Showing papers on "Selective catalytic reduction published in 2001"

Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO2 at Low Temperatures

Abstract: The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO2 was investigated. The two main reactions are 4NH3 + 2NO + 2NO2 → 4N2 + 6H2O and 2NH3 + 2NO2 → NH4NO3 + N2 + H2O. The “fast SCR reaction” exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the “ammonium nitrate route” becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole NOx conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNOx mechanisms: This reac...

Low-Temperature Selective Catalytic Reduction (SCR) of NO with NH3 by Using Mn, Cr, and Cu Oxides Supported on Hombikat TiO2

Abstract: A Highly active, time stable, and water resistant, Hombikat TiO2 supported Mn catalyst has been developed for the selective reduction of NO by NH3 [Eq. (1)]. The analogous Cu and Cr supported catalysts also provide 100 % N2 selectivity at ≤120°C. Lewis acidity, redox properties, and a high surface metal oxide concentration are essential for good catalytic performance.

Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism

Abstract: Silver-alumina catalysts prepared by sol–gel method (Ag-Al2O3) and impregnation method (Ag/Al2O3) were studied for the selective catalytic reduction (SCR) of NO by higher alkanes (n-hexane and n-octane). UV–VIS and Ag K-edge XAFS results established the structure of the catalysts; below 2 wt.%, highly dispersed Ag+ ions are predominant Ag species, while at higher Ag loading, Agn clusters are predominant. A relationship between the structure of Ag species and their catalytic function for SCR by n-octane was clarified. Ag+ ions are responsible for the selective reduction of NO to N2, while the Agn clusters are responsible for the hydrocarbon combustion and N2O formation. The mechanism of SCR by n-hexane on Ag-Al2O3, which mainly consists of Ag+ ions, were investigated by in situ FTIR spectroscopy. During NO+n-hexane+O2 reaction, the acetate produced via the partial oxidation of n-hexane and the nitrates produced via the oxidation of NO were main adspecies in the steady-state condition at 473–623 K. The acetate, which was stable in O2 or NO, was reactive in NO+O2. Nitrates, which were relatively stable in n-hexane, were quite reactive toward n-hexane+O2. The rate of nitrates reaction in n-hexane+O2 was close to the steady-state rate of NO reduction over wide range of temperature, indicating that the nitrate is a possible intermediate in the SCR. A proposed mechanism, suggesting the reaction of nitrates with partially oxidized hydrocarbon species as a crucial step, explains the steady-state kinetic results. At relatively high NO concentration or at low temperature, nitrates should inhibit the reaction by strongly adsorbing on the catalyst surface.

Copper ion-exchanged zeolite catalysts in deNOx reaction

Abstract: Copper ion-exchanged zeolites have widely been studied as the catalysts for NOx emission control. Cu-MFI zeolites, especially over-exchanged Cu-MFI, are very active for the catalytic decomposition of NO. The reaction mechanisms and the active sites suggested are summarized. The newly developed selective catalytic reduction of NO with hydrocarbons in the presence of excess oxygen is then introduced. The role of oxygen and the deactivation of the zeolite catalysts are discussed. In the last chapter, the high ability of Cu-zeolites for NO adsorption is overviewed. Here, some fundamental aspects of the bonding and the molecular diffusion of NO or NO2 molecules in zeolites are also summarized.

Low-temperature NOx reduction processes using combined systems of pulsed corona discharge and catalysts

Abstract: In this paper, we will report NOx removal via reduction processes using two types of combined system of pulse corona discharge and catalysts: the single-stage plasma-driven catalyst (PDC) system, and the two-stage plasma-enhanced selective catalytic reduction (PE-SCR) system. Several catalysts, such as γ-alumina catalysts, mechanically mixed catalysts of γ-alumina with BaTiO3 or TiO2, and Co-ZSM-5 were tested. In the PDC system, which is directly activated by the discharge plasma, it was found that the use of additives was necessary to achieve NOx removal by reduction. Removal rates of NO and NOx were linearly increased as the molar ratio of additive to NOx increased. The dependence of NO and NOx removal on the gas hourly space velocity (GHSV) at a fixed specific input energy (SIE) indicates that plasma-induced surface reaction on the catalyst plays an important role in the PDC system. It was found that the optimal GHSV of the PDC system with the γ-alumina catalyst was smaller than 6000 h-1. Mechanical mixing of γ-alumina with BaTiO3 or TiO2 did not enhance NO and NOx removal and γ-alumina alone was found to be the most suitable catalyst. The dielectric constant of the catalyst only influenced the plasma intensity, not the NOx removal. In the PE-SCR system, plasma-treated NOx (mostly NO2) was reduced effectively with NH3 over the Co-ZSM-5 catalyst at a relatively low temperature of 150 °C. Under optimal conditions the energy cost and energy yield were 25 eV/molecule and 21 g-N (kWh)-1, respectively.

Low pressure injection and turbulent mixing in selective catalytic reduction system

Abstract: A selective catalytic reduction system for engine exhaust injects a source of ammonia, preferably an aqueous urea solution, or other ammonia solution, to preferably evaporate, decompose and hydrolyze to produce ammonia to react with and reduce NOx in the exhaust. A turbulence generator between the injector and a downstream catalyst enhances ammonia mixing upstream of the catalyst. A perforated reflector between the injector and the catalyst passes the exhaust through the perforations and reflects exhaust back toward the injector, to generate the turbulence to achieve enhanced ammonia mixing. A low pressure generator creates a low pressure evaporative diffusion enhancement zone at the injector accelerating evaporation rate and accelerating ammonia diffusion and mixing with the exhaust.

Fe-ZSM-5 for Selective Catalytic Reduction of NO with NH3: A Comparative Study of Different Preparation Techniques

Abstract: Fe-ZSM-5 are prepared by using four different techniques: conventional aqueous ion-exchange (CA), improved aqueous ion-exchange (IA), solid-state ion-exchange (SS) and chemical vapor ion-exchange (CV). All of the catalysts show very high activities for selective catalytic reduction (SCR) of NO with ammonia. However, the activities are different and follow the sequence of Fe-ZSM-5 (IA) > Fe-ZSM-5 (CA), Fe-ZSM-5 (SS) > Fe-ZSM-5 (CV). ESR results indicate that Fe3+ ions with tetrahedral coordination are the active sites for the SCR reaction.

Study of thermal deactivation of a de-NOx commercial catalyst

Abstract: In the present paper, the thermal deactivation of a commercial de-NO x V 2 O 5 –WO 3 /TiO 2 catalyst is investigated. In order to simulate long-term operation in gas firing samples of the catalyst are calcined at different temperatures from 773 to 1173 K. The structural, morphological and chemico-physical changes caused by calcination at high temperatures are investigated by XRD, BET and Hg porosimetry measurements, FT-IR, FT-Raman and EPR spectroscopies, and the activity in the reduction of NO with ammonia, the direct oxidation of ammonia and the oxidation of SO 2 is measured over the different catalyst samples. It is shown that sintering of the TiO 2 support leads to aggregation of isolated vanadium ions; this favours the selective catalytic reduction (SCR) reaction at low temperatures, the oxidation of ammonia at high temperatures and the undesired oxidation of SO 2 as well. Thus, it is expected that catalysts operated for long-term in gas firing at high temperatures are no longer appropriate for commercial use in the SCR process, because in spite of the greater activity in the de-NO x reaction, the temperature window is too narrow and the activity in the oxidation of SO 2 is too high.

Selective catalytic reduction of nitrogen oxides with NH3 over natural manganese ore at low temperature

Abstract: The selective catalytic reduction of NOx (NO + NO2) at low temperature with ammonia has been investigated with natural manganese ore, pure manganese dioxide, and manganese dioxide supported alumina. The catalysts showed high activities for NOx reduction with NH3 in the presence of O2 at temperatures below 250 °C. The decrease of SCR activity without oxygen in the gas phase differed among the three catalysts in accord with their differing amounts of lattice oxygen. The SCR activity of the catalysts, in that case, decreased in the order MnO2 > NMO > MnO2/γ-Al2O3. Small amounts of SO2 deactivated the catalysts in the low-temperature range. An XRD analysis of the sulfated catalysts with differing temperature provides evidence that the formation of ammonium sulfates is the main poisoning route. These results could restrict the practical application of these catalysts to sulfur-free conditions.

Impactor for selective catalytic reduction system

Abstract: A selective catalytic reduction system for engine exhaust injects a solution comprising a source of ammonia, preferably an aqueous urea solution, or other ammonia solution, to preferably evaporate, decompose and hydrolyze to produce ammonia to react with and reduce NO x in the exhaust. An inertial impactor in the housing between the injector and the catalyst is impacted by the solution droplets and holds same until evaporated, decomposed and hydrolyzed to ammonia.

Catalytic NO reduction with ammonia at low temperatures on V2O5/AC catalysts: effect of metal oxides addition and SO2

Abstract: The catalytic behavior of the V-M/AC (M=W, Mo, Zr, and Sn) catalysts were studied for the NO reduction with ammonia at low temperatures, especially in the presence of SO2. The presence of the metal oxides does not increase the V2O5/AC activity but decreases it. Except V-Mo/AC, the other catalysts are promoted by SO2 at 250°C, especially for V-Sn/AC. However, the promoting effect of SO2 is gradually depressed by catalyst deactivation. Changes in catalyst preparation method can improve the catalyst stability in short-term but cannot completely prevent the catalyst from a long-term deactivation. Mechanisms of the promoting effect and the deactivation of V-Sn/AC catalyst by SO2 were studied using Fourier transform infrared spectroscopy (FT-IR) spectra and measurement of catalyst surface area and pore volume. The results showed that both the SO2 promotion and deactivation are associated with the formation of sulfate species on the catalyst surface. In the initial period of the selective catalytic reduction (SCR) reaction in the presence of SO2, the formed sulfate species provide new acid sites to enhance ammonia adsorption and thus the catalytic activity. However, as the SCR reaction proceeds, excess amount of sulfate species and then ammonium-sulfate salts are formed which is stabilized by the presence of tin oxide, resulting in gradual plugging of the pore structures and the catalyst deactivation.

Microwave assisted catalytic reduction of sulfur dioxide with methane over MoS2 catalysts

Abstract: The catalytic reduction of sulfur dioxide with methane to form carbon dioxide and sulfur has been studied over MoS2/Al2O3 catalysts. The reaction has been found to occur with microwave (2.45 GHz) heating at recorded temperatures as much as 200 ◦ C lower than those required when conventional heating was used. An activation energy of 117 kJ mol −1 has been calculated for the conventionally heated reaction, but an Arrhenius analysis of the data obtained with microwave heating was not possible, probably because of temperature variations in the catalyst bed. The existence of hot spots in the catalysts heated by microwave radiation has been verified by the detection of -alumina at a recorded temperature some 200 ◦ C lower than the temperature at which the -t o-alumina phase transition is normally observed. Among four catalysts prepared in different ways, a mechanically mixed catalyst showed the highest conversion of SO2 and CH4 for microwave heating at a given temperature. Supported catalysts, sulfided either by conventional heating or under microwave conditions, showed little difference in the extent of SO2 and CH4 conversions. The highest conversions to carbon dioxide and sulfur, combined with low production of undesirable side products, was obtained when the molar ratio of SO2 to CH4 was equal to two, the stoichiometric ratio. © 2001 Elsevier Science B.V. All rights reserved.

Catalytic removal of N2O in model flue gases of a nitric acid plant using a promoted Fe zeolite

Abstract: Three options for removal of N 2 O using a promoted Fe-ZSM-5 catalyst have been explored under conditions representative for the off gases of a nitric acid plant: catalytic decomposition and selective catalytic reduction (SCR) using propane or methane as a reductant. Catalytic decomposition of N 2 O takes place at temperatures above 400°C at a space velocity of 10,000 h −1 . The addition of propane lowers the temperature for N 2 O conversion with about 100°C. In propane-assisted SCR of N 2 O, the emissions of unreacted hydrocarbons and of CO are low and also NO x reduction takes place. Methane is more difficult to be activated by the catalyst, which causes lower N 2 O destruction efficiency and the output of unreacted methane. In all cases, N 2 O conversion is higher at elevated pressure.

High efficiency conversion of nitrogen oxides in an exhaust aftertreatment device at low temperature

Abstract: A system and method for delivering reductant to a lean NOx catalyst receiving exhaust gases from an internal combustion engine operating at a lean air-fuel ratio in disclosed. Reductant supply is based on an amount of reductant stored within the catalyst. Furthermore, reductant is supplied under prescribed conditions including an exhaust gas concentration upstream of the catalyst of less than 25 ppm NOx or the catalyst at a temperature greater than 300° C.

Redox and acid reactivity of wolframyl centers on oxide carriers: Brønsted, Lewis and redox sites

Abstract: Catalysts prepared by impregnating tungsten oxide on alumina, titania and zirconia and their mixed oxides have been characterized by skeletal IR, IR of adsorbed ammonia, Raman and UV–VIS–NIR spectroscopies and by temperature programmed reduction In all cases catalysts with W loading well below the monolayer have been taken into consideration Surface mono-oxo wolframyl species with similar low coordination structure have been found to largely predominate in all the supports However, the WO bond length, the Lewis acidity, the charge transfer transition energies and the reducibility of the WOx species strongly depend on the support nature In particular, the wolframyls on alumina are most acidic, have higher charge transfer transition energies and are less easily reducible than those on titania The wolframyls on zirconia show intermediate properties Evidence is given for the different behavior of wolframyl centers in spite of their similar geometric “molecular” structure The different properties of wolframyl centers on the supports used here explain the different behavior of these materials for hydrocarbon conversion, in the selective catalytic reduction of NO by ammonia and as precursors of hydrodesulphurization catalysts

The selective catalytic reduction of nitrogen oxides by methane on noble metal-loaded sulfated zirconia

Abstract: The selective catalytic reduction of NO x by methane on noble metal-loaded sulfated zirconia (SZ) catalysts was studied. Ru, Rh, Pd, Ag, Ir, Pt, and Au-loaded sulfated zirconia catalysts were compared with the intact sulfated zirconia. For the NO–CH 4 –O 2 reaction, Ru, Rh, Pd, Ir, and Pt showed promotion effect on NO x reduction, while for the NO 2 –CH 4 –O 2 reaction, only Rh and Pd showed promotion effect. Over intact and Rh, Pd, Ag, and Au-loaded sulfated zirconia, NO x conversion in NO 2 –CH 4 –O 2 reaction was significantly higher than that in NO–CH 4 –O 2 reaction, while clear difference was not observed over Ru, Ir, and Pt-loaded sulfated zirconia. Comparison of [NO 2 ]/([NO]+[NO 2 ]) in the effluent gases in NO–O 2 and NO 2 –O 2 reactions showed that Ru, Ir, and Pt has high activity for NO oxidation under the reaction conditions. These facts suggest that effects of these metals toward NO x reduction by methane can be categorized into the following three groups: (i) low activity for NO oxidation to NO 2 , and high activity for NO 2 reduction to N 2 (Pd, Rh); (ii) high activity for NO oxidation to NO 2 , and low activity for NO 2 reduction to N 2 (Ru, Ir, Pt); (iii) low activity for both reactions (Ag, Au). To confirm these suggestions, combination of these metals were investigated on binary or physically-mixed catalysts. The combination of Pd or Rh with Pt or Ru gave high activity for the selective reduction of NO x by methane.

Method for removing nitrogen oxides and particulates from the lean exhaust gas of an internal combustion engine and exhaust gas emission system

Abstract: A method for removing nitrogen oxides and particulate matter from the lean exhaust gas of a combustion engine that also contains low concentrations of sulfur oxides. The exhaust gas stream is passed over a nitrogen oxide storage catalyst and a particulate filter, where nitrogen oxides and sulfur oxides are adsorbed by the storage catalyst under lean exhaust gas conditions and the particulate matter is deposited on the particulate filter. The storage catalyst is in a first cycle is periodically denitrated by enriching the exhaust gas regeneration of the particulate filter. Desulfurization of the nitrogen oxide storage catalyst is carried out in a second cycle by raising the temperature of the lean exhaust gas to a value at which the particulate matter combustion on the particulate filter is initiated and then the storage catalyst can be desulfurized by enriching the exhaust gas.

Mechanistic causes of the hydrocarbon effect on the activity of Ag–Al2O3 catalyst for the selective reduction of NO

Abstract: Ag–Al2O3 catalyst prepared by the sol–gel method was studied for the selective catalytic reduction (SCR) of NO by various hydrocarbons (HC-SCR). The rates of NO reduction and hydrocarbon conversion depend strongly on the type of hydrocarbon reductant; the rates were higher for the higher normal alkanes, higher for alkene than alkane, and higher for normal alkane than branched alkane. The formation and reaction of hydrocarbon-derived species adsorbed on the surface were investigated by in situ FTIR spectroscopy. The apparent rate of acetate formation in the hydrocarbon + O2 reaction and the rate of nitrate reaction in hydrocarbon + O2 also depend on the nature of the hydrocarbon with a similar activity pattern to that for the SCR reaction. For the various hydrocarbons used, the rate of nitrate reaction in hydrocarbon + O2 was close to the steady-state rate of NO reduction, indicating that the nitrate is a possible intermediate in this reaction. The reaction of nitrates with oxygenated hydrocarbon species, possibly the acetate, was proposed to be a crucial step in HC-SCR. A proposed mechanism explains the hydrocarbon effect on the de-NOx activity: the NO reduction activity depends on the reactivity of the hydrocarbon molecule, which affects the rate of hydrocarbon oxidation to oxygenates and hence the rate of nitrate reaction with oxygenated hydrocarbons.

Surface species during catalytic reduction of NO by propene studied by in situ IR-spectroscopy over Pt supported on mesoporous Al2O3 with MCM-41 type structure

Abstract: The selective catalytic reduction of NO by propene (HC-SCR) in the presence of excess oxygen over Pt/MCM-41 (Al 2 O 3 ) and MCM-41 (Al 2 O 3 ) catalysts has been studied by simultaneous in situ IR-spectroscopy and catalytic activity measurements. IR-spectra of Pt/MCM-41 (Al 2 O 3 ) and MCM-41 (Al 2 O 3 ) were compared during the adsorption of the single reactants (NO and C 3 H 6 ) and during the NO reduction with C 3 H 6 in presence of excess O 2 (NO+C 3 H 6 +O 2 reaction). The adsorption of NO resulted in the formation of surface nitrate species on the Al 2 O 3 support and surface NO species on Pt. Adsorption of propene led to the formation of carboxylate species on Al 2 O 3 . Under reaction conditions nitrates, carboxylates, acetates, hydrocarbon fragments, isocyanate, cyanide species and CO were present on the catalyst surface. The surface concentration of the isocyanate species was found to be strongly correlated to the activity of the Pt/MCM-41 catalyst. Besides the formation of isocyanate, cyanide species were also observed on the surface. Both species increased in concentration with increasing oxygen concentration in the feed. The isocyanate species were found to be a reaction intermediate during the NO reduction, whereas the cyanide species were stable against further reaction. For the cyanide formation, a reaction pathway via isocyanate by abstraction of an oxygen atom by propene was observed.

Process and catalyst for reducing nitrogen oxides

Abstract: A process for reducing the nitrogen oxides present in a lean exhaust gas from an internal combustion engine by selective catalytic reduction on a reduction catalyst using ammonia, wherein a fraction of the nitrogen monoxide present in the exhaust gas is oxidized to nitrogen dioxide before the exhaust gas, together with ammonia, is passed over the reduction catalyst. The reduction catalyst contains a zeolite exchanged with transition metals and oxidation of the nitrogen monoxide is performed in such a way that the exhaust gas contains 30 to 70 vol. % of nitrogen dioxide before contact with the reduction catalyst.

Method for the combined reduction of nitrogen oxide and sulfur dioxide concentrations in the furnace region of boilers

Abstract: A method for the combined reduction of sulfur dioxide, SO 2 , and nitrogen oxides, NO x , in the gas stream of a furnace from the combustion of fossil fuels is disclosed In a narrow gas temperature zone in a furnace, NO x is converted to nitrogen by reaction with a reducing agent such as urea or ammonia with negligible residual ammonia and other reaction pollutants In about this same temperature zone, SO 2 will react with calcium oxide particles, derived from the calcination of lime, Ca(OH) 2 , or limestone, CaCO 3 , to form CaSO 4 particles A wide size distribution of aqueous droplets, containing dispersed lime or very fine limestone particles and dissolved urea or ammonia, is injected at the outer edge of the furnace gas zone at which the SO 2 and NO x reduction reaction are effective The key element in this invention is that the aqueous droplet size distribution is optimized for the specific furnace dimension while the concentration of the reactants, urea or ammonia and lime or very fine limestone, is optimized for optimum reaction rates Special injectors produce the different size droplets that vaporize throughout said gas zone, thereby distributing said lime or limestone particles and urea or ammonia gas molecules exclusively throughout the combustion gas zone being treated Also disclosed is a system to produce said aqueous mixture and effectively accomplish this injection This method can be combined with other NO x and SO 2 reduction processes to sharply reduce overall NO x and SO 2 emissions from the combustion gas effluent