Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons

The interactions of nitrogen oxides with carbons differing in the chemical structure of surface functional groups were studied using in situ FTIR combined with the measurements of catalytic activity. Microporous carbon samples with similar pore size distribution were prepared from cellulose. The structure and coverage of adsorbates during reactions at temperatures between 295 and 573 K are determined by FTIR. No significant changes in NO_x reaction with carbon surface were found by oxidation of the carbonized film. During the study of the reaction of NO/O₂ mixture with carbons, the infrared absorption bands for the surface species formed are similar to the IR bands observed after the reaction of carbon samples with NO₂. For both reactions, surface species, including C-NO₂, C-ONO, C-NCO and anhydride structures are formed. Catalytic NO_x reduction by carbons has been investigated in the temperature range 295-623 K in the flow reactor equipped with an FTIR gas analyzer. As the surface of carbon is exposed to NO₂ gaseous NO is formed. The reduction of NO₂ to N₂ without the use of an externally supplied reductant can be achieved with microporous carbons. Significant NO₂ conversion to N₂ occurred at 623 K on both oxidized and non-oxidized carbons.     

deNOx Performance and Reaction Mechanism of the Di-Air System

The detailed reaction mechanism of Di-Air, which showed the unprecedented high deNO_x performance at high temperature conditions, was studied in this work. Since the Di-Air phenomenon occurs with continuous short pulse injections of hydrocarbons over NSR (NO_x storage and reduction) catalyst, this study focused on the specific function of HC as an effective reductant for NO_x reduction reaction. As a first step, the deNO_x performance was compared with three different reductant gases including CO, H₂ and C₃H₆ in a modified synthetic gas bench equipped with a gas injector which enables continuous small amount of injections. At inlet gas temperature of 450 °C C₃H₆ showed the best deNO_x performance whereas H₂ was the best at 150 °C. Moreover the result of temperature programmed desorption showed that intermediate species represented by –NCO (isocyanates) produced from C₃H₆ was thermally more stable than that produced from CO. These results confirmed that the injected HC reacts with adsorbed NO_x on NSR catalyst generating thermally stable intermediates, which could contribute the high deNO_x performance at high temperature conditions.     

Harmful by-products in selective catalytic reduction of nitrogen oxides by olefins over alumina

The selective reduction of nitrogen dioxide and nitrogen monoxide by olefins (ethene, propene) has been studied over two different -aluminium oxides in the temperature range 473–873 K. Nitrogen dioxide was reduced more effectively than nitrogen monoxide with both, ethene and propene, as a reductant. At temperatures exceeding 700 K, ammonia was formed as a by-product over one type of alumina. Concentrations in the range 30–40 ppm were determined for propene in combination with both, NO and NO₂, while no ammonia was produced with ethene as a reductant. In addition, significant formation of hydrogen cyanide up to 70 ppm was observed with propene over both aluminium oxides starting from either NO or NO₂. In contrast, hydrogen cyanide formation remained below 10 ppm with ethene as a reductant. Nitrous oxide formation did not exceed 10 ppm for all investigations. The results show that for alumina catalysts ethene is a more suitable reductant than propene due to its lower tendency to form undesired by-products.     

NO2 formation over Cu-ZSM-5 and the selective catalytic reduction of NO

The extent of the selective catalytic reduction (SCR) of nitric oxide to dinitrogen in the presence of excess oxygen is enhanced by the oxygen on several zeolite-based catalysts and using different reductants. When the catalyst is Cu-ZSM-5 and the reductant is a hydrocarbon, an NO₂ intermediate has been suggested by several investigators. This work shows that at short residence times, with excess reductant and in the absence of oxygen, the NO₂ itself is reduced only back to NO. Thus, for the selective reduction of NO₂ to N₂ (N-pairing) strongly oxidizing conditions are required, same as for the complete reduction of NO. In the presence of excess oxygen the activity of Cu-ZSM-5 in the NO + O₂ reaction to form NO₂ parallels the SCR in every respect. It is higher over Cu-ZSM-5 than on Cu/Al₂O₃ or on H-ZSM-5. The coppercontaining zeolite is also active in the decomposition of NO₂ back to NO and O₂ while the other catalysts are much less active. The inhibiting effect of water on the NO + O₂ catalytic reaction is also parallel to the effect on SCR. This evidence strengthens the notion of an NO₂ intermediate.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

Effect of Silver Loading on the Lean NOx Reduction with Methanol Over Ag–Al2O3

The influence of silver loading on the lean NO_x reduction activity using methanol as reductant has been studied for alumina supported silver catalysts. In general, increasing the silver loading (0–3 wt%), in Ag–Al₂O₃, shifts or extends the activity window, for lean NO_x reduction towards lower temperatures. In particular Ag–Al₂O₃ with 3 wt% silver is active for NO_x reduction under methanol-SCR conditions in a broad temperature interval (200–500 °C), with high activity in the low temperature range (maximum around 300 °C) typical for exhaust gases from diesel and other lean burn engines. Furthermore, increasing the C/N molar ratio enhances the reduction of NO_x. However, too high C/N ratios results in poor selectivity to N₂.     

Oxidation of Acetaldehyde and Propionaldehyde on a VOx/TiO2 Catalyst in the Presence of Water Vapor

The partial oxidation of acetaldehyde and propionaldehyde on a TiO₂ supported VO_x catalyst in the presence of water vapor was investigated at temperatures from 120 to 280 °C. Depending on the kind of aldehyde and reaction temperature, the selective oxidation to the appropriate carboxylic acid and an oxidative splitting to lower carboxylic acids took place. Acetaldehyde was oxidized to acetic acid with selectivities up to 82 % at ～ 200 °C whereas propionic acid was formed only with selectivities of about 20 % at ～ 140 °C in the oxidation of propionaldehyde. The oxidative cleavage of propionaldehyde led to the formation of more acetic acid than formic acid, which was in agreement with the higher formation of CO_x compared to that in the acetaldehyde oxidation. The presence of water and the increasing concentration of oxygen in the feed was found to enhance the selectivity towards the formation of C₁ to C₃ carboxylic acids by inhibiting the total oxidation of aldehydes and carboxylic acids.     

FTIR and Transient Reactivity Experiments of the Reduction by H2, CO and HCs of NO x Stored Over Pt–Ba/Al2O3 LNTs

The reduction of NO _x stored over a Pt–Ba/Al₂O₃ Lean NO _x Trap is analysed when H₂, CO or heptane are used as reductants. In all cases, the reduction proceeds via a Pt-catalyzed process involving the formation of intermediate species like ammonia and isocyanates in the case of H₂ and CO, respectively. No specific intermediates have been observed when heptane is used as reductant. It is claimed that the role of the reductant is to keep Pt in a reduced state; this favours nitrate decomposition and reduction over the Pt sites. The effect of water on the reaction is also investigated.     

Catalytic process for decolorizing yellow plume

Yellow-colored exhaust gas streams from internal engines or gas turbines, frequently referred to as “yellow plume,” contain nitrogen dioxide (NO₂) at concentrations as low as 15 ppm. The process developed in this work for decolorizing the yellow plume is based on reduction of NO₂ to NO utilizing a combination of a Pt catalyst and a reducing agent. A stoichiometric excess of carbon monoxide, diesel oil, methanol or ethanol were used as reducing agents. Depending on the type of the reductant, the active temperature window of NO₂ reduction was varied with methanol and CO being active at lower temperatures and ethanol and diesel oil at higher temperatures. By changing the Pt loading of the catalysts the active temperature window of NO₂ reduction was also changed, higher loading Pt catalysts being active at lower temperatures. This scheme of NO₂ reduction process was verified in a pilot-scale test with the real exhaust gas from the gas turbine power plant, showing 96% of NO₂ reduction at the stack temperatures of 102–123 °C and at space velocities of 28,000–95,000 h⁻¹ with inherent CO in the exhaust gas as the reducing agent.     

NO2 Reduction with Nitromethane over Ag/Y: A Catalyst with High Activity over a Wide Temperature Range

Nitromethane (NM) is a very efficient reductant for converting NO₂ to N₂ over Ag/Y: Between 140 °C and 400 °C, the N₂ yield is close to 100%. This high N₂ yield results from the ability of Ag/Y to effectively catalyze the reaction between NM and NO₂. This high catalytic activity of Ag/Y is minimally affected by surface bound CN⁻, NC⁻, or acetate, all of which are stable at temperatures below ∼300 °C. At T ≥ 400 °C, there is a reaction path that yields N₂ from NM even in the absence of NO₂. However even at 400 °C, under typical deNO _x conditions, most N₂ molecules are formed as a result of the reaction of NM and NO₂.     

How to control the selectivity in the reduction of NOx with H2 over Pt-Ba/Al2O3 Lean NOx Trap catalysts

The reduction process of NO_x species stored over Pt-Ba/Al₂O₃ Lean NO_x Trap systems is analysed in this paper when H₂ is used as a reductant. The effect of different experimental conditions (temperature, reductant concentration, adsorption lengths, etc.) is addressed and discussed in relation to the selectivity and the efficiency of the reduction process.     

Denitrification in aqueous FeEDTA solutions

The biological reduction of nitric oxide (NO) in aqueous solutions of FeEDTA is an important key reaction within the BioDeNOx process, a combined physico‐chemical and biological technique for the removal of NO_x from industrial flue gasses. To explore the reduction of nitrogen oxide analogues, this study investigated the full denitrification pathway in aqueous FeEDTA solutions, ie the reduction of NO₃^?, NO₂^?, NO via N₂O to N₂ in this unusual medium. This was done in batch experiments at 30 °C with 25 mmol dm^?3 FeEDTA solutions (pH 7.2 ± 0.2). Also Ca²⁺ (2 and 10 mmol dm^?3) and Mg²⁺ (2 mmol dm^?3) were added in excess to prevent free, uncomplexed EDTA. Nitrate reduction in aqueous solutions of Fe(III)EDTA is accompanied by the biological reduction of Fe(III) to Fe(II), for which ethanol, methanol and also acetate are suitable electron donors. Fe(II)EDTA can serve as electron donor for the biological reduction of nitrate to nitrite, with the concomitant oxidation of Fe(II)EDTA to Fe(III)EDTA. Moreover, Fe(II)EDTA can also serve as electron donor for the chemical reduction of nitrite to NO, with the concomitant formation of the nitrosyl‐complex Fe(II)EDTA–NO. The reduction of NO in Fe(II)EDTA was found to be catalysed biologically and occurred about three times faster at 55 °C than NO reduction at 30 °C. This study showed that the nitrogen and iron cycles are strongly coupled and that FeEDTA has an electron‐mediating role during the subsequent reduction of nitrate, nitrite, nitric oxide and nitrous oxide to dinitrogen gas. Copyright © 2004 Society of Chemical Industry     

Identification of adsorbed species on Cu-ZSM-5 under NH3 SCR conditions

The selective catalytic reduction of nitrogen oxides with ammonia as the reducing agent was studied using Fourier transform infrared (FTIR) spectroscopy. The adsorbed species found on a Cu-ZSM-5 powder during exposure to NO, NO₂ or NH₃ was compared to the adsorbed species identified during SCR conditions. A blocking effect caused by ammonia at 175 °C was investigated by a stepwise increase of the ammonia concentration, and the spectra indicated that the formation of nitrites or nitrates decreased as surface coverage of ammonia increased. No such effect was observed at 350 °C, since the oxidation of ammonia results in very low ammonia coverage. The effect of changes in the NO to NO₂ ratio was also studied at 350 °C, and the species identified during SCR reaction indicated that the enhanced activity at equimolecular amounts of NO and NO₂ possibly involves gas phase components as well as adsorbed species.     

Selective reduction of NOx by hydrogen and methane in natural gas stationary sources over alumina-supported Pd, Co and Co/Pd catalysts: Part A. On the effect of palladium precursors and catalyst pre-treatment

The aim of the present work is to study the selective reduction of NO_x from natural gas sources. The unburned methane can be used as reductant. Another reductant such as hydrogen can be created in situ, using a microreformer. The results suggest that the NO_x are reduced by H₂ at low temperature, when methane is not activated and at higher temperature the methane is then the main reductant. However, the catalytic behaviour depends on the metal precursor and the catalyst treatment. The most prominent result is obtained on the palladium catalyst prepared from Pd(NH₃)₄(NO₃)₂ precursor. Comparing the reduction and the calcination step in the course of catalyst preparation, one can conclude that calcination lead to the higher activity in deNO_x, since reduced catalysts are oxidized during the deNO_x process.     

Effect of Low Concentration of SO x on NO Reduction with Propene Over Ag/Al2O3

Catalytic activity for NO reduction with propene was investigated at 0–80 ppm SO₂. NO was reduced more efficiently by propene on SO₂-treated than untreated catalyst. Simultaneously, combustion of reductant was observed to lower NO reduction efficiency. Thus, the role of surface-adsorbed SO _x species was regarded as depressing reductant combustion. NH₃ adsorption revealed that SO₂ treatment increased Bronsted acidity of the Ag/Al₂O₃ catalyst, which promoted propene activation. Reductant activation is a more important step, compared with NO activation to oxidative nitrate species. The NCO species, an index intermediate in NO _x reduction, was produced on SO₂-adsorbed Ag/Al₂O₃ at a lower temperature (473 K) than on the untreated catalyst. The reductive intermediates at low temperature are suggested to be alcohol, or aldehyde-adsorbed species, based on observed C=O band.     

Investigating the Effect of NO Versus NO2 on the Performance of a Model NO X Storage/Reduction Catalyst

The effects of using NO or NO₂ as the NO _X source on the performance of a NO _X storage/reduction catalyst were investigated from 200 to 500 °C. The evaluation included comparison with constant cycling times and trapping the same amount of NO _X during the lean phase. With NO₂ as the NO _X source, better trapping and reduction performance was attained in comparison to NO, at all operating temperatures except 300 °C. This exception, under the conditions tested, was likely due to high NO oxidation activity and rapid trapping of NO₂, although it is expected that extending the trapping time would lead to consistent differences. Several reasons for the observed improvements at 200, 400 and 500 °C with NO₂ relative to NO are discussed. One that can explain the data, for both trapping and release improvement, is treating the monolith as an integral reactor. With NO₂, more NO _X is trapped at the very inlet of the catalyst, whereas with NO, the maximum in trapping during cycling occurs slightly downstream. Thus more of the catalyst can be used for trapping with NO₂ as the NO _X source. The decreased release during catalyst regeneration is similarly explained; with more being released at the very inlet, there is more residence time and therefore contact with downstream Pt sites, but more importantly more interaction between reductant and stored NO _X . NH₃ and N₂O measurements support this conclusion.     

The NO x Reduction Mechanism by H2 under near Isothermal Conditions over Pt–K/Al2O3 Lean NO x Trap Systems

The reduction of NO _x stored over a Pt–K/Al₂O₃ Lean NO _x Trap is analyzed when H₂ is used as reductant. An in series 2-steps process is herein proposed, involving at first the formation of NH₃ upon reaction of nitrates with H₂ (step 1), followed by the reaction of NH₃ with residual nitrates to give N₂ (step 2).     

Reduction of NOx over a NOx-Trap Catalyst and the Regeneration Behaviour of Adsorbed SO2

A conventional NO _x -trap catalyst containing platinum, rhodium, barium and lanthanum was conditioned with oxygen at 500°C, preloaded with NO under standard oxidising conditions and then subjected to regeneration with the reductants H₂, CO and C₃H₆, either alone or as a mixture. Hydrogen is the most efficient reductant in terms of NO _x conversion efficiency and reductant usage efficiency. There is a temperature optimum for CO between 300 and 400°C and a catalyst loading optimum (mols reductant added)/(mols NO _x adsorbed) between 1.5 and 3.0. The behaviour of the catalyst towards sulphur poisoning was examined in supplementary trials with the adsorption of SO₂ in the presence or absence of water vapour. When water is not present in both adsorption and reduction steps, very stable sulphates are formed, unattacked by reductants even at 1000°C. Sulfates are more easily reduced when water is present in the reductant mixture.     

Reduction of lean NO2 with diesel soot over metal-exchanged ZSM5, perovskite and γ-alumina catalysts

The catalytic performances of metal-exchanged ZSM5, perovskite and γ-alumina catalysts for the reduction of nitrogen dioxide (NO₂) by diesel soot were investigated. The reaction tests were performed through temperature-programmed reaction (TPR), in which NO₂ and O₂ were passed through a fixed bed of catalyst-soot mixture. On the three types of catalyst, NO₂ was reduced to N₂ by model soot (Printex-U) and most of the soot was converted into CO₂. Pt-, Cu- and Co-exchanged ZSM5 catalysts exhibited reduction activities with conversions of NO₂ into N₂ of about 20%. Among the perovskite catalysts tested, La_0.9K_0.1FeO₃ showed a 32% conversion of NO₂ into N₂. The catalytic activities of the perovskite catalysts were largely influenced by the number and stability of oxygen vacancies. For the γ-alumina catalyst, the peak reduction activity appeared at a relatively high temperature of around 500 °C, but the NO₂ reduction was more effective than the NO reduction, in contrast to the results of the ZSM-5 and perovskite catalysts.