Presence Of Excess Oxygen Research Articles

Low current density electrochemical cell for NO decomposition

Solid-state electrochemical cells with two cathode layers were designed for effective NO x decomposition. It was found that covering of the cathode by a mixed oxide electrode increased NO decomposition even in the presence of excess oxygen. A strong correlation between the microstructure of the working electrode and conversion rate of NO x gas was observed and studied. It was shown that there was a linear dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nano-porous working electrode and that the value of current efficiency depended on the NO and O 2 gas concentrations only. A mechanism for the process of NO decomposition on nano-porous ceramics electrode was proposed and the theoretical value of the current efficiency was calculated. A good numerical correlation between the calculated and measured characteristics of the electrochemical cell were observed.

Novel low voltage electrochemical cell for NO decomposition

In recent years, an intense research effort has been focused on electrochemical cells for reduction of gases of nitrogen oxide radicals (NOx) due to the need to design an effective method for the purification of the exhaust gas from lean burn and diesel engines. A new type of solid-state electrochemical cell with composite Pt–YSZ cathode and nano-porous NiO–YSZ working electrode for decomposition of NOx gas in the presence of excess oxygen was proposed and investigated. It was shown that the dependence of the NO conversion on the value of the current passed through the electrochemical cell with a nano-porous NiO– YSZ working electrode was linear and that the value of current efficiency is dependent on the NO and O2 gas concentrations only. For the first time, an electrochemical cell for NO decomposition operating at dc voltage lower than 1.7 V in the temperature range of 550–700 jC was designed. The combination of the new type of composite cathode with a nano-porous working electrode provides the possibility of large reductions in the electrical power required for NO decomposition in the presence of excess oxygen. D 2002 Elsevier Science B.V. All rights reserved.

Catalytic reduction of nitric monoxide by ethene over Nb/TiO2 in the presence of excess oxygen

Selective catalytic reduction of nitrogen monoxide over TiO2 with different niobium loadings (between 0 and 15wt.%) by ethene in an oxidizing atmosphere was studied. Nb/TiO2 catalyst showed high activity and selectivity for the reduction of NO to N2. The maximum conversion of NO to N2 was observed on the Nb (10wt.%)/TiO2. NO2 and N2O were hardly created at the temperature the selective catalytic reduction of NO to N2 occurred. On increasing the concentration of ethene, the catalytic activity for the reduction of NO to N2 was enhanced. The effect of water vapor on the catalytic activity was studied.

Selective Catalytic Oxidation of Ammonia to Nitrogen over Fe2O3–TiO2 Prepared with a Sol–Gel Method

Fe2O3–Al2O3, Fe2O3–TiO2, Fe2O3–ZrO2, and Fe2O3–SiO2 were prepared with a sol–gel method and they showed high activities for selective catalytic oxidation (SCO) of ammonia to nitrogen in the presence of excess oxygen. The Fe2O3–TiO2 catalysts prepared from iron sulfate yielded a higher selectivity for N2 than those prepared from nitrate. More than 92% of N2 yields were obtained on the 10 wt% Fe2O3–TiO2 (SO42−) and 20 wt% Fe2O3–TiO2 (SO42−) at 400–450°C under the condition of GHSV=2.0×105 h−1. Also, after the Fe2O3–TiO2 prepared from nitrate was treated with SO2+O2 at 450°C, the N2 selectivity and yield were enhanced significantly at 450–500°C, suggesting a promoting role by SO2. But H2O decreased the N2 yield slightly. The N2 selectivity for the SCO reaction is in good agreement with their surface acidity and the activity for selective catalytic reduction (SCR) of NO with ammonia. This further supports the two-step SCO mechanism in which NH3 is first oxidized to NO and then NO is reduced to N2 by unreacted NH3 adsorbed species through a SCR reaction. The presence of sulfate species on Fe2O3–TiO2 (SO42−) increased surface acidity and thus improved SCO performance. Temperature-programmed desorption and temperature-programmed surface reaction of ammonia showed that gaseous, adsorbed, and lattice oxygen may participate in the SCO reaction.

Superior performance of nano-Au supported over Co3O4 catalyst in direct N2O decomposition.

Au nanoparticles supported over Co3O4 lead to a highly active and stable catalyst for N2O decomposition even in the presence of excess oxygen and steam.

Bimetallic PtAu cluster-derived catalysts for the selective catalytic reduction of NO by propylene

The selective catalytic reduction of nitric oxide by propylene in the presence of excess oxygen has been investigated over a series of silica supported Pt and PtAu catalysts. Cluster-derived catalysts were prepared by wet impregnation using a Pt 2Au 4(CC tBu) 8 organometallic cluster precursor, and compared to catalysts obtained by incipient wetness impregnation using individual Pt and Au salts. The addition of Au by co-impregnation resulted only to a small shift (20–25 °C) in the light-off of propylene towards higher temperatures. A markedly different catalytic behavior was observed in the case of cluster-derived catalysts, where a 150 °C delay was observed both in the temperature of maximum NO reduction and the light-off of propylene in the presence and in the absence of NO. Most importantly, the selectivity towards N 2 increased by 50% compared to the monometallic Pt catalysts. The results of characterization studies indicate that the monometallic Pt/SiO 2 and the co-impregnated PtAu/SiO 2 samples have similar metal particle sizes. A more narrow particle size distribution was observed with the cluster-derived bimetallic sample.

Physicochemical Investigation of the Reaction of Components of the Si — Al — O — N — Ti System in the Si3N4 — Al2O3 — AlN — SiO2 — TiN — TiO2 Region. Part 2. Subsystems Si3N4 — AlN, TiN — AlN, Al2O3 — AlN

The reaction of components of the Si — Al — O — N — Ti system in its elements Si 3N4 — AlN, TiN — AlN, and Al 2O 3 — AlN was investigated by differential thermal and x-ray diffraction analysis. It was established that upon hot pressing mixtures of Si3N 4 and AlN (up to ∼1950°C) free silicon is formed by the decomposition of Si 3N4, which reacts with oxygen present as an impurity to form SiO. When TiN reacts with AlN a phase with the spinel structure (Al23O 27N 5), which can form only in the presence of excess oxygen, appears in addition to the initial components. Spinel is also produced by the reaction of Al 2O3 with AlN. In this case a eutectic between Al 23O27N5 and Al2O3 is observed.

Selective Catalytic Reduction of NO by C3H6 over Co/Al2O3 Catalyst with Extremely Low Cobalt Loading

In order to reveal the optimum Co loading, the selective catalytic reduction of NO with C3H6 over Co/Al2O3 catalyst was studied in a systematic fashion by varying the amount of cobalt oxide. It was found that upon loading a small amount of cobalt oxide (namely 0.5 wt% on a Co metal basis), the combination between Co(II) acetate salt and a high-purity alumina provided an active catalyst in the presence of excess oxygen and water. TPR measurement showed the presence of Co species other than CoAl2O4 spinel in the most excellent performance catalyst, from which the active sites should be produced.

A Sol–Gel Derived CuOx/Al2O3–ZrO2 Catalyst for the Selective Reduction of NO by Propane in the Presence of Excess Oxygen

Copper catalysts supported on alumina-doped zirconia were prepared by sol–gel processing followed by supercritical drying or aging in the mother solution at 100 °C. After drying and calcination, the catalyst supports were impregnated with a copper(II) nitrate aqueous solution by the incipient wetness method to achieve a Cu loading of about 2%. The samples showed ∼90% NO conversion at 350–400 °C. The catalytic performance of these systems appears to be determined by the degree of clustering of copper cations as probed by FTIR spectroscopy of adsorbed CO.

Enhancement of C2H6 Oxidation by O2 in the Presence of N2O over Fe Ion-Exchanged BEA Zeolite Catalyst

Selective catalytic reduction (SCR) of N2O with C2H6 took place effectively over Fe ion-exchanged BEA zeolite catalyst (Fe-BEA) even in the presence of excess oxygen. The mechanism in the SCR of N2O with C2H6 over Fe-BEA catalyst was studied by a transient response experiment and an in situ DRIFT spectroscopy. No oxidation of C2H6 by O2 took place below 350 °C (in C2H6/O2). In the N2O/C2H6/O2 system, however, it was found that the reaction of C2H6 with O2 was drastically enhanced by the presence of N2O even at low temperatures (200-300 °C). Therefore, it was concluded that N2O played an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). On the basis of these findings, the mechanism in the SCR of N2O with C2H6 is discussed.

Metalorganic chemical vapor deposition of aluminum oxide on silicon nitride

ABSTRACTThin films of aluminum oxide were deposited on silicon nitride thin films using trimethylaluminum and oxygen at 0.5 Torr and 300 °C. Fourier transform infrared (FTIR) and x-ray photoelectron spectroscopic (XPS) analyses of these films showed no aluminum silicate phase at the film-substrate interface. The O/Al ratio in the deposited film was found to be higher than that in stoichiometric Al2O3 indicating the presence of excess oxygen. FTIR spectroscopy and XPS of the annealed samples did not show any formation of silicon oxide, oxynitride or silicate at the aluminum oxide/silicon nitride interface. In contrast to aluminum oxide on clean silicon substrates, using ultrathin silicon nitride as a barrier layer could prevent excess oxygen migration towards the Si substrate and formation of any interfacial layers.

NOx-sorbing metal oxides, MnOx-CeO2. Oxidative NO adsorption and NOx-H2 reaction

(n)MnOx–(1−n)CeO2 binary oxides have been studied for the sorptive NO removal and subsequent reduction of NOx sorbed to N2 at low temperatures (≤150 °C). The solid solution with a fluorite-type structure was found to be effective for oxidative NO adsorption, which yielded nitrate (NO−3) and/or nitrite (NO−2) species on the surface depending on temperature, O2 concentration in the gas feed, and composition of the binary oxide (n). A surface reaction model was derived on the basis of XPS, TPD, and DRIFTS analyses. Redox of Mn accompanied by simultaneous oxygen equilibration between the surface and the gas phase promoted the oxidative NO adsorption. The reactivity of the adsorbed NOx toward H2 was examined for MnOx–CeO2 impregnated with Pd, which is known as a nonselective catalyst toward NO–H2 reaction in the presence of excess oxygen. The Pd/MnOx–CeO2 catalyst after saturated by the NO uptake could be regenerated by micropulse injections of H2 at 150 °C. Evidence was presented to show that the role of Pd is to generate reactive hydrogen atoms, which spillover onto the MnOx–CeO2 surface and reduce nitrite/nitrate adsorbing thereon. Because of the lower reducibility of nitrate and the competitive H2–O2 combustion, H2–NO reaction was suppressed to a certain extent in the presence of O2. Nevertheless, Pd/MnOx–CeO2 attained 65% NO-conversion in a steady stream of 0.08% NO, 2% H2, and 6% O2 in He at as low as 150 °C, compared to ca. 30% conversion for Pd/γ–Al2O3 at the same temperature. The combination of NOx-sorbing materials and H2-activation catalysts is expected to pave the way to development of novel NOx-sorbing catalysts for selective deNOx at very low temperatures.

Copper ion-exchanged zeolite catalysts in deNO x reaction

Copper ion-exchanged zeolites have widely been studied as the catalysts for NO x emission control. Cu-MFI zeolites, especially over-exchanged Cu-MFI, are very active for the catalytic decomposition of NO. The reaction mechanisms and the active sites suggested are summarized. The newly developed selective catalytic reduction of NO with hydrocarbons in the presence of excess oxygen is then introduced. The role of oxygen and the deactivation of the zeolite catalysts are discussed. In the last chapter, the high ability of Cu-zeolites for NO adsorption is overviewed. Here, some fundamental aspects of the bonding and the molecular diffusion of NO or NO 2 molecules in zeolites are also summarized.

Copper supported on mixed alumina/gallium oxide pillared α-tin phosphate for De-NOx applications

Alumina- and mixed alumina/gallium oxide-pillared α-tin phosphate have been impregnated with different amounts of copper via the incipient wetness method. Their characterization by X-ray photoelectron spectroscopy, H2 temperature-programmed reduction and NO temperature-programmed desorption has allowed to gain insight into the nature of the copper species. These are present as small CuO clusters (not detectable by XRD) as well as forming part of spinel-like structures. NO-TPD studies show that only NO is desorbed from all the catalysts. Ga3Al11-SnP with 4.9 wt% Cu interacts most strongly with NO and this catalyst exhibits the highest degree of reduction of Cu2+ to Cu+ after the catalytic reaction. The catalysts are active for the selective catalytic reduction (SCR) of nitric oxide with propane in the presence of excess oxygen.

Acetic acid stability in the presence of oxygen over vanadium phosphate catalysts: comments on the design of catalysts for the selective oxidation of ethane

The stability of acetic acid in the presence of excess oxygen is evaluated for a range of vanadium phosphate and oxide catalysts. In the temperature range 300–400 °C most oxides catalyse the total oxidation of acetic acid to carbon dioxide and water. However, over molybdenum oxide acetic acid oxidation is not significant. This is consistent with Mo being a major component of oxide catalysts that have, to date, been identified for the selective oxidation of ethane to acetic acid at this temperature. Interestingly, VO(H 2PO 4) 2, a phase that is very non-selective for the partial oxidation of n-butane, exhibits the highest stability for acetic acid under oxidising conditions. It is proposed that catalysts based on VO(H 2PO 4) 2 may provide the basis of improved ethane partial oxidation catalysts.

Influence of low concentration of SO 2 for selective reduction of NO by C 3H 8 in lean-exhaust conditions on the activity of Ag/Al 2O 3 catalyst

The effect of SO 2 for the selective reduction of NO by C 3H 8 on Ag/Al 2O 3 was investigated in the presence of excess oxygen and water vapor. The NO x conversion decreased permanently even in the presence of a low concentration of SO 2 (0.5–10 ppm) at <773 K. The increase in SO 2 concentration resulted in a large decrease in NO x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al 2O 3 remained constant even in the presence of 10 ppm of SO 2. The sulfate species formed on the used Ag/Al 2O 3 were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO x activity on the Ag/Al 2O 3. The sulfated Ag/Al 2O 3 was appreciably regenerated by thermal treatment in the deNO x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO 2 for long time by the heat treatment at every 20 h intervals.

Plasma-enhanced HC-SCR of NOx in the presence of excess oxygen

The oxidative potential of a non-thermal plasma (NTP) in engine off-gases with excess oxygen results in an effective conversion of NO to NO2 that can be converted synergistically to molecular nitrogen with appropriate catalysts by selective catalytic reduction of NO2 with hydrocarbons (HC-SCR). The hydrocarbon added has two essential functions: first, it assists the gas-phase oxidation of NO to NO2 by the electric discharge in excess oxygen and, secondly, it reacts with NO2 in the hydrocarbon SCR. Besides CO2 and CO, significant amounts of formaldehyde and acetaldehyde are formed in the plasma-initiated gas-phase reaction. These and other by-products are involved together with the remaining propene in the subsequent catalytic reaction.The combination of SCR and cold plasma enhances the overall reaction and allows an effective removal of NOx at relatively low temperatures. Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in this reaction. With an energy effort of ca. 30eV per NO-molecule, a temperature of 300°C and a space velocity of 20,000h−1 at the catalyst, it is possible to reduce 500ppm NO in excess oxygen by more than half.The synergistic combination of NTP and HC-SCR has been verified under real conditions with exhaust gas from a diesel engine.

Surface species during catalytic reduction of NO by propene studied by in situ IR-spectroscopy over Pt supported on mesoporous Al2O3 with MCM-41 type structure

The selective catalytic reduction of NO by propene (HC-SCR) in the presence of excess oxygen over Pt/MCM-41 (Al2O3) and MCM-41 (Al2O3) catalysts has been studied by simultaneous in situ IR-spectroscopy and catalytic activity measurements. IR-spectra of Pt/MCM-41 (Al2O3) and MCM-41 (Al2O3) were compared during the adsorption of the single reactants (NO and C3H6) and during the NO reduction with C3H6 in presence of excess O2 (NO+C3H6+O2 reaction). The adsorption of NO resulted in the formation of surface nitrate species on the Al2O3 support and surface NO species on Pt. Adsorption of propene led to the formation of carboxylate species on Al2O3. Under reaction conditions nitrates, carboxylates, acetates, hydrocarbon fragments, isocyanate, cyanide species and CO were present on the catalyst surface. The surface concentration of the isocyanate species was found to be strongly correlated to the activity of the Pt/MCM-41 catalyst. Besides the formation of isocyanate, cyanide species were also observed on the surface. Both species increased in concentration with increasing oxygen concentration in the feed. The isocyanate species were found to be a reaction intermediate during the NO reduction, whereas the cyanide species were stable against further reaction. For the cyanide formation, a reaction pathway via isocyanate by abstraction of an oxygen atom by propene was observed.

Promotional Effect of Reduction Treatments of PtIn(ferrierite) on Its Activity in the SCR of NO with Methane. Kinetics and Novel Characterization Studies

K−ferrierite co-exchanged with Pt and In (PtIn(ferrierite)), with different pretreatments, is characterized by EXAFS (extended X-ray absorption fine structure), TDPAC (time differential perturbed angular correlation), TPR (temperature-programmed reduction), and hydrogen chemisorption. The presence of different In and Pt species is correlated with activity and selectivity during the NO selective reduction (SCR) with CH4 in the presence of excess oxygen. The reduction of PtIn(ferrierite) catalyst under controlled conditions (hydrogen at 350 or 500 °C) strongly promotes the activity of the said catalyst. The reduction at 350 °C results in the formation of small Pt° crystals, stable under wet reaction conditions, which probably favor both the oxidation of NO to NO2 and the adsorption of the said species, thus increasing the overall reaction rate. The reduction at 500 °C further increases the reaction rate because it promotes the formation of new (InO)+ active species from the reduction of In2O3 at the catalys...

Peculiarity of NO Decomposition by Electrochemical Cell with a Mixed Oxide Working Electrode

The decomposition activity was measured on nonsymmetric electrochemical cell oxide (cathode) yttria-stabilized zirconia (YSZ) (anode) by applying dc voltage lower then 3.5 V in the temperature range from 550 to 700°C. It was found that covering the Pt cathode by a mixture of oxygen ionic conductor (YSZ) and electronic conductor (NiO) increased the NO decomposition even in the presence of excess oxygen. The microstructure of YSZ-NiO mixed oxide electrodes was investigated depending on the mixed oxide electrode sintering temperature. A strong correlation between the microstructure of the mixed oxide electrode and the conversion rate of NO decomposition was observed and investigated. A mechanism for the NO decomposition is proposed and discussed. © 2001 The Electrochemical Society. All rights reserved.