NOx adsorption on MnO2/NaY composite: an in situ FTIR and EPR study

The NO, NO/O₂, and NO/O₂/H₂O adsorption on MnO₂/NaY (5 and 15 wt.% MnO₂) composite catalyst and NaY has been studied by means of in situ FTIR and EPR spectroscopy at elevated temperatures and during heating under reaction-like conditions. NO adsorption and co-adsorption of NO and O₂ on NaY and MnO₂/NaY proceeds via oxidation of NO forming NO₂⁻ and NO₃⁻ species. Whereas the manganese dioxide preferably acts as oxidising agent, the zeolite stores the NO_x species as nitrite and nitrate ions in the solid. In the presence of oxygen, the nitrate formation is enhanced due to additional oxidation of NO through gaseous oxygen leading to NO₂. Dimerisation of NO₂ to N₂O₄ and following disproportionation of the latter causes the formation of NO⁺ and NO₃⁻ species which are associated with nucleophilic zeolitic oxygen and especially alkali cations of the zeolite, respectively. The presence of oxygen facilitates reoxidation of Mn²⁺ which keeps more Mn ions in the active state. Pre-adsorbed water and higher amounts of water vapour in the feed hinder the NO adsorption by blocking the adsorption sites and shift the nitrate formation to higher temperatures. The quantities and thermal stability of the nitrates formed during NO and NO/O₂ adsorption differs which points to a different mechanism of nitrate formation. In the absence of gaseous oxygen, nitrates are formed by participation of only lattice oxygen. In the presence of oxygen, nitrate formation by dimerisation and disproportionation reactions of NO₂ dominates. The manganese component of the composite catalyst supports the oxidation of NO to nitrite and subsequently to nitrate. During this process Mn⁴⁺ is reduced to Mn²⁺ as evidenced by in situ EPR measurements.     

The role of Bronstead acidity in poisoning the SCR-urea reaction over FeZSM-5 catalysts

Two series of FeZSM-5 catalysts prepared from Na⁺ and NH₄⁺ ZSM-5 precursors are studied in the selective reduction of NO_x using NH₃ and urea as reducing agents. All Fe-containing catalysts are active for NO_x reduction in the SCR-NH₃ reaction with ex-NH₄⁺ catalysts being more active than ex-Na⁺ materials and the activity depending (to a minor extent within each series of catalysts) upon [Fe]. Catalysts with Bronstead acid sites also show a small transient deNO_x activity at low temperatures. All catalysts are less active for the SCR-urea reaction but the ex-Na⁺ catalysts retain far more deNO_x activity than the ex-NH₄⁺ materials. NH₃ TPD shows that strongly binding Bronstead acid sites are present on the ex-NH₄⁺ materials and H⁺-treated parent zeolites while Urea TPD shows that the mode of decomposition of urea differs as a function of initial zeolite counter-ion. Urea TPSR shows that the reaction between adsorbed urea and gaseous NO/O₂ is related to [Fe]. It is proposed that the decreased activity of the ex-NH₄⁺ catalysts in the SCR-urea reaction is due to a less favourable mode of decomposition over these catalysts. Furthermore it is suggested that the Bronstead acidity plays some part in this less favoured decomposition.     

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.     

Catalytic reduction of NOx with H2/CO/CH4 over PdMOR catalysts

Conversion of NO_x with reducing agents H₂, CO and CH₄, with and without O₂, H₂O, and CO₂ were studied with catalysts based on MOR zeolite loaded with palladium and cerium. The catalysts reached high NO_x to N₂ conversion with H₂ and CO (>90% conversion and N₂ selectivity) range under lean conditions. The formation of N₂O is absent in the presence of both H₂ and CO together with oxygen in the feed, which will be the case in lean engine exhaust. PdMOR shows synergic co-operation between H₂ and CO at 450–500 K. The positive effect of cerium is significant in the case of H₂ and CH₄ reducing agent but is less obvious with H₂/CO mixture and under lean conditions. Cerium lowers the reducibility of Pd species in the zeolite micropores. The catalysts showed excellent stability at temperatures up to 673 K in a feed with 2500 ppm CH₄, 500 ppm NO, 5% O₂, 10% H₂O (0–1% H₂), N₂ balance but deactivation is noticed at higher temperatures. Combining results of the present study with those of previous studies it shows that the PdMOR-based catalysts are good catalysts for NO_x reduction with H₂, CO, hydrocarbons, alcohols and aldehydes under lean conditions at temperatures up to 673 K.     

A combination of NOx trapping materials and urea-SCR catalysts for use in the removal of NOx from mobile diesel engines

Preliminary studies on a series of nanocomposite BaO–Fe ZSM-5 materials have been carried out to determine the feasibility of combining NO_x trapping and SCR-NH₃ reactions to develop a system that might be applicable to reducing NO_x emissions from diesel-powered vehicles. The materials are analysed for SCR-NH₃ and SCR-urea reactivity, their NO_x trapping and NH₃ trapping capacities are probed using temperature programmed desorption (TPD) and the activities of the catalysts for promoting the NH_{3 ads} + NO/O₂ → N₂ and NO_{x ads} + NH₃ → N₂ reactions are studied using temperature programmed surface reaction (TPSR).     

Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst

The release and reduction of NO_x in a NO_x storage-reduction (NSR) catalyst were studied with a transient reaction analysis in the millisecond range, which was made possible by the combination of pulsed injection of gases and time resolved time-of-flight mass spectrometry. After an O₂ pulse and a subsequent NO pulse were injected into a pellet of the Pt/Ba/Al₂O₃ catalyst, the time profiles of several gas products, NO, N₂, NH₃ and H₂O, were obtained as a result of the release and reduction of NO_x caused by H₂ injection. Comparing the time profiles in another analysis, which were obtained using a model catalyst consisting of a flat 5 nmPt/Ba(NO₃)₂/cordierite plate, the release and reduction of NO_x on Pt/Ba/Al₂O₃ catalyst that stored NO_x took the following two steps; in the first step NO molecules were released from Ba and in the second step the released NO was reduced into N₂ by H₂ pulse injection. When this H₂ pulse was injected in a large amount, NO was reduced to NH₃ instead of N₂.

A only small amount of H₂O was detected because of the strong affinity for alumina support. We can analyze the NO_x regeneration process to separate two steps of the NO_x release and reduction by a detailed analysis of the time profiles using a two-step reaction model. From the result of the analysis, it is found that the rate constant for NO_x release increased as temperature increase.     

Characterization and performance of Pt-USY in the SCR of NOx with hydrocarbons under lean-burn conditions

Pt-USY was used for the selective catalytic reduction of NO_x with hydrocarbons in the presence of excess oxygen. The catalyst was prepared by an ion-exchange method and characterized by XRD, TEM, CO chemisorption, and Ar adsorption at 87 K. The platinum particle size distribution was found to be broad (2–20 nm), with no apparent sintering of the active phase during the HC-SCR process after 25 h time-on-stream. Generally, large metal clusters (>15 nm) are situated at the external surface of the zeolite, while the smaller ones are located in the pores of the support. Pt-USY shows an excellent activity in the deNO_x reaction (molar NO_x conversion 90% at 475 K) with propene as the reductant in 5 kPa O₂, as well as stable operation during time-on-stream. Propane only yields a low NO_x conversion compared to propene. The presence of high oxygen contents (5–10 kPa O₂) slightly inhibits the reaction. No significant decrease in deNO_x activity was observed at high space velocities (up to 100,000 h⁻¹). The presence of SO₂ and H₂O in the feed stream did not significantly affect the deNO_x activity. Pt-USY performs better under lean-burn conditions than other Pt-catalysts supported on e.g. ZSM-5, Al₂O₃, or SiO₂. The selectivity to N₂ was similar to the other Pt-based catalysts (30%), the other major product being N₂O.     

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.     

Alumina- and titania-based monolithic catalysts for low temperature selective catalytic reduction of nitrogen oxides

The selective catalytic reduction of NO+NO₂ (NO_x) at low temperature (180–230°C) with ammonia has been investigated with copper-nickel and vanadium oxides supported on titania and alumina monoliths. The influence of the operating temperature, as well as NH₃/NO_x and NO/NO₂ inlet ratios has been studied. High NO_x conversions were obtained at operating conditions similar to those used in industrial scale units with all the catalysts. Reaction temperature, ammonia and nitrogen dioxide inlet concentration increased the N₂O formation with the copper-nickel catalysts, while no increase was observed with the vanadium catalysts. The vanadium-titania catalyst exhibited the highest DeNO_x activity, with no detectable ammonia slip and a low N₂O formation when NH₃/NO_x inlet ratio was kept below 0.8. TPR results of this catalyst with NO/NH₃/O₂, NO₂/NH₃/O₂ and NO/NO₂/NH₃/O₂ feed mixtures indicated that the presence of NO₂ as the only nitrogen oxide increases the quantity of adsorbed species, which seem to be responsible for N₂O formation. When NO was also present, N₂O formation was not observed.     

Mn/MFI catalyzed reduction of NOx with alkanes

Mn/MFI catalysts were prepared by different methods and probed as catalysts for the catalytic reduction of NO_x with CH₄ or iso-butane in a gas flow containing excess O₂. Mn/MFI with high manganese loading was obtained by solid state ion exchange (SSI). The intensity of an IR band at 957 cm⁻¹, which is due to the perturbation of zeolite lattice vibrations by Mn ions attached to cage walls is proportional to the Mn content of the catalysts. Conversely, the intensity of the 3610 cm⁻¹ band, assigned to Brønsted acid sites decreases linearly with the Mn loading. A catalyst obtained by exchanging Na/MFI with an aqueous solution of Mn acetate is found most active for NO_x reduction with methane. Transport by surface diffusion of Mn ions from MnI₂ to exchange positions in MFI is more efficient than their transport through the gas phase. High NO conversion over proton-free catalysts indicates that protons are not instrumental in NO_x reduction over Mn/MFI.     

Analysis of the thermodynamic feasibility of NOx decomposition catalysis to meet next generation vehicle NOx emissions standards

Free energy minimization calculations are used to determine the thermodynamic equilibrium concentrations of NO_x and other species in stoichiometric and lean gas mixtures over a range of temperatures and compositions. Under lean (excess N₂ and O₂) conditions, the NO decomposition (NO↔(1/2)N₂+(1/2)O₂) and NO oxidation (NO+(1/2)O₂↔NO₂) equilibria impose lower bounds on the NO_x concentrations achievable by thermodynamic equilibration or NO_x decomposition, and these equilibrium NO_x concentrations can be practically significant. Assuming a perfect isothermal catalyst acting on a representative diesel exhaust stream collected over the federal test procedure (FTP) cycle, equilibrium NO_x levels exceed upcoming California Low Emission Vehicle II (LEV-II) and Tier II NO_x emissions standards for automobiles and trucks at temperatures above approximately 800 K. Consideration of a perfect adiabatic catalyst acting on the same diesel exhaust shows that equilibrium NO_x values can fall below NO_x emissions standards at lower temperatures, but to achieve these low concentrations would require the catalyst to attain 100% approach to equilibrium at very low temperatures. It is concluded that NO_x removal based on a thermodynamic equilibrating catalyst under lean exhaust conditions is not practically viable for automotive application, and that to achieve upcoming NO_x standards will require selective NO_x catalysts that vigorously promote NO_x reactions with reductant and do not promote NO decomposition or oxidation. Finally, the ability of a selective NO_x catalyst system to reduce NO_x concentrations to or below thermodynamic equilibrium values is proposed as a useful measure for selective catalytic reduction (SCR) activity.     

The bifunctional reaction pathway and dual kinetic regimes in NOx SCR by methane over cobalt mordenite catalysts

The pathway for selective reduction of NO_x by methane over Co mordenite cataysts has been studied by comparing the rates of the individual reactions (NO oxidation, CH₄ oxidation, NO₂ reduction) with that of the combined reaction (NO + O₂ + CH₄). Co⁽⁺²⁾ was exchanged into H-MOR and Na-MOR to give catalysts with different metal loading and number of support protons. Additionally, exchanged Co⁽⁺²⁾ ions were precipitated with NaOH to produce dispersed cobalt oxide on Na-MOR. The NO oxidation rate is the same for ion exchanged Co⁽⁺²⁾ ions in H-MOR and Na-MOR, but the rate of Co⁽⁺²⁾ ions is much lower than that of cobalt oxide. NO oxidation equilibrium is obtained only for those catalysts with high metal loading, cobalt oxide or run at low GHSV. Under the conditions of selective catalytic reduction, methane oxidation by O₂ is low for all catalysts. The turnover frequency of Co on Na-MOR, however, is higher than that on H-MOR. The rate of NO₂ reduction to N₂ is directly proportional to the number of support acid sites and independent of the amount of Co. Comparison of the rates and selectivities for the individual reactions with the combined reaction of NO + O₂ + CH₄ indicates that there are two types of catalysts. For the first, the NO oxidation is in equilibrium and the rate determining step is reduction of NO₂. For these catalysts, the rate (and selectivity) for formation of N₂ is identical from NO + O₂ + CH₄ and NO₂ + CH₄. These catalysts have high metal loading and few acid sites. Nevertheless, the rate of N₂ formation increases with increasing number of protons. For the second type of catalyst, NO oxidation is not in equilibrium and is the rate limiting step. For these catalysts the rate of N₂ formation increases with increasing metal loading. Neither catalyst type, however, is optimized for the maximum formation of N₂. By using a mixture of catalysts, one with high NO oxidation activity and one with a large number of Brønsted acid sites, the rate of N₂ is greater than the weighted sum of the individual catalysts. The current results support the proposal that the pathway for selective catalytic reduction is bifunctional where metal sites affect NO oxidation, while support protons catalyze the formation of N₂.     

Effect of oxygen concentration on the NOx reduction with ammonia over V2O5–WO3/TiO2 catalyst

The catalytic reduction of NO_x in the typical operation temperatures and oxygen concentrations of diesel engines has been studied in the presence of V3W9Ti in a tubular flow reactor. The results have shown that the selective catalytic reduction is strongly affected by the oxygen concentration in low temperature range (150–275 °C). At higher temperatures, the reaction becomes independent of the O₂ concentration. The rate of the selective catalytic reduction of NO with ammonia may be considerably enhanced by converting part of the NO into NO₂. DRIFT measurements have shown that NH₃ and NO₂ are adsorbed on the catalyst surface on the contrary of NO. The experiments have shown that the decrease in N₂ selectivity of the SCR reaction is mainly due to the SCO of ammonia and to the formation of nitrous oxide.     

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.     

Catalytic reduction of NOx by hydrocarbons over Co/ZSM-5 catalysts prepared with different methods

Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl₂ vapor is deposited. TPR shows peaks for Co²⁺ ions at 695–705°C and for Co₃O₄ at 385–390°C, but a peak in the 220–250°C region appears to indicate Co²⁺ oxo-ions.

The catalysts have been tested for the selective reduction of NO_x with iso-C₄H₁₀ under O₂-rich conditions and in the absence of O₂, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co₃O₄ clusters first oxidize NO to NO₂, which is chemisorbed as NO_y (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N₂ yield 90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co²⁺ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O₂-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co²⁺ ions without intermediate formation of NO₂.

The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO₂ formation.     

Role of zeolite structure on reduction of NOx with methane over In- and Pd-based catalysts

Catalytic performances of ZSM-5 based catalysts containing indium or palladium were examined for NO reduction with CH₄ and NO_x chemisorption. The amounts of NO_x chemisorbed on In/H-ZSM-5 were well proportional to the catalytic activities for NO_x reduction. Pd/H-ZSM-5, on the other hand, hardly chemisorbed NO₂, while the catalytic activity for NO₂ reduction with CH₄ is very high. Furthermore, Pd loaded on SiO₂ showed comparably high catalytic activity for NO₂ reduction with CH₄ at 400°C in the absence of oxygen as Pd/H-ZSM-5. CH₄ combustion during NO_x reduction with CH₄ in the presence of oxygen significantly occurred over PdO on SiO₂, while less over Pd/H-ZSM-5. The role of zeolite might be slightly different between In/H-ZSM-5 and Pd/H-ZSM-5: the zeolitic porous structure is needed for In/H-ZSM-5 in order to concentrate NO₂ adspecies on InO⁺ sites, which is important for NO reduction with CH₄ on In/H-ZSM-5 based catalysts, while the ion-exchangeable ability of zeolite is needed for Pd/H-ZSM-5 in order to make Pd²⁺ located in a highly dispersed state, on which NO is strongly chemisorbed.     

CO2, NOx and SO2 emissions from the combustion of coal with high oxygen concentration gases

The emissions of CO₂, NO_x and SO₂ from the combustion of a high-volatile coal with N₂- and CO₂-based, high O₂ concentration (20, 50, 80, 100%) inlet gases were investigated in an electrically heated up-flow-tube furnace at elevated gas temperatures (1123–1573 K). The fuel equivalence ratio, φ, was varied in the range of 0.4–1.6. Results showed that CO₂ concentrations in flue gas were higher than 95% for the processes with O₂ and CO₂-based inlet gases. NO_x emissions increased with φ under fuel-lean conditions, then declined dramatically after φ=0.8, and the peak values increased from about 1000 ppm for the air combustion process and 500 ppm for the O₂(20%)+CO₂(80%) inlet gas process to about 4500 ppm for the oxygen combustion process. When φ>1.4 the emissions decreased to the same level for different O₂ concentration inlet gas processes. On the other hand, NO_x emission indexes decreased monotonically with φ under both fuel-lean and fuel-rich combustion. SO₂ emissions increased with φ under fuel-lean conditions, then declined slightly after φ>1.2. Temperature has a large effect on the NO_x emission. Peak values of the NO_x emission increased by 50–70% for the N₂-based inlet gas processes and by 30–50% for the CO₂-based inlet gas process from 1123 to 1573 K. However, there was only a small effect of temperature on the SO₂ emission.     

On the nature of active sites in Fe/ZSM-5 catalysts for NOx abatement

Fe/ZSM-5 catalysts with high Fe loading (Fe/Al1) have been prepared by sublimation of FeCl₃ onto H-ZSM-5 samples of different Si/Al ratios. They catalyze NO_x reduction with hydrocarbons in an excess of O₂ and H₂O. TPR shows that the Fe in the zeolite cavities is different from Fe₂O₃ particles. Naked Fe³⁺ ions are absent; oxo-ions, which are equally well reducible by CO and H₂, prevail. A minority of the Fe complexes lose oxygen upon mere heating to 500°C; some of the reduced sites are reoxidized only by N₂O. The population of oxo-complexes that lose oxygen by heating depends on the Si/Al ratio, this dependence is in qualitative agreement with the model of (2+) charged binuclear ions [HO–Fe–O–Fe–OH]²⁺. Upon reacting with NO, the bridging O atom is transferred and NO₂ is formed. This step is not rate limiting for active catalysts with high Al/Si ratio and high Fe loading, but it becomes critical with zeolites of low Al/Si ratio.     

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.