Showing papers on "Selective catalytic reduction published in 2000"

Catalytic performance of Ag-Al2O3 catalyst for the selective catalytic reduction of NO by higher hydrocarbons

Abstract: Silver–aluminum mixed oxide catalyst (Ag–Al2O3) prepared by the sol–gel method was studied for the selective reduction of NO by various alkanes in the presence of water vapor. As the carbon number of alkanes increases, the de-NOx activity and water tolerance were markedly increased. In the case of n-octane as a reductant, the presence of water vapor markedly promoted NO reduction. The results of reaction studies and in situ IR experiment showed that the possible reasons for the promoting effect by water vapor are the inhibition of the n-octane oxidation by O2 and the suppression of the poisoning effect caused by carboxylate and carbonate species. Among various alumina-supported transition metal catalysts, Ag–Al2O3 showed the highest activity for SCR by n-octane. Ag–Al2O3 showed higher NO conversion to N2 and selectivity than alumina-supported Pt and Cu-ZSM-5 catalysts for the selective reduction of NO by n-octane and i-octane.

Catalytic reduction of nitrates and nitrites in water solution on pumice-supported Pd–Cu catalysts

Abstract: Two series of pumice-supported palladium and palladium–copper catalysts, prepared by impregnation with different palladium and copper precursors, were tested for the hydrogenation of aqueous nitrate and nitrite solutions. Measurements were performed in a stirred tank reactor, operating in batch conditions, in buffered water solution at atmospheric pressure and at 293 K. The activities of the catalysts were calculated in terms of nitrate and/or nitrite removal. With the monometallic Pd/pumice, the reduction of nitrite is highly selective; only 0.2% of the initial nitrite content is converted to ammonium ions. The activity in terms of turn over frequency (TOF) is higher as compared to a catalyst of Pd on silica. Addition of copper to the palladium catalyst is essential for the reduction of nitrates, although it decreases the nitrite reduction activity and increases the production of ammonium ions. Nitrate reduction appears to be structure-insensitive and a volcano-type dependence of the activity versus the overall Cu atomic weight percentage is observed for the two series of catalysts.

Selective catalytic reduction of nitrogen oxides by combining a non-thermal plasma and a V2O5-WO3/TiO2 catalyst

Abstract: The selective catalytic reduction (SCR) of nitrogen oxides (NOx) by a combination of dielectric barrier discharge plasma and a monolithic V2O5-WO3/TiO2 catalyst using ammonia as a reducing agent was studied in synthetic gas mixtures at temperatures between 100 and 250°C. The mixtures were similar to diesel exhaust gases. For gas mixtures (NOx,0=NH3,0=500 ppm) in which 95% of the nitrogen oxides were in the form of NO, the removal of NOx below 140°C without plasma treatment was negligible. Treating the gas mixture with dielectric barrier discharges before the catalytic conversion, about 70% of the NOx was reduced at temperatures as low as 100°C. By plasma treatment 170 ppm of the NO was converted, 110 ppm by oxidation to NO2 and 60 ppm by reduction with products of NH3. Due to the coexistence of NO and NO2 on the catalyst, the selective catalytic reduction was enhanced. Similar effects were observed for the selective catalytic reduction in gas mixtures containing equal amounts of NO and NO2 without plasma treatment. Relative reaction rates for different NOx-reducing reactions over the catalyst are evaluated using a macroscopic model.

Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5–WO3 and V2O5–MoO3 catalysts

Abstract: A comparison between commercial and model WO3–V2O5/TiO2 and MoO3–V2O5/TiO2 SCR catalysts is considered in this study. The data indicate that WO3 and MoO3 behave as “structural” and “chemical” promoters for the catalysts. MoO3-based catalysts are more active but less selective than WO3–V2O5/TiO2 catalysts in the SCR reaction, although in the presence of water the catalytic performances of the investigated samples are comparable.

Characteristics of V2O5 supported on sulfated TiO2 for selective catalytic reduction of NO by NH3

Abstract: V2O5 supported on sulfated TiO2 catalyst was investigated by using Raman and infrared spectroscopies to examine the surface structure of vanadia and the hydroxyl groups of titania along with the sulfate species on the catalyst surface. The surface structure of vanadia plays a critical role, particularly for the reduction of NO by NH3. The polymeric vanadate species on the catalyst surface is the active reaction site for this reaction system. The surface sulfate species enhanced the formation of the polymeric vanadate by reducing the available surface area of the catalyst. The formation of the polymeric vanadate species on the catalyst surface also depends on the number of hydroxyl groups on the support. Both the sulfate and the vanadate species strongly interacted with the hydroxyl groups on titania. The fewer the number of the hydroxyl sites on the catalyst surface became by increasing the calcination temperatures, the more the polymeric vanadate species formed. A model was proposed to elucidate the progressive alteration of the surface structure of vanadia by the amounts of V2O5 loadings and the sulfate species on the catalyst surface.

Catalytic reduction of NOx by hydrocarbons over Co/ZSM-5 catalysts prepared with different methods

Abstract: Co/ZSM-5 catalysts were prepared by several methods, including wet ion exchange (WIE), its combination with impregnation (IMP), solid state ion exchange (SSI) and sublimation (SUB). FTIR results show that the zeolite protons in H-ZSM-5 are completely removed when CoCl2 vapor is deposited. TPR shows peaks for Co2+ ions at 695–705°C and for Co3O4 at 385–390°C, but a peak in the 220–250°C region appears to indicate Co2+ oxo-ions. The catalysts have been tested for the selective reduction of NOx with iso-C4H10 under O2-rich conditions and in the absence of O2, both with dry and wet feeds. A bifunctional mechanism appears to operate at low temperature: oxo-ions or Co3O4 clusters first oxidize NO to NO2, which is chemisorbed as NOy (y≥2) and reduced. In this modus operandi catalyst SUB shows the highest N2 yield ∼90% near 390°C for dry and wet feeds. It is found to be quite stable in a 52 h run with a wet feed. In contrast, the WIE catalyst, which mainly contains isolated Co2+ ions and has poor activity below 400°C, excels at T>430°C. This and the observation that, at high temperature, NO is reduced in O2-free feeds over Co/MFI catalysts, suggest that NO can be reduced over Co2+ ions without intermediate formation of NO2. The bifunctional mechanism at low temperature is supported by the fact that a strongly enhanced performance is obtained by mixing WIE with Fe/FER, a catalyst known to promote NO2 formation.

Low temperature selective catalytic reduction of NO over modified activated carbon fibres

Abstract: Activated carbon fibres (ACFs) have been used for the selective catalytic reduction (SCR) of NO with NH 3 in the temperature range 100–400°C. A model flue gas (700 ppm v NO, 800 ppm v NH 3 , 3 vol.% O 2 and He to balance) was used in all experiments. Among all the ACFs studied, polyacrilonitrile (PAN)-based ACFs showed the highest NO reduction activity which is ascribed to the effect of nitrogen functionalities existing on the surface of this material. The curve of NO conversion versus temperature exhibits a minimum in the temperature range 150–200°C. The presence of water in the flue gases dramatically diminishes the NO reduction activity at temperatures below 250°C. On the other hand, the presence of O 2 shows a positive effect on conversion. NO reduction activity of ACFs can be enhanced by means of different pre-treatments which introduce new superficial groups in the fibres. Among all the treatments studied, the best results were obtained by oxidation of the fibres followed by reaction with NH 3 . The reaction rate seems to be first-order with respect to NO and zero-order with respect to NH 3 . A Langmuir-type dependence between NO conversion and oxygen concentration was observed.

Effect of ex situ treatments with SO2 on the activity of a low loading silver–alumina catalyst for the selective reduction of NO and NO2 by propene

Abstract: A 1.2 wt.% Ag/γ-Al 2 O 3 catalyst for the selective catalytic reduction of NO with propene (C 3 H 6 -SCR) was rapidly and permanently deactivated following the addition of 100 ppm of SO 2 to the gas stream. DRIFTS studies performed on the deactivated silver–alumina material showed the formation of two different types of surface sulphate species. One of these surface species was a surface aluminium sulphate whereas the other corresponded to a sulphate associated with the silver phase. Treatment at higher temperatures in the deNO x feed did not regenerate the catalytic activity fully whereas reduction with H 2 was able to do so by removing only the sulphate species associated with the silver phase. When NO 2 was used as reactant, there was no need for a promoter for either the alumina or the sulphated alumina, both of those giving high conversions of the NO 2 to N 2 over a wide temperature window. The decrease in the SCR activity observed over the Ag/γ-Al 2 O 3 upon sulphation is thought to be associated with a decrease of the activity for the oxidation of NO to ad-NO x species ( x > 1) promoted by the silver. It is suggested that the reduction of ad-NO x species and NO 2 by propene over the 1.2 wt.% Ag/γ-Al 2 O 3 catalyst (plain or sulphated) mostly occurred on the alumina (clean or sulphated).

Intermediates in the Selective Reduction of NO by Propene over Cu−Al2O3 Catalysts: Transient in−Situ FTIR Study

Abstract: The mechanism of the selective catalytic reduction (SCR) of NO by C3H6 on Cu−Al2O3 catalysts, which consist of highly dispersed Cu2+ ions in the surface aluminate phase, are investigated by in-situ FTIR spectroscopy. During NO + C3H6 + O2 reaction, the acetate is produced via the partial oxidation of C3H6 and becomes the predominant adspecies in the steady-state condition at 473−623 K. The acetate, which is stable in NO, is quite reactive with NO + O2, leading to the formation of isocyanate species (Cu−NCO) on the surface and N2 and CO2 in the gas phase. The rate of acetate reaction in NO + O2 is close to the steady-state rate of NO reduction over wide range of temperature, indicating that the acetate is an intermediate in the SCR and takes part in the rate-determining stage. A mechanism is proposed; the acetate and nitrates, formed by NO + O2, react to generate the Cu−NCO species, then Cu−NCO reacts with nitrates or NO to produce N2 and CO2. This mechanism explains the role of oxygen in facilitating SCR....

Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts

Abstract: A series of vanadia doped TiO 2 -pillared clay (TiO 2 -PILC) catalysts with various amount of vanadia were studied for selective catalytic reduction (SCR) of NO by ammonia in the presence of excess oxygen. It was found that the V 2 O 5 /TiO 2 -PILC catalysts were highly active for the SCR reaction. The catalysts showed a broad temperature window, and the maximum NO conversion was higher than that on V 2 O 5 /TiO 2 catalyst and was the same as the commercial V 2 O 5 + WO 3 /TiO 2 catalyst. The V 2 O 5 /TiO 2 -PILC catalysts also had higher N 2 /N 2 O product selectivities as compared to V 2 O 5 doped TiO 2 catalysts. In addition, H 2 O + SO 2 slightly increased the activities at high temperatures (>350°C) for the V 2 O 5 /TiO 2 -PILC catalysts. Addition of WO 3 to V 2 O 5 further increased the activities of the PILC catalysts. These results indicate that TiO 2 -PILC is a good support for vanadia catalysts for the SCR reaction. In situ FT–IR experiment indicated that both Bronsted acid sites and Lewis acid sites exist on the catalyst surface, but with a large proportion being Bronsted acid sites at low temperatures (e.g., 100°C). The reaction path for NO reduction by NH 3 on the V 2 O 5 /TiO 2 -PILC is similar to that on V 2 O 5 /TiO 2 catalyst, i.e., N 2 originates from the reaction between gaseous NO and NH 3 adspecies.

The simultaneous catalytic reduction of NO and N2O by NH3 using an Fe-zeolite-beta catalyst

Abstract: An Fe-Zeolite-beta (Fe-BEA) catalyst, composed only of Fe cations or oxocations in charge compensation sites of the zeolite, is active in the simultaneous removal of NO (1500 ppm) and N2O (1000 ppm) by NH3 (2500 ppm) in the presence of O2 (3 vol.%). In temperature-programmed surface reaction (TPSR) experiments (ramp: 10 K min−1 from 423 to 823 K, space velocity: 200,000 h−1), the light-off temperature, at 50% conversion, is shifted to lower values viz. by 20 K for NO (≈590 K) and 40 K for N2O (≈670 K), compared to those found when processing NO and N2O alone. It is proposed that the removal of surface oxygen O*, coming from the interaction of N2O with iron sites, is faster with NO than with NH3. NO2 which is then formed reacts in turn very fast with NO and NH3 through the classical selective catalytic reduction (SCR) of NOx by NH3 in the presence of O2.

Selective catalytic reduction of NO with methane over Ag-alumina catalysts

Abstract: Silver-alumina catalysts prepared by a single-step co-gelation technique are reported in this letter as highly active and stable for the selective catalytic reduction of NO with methane (CH4–SCR). This type catalyst is active over a wide-temperature range (450–650°C), and for a relatively wide range of Ag loading (1–7 wt.%). Excess O2 in the feed gas favors the SCR reaction. Catalysts capable of high conversions of NOx by CH4 in excess O2-containing gas streams would be an attractive alternative to high temperature NH3–SCR, which is commercially practised for NOx reduction in gas turbine exhausts at >500°C. The structural stability of Ag was studied in reduction-oxidation cycles from 25–600°C by UV–VIS diffuse reflectance spectroscopy. Totally reversible structures were observed. The catalyst is active both for CH4–SCR as well as for the complete oxidation of CH4. This bifunctionality appears to be related to the coexistence of highly dispersed silver clusters and silver nanoparticles in the alumina matrix. The catalyst activity/stability remained high under demanding reaction conditions; namely in 10%O2, 30 ppm SO2 and 10%H2O and high space velocities.

Catalytic reduction of nitrate and nitrite ions by hydrogen: investigation of the reaction mechanism over Pd and Pd–Cu catalysts

Abstract: The catalytic behavior of mono- and bimetallic catalysts with Pd and/or Cu supported over γ-Al2O3 in the reduction of aqueous nitrate and nitrite ions by hydrogen was investigated. The composition of the supported metal catalysts was analysed using secondary ion mass spectroscopy (SIMS) and X-ray diffraction (XRD) techniques. Surface enrichment of the bimetallic Pd–Cu particles in copper was revealed. Interactions of NO (possible intermediate in reduction of nitrate and nitrite ions) with H2 over Pd were studied by thermal desorption spectroscopy (TDS) under high-vacuum conditions. The molecular mechanism of NO3− and NO2− reduction by H2 as well as the role of Pd and Cu active sites are discussed.

Reduction of NOx over various Fe/zeolite catalysts

Abstract: A systematic comparison was made of the catalytic activity of a variety of Fe/zeolite catalysts in the selective catalytic reduction of NO x with iso -butane or propane, both with dry and wet feeds. All catalysts were prepared by subliming FeCl 3 vapor onto the H-form of the zeolite, followed by Cl removal. The activities decrease in the order Fe/BEA > Fe/MFI ⪢ Fe/FER > Fe/MOR ≈ Fe/Y. These differences are rationalized in terms of our mechanistic model and the known pore geometry of the zeolites. FT-IR shows that some Fe loaded narrow pore zeolites are able to oxidize NO to NO 2 and form NO y chemisorption complexes, but unable to let iso -butane enter the pores. Unlike some wider pore zeolites that become covered by a carbonaceous deposit at low temperature, these materials retain their high activity for NO oxidation. Mixtures of Fe loaded wide and narrow pore zeolites, therefore, lead to higher steady-state activities at low temperature than single catalysts. In particular, the combination of Fe/MFI with Fe/FER displays a spectacular activity enhancement.

Emission abatement system

Abstract: The system includes a source of emissions (26) and a catalyst (32) for receiving the emissions. Suitable catalysts are absorber catalysts and selective catalytic reduction catalysts. A plasma fuel converter (12) generates a reducing gas from a fuel source (18) and is connected to deliver the reducing gas into contact with the absorber catalyst (32) for regenerating the catalyst. A preferred reducing gas is a hydrogen rich gas and a preferred plasma fuel converter is a plasmatron. It is also preferred that the absorber catalyst be adapted for absorbing NOx.

Catalytic selective reduction of NO with ethylene over a series of copper catalysts on amorphous silicas

Abstract: Catalytic selective reduction of NO to N 2 was studied comparing a series of Cu-based catalysts (ca. 8 wt.%) supported over amorphous pure and modified silicas: SiO 2 , SiO 2 -Al 2 O 3 , SiO 2 -TiO 2 , SiO 2 -ZrO 2 . The catalysts were prepared by the chemisorption-hydrolysis method which ensured the formation of a unique copper phase well dispersed over all supports, as confirmed by scanning electron micrographs (SEMs). Temperature-programmed reduction (TPR) analyses confirmed the presence of dispersed copper species which underwent complete reduction at a temperature of about 220°C, independently of the support. It was found that the support affects the extent of NO reduction as well as the selectivity to N 2 formation. Maximum N 2 yield was found in the range 275–300°C. The catalyst prepared over SiO 2 -Al 2 O 3 was the most active and selective with respect to the other silicas. Competitiveness factors (c.f.’s) as high as 13–20% in the temperature range 200–250°C could be calculated. For all catalysts, the temperature of the N 2 peak maximum did not correspond to that of the maximum C 2 H 4 oxidation to CO 2 , suggesting the presence of two different sites for the oxidation and the reduction activity. On the catalyst prepared on SiO 2 -Al 2 O 3 , a kinetic interpretation of catalytic data collected at different contact times and temperatures permitted evaluating the ratio between kinetic coefficients as well as the difference between activation energies of NO reduction by C 2 H 4 and C 2 H 4 oxidation by O 2 .