Catalytic Reduction Of Nitric Oxide Research Articles

Cation synergies affect ammonia adsorption over VOX and (V,W)OX dispersed on α-Al2O3 (0001) and α-Fe2O3 (0001)

The catalytic behavior of oxide-supported metal oxide species depends on the nature of the support and the presence of co-catalysts. We use density functional theory (DFT) to explore the relationship between the structure and chemical behavior of vanadium oxide in light of its industrial use for the selective catalytic reduction of nitric oxide with ammonia (NO-SCR). The relative stabilities of dispersed VOX monomers, dimers, and long-chain oligomers on two model oxide support surfaces with similar structure but drastically different chemical behavior, α-Al2O3 (0001) and α-Fe2O3 (0001), are determined. The effect of added tungsten, known to promote NO-SCR, is also investigated on the relatively inert α-Al2O3 (0001) support. We find that the adsorption behavior of NH3, representing the first step of the NO-SCR reaction, depends strongly on the VOX local structure. Protonation of NH3 to NH4+ over surface hydroxyls is energetically favorable over VOX-WOX dimers and VOX oligomers, which are stabilized by the reducible α-Fe2O3 (0001) support.

A comparative study of the H2-assisted selective catalytic reduction of nitric oxide by propene over noble metal (Pt, Pd, Ir)/γ-Al2O3 catalysts

The impact of H2 as additional reducing agent on the SCR of NO with C3H6 in excess oxygen, was comparatively explored over low noble metal loading (0.5wt%), Pt/γ-Al2O3, Pd/γ-Al2O3, Ir/γ-Al2O3 catalysts. To gain insight into the role of H2, the reactions NO+C3H6+O2 (R#1), NO+C3H6+O2+H2 (R#2), NO+H2+O2 (R#3) were employed. In respect to propene oxidation, the Pd>Pt>Ir sequence was obtained under R#1, since they exhibit complete conversion at 220, 250, 325°C, respectively; all metals exhibit moderate deNOx performances (XNO, <40%). H2 co-presence (R#2) promotes both the NO and C3H6 conversions, which is valid in the whole temperature interval investigated (50–400°C), being more substantial for Pt/γ-Al2O3 and Ir/γ-Al2O3, less beneficial for Pd/γ-Al2O3. A two-maxima feature is obtained on XNO pattern (at ∼100 and ∼230°C) of Pt and Pd during R#2. The low temperature maximum −attributed to NO reduction by H2- is substantially more pronounced on Pt than Pd, offering XNO ∼90% and SN2 ∼85%; the high temperature maximum—attributed to NO reduction by C3H6—is higher by ∼15% on both Pt and Pd, in respect to the values obtained during R#1, while SN2 remained unaffected. Different XNO pattern with one maximum is obtained over Ir, implying a synergistic interaction between H2 and C3H6. This synergy is accompanied by a substantial widening of the NO reduction window toward lower temperatures and a considerable increase on both XNO,max and SN2 (from XNO ∼30% with SN2 ∼55% during R#1 to XNO ∼70% with SN2 ∼95% during R#2). The specific features of all reactions and metals employed are comparatively discussed.

Selective catalytic reduction of nitric oxide with hydrogen on supported Pd: Enhancement by hydrogen spillover

Two high-activity supported-Pd catalysts (Pd/V2O5/TiO2/SBA-15 and Pd/V2O5/TiO2/MCM-41) for selective catalytic reduction of NO with H2 were studied. They differ only by the mesoporous-silica support: SBA-15 vs. MCM-41. Although the SBA-15 supported catalyst had a lower surface area, it showed a higher NO conversion (95% vs. 84%, at a high space velocity of 4.6×104 1/h). Two pieces of direct and related evidence are shown for the role of hydrogen spillover in enhancing the catalytic activities of these two catalysts. A comparison of hydrogen adsorption isotherms (via the Benson-Boudart method) show more spillover on the SBA-15 supported catalyst. The results from the transient kinetics experiments also showed that the amount of spiltover hydrogen stored on the Pd/V2O5/TiO2/SBA-15 under dynamic reaction conditions was significantly higher than that on Pd/V2O5/TiO2/MCM-41 (7.3μmol/g vs. 2.1μmol/g). Catalytic performance of the two catalysts for practical applications is reported. The favorable effect of hydrogen spillover on H2-SCR could be used as a new strategy for improving the performance of noble-metal H2-SCR catalysts.

Promotional effect of iron oxide on the catalytic properties of Fe–MnOx/TiO2 (anatase) catalysts for the SCR reaction at low temperatures

The surface interaction of the iron-improved MnOx/TiO2 (anatase) catalyst for the selective catalytic reduction of nitric oxide was studied. The role of iron was investigated through detailed experiments.

Selective catalytic reduction of nitric oxide with ammonia over high-activity Fe/SSZ-13 and Fe/one-pot-synthesized Cu-SSZ-13 catalysts

Fe-impregnated one-pot-synthesized Cu-SSZ-13 catalysts showed excellent catalytic activities for the selective catalytic reduction (SCR) of NO with ammonia.

Selective catalytic reduction of nitric oxide over cerium-doped activated carbons

An affordable selective catalytic reduction (SCR) catalyst for nitrogen oxides (NOx) from diesel engines is needed, which is effective at low as well as high temperatures. The contribution of this study was to directly compare the effectiveness of cerium-doped activated carbons as potential diesel SCR catalysts, via testing under similar conditions. Unmodified and cerium-doped granular activated carbon (GAC), activated carbon fiber (ACF), and multiwall carbon nanotubes (MWCNTs) were tested in a fixed bed column from 100–400°C for 150 and 500ppm concentration NO. For 150ppm NO, CeGAC (with the highest cerium surface weight percent) achieved the highest reduction efficiencies over all temperatures. It maintained these efficiencies over 12h of durability testing. However, CeGAC was only thermally stable to 310°C, and its reduction efficiency did not increase at 500ppm NO, due to limited accessibility of internal pore space. For high NOx conditions, CeMWCNTs (thermally stable until 412°C) achieved the highest efficiencies above 280°C (85% at 300°C). Additional research is needed to extend the thermal stability of CeGAC or CeACF to higher temperatures, or increase the reduction efficiencies of CeMWCNTs at lower temperatures.

Selective catalytic reduction of NO by methane over the Co/MOR catalysts in the presence of oxygen

AbstractA series of Co/MOR catalysts were prepared by impregnation method and used in the selective catalytic reduction of nitric oxide with methane (CH4-SCR). These catalysts were characterized by XRD, BET, TG-MS, H2-TPR, NH3-TPD and NO-TPD; their performance in the CH4-SCR of NO was investigated. The results showed that cobalt species exist as Co3O4 spinal in the Co/MOR catalysts; the acidity and redox and NO absorption/desorption ability of the Co/MOR catalysts are changed after the incorporation of cobalt in MOR zeolite, in comparison with pure MOR zeolite. The catalytic performance of Co/MOR is closely related to its redox and NO adsorption/desorption ability, which are dependent on the cobalt loading. The Co(10)/MOR catalyst with a cobalt loading of 10% exhibits high activity in the CH4-SCR of NO; over it the conversion of nitric oxide reaches 54.2% at 330°C.

Selective catalytic reduction of nitric oxide by hydrogen over NiFe2−xPdxO4 catalysts at low temperature

Selective catalytic reduction of NOx to nitrogen by hydrogen (H2-SCR) does not result in any secondary pollutants and can reduce NOx at the lowest possible temperatures. To achieve this goal, proper catalysts are critical. In this paper, NiFe2−xPdxO4 catalysts were prepared using a sol–gel auto-ignition method for the selective catalytic reduction of NO with hydrogen (H2-SCR) in the presence of oxygen at 150–250°C. The NiFe2−xPdxO4 catalysts exhibited high SCR activity providing 100% NO conversion at 190°C, and the activity improved as the palladium content increased. NiFe1.90Pd0.10O4 exhibited a slightly lower activity in the 180–220°C range. NiFe1.93Pd0.07O4 and NiFe1.95Pd0.05O4 exhibited larger acidity, higher level of reducibility and catalytic activity, which was validated through NH3-TPR, H2-TPR analysis and SCR activity tests. The XRD results indicated that the samples had a spinel cubic structure with good crystallization. The EDS results indicated that the Fe/Ni and Pd/Ni atomic ratio in the samples were consistent with the theoretical molecular formula (i.e., NiFe2−xPdxO4). The effects of GHSV, H2/NO ratios and O2 feed concentration on the NO conversion over the NiFe1.95Pd0.05O4 catalyst were also investigated. An increase in the O2 concentration from 0% to 6% and a decrease in the H2/NO ratio from 10:1 to 5:1 produced only a slight decrease in the NO conversion. Studies on the effect of 3% and 5% H2O on the SCR activity concluded that the slight activity decrease, was due to reversible inhibition and weakly irreversible deactivation, and that the catalysts contained excellent water tolerance.

ChemInform Abstract: Visible‐Light‐Assisted Selective Catalytic Reduction of Nitric Oxide with Ammonia over Dye‐Modified Titania Photocatalysts.

AbstractDye‐modified TiO2 photocatalysts show a high photocatalytic activity for the selective catalytic reduction of NO with NH3 in the presence of O2 under visible‐light irradiation.

Utilization of sargassum based activated carbon as a potential waste derived catalyst for low temperature selective catalytic reduction of nitric oxides

The use of activated carbon derived from sargassum (SAC), normally disposed as waste, has been examined as potential catalyst for selective catalytic reduction of NO with NH3 (NH3-SCR) in the temperature range of 50–250°C. The influence of important process variables, such as preparation methods, phosphoric acid impregnation ratios and surface nitrogen functional groups, were investigated. The impacts of water (H2O) and sulfur dioxide (SO2) on the SCR activity of the C/SAC-2/U-6 catalyst were also discussed. An array of analytical techniques, including Brunauer–Emmett–Teller method (BET), Elemental Analysis (EA), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) were also employed to study the structural properties, elemental content, and distribution of nitrogen-containing groups of the catalyst prepared. Experimental results suggested that the urea modified C/SAC-2/U-6 sample exhibited higher NOx conversion, better N2 selectivity and broader operation temperature window than all the SAC samples and the commercial activated carbon sample. Moreover, it has good water and sulfur tolerance, making the sargassum derived catalyst as a promising NH3-SCR catalyst for NOx emission from fired power plant.

Atomic-Scale View of VOX–WOX Coreduction on the α-Al2O3 (0001) Surface

The catalytic activity of oxide-supported vanadium oxide is improved by the presence of tungsten oxide for the selective catalytic reduction of nitric oxides. We propose a mechanism for V–W synergy through studies of the reduction–oxidation behavior of near-monolayer VOX and WOX species grown by atomic layer deposition on the α-Al2O3 (0001) single crystal surface. In situ X-ray standing wave measurements reveal an overlayer of W6+ species that is correlated with the substrate lattice as well as a redox-reversible shift from uncorrelated V5+ to correlated V4+. X-ray photoelectron spectroscopy and electronic structure calculations show a partial reduction of W6+ in the presence of V4+, improving the Bronsted acidity in mixed V–W catalyst systems. This mechanism of V–W synergy suggests that control of W d-states might be used as a design parameter for Bronsted acid sites in multicomponent oxide catalysts.

Cover Picture: Visible-Light-Assisted Selective Catalytic Reduction of Nitric Oxide with Ammonia over Dye-Modified Titania Photocatalysts (ChemCatChem 12/2015)

Cover Picture: Visible-Light-Assisted Selective Catalytic Reduction of Nitric Oxide with Ammonia over Dye-Modified Titania Photocatalysts (ChemCatChem 12/2015)

Visible-Light-Assisted Selective Catalytic Reduction of Nitric Oxide with Ammonia over Dye-Modified Titania Photocatalysts

The front cover artwork for Issue 12/2015 is provided by a group at Kyoto University. The image shows a high-performance photocatalyst for selective catalytic reduction (SCR) of NOx with NH3 under visible-light irradiation (photo-SCR). See the Full Paper itself at http://dx.doi.org/10.1002/cctc.201500207.

Visible‐Light‐Assisted Selective Catalytic Reduction of Nitric Oxide with Ammonia over Dye‐Modified Titania Photocatalysts

AbstractDye‐modified TiO2 photocatalysts showed a high photocatalytic activity for the selective catalytic reduction of NO with NH3 in the presence of O2 under visible‐light irradiation. Among the 15 dyes investigated, the maximum conversion was achieved using a Ru(2,2′‐bipyridyl‐4,4′‐dicarboxylic acid)2(NCS)2 complex (N3‐dye) for the modification of TiO2 (NO conversion &gt;99 %, N2 selectivity &gt;99 %). Diffuse reflectance infrared Fourier transform spectroscopy showed that nitrite (NO2−) and nitrate (NO3−) species were generated on the N3‐TiO2 surface under visible‐light irradiation. In gas‐switching reactions, NO2− on the N3‐TiO2 surface became N2 by a reaction with adsorbed NH3 under visible‐light irradiation, although NO3− reacted with NH3 to form N2 and N2O. Based on the reactivity with NH3, we concluded that the NO2− species is an intermediate in the selective photocatalytic reduction and reacts with NH3 adsorbed on the surface of N3‐TiO2 under visible‐light irradiation to form N2 selectively.

Influence of VOx surface density and vanadyl species on the selective catalytic reduction of NO by NH3 over VOx/TiO2 for superior catalytic activity

Because VOx/TiO2 is widely used as a catalyst for the selective catalytic reduction of nitric oxide with NH3 (NH3-SCR), a number of studies have been performed to date. VOx/TiO2 is prepared by using various vanadium loading and TiO2 supports with different surface areas. The surface density of the vanadyl species is a factor in determining the optimal activity, depending on the VOx/TiO2. We studied the relationship between NH3-SCR activity and the VOx surface density of VOx/TiO2 using various vanadium loading and TiO2 supports. The optimal activity is observed when VOx/TiO2 has an approximate VOx surface density of 4.5VOxnm−2. In addition, the ideal vanadyl species for favorable SCR activity was examined using diffuse reflectance UV-visible (DRUV-vis) spectroscopy. In the VOx/TiO2 catalyst that provided the optimal VOx surface density and best SCR activity, polymeric tetrahedral VOx was the most abundant vanadium species. When the vanadium loading is determined by the manufacturing process of the catalyst, TiO2 with an appropriate surface area should be prepared in order to manufacture VOx/TiO2 with maximum activity. In contrast, when VOx/TiO2 is prepared using commercial TiO2, the vanadium loading of VOx/TiO2 that provides the maximum activity can be determined.

A CFD-based evaluation of selective non-catalytic reduction of nitric oxide in iron ore grate-kiln plants

The overall goal of this study is to explore the function of selective non–catalytic reduction of nitric oxide (NO) in iron ore grate–kiln plants. Computational fluid dynamics is used to model the flow and the injection of urea and cyanuric acid reagents. Temperature, residence time and inlet NO content are varied, and the effects on the reduction of NO are studied. The simulations show that the use of urea results in a higher reduction of NO within the temperature window investigated as compared to cyanuric acid; however, urea cannot be used for NO reduction over the entire range of applicable temperatures. Cyanuric acid is usable within the entire temperature range for high NO content (i.e., 300 ppm) in the flue gas, but it is not effective for low NO concentration (i.e., 150 ppm). Moreover, the simulations indicate that temperature has a larger influence on the NO reduction than the residence time.

MnO2catalysts uniformly decorated on polyphenylene sulfide filter felt by a polypyrrole-assisted method for use in the selective catalytic reduction of NO with NH3

A manganese dioxide (MnO2)/polypyrrole (PPy) nanocoating was uniformly decorated on the surface of polyphenylene sulfide (PPS) filter felt via an in situ synthesis method to fabricate a catalytic filter material. The pyrrole functioned as a dispersant for the MnO2 catalysts and the PPy generated acted as a binder to adhere the MnO2 catalysts and filter felt together. The catalytic filter material obtained, had a high adhesive strength between that of the MnO2/PPy nanocoating and the PPS filter felt, and was used for the selective catalytic reduction of nitric oxide (NO) with ammonia under model conditions without any sulfur dioxide or water vapor in the gas. More than 70% conversion of NO was achieved at 160–180 °C at a high space velocity of 38 000 h−1.

Flavodiiron nitric oxide reductases: Recent developments in the mechanistic study and model chemistry for the catalytic reduction of NO

Inducible NO synthase in mammals helps to produce up to micromolar concentration of nitric oxide (NO) which acts as a key immune defense agent to kill invading pathogens. In order to counter the toxic effects of NO, the pathogens have expressed flavodiiron nitric oxide reductases (FNORs). The FNORs reduce the toxic NO into much less toxic N2O and thus help the pathogens to survive under nitrosative stress. As a consequence, these pathogens proliferate in the human body and cause harmful infections. An appreciable amount of research work has been performed to discover the true mechanism of the FNORs. Different mechanisms involving both mononitrosyl and dinitrosyl diiron complexes as key intermediates are proposed. Evidences for the involvement of new intermediates and more and more experimental evidences for existing ones in the proposed catalytic cycle of FNORs are coming up. These interesting biochemical events have recently boosted the biomimetic chemistry of the FNOR activity as well. This article discusses the importance and the currently understood mechanistic aspects of FNORs. Structural and functional models for the active site of FNORs are discussed along with their success and limitations. Possible future prospects of the modeling chemistry are also suggested.

Selective Catalytic Reduction of Nitric Oxide by Ammonia over V2O5/TiO2 in a Hollow Cylindrical Catalyst under Enhancing Effect of Electrohydrodynamics: A Kinetic Modeling Study

Emissions of nitrogen oxides (NOx) in fuel combustion from stationary and mobile sources contribute to the greenhouse effects and cause a variety of environmentally harmful results. Meanwhile, the selective catalytic reduction (SCR) of NOx using ammonia over a vanadia-based catalyst in heterogeneous reactors is still a well-proven technique for NOx abatement. However, to meet forthcoming restrictive legislation, greater De-NOx performance can be achieved by eliminating operational setbacks such as diffusion resistance, poor catalyst activity at low temperatures, etc. In this study, an electrohydrodynamic (EHD)-SCR model was developed to evaluate the enhancing effect of the EHD technique on NH3-SCR of NO through a hollow cylindrical catalyst. Computational investigations were performed based on the proposed model in different operational conditions to examine the effect of various operating parameters on SCR enhancement. Simulation results showed that catalyst utilization was intensified significantly by EHD application and generation of additional flow known as EHD-induced secondary flow through the catalyst porous layer, which undermined the proposed drawbacks of the catalyst medium and provided higher catalyst effectiveness with greater NO conversion. Results also indicated that the maximum enhancement of almost 4.2-fold could be obtained for NO conversion with electric potentials and operating temperatures of 150–270 V and 150–165 °C, respectively.

Selective catalytic reduction of NO with CO using different metal-oxides incorporated in MCM-41

Metal oxide doped mesoporous silica (MCM-41) were prepared by a co-precipitation method and utilized for the selective catalytic reduction of nitric oxide with carbon monoxide at temperatures in the range 423–723 K. The supported metal oxides investigated included oxides of ruthenium, copper, cobalt, nickel and iron. This selection offered a wide range of metal–oxygen bond strength within the catalyst. The catalysts were characterized by powder XRD, N2 sorption analysis and temperature programmed reduction (TPR) in hydrogen. At reaction temperatures less than 550 K the activity of the catalysts was in the order of Ru-MCM-41 > Co-MCM-41 > Ni-MCM-41 ≈ Fe-MCM-41 ≈ Cu-MCM-41. At reaction temperatures above 650 K, the Ru-MCM-41 was still the most active catalyst followed by Cu-MCM-41.