Selective Catalytic Reduction Of Nitrogen Oxides Research Articles

Influence of Supports on Pd Catalysts for the Selective Catalytic Reduction of NOx with H2 and CO

A major concern of current commercial catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NOx) (DeNOx) is the lack of sufficient catalytic activity at low temperatures (below 200°C) that typically occur during cold start and urban driving conditions. To address this issue, a study was undertaken to develop an (H2 + CO)-SCR Pd-based catalyst to obtain high DeNOx activity in this low-temperature range. A series of catalysts with Pd loaded on supports containing various amounts of γ-alumina (γ-Al2O3) (0–65 wt%), CBV-2314 (15–100 wt%), and TiO2 (0–20 wt%) were tested for DeNOx activity and aging in a catalyst testing unit (CTU). The results showed that each component in the support played a role in the NOx conversion, depending on the temperature. Catalysts with Pd supported on {[γ-Al2O3 (60 wt%) + CBV-2314 (20 wt%)]–TiO2 (20 wt%)} achieved the best overall DeNOx activity. Catalysts with various Pd loadings (0.05 wt% to 0.5 wt%) were prepared with a {[γ-Al2O3 (60 wt%) + CBV-2314 (20 wt%)] − TiO2 (20 wt%)} support. The catalysts loaded with 0.05 wt% and 0.2 wt% Pd demonstrated the best overall DeNOx activity (up to 90 % NOx conversion starting at 150 °C). These catalysts were subjected to an accelerated aging regime. Based on the aging studies, the 0.2 wt% Pd / {[γ-Al2O3 (60 wt%) + CBV-2314 (20 wt%)] − TiO2 (20 wt%)} composition was the most active and stable catalyst.

Hydrogen-assisted SCR of NOx over alumina-supported silver and indium catalysts using C2-hydrocarbons and oxygenates

This work focuses on the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ethane, ethene, acetic acid, dimethyl ether (DME) and ethanol, respectively, over alumina-supported silver and indium catalysts. The impact of hydrogen and the nature of the reductant on the catalyst structure and SCR activity are studied. The ability to reduce NOx differs significantly among the studied reductants, but also between the two catalysts, of which Ag/Al2O3 is the overall most active catalyst. However, the In/Al2O3 catalyst exhibits high SCR activity with DME as reductant in contrast to the Ag/Al2O3 catalyst, which shows negligible activity. We attribute this difference to the higher number of acidic sites on the In/Al2O3 catalyst and the higher degree of DME combustion over Ag/Al2O3. Moreover, the number of sites that previously have been identified to promote hydrocarbon activation, i.e. silver cluster sites (Agnδ+) in Ag/Al2O3 catalysts and indium cluster sites (In3+) in In/Al2O3 catalysts, are shown to increase upon addition of small amounts of hydrogen in the feed. We suggest that the increased SCR activity over In/Al2O3 originates from an increased number of sites for activation of hydrocarbons, analogous to previous proposals for Ag-based catalysts, in combination with a direct effect on the reaction mechanism.

Quantification of Urea-spray Non-uniformity Effects on the H2-assistedNO Reduction and NH3 Slip over an Ag/Al2O3 Catalyst

Selective catalytic reductionof nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient technology for lean NOx removal in automotive applications. In the current work, a kinetic model forhydrogen-assisted NH3-SCR over a silver-alumina (Ag/Al2O3) monolith catalyst is adapted to the corresponding experimental results. The degree of non-uniformity in the NH3 dose to individual catalyst channels during operation of a urea-SCR system is also investigated and the NH3 dose probability distribution functions are derived. The effects ofNH3non-uniformity on the NO conversion and NH3 slip are studied over an otherwise optimal Ag/Al2O3 system. Above the stoichiometric point, the NH3 slip is shown to increase almost linearly with increasing NH3 dosage. Channels that receive significantly lower NH3 dose than the stoichiometric one exhibit low NO conversion, whereas there is no discernable increase in the NO conversion above an NH3 dose four times the average. These results highlight the importance of the interaction between the design of the NH3dosing system and the performance of the catalytic system.

Evaluation of carbon nanotubes produced from toluene steam reforming as catalyst support for selective catalytic reduction of NOx

There is current interest in developing low cost, effective catalysts for the low temperature selective catalytic reduction (SCR) of nitrogen oxides (NOx). In this work, we have applied carbon nanotubes (CNTs), produced as a by-product of hydrogen production from the steam reforming of toluene (as a representative hydrocarbon), as a catalyst support for a V2O5–WO3 catalyst for SCR of NOx. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis showed well dispersed metals on the surface of the CNTs. The V2O5–WO3/CNT catalyst has exhibited NOx reduction efficiency higher than 95% at reaction temperatures between 340 and 400 °C. However, there was a low NOx reduction at SCR reaction temperature of less than 200 °C which is suggested to be due to the lack of Lewis acid sites, as determined from NH3-TPD (temperature program desorption) analysis. Future work to lower the SCR reaction temperature with high NOx efficiency is suggested.

Physical mixture of Ag/Al2O3 and Zn/ZSM-5 as an active catalyst component for selective catalytic reduction of NOx with n-C10H22

The activity of single and two-component catalyst systems consisting of a partial oxidation catalyst and a reduction catalyst (Ag/Al2O3), were investigated for selective catalytic reduction (SCR) of nitrogen oxides (NOx) with n-decane (HC) as a reductant. The single component Ag/Al2O3 catalyst showed a maximum NOx conversion of 28% at 400°C and formation of minor amounts of hydrocarbon cracking products. Co-presence of partial oxidation catalyst, especially metal/ZSM-5, along with Ag/Al2O3 enhanced the NOx conversion of the composite catalysts. Among the composite catalysts, Zn/ZSM-5–Ag/Al2O3 combination led to the most increase in NOx conversion. It was identified that the n-decane can be more effectively converted, on Zn/ZSM-5, to smaller hydrocarbons (such as ethene, propane and propene) intermediates. These in situ generated intermediates were found to enhance the NOx conversion of Zn/ZSM-5–Ag/Al2O3 composite catalysts. Catalyst optimization showed that the NOx conversion increased with increase in zinc loading of up to 5wt% in ZSM-5 and by increasing Zn/ZSM-5 content in the composite mixture catalyst. A combination of 17mg of Ag/Al2O3 and 33mg of 5%Zn/ZSM-5 led to the most active catalyst showing a maximum NOx conversion of about 67% at 400°C.

Removal of NOx with Selective Catalytic Reduction Based on Nonthermal Plasma Preoxidation

An extensive series of experiments have been conducted using a nonthermal plasma generated by dielectric barrier discharge (DBD) process combined with vanadium pentoxide catalyst to reduce the nitrogen oxides (NOx) from diesel engine exhaust over a broad reaction temperature (100−500 °C). In this system, the effects of input voltage, propylene (C3H6) concentration, and sulfur content, etc. on the plasma facilitated (PF) selective catalytic reduction of NOx with NH3 were examined. In the presence of C3H6 as an additive, the oxidation of NO to NO2 is largely enhanced even with lower input voltages. The PF NH3−SCR system enhanced the overall reaction and showed a remarkable improvement in NOx removal efficiency at temperatures of 100−250 °C. The removal of NOx was found to be largely increased by the input voltage and the addition of propylene. Besides the small amount of nitrous oxide and the significant amount of carbon monoxide, aldehydes-type unregulated byproduct such as formaldehyde and acetaldehyde were also observed at the outlet of the DBD reactor, while formaldehyde and acetaldehyde could be almost completely removed in the NH3−SCR reactor. The NOx conversion decreases at lower temperatures but increases at higher temperatures with SO2 concentration increases. The PF NH3−SCR hybrid system can be used stably with several hundreds of ppm of SO2 in durability tests. Moreover, the presence of SO2 inhibits N2O formation at all employed reaction temperatures.

Experimental and Kinetic Study of Selective Catalytic Reduction of NO with NH 3 over CuO/Al 2O 3/Cordierite Catalyst

The CuO/γ-Al 2O 3/cordierite catalyst, after being sulfated by sulfur dioxide (SO 2) at 673 K, exhibits high activities for selective catalytic reduction (SCR) of nitrogen oxide (NO) with ammonia (NH 3) at 573-723 K. The intrinsic kinetics of SCR of NO with NH 3 over CuO/γ-Al 2O 3/cordierite catalyst has been measured in a fixed-bed reactor in the absence of internal and external diffusions. The experimental results show that the reaction rate can be quantified by a first-order expression with activation energy E′ a of 94.01 kJ·mol −1 and the corresponding pre-exponential factor A′ of 3.39×10 8 cm 3·g −1·s −1 when NH 3 is excessive. However, when NH 3 is not enough, an Eley-Rideal kinetic model based on experimental data is derived with E a of 105.79 kJ·mol −1, the corresponding A of 2.94×10 9 cm 3·g −1·s −1, heat of adsorption Δ H ads of 87.90 kJ·mol −1 and the corresponding A ads of 9.24 cm 3·mol −1. The intrinsic kinetic model obtained was incorporated in a 3D mathematical model of monolithic reactor, and the agreement of the prediction with experimental data indicates that the present kinetic model is adequate for the reactor design and engineering scale-up.

Silver supported mesoporous SBA-15 as potential catalysts for SCR NOx by ethanol

Reduction of nitrogen oxides (NOx) in a lean exhaust gases has become one of the most important environmental concerns. This study compared the performances of DeNOx and the properties of silver/mesoporous aluminosilica synthesized by different methods. Silver nanoparticles were obtained after calcination of the materials prepared by incipient wetness or by the excess solvent impregnation of Al-SBA-15 support by silver nitrate. On the other hand, the silver nitrate was introduced on the synthesis gel of SBA-15. The solid product was used as support to deposit aluminium. The effect of synthesis method on silver incorporation and the porous structure of the resulting solids has been examined. Some techniques had been applied, such as: elemental analysis, X-ray diffraction (XRD), N2 sorption measurements and Transmission Electron Microscopy (TEM). The nature of silver species in these catalysts was investigated by XPS measurements, high angle XRD, high resolution TEM and TPR/H2 (temperature-programmed reduction). The resulting materials were tested in the selective catalytic reduction of NOx by ethanol in the presence of oxygen. Finally, the effect of H2 on the DeNOx activity was also investigated.The impregnation method of Al-SBA-15 by silver nitrate influences the size, the location of the particles and the catalytic activity. To maintain a higher DeNOx activity, the percentage of aluminium loading and the feed of H2 gases should be increased.

Effect of vanadium on the behaviour of unsulfated and sulfated Ti-pillared clay catalysts in the SCR of NO by NH 3

Vanadium supported on unsulfated and sulfated Ti-pillared clays are developed for the selective catalytic reduction (SCR) of nitrogen oxide (NO) by ammonia. The unsulfated Ti-pillared clays (TiM-PILC) were prepared by hydrolysis of titanium methoxide (Ti(OCH 3) 4) with HCl. The same procedure was employed using H 2SO 4 in order to obtain the sulfated Ti-pillared clays (STiM-PILC). All the pillared materials were characterized before and after vanadium addition by chemical analysis, XRD, N 2-physisorption, TGA, NH 3-TPD, H 2-TPR and UV–vis. It was found that the sulfation or the addition of a low amount of vanadium into the TiM-PILC reduces the surface area but improves the acidity of the material. The addition of vanadium to the STiM-PILC decreases the surface area, the number of acid sites and the stability of the sulfate species but enhances the redox properties of the material. The STiM-PILC and the V-TiM-PILC exhibit higher NO removal activity at high temperature than the TiM-PILC. The presence of both sulfate and a low amount of vanadium (2 wt.%) in the TiM-PILC leads to an excellent active and selective catalyst for SCR-NO with NH 3.

Selective catalytic reduction of NO with C1–C3 reductants over solvothermally prepared Ga2O3–Al2O3 catalysts: Effects of water vapor and hydrocarbon uptake

The influence of reducing agents (C1–C3 hydrocarbons) upon the selective catalytic reduction (SCR) of nitrogen oxide (NO) on γ-Ga2O3–Al2O3 solid solutions was explored. Under dry conditions, the γ-Ga2O3–Al2O3 solid solutions having large crystallite size had high activity for the SCR of NO with various hydrocarbons. However, under wet conditions, NO conversion on the catalyst decreased drastically for all the hydrocarbons. The decrease of NO conversion caused by the presence of water is due to the preferential adsorption of water which inhibits hydrocarbon adsorption on the catalyst surface. Among reductants of C1–C3 hydrocarbons, the most atom-efficient NO abatement was achieved with C2 hydrocarbons. C3 hydrocarbons were less selective for NO reduction than the other hydrocarbons. It was suggested that only one carbon atom per molecules were used for the SCR reaction, and the other carbon atoms were consumed by the simple combustion reaction.

FTIR study of the selective catalytic reduction of NO 2 with ammonia on nanocrystalline NaY and CuY

In this study, the selective catalytic reduction (SCR) of NO 2 to N 2 and O 2 with ammonia at 298 K on nanocrystalline NaY, Aldrich NaY and nanocrystalline CuY was investigated using in situ Fourier transform infrared (FTIR) spectroscopy. It was determined that the kinetics of SCR were 30% faster on nanocrystalline NaY compared to Aldrich NaY. The superior performance of the nanocrystalline zeolite was attributed to an increase in external surface reactivity. External surface sites, which include silanol groups and extra framework alumina (EFAL), gave rise to differences in the adsorption of NO 2 and NH 3 on nanocrystalline NaY compared to commercial NaY. Copper cation-exchanged nanocrystalline Y resulted in an additional increase in the rate of SCR as well as distinct NO 2 and NH 3 adsorption sites associated with the copper cation. This is the first study of a transition metal cation-exchanged nanocrystalline zeolite and its potential use as a catalyst in the SCR of nitrogen oxides.

Recent Advances in Catalytic DeNOX Science and Technology

The catalytic removal of nitrogen oxide (NOx) under lean‐burn conditions is one of the most important targets in catalysis research. Some lean‐NOx control technologies such as the direct decomposition of NOx, NOx storage‐reduction (NSR), and selective catalytic reduction (SCR) using different reducing agents (diesel soot, NH3, or hydrocarbon) are described. The reaction mechanism of NSR, which is the most promising technology, together with some novel NSR catalysts is discussed. Some mechanisms of SCR of NOx by hydrocarbon (HC‐SCR) were classified into two categories: one is the adsorption/dissociation mechanism, and the other is the oxidation‐reduction mechanism. Based on the discussion of the reaction mechanism, the influence of some factors (catalyst support, metal loading, calcination temperature, catalyst preparation method, oxygen, reducing agents, water, and sulfur) on the activity of HC‐SCR catalyst is discussed. It seems that Ag/Al2O3 catalyst offers the most promising for SCR of NOx by hydrocarbon due to the exciting promotion effect of H2. Plasma or microwave assisted zeolite‐based catalyst may also lead to a new approach for HC‐SCR of NOx. Combinatorial catalysis, which was developed recently for discovering the practical combined catalyst quickly, also was introduced. Finally, future research directions in the area of lean‐NOx is proposed.

N 2O formation in the ammonia oxidation and in the SCR process with V 2O 5-WO 3 catalysts

Commercial catalysts based on vanadia and tungsta supported on titania are generally employed in the selective catalytic reduction (SCR) of nitrogen oxides present in the stack gases of power stations; similarly, they are active for the oxidation of traces of ammonia and the elimination of dioxins and furans. When these catalysts are used for gas purification in the presence of ammonia, they can produce nitrous oxide that is released to the atmosphere. In order to evaluate the formation of nitrous oxide during the total ammonia oxidation and SCR of nitrogen oxides with ammonia, a series of V 2O 5-WO 3/TiO 2 catalysts were prepared and tested under conditions close to those found in their industrial use. The results confirm that nitrous oxide formation mechanisms in both processes were clearly related to the active phase composition, increasing the formation of this undesired compound as the vanadia loading in the catalyst was raised. With the oxidation of trace ammonia, nitrous oxide is formed from the adsorption of two ammonia molecules over polymeric vanadyl species. For the SCR process, nitrous oxide is also formed by nitric oxide molecules adsorbed on adjacent centres of the polymeric vanadia.

The effect of hydrogen on the selective catalytic reduction of NO in excess oxygen over Ag/Al2O3

The selective catalytic reduction (SCR) of nitrogen oxides (NOx) by propane in the presence of H2 on sol–gel prepared Ag/Al2O3 catalysts (0.5–5wt.% Ag) was investigated. It was confirmed that hydrocarbon-assisted SCR of NOx is remarkably enhanced by co-feeding hydrogen to a lean exhaust gas mixture (λ>1), attaining considerable activity within a wide temperature window (470–825K). The samples had marginal activity at 575K without co-fed H2, but achieved up to 60% NOx conversion in the presence of H2 at a space velocity of 30,000h−1. NO2 as NOx feed component is not converted to N2 by C3H8 to a substantial extent under lean conditions. This points to an activation route of NO through direct conversion to adsorbed nitrite/nitrate or to a dissociation of NO over Ag0, formed through short-term reduction by H2. The nature of Ag species was characterized by X-ray diffraction, temperature-programmed reduction, pulse thermoanalytical measurements, electron microscopy and FTIR spectroscopy. It could be shown that Ag2O nano-sized clusters are predominantly present on all samples, whereas formation of silver aluminate could not be confirmed. Nano-sized Ag2O clusters can reversibly be reduced/reoxidized by H2. A silver loading higher than 2wt.% leads to a part of Ag2O particles, which are thermally decomposed during calcination at 800K or higher. The catalytic role of this metallic silver is still unclear. Formal kinetic analysis of catalytic data revealed that the activation energy of the overall reaction is significantly lowered in the presence of H2. The presence of water does not change the activation energy. It is concluded that hydrogen reduces the nano-sized Ag2O clusters to Ag0 on a short-term scale. Zero-valent silver promotes a dissociation pathway of NOx conversion. The fact that more oxidized ad-species (nitrite/nitrate) are observed in the presence of H2 is attributed to a dissociative activation of gas-phase oxygen on Ag0.

Selective catalytic reduction of NOx with CH3OH, C2H5OH and C3H6 in the presence of O2 over Ag/Al2O3 catalyst: Role of surface nitrate species

The involvement of the reaction of surface nitrate [NO3−(ads)] species with different reductants (C3H6, C2H5OH and CH3OH) in the selective catalytic reduction of nitrogen oxides (NOx) over a Ag/Al2O3 catalyst has been studied by in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and temperature-programmed desorption (TPD). When an NO/O2 mixture was exposed to a Ag/Al2O3 catalyst at 150 °C, three kinds of nitrate species (bridging, monodentate and bidentate) were observed by DRIFT. The thermal stability of the monodentate NO3−(ads) species was higher than that of the bridging and bidentate NO3−(ads) species, which was confirmed by DRIFT and TPD measurements. The monodentate NO3−(ads) species reacted effectively with C2H5OH and CH3OH in the presence of O2 to form surface isocyanate [NCO(ads)] species at 250 °C, whereas the bridging and bidentate NO3−(ads) species reacted minimally. All NO3−(ads) species were largely unreactive with C3H6 in the presence of O2 at temperatures below 250 °C. On the other hand, the order of reactivity in the reduction of NOx at temperatures below 350 °C was in good agreement with that of both the reactivity of the monodentate NO3−(ads) species and the ease of NCO(a) formation (C2H5OH>CH3OH>C3H6). Based on these findings, the involvement of the reactivity of NO3−(ads) species and the formation of NCO(ads) species in the selective reduction of NOx are discussed.

高温触媒燃焼

This paper describes concept and current status of development of catalytic combustion for NOx (nitrogen oxides) abatement. Catalytic combustion is an alternative technology, currently under development, which has been offering the potential of meeting SCR (Selective Catalytic Reduction of NOx) emission level and cost reduction. A conventional combustion process produces a peak temperature which is far into the thermal NOx formation region. In contrast, a catalytic combustion process maintains a peak temperature well below the NOx formation region without compromising combustion efficiency.Two types of the catalytic combustion concept have been proposed. One employs both homogeneous (gas phase) reaction and heterogeneous (catalyst surface) reaction to accomplish complete combustion within a honeycomb catalyst bed. It suggests that the catalyst temperature reaches the adiabatic temperature of fuelair mixtures and that the catalyst shoud withstand above 1300°C. The other employs only heterogeneous reaction, within a honeycomb catalyst bed, to reduce maximum catalyst temperature. And, complete combustion takes place in a post catalyst bed without a catalyst. The maximum catalyst temperature is maintained around 1000°C.Based on these concepts, several development programs for gas turbine applications have been conducted in USA and Japan. They demonstrated very low NOx emission performance (lower than 10ppmv). Despite the low NOx capability of catalytic combustion, further development efforts are needed in catalyst material, combustor design and control system.