Catalytic performance of various mesoporous silicas modified with copper or iron oxides introduced by different ways in the selective reduction of NO by ammonia

Mesoporous silicas (MCM-48, SBA-15, MCF), reflecting various porous structures, were modified with copper and iron oxides by two different methods. For a first series of the samples the molecular designed dispersion (MDD) method using acetylacetonate complexes of copper and iron was applied for the deposition of transition metal oxides on the silica supports. A second series of the catalysts was obtained by the incipient impregnation technique using aqueous solutions of the suitable metal nitrates. The modified materials were characterized with respect to the texture (BET), composition (electron microprobe analysis), coordination of the transition metals (UV–vis–DRS) and surface acidity (NH₃-TPD, FTIR). The mesoporous silica supports modified with transition metal oxides were tested as catalysts of the selective reduction of NO with ammonia. The catalytic performance of the studied samples depended on the method used for the deposition of transition metal oxide as well as the kind of mesoporous silica used as a catalytic support. In general, the Cu-containing mesoporous samples effectively operated at lower temperatures than silicas modified with iron. The samples obtained by the MDD method have been found to be more active and selective compared to the analogous samples prepared by the impregnation technique. An introduction of water vapor into the reaction mixture only slightly decreased the NO conversion and selectivity towards N₂ over the MCF mesoporous silica modified with copper or iron oxide.     

The role of Brønsted acidity in the SCR of NO over Fe-MFI catalysts

The selective catalytic reduction (SCR) of NO with isobutane and with NH₃ was studied over Fe-MFI catalysts which differ strongly in Brønsted acidity but are similar in Fe content and structure of Fe sites, having shown similar activity in N₂O decomposition in related work. The catalysts were prepared by exchange of Na-ZSM-5 (Si/Al ca. 14) with Fe²⁺ ions formed in situ by acidic dissolution of Fe powder and by steam extraction of framework iron from Fe-silicalite or from H-[Fe]-ZSM-5 (Si/Al ca. 30). The characterization of acidic properties by ammonia TPD and by IR of adsorbed pyridine at different temperatures revealed marked differences in acidity between exchanged and steam-activated samples, the latter being (almost) void of strong Brønsted sites. The structural similarity of the iron sites was confirmed by UV–Vis and EPR spectroscopic results. The weakly acidic samples were inferior both in isobutane-SCR and in ammonia-SCR. With isobutane, dramatic differences over the whole range of parameters studied imply a vital role of Brønsted acidity in the reaction mechanism (e.g. in isobutane activation). In NH₃-SCR, large reaction rates were achieved with non-acidic catalysts as well, but a promoting effect of acidity was noted for catalysts that contain the iron in the most favorable site structure (oligomeric Fe oxo clusters). This suggests that an acid-catalyzed step (e.g. the decomposition of NH₄NO₂) may be rate-limiting at low temperatures.     

Selective catalytic reduction of NO with NH3 at low temperatures over iron and manganese oxides supported on mesoporous silica

A series of catalysts of iron–manganese oxide supported on mesoporous silica (MPS) with different Mn/Fe ratio were studied for low-temperature selective catalytic reduction (SCR) of NO with ammonia in the presence of excess oxygen. Effects of amounts of iron–manganese oxide and calcination temperatures on NO conversion were also investigated. It was found that the Mn–Fe/MPS with Mn/Fe = 1 at the calcination temperature of 673 K showed the highest activity. The results showed that this catalyst yielded 99.1% NO conversion at 433 K at a space velocity of 20,000 h⁻¹. H₂O has no adverse impact on the activity when the SCR reaction temperature is above 413 K. In addition, the SCR activity was suppressed gradually in the presence of SO₂ and H₂O, while such effect was reversible after heating treatment.     

Selective catalytic reduction of nitric oxide by ammonia over Cu-exchanged Cuban natural zeolites

The catalytic selective reduction of NO over Cu-exchanged natural zeolites (mordenite (MP) and clinoptilolite (HC)) from Cuba using NH₃ as reducing agent and in the presence of excess oxygen was studied. Cu(II)-exchanged zeolites are very active catalysts, with conversions of NO of 95%, a high selectivity to N₂ at low temperatures, and exhibiting good water tolerance. The chemical state of the Cu(II) in exchanged zeolites was characterized by H₂-TPR and XPS. Cu(II)-exchanged clinoptilolite underwent a severe deactivation in the presence of SO₂. However, Cu(II)-exchanged mordenite not only maintained its catalytic activity, but even showed a slight improvement after 20 h of reaction in the presence of 100 ppm of SO₂.     

Selective catalytic reduction of nitric oxide with ammonia at low temperatures

Selective catalytic reduction of nitric oxide with ammonia in synthetic low temperature flue gases has been investigated on a commercially available precious metal catalyst, NO_xCAT 920 LT^TM. It has been found that this catalyst is capable of achieving up to 90% conversion at temperatures below 300°C and low space velocities (12 000 h⁻¹), even in the presence of 20 ppm sulfur dioxide. The ideal ammonia concentration to reduce slip and achieve maximum conversion seems to be a stoichiometric match between ammonia concentration and nitric oxide concentration. A dual site model is proposed to explain the selectivity dependence on the presence of water vapor or sulfur dioxide.     

A kinetic model of the hydrogen assisted selective catalytic reduction of NO with ammonia over Ag/Al2O3

A global kinetic model which describes H₂‐assisted NH₃‐SCR over an Ag/Al₂O₃ monolith catalyst has been developed. The intention is that the model can be applied for dosing NH₃ and H₂ to an Ag/Al₂O₃ catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH₃‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO_x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH₃ and H₂ of 1:1:2. Further reactions included in the model are the oxidation of NH₃ to N₂ and NO, oxidation of H₂, and the adsorption and desorption of NH₃. The model was fitted to the results of an NH₃‐TPD experiment, an NH₃ oxidation experiment, and a series of H₂‐assisted NH₃‐SCR steady‐state experiments. The model predicts the conversion of NO_x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013     

Selective catalytic reduction of NO by ammonia with fly ash catalyst

This paper investigates the selective catalytic reduction (SCR) of NO with NH₃ using fly ash catalyst. The catalytically active elements investigated here included Fe, Cu, Ni and V. The results indicated that fly ash, after pre-treatment, can be reasonably used as the SCR catalyst support to remove NO from flue gas. Cu gave the highest catalytic activity and NO conversion, compared with Fe, Ni and V. In the pre-treatment process, the nitric acid treatment and drying temperatures for the fly ash particles had little effect on the NO conversion. However, the calcination temperature had an important effect on the catalyst preparation process.     

Selective catalytic reduction of NO by alcohols on Co- and Fe-Siβ catalysts

Co–Siβ and Fe–Siβ catalysts prepared by a two-step post-synthesis method were characterised by EPR, diffuse reflectance UV–vis, XRD and N₂-physisorption. Iron and cobalt ions are present as isolated lattice tetrahedral Co^II and Fe^III species for low metal contents (0.7 and 0.9 wt.%, respectively). For higher iron content, FeO_x oligomers appear. Zeolites with isolated Co^II and Fe^III species are active in selective catalytic reduction of NO with ethanol. On FeO_x oligomers the oxidation of NO to NO₂ starts to dominate in the selective catalytic reduction of NO with ethanol at the temperatures higher than 700 K.     

DRIFTS investigation of V=O behavior and its relations with the reactivity of ammonia oxidation and selective catalytic reduction of NO over V2O5 catalyst

The behavior of V=O band over V₂O₅ crystallite during NH₃ adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH₃ oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V₂O₅ surface and/or to the adsorption of ammonia. The 70% intensity of original V=O band is preserved up to 573 K under the conditions of SCR reaction. The vanadium oxidation state is about +4.4. When the temperature reached 673 K, almost all the V=O band was recovered. From these results, it can be suggested that the decrease of the apparent SCR activity due to the increase of NO amount through NH₃ oxidation above 673 K be attributed to the increase of two neighboring V=O sites, which favor the NO formation in ammonia oxidation.     

Selective catalytic reduction of NO and NO2 at low temperatures

The fast SCR reaction using equimolar amounts of NO and NO₂ is a powerful means to enhance the NO_x conversion over a given SCR catalyst. NO₂ fractions in excess of 50% of total NO_x should be avoided because the reaction with NO₂ only is slower than the standard SCR reaction.

At temperatures below 200 °C, due to its negative temperature coefficient, the ammonium nitrate reaction gets increasingly important. Half of each NH₃ and NO₂ react to form dinitrogen and water in analogy to a typical SCR reaction. The other half of NH₃ and NO₂ form ammonium nitrate in close analogy to a NO_x storage-reduction catalyst. Ammonium nitrate tends to deposit in solid or liquid form in the pores of the catalyst and this will lead to its temporary deactivation.

The various reactions have been studied experimentally in the temperature range 150–450 °C for various NO₂/NO_x ratios. The fate of the deposited ammonium nitrate during a later reheating of the catalyst has also been investigated. In the absence of NO, the thermal decomposition yields mainly ammonia and nitric acid. If NO is present, its reaction with nitric acid on the catalyst will cause the formation of NO₂.     

Effect of Co content on the catalytic activity of CoSiBEA zeolite in the selective catalytic reduction of NO with ethanol: Nature of the cobalt species

The effect of Co content on the catalytic activity of CoSiBEA zeolites in the selective catalytic reduction (SCR) of NO with ethanol is investigated. The Co_xSiBEA zeolites (x = 0.3, 0.7, 3.6 and 6.75 Co wt.%) are prepared by a two-step postsynthesis method which allows to control the introduction of cobalt into zeolite and thus to obtain catalysts with specific Co sites. The nature of the active sites is characterized by XRD, diffuse reflectance UV–vis, H₂-TPR and XPS.

The catalytic activity of Co_xSiBEA strongly depends on the nature and environment of Co species. Zeolites with isolated lattice tetrahedral Co(II) (Co_0.3SiBEA and Co_0.7SiBEA samples) are active in SCR of NO with ethanol with selectivity toward N₂ exceeding 85% for NO conversion from 20 to 70%. When additional isolated extra-lattice octahedral Co(II) species appear (Co_3.6SiBEA sample), the full oxidation of ethanol by dioxygen becomes a very important reaction pathway. In presence of additional cobalt oxides (Co_6.75SiBEA sample), the activity and selectivity toward N₂ substantially change and full oxidation of ethanol to CO₂ is the main reaction pathway and full NO oxidation also takes place in the temperature range 550–775 K. The lack of correlation between the activity in SCR of NO with ethanol and NO oxidation to NO₂ suggests that the two reactions are more competitive than sequential.     

Selective reduction of NO by NH3 over manganese–cerium mixed oxides: Relation between adsorption, redox and catalytic behavior

Manganese–cerium mixed oxide catalysts with different molar ratio Mn/(Mn + Ce) (0, 0.25, 0.50, 0.75, 1) were prepared by citric acid method and investigated concerning their adsorption behavior, redox properties and behavior in the selective catalytic reduction of NO_x by NH₃. The studies based on pulse thermal analysis combined with mass spectroscopy and FT-IR spectroscopy uncovered a clear correlation between the dependence of these properties and the mixed oxide composition. Highest activity to nitrogen formation was found for catalysts with a molar ratio Mn/(Mn + Ce) of 0.25, whereas the activity was much lower for the pure constituent oxides. Measurements of adsorption uptake of reactants, NO_x (NO, NO₂) and NH₃, and reducibility showed similar dependence on the mixed oxide composition indicating a clear correlation of these properties with catalytic activity. The adsorption studies indicated that NO_x and NH₃ are adsorbed on separate sites. Consecutive adsorption measurements of the reactants showed similar uptakes as separate measurements indicating that there was no interference between adsorbed reactants. Mechanistic investigations by changing the sequence of admittance of reactants (NO_x, NH₃) indicated that at 100–150 °C nitrogen formation follows an Eley–Rideal type mechanism, where adsorbed ammonia reacts with NO_x in the gas phase, whereas adsorbed NO_x showed no significant reactivity under conditions used.     

Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5

The influence of NO₂ on the selective catalytic reduction (SCR) of NO with ammonia was studied over Fe-ZSM5 coated on cordierite monolith. NO₂ in the feed drastically enhanced the NO_x removal efficiency (DeNOx) up to 600 °C, whereas the promoting effect was most pronounced at the low temperature end. The maximum activity was found for NO₂/NO_x = 50%, which is explained by the stoichiometry of the actual SCR reaction over Fe-ZSM5, requiring a NH₃:NO:NO₂ ratio of 2:1:1. In this context, it is a special feature of Fe-ZSM5 to keep this activity level almost up to NO₂/NO_x = 100%. The addition of NO₂ to the feed gas was always accompanied by the production of N₂O at lower and intermediate temperatures. The absence of N₂O at the high temperature end is explained by the N₂O decomposition and N₂O-SCR reaction. Water and oxygen influence the SCR reaction indirectly. Oxygen enhances the oxidation of NO to NO₂ and water suppresses the oxidation of NO to NO₂, which is an essential preceding step of the actual SCR reaction for NO₂/NO_x < 50%. DRIFT spectra of the catalyst under different pre-treatment and operating conditions suggest a common intermediate, from which the main product N₂ is formed with NO and the side-product N₂O by reaction with gas phase NO₂.     

Ammonia activation over catalysts for the selective catalytic reduction of NOx and the selective catalytic oxidation of NH3. An FT-IR study

The adsorption and transformation of ammonia over V₂O₅, V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and CuO/TiO₂ systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH₂ amide species or to its dimeric form N₂H₄, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N₂⁻ and azide anions N₃⁻ are further observed over CuO/TiO₂. The comparison of the infrared spectra of the species arising from both NH₃ and N₂H₄ adsorbed over CuO/TiO₂ strongly suggest that N₂H₄ is an intermediate in NH₃ oxidation over this active selective catalytic reduction (SCR) and selective catalytic oxidation (SCO) catalysts. This implies that ammonia is activated in the form of NH₂ species for both SCR and SCO, and it can later dimerize. Ammonia protonation to ammonium ion is detected over V₂O₅-based systems, but not over CuO/TiO₂, in spite of the high SCR and SCO activity of this catalyst. Consequently Brönsted acidity is not necessary for the SCR activity.     

Selective catalytic reduction of NO with propene over CuMFI zeolites: dependence on Si/Al ratio and copper loading

Selective catalytic reduction of nitrogen oxide by propene in an oxidising atmosphere was studied on several CuMFI catalysts with different Si/Al ratios (11 Si/Al 100) and different copper loadings (between 0 and 5.5 wt.-%). From the results it was observed that the influence of zeolite Si/Al ratio on CuMFI catalytic activity for NO SCR by propene is dependent on the catalyst copper loading. Furthermore, the effect of catalyst copper loading on catalytic performance depended on the catalyst Si/Al ratio. The results also demonstrated that CuMFI catalysts with different Si/Al ratios and copper loadings, but with the same Cu/Al ratio and, therefore, the same copper exchange level have similar catalytic activity profiles for NO SCR.

It was further observed that not all Cu cations exchanged into MFI catalysts have equivalent catalytic activity for NO SCR, which made the existence of different copper environments on CuMFI catalysts evident, isolated Cu²⁺ ions being the most active species for NO SCR by propene.

Moreover, the results showed an improvement of the CuMFI catalytic activity at low temperatures by increasing the catalyst copper exchange level and, consequently, decreasing the number of Brönsted acid sites, which can be performed either by increasing the zeolite Si/Al ratio or copper loading.     

Selective catalytic reduction of NO with propane over CoZSM-5 containing alkaline earth cations

Selective catalytic reduction (SCR) of nitric oxide (NO) with propane is studied over CoZSM-5 catalysts with a series of exchanged cobalt concentrations (0.9-7.5 wt.-%). The overall activity for SCR of NO is found to increase linearly with the cobalt content in the range below the maximum exchange capacity (CoAl= 0.5). However, when the cobalt loading exceeds the exchange capacity of the zeolite, viz.CoAl 0.5, the combustion of propane is favored significantly, resulting in a decrease of the NO conversion. The presence of excess Co²⁺ in zeolite appears to bring about the marked falls in adsorption of NO. In this case cobalt oxide particles are presumed to form, which promote the oxidation of propane. Nevertheless, the addition of alkaline-earth metal cations (Ba, Ca) resulted in the suppression of propane oxidation over CoZSM-5, and improved the NO conversion dramatically.     

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.     

Selective catalytic reduction of NO with NH3 over Nb2O5-promoted V2O5/TiO2 catalysts

In contrast to previous claims, the addition of niobia to catalysts containing vanadia supported on titania resulted in much enhanced activity for low-temperature SCR of NO with NH₃ only at low vanadia loadings. Niobia promoted catalysts could also be demonstrated to show higher selectivities to N2, especially at high temperatures and low vanadia loading. This enhancement of the activity cannot be explained only on the basis of the observation that niobia stabilized the surface area of the catalyst: calculations of the activation energy suggest that a different mechanism of the reaction may be at work at low vanadia loadings.     

Characterization and activity of novel copper-containing catalysts for selective catalytic reduction of NO with NH3

Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH₃, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu²⁺ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu²⁺ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V₂O₅–WO₃TiO₂ catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH₃ combustion on the free Mg/Al mixed oxide surface.     

Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

ZSM-5 zeolite was loaded with vanadyl ions (VO²⁺) by treatment of Na–ZSM-5 with aqueous VOSO₄ solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH₃ and 2% O₂ in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g⁻¹ h⁻¹ (≈400 000 h⁻¹) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N₂O formation at NH₃ excess. Admixture of SO₂ (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO₂ conversion was noted at T ≤ 723 K and 450 l g⁻¹ h⁻¹. The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.