could play a role only as a surface intermediate in the CO + NO reaction.     

Low-pressure chemical and photochemical reactions of oxides of nitrogen on alumina taken as a model substance for mineral dust in relation to air pollution

The interaction of γ-Al₂O₃, taken as a model substance of tropospheric mineral dust, with N₂O, NO and NO₂ has been studied using kinetic and temperature-programmed desorption (TPD) mass-spectrometry in presence and absence of UV irradiation. At low surface coverages (<0.001 ML), adsorption of N₂O and NO₂ is accompanied by dissociation and chemiluminescence, whereas adsorption of NO does not lead to appreciable dissociation. Upon UV irradiation of Al₂O₃ in a flow of N₂O, photoinduced decomposition and desorption of N₂O take place, whereas in a flow of NO, only photoinduced desorption is observed. Dark dissociative adsorption of N₂O and NO and photoinduced N₂O dissociation apparently occur by a mechanism involving electron capture from surface F- and F⁺-centers. Photoinduced desorption of N₂O and NO may be associated with decomposition of complexes of these molecules with Lewis acid sites, V-centers or OH-groups. TPD of N₂O and NO proceeds predominantly without decomposition, while NO₂ partially decomposes to NO and O₂.