On the role of H2 to modify surface NOx species over Ag-Al2O3 as lean NOx reduction catalyst: TPD and DRIFTS studies

Formation and stability of surface NOx species related to the promotional effect of H2 over Ag–Al2O3 as NOx reduction catalyst were investigated with temperature-programmed desorption and DRIFT spectroscopy. Formation of two groups of surface NOx species was found: a less thermally stable group of “low temperature (LT) species” and a more thermally stable group of “high temperature (HT) species”. The LT NOx was attributable to the decomposition of surface NOx species formed on the active sites where its elimination by addition of H2 or thermal decomposition correlated with higher NO oxidation and NOx reduction conversion. Under reaction conditions, these possibly inhibiting LT NOx species were stable up to about 300 °C and their formation depended on donation of oxygen from surface oxides. Removal of LT nitrate species by H2 accounted for only a fraction of the increased NO oxidation and NOx reduction conversion by co-feeding H2. Furthermore, it was also found that H2 facilitates formation of HT NOx that primarily corresponded to the decomposition of spectator species on the Al2O3 support identified as monodentate nitrate species. From TPD studies of C3H6-SCR, it was shown that H2 not only eliminated LT NOx but also promoted formation of greater quantities of adsorbed hydrocarbons

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

Quantification of urea-spray non-uniformity effects on the H2-assisted NO reduction and NH3 slip over an Ag/Al2O3 catalyst

Selective catalytic reduction of nitrogen oxides (NOx) with ammonia (NH3-SCR) is an efficient technology for lean NOx removal in automotive applications. In the current work, a kinetic model for hydrogen-assisted NH3-SCR over a silver-alumina (Ag/Al2O3) monolith catalyst is adapted to the corresponding experimental results. The degree of non-uniformity in the NH3 dose to individual catalyst channels during operation of a urea-SCR system is also investigated and the NH3 dose probability distribution functions are derived. The effects of NH3 non-uniformity on the NO conversion and NH3 slip are studied over an otherwise optimal Ag/Al2O3 system. Above the stoichiometric point, the NH3 slip is shown to increase almost linearly with increasing NH3 dosage. Channels that receive significantly lower NH3 dose than the stoichiometric one exhibit low NO conversion, whereas there is no discernable increase in the NO conversion above an NH3 dose four times the average. These results highlight the importance of the interaction between the design of the NH3 dosing system and the performance of the catalytic system

On the performance of Ag/Al2O3 as a HC-SCR catalyst – influence of silver loading, morphology and nature of the reductant

This study focuses on the performance of Ag/Al2O3 catalysts for hydrocarbon selective catalytic reduction (HC-SCR) of NOx under lean conditions, using complex hydrocarbons as reductants. The aim is to elucidate the correlation towards the silver loading and morphology, with respect to the nature of the reductant. Ag/Al2O3 samples with either 2 or 6 wt% silver loading were prepared, using a sol–gel method including freeze-drying. The catalytic performance of the samples was evaluated by flow reactor experiments, with paraffins, olefins and aromatics of different nature as reductants. The physiochemical properties of the samples were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy, scanning transmission electron microscopy/high angle annular dark field imaging, X-ray photoelectron spectroscopy and N2-physisorption. The 2 wt% Ag/Al2O3 sample was found to be the most active catalyst in terms of NOx reduction. However, the results from the activity studies revealed that the decisive factor for high activity at low temperatures is not only connected to the silver loading per se. There is also a strong correlation between the silver loading and morphology (i.e. the ratio between low- and high- coordinated silver atoms) and the nature of the hydrocarbon, on the activity for NOx reduction. Calculated reaction rates over the low-coordinated step and high- coordinated terrace sites showed that the morphology of silver has a significant role in the HC-SCR reaction. For applications which include complex hydrocarbons as reductants (e.g. diesel), these issues need to be considered when designing highly active catalysts

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

Effect of silver loading on the lean NOx reduction with methanol over Ag-Al2O3

The influence of silver loading on the lean NO x reduction activity using methanol as reductant has been studied for alumina supported silver catalysts. In general, increasing the silver loading (0-3 wt%), in Ag-Al 2 O 3 , shifts or extends the activity window, for lean NO x reduction towards lower temperatures. In particular Ag-Al 2 O 3 with 3 wt% silver is active for NO x reduction under methanol-SCR conditions in a broad temperature interval (200-500 C), with high activity in the low temperature range (maximum around 300 C) typical for exhaust gases from diesel and other lean burn engines. Furthermore, increasing the C/N molar ratio enhances the reduction of NO x . However, too high C/N ratios results in poor selectivity to N 2 . \ua9 2013 Springer Science+Business Media New York

Role of hydrogen formation and silver phase for methanol-SCR over silver/alumina

Formation of reaction products over silver/alumina during lean NOx reduction with methanol was studied, with and without addition of hydrogen to the feed. The silver phase in silver/alumina was analyzed by UV–vis spectroscopy in order to elucidate the influence of silver loading and different gas environments. Formation of molecular hydrogen and reduction of supported silver species were observed during selective catalytic reduction (SCR) with methanol, without hydrogen in the feed. This availability of hydrogen and the reduction of silver are suggested to contribute to the high low-temperature activity connected to oxygenates (like alcohols) as reducing agents for NOx

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