Showing papers on "Selective reduction published in 2003"

Promotion effect of H2 on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al2O3

Abstract: The effect of H2 on the selective reduction of NO by light hydrocarbons over Ag/Al2O3 catalyst under lean-exhaust conditions was investigated. The NO reduction activity at the low temperature region was markedly increased by adding H2. The NO reduction activity at high GHSV condition and SO2 tolerance were also improved by adding H2. Since the NO was reduced little by H2 in the absence of C3H8, H2 should act as a promoter of NO reduction by hydrocarbons over Ag/Al2O3. The results of reaction studies and in situ IR experiments showed that the possible reasons for promoting the effect by H2 on the HC-SCR were mainly caused by the activation of hydrocarbons.

Catalytic decomposition of N2O and catalytic reduction of N2O and N2O + NO by NH3 in the presence of O2 over Fe-zeolite

Abstract: The decomposition of N2O, and the catalytic reduction by NH3 of N2O and N2O + NO, have been studied on Fe-BEA, -ZSM-5 and -FER catalysts. These catalysts were prepared by classical ion exchange and characterized by TPR after various activation treatments. Fe-FER is the most active material in the catalytic decomposition because “oxo-species” reducible at low temperature, appearing upon interaction of FeII-zeolite with N2O (α-oxygen), are formed in largest amounts with this material. The decomposition of N2O is promoted by addition of NH3, and even more with NH3 + NO in the case of Fe-FER and -BEA. It is proposed that the NO-promoted reduction of N2O originated from the fast surface reaction between α-oxygen O∗ and NO∗ to yield NO2∗, which in turn reacts immediately with NH3.

Study of Ag/La0.6Ce0.4CoO3 catalysts for direct decomposition and reduction of nitrogen oxides with propene in the presence of oxygen

Abstract: Perovskite-type oxide La 0.6 Ce 0.4 CoO 3 and its doped Ag catalysts were prepared and their catalytic performances were evaluated for the direct decomposition of NO and the selective reduction of NO with propene in the presence of oxygen. A noticeable enhancement in activity was achieved by doping Ag and the optimum Ag loading was 1%. The effects of H 2 O, SO 2 , CO 2 and O 2 on the performances of Ag/La 0.6 Ce 0.4 CoO 3 catalysts for NO decomposition were also investigated. The resistance against H 2 O and SO 2 appears satisfactory. The inhibition by CO 2 is strong, although it is reversible. Oxygen did not inhibit the NO decomposition reaction but significantly promoted it. Compared with other perovskite-type oxides reported previously, higher conversions were obtained over the present catalysts for the NO reduction by propene. We speculate that the decomposition of NO is the predominant process even in the presence of propene. The catalysts were characterized by N 2 -adsorption, XRD, XPS and NO-TPD and some explanations were put forward.

Remarkable promoting effect of rhodium on the catalytic performance of Ag/Al2O3 for the selective reduction of NO with decane

Abstract: The addition of small amounts of rhodium enhanced the activity of Ag/Al 2 O 3 catalyst for the selective reduction of NO with decane at low temperatures. The Rh-promoted Ag/Al 2 O 3 showed its high performance even in the presence of low concentrations of SO 2 . Based on the catalytic activity for elementary reactions, it was suggested that the role of added rhodium is to enhance the reaction between NO x and decane-derived species, leading to NO reduction. Catalyst characterization by UV-Vis spectroscopy indicated that the major silver species on Rh-promoted Ag/Al 2 O 3 is Ag n δ + clusters, which would be responsible for the high activity. FT-IR measurements revealed that the formation rate of isocyanate species, which is a major reaction intermediate, is higher on Rh-promoted Ag/Al 2 O 3 .

Selective catalytic reduction of NO by hydrocarbons in the presence of excess oxygen using Pt/MCM-41 catalysts

Abstract: The selective catalytic reduction of NO by C3H6 and C3H8 in the presence of 5% O2 over 0.5–2% Pt supported on siliceous MCM-41 has been studied in detail. Pt/MCM-41 shows better activity for NO reduction than Pt/SiO2 and Pt/Al2O3 under the same reaction conditions due to the larger surface area and pore volume of MCM-41. Two types of Pt species having different reduction temperatures (50–200, 350–500 °C, respectively) are formed on Pt/MCM-41. The Pt species having lower reduction temperature is suggested to be responsible for the selective reduction of NO. A redox mechanism is used to explain this behavior. In addition to its high stability in the presence of water vapor, Pt/MCM-41 shows high resistance to SO2 without irreversible deactivation due to its weak interaction with SO2. The activity of Pt/MCM-41 catalyst is found to be improved in the presence of Al species both in the framework or on the surface of MCM-41 support, possibly due to the improvement of the acidity of Pt/MCM-41 by Al species. In situ XRD result indicates that the surface Al-species prevent the active Pt from sintering at high temperature, hence improving the thermal stability of Pt/MCM-41. Having additive Ru species, Pt/Ru/Si-MCM-41 is found to widen the reaction window and decrease the selectivity of the conversion of NO to N2O during the SCR of NO; this is an important finding for overcoming the main shortcoming of Pt-based catalyst.

Improvements in the N2 selectivity of Pt catalysts in the NO–H2–O2 reaction at low temperatures

Abstract: The selective reduction of NO with H 2 in the presence of excess O 2 (the NO–H 2 –O 2 reaction) over noble metal catalysts, with a particular emphasis on the use of Pt, was investigated. The conversion of NO x exceeded 98% with Pt catalysts at between 350 and 500 K in the presence of 1% H 2 . Pd, Ir, and Rh catalysts were also highly active in the presence of 1% H 2 . NH 3 was formed as a byproduct over Pt catalysts between 350 and 400 K. The formation of NH 3 increased over Pt supported on ZrO 2 . The NH 3 formed at around 400 K was effectively converted to N 2 by the selective reduction of NO with NH 3 over H-ZSM-5, which was physically mixed with the Pt catalyst.

Novel Pd promoted Ag/Al2O3 catalyst for the selective reduction of NOx

Abstract: A novel palladium promoted Ag/Al2O3 catalyst (denoted Ag-Pd/Al2O3) has been developed for the selective catalytic reduction of NO by C3H6. The Ag-Pd/Al2O3 shows a higher NOx conversion than Ag/Al2O3, especially at the temperatures ranging from 300 to 450degreesC. The addition of a small amount of Pd (0.01 wt.%) to Ag/Al2O3 is considered to be favorable for the partial oxidation of C3H6. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggest that the presence of Pd catalyzes the formation of enolic species which is converted from C3H6. The enolic species is very active towards NO2 and NO3-, resulting in the formation of -NCO species which is the key reaction intermediate in the selective catalytic reduction of NO. (C) 2003 Elsevier B.V. All rights reserved.

Novel Pd promoted Ag/Al 2 O 3 catalyst for the selective reduction of NO x

Abstract: A novel palladium promoted Ag/Al2O3 catalyst (denoted Ag-Pd/Al2O3) has been developed for the selective catalytic reduction of NO by C3H6 The Ag-Pd/Al2O3 shows a higher NOx conversion than Ag/Al2O3, especially at the temperatures ranging from 300 to 450 ◦ C The addition of a small amount of Pd (001 wt%) to Ag/Al2O3 is considered to be favorable for the partial oxidation of C3H6 In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) suggest that the presence of Pd catalyzes the formation of enolic species which is converted from C3H6 The enolic species is very active towards NO2 and NO3 − , resulting in the formation of –NCO species which is the key reaction intermediate in the selective

The Reaction Mechanism of Selective Catalytic Reduction of Nitrogen Oxides by Hydrocarbons in Excess Oxygen: Intermediates, Their Reactivity, and Routes of Transformation

Abstract: The main features of the mechanism of selective reduction of nitrogen oxides by hydrocarbons (methane, propane, and propylene) in excess oxygen catalyzed by systems containing transition metal cations are considered. A combination of steady-state and non-steady-state kinetic studies, in situ Fourier-transform infrared (FTIR) spectroscopy, temperature-programmed desorption, and theoretical analysis of bond strengths and spectral data for adsorption complexes made it possible to determine reliably that surface nitrate complexes are key intermediates at real temperatures of catalysis. The rate-limiting step in these reactions includes the interaction of these complexes with hydrocarbons or their activated forms. Factors are considered that determine the structure, bond strength, and routes of nitrate complexes transformations under the action of hydrocarbons. Mechanistic schemes are proposed for the reaction of various types of hydrocarbons in which the determining role belongs to the formation of organic nitro compounds in a rate-limiting step. Their further fast transformation with the participation of surface acid sites resulting in the formation of ammonia, which is a highly efficient reducing agent, though not limiting the whole process, but determines nevertheless both the selectivity to the target product, molecular nitrogen, and the selectivity of hydrocarbon consumption for nitrogen oxide reduction.

Selective reduction of aldehydes to alcohols by calcined Ni-Al hydrotalcite

Abstract: Several aromatic and heterocyclic aldehydes are reduced at atmospheric pressure by calcined Ni-Al hydrotalcite and the catalyst is reused for several cycles with consistent activity and selectivity.

Selective reduction of NOx with C3H6 over Cu and Cr promoted CeO2 catalysts

Abstract: The Cu/CeO2 and Cu/Cr/CeO2 catalysts synthesized using the impregnation method were employed to investigate the selective reduction of NOx with C3H6 at 673.15 K and 1 atm. The activity of CeO2 was greatly induced by loading 4 wt.% of Cu onto the bare support. The presence of C3H6 was a crucial factor for elevating the NOx conversion under lean conditions. The modification of the optimal Cu(4)/CeO2 catalyst with 3 wt.% of Cr steadily improved the performance of the catalysts. Both the optimal Cu(4)/CeO2 and Cu(4)/Cr(3)/CeO2 catalysts were characterized using XRD, SEM and TPR-H2. The co-existence of CuO, Cr2O3 and CuCr2O4 was believed to grant the promising performance of the Cu(4)/Cr(3)/CeO2 catalyst in SCR-HC under net oxidizing conditions.

Performance of Pd-Mo/γ-Al2O3 catalysts for the selective reduction of NO by methane

Abstract: Pd-Mo/γ-Al 2 O 3 catalysts were prepared by wet impregnation using organometallic precursors. The performance of these catalysts in the selective catalytic reduction of NO by methane (denoted NO–CH 4 ) was compared with corresponding palladium- and molybdenum-supported catalysts prepared by the same method. Characterization by TPR, hydrogen chemisorption and FTIR of CO and NO showed evidence of palladium–molybdenum interaction. The experimental results indicated an encapsulation effect of partially reduced Mo species over Pd particle. The catalytic tests revealed that the binary catalysts have different behaviors for the studied reaction. The activity for NO–CH 4 reaction is higher for the binary samples in the temperature range 200–500 °C. On the other hand, a decrease in the activity (at 500 °C) was observed over the Pd-Mo/Al 2 O 3 samples after 2 h under reaction conditions, being similar to the activity displayed by Pd/Al 2 O 3 . This is thought to be associated with a poisoning of MoO x sites by the oxygen formed during the dissociation of NO. The oxygen taken up by the MoO x species does not seem to be removed by methane. Higher initial activity on bimetallic catalysts is attributed to the greater number of active sites and a synergetic effect between Pd and Mo.

Zinc/Ammonium Formate: A New Facile System for the Rapid and Selective Reduction of Oximes to Amines†:

Abstract: Various oximes, both aldoximes and ketoximes, are selectively reduced to corresponding amines employing low cost zinc dust and ammonium formate despite presence of other functional groups such as h...

Comparison of Ag/Al2O3 and Ag-ZSM5 catalysts for the selective reduction of NO with propylene in the presence of oxygen

Abstract: The selective reduction of NO (0.1%) with propylene (0.1%) in the presence of oxygen (5%) was studied using two Ag catalysts (Ag-ZSM5, Ag/Al2O3) in view of the reaction mechanism within the temperature range of 473?773 K. Ag/Al2O3 showed high and stable activity more than 24 h, while the activity of Ag-ZSM5 decreased with time on streams. The deactivation in the latter case has been attributed to the deposition of coke in the form of oligomers evoked on Bronsted acid sites of Ag-ZSM5. The ability of Ag/Al2O3 for the oxidation of NO (without the existence of reductant) was rather lower than that of Ag-ZSM5 catalyst. It was explained that the reaction is retarded on Ag/Al2O3 by the formation of nitrate species, which were produced by the reaction of NO2 with Ag2O phases. Significant differences on the ability for activation of C3H6 over both Ag catalysts were observed. It was concluded that the reaction of nitrate species (or NO2) with partial oxidized C3H6 is essential to drive the effective reduction of NOx to N2 and this was attributed to have high and stable activity over Ag/Al2O3 catalyst.

Durable and selective activity of Pd loaded on WO3/ZrO2 for NOCH4O2 in the presence of water vapor

Abstract: Pd loaded on WO3/ZrO2 was applied for the selective reduction of NO by methane in the presence of O2. The catalyst exhibited the highest NO and methane conversion when WO3 monolayer covered over ZrO2 surface. NO reduction activity was observed when Pd loading was lower than 0.17 wt.%. However, further increase in the Pd concentration lowered the NO conversion due to the occurrence of direct oxidation of methane. The Pd loaded on monolayer-WO3/ZrO2 catalyst exhibited durable and selective activity for the NOCH4O2 reaction. The selectivity was considerably improved in an atmosphere of 10% water vapor. The durability of the catalysis was responsible for the stability and enhanced concentration of Bronsted acidity in WO3/ZrO2 as proved by NH3 temperature programmed desorption (TPD) and measurement of adsorbed pyridine.

Experimental and theoretical analysis of selective hydrogenation of p-nitroanisole to p-anisidine over Pd/C: kinetics and catalyst deactivation

Abstract: The selective reduction of nitro compounds to the corresponding amines in multi-functional moieties is attempted in a variety of industries using polluting processes including iron–acid and sulfide/polysulfide reduction processes. In the current work, selective hydrogenation of p-nitroanisole (PNA) to p-anisidine (PA) has been carried out using supported metal catalysts. 5% Pd/C was found to be highly effective in comparison with other metal catalysts used. Kinetic interpretation has been made by studying the important process parameters using 5% Pd/C as the catalyst. The experimental results show that conversion of PNA reaches 100% under the appropriate reaction conditions. The reaction was found to be 100% selective towards PA. It was possible to determine both the rate constant and adsorption equilibrium constant for the reaction. The activation energy of reaction and free energy of adsorption were found to be 10.25 and –2.4 kcal mol–1, respectively, indicating that the reaction is Langmuir-Hinshelwood kinetically controlled. The catalyst was found to be deactivated due to the blocking of channels by product precipitation, which was considered as independent deactivation. A deactivation model was developed and it was found to fit the data very well. A complete theoretical and experimental analysis is presented.

Exhaust gas cleaning system, from internal combustion motor, has storage catalyst to hold nitrogen oxide in buffer until selective reduction catalyst has reached its working temperature

Abstract: The catalyst assembly to clean the exhaust gas from a motor (2), especially with spontaneous ignition and/or direct fuel injection, has a storage catalyst (111) which can store nitrogen oxide from the exhaust at least partially and especially on starting a cold motor, as long as the unit (6) for the selective catalytic reduction has not achieved its operating temperature. The nitrogen oxide is released when the selective reduction unit is at its working temperature and allows nitrogen oxide to pass through with the exhaust gas. An oxidation catalyst (4) is between the storage catalyst and the selective reduction unit. A valve (83) can deliver the reduction agent selectively into the oxidation catalyst.

102 NOx selective catalytic reduction over Pt supported catalyst promoted by zeolite and CeO2-ZrO2

Abstract: Publisher Summary This chapter proposes a new concept of NOx selective reduction catalyst with high durability. The NOx reduction activities on the catalysts were investigated using a simulated diesel engine exhaust gas. X-ray photoelectron spectroscopy (XPS) measurements were performed to characterize the oxidation state of supported Pt. The chapter reveals NOx reduction activity on the catalyst containing mordenite in light-off test with increasing temperature at 20K/min from 423K to 773K after hydrocarbon (HC) adsorption pre-treatment at 423K. Maximum of NOx conversion increased with increasing HC adsorption amount. This result suggests that the adsorbed HC on catalyst at low temperature should promote NOx reduction at elevating temperature. However, HC adsorption treatment degraded catalytic activity at low temperature because of HC poisoning. To suppress this poisoning some oxides that have redox activity were added. The addition of CeO2–ZrO2 improved the NOx reduction activity below 473K.

Interaction of mixed-metal oxides with supercritical isopropanol

Abstract: Interaction between mixed-metal oxides and supercritical isopropanol (SCF-i-PrOH) was studied. It was shown that the following reactions are possible: reduction of a mixed-metal oxide with the formation of a eutectic alloy of two metal components, reduction of the mixed-metal oxide with the formation of an intermetallide, selective reduction of an oxide component with the formation of a metal and an oxide of another component, and change in the oxygen stoichiometry of the oxide (extraction of oxygen from single crystals).

The Mechanism of Selective NOx Reduction by Hydrocarbons in Excess Oxygen on Oxide Catalysts: IV. Spectrokinetic Characteristics of Intermediate Complexes on the Commercial STK Catalyst

Abstract: Surface nitrite-nitrate and acetate compounds were detected under conditions of the selective reduction of nitrogen oxides by propane on the STK iron-chromium oxide catalyst using spectrokinetic measurements. The rate of conversion of these complexes under reaction conditions was measured. The resulting values were compared to the rate of the process. The results of this comparison indicated that, at low temperatures (to ∼250°C), the rate of reduction of nitrogen oxides was determined by the interaction of surface nitrite-nitrate complexes with the activated hydrocarbon. The amount of acetate complexes on the surface increased with temperature. A reduction of the surface was also observed as the temperature was increased. The reaction of NO decomposition began on the reduced surface. Nitrogen atoms recombined, and oxygen atoms reacted with the hydrocarbon to form CO2 in a gas phase. A distinctive feature of the STK catalyst is its ability to form the products of propane mild oxidation and/or oxidative dehydrogenation. From a synergistic standpoint, the STK catalyst is an effective supplier of the activated hydrocarbon, whereas the NTK-10-1 catalyst well activates NOx. This fact explains the nonadditivity effect observed in the catalytic properties of mechanical mixtures of these catalysts.

Selective Reduction of an Aromatic Nitro Group to an Azoxy Unit in the Presence of an Aliphatic Nitro Group

Abstract: Chemoselective reduction of 1-nitro-2-(2-nitro-2-methylpropyl)-benzene to 2,2′-di-(2-nitro-2-methylpropyl)-azoxybenzene was achieved with sodium borohydride in methanol in the presence of substoichiometric amounts of bismuth, whereas reduction with zinc in hydrochloric acid gave a mixture of the latter, 1-amino-2-(2-amino-2-methylpropyl)-benzene, and 3,3-dimethyl-3,4-dihydrocinnoline, and showed poor reproducibility. The crystal structure of the azoxybenzene was determined by single-crystal X-ray diffraction.

Rh/SiO2 Catalysts for Selective Reduction of NO with H2 in the Presence of SO2 and O2

Abstract: 酸素存在下, Rh/SiO2触媒上での水素によるNO選択還元反応において, 共存SO2は反応を促進する効果を示した。SO2は酸素が存在しない場合には触媒を被毒し, またSO2が存在しないと酸素はNO還元を阻害した。FT-IRによる触媒吸着種の観察から, 酸素非存在下ではSO2が優先して吸着しNO吸着を阻害するのに対して, 酸素存在下ではSO3がRh上に形成されることでNO吸着が可能となり, アンモニア脱硝と類似のNHx中間体を経て反応が進行すると推察した。ロジウムの担体としてはシリカが最も有効であった。シリカ以外の担体上で効果が発揮されないのは, 担体上に吸着するNO3あるいはSO3等の強いアニオン種によって, ロジウム金属の電子状態がよりカチオン性 (Rhδ+) を強めることで, 水素の活性化が阻害されるためと推察した。