Showing papers on "Selective catalytic reduction published in 1997"

An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides

Abstract: The adsorption and the oxidation of ammonia over sub-monolayer TiO2-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NOx by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SCO process). For comparison, the adsorption of ammonia and hydrazine over the TiO2-anatase support has also been studied. CrOx TiO2 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe2O3 TiO2, CrOx TiO2, CoOx TiO2 and CuO TiO2, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions,N−2. NiOx TiO2 and MnOx TiO2 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SCO processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SCO activity.

Method and device for introducing liquid into an exhaust-gas purification system

Abstract: Nitrogen oxides emitted by an internal-combustion engine operated with excess air are normally converted by the method of selective catalytic reduction by bringing the nitrogen oxides, together with ammonia, into contact with a selective catalyst. Due to the dangers associated with the use of ammonia, in a motor vehicle ammonia should only be carried in the form of a substance which liberates ammonia, generally an aqueous urea solution. A method and a device for introducing liquid into an exhaust-gas purification system according to the invention avoids frost damage to sections of the system during shutdown times and permits operation of the system at temperatures below the freezing point of the reducing agent solution being used. The method and device include a (thermally insulated) reservoir for the reducing agent liquid and a liquid supply line which is connected thereto and terminates in an outlet opening for the liquid. The reservoir and the liquid supply line can be heated. Furthermore, a heater is provided for liquefying a starting volume which is small as compared with the volume of the reservoir. The liquid supply line may also have a back-flush valve to which a gas that is under pressure can be applied. The supply line can consequently be blown free.

Urea pyrolysis chamber and process for reducing lean-burn engine NOx emissions by selective catalytic reduction

Abstract: Urea is pyrolyzed in a chamber designed to facilitate gasification of the urea by pyrolysis with conversion of urea to ammonia and isocyanic acid (HNCO) with water vapor and carbon dioxide The product gases are introduced into exhaust gases from a lean-burn engine, preferably upstream of a turbocharger The exhaust gases are then contacted with an SCR catalyst

Reducing NOx emissions from an engine by selective catalytic reduction utilizing solid reagents

Abstract: Urea or other solid NO x -reducing reagent is employed in a selective catalytic reduction process on emissions from diesel and lean-burn gasoline engines. The solid reagent is fed to a gas generator that produces a reactant gas by heating. In one embodiment the reactant gas is maintained at elevated temperatures to prevent condensation products from forming. The reactant gas contains ammonia and is fed to the exhaust on an as-needed basis.

Reducing no emissions from an engine by on-demand generation of ammonia for selective catalytic reduction

Abstract: A safe, reliable SCR system for reducing NOx emissions from an internal combustion engine hydrolyzes urea or like reagent under sufficient pressure to assure generation of ammonia, without production of solids that could foul injectors or catalysts. The heat for hydrolysis can be provided by the exhaust or an auxiliary means.

Infrared study of catalytic reduction of nitrogen monoxide by propene over Ag/TiO2-ZrO2

Abstract: The paper discusses the mechanism of the selective reduction of NO with propene based on the behavior of adsorbed species formed on Ag/TiO 2 -ZrO 2 catalyst, observed by in-situ FT-IR spectroscopy

IR spectroscopic study of NOxadsorption and NOx–O2coadsorption on Co2+/SiO2catalysts

Abstract: Adsorption of nitrogen oxides (NO, NO2) and their coadsorption with oxygen on Co2+/SiO2 samples has been investigated by IR spectroscopy with a view to elucidating the mechanism of selective catalytic reduction (SCR) of NOx with hydrocarbons. A Co2+/SiO2 sample synthesized by ion exchange is characterized by a highly dispersed cobalt and a very weak surface acidity: CO is adsorbed only at low temperature (100 K) forming Co2+–CO carbonyls [ν(CO) = 2180 cm−1]. Adsorption of NO on Co2+/SiO2 leads to the formation of Co2+(NO)2 dinitrosyl complexes (1872 and 1804 cm−1) which are decomposed upon evacuation. Adsorption of NO2, as well as coadsorption of NO and O2, produce NO2 species weakly bound to the support (a band at 1681 cm−1) and N2O4 (a band at 1744 cm−1 with a shoulder at 1710 cm−1), the latter being adsorbed reversibly on both the support and the Co2+ ions. In the second case N2O4 is transformed into surface monodentate nitrates of Co2+ (a band at 1550–1526 cm−1) and partly into bridged nitrates (a band at ca. 1640 cm−1). The monodentate nitrates are stable with respect to evacuation up to 125 °C and act as strong oxidising agents: they are reduced by NO, even at room temperature, and by methane at 100 °C. In the latter case, organic nitro-compounds and isocyanate groups are registered as reaction products (probably intermediate compounds in SCR). The surface species obtained after NO and NO2 adsorption on Co2+/SiO2 prepared from cobalt acetate (active SCR catalyst) are essentially the same as those observed with the ion-exchanged sample. No monodentate nitrates, however, are formed during NO2 adsorption on a Co2+/SiO2 sample synthesized by impregnation with cobalt nitrate, which accounts for the lack of activity of this sample in the SCR.

Selective catalytic reduction of NOx on Cu-beta zeolites

Abstract: A series of Cu-beta zeolites with different Cu contents have been prepared. Their activity for selective catalytic reduction (SCR) of NO with C3H8 has been studied and compared with that of Cu-ZSM5 zeolites. Cu-beta samples are stable SCR catalysts giving activities as high as those of Cu-ZSM5 zeolites. Their behaviour toward reaction temperature and oxygen content is practically the same as that of Cu-ZSM5. Similar results were achieved when the adsorption of NO over the zeolites was followed by i.r. spectroscopy.

Plasma-assisted catalytic reduction system

Abstract: A two-stage catalyst (100) comprises an oxidative stage (102) and a reductive second stage (104). The first stage (102) is intended to convert NO to NO2 in the presence of O2. The second stage (104) serves to convert NO2 to environmentally benign gases that include N2, CO2, and H2O. By preconverting NO to NO2 in the first stage (102), the efficiency of the second stage (104) for NOX reduction is enhanced. For example, an internal combustion engine exhaust is connected by a pipe (106) to a first chamber (108). A first oxidizing catalyst (110) converts NO to NO2 in the presence of O2 and includes platinum/alumina, e.g., Pt/Al2O3 catalyst. A flow of hydrocarbons (CxHy) is input from a pipe (112) into a second chamber (114). For example, propene can be used as a source of hydrocarbons. The NO2 from the first catalyst (110) mixes with the hydrocarbons in the second chamber (114). The mixture proceeds to a second reduction catalyst (116) that converts NO2 to N2, CO2, and H2O, and includes a gamma-alumina. The hydrocarbons and NOX are simultaneously reduced while passing through the second catalyst (116).

Selective Reduction of NOx with Propene under Oxidative Conditions: Nature of the Active Sites on Copper-Based Catalysts

Abstract: The chemical changes that occurred in two copper-based catalysts (Cu-ZSM-5 and Cu-Al2O3) during the selective reduction of NO with propene in the presence of oxygen were studied using in situ X-ray absorption near edge structure (XANES). For the quantitative analysis of the XANES spectra, a mathematical procedure based on evolving factor analysis (EFA) has been applied. Correlation with catalytic data shows that copper is fully oxidized in both systems when the conversion of propene is complete and the conversion of NO to N2 reaches its maximum value. The XANES analysis together with comparison of the catalytic behavior of Cu-ZSM-5 with Cu-Al2O3 and Cu-SiO2 systems indicates that the rate limiting step of the reaction takes place on cupic oxides (although other species may also participate). This conclusion is of general significance because it establishes the physical basis for the role of Cu in selective catalytic reduction of NO by hydrocarbons; this explains the recently recognized fact that zeolitic ...

A study on selective reduction of NOx by propane on Co-Beta

Abstract: Selective catalytic reduction of NOx by propane was investigated on Co-Beta to clarify the loaded states of cobalt and their role in catalytic activity. At low ion exchange levels less than 100%, loaded cobalt is highly dispersed, which has a high selectivity for NOx reduction. At ion exchange levels much higher than 100%, Co3O4 appears as identified by Raman spectroscopy, and it contributes to propane oxidation by oxygen and lowers the selectivity especially at high temperatures.

Characterization of highly active silver catalyst for NOx reduction in lean-burning engine exhaust

Abstract: A new Ag/Al2O3 catalyst for removing NOx in lean exhaust gas was developed. Oxidized Ag/Al2O3 catalyst is highly active for reduction of NOx with ethanol and propene, whereas reduced Ag/Al2O3 catalyst is less active for these reactions. Selectivity to N2 is also high on the oxidized Ag/Al2O3 compared to that on the reduced Ag/Al2O3. XRD and SEM studies of these two types of Ag catalysts suggest that oxidation induces an interaction between Ag and the support, where the particles are grown in large size. In contrast, the metallic Ag particles are finely dispersed by the reduction process. Although dispersion of Ag particles is decreased by the oxidation process, the catalytic activity is increased. This suggests that the Ag-alumina sites created in the high temperature oxidizing environment are active in catalytic reduction of NOx.

Selective catalytic reduction of nitrogen monoxide by decane on copper-exchanged beta zeolites

Abstract: The selective catalytic reduction (SCR) of NO by decane has been investigated on a series of copper exchanged beta zeolite (BEA), and the catalytic behaviour has been compared to Cu-MFI. The different solids were characterised by temperature programmed reduction by H2 and temperature programmed desorption of NO in order to determine the nature, reducibility and accessibility of copper species. Only CuO aggregates are formed on Cu/H-BEA whatever the Cu content. By contrast, isolated Cu2+ are the main species occurring on Cu/Na-BEA when Cu exchange level is lower than 120% and the accessibility to Cu sites is high (NO/Cu≥ 0.45). At higher content, CuO aggregates exists too, and the accessibility decreases to NO/Cu≈ 0.29. The activity in SCR of NO by decane of Cu/Na-BEA goes through a maximum for 113% Cu exchange. This sample is less active than Cu/MFI at high temperature, but much more active at low temperature. The intrinsic activity at 673 K per accessible Cu site remains very similar for the different Cu/Na-BEA samples. This is attributed to temperatures of the reduction step Cu2+ → Cu2+ very close whatever the copper content.

Ammonia injection in NOx control

Abstract: Improvements are described in the injection of ammonia (NH3) into the exhaust gases of an engine to reduce nitrogen oxides. Instead of merely injecting ammonia into the exhaust gas conduit through a hole in its side, an ammonia injector (90) is provided that projects considerably into the exhaust conduit (16), with the injector having a plurality of holes (94). The ammonia is activated to decompose it into its reactive components, including NH2 and NH prior to injecting it into the exhaust conduit. Such activation prior to injection can be accomplished by heating the ammonia in the presence of a catalyst such as a metal of the platinum group, iron, nickel, or zinc. In an engine that has a fuel injection system wherein electrical pulses are delivered to fuel injectors to control the fuel flow rate, the durations of these electrical signals are used to control the opening of a valve (72) that controls the flow rate of ammonia into the exhaust gas conduit.

X-ray Absorption Spectroscopic Studies at the Cobalt K-Edge on a Reduced Al2O3-Supported Rhenium-Promoted Cobalt Fischer−Tropsch Catalyst

Abstract: In situ XAFS spectroscopic studies have been carried out at 450 °C on the hydrogen reduction of a rhenium-promoted Co3O4/Al2O3 catalyst. The results show that reduction at this temperature yields a...

The catalytic removal of CO and NO over Co-Pt(Pd, Rh)/γ-Al2O3 catalysts and their structural characterizations

Abstract: A series of Co-Pt(Pd, Rh)/γ- Al2O3 catalysts were prepared by successive wetness impregnation. The catalytic activities for CO oxidation, NO decomposition and NO selective catalytic reduction (SCR) by C2H4 over the samples calcined at 500°C and reduced at 450°C were determined. The activities of the samples calcined at 750°C and reduced at 450°C for NO selective catalytic reduction (SCR) by C2H4 were also determined. All the samples were characterized by XRD, XPS, XANES, EXAFS, TPR, TPO and TPD techniques. The results of activity measurements show that the presence of noble metals greatly enhances the activity of Co/γ-Al2O3 for CO or C2H4 oxidation. For NO decomposition, the H2-reduced Co-Pt(Pd, Rh)/γ- Al2O3 catalysts exhibit very high activities during the initial period of catalytic reaction, but with the increase of reaction time, the activities decrease obviously because of the oxidation of surface cobalt phase. For NO selective reduction by C2H4, the reduced samples are oxidized more quickly by the excess oxygen in reaction gas. The oxidized samples possess very low activities for NO selective reduction. The results of XRD, XPS and EXAFS indicate that all the cobalt in Co-Pt(Pd, Rh)/γ-Al2O3 has been reduced to zero valence during reduction by H2 at 450°C, but in Co/γ-Al2O3 only a part of the cobalt has been reduced to zero valence, the rest exists as CoAl2O4-like spinel which is difficult to reduce. For the samples calcined at 750°C, the cobalt exists as CoAl2O4 which cannot be reduced by H2 at 450°C and possesses better activities for NO selective reduction. The results of XANES spectra show that the cobalt in Co/γ- Al2O3 has lower coordination symmetry than that in Co-Pt(Pd, Rh)/γ-Al2O3. This difference mainly results from the distorting tetrahedrally- coordinated Co2+ ions which have lower coordination symmetry than Co0 in the catalysts. The coordination number for the Co-Co shell from EXAFS has shown that the cobalt phase is highly dispersed on Co-Pt(Pd, Rh)/γ- Al2O3 catalysts. The TPR results indicate that the addition of noble metals to Co/γ- Al2O3 makes the TPR peaks shift to lower temperatures, which implies the spillover of hydrogen species from noble metals to cobalt oxides. The oxygen spillover from noble metals to cobalt is also inferred from the shift of TPO peaks to lower temperatures and the increased amount of desorbed oxygen from TPD. For CO oxidation, the Co0 is the main active phase. For NO decomposition and selective reduction, Co0 is also catalytically active, but it can be oxidized into Co3O4 by oxygen at high reaction temperature.

Catalyst and method for catalytic reduction of nitrogen oxides

Abstract: The invention provides a catalyst for catalytic reduction of nitrogen oxides contained in exhaust gases wherein fuel is supplied and subjected to combustion under periodic rich/lean conditions and the resulting exhaust gases are brought into contact therewith, which catalyst comprises: (A) a catalyst component A comprising (c) ceria or (d) praseodymium oxide or (e) an oxide and/or a composite oxide of at least two elements selected from the group consisting of cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium and lanthanum; (B) a catalyst component B comprising (d) a noble metal catalyst component selected from the group consisting of platinum, rhodium, palladium and oxides thereof and (e) a carrier; and (C) a catalyst component C comprising (f) a solid acid, and (g) a solid acid supporting an oxide of at least one element selected from the group consisting of vanadium, tungsten, molybdenum, copper, iron, cobalt, nickel and manganese. The catalyst reduces NOx contained in exhaust gases wherein fuel is supplied and subjected to combustion with a periodic rich/lean excursions, whereby NOx is generated in the exhaust gases, with high durability in a wide temperature range even in the presence of oxygen, sulfur oxides or water.

Selective catalytic reduction of nitric oxide by methane over cerium and silver ion-exchanged ZSM-5 zeolites

Abstract: A new catalyst comprising cerium and silver ion-exchanged ZSM-5 zeolite is reported in this paper, for the reduction of nitric oxide by methane in the presence of excess oxygen. The bi-cation exchanged Ce—Ag-ZSM-5 catalyst was very active for this reaction, while either Ce-ZSM-5 or Ag-ZSM-5 alone showed low activity. The presence of oxygen in the feed gas mixture enhanced the activity of the catalyst and the NO conversion to N2 increased with the CH4/NO ratio and Ag loading of the zeolite. The presence of water vapor had a small adverse effect on the catalyst activity. The coexistence of Ce and Ag ions in the zeolite is crucial for achieving high NO conversion to N2. A small amount of cerium is adequate to promote the selective catalytic reduction of NO. The two main functions of Ce ions are (i) to provide the Ag ion sites with NO2 by catalyzing the oxidation of NO to NO2 and (ii) to suppress the direct CH4 oxidation to CO2. The Ag sites are the active centers where the reaction of NO2 with CH4 takes place.

Laboratory investigation of the catalytic reduction technique for measurement of atmospheric NO y

Abstract: We report laboratory studies of the detection scheme employed for in situ measurement of NOy in the atmosphere. In this technique, an air stream is passed over a hot metal (usually 24 karat (k) Au) catalyst in the presence of a reducing agent (usually CO), which converts the NOx compounds to NOx Using the NOy species NO, NO2, HNO3, and isopropyl nitrate and the potential interferences HCN, CH3CN, NH3, and N2O, we investigated: (1) conversion efficiencies as a function of pressure and catalyst temperature; (2) conversion efficiencies as a function of reducing-agent concentration with both H2 and CO; (3) the effect of humidity and O3 on conversion efficiency; (4) loss of NO in the catalyst; and (5) the efficacy and suitability as catalytic converters (or inlets) of several metals (24 k Au, 18k Au, Au with 1% Co, Ag, Pt, stainless steel) and quartz. The most significant results are the discovery of a gas-phase process that contributes to the conversion of HNO3 to NO and the identification of conditions under which HCN, CH3CN, and NH3 are converted to NO with high efficiency. We discuss the implications of these results for in situ measurement of atmospheric NOy.

Pre-converted nitric oxide gas in catalytic reduction system

Abstract: A two-stage catalyst comprises an oxidative first stage and a reductive second stage. The first stage is intended to convert NO to NO 2 in the presence of O 2 . The second stage serves to convert NO 2 to environmentally benign gases that include N2, CO2, and H 2 O. By preconverting NO to NO 2 in the first stage, the efficiency of the second stage for NO x reduction is enhanced. For example, an internal combustion engine exhaust is connected by a pipe to a first chamber. An oxidizing first catalyst converts NO to NO 2 in the presence of O 2 and includes platinum/alumina, e.g., Pt/Al 2 O 3 catalyst. A flow of hydrocarbons (C x H y ) is input from a pipe into a second chamber. For example, propene can be used as a source of hydrocarbons. The NO 2 from the first catalyst mixes with the hydrocarbons in the second chamber. The mixture proceeds to a second reduction catalyst that converts NO 2 to N2, CO2, and H 2 O, and includes a gamma-alumina γ-Al 2 O 3 . The hydrocarbons and NO x are simultaneously reduced while passing through the second catalyst.

Characterization and activity of novel copper-containing catalysts for selective catalytic reduction of NO with NH3

Abstract: Cu/Mg/Al mixed oxides (CuO = 4.0–12.5 wt%), obtained by calcination of hydrotalcite-type (HT) anionic clays, were investigated in the selective catalytic reduction (SCR) of NO by NH3, either in the absence or presence of oxygen, and their behaviours were compared with that of a CuO-supported catalyst (CuO = 10.0 wt%), prepared by incipient wetness impregnation of a Mg/Al mixed oxide also obtained by calcination of an HT precursor. XRD analysis, UV-visible-NIR diffuse reflectance spectra and temperature-programmed reduction analyses showed the formation, in the mixed oxide catalysts obtained from HT precursors, mainly of octahedrally coordinated Cu2+ ions, more strongly stabilized than Cu-containing species in the supported catalyst, although the latter showed a lower percentage of reduction. The presence of well dispersed Cu2+ ions improved the catalytic performances, although similar behaviours were observed for all catalysts in the absence of oxygen. On the contrary, when the mixture with excess oxygen was fed, very interesting catalytic performances were obtained for the catalyst richest in copper (CuO = 12.5 wt%). This catalyst exhibited a behaviour comparable to that of a commercial V2O5–WO3TiO2 catalyst, without any deactivation phenomena after four consecutive cycles and following 8 h of time-on-stream at 653 K. Decreasing the copper content or increasing the calcination time and temperature led to considerably poorer performances and catalytic behaviours similar to that of the CuO-supported catalyst, due to the side-reaction of NH3 combustion on the free Mg/Al mixed oxide surface.