Showing papers on "Selective reduction published in 1996"

A role of surface nitrite and nitrate complexes in NOx selective catalytic reduction by hydrocarbons under oxygen excess

Abstract: NO adsorption over three types of catalytic systems, such as cation-exchanged zeolites, transition metal oxides supported on γ-Al2O3, and partially stabilised tetragonal ZrO2 (PSZ) was studied by using the TPD method. NO forms several surface complexes having different desorption temperatures. TPD results compared with catalytic properties of these systems in the selective reduction of NO x by propane under oxygen excess showed that strongly bound nitrites and nitrates appeared to be true intermediates in this reaction.

Cooperative effect of platinum and alumina for the selective reduction of nitrogen monoxide with propane

Abstract: Reduction of nitrogen monoxide with propane in the presence of oxygen proceeded not only on alumina-supported platinum but also on physical mixtures of alumina and silica-supported platinum, both of which are inactive for the selective NO reduction. Spillover, gas phase transfer of some reaction intermediates, or homogeneous propane oxidation seems responsible for the cooperative effect.

Silica-supported cobalt catalysts for the selective reduction of nitrogen monoxide with propene

Abstract: Catalytic behavior of silica-supported transition metals for NO reduction with propene in the presence of oxygen was investigated. While both silica and cobalt oxides did not show any activity for the selective NO reduction, impregnated CoO/SiO2 prepared from cobalt acetate showed good activity although the preparation conditions had significant effect on the activity. It was suggested that highly dispersed surface Co2+ ions on silica are responsible for the catalytic activity.

Selective NO reduction by propane and propene over a Pt/ZSM-5 catalyst: a transient study of the reaction mechanism

Abstract: The selective reduction of nitric oxide by propane and propene in an excess of oxygen has been studied over a Pt/ZSM-5 catalyst to elucidate the role of the reducing hydrocarbons in the reaction mechanism. Temperature-programmed reaction (TPR) and transient studies using the temporal-analysis-of-products (TAP) reactor have been performed. Propene is found to be the more efficient reductant compared to propane at T ⩽ 600K. Mechanistic studies demonstrate that even in an excess of oxygen carbon-containing species, formed from propene, are adsorbed on the catalyst, which further react with nitric oxide to N2, N2O and CO2; no such intermediates are formed from propane, giving rise to its far lower reduction efficiency. It is concluded that the main reaction pathway over Pt/ZSM-5 involves a surface reaction between propene-derived adsorbates and NO or, possibly, NO2. Catalytic surface reduction by hydrocarbons, followed by NO decomposition on reduced platinum sites, is proposed as a second, minor, mechanistic pathway at low reaction temperatures (T ⩽ 600K).

Selective catalytic reduction of NOx by hydrocarbons on Pt/Al2O3 catalysts at low temperatures without the formation of N2O

Abstract: The selective reduction of nitrogen oxides by hydrocarbons on a Pt catalyst has been investigated. Some variations are observed between different reductants. However, the most significant result is the observation that no N 2 O is observed as a by-product when toluene is used as a reductant.

Selective reduction of organic compounds with aluminum and boron hydrides

Abstract: Several new selective reductions of organic compounds using complex metal hydrides and hydride reducing systems are discussed. Sodium diethylpiperidinoaluminate is an excellent agent for the partial reduction of esters to the corresponding aldehydes. Borohydride exchange resin (BER) is a convenient reagent for the chemoselective reduction of carbonyl compounds and also for reductive amination. BER-N$B has proved to be an excellent chemoselective reducing agent for olefins, halides, azides, and nitro compounds in the presence of many functional groups such as epoxides, esters, amides, and nitriles. On the other hand, BER-Cu is a reagent of choice for the reduction of a,kunsaturated acid derivatives and amine N-oxides. The discovery of sodium borohydride' in 1942 and of lithium aluminum hydride' in 1945 brought about a revolutionary change in procedures utilized for the reduction of functional groups in organic molecules. However, NaBH, is a very mild reducing agent which reduces only aldehydes, ketones, and acyl chlorides, whereas LiAM, is an exceedingly powerful reducing agent, capable of reducing practically all organic functional gro~ps.~ In order to find more selective reducing agents, considerable efforts have been made to modify these two extremes for more than forty years? Many new metal hydrides have been prepared, studies of the reducing characteristics of these hydrides have developed a number of selective reduction methods valuable to organic synthesis. Among them, several recent developments in our laboratory are discussed in this conference. 1. Sodium Diethylpiperidinohydroaluminate (SDPA)'

Catalytic behaviour of Cu/ZrO2 and Cu/ZrO2(SO42−) in the reduction of nitric oxide by decane in oxygen-rich atmosphere

Abstract: The selective catalytic reduction (SCR) of NO by decane under an oxidising atmosphere has been carried out on Cu/ZrO2 and Cu/ZrO2(SO42−). Zirconia-supported Cu catalysts were prepared by ligand exchange with Cu acetylacetonate followed by calcination at 773 K. The solids obtained were characterised by temperature programmed reduction (TPR) by hydrogen and temperature programmed desorption (TPD) of NO. Cu/ZrO2 is active and selective in the reduction of NO by decane at low temperature (< 600 K) but oxidises NO to NO2 between 640 and 770 K. By contrast, whatever the temperature, a total selectivity to nitrogen is obtained with Cu/ZrO2(SO42−). About 40% NO conversion to N2 is observed with GHSV of 70 000 h−1. The promoting effect of sulfate is attributed to the large increase of acidity and the strong interaction between copper and sulfur species which is evidenced by TPD of NO and TPR by H2.

Reaction Mechanism of NO Reduction by CH4over Rare Earth Oxides in Oxidizing Atmosphere

Abstract: Rare earth oxides have been examined as catalysts for the catalytic reduction of NO by CH4 in the presence of oxygen. Among the rare earth oxides examined, Sc2O3 and Sm2O3 were found to be active for this reaction. For the Sm2O3 catalyst, ZrO2 was the most effective support. The mechanism of the reaction was studied over the Sm2O3/ZrO2. Kinetic studies indicated that CH4 oxidation is initiated by O2, and NO can react only with CH3 radicals formed from the reaction of CH4 with O2. Ab-initio MO calculations also suggested that CH3 radical is a more favorable reducing species for selective reduction of NO than CH4. It is thus proposed that NO is reduced into N2 by the CH3 radical formed from the activation of methane by oxygen.

Intercalation compounds of α-zirconium hydrogen phosphate with Rh3+ ions and Rh3+-diamine complexes. Part II. Their behaviour towards CO, CO2 and H2 and their use in the CO catalytic oxidation

Abstract: The reactivity of Rh3+ ions and Rh3+-diamine α-Zr(HPO4)2·H2O complexes intercalated in α-zirconium hydrogen phosphate towards small molecules (CO, O2, H2) was studied. The compounds only containing Rh3+ ions, of composition ZrHxRhy(PO4)2·4H2O (x = 2 – 3y; 0 < y ≤ 0.66) react with CO at atmospheric pressure and temperatures ranging from 80 to 100°C, and undergo selective reduction of Rh3+ to Rh1+. The resulting materials containing Rh1+ are reoxidized to Rh3+ by molecular dioxygen under the same pressure and temperature conditions. The simultaneous action of a COO2 mixture determines the catalytic oxidation of the CO to CO2 and the system acts as a stable catalyst of this reaction. At higher temperatures, the reduction of Rh3+ is no longer selective and in these conditions Rh0 is formed, which escapes from the support and causes its deactivation. Similar behaviour is found in systems containing Rh3+-diamine complexes, which react with CO at temperatures higher than 120°C and undergo an irreversible reduction of Rh3+ to Rh0. The reaction with H2 (70 < T < 100°C) also causes a non selective reduction of the Rh3+ to Rh1+ and Rh0. The progress over time of the catalytic activity of some compounds with different contents of Rh3+ in converting CO to CO2 has shown not only that these materials maintain a constant catalytic activity, indicating the stability of the systems to the loss of metal during working cycles, but also that Rh3+ supported in these matrixes is more active and selective in this type of reaction than Rh3+ in solution.

KGa-priderite and related compound for the selective reduction of nitrogen monoxide with propylene

Abstract: The catalytic activity of KGa-priderite, K1.6Ga1.6Ti6.4O16, and its related compound KGa8Ga9Ti15O56 was investigated for the selective reduction of nitrogen monoxide (NO) with propylene (C3H6) in the presence of high oxygen concentrations. The KGa-priderite showed significant activity during this reaction, but the related compound showed only a little activity. These compounds are quite different from the conventional catalysts for NOx selective reduction and are characterized by the fact that their properties are free from the effects of solid acidity and support metals. This difference was attributable to the NO desorption rate at the surface of these compounds. It has become clear that the KGa-priderite catalyst remarkably adsorbed NO, and it is suggested that the amount of NO adsorbed and the amount of catalytic activity are able to be increased by the design of priderite structure.

Precious metal loaded In/H-ZSM-5 for reduction of nitric oxide with methane in the presence of water vapor

Abstract: The catalytic activity of In/H-ZSM-5 for the selective reduction of nitric oxide (NO) with methane was improved by the addition of Pt and Ir which catalyzed NO oxidation, even in the presence of water vapor. It was also found that the precious metal, particularly Ir loaded In/H-ZSM-5 gave a low reaction order with respect to NO, and then showed a high catalytic activity for the reduction of NO at low concentrations, if compared with In/H-ZSM-5. The latter effect of the precious metal is attributed to the enhancement of the chemisorption of NO and also to the increase in the amount of NO 2 adsorbed on In sites.

Synthesis and Structural Determination of 1,1′-Spirobi(3H-2,1-benzoxatellurole)-3-one ([10-Te-4(C2O2)]): Conversion of Diacyloxyspirotellurane to Alkoxyspirotellurane by Reduction with Lithium Aluminium Hydride

Abstract: Selective reduction of the diacyloxyspirotellurane 1 with lithium aluminium hydride gave the acyloxyalkoxyspirotellurane 2 and dialkoxyspirotellurane 3, respectively. The X-ray crystal structure analysis of 2 (R = 0.020) reveals distorted trigonal-bipyramidal geometry about tellurium, similar to structures previously determined for other spirotelluranes.