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.     

The effect of NO2 on the activity of fresh and aged zeolite catalysts in the NH3-SCR reaction

The activity of fresh and hydrothermally aged zeolite-based catalysts in the NH₃-selective catalytic reduction (SCR) reaction with excess of oxygen were studied. In addition, the effect of NO₂ in the gas feed as well as the acidity of the catalysts for the SCR activity was investigated. The studied catalysts were hydrogen, copper, iron and silver ion exchanged ZSM-5, mordenite, beta, ferrierite, and Y-zeolites. The investigation verifies that the zeolite-based catalysts are very promising for the ammonia SCR reaction. Especially, the activity at low and high temperatures was higher than the activity of commercial vanadia-based catalysts. From the studied catalysts, Fe-beta was the most potential one. The presence of NO₂ in the inlet flow enhanced significantly the catalytic activity of fresh and hydrothermally aged zeolite catalysts. This suggests that the oxidation of NO to NO₂ is probably the rate-determining step for the SCR reaction.     

Selective catalytic reduction of NOx with NH3 over Cu-ZSM-5—The effect of changing the gas composition

The selective catalytic reduction of nitrogen oxides (NO_x) with ammonia over ZSM-5 catalysts was studied with and without water vapor. The activity of H-, Na- and Cu-ZSM-5 was compared and the result showed that the activity was greatly enhanced by the introduction of copper ions. A comparison between Cu-ZSM-5 of different silica to alumina ratios was also performed. The highest NO conversion was observed over the sample with the lowest silica to alumina ratio and the highest copper content. Further studies were performed with the Cu-ZSM-5-27 (silica/alumina = 27) sample to investigate the effect of changes in the feed gas. Oxygen improves the activity at temperatures below 250 °C, but at higher temperatures O₂ decreases the activity. The presence of water enhances the NO reduction, especially at high temperature. It is important to use about equal amounts of nitrogen oxides and ammonia at 175 °C to avoid ammonia slip and a blocking effect, but also to have high enough concentration to reduce the NO_x. At high temperature higher NH₃ concentrations result in additional NO_x reduction since more NH₃ becomes available for the NO reduction. At these higher temperatures ammonia oxidation increases so that there is no ammonia slip. Exposing the catalyst to equimolecular amounts of NO and NO₂ increases the conversion of NO_x, but causes an increased formation of N₂O.     

The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3

The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C₃H₆ over CuO/γ-Al₂O₃ has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu⁰/Cu⁺ to Cu²⁺ by NO and O₂, (ii) the co-adsorption of NO/NO₂/O₂ to produce Cu²⁺(NO₃⁻)₂, and (iii) the reaction of Cu²⁺(NO₃⁻)₂ with C₃H₆ to produce N₂, CO₂, and H₂O. Increasing the O₂/NO ratio from 25.0 to 83.4 promotes the formation of NO₂ from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO₂/O₂. This reactant mixture allows the formation of Cu²⁺(NO₃⁻)₂ and its reaction with the C₃H₆ to occur at a higher rate with a higher selectivity toward N₂ than the low O₂/NO flow. Both the high and low O₂/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C₃H₇---NO₂, C₃H₇---ONO, CH₃COO⁻, Cu⁰---CN, and Cu⁺---NCO intermediates toward N₂, CO₂, and H₂O products. High O₂ concentration in the high O₂/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O₂/NO SCR at 523–698 K. High O₂ concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O₂ concentration at temperatures above 723 K.     

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.     

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.     

Co3O4 based catalysts for NO oxidation and NOx reduction in fast SCR process

Reaction activities of several developed catalysts for NO oxidation and NO_x (NO + NO₂) reduction have been determined in a fixed bed differential reactor. Among all the catalysts tested, Co₃O₄ based catalysts are the most active ones for both NO oxidation and NO_x reduction reactions even at high space velocity (SV) and low temperature in the fast selective catalytic reduction (SCR) process. Over Co₃O₄ catalyst, the effects of calcination temperatures, SO₂ concentration, optimum SV for 50% conversion of NO to NO₂ were determined. Also, Co₃O₄ based catalysts (Co₃O₄-WO₃) exhibit significantly higher conversion than all the developed DeNO_x catalysts (supported/unsupported) having maximum conversion of NO_x even at lower temperature and higher SV since the mixed oxide Co-W nanocomposite is formed. In case of the fast SCR, N₂O formation over Co₃O₄-WO₃ catalyst is far less than that over the other catalysts but the standard SCR produces high concentration of N₂O over all the catalysts. The effect of SO₂ concentration on NO_x reduction is found to be almost negligible may be due to the presence of WO₃ that resists SO₂ oxidation.     

Catalytic activity of dispersed CuO phases towards nitrogen oxides (N2O, NO, and NO2)

A systematic reactivity study of N₂O, NO, and NO₂ on highly dispersed CuO phases over modified silica supports (SiO₂–Al₂O₃, SiO₂–TiO₂, and SiO₂–ZrO₂) has been performed. Different reaction paths for the nitrogen oxide species abatement were studied: from direct decomposition (N₂O) to selective reductions by hydrocarbons (N₂O, NO, and NO₂) and oxidation (NO to NO₂). The oxygen concentration, temperature, and contact time, were varied within suitable ranges in order to investigate the activity and in particular the selectivity in the different reactions studied. The support deeply influenced the catalytic properties of the active copper phase. The most acidic supports, SiO₂–Al₂O₃ and SiO₂–ZrO₂, led to a better activity and selectivity of CuO for the reactions of N₂O, NO, and NO₂ reductions and N₂O decomposition than SiO₂–TiO₂. The catalytic results are discussed in terms of actual turnover frequencies starting from the knowledge of the copper dispersion values.     

Influence of NH3 and NO oxidation on the SCR reaction mechanism on copper/nickel and vanadium oxide catalysts supported on alumina and titania

The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N₂O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H₂-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO₂ and N₂O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO₂ intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO₂ species could be related to the N₂O formation.     

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.     

Catalytic effect of platinum on the kinetics of carbon oxidation by NO2 and O2

The effect of a commercial Pt/Al₂O₃ catalyst on the oxidation by NO₂ and O₂ of a model soot (carbon black) in conditions close to automotive exhaust gas aftertreatment is investigated. Isothermal oxidations of a physical mixture of carbon black and catalyst in a fixed bed reactor were performed in the temperature range 300–450 °C. The experimental results indicate that no significant effect of the Pt catalyst on the direct oxidation of carbon by O₂ and NO₂ is observed. However, in presence of NO₂–O₂ mixture, it is found that besides the well established catalytic reoxidation of NO into NO₂, Pt also exerts a catalytic effect on the cooperative carbon–NO₂–O₂ oxidation reaction. An overall mechanism involving the formation of atomic oxygen over Pt sites followed by its transfer to the carbon surface is established. Thus, the presence of Pt catalyst increases the surface concentration of –C(O) complexes which then react with NO₂ leading to an enhanced carbon consumption. The resulting kinetic equation allows to model more precisely the catalytic regeneration of soot traps for automotive applications.     

The activity and characterization of sol–gel Sn/Al2O3 catalyst for selective catalytic reduction of NOx in the presence of oxygen

Catalytic performance of Sn/Al₂O₃ catalysts prepared by impregnation (IM) and sol–gel (SG) method for selective catalytic reduction of NO_x by propene under lean burn condition were investigated. The physical properties of catalyst were characterized by BET, XRD, XPS and TPD. The results showed that NO₂ had higher reactivity than NO to nitrogen, the maximum NO conversion was 82% on the 5% Sn/Al₂O₃ (SG) catalyst, and the maximum NO₂ conversion reached nearly 100% around 425 °C. Such a temperature of maximum NO conversion was in accordance with those of NO_x desorption accompanied with O₂ around 450 °C. The activity of NO reduction was enhanced remarkably by the presence of H₂O and SO₂ at low temperature, and the temperature window was also broadened in the presence of H₂O and SO₂, however the NO_x desorption and NO conversion decreased sharply on the 300 ppm SO₂ treated catalyst, the catalytic activity was inhibited by the presence of SO₂ due to formation of sulfate species (SO₄²⁻) on the catalysts. The presence of oxygen played an essential role in NO reduction, and the activity of the 5% Sn/Al₂O₃ (SG) was not decreased in the presence of large oxygen.     

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.     

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.     

The mechanism of SO2 effect on NO reduction with propene over In2O3/Al2O3 catalyst

An In₂O₃/Al₂O₃ catalyst shows high activity for the selective catalytic reduction of NO with propene in the presence of oxygen. The presence of SO₂ in feed gas suppressed the catalytic activity dramatically at high temperatures; however it was enhanced in the low temperature range of 473–573 K. In TPD and FT-IR studies, the formation of sulfate species on the surface of the catalyst caused an inhibition of NO_X adsorption sites, and the absorbance ability of NO was suppressed by the presence of SO₂, and the amount of ad-NO₃⁻ species decreased obviously. This leads to a decrease of catalytic activity at higher temperatures. However, addition of SO₂ enhanced the formation of carboxylate and formate species, which can explain the promotional effect of SO₂ at low temperature, because active C₃H₆ (partially oxidized C₃H₆) is crucial at low temperature.     

Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content

We present a systematic study of the NH₃-SCR reactivity over a commercial V₂O₅–WO₃/TiO₂ catalyst in a wide range of temperatures and NO/NO₂ feed ratios, which cover (and exceed) those of interest for industrial applications to the aftertreatment of exhaust gases from diesel vehicles. The experiments confirm that the best deNO_x efficiency is achieved with a 1/1 NO/NO₂ feed ratio. The main reactions prevailing at the different operating conditions have been identified, and an overall reaction scheme is herein proposed.

Particular attention has been paid to the role of ammonium nitrate, which forms rapidly at low temperatures and with excess NO₂, determining a lower N₂ selectivity of the deNO_x process. Data are presented which show that the chemistry of the NO/NO₂–NH₃ reacting system can be fully interpreted according to a mechanism which involves: (i) dimerization/disproportion of NO₂ and reaction with NH₃ and water to give ammonium nitrite and ammonium nitrate; (ii) reduction of ammonium nitrate by NO to ammonium nitrite; (iii) decomposition of ammonium nitrite to nitrogen. Such a scheme explains the peculiar deNO_x reactivity at low temperature in the presence of NO₂, the optimal stoichiometry (NO/NO₂ = 1/1), and the observed selectivities to all the major N-containing products (N₂, NH₄NO₃, HNO₃, N₂O). It also provides the basis for the development of a mechanistic kinetic model of the NO/NO₂–NH₃ SCR reacting system.     

Effect of Na-addition on catalytic activity of Pt-ZSM-5 for low-temperature NO–H2–O2 reactions

The effect of additives on Pt-ZSM-5 catalysts was studied for the selective NO reduction by H₂ in the presence of excess O₂ (NO–H₂–O₂ reaction) at 100 °C. The reaction of NO in a stream of 0.08% NO, 0.28% H₂, 10% O₂, and He balance yielded N₂ with less than 10% selectivity, which could not be increased by changing Pt loading or H₂ concentration in the gas feed. Co-impregnation of NaHCO₃ and Pt onto ZSM-5 decreased the BET surface area and the Pt dispersion. Nevertheless, the Na-loaded catalyst (Na-Pt-ZSM-5) exhibited the higher NO_x conversion (>90%) and the N₂ selectivity (ca. 50%). Such a high catalytic activity even at high Na loadings (≥10 wt.%) is completely contrast to other Na-added Pt catalyst systems reported so far. Further improvement of N₂ selectivity was attained by the post-impregnation of NaHCO₃ onto Pt-ZSM-5. In situ DRIFT measurements suggested that the addition of Na promotes the adsorption of NO as NO₂⁻-type species, which would play a role of an intermediate to yield N₂. The introduction of Lewis base to the acidic supports including ZSM-5 would be applied to the catalyst design for selective NO–H₂–O₂ reaction at low temperatures.     

Fourier transform IR study of NOx adsorption on a CuZSM-5 DeNOx catalyst

The adsorption and coadsorption of selective catalytic reduction (SCR) reactants and reaction products on CuZSM-5-37 containing 11 wt.-% CuO have been studied by FTIR spectroscopy. The catalyst surface is characterized by both weak acidity and weak basicity as revealed by testing with probe molecules (CO₂, NH₃, H₂O). NO₂ adsorption results in formation of different kinds of nitrates. The same species are formed when NO is coadsorbed with oxygen at 180°C. NO adsorption at ambient temperature also leads to formation of nitrates as well as of Cu²⁺NO species. In the presence of oxygen the latter are converted according to the scheme: NO → N₂O₃ → N₂O₄ → NO₂ → NO₃. It is concluded that the surface nitrates are important intermediates in the SCR process. They are thermally stable and resistant towards interaction with CO₂, N₂, O₂, and are only slightly affected by H₂O and NO. However, they posses a high oxidation ability and are fully reduced by propane at 180°C. It is concluded that one of the most important roles of oxygen in SCR by hydrocarbons is to convert NO_x into highly active surface nitrates.     

Storage of NO2 on BaO/TiO2 and the influence of NO

The influence of NO on the adsorption and desorption of NO₂ on BaO/TiO₂ has been studied under lean conditions. The adsorption of NO₂ involves the disproportionation of NO₂ into an adsorbed nitrate species and NO released to the gas phase with a 3:1 ratio,

BaO+3NO₂→NO+Ba(NO₃)₂.

Three different nitrate species form on the catalyst: surface nitrates on the TiO₂ support, surface nitrates on BaO, and bulk barium nitrate. The stability of the three species in different gas feeds was investigated by temperature-programmed desorption (TPD).

The reverse reaction of the NO₂ disproportionation has also been observed. If NO is added to the feed, nitrates previously formed on the sorbent will decompose into NO₂. Therefore, the above chemical equation should be considered as an equilibrium reaction. Applying this finding to the NO_x storage and reduction catalyst means that NO probably reacts with the previously formed nitrates yielding NO₂ as an intermediate product. This NO₂ is subsequently reduced by the reducing agents (hydrocarbons and CO) present during the regeneration period.     

A new active zeolite structure for the selective catalytic reduction (SCR) of nitrogen oxides: ITQ7 zeolite: The influence of NO2 on this reaction

The activity of a new zeolite material, ITQ7, has been studied for the selective catalytic reduction (SCR) of NO. The pore topology of this material is similar to the structure of a beta zeolite, with a tridirectional system with 12-member rings. ITQ7 exchanged with copper or cobalt shows a catalytic behaviour very similar to a beta zeolite exchanged with copper or cobalt, probably due to its similar structure. The presence of oxygen, water, sulphur dioxide and NO₂ has been studied, obtaining the best results at low oxygen concentration and in the absence of water and SO₂. Nevertheless if NO₂ is present in the reaction mixture, the maximum activity of the catalyst shifts towards higher oxygen concentration.