Efficient low temperature lean NOx reduction over Ag/Al2O3-A system approach

This study focuses on lean NOx reduction (LNR) by n-octane using silver-alumina based catalysts, with the addition of hydrogen. The work takes a system approach, where parameters such as temperature, reformate gas composition, fuel penalty and realistic monolith samples are considered. The LNR catalyst samples were prepared by impregnation and sol-gel methods and the NOx reduction performance was characterized by flow-reactor experiments, where realistic engine-out gas compositions were used. The hydrogen feed over the LNR catalyst samples was determined via data achieved by autothermal reforming experiments over a rhodium based catalyst, using real diesel as feedstock. The LNR catalyst samples generally show an enhanced NOx reduction when hydrogen is added to the gas feed. In particular, a 2 wt% silver-alumina sample with the addition of minute amounts of platinum, shows a high increase in NOx reduction when hydrogen is added to the feed. The addition of CO, a potential poison in the reaction and a by-product from the reforming, did not show any significant effect on the LNR catalyst performance at the conditions used. This is beneficial, since it renders a CO clean-up step in the reformer system unneeded. Ammonia formation is discussed in terms of a possible dual-SCR system. Finally, the fuel penalty for hydrogen production and hydrocarbon addition is taken into consideration. It is found that an addition of 1000 ppm H-2 leads to unacceptable fuel penalties. (C) 2011 Elsevier B.V. All rights reserved

Mechanistic Study of Lean NO2 Reduction by Propane Over HZSM-5 in the Presence of Water

This study focuses on the mechanism of lean NO2 redn. by propane in the presence of water, over an acidic zeolite (HZSM-5). Fourier Transform IR spectroscopy measurements with NO2 and propane in excess oxygen show formation of surface bound NO+, isocyanate, unsatd. hydrocarbons and traces of amine species. Upon addn. of water the isocyanate species disappear and amine species are formed. Hence, it seems likely that the isocyanate species are hydrolyzed to amine species, which are possible reaction intermediates in the HC-SCR reaction over HZSM-5

Influence of the Support Acidity of Pt/Aluminum-Silicate Catalysts on the Continuous Reduction of NO under Lean Conditions

This study focuses on the influence of the support acidity on the continuous reduction of nitric oxide by hydrocarbons over supported Pt catalysts under lean conditions. The support materials used were alumina, silica and co-precipitated aluminum-silicates. Activity studies of NO reduction with either propene or propane as the reducing agent, CO chemisorption, CO oxidation, isopropylamine temperature programmed desorption (TPD) and ammonia TPD experiments have been performed. The isopropylamine TPD experiments indicated the presence of Bronsted acid sites in the samples that contained alumina. The activity for NO reduction and the selectivity for N-2 formation with propene as the reducing agent showed no pronounced differences between the samples studied. For propane, on the other hand, expressed differences in NO reduction activity between the samples were observed. The activation of propane and consequently the reduction of NO, seemed to be promoted by the presence of acidic sites on the surface of the support. The selectivity towards N2 formation seemed to be strongly connected to the amount of Bronsted acid sites present in the samples

Mechanistic aspects of lean NO2 reduction by propane over H-ZSM-5

This study focuses on the mechanism of lean NO2 reduction by propane. In particular the role of isocyanate- and amine species has been studied in transient experiments by in situ Fourier Transform Infrared (FTIR) spectroscopy. The results imply that these species are possible reaction intermediates over acidic HZSM-5

Mechanistic Aspects of HC-SCR over HZSM-5 - Hydrocarbon Activation and Role of Carbon-Nitrogen Intermediates

This study focuses on the mechanism of lean NO 2 reduction by hydrocarbons (propane, propene, and isobutane) over HZSM-5. In-situ FTIR measurements indicate a close correlation between formation of isocyanate species, consumption of water (formed in the reaction), and formation of amine species. The results in this investigation confirm our previously suggested reaction mechanism, which involves reaction of NO + species and hydrocarbon-derived species over Br\uf8nsted acid sites, forming isocyanate species. These species are hydrolyzed by water, forming amine species and, finally, N 2 . Experiments with 18 O 2 show an enhanced oxidation of propane by oxygen, in the presence of NO 2 . This effect can possibly be explained by a type of reaction mechanism where gas-phase and/or loosely bound NO 2 react with the adsorbed hydrocarbon-derived species (i.e., carbenium ion adsorbates and/or alkenes), which then more easily react with oxygen. \ua9 2006 American Chemical Society

Mechanistic Considerations of the NOx source and the Reducing Agent for Lean NOx Reduction over H-ZSM-5

none3H. Ingelsten; R. Matarrese; M. SkoglundhH., Ingelsten; Matarrese, Roberto; M., Skoglund

Ag-Al2O3 catalysts for lean NOx reduction - Influence of preparation method and reductant

Alumina supported silver catalysts for selective catalytic reduction of NOx with hydrocarbons under excess oxygen (HC-SCR) were prepared according to three different methods. These are impregnation (I) and two different sol-gel routes, via thermal drying (II) or freeze-drying (III). The latter method has not previously been reported for preparation of Ag-Al2O3 catalysts. The prepared samples were characterized by N2-sorption, TEM, XRD and XPS, and the catalytic properties were further investigated in flow reactor experiments. Two different hydrocarbons (propene and n-octane) were used separately as reductants for the SCR reaction. The TEM, XRD and XPS analyses confirmed that the sol-gel samples contain more dispersed silver of higher oxidation state than the corresponding impregnated sample. Further, it was shown that the freeze-dried sol-gel samples most likely contain well dispersed silver throughout the alumina matrix, mainly as oxidized silver. The XPS results also strongly indicate the presence of silver clusters on the surface of samples prepared by routes (I) and (II). The results from the flow reactor study imply that the ratio between silver clusters and oxidized silver species is crucial to achieve high NO reduction

Influence of Gas Phase Reactions on DME-SCR over gamma-alumina

Above 300 A degrees C, gas phase reactions occur between dimethyl ether (DME), NO and O-2 generating NO2. The influence of these reactions on DME-SCR was studied in a setup that decoupled the occurrence of gas phase reactions from catalyst temperature. NOx conversion decreased at 350 A degrees C and increased at 250 A degrees C due to less DME available and higher NCO conversion with NO2, respectively

Kinetic modeling of H2-assisted C3H6 selective catalytic reduction of NO over silver alumina catalyst

A global kinetic model was developed for C3H6-SCR consisting of NO oxidation, C3H6 oxidation and C3H6-SCR reactions with and without H-2 over Ag-Al2O3. The model is based on a dual role of H-2 to remove inhibiting nitrates from the active sites as well as to modify/form new active Ag sites. For model development, a range of temperature programmed reaction (TPR) and transient feed experimental conditions were used. The proposed model was eventually also validated with additional transient experimental data. The kinetic model proposed in this study predicts the experimental data well for a wide-range of feed conditions. Evaluation of mass transfer resistance in the washcoat indicated that during H-2-assisted C3H6-SCR, mild internal mass transfer resistance for NO was predicted to be important already at 250 degrees C