Showing papers on "Selective catalytic reduction published in 1995"

Bifunctionality of palladium-based catalysts used in the reduction of nitric oxide by methane in the presence of oxygen

Abstract: We have studied the selective catalytic reduction (SCR) of nitric oxide by methane over palladium on several acidic and non-acidic supports in the presence of excess oxygen. We have found that the acidity of the support promotes the nitric oxide conversion. Both palladium and acid sites are necessary to achieve high catalytic activity. On Pd/H-ZSM-5 catalysts, the level of palladium loading is important. The activity and selectivity for nitric oxide reduction pass through a maximum with increasing Pd wt.-%. On low palladium loading catalysts, the activity and selectivity also reach a maximum with increasing oxygen concentration. The H-ZSM-5 support was not unique in promoting the activity of palladium, but it was the most effective among the acidic supports investigated when palladium was directly supported on it. Enhancements in activity and selectivity were also observed when acidic materials were mechanically mixed with a Pd/SiO 2 catalyst. In this case, the most effective material to promote the activity of palladium was sulfated zirconia. Therefore, it seems that the acidity is more important than the zeolitic structure in determining the activity of these catalysts. The observed catalytic behavior is consistent with a two-step bifunctional mechanism previously proposed for the nitric oxide reduction on Ga and In/H-ZSM-5 catalysts.

Conditions in which Cu-ZSM-5 outperforms supported vanadia catalysts in SCR of NOxby NH3

Abstract: Continuous flows of a standard reactant mixture, featuring 0.6% nitric oxide, 0.6% ammonia and 3.3% oxygen at moderate space velocities over 4 different catalysts, have been used to compare relative activities for selective catalytic reduction of NO x at 373–773 K. The catalysts tested were: Cu-ZSM-5 featuring > 100% ion-exchange; a conventionally prepared vanadia-titania-tungstate (VTT) material and two unconventional catalysts prepared by vanadia deposition onto ex-sol-gel WO 3 TiO 2 supports. At catalytic temperature 473 K, higher conversion to N 2 was achieved over Cu-ZSM-5 than over the other three materials. Tests without NO at 473 K showed insignificant contributions to N 2 formation from ammonia oxidation over any of the catalysts, whereas tests at 573, 623, 673 and 773 K revealed larger progressive increases in such contributions over Cu-ZSM-5 than over the other catalysts. Values for SCR activities corrected for such contributions demonstrated that activity of Cu-ZSM-5 for SCR conversion of the standard NO + NH 3 + O 2 reactant mixture to N 2 at 473 K was ca. twice as great as the other three catalysts at that temperature, but that increasing the reaction temperature to 573 K caused only a slight further increase. ‘Corrected’ SCR activities in the standard reactant mixtures were rather similar for all four materials at 573 K, but with Cu-ZSM-5 marginally out-performed by one of two unconventional catalysts featuring vanadia upon an ex-sol-gel WO 3 TiO 2 support having tungsten incorporated into the TiO 2 anatase structure. Both of these unconventional catalysts outperformed a conventional ‘VTT’ catalyst. Observations upon variations in conversion to N 2 with variation in the oxygen content of the reactant gas mixture from 1 to 6% established another unique feature of the Cu-ZSM-5 catalyst at 473 K, viz. the need for ca. 4.5% O 2 to raise conversion to the maximum attainable over that catalyst at this temperature. No deactivation was observed after short-term runs at temperatures up to 823 K. Introduction of water vapour into the standard reactant mixture slightly enhanced the activity of Cu-ZSM-5 at 473 K.

Method for removing nitrogen oxides in exhaust gas by selective catalytic reduction and catalyst for reduction of nitrogen oxides

Abstract: The present invention provides a catalyst for reduction of nitrogen oxides represented by the formula (A a O x .B b O y ).(C c O z .C' c' O z' )/S which is produced by supporting mixed metal oxides represented by the formula (A z O x .B b O y ).(C c O z .C' c' O z' ) in amorphous state on aluminum or silicon-containing support at calcination temperature of 400° to 700° C. in a molar ratio of 0.01:1 to 5:1, and a method for removing nitrogen oxides through selective catalytic reduction which comprises converting 50 to 50,000 ppm of nitrogen oxides in exhaust gas from automobile or from fixed source such as plant turbine and boiler, and other industry in a state of having 0.1 to 20% of excessive oxygen over (A a O x .B b O y ).(C c O z .C' c' O z' )/S catalyst by using 100 to 100,000 ppm of hydrocarbon reducing agent having 1 to 5 carbons under the reaction condition of 200° to 800° C. of reaction temperature, 1 to 10 atmosphere of reaction pressure and 1000 to 100,000/hour of space velocity into nitrogen in which A and B are lanthanide metals, such as lanthanum, cerium, praseodymium or neodymium, and alkali metals or alkaline earth metals, such as sodium, potassium, rubidium, cesium, magnesium, calcium, strontium or barium, C and C' are transition metals of the first period, such as cobalt, copper, nickel, manganese, iron, vanadium, titanium, chromium and zinc, and noble metals, such as platinum, rhodium, iridium, ruthenium, rhenium, palladium and silver, a, b, c and c' have stoichiometrically 0 to 1, provided with a+b=1, c+c'=1 and is in the range of 0.1-3.0:1.0, and S is aluminum or silicon-containing support, and is zeolite, silica, alumina or silica-alumina.

Selective catalytic reduction of nitric oxide with ammonia overV2O5/TiO2 catalyst: A steady-state and transient kinetic study

Abstract: The kinetics of the selective catalytic reduction (SCR) of nitric oxide with ammonia over an 8 mol-% V 2 O 5 /TiO 2 catalyst was studied in the temperature range 180–380°C, nitric oxide and ammonia feed concentrations in the range 500–2500 ppm with excess of oxygen. It was found that the reaction order with respect to ammonia strongly depends on reaction temperature, in contrast to the case of the reaction order with respect to nitric oxide. The apparent activation energy of the reaction for nitrogen formation depends more on the feed concentration of ammonia than of nitric oxide. This activation energy varies between 12 and 9 kcal mol −1 for ammonia concentrations in the range 500–2000 ppm. Temperature-programmed desorption (TPD) studies revealed the presence of three well-resolved ammonia peaks corresponding to desorption energies in the range 22–28 kcal mol −1 . Transient isotopic experiments with 18 O 2 showed that at 400°C only small amounts of lattice oxygen of V 2 O 5 can be exchanged with gaseous oxygen. Similar experiments with 15 NO showed also that only very small quantities of nitric oxide adsorbed on the catalyst surface from a mixture containing 15 NO/O 2 /He. The partial oxidation reaction of ammonia to nitrogen and nitrous oxide at 350°C was studied by steady-state tracing techniques. The results obtained suggest that at the level of ammonia conversion of 75% there is an appreciable amount of NH x intermediate species (8.3 μmol/g) which are found in the reaction pathway of nitrogen formation, but a small amount (0.4 μmol/g) is found in the reaction pathway of nitrous oxide formation. In addition, adsorption and desorption steps of ammonia must be considered as faster steps than those involved in reaction between adjacent adsorbed NH x species to form nitrogen and nitrous oxide.

Relationship between methane adsorption and selective catalytic reduction of nitrogen oxide by methane on gallium and indium ion-exchanged ZSM-5

Abstract: Methane adsorption on Ga ion-exchanged ZSM-5 (Ga-ZSM-5) and In ion-exchanged ZSM-5 (In-ZSM-5) was investigated by temperature-programmed desorption in ultra high vacuum (UHV-TPD) to clarify the difference in the influence of water vapor on the selective catalytic reduction of NOx by methane. Methane was adsorbed on Ga-ZSM-5 and In-ZSM-5, and the adsorption heats of methane were estimated to be − 98 kJ/mol and − 132 kJ/mol for Ga-ZSM-5 and In-ZSM-5, respectively. The amount of methane adsorbed on Ga-ZSM-5 decreased considerably after the preadsorption of water, while little decrease was observed on In-ZSM-5. These results suggest that one of the reasons that the suppression of HC-SCR activity by water vapor is less severe on In-ZSM-5 than on Ga-ZSM-5, may be that competitive water adsorption is much more inhibiting to methane adsorption on Ga-ZSM-5 than it is on In-ZSM-5.

Method of removing chlorine and halogen-oxygen compounds from water by catalytic reduction

Abstract: The invention concerns a method of removing substances present in water, in particular halogen-oxygen compounds which remain in the water as residues of disinfecting or are formed as by-products of oxidative water treatment. According to the invention, the substances present in water are removed by catalytic reduction in the presence of hydrogen on a supported precious metal catalyst.

Reduction of NOx in diesel exhaust with methanol over alumina catalyst

Abstract: NO x reduction in diesel exhaust using selective catalytic reduction by methanol over alumina catalyst was investigated. Although a slight decrease in catalytic activity took place at the initial stage of the catalyst evaluation, more than 40% NO x conversion was obtained at 400°C and a space velocity of 10 000 h −1 by using a ball type alumina catalyst and methanol ( CH 3 OH/NO x mole ratio = 2) as a reducing agent. The experimental results using synthetic exhaust gases containing SO 2 indicated that the initial catalytic deactivation is ascribed to the formation of Al 2 (SO 4 ) 3 -like species on alumina surface through reaction with SO 2 . However, the formation stopped at a certain point in time. The stable activity following the initial deactivation did not depend on SO 2 concentration. A durability test of a honeycomb type alumina was also conducted, and more than 70% of NO x conversion maintained for more than 4000 h at a CH 3 OH/NO x mole ratio of 6. In this case a gradual decrease in the activity of alumina occurred due to catalyst sintering.

A method of purifying gases containing nitrogen oxides and an apparatus for purifying gases in a steam generation boiler

Abstract: A method of reducing the nitrogen oxide level in the flue gases issuing from combustion units by introduction of reducing agents into contact with gases containing nitrogen oxides in first and second reducing stages, is provided. The first reducing stage is a non-catalytic stage (e.g. at temperatures over 800 °C), while the second stage is a catalytic stage (e.g. at temperatures of about 300-400 °C). A steam generation boiler with improved nitrogen reduction facilities is also provided. The amount of nitrogen oxides in the hot gases is reduced in the combination of the first and second reducing stages while producing steam in a steam generation boiler system, thus resulting in gases essentially free from nitrogen oxides while eliminating the possibility of NH3 (or other reducing agent) slip in the exhausted flue gases. Heat transfers in a convection section are used to establish stabilized temperature conditions for catalytic reduction.

Selective catalytic reduction reactor integrated with condensing heat exchanger for multiple pollutant capture/removal

Abstract: A system is disclosed for minimizing equipment and flue fouling for any boiler, turbine, or combustion process in which heat recovery is advantageous and pollutant removal is necessary. The process uses an SCR located upstream of a condensing heat exchanger and maintains the temperatures in the flue gas duct such that ammonia slip and ammonium salt products are collected only on the heat exchanger surfaces which are periodically washed with water and this wash water being collected.

Influence of PtRe interaction on activity and selectivity of reforming catalysts

Abstract: The influence of the preparation procedure on the Pt and Re interaction in Pt Re/Al2O3 reforming catalysts has been studied. Three different preparation procedures have been used: the classical coimpregnation and successive impregnation techniques, and the recently reported catalytic reduction method. Catalyst activation was done either by direct reduction after metal deposition, or by calcination and reduction. The degree of interaction of the metals was indirectly measured by the cyclopentane hydrogenolysis reaction. It has been found that interaction between Pt and Re increases according to the sequence: coimpregnation (calcined and reduced), catalytic reduction (calcined and reduced), successive impregnations (reduced catalysts) and catalytic reduction (reduced catalysts). Calcination greatly diminishes the Pt Re metal-metal interaction, as measured by cyclopentane hydrogenolysis. After sulfiding, the catalysts prepared by catalytic reduction with a calcination and reduction treatment display the highest n-heptane dehydrocyclization activity. Catalysts only reduced also have good activity for this reaction, but with poor stability.

IR Investigation of Selective Reduction of NO by Ethene on Cu-ZSM-5

Abstract: Adsorbed species during the reaction of selective catalytic reduction of NOx by hydrocarbons (HC-SCR) on Cu ion-exchanged ZSM-5 (Cu-ZSM-5) were investigated by in situ FT-IR using isotopes as well as gas phase analysis. No oxygen-containing species was observed by IR and no reaction proceeded in the presence of gaseous O2 and C2H4 below 473 K, at which HC-SCR occurs. However, an extra NOx species (tentatively assigned to organic nitrite or nitrate species), other than inorganic NOx species derived from O2 and NO, appeared very rapidly in IR at 1670 cm−1 at room temperature, in the presence of adsorbed NO2 species, gaseous C2H4 and NO. Afterwards, a carbonyl species (1677 cm−1) was formed gradually at room temperature. When the extra NOx species was observed, N2 and N2O evolved even at room temperature, therefore, the extra NOx species is considered to be related to HC-SCR. In the O2 + C2H4 + NO reaction at 473 K, no nitrogen-containing species, except for quite a small amount of nitrile (2168 cm−1), was o...

Effect of Copper Contents on Sulfur Poisoning of Copper Ion-Exchanged Mordenite for NO Reduction by NH3

Abstract: Deactivation of copper ion-exchanged mordenite (CuHM) catalysts with different copper contents by SO{sub 2} for NO reduction with NH{sub 3} was examined in a fixed bed flow reactor. A larger amount of deactivating agent deposits on the catalyst surface with the increase of copper contents under the same operating conditions. The deactivating agents deposited on the copper-exchanged mordenite catalysts were mainly (NH{sub 4}){sub 2}SO{sub 4} and/or NH{sub 4}HSO{sub 4} from the results of thermal analyses such as thermogravimetric analysis (TGA), differential thermal analysis (DTA), and temperature-programmed desorption (TPD). The formation of the ammonium salts largely depends on the reaction temperature and SO{sub 3} concentration generated from SO{sub 2} oxidation which is a reaction catalyzed primarily by copper ions on the catalyst surface. The catalytic activity and surface area of the deactivated catalysts are well-correlated with the sulfur content deposited on the catalyst surface, depending upon the reaction temperatures and its catalyst copper contents.

Selective catalytic reduction of nitrogen oxide by hydrocarbons over mordenite-type zeolite catalysts

Abstract: The reduction of NO by hydrocarbons such as C 2 H 4 , C 2 H 6 , C 3 H 6 , and C 3 H 8 has been investigated over mordenite-type zeolite catalysts including HM, CuHM, NZA (natural zeolite), and CuNZA prepared by an ion-exchange method in a continuous flow fixed-bed reactor. NO conversion over CuNZA catalyst reaches about 94% with 2000 ppm of C 3 H 6 at 500°C. As reductants, alkenes seem to exhibit a higher performance for NO conversion than alkanes regardless of the catalysts. No deterioration of the catalytic activity due to carbonaceous deposits for CuNZA was observed above 400°C even after 30 h of on-stream time, but SO 2 in the feed gas stream causes a severe poisoning of the CuNZA catalyst. The effect of H 2 O on NO conversion was significant regardless of the catalysts and the reductants employed in this study. However, CuNZA catalyst shows a unique water tolerance with C 3 H 6 . The reaction path of NO to N 2 is the most important factor for high performance of this catalytic system. NO is directly reduced by a reaction intermediate, C n ′ H m ′ (O) formed from hydrocarbon and O 2 , N 2 O is another reaction intermediate which can be easily removed by C n ′ H m ′ (O).