Showing papers on "Selective catalytic reduction published in 1994"

Activity and selectivity of pure manganese oxides in the selective catalytic reduction of nitric oxide with ammonia

Abstract: Manganese oxides of different crystallinity, oxidation state and specific surface area have been used in the selective catalytic reduction (SCR) of nitric oxide with ammonia between 385 and 575 K. MnO2 appears to exhibit the highest activity per unit surface area, followed by Mn5O8, Mn2O3, Mn3O4 and MnO, in that order. This SCR activity correlates with the onset of reduction in temperature-programmed reduction (TPR) experiments, indicating a relation between the SCR process and active surface oxygen. Mn2O3 is preferred in SCR since its selectivity towards nitrogen formation during this process is the highest. In all cases the selectivity decreases with increasing temperature. The oxidation state of the manganese, the crystallinity and the specific surface area are decisive for the performance of the oxides. The specific surface area correlates well with the nitric oxide reduction activity. The nitrous oxide originates from a reaction between nitric oxide and ammonia below 475 K and from oxidation of ammonia at higher temperatures, proven by using 15NH3. Participation of the bulk oxygen of the manganese oxides can be excluded, since TPR reveals that the bulk oxidation state remains unchanged during SCR, except for MnO, which is transformed into Mn3O4 under the applied conditions. In the oxidation of ammonia the degree of oxidation of the nitrogen containing products (N2, N2O, NO) increases with increasing temperature and with increasing oxidation state of the manganese. A reaction model is proposed to account for the observed phenomena.

Catalytic Air Pollution Control: Commercial Technology

Abstract: Preface. ACKNOWLEDGEMENTS. ACKNOWLEDGEMENTS, FIRST EDITION. ACKNOWLEDGEMENTS, SECOND EDITION. I. FUNDAMENTALS. 1. Catalyst Fundamentals. 1.1 Introduction. 1.2 Catalyzed Verses Non-Catalyzed Reactions. 1.3 Catalytic Components. 1.4 Selectivity. 1.5 Promoters and their Effect on Activity and Selectivity. 1.6 Dispersed Model for Catalytic Component on Carrier: Pt on Al 2 O 3 . 1.7 Chemical and Physical Steps in Heterogeneous Catalysis. 1.8 Practical Significance of knowing the Rate-Limiting Step. 2. The Preparation of Catalytic Materials: Carriers, Active Components, and Monolithic Substrates. 2.1 Introduction. 2.2 Carriers. 2.3 Making the Finished Catalyst. 2.4 Nomenclature for Dispersed Catalysts. 2.5 Monolithic Materials as Catalyst Substrates. 2.6 Preparing Monolithic Catalysts. 2.7 Catalytic Monoliths. 2.8 Catalyzed Monoliths Nomenclature. 2.9 Precious Metal Recovery from Monolithic Catalysts. 3. Catalyst Characterization. 3.1 Introduction. 3.2 Physical Properties of Catalysts. 3.3 Chemical and Physical Morphology Structures of Catalytic Materials . 3.4 Techniques for Fundamental Studies. 4. Monolithic Reactors for Environmental Catalysis. 4.1 Introduction. 4.2 Chemical Kinetic Control. 4.3 The Arrhenius Equation and Reaction Parameters. 4.4 Bulk Mass Transfer. 4.5 Reactor Bed Pressure Drop. 4.6 Summary. 5. Catalyst Deactivation. 5.1 Introduction. 5.2 Thermally Induced Deactivation. 5.3 Poisoning. 5.4 Washcoat Loss. 5.5 General Comments on Deactivation Diagnostics in Monolithic Catalysts for Environmental Applications. II. MOBILE SOURCE. 6. Automotive Catalyst. 6.1 Emissions and Regulations. 6.2 The Catalytic Reactions for Pollution Abatement. 6.3 The Physical Structure of the Catalytic Converter. 6.4 First-Generation Converters: Oxidation Catalyst (1976-1979). 6.5 NOx, CO and HC Reduction: The Second Generation: The Three Way Catalyst (1979 - 1986). 6.6 Vehicle Test Procedures (U.S., European and Japanese). 6.7 NOx, CO and HC Reduction: The Third Generation (1986 - 1992). 6.8 Palladium TWC Catalyst: The Fourth Generation (Mid-1990s). 6.9 Low Emission Catalyst Technologies. 6.10 Modern TWC Technologies for the 2000s. 6.11 Towards a Zero-Emission Stoichiometric Spark-Ignit Vehicle. 6.12 Engineered Catalyst Design. 6.13 Lean-Burn Spark-Ignited Gasoline Engines. 7. Automotive Substrates. 7.1 Introduction to Ceramic Substrates. 7.2 Requirements for Substrates. 7.3 Design Sizing of Substrates. 7.4 Physical Properties of Substrates. 7.5 Physical Durability. 7.6 Advances in Substrates. 7.7 Commercial Applications. 7.8 Summary. 8. Diesel Engine Emissions. 8.1 Introduction. 8.2 Worldwide Diesel Emission Standards. 8.3 NO x -Particulate Tradeoff. 8.4 Analytical Procedures for Particulates. 8.5 Particulate Removal. 8.6 NOX Reduction Technologies. 8.7 2007 Commercial System Designs (PM Removal Only). 8.8 2010 Commercial System Approaches under Development (PM and NO x Removal). 8.9 Retrofit and Off-Highway. 8.10 Natural Gas Engines. 9. Diesel Catalyst Supports and Particulate Filters. 9.1 Introduction. 9.2 Health Effects of Diesel Particulate Emissions. 9.3 Diesel Oxidation Catalyst Supports. 9.4 Design/Sizing of Diesel Particulate Filter. 9.5 Regeneration Techniques. 9.6 Physical Properties and Durability. 9.7 Advances in Diesel Filters. 9.8 Applications. 9.9 Summary. 10. Ozone Abatement within Jet Aircraft. 10.1 Introduction. 10.2 Ozone Abatement. 10.3 Deactivation. 10.4 Analysis of In-Flight Samples. 10.5 New Technology. III. STATIONARY SOURCES. 11. Volatile Organic Compounds . 11.1 Introduction. 11.2 Catalytic Incineration. 11.3 Halogenated Hydrocarbons. 11.4 Food Processing. 11.5 Wood Stoves. 11.6 Process Design. 11.7 Deactivation. 11.8 Regeneration of Deactivated Catalysts. 12. Reduction of NO x . 12.1 Introduction. 12.2 Nonselective Catalytic Reduction of NOx. 12.3 Selective Catalytic Reduction of NOx. 12.4 Commercial Experience. 12.5 Nitrous Oxide (N 2 O). 12.6 Catalytically Supported Thermal Combustion. 13. Carbon Monoxide and Hydrocarbon Abatement from Gas Turbines. 13.1 Introduction. 13.2 Catalyst for CO Abatement. 13.3 Non-Methane Hydrocarbon (NMHC) Removal. 13.4 Oxidation of Reactive Hydrocarbons. 13.5 Oxidation of Unreactive Light Paraffins. 13.6 Catalyst Deactivation. 14. Small Engines. 14.1 Introduction. 14.2 Emissions. 14.3 EPA Regulations. 14.4 Catalyst for Handheld and Nonhandheld Engines. 14.5 Catalyst Durability. IV. NEW AND EMERGING TECHNOLOGIES. 15. Ambient Air Cleanup. 15.1 Introduction. 15.2 Premair (R) Catalyst Systems. 15.3 Other Approaches. 16. Fuel Cells and Hydrogen Generation. 16.1 Introduction. 16.2 Low-Temperature PEM Fuel Cell Technology. 16.3 The Ideal Hydrogen Economy. 16.4 Conventional Hydrogen Generation. 16.5 Hydrogen Generation from Natural Gas for PEM Fuel Cells. 16.6 Other Fuel Cell Systems. INDEX.

Mechanism of the Selective Catalytic Reduction of Nitric Oxide by Ammonia Elucidated by in Situ On-Line Fourier Transform Infrared Spectroscopy

Abstract: The selective catalytic reduction reaction of nitric oxide bv ammonia over vanadia-titania catalysts is one of the methods of removing NOx pollution. In the present study, it has been possible to identify the reaction mechanism and the nature of the active sites in these catalysts by combining transient or steady-state in situ (Fourier transform infrared spectroscopy) experiments directly with on-line activity studies. The results suggest a catalytic cycle that consists of both acid and redox reactions and involves both surface V-OH (Bronsted acid sites) and V=O species. A fundamental microkinetic model is proposed, which accounts for the observed industrial kinetics performance.

Selective catalytic reduction of nitric oxide with ethane and methane on some metal exchanged ZSM-5 zeolites

Abstract: The selective reduction of nitric oxide by methane or ethane, in the presence and in the absence of a large excess of oxygen, has been investigated on Cu/ZSM-5, Co/ZSM-5, Rh/ZSM-5 and Pt/ZSM-5 catalysts over a wide range of temperatures. It has been found that the maximum nitric oxide conversion is higher with ethane than with methane and the temperature of this maximum is lower with ethane. In the absence of oxygen the order of activity is Rh/ZSM-5>Pt/ZSM-5>Co/ZSM-5 > Cu/ZSM-5 with the Cu/ZSM-5 being essentially inactive, while in the presence of oxygen the order is: Rh/ZSM-5>Co/ZSM-5>Cu/ZSM-5 > Pt/ZSM-5 when ethane is used as reductant and: Rh/ZSM-5>Co/ZSM-5 > Cu/ZSM-5>Pt/ZSM-5 when methane is used. The effect of the oxygen content has been investigated for the Co/ZSM-5 catalyst. It has been found that with a small quantity of oxygen the catalytic activity decreases markedly; with higher oxygen content the activity of the catalyst rises again. It appears that two different reaction schemes may be operative, one in the absence of oxygen the other in the presence of oxygen. It is concluded that neither carbonaceous deposits, nor nitrogen dioxide formation in the gas phase are important in the reaction mechanism on metal-containing zeolites. It is proposed that the reaction is essentially a redox process in which decomposition of nitric oxide occurs on reduced metallic or metal ion sites (the relative activity of each of these depending on the choice of metal), leading to the formation of gaseous nitrogen and adsorbed oxygen, followed by the removal of the adsorbed oxygen by the hydrocarbon, thus recreating the active centres.

Removal of NO from flue gases by absorption to an iron(ii) thiochelate complex and subsequent reduction to ammonia

Abstract: THE combustion of fossil fuels generates SO2 and NOX pollutants which cause acid rain and urban smog1 Existing flue-gas desulphurization scrubbers involve wet limestone processes which are efficient for controlling SO2 emissions but are incapable of removing water-insoluble nitric oxide The current technique for postcombustion control of nitrogen oxide emissions, ammonia-based selective catalytic reduction, suffers from various problems2,3, including poisoning of the catalysts by fly ash rich in arsenic or alkali, disposal of spent toxic catalysts and the effects of ammonia by-products on plant components downstream from the reactor To circumvent the need for separate schemes to control SO2 and NOX, we have developed an iron(ii) thiochelate complex that enhances the solubility of NO in aqueous solution by rapidly and efficiently absorbing NO to form iron nitrosyl complexes The bound NO is then converted to ammonia by electrochemical reduction, regenerating the active iron(ii) catalyst for continued NO capture Our results suggest that this process can be readily integrated into existing wet limestone scrubbers for the simultaneous removal of SO2 and NOX

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

Abstract: 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.

Nature, distribution and reactivity of copper species in over-exchanged Cu-ZSM-5 catalysts: an XPS/XAES study

Abstract: Cu-ZSM-5 catalysts prepared by “over-exchange” (degree of exchange = 130%) and by impregnation have been studied by X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger spectroscopy (XAES). In both preparations, the external surface region of the catalysts is highly enriched in copper. In the over-exchanged zeolite, the observation of high XPS binding energies and unexpectedly low Auger kinetic energies shows that copper is present as isolated ions or small clusters. In the impregnated material large aggregates of metallic copper at the external zeolite surface are produced by reduction in hydrogen, which are not dispersed by treatment in oxygen or NO. In the exchanged materials, the copper becomes more evenly distributed across the zeolite crystal as a result of calcination or reductive treatment. Changing the composition of the atmosphere to which the exchanged Cu-ZSM-5 is exposed shows that there is ready conversion between Cu(II) and Cu(I). Copper is predominantly in the +2 oxidation state under conditions relevant to selective catalytic reduction of NO in lean-burn engine exhaust gas purification.

Catalytic system for the reducton of nitrogen oxides

Abstract: There is presented a process for the treatment of exhaust gas, which process uses a specially prepared catalyst composition, for the selective catalytic reduction of NOx contained in the exhaust gas An embodiment of the process of this invention comprises a catalytic stage to selectively catalytically reduce NOx over a catalyst composition comprising a molecular sieve that has been treated with a metal in a way effective to maximize metal dispersion The catalyst of this invention typically comprises a silica, titania, or zirconia binder, eg a binder including a high molecular weight, hydroxyl functional silicone resin The catalyst of this invention may be formed into a desired shape, eg, by extrusion, and finished in a humidified atmosphere after forming

Catalytic reduction of NO by propene in the presence of oxygen over mechanically mixed metal oxides and Ce-ZSM-5

Abstract: The mechanical mixing of Mn2O3 or CeO2 to Ce-ZSM-5 considerably enhanced the rate of the reduction of NO by propene in the low to medium temperature region, although Mn2O3 or CeO2 itself was much less active for this reaction. In contrast, Mn2O3 was highly active and CeO2 was moderately active for the oxidation of NO to NO2. On the basis of the comparison of the rates of the C3H6 + O2, NO + C3H6 + O2 and NO2 + C3H6 + O2 reactions over these catalysts, a bifunctional mechanism is proposed, in which Mn2O3 and CeO2 accelerate the oxidation of NO and the subsequent reaction steps between NO2 and propene proceed on Ce-ZSM-5.

Method for reducing the nitrogen oxide concentration in the exhaust of an internal combustion engine or of a firing system

Abstract: The method reduces the nitrogen oxide concentration in the exhaust of an internal combustion engine. A noticeable reduction in the NOx emission of a diesel motor can be achieved by applying the Selective Catalytic Reduction (SCR) method. In this method, ammonia is injected into a catalyzer through which the exhaust gas flows, this ammonia reacting with nitrogen monoxide or, respectively, nitrogen dioxide to form nitrogen and water. Since the exhaust gas should contain neither nitrogen monoxide nor excess ammonia, suitable methods are required for regulating the metering of NH3. For controlling the amount of urea added to the exhaust gas as a reduction agent, the NO and NH3 concentration is measured using a detector located in the exhaust systems following the SCR catalyzer. The detector contains a vanadate layer manufactured on the basis of a specific sputtering method as a sensitive element. The electrical resistance thereof is highest when the conversion of nitrogen monoxide to form nitrogen and water occurs stoichiometrically.

Catalytic reduction of nitrogen oxides in methane-fueled engine exhaust by controlled methane injections

Abstract: Nitrogen oxides are removed from the exhaust of an internal combustion engine which operates on a methane-containing fuel by reacting the nitrogen oxides and oxygen in the exhaust gas with a controlled amount of methane in the presence of a reducing catalyst The catalyst preferably comprises a crystalline zeolite having a silicon to aluminum ratio of equal to or greater than about 25 which is prepared from a non-acid zeolite which is ion exchanged with a cation selected from the group consisting of cobalt, nickel, iron, chromium, and manganese Methane for the reaction is provided as a portion of the methane-containing fuel Carbon monoxide present in the exhaust gas is converted to carbon dioxide simultaneously with the reduction of nitrogen oxides

Performance and durability of Pt-MFI zeolite catalyst for selective reduction of nitrogen monoxide in actual diesel engine exhaust

Abstract: Selective catalytic reduction of nitrogen monoxide (NO) by hydrocarbon in an oxidizing atmosphere has been studied over platinum-MFI zeolite (Pt-MFI) in synthesized or actual diesel engine exhaust gases. The activity of Pt-MFI in the synthesized gas, containing 10% water, changed in the early stage of the use, leveled off after 150–200 h, and remained constant for more than 800 h. The Pt-MFI catalyst also showed stable activity at 423–773 K and 10 000–150 000 h − (gas hourly space velocity) in actual engine exhaust with light oil as a fuel. The degree of nitrogen monoxide reduction increased linearly upon addition of ethylene into the exhaust gas.

Properly apply selective catalytic reduction for NOx removal

Abstract: Selective catalytic reduction (SCR) is a process in which nitrogen oxides (NO{sub x}) are removed by the injection of ammonia (NH{sub 3}) into the flue gas. Chemical reactions in the presence of a catalyst produce nitrogen and water vapour. The article discusses the design and operating experience of SCR systems for denitrification of flue gas in chemical process industries (CPI) and petroleum refinery heaters and boilers, gas turbine systems, and coal-fired steam plants. It provides an overview of the general SCR design approaches, the effects on the denitrification efficiency of major design and operating parameters, various ammonia vaporization and injection methods, and catalyst management strategies. Special precautions and design features associated with coal-fired units are also covered. Although the technical information here is primarily based on the experience of the author`s company, it is considered to be typical of SCR industrial practice. 3 refs., 6 figs., 4 tabs.

In situ characterization of Cu-ZSM-5 by X-ray absorption spectroscopy: XANES study of the copper oxidation state during selective catalytic reduction of nitric oxide by hydrocarbons

Abstract: Reported here is our recent investigation of the mechanism of the selective nitric oxide reduction by hydrocarbons on Cu-ZSM-5 catalysts in an oxygen-rich gas mixture. We studied the copper oxidation state change during the catalytic reaction using the X-ray Absorption Near Edge Structure (XANES) method. We observe that even under strongly net oxidizing conditions, a significant fraction of the copper ions in ZSM-5 is reduced to Cu I at elevated temperature, when propene is present in the reactant stream. XANES spectra show that the Cu I 1s → 4p transition intensity, which is proportional to cuprous ion concentration, changes with the reaction temperture in a pattern similar to the NO conversion activity. For comparison purposes, we also studied the Cu I concentration change using a gas mixture in which propene was replaced by a stoichiometrically equivalent concentration of methane. Unlike propene, methane provides no NO selective reduction pathway over Cu-ZSM-5. No window of enhanced Cu I concentration was observed using methane as the reductant. Our study indicates that, even in a strongly oxidizing environment, cupric ion can be partially reduced by propene to form Cu I , possibly by way of allylic intermediate, which may be a crucial step for effective NO conversion through a redox mechanism.

Integrated regenerative catalytic oxidation/selective catalytic reduction abatement system

Abstract: A system for the abatement of industrial process emissions comprises a catalytic oxidizer and a selective catalytic reduction bank integrated into a single housing. The system utilizes at least two regenerative chambers in a controlled abatement process. The integrated system removes volatile organic compounds (VOCs), carbon monoxide (CO), and toxic oxides of nitrogen (NO x ) from the process emissions. The singular housing of the present invention is efficient and eliminates the need for more than one supplementary heat source, while eliminating major pollutants from the emissions.

Process for the selective catalytic NOx reduction in water vapour and oxygen containing gases

Abstract: of EP0615777For the catalytic reduction of NOx in oxygen-containing exhaust gases (18), solid urea (12) is used as reducing agent instead of an aqueous urea solution. Significant advantages result from this, especially for mobile use. The catalyst system used is a combination of urea hydrolysis and SCR catalyst (34, 5).