Co-ZSM-5 catalysts for N2O decomposition

The catalytic properties of cobalt containing ZSM-5 zeolites prepared by various methods were compared. TPR, XRD, N₂-BET, XPS, FTIR and UV–vis spectroscopy were used for characterizing the samples. Well-dispersed cobalt oxide-like species and isolated Co²⁺ ions in charge compensation positions were found in the zeolite. Catalysts prepared using a single step cation exchange method showed high activity for N₂O decomposition in a temperature range 300–550°C, in the presence of 0–5% O₂, and high stability in the presence of 10% H₂O to the feed. UV–vis spectra and TPR experiments indicated the presence of some cobalt oxides, not detected by DRX, in a Co-ZSM-5 catalyst containing 3.76 wt% Co, prepared by a solid-state reaction procedure. The N₂O conversion over this catalyst was strongly affected by addition of both O₂ and H₂O to the feed.     

Nitrous oxide as an oxidant for ethane oxydehydrogenation

Waste nitrous oxide was used as an oxidant for ethane oxydehydrogenation performed at the range of temperature from 350 to 450 °C over iron modified zeolite catalysts. Different zeolite matrices (zeolite ZSM-5 of different Si/Al ratio, H-Y, mordenite) modified with iron cations introduced into zeolite by means of ionic exchange were applied as catalysts for the reaction under study. Additionally, amorphous silica and alumina silica as well as silicalite of MFI structure were also used as a matrix for iron ions accommodation and they were tested for oxydehydrogenation reaction. It was found that only iron modified zeolites showed activity for reaction under study. Amorphous oxide supports and crystalline neutral silicalite modified with iron cations by means of impregnation were completely inactive for oxydehydrogenation reaction. The best catalytic performance was found on iron modified zeolites of MFI structure. The Si/Al ratio of the ZSM-5 matrix influenced the activity for ethane oxydehydrogenation reaction insignificantly. N₂O oxidant was partly utilized for ethane oxidation (towards ethene or carbon oxides), while some part of the oxidant was decomposed to nitrogen and oxygen. Performing the reaction at 450 °C resulted in a high ethene yield and complete N₂O removal.     

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.     

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.     

NOX removal in excess oxygen by plasma-enhanced selective catalytic reduction

In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO₂. In combination with appropriate catalysts and ammonia (NH₃-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO₂) synergistically to molecular nitrogen.

The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NO_X at low temperatures. Using NH₃ as a reducing agent, NO_X was converted to N₂ on zeolites or NH₃-SCR catalysts like V₂O₅–WO₃/TiO₂ at temperatures as low as 100–200 °C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH₃ stored by ion exchange on the zeolite and for continuous NH₃ supply.

Certain modifications of Al₂O₃ and ZrO₂ have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30 eV/NO-molecule, 500 ppm NO was reduced by more than half at a temperature of 300 °C and a space velocity of 20 000 h⁻¹ at the catalyst. The synergistic combinations of NTP and both NH₃- and HC-SCR have been verified under real diesel engine exhaust conditions.     

NO reduction with NH3 over chromia–vanadia catalysts supported on TiO2: an in situ Raman spectroscopic study

In situ Raman spectroscopy was used for studying the ternary 2% CrO₃–6% V₂O₅/TiO₂ catalyst, for which a synergistic effect between vanadia and chromia leads to enhanced catalytic performance for the selective catalytic reduction (SCR) of NO with NH₃. The structural properties of this catalyst were studied under NH₃/NO/O₂/N₂/SO₂/H₂O atmospheres at temperatures up to 400 °C and major structural interactions between the surface chromia and vanadia species are observed. The effects of oxygen, ammonia, water vapor and sulfur dioxide presence on the in situ Raman spectra are presented and discussed.     

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.     

Formation of nitrogen oxides from fuel-N through HCN and NH3: a model-compound study

The conversion of fuel-nitrogen to HCN and NH₃ and to nitrogen oxides was studied with nitrogen-containing model compounds, chosen to represent the main nitrogen and oxygen functionalities in fossil fuels. Two kinds of experiments were performed in an entrained-flow reactor at 800 °C. The conversion of model-compound-N to HCN and NH₃ was determined under inert conditions, and the formation of NO, N₂O and NO₂ was determined under oxidizing conditions. In inert atmosphere, oxygen-containing functional groups had an important effect on the ratio of HCN to NH₃. In particular, OH groups bound directly in the ring structure increased the conversion of nitrogen to NH₃. In oxidizing atmosphere, the conversions of model-compound-N to N₂O were high, but the substituent groups had no well-defined effect on the ratio of N₂O to NO. The formation of NO₂ was insignificant.     

Selective catalytic reduction of NO by ammonia using mesoporous Fe-containing HZSM-5 and HZSM-12 zeolite catalysts: An option for automotive applications

Mesoporous and conventional Fe-containing ZSM-5 and ZSM-12 catalysts (0.5–8 wt% Fe) were prepared using a simple impregnation method and tested in the selective catalytic reduction (SCR) of NO with NH₃. It was found that for both Fe/HZSM-5 and Fe/HZSM-12 catalysts with similar Fe contents, the activity of the mesoporous samples in NO SCR with NH₃ is significantly higher than for conventional samples. Such a difference in the activity is probably related with the better diffusion of reactants and products in the mesopores and better dispersion of the iron particles in the mesoporous zeolite as was confirmed by SEM analysis. Moreover, the maximum activity for the mesoporous zeolites is found at higher Fe concentrations than for the conventional zeolites. This also illustrates that the mesoporous zeolites allow a better dispersion of the metal component than the conventional zeolites. Finally, the influence of different pretreatment conditions on the catalytic activity was studied and interestingly, it was found that it is possible to increase the SCR performance significantly by preactivation of the catalysts in a 1% NH₃/N₂ mixture at 500 °C for 5 h. After preactivation, the activity of mesoporous 6 wt% Fe/HZSM-5 and 6 wt% Fe/HZSM-12 catalyst is comparable with that of traditional 3 wt% V₂O₅/TiO₂ catalyst used as a reference at temperatures below 400 °C and even more active at higher temperatures.     

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.     

Inhibition effect of H2O on V2O5/AC catalyst for catalytic reduction of NO with NH3 at low temperature

The inhibition effect of H₂O on V₂O₅/AC catalyst for NO reduction with NH₃ is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H₂O does not reduce NO and NH₃ adsorption on V₂O₅/AC catalyst surface, but promotes NH₃ adsorption due to increases in Brønsted acid sites. Many kinds of NH₃ forms present on the catalyst surface, but only NH₄⁺ on Brønsted acid sites and a small portion of NH₃ on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH₃ on Lewis acid sites does not react with NO, regardless the presence of H₂O in the feed gas. H₂O inhibits the SCR reaction between the NH₃ on the Lewis acid sites and NO, and the inhibition effect increases with increasing H₂O content. The inhibition effect is reversible and H₂O does not poison the V₂O₅/AC catalyst.     

The reactions of possible intermediates in the selective catalytic reduction of nitrogen oxides by hydrocarbons

The reactions of formamide and nitromethane, two possible intermediates in the selective catalytic reduction of nitrogen oxides by methane, have been studied over Co-ZSM5, H-ZSM5 and Cu-ZSM5. Formamide, a possible surrogate for nitrosomethane, reacts completely below 250°C over Co-ZSM5 with formation of NH₃ and CO by one route and HCN and H₂O by another. Inclusion of NO causes partial conversion of NH₃ to N₂ at 300°C. H-ZSM5 behaves similarly but with a higher conversion of NH₃ in the presence of NO. Cu-ZSM5 gives CO₂ and N₂ alone, apparently because of its high oxidation activity. The reaction of nitromethane over H-ZSM5 is similar to that previously established for Co-ZSM5 with NH₃ and CO₂ as the initial products and subsequent N₂ formation by the NH₃-SCR reaction. Again N₂ formation is more extensive with H-ZSM5 than Co-ZSM5 when NO is present while Cu-ZSM5 gives only CO₂ and N₂. Deactivation is characteristic of the reaction of nitromethane over all three zeolites at temperatures below ≈280°C with eventual breakthrough of isocyanic acid (HNCO) as a product. In situ FTIR measurements with H-ZSM5 indicate that deactivation is due to reactions of HNCO to form deposits of s-triazine compounds which can be removed by NO₂. The overall conclusion is that nitromethane and formamide, and by inference nitrosomethane, react in ways which are consistent with the possibility that species of these types could be intermediates in the methane-SCR reaction over zeolite catalysts. Distinction between them is possible only with catalysts of low oxidation capability when CO formation is consistent with the involvement of nitrosomethane and CO₂ formation with that of nitromethane.     

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.     

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.     

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.     

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.     

Surface-nitrogen removal in NO and N2O reduction on Pd(1 1 0): Angular distribution studies of desorbing products

This paper is a report of angle-resolved product desorption measurements in the course of catalyzed NO and N₂O reduction on Pd(1 1 0). Surface-nitrogen removal processes show different angular distributions, i.e. normally directed N₂ desorption takes place in process (i) 2N(a) → N₂(g). Highly inclined N₂ desorption towards the [0 0 1] direction is induced in process (ii) N₂O(a) → N₂(g) + O(a). N₂O or NH₃ desorption follows the cosine distribution characterizing the desorption after the thermalization in process (iii) N₂O(a) → N₂O(g) or (iv) N(a) + 3H(a) → NH₃(a) → NH₃(g). Thus, a combination of the angular and velocity distributions provides the analysis of most of surface-nitrogen removal processes in the course of catalyzed NO reduction.

At temperatures below 600 K, processes (ii) and (iii) dominate and process (iv) is enhanced at H₂ pressures higher than NO. Process (i) contributes significantly above 600 K. Only three processes except for NH₃ formation are operative when CO is used. Only process (ii) was observed in a steady-state N₂O + CO (or H₂) reaction.     

Revision of Charnell’s procedure towards the synthesis of large and uniform crystals of zeolites A and X

A critical replication and re-evaluation of Charnell’s procedure to the synthesis of zeolites A and X has been carried out, aiming at reliable protocols for preparation of large and uniform crystals of the respective zeolites in a scale of 50 g per batch. Triethanolamine, as an organic additive to the reacting sodium aluminosilicate hydrogel, increases the viscosity of the system, and reduces the reactivity of aluminium species towards nucleation and crystal growth through forming chelated complex compounds. The reactivity of silicate species has to be accordingly adjusted, by choosing proper source materials. Zeolite A crystals, which possess shapes of edge-truncated cubes, and sizes of 35–40 μm in edge-length, have been synthesized with a starting gel having the composition 1.7Na₂O:Al₂O₃:0.7SiO₂:165H₂O:6.1TEA in 2-l batch-size, using dissolved metallic aluminium and colloidal silica. The crystallization has been accomplished at 85 °C within 21 days. When the gel has a starting composition 2.3Na₂O:Al₂O₃:1.3SiO₂:300H₂O:10TEA (1-l batch), 70–80 μm large zeolite X crystals can be obtained at 85 °C in 35 days. Both starting gels are prepared at 0 °C, then quickly heated to the crystallization temperature.     

Zeolite-mediated removal of NOx by NH3 from exhaust streams at low temperatures

NH₃ stored on zeolites in the form of NH₄⁺ ions easily reacts with NO to N₂ in the presence of O₂ at temperatures <373 K under dry conditions. Wet conditions require a modification of the catalyst system. It is shown that MnO₂ deposited on the external surface of zeolite Y by precipitation considerably enhances the NO_x conversion by zeolite fixed NH₄⁺ ions in the presence of water at 400–430 K. Particle-size analysis, temperature-programmed reduction, textural characterization, chemical analysis, ESR and XRD gave a subtle picture of the MnO₂ phase structure. The MnO₂ is a non-stoichiometric, amorphous phase that contains minor amounts of Mn²⁺ ions. It loses O₂ upon inert heating up to 873 K, but does not crystallize or sinter. The phase is reducible by H₂ in two stages via intermediate formation of Mn₃O₄. The manufacture of extrudates preserving stored NH₄⁺ ions for NO_x reduction is described. It was found that MnO₂ can oxidize NO by bulk oxygen. This enables the reduction of NO to N₂ by the zeolitic NH₄⁺ ions without gas-phase oxygen for limited time periods. The composite catalyst retains storage capacity for both, oxygen and NH₄⁺ ions despite the presence of moisture and allows short-term reduction of NO without gaseous O₂ or additional reductants. The catalyst is likewise suitable for steady-state DeNO_x operation at higher space velocities if gaseous NH₃ is permanently supplied.