1,720,999 research outputs found
The low-temperature interaction of NH3/NO/NO2 + O2 with Fe-ZSM-5 + BaO/Al2O3 and H-ZSM-5 + BaO/Al2O3: Influence of phase separation and relevance for the NH3-SCR chemistry
Full text linkIn an effort to elucidate mechanism and intermediates of Standard SCR over metal-zeolite catalysts, we apply Transient Response Methods (TRM) to identify the mediating species in the low-temperature (120 °C) interaction of NO2 + O2, NO + O2 and NO + NO2 + O2 (NO/NO2 = 10/1 v/v) with a composite Fe-ZSM-5 (Fe = 1% w/w) + BaO/Al2O3 system in different configurations (physical mixture versus double-bed), corresponding to different degrees of separation of the two component phases. The results clearly indicate for the first time that the strong interaction between the two system components, already demonstrated in previous work, survives their complete segregation, proceeds via the gas phase, and is mediated by stable gaseous NOx species. The nature of the NOx species trapped on the BaO phase is identified by TPD experiments: in line with previous data, they include primarily nitrates for NO2 adsorption, and nitrites for NO + O2 adsorption at short exposure times. A new, striking finding is that formation of nitrites on BaO upon exposure of Fe-ZSM-5 + BaO/Al2O3 to NO + O2, which involves the oxidative activation of NO on Fe-sites, is fully equivalent to the formation of nitrites observed upon exposing only BaO/Al2O3 to NO2 in excess NO. This suggests that NO2 (possibly in the form of N2O3) may play the role of mediating gas-phase species generated by the oxidative activation of NO on Fe centers. The reactivity with NH3 of nitrites trapped on BaO is probed by Temperature Programmed Surface Reaction (NH3-TPSR) runs, which show rapid dinitrogen formation from low temperatures when Fe-ZSM-5 is not only mixed with, but also placed downstream from BaO/Al2O3, thus confirming the stability of the NOx intermediate formed on Fe-centers, and linking it to the Standard SCR reactivity. Finally, in order to study the role of the metal redox sites in the reactivity of nitrites stored on BaO with ammonia, we compare NH3-TPSR experiments over a Fe-ZSM-5 catalyst and over a parent H-ZSM-5 zeolite with a drastically reduced Fe content (Fe ≅ 0.02% w/w). Results show that nitrites on BaO react with NH3 to dinitrogen equally well on Fe- and on H-ZSM-5, which questions the role of the metal sites and therefore of the oxidative activation of NH3 in such a step. The present data emphasize the bifunctional (redox + acid) nature of the NH3-SCR catalytic chemistry at low temperatures, and should be considered in the development of comprehensive mechanisms for the Standard SCR reaction over Fe-zeolite catalysts
The Low Temperature Interaction of NO + O2 with a Commercial Cu-CHA Catalyst: A Chemical Trapping Study
No full textModelling Inhibition Effects of Short-Chain Hydrocarbons on a Small-Pore Cu-Zeolite NH3-SCR Catalyst
No full textNH3-SCR currently represents the leading technology for NOx abatement from lean exhaust gases, e.g. from diesel engines. Hydrocarbon (HC) slip onto the SCR catalyst can have a negative impact on its DeNOx performance. In order to better understand hydrocarbon poisoning, the influence of different light HCs on the catalytic activity of a small-pore Cu-zeolite has been investigated in a synthetic gas test rig. Based on these data and on further indications from the literature, an existing SCR kinetic model has been extended to represent major effects of light HCs on the behavior of zeolite SCR catalysts. The developed model is able to describe such effects in a simple but effective way
NO oxidation on Fe- and Cu-zeolites mixed with BaO/Al 2 O 3 : free oxidation regime and relevance for the NH 3 -SCR chemistry at low temperature
Full text linkThe impact of light and heavy hydrocarbons on the NH3-SCR activity of commercial Cu- and Fe-zeolite catalysts
No full textLow temperature operation of NH3-SCR systems for NOx abatement in lean burn and Diesel engines poses new demanding challenges due to the necessity of solving issues related to poor catalyst activity, urea injection management and system control. A typical example of these problems is represented by unconverted hydrocarbons (HCs) slip (e.g. during cold start or long idling) onto the SCR catalyst leading to a subsequent decrease of the DeNOx activity. In the present work, a detailed evaluation of the impact of different hydrocarbons (namelyC3H6and C10H22) on the performances of two new generation commercial zeolite SCR catalysts promoted with Fe and Cu is carried out by means of transient response method (TRM) experiments. Particular attention is devoted to elucidate the interaction between HCs and the typical constituents of SCR feed such as NH3, NO, NO2, O2and H2O and their influence on the overall reactivity, in order to possibly clarify the associated mechanistic steps
Investigation of NO2 and NO interaction with an Fe-ZSM-5 catalyst by transient response methods and chemical trapping techniques This paper is dedicated to the memory of Haldor Topsøe
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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