Catalytic reduction of nitrogen oxides on mordenite some aspect on the mechanism

doi:10.1016/0920-5861(89)85050-3

Catalysis Today

Volume 4, Issue 2, January 1989, Pages 187-203

https://doi.org/10.1016/0920-5861(89)85050-3 Get rights and content

Abstract

The emission of nitrogen oxides is a global environmental problem. The ultimate solution would be a catalytic decomposition of NO to N₂ and O₂. Presently no success has been achieved in developing a suitable catalyst. A working technology to eliminate nitrogen oxides from stationary sources is the Selective Catalytic Reduction (SCR) of nitrogen oxides with ammonia.

Since 1983 we have been working with SCR and then initially using V₂O₅ catalysts. We have found that the reaction rate for equimolar mixtures of NO and NO₂ is much higher than that for each gas alone. Since the NO_x emissions in flue gases consist of 95% No it is necessary to convert 45% to NO₂ to take advantage of the increased activity. The idea was to combine a catalyst for the oxidation of No to NO₂ and a catalyst for the reduction step. The chosen catalyst, Nortons Zeolon 900 H, is a good oxidation catalyst for NO and a fairly good reduction catalyst. To enhance the oxidation activity, the zeolite was exchanged with transition metal ions. In contradiction to our expectations the result was a decrease in the activity. However the activity in the reduction of NO to water and nitrogen was greatly enhanced. This is an interesting coupling between the oxidation and the reduction activity, and a link between mordenite and V₂O₅. V₂O₅ is also a very good reduction catalyst and a very poor oxidation catalyst for NO.

Both the oxidation and reduction activities are depending on the aluminium content in the H-mordenite. The metallic ion is bonded to the zeolite framework on the site where strong Lewis acids are formed on dehydroxylation. The decrease in the oxidation activity is caused by this decreased formation of strong Lewis acids. The ionization of NO-NO₂ mixtures to NO⁺ and NO₂⁻ species attached to the surface can explain their catalytic behaviour. In a similar way NO₂ alone forms an ion pair with itself. Eventually the same thing applies to mixtures of NO and O₂.

References (23)

C.U.I. Odenbrand et al.
Applied Catalysis
(1986)
W.M. Meier
Z. Kristallogr.
(1961)
W.E. Addison et al.
J. Chem. Soc.
(1955)
F.R. Channings
J. Phys. Chem.
(1968)
G. Hägg
K. Segawa et al.
J. Catal.
(1982)
L.M. Aparicio et al.
J. Catal
(1987)
J.C. Oudejans et al.
C.U.I. Odenbrand et al.
Applied Catalysis
(1985)
C.U.I. Odenbrand, J.G.M. Brandin, L.A.H. Andersson and S.T. Lundin, Swedish Patent Appl. nr 8701159-9, March...

H. Arai et al.H. Arai et al.

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A series of CrO_x–WO_x mixed-oxide catalysts was synthesized through the citric acid-assisted sol–gel method and used in the selective catalytic reduction of NO_x with NH₃ to eliminate NO_x. The pristine CrO_x displayed certain deNO_x activity, achieving >90 % NO_x conversion efficiency at the temperature range of 210 °C–230 °C. Its robust oxidative ability facilitated efficient activation of adsorbed NH₃, contributing to relatively high low-temperature activity. However, at higher temperatures, the increased over-oxidation of NH₃ led to additional NO_x formation. Moreover, the comparatively weak acidity resulted in a lower NH₃ coverage on the CrO_x surface, causing a deficiency in reductant NH₃ for NO_x reduction and consequently a decrease in high-temperature activity. With WO_x addition, the acidity of the catalysts was enhanced, which was primarily responsible for the high NH₃ coverage on the catalysts’ surfaces. Furthermore, the reduction in the reactivity of Cr⁶⁺ and the number of chemisorbed oxygen species slowed NH₃ activation and inhibited NH₃ over-oxidation, shifting the catalyst’s operating temperature window toward higher ranges. At a fixed W/Cr molar ratio of 0.8, >90 % NO_x was eliminated at the temperature range of 230 °C–390 °C. However, the relatively weak resistance of the CrO_x–WO_x mixed oxide to SO₂ or Na, together with the irrecoverable deactivation upon long exposure to these two components, suggests the need for further modification to enhance the practicality of the catalyst.
Review on the NO removal from flue gas by oxidation methods
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Citation Excerpt :
Some studies on SCR denitrification showed that molecular sieve catalysts had gooddenitrification performance, hydrothermal resistance and good SO2 resistance (Wang et al., 2019; Chen et al., 2018; Leistner and Olsson, 2015). Brandin et al. (1989) used H-mordenite for the catalytic oxidation of NO and found that the oxidation activity of NO was related to the content of aluminum in the H-mordenite. Odenbrand et al. (1989) found that NO adsorbed on the H-mordenite catalyst to form NO+, and the adsorption capacity decreased with the decrease of the aluminum content in H-mordenite, which led to the NO oxidation efficiency decrease.
Due to the increasingly strict emission standards of NOx on various industries, many traditional flue gas treatment methods have been gradually improved. Except for selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) methods to remove NOx from flue gas, theoxidation method is paying more attention to NOx removal now because of the potential to simultaneously remove multiple pollutants from flue gas. This paper summarizes the efficiency, reaction conditions, effect factors, and reaction mechanism of NO oxidation from the aspects of liquid-phase oxidation, gas-phase oxidation, plasma technology, and catalytic oxidation. The effects of free radicals and active components of catalysts on NO oxidation and the combination of various oxidation methods are discussed in detail. The advantages and disadvantages of different oxidation methods are summarized, and the suggestions for future research on NO oxidation are put forward at the end. The review on the NO removal by oxidation methods can provide new ideas for future studies on the NO removal from flue gas.
Insights into the effects of sulfate species on CuO/TiO<inf>2</inf> catalysts for NH<inf>3</inf>-SCR reactions
2020, Molecular Catalysis
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In this case, the NH3-SCR rate-limiting step, involving a redox catalyst function, would be adversely affected by NH3 adsorbed onto nearby acidic sites because of electronic interaction or possibly direct blocking of the redox sites [35–38]. This “NH3 inhibition effect”, which has been discovered in metal oxide- and zeolite-based catalysts by some research groups and only occurs at lower temperatures, could explain the relatively high activity of the CuSO4/TiO2 catalyst below 300 °C [35–38]. As the temperature increases, thanks to the similar oxidative ability, these two sulfate-containing samples exhibit a similar NH3 oxidation behavior, thus resulting in a similar high-temperature deNOx performance.
In this study, a series of copper species supported on TiO₂ catalysts were synthesized. With respect to the CuO/TiO₂ catalyst, around 75% NO_x was reduced between 200 and 250 °C and a negative deNO_x efficiency was achieved above 350 °C. The existence of plentiful reactive surface oxygen species favored the activation of NO_x and NH₃, thus leading to satisfactory activity levels at lower temperatures. Due to this, NH₃ over-oxidized to NO_x was also facilitated and the portion of NH₃ reducing NO_x to N₂ or N₂O in the reactions showed a downward trend as temperature increased, which explained the decreasing high-temperature NO_x removal efficiency of this sample. For CuSO₄/TiO₂ and SO₂-treated CuO/TiO₂ catalysts, the generation of plentiful strong acid sites at the expense of the reactive surface oxygen species suppressed the activation of NH₃ and NO_x to some extent at lower temperatures and also negatively affected the over-oxidation of NH₃ to NO_x at elevated temperatures, which explained the changes in the operating temperature window to a high temperature region.
A comparative study of N<inf>2</inf>O formation during the selective catalytic reduction of NOx with NH<inf>3</inf> on zeolite supported Cu catalysts
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A comparative study was carried out on a small-pore Cu-CHA and a large-pore Cu-BEA zeolite catalyst to understand the lower N₂O formation on small-pore zeolite supported Cu catalysts in the selective catalytic reduction (SCR) of NOx with NH₃. On both catalysts, the N₂O yield increases with an increase in the NO₂/NOx ratios of the feed gas, suggesting N₂O formation via the decomposition of NH₄NO₃. Temperature-programmed desorption experiments reveal that NH₄NO₃ is more stable on Cu-CHA than on Cu-BEA. In situ FTIR spectra following stepwise (NO₂ + O₂) and (¹⁵NO + NH₃ + O₂) adsorption and reaction, and product distribution analysis using isotope-labeled reactants, unambiguously prove that surface nitrate groups are essential for the formation of NH₄NO₃. Furthermore, Cu-CHA is shown to be considerably less active than Cu-BEA in catalyzing NO oxidation and the subsequent formation of surface nitrate groups. Both factors, i.e., (1) the higher thermal stability of NH₄NO₃ on Cu-CHA, and (2) the lower activity for this catalyst to catalyze NO oxidation and the subsequent formation of surface nitrates, likely contribute to the higher SCR selectivity with less N₂O formation on this catalyst as compared to Cu-BEA. The latter is determined as the primary reason since surface nitrates are the source that leads to the formation of NH₄NO₃ on the catalysts.
Oxidation of zeolite acid sites in NO/O<inf>2</inf> mixtures and the catalytic properties of the new site in NO oxidation
2015, Journal of Catalysis
Citation Excerpt :
Early reports by Halasz et al. [22,23] showed that H-ZSM-5 was capable of catalyzing the NO oxidation, while Li-ZSM-5 showed minimal activity at temperatures above 473 K. The authors suggested that the Brønsted acid sites were the likely active sites for the reaction above 473 K. Järås and co-workers [13,24,25] studied NO oxidation over mordenite (MOR) zeolites and ranked zeolite catalysts based on observed reaction rates in the order of H-MOR > Fe-MOR > Cu-MOR for NO oxidation, but in the reverse order for the NO reduction using NH3-SCR. The NO oxidation rate was dependent on the aluminum content for the H-MOR catalyst, and the authors suggested that surface NO+ is involved in the catalytic cycle [25].
The NO oxidation reaction kinetics over a number of zeolite structures were investigated at temperatures above 423 K using plug-flow microreactors and in situ FTIR spectroscopy. Fast catalytic NO oxidation rates (up to 1000×) are observed over H- and Na-exchanged zeolite frameworks (BEA, MFI, and CHA) at temperatures above 423 K with respect to the gas-phase reaction. NO oxidation rates increase with temperature over H- and Na-zeolites, while siliceous zeolites display little activity. Activation energies scale in the order of CHA < MFI < BEA (40.8–55.5 kJ mol⁻¹), and application of transition state theory shows that the formation of the activated complex is an enthalpically controlled process that benefits from smaller zeolite pore size. In all samples, rates are proportional to NO and O₂ concentrations, in contrast to low-temperature catalytic rates Loiland et al. [33], which are second order in NO and first order in O₂. This difference indicates that a new reaction mechanism is operative above 423 K. In-situ FTIR studies reveal that NO⁺ coordinated at framework sites in the zeolite pores (SiO⁻(NO⁺)Al) plays a direct role in the catalysis, and they also reveal that the NO oxidation rate is proportional to the amount of NO⁺ present in the sample. Furthermore, it is found that NO⁺ is in equilibrium with gas-phase NO and that desorption of NO⁺ (as NO) yields an oxidized acid site (SiOAl). No evidence of NO⁺ formation is observed over the siliceous zeolite samples. A working model of the reaction mechanism for the high-temperature NO oxidation reaction is proposed for acidic zeolites that is consistent with the observed form of the rate equation and the observed NO⁺ reaction intermediate.
Selective catalytic oxidation of ammonia by nitrogen oxides in a model synthesis gas
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As mentioned above, the proportion of NO2 in the NOX content in flue gas is normally low and the effects of the fast SCR reaction are usually undetectable. In some diesel automotive SCR applications a part of the NO content in the flue gas is catalytically oxidised to NO2 [22] to improve low-temperature SCR performance and improve the oxidation of soot in the particulate filter [19]. If the opposite reaction is considered, i.e. the removal, inverse SCR or selective catalytic oxidation (SCO), of ammonia in a gas stream by injection of NOX over a suitable catalyst.
Synthesis gas generated by the gasification of nitrogen-containing hydrocarbons will contain ammonia. This is a catalyst poison and elevated levels of nitrogen oxides (NO_X) will be produced if the synthesis gas is combusted. This paper presents a study of the selective oxidation of ammonia in reducing environments. The concept is the same as in traditional selective catalytic reduction, where NO_X are removed from flue gas by reaction with injected ammonia over a catalyst. Here, a new concept for the removal of ammonia is demonstrated by reaction with injected NO_X over a catalyst. The experiments were carried out in a model synthesis gas consisting of CO, CO₂, H₂, N₂ and NH₃/NO_X. The performance of two catalysts, V₂O₅/WO₃/TiO₂ and H-mordenite, were evaluated. On-site generation of NO_X by nitric acid decomposition was also investigated and tested. The results show good conversion of ammonia under the conditions studied for both catalysts, and with on-site generated NO_X.

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Catalytic reduction of nitrogen oxides on mordenite some aspect on the mechanism

Abstract

Applied Catalysis

Z. Kristallogr.

J. Chem. Soc.

J. Phys. Chem.

J. Catal.

J. Catal

Applied Catalysis