Showing papers on "Selective catalytic reduction published in 1993"

Alumina-supported silver catalysts for the selective reduction of nitric oxide with propene and oxygen-containing organic compounds

Abstract: The selective catalytic reduction of nitric oxide with propene and oxygen-containing organic compounds over several alumina-supported metal catalysts was investigated. Alumina-supported silver catalysts showed high activities in the presence of water and excess oxygen, while water vapor significantly decreased the activities of alumina and Co/Al2O3 which were highly active in the absence of water. It was also found that oxygen-containing organic compounds such as ethanol and acetone were more effective than propene in reducing nitric oxide over Ag/Al2O3 in the presence of water and excess oxygen.

Selective catalytic reduction of NOx with methane over metal exchange zeolites

Abstract: Catalytic reduction of NO x with methane in an oxidizing atmosphere was studied over metal exchanged zeolites. We found that the combinations of Co 2+ , Mn 2+ and Ni 2+ with certain types of zeolites, e.g., ZSM-5 and mordenite, are active catalysts for this reaction. For a Co-ZSM-5 catalyst, the nitric oxide conversion displays a volcano-shape curve as temperature increases, which is reversible upon decreasing temperature. The nitric oxide reduction activity is proportional to the level of Co 2+ exchanged into ZSM-5, but excess amounts of cobalt do not contribute to the activity. Mn-ZSM-5 is very similar to Co-ZSM-5 in the nitric oxide reduction activity, and Ni-ZSM-5 has slightly lower activity compared to Co-ZSM-5. Under an oxidizing conditions, Cu-ZSM-5, however, is ineffective for the nitric oxide reduction. Co-Y, which has much more Co 2+ , is much less active compared to Co-ZSM-5, or Co-mordenite. The amount of nitric oxide adsorbed, measured by TPD, on Co-Y is extremely small (NO/Co=0.06) compared to Co-ZSM-5 (NO/Co > 1.1) and Co-mordenite (NO/Co=0.8).

Catalytic Reduction of Nitrogen Monoxide by Methane over Palladium-Loaded Zeolites in the Presence of Oxygen

Abstract: Palladium ion-exchanged H-ZSM-5 showed a high catalytic activity for the removal of dilute nitrogen monoxide in the presence of methane and oxygen at 350–550 °C. The presence of palladium and (protonic) acidity is essential for the high activity.

Device for catalytic NOx reduction

Abstract: For the effective catalytic reduction of NOx from oxygen-containing exhaust gases using urea, a device is proposed which contains a hydrolysis catalyst (3 and 4), which is composed of fine flow channels, which admit part-streams through diversions and through holes or slots, which part-streams are oriented approximately perpendicularly to the main stream. By this means, a uniform distribution of the urea solution and very rapid heating of the solution is to be effected. The urea solution can be quantitatively converted in this manner into ammonia and carbon dioxide without formation of harmful byproducts.

NOx emission control in oxygen-rich exhaust through selective catalytic reduction by hydrocarbon

Abstract: Selective catalytic reduction of nitrogen monoxide by hydrocarbon in the presence of oxygen and sulphur dioxide has very recently been found on copper ion-exchanged ZSM-5 zeolite (Cu-ZSM-5). The reaction proceeds even in the oxidizing atmosphere at temperatures as low as 573 K. Addition of oxygen to the reactant stream is necessary to achieve the selective reduction of nitrogen monoxide. The increment of concentration of hydrocarbon increased the conversion into nitrogen and expanded the active temperature region. After summarizing the new findings on Cu-ZSM-5, the present review introduces catalytic activities of various metal-ion exchanged zeolites and metal-loaded aluminas. Based on the temperature dependence of each catalyst and the change of conversion of nitrogen monoxide into nitrogen with space velocity, Cu-ZSM-5 is suggested as one of the strong candidates for catalysts applicable to lean-burn gasoline and diesel engines, though several problems such as durability to water and overheating remain ...

Selective reduction of nitrogen oxides (NOx) by ammonia over honeycomb selective catalytic reduction catalysts

Abstract: Data are presented for the effects of feed composition (O[sub 2], NO [sub x], NH[sub 3], H[sub 2]0, SO[sub 2] concentrations) and of the operating conditions (area velocity, reaction temperature, and Re number) on the NO[sub x] removal efficiency over commercial-type honeycomb DeNOxing catalysts with different compositions and geometric characteristics. All of these data are explained by a Rideal type kinetic expression, derived assuming reaction between strongly adsorbed ammonia and gaseous or weakly bonded NO. The significance of interphase and intraparticle diffusion limitations is discussed, and a comprehensive model of the SCR monolith reactor is illustrated which fully accounts for both external and internal transport of the reactants as well as for the effects of the operating variables and of the feed composition under representative SCR conditions.

Control of nox reduction in flue gas flows

Abstract: The NOx content in a flow of flue gas is reduced by passing the flue gas through a first treatment zone and a second treatment zone. A nitrogeneous treatment agent is introduced into the first treatment zone for the selective non-catalytic reduction of part of the NOx, and the flue gas is thereafter passed through the second treatment zone which includes a catalyst for further selective catalytic reduction of the NOx. Optionally, a second nitrogeneous treatment agent is added to the flue gas in the second treatment zone. The quantity of NOx in the flue gas is detected intermediate the first and second treatment zones and, optionally, after the flue gas has left the second treatment zone. The quantity of ammonia in the flue gas exiting from the second treatment zone is also detected. The amounts of the treatment agents added to the flue gas are controlled responsive to the variations and absolute levels determined by these measurements.

Exhaust gas aftertreatment device for internal combustion engines

Abstract: An exhaust gas aftertreatment device for internal combustion engines having a catalyzer for the selective catalytic reduction of oxides of nitrogen from exhaust gases of motor vehicle diesel engines, provides overstoichiometric supply of NH3 or materials releasing NH3. A first sensor records the NH3 concentration contained in the exhaust gas and interrupts the supply of the NH3 quantity when a specified upper threshold value is reached. A second sensor records the NH3 adsorbed in the catalyzer, by way of which the NH3 supply is resumed on reaching a specified lower threshold value. Alternatively, only one NH3 sensor is provided in the exhaust gas aftertreatment device. The NH3 concentration determined by this single sensor is compared, as the actual value, with a required value corresponding to a specified NH3 concentration in order to form a correction signal which is used for triggering the metering appliance continuously connected into the gas phase.

Method for treating combustion exhaust gas

Abstract: A method for recovering carbon dioxide by absorbing carbon dioxide present in a combustion exhaust gas using an aqueous alkanolamine solution, comprising the step of bringing a combustion exhaust gas from which carbon dioxide has been absorbed and removed into contact with water containing carbon dioxide. A method for treating a combustion exhaust gas for denitration using ammonia as a reducing agent and for removal of carbon dioxide by absorption with an aqueous alkanolamine solution, which method comprising the steps of recovering ammonia present in the combustion exhaust gas after the carbon dioxide removal, and using the recovered ammonia as a reducing agent for the denitration. A method for removing CO₂ from a combustion exhaust gas comprising the step of bringing the combustion exhaust gas into contact under atmospheric pressure with an aqueous monoethanolamine solution having a concentration of 35% by weight or more.

Selective Catalytic Reduction of no by Hydrocarbon in Oxidizing Atmosphere

Abstract: In the presence of O 2 , SO 2 , and H 2 O selective reduction of NO by hydrocarbon over various catalysts, especially, over copper ion-exchanged zeolite has been studied. Simultaneous presence of O 2 and hydrocarbon such as ethene, propene, and propane with NO in the reactant gas resulted in the great enhancement of the catalytic activity for the removal of NO at low temperature (473–673 K). On the other hand, the addition of CO, H 2 , or CH 4 to the NO+O 2 system did not cause any selective reduction of NO. In the former selective reduction, the increment of concentration of hydrocarbon increased the conversion into N 2 and expanded the active temperature region. Addition of oxygen to reactant stream is necessary to achieve the selective reduction of NO, and with Cu-MFI zeolite the maximum activity was observed in the range of oxygen concentration of 0.8–2.0%. No deterioration in the catalytic activity was observed at 573 K even after 30 h of continuous service. When SO 2 or H 2 O was added to NO–O 2 +hydrocarbon gas stream, a certain decrement in the catalytic activity of Cu-MFI zeolites was observed. Within the present experiments, the activity was restored in its absence. Various metal ion-exchanged zeolites, metal loading alumina, and oxides have been screened as catalysts for the reaction and it was clarified that Cu-MFI zeolite catalysts are the most active at high SV region. The addition of copper to zeolites, alumina, and silica-alumina greatly enhanced the activities.

Selective catalytic reduction of nitrogen oxides

Abstract: There is provided a catalytic method for converting nitrogen oxides to nitrogen (i.e., N 2 ). The catalyst for this method comprises an acidic solid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal and further comprising at least one metal selected from the group consisting of Group IB, Group IVA, Group VB, Group VIIB, Group VIII, and mixtures thereof. An example of this catalyst is zirconia, modified with tungstate, and iron. This method may be used for reducing emissions of nitrogen oxides from waste gases, including industrial exhaust gases and automobile exhaust gases. In a particular embodiment, nitrogen oxides in waste gases may be reacted with ammonia before the waste gases are discharged to the atmosphere.

Selective catalytic reduction of nitric oxide by propane in oxidizing atmosphere over copper-exchanged zeolites

Abstract: Selective catalytic reduction of nitric oxide with propane and oxygen was investigated on Cu-exchanged ZSM-5, mordenite, X-type and Y-type zeolites at temperatures in the range of 200 to 600°C. Catalytic activities of Cu-X and Cu-Y are negligible, activity of Cu-mordenite moderate, and that of Cu-ZSM-5 very high, converting > 90% of nitric oxide to nitrogen at 400°C and at a space velocity of 102 000 h −1 . Effects of space velocity, nitric oxide concentration, C 3 H 8 /NO ratio, oxygen concentration, and water vapor on the activities of Cu-ZSM-5 and Cu-mordenite were investigated. Nitric oxide conversion decreases with increasing space velocity, decreasing propane and nitric oxide concentrations, and decreasing propane/NO ratio. Water vapor decreases the activity significantly at all temperatures. At temperatures above 400°C, propane oxidation by oxygen is a significant competing reaction in decreasing the selectivity for nitric oxide reduction. The results indicate that Cu-ZSM-5 is a promising catalyst for selective catalytic reduction of nitric oxide by hydrocarbons.

Catalytic reduction of nitric oxide on PdO—MoO3/γ-Al2O3

Abstract: The activity and selectivity of PdO—MoO 3 /γ-Al 2 O 3 catalysts containing 2% Pd and 20% Mo for the reduction of nitric oxide by carbon monoxide, by hydrogen, and by a mixture of CO + H 2 was investigated in the presence of different amounts of oxygen at reaction temperatures from 25 to 550°C. Results are compared with PdO/γ-Al 2 O 3 , MoO 3 /γ-Al 2 O 3 , and a commercial Pt—Rh catalyst. The palladium determines the performance of PdO—MoO 3 /γ-Al 2 O 3 under oxygen free reaction conditions up to 400°C, notwithstanding the high loading of the molybdena. At higher temperatures the molybdena can improve the performance of the catalyst, especially at slightly oxidizing conditions. Neither the activity nor the selectivity of PdO—MoO 3 /γ-Al 2 O 3 resemble those of the monometallic catalysts in the presence of oxygen when CO + H 2 is used for reduction. In the case of the present catalyst in fresh condition, nitric oxide is reduced selectively to nitrogen and nitrous oxide with a conversion > 70% at temperatures from 300 to 550°C under either slightly reducing or slightly oxidizing conditions.

Selective catalytic reduction of nitric oxide by ethene on gallium ion-exchanged ZSM-5 under oxygen-rich conditions

Abstract: Selective reduction of nitric oxide by ethene in the presence of excess oxygen was investigated using a gallium ion-exchanged ZSM-5 catalyst. Ga-ZSM-5 showed high activity in a wide range of reaction temperatures. In addition, Ga-ZSM-5 showed extremely high selectivity for this reaction under oxygen-rich conditions (10%). The limiting molar ratio of reacted NO to consumed ethylene was found to be 2. High catalytic activity was maintained even under high space velocity at 500°C on Ga-ZSM-5.

Influence of the binder on the properties of catalysts based on titanium-vanadium oxides

Abstract: The influence of the nature of the binder and its concentration in V2O5/TiO2 catalysts on their mechanical and catalytic properties has been studied. The characterization analysis showed that the agglomeration mechanism is different when an inorganic acid, such as H3PO4, or a natural silicate, such as sepiolite, were used. Two different patterns are proposed, which explain the effect of these binders on the performance of this type of catalyst in the selective catalytic reduction of NOx with NH3.

Effect of crystal morphology in selective catalytic reduction of nitric oxide over V2O5 catalysts

Abstract: Structural specificity of vanadium pentoxide catalysts was investigated in the selective catalytic reduction (SCR) of nitric oxide. Catalysts were prepared by different temperature-programmed methods to obtain particles having preferential exposure of different crystal planes. Catalyst samples were characterized using the BET surface area technique, X-ray diffraction, laser Raman spectroscopy. X-ray photoelectron spectroscopy, scanning electron microscopy and 3-D imaging techniques. A steady-state fixed-bed reactor system was used for activity and selectivity measurements. Identification and quantification of reaction effluents were achieved using an analytical scheme that combined gas chromatography, chemiluminescence, gas chromatography-mass spectroscopy and wet chemistry techniques. Samples that preferentially exposed the (010) basal planes were found to promote direct oxidation of ammonia more readily than nitric oxide reduction as evidenced by the differences in nitric oxide and ammonia conversions and in nitrogen and nitrous oxide yields. The difference in catalytic activity and selectivity was related to the relative abundance of V O sites exposed on the catalyst surface and the competing reactions that are involved in SCR and direct ammonia oxidation reaction schemes.

Catalyst for elemental sulfur recovery process

Abstract: A catalytic reduction process for the direct recovery of elemental sulfur from various SO2 -containing industrial gas streams. The catalytic process provides high activity and selectivity, as well as stability in the reaction atmosphere, for the reduction of SO2 to elemental sulfur product with carbon monoxide or other reducing gases. The reaction of sulfur dioxide and reducing gas takes place over a metal oxide composite catalyst having one of the following empirical formulas: [(OF.sub.2).sub.1-n (RO.sub.1)n].sub.1-k M.sub.k, [(FO.sub.2).sub.1-n (RO.sub.1.5).sub.n ].sub.1-k M.sub.k, or [Ln.sub.x Zr.sub.1-x O.sub.2-0.5x ].sub.1-k M.sub.k wherein FO2 is a fluorite-type oxide; RO represents an alkaline earth oxide; RO1.5 is a Group IIIB or rare earth oxide; Ln is a rare earth element having an atomic number from 57 to 65 or mixtures thereof; M is a transition metal or a mixture of transition metals; n is a number having a value from 0.0 to 0.35; k is a number having a value from 0.0 to about 0.5; and x is a number having a value from about 0.45 to about 0.55.

Influence of metal particle size and effect of gold addition on the activity and selectivity of Pt/Al2O3 catalysts in the reduction of nitric oxide by methane

Abstract: The catalytic reduction of nitric oxide by methane over alumina supported platinum and Pt-Au catalysts was investigated using a gas mixture of 2 vol.-% NO and CH 4 in helium at 250–350°C. With monometallic platinum catalysts, the specific reaction rate was found to increase with increasing particle sizes from 1.5 to 15 nm, indicating that the reaction can occur preferentially on large platinum planes. This result was confirmed through addition of gold selectively deposited on the edges and corners of platinum particles. The selectivity pattern obtained suggests that the formation of strongly adsorbed species (NO 2 2− ) is favoured on small platinum particles of higher oxidation state.

Influence of nitrogen dioxide on the selective reduction of NOx with a catalyst of copper and nickel oxides

Abstract: The selective reduction of NOx, with ammonia on alumina-supported copper catalysts is shown to be effective when O2 or NO2 are present in the feed. Under steady state conditions, the presence of NO2 in the feed stream increases the overall rate of reduction of NOx and simultaneously reduces its dependence on the oxygen concentration. A maximum in activity is found for a molar inlet ratio NO2/NO ≈ 1. It has also been observed that the stoichiometry of the process is a function of the reaction temperature, with a secondary NH3 oxidation reaction appearing at temperatures above 500 K. As revealed by X-ray photoelectron spectroscopy, the copper remains mainly as Cu+ species at steady state conditions. Such species are reoxidized by NO2 more easily than by O2 or NO. An analysis of the transitional states when the feed composition is changed and infrared spectra of the adsorbed species allows us to propose a mechanism which explains the observed results.