1,720,995 research outputs found
Kinetic Studies of NO Oxidation and Reduction over Silver-Alumina Catalyst [Elektronisk resurs]
No full textIn line with growing concerns to manufacture more environmentally friendly vehicles, the use of internal combustion engines operating with oxygen excess or so called lean-burn engines will continue to be increasingly used. For lean-burn operation, reduction of NOx (NO+NO2) emissions is a major challenge and it is therefore urgently required to develop efficient and reliable NOx reduction aftertreatment systems for a wide variety of lean-burn or diesel engines. The main goal of this thesis is to increase the understanding of the reaction mechanism of selective catalytic reduction (SCR) of NOx with a hydrocarbon (HC) reductant over Ag-Al2O3 catalysts. As an important subsystem in the HC-SCR mechanism, H2 assisted NO oxidation over a monolith-supported Ag-Al2O3 catalyst was investigated by constructing a microkinetic model that accounted for heat and mass transport in the catalyst washcoat. The effect of H2 examined in the kinetic model, was to reduce self-inhibiting surface nitrate species on active sites. A reduced factorial design of the inlet experimental conditions was used to generate transient experimental data. In general, the modelling results could reproduce the transient experimental data well with correct levels of outlet concentrations and time scales for transient responses. When H2 was present in the feed, the kinetic model showed that H2 was consumed rapidly in the front part of the monolith. This indicated that the H2 promotion of the NO oxidation reaction may have been isolated to only a portion of the catalyst. A series of temperature-programmed desorption (TPD) studies of NOx were conducted over Ag-Al2O3 catalysts to quantify and characterize the stability of surface NOx species. Formation of two general groups of surface NOx species were found to be present: a less thermally stable group of so called “low temperature (LT) nitrates” and a more thermally stable group of “high temperature (HT) nitrates”. The LT NOx desorption peak could be attributed to the decomposition of nitrate species formed on the active sites. Elimination or decrease in quantities of these LT nitrates either thermally or by reaction with H2 resulted in higher NO oxidation and NOx reduction conversion. The HT NOx desorption peak primarily corresponded to the decomposition of nitrates on the Al2O3 support and could be considered spectator surface species. It was also found that H2 facilitates formation of nitrate on the Al2O3 support and it was indicative that the mechanism of NOx storage on the Al2O3 support was mainly via NO2 readsorption. From TPD studies of C3H6-SCR in the presence and absence of H2, it was shown that the presence of H2 not only eliminated LT nitrates but also promoted the formation of adsorbed hydrocarbons. Therefore, the dual role of H2 to both eliminate nitrates from active sites and promote NOx storage was elucidated
Kinetic Studies of NO Oxidation and Reduction over Silver-Alumina Catalyst
No full textIn line with growing concerns to manufacture more environmentally friendly vehicles, the use of internal combustion engines operating with oxygen excess or so called lean-burn engines will continue to be increasingly used. For lean-burn operation, reduction of NOx (NO+NO2) emissions is a major challenge and it is therefore urgently required to develop efficient and reliable NOx reduction aftertreatment systems for a wide variety of lean-burn or diesel engines. The main goal of this thesis is to increase the understanding of the reaction mechanism of selective catalytic reduction (SCR) of NOx with a hydrocarbon (HC) reductant over Ag-Al2O3 catalysts. As an important subsystem in the HC-SCR mechanism, H2 assisted NO oxidation over a monolith-supported Ag-Al2O3 catalyst was investigated by constructing a microkinetic model that accounted for heat and mass transport in the catalyst washcoat. The effect of H2 examined in the kinetic model, was to reduce self-inhibiting surface nitrate species on active sites. A reduced factorial design of the inlet experimental conditions was used to generate transient experimental data. In general, the modelling results could reproduce the transient experimental data well with correct levels of outlet concentrations and time scales for transient responses. When H2 was present in the feed, the kinetic model showed that H2 was consumed rapidly in the front part of the monolith. This indicated that the H2 promotion of the NO oxidation reaction may have been isolated to only a portion of the catalyst. A series of temperature-programmed desorption (TPD) studies of NOx were conducted over Ag-Al2O3 catalysts to quantify and characterize the stability of surface NOx species. Formation of two general groups of surface NOx species were found to be present: a less thermally stable group of so called “low temperature (LT) nitrates” and a more thermally stable group of “high temperature (HT) nitrates”. The LT NOx desorption peak could be attributed to the decomposition of nitrate species formed on the active sites. Elimination or decrease in quantities of these LT nitrates either thermally or by reaction with H2 resulted in higher NO oxidation and NOx reduction conversion. The HT NOx desorption peak primarily corresponded to the decomposition of nitrates on the Al2O3 support and could be considered spectator surface species. It was also found that H2 facilitates formation of nitrate on the Al2O3 support and it was indicative that the mechanism of NOx storage on the Al2O3 support was mainly via NO2 readsorption. From TPD studies of C3H6-SCR in the presence and absence of H2, it was shown that the presence of H2 not only eliminated LT nitrates but also promoted the formation of adsorbed hydrocarbons. Therefore, the dual role of H2 to both eliminate nitrates from active sites and promote NOx storage was elucidated
Experimental and kinetic studies of H2 effect on lean exhaust aftertreatment processes: HC-SCR and DOC
No full textWith a growing concern to lower greenhouse gas emissions from road transportation, lean burn and diesel engines will keep playing an important role in the future. Development of a highly efficient and durable process to reduce NOx to N2 becomes a challenging issue especially in the presence of ample O2 concentration as in lean burn exhaust. One way to reduce NOx emissions in lean exhaust is by using hydrocarbon-selective catalytic reduction (HC-SCR). HC-SCR over Ag/Al2O3 catalysts appears to be a promising technology to abate NOx emission in lean burn exhaust. The function of the Diesel oxidation catalyst (DOC), as a part of a lean exhaust aftertreatment process, is to oxidize CO, HC and NO. Interestingly, addition of H2 has been shown to promote the HC-SCR activity over Ag/Al2O3 and NO oxidation activity over Pt/Al2O3 catalyst. The overall focus of this thesis was to increase understanding of the mechanisms of the H2 effect on the model catalysts of Ag-Al2O3 and Pt/Al2O3. A combination of experimental and kinetic modeling approaches was utilized as a way to examine mechanistic effects of H2.Temperature-programmed desorption (TPD) technique was used to characterize thermal stabilities of various surface NOx species formed during NO oxidation and C3H6-SCR conditions over Ag/Al2O3 catalyst. In addition, DRIFTS analysis was used to identify different types of nitrate species. These TPD results elucidated the dual roles of H2 to remove inhibiting nitrate on active sites and facilitate formation of inactive nitrate species mainly on the Al2O3 support. An initial development of a microkinetic model to describe H2-assisted NO oxidation over Ag/Al2O3 was conducted using a set of transient data. The single role of H2 to remove inhibiting nitrate species on active sites was examined. In the further model development, a global kinetic model of H2-assisted C3H6-SCR, including NO oxidation, C3H6 oxidation and C3H6-SCR in the presence and absence of H2, was proposed. This model was based on dual roles of H2 to remove inhibiting nitrates from active sites and simultaneously form more active Ag sites. The model could effectively capture a wide range of feed concentrations and temperatures, including temperature-programmed and transient experiments.The influence of H2 on NO oxidation over Pt/Al2O3 as a DOC catalyst was evaluated with various feed mixtures. Formation of Pt oxide has been known to lower the NO oxidation activity over Pt/Al2O3. The role of H2 to retard the Pt oxide formation was investigated. This resulted in a temporal enhancement in NO2 yield due to H2 addition during temperature ramp experiments. In addition, the effect of C3H6 and CO to influence the NO oxidation was also investigated. Addition of H2 mainly serves to weaken the inhibition effect of C3H6 and to a much lesser degree CO. This is mainly due to an enhancement of lower temperature C3H6 oxidation. The promotional effects of H2 to increase NO2 yield was proposed as a result of effects of H2 on surface chemistry and/or reactions. These effects could be clearly distinguished from exothermal heat effects from mainly H2 but also C3H6 and CO oxidation
Experimental and kinetic studies of H2 effect on lean exhaust aftertreatment processes: HC-SCR and DOC [Elektronisk resurs]
No full textWith a growing concern to lower greenhouse gas emissions from road transportation, lean burn and diesel engines will keep playing an important role in the future. Development of a highly efficient and durable process to reduce NOx to N2 becomes a challenging issue especially in the presence of ample O2 concentration as in lean burn exhaust. One way to reduce NOx emissions in lean exhaust is by using hydrocarbon-selective catalytic reduction (HC-SCR). HC-SCR over Ag/Al2O3 catalysts appears to be a promising technology to abate NOx emission in lean burn exhaust. The function of the Diesel oxidation catalyst (DOC), as a part of a lean exhaust aftertreatment process, is to oxidize CO, HC and NO. Interestingly, addition of H2 has been shown to promote the HC-SCR activity over Ag/Al2O3 and NO oxidation activity over Pt/Al2O3 catalyst. The overall focus of this thesis was to increase understanding of the mechanisms of the H2 effect on the model catalysts of Ag-Al2O3 and Pt/Al2O3. A combination of experimental and kinetic modeling approaches was utilized as a way to examine mechanistic effects of H2.Temperature-programmed desorption (TPD) technique was used to characterize thermal stabilities of various surface NOx species formed during NO oxidation and C3H6-SCR conditions over Ag/Al2O3 catalyst. In addition, DRIFTS analysis was used to identify different types of nitrate species. These TPD results elucidated the dual roles of H2 to remove inhibiting nitrate on active sites and facilitate formation of inactive nitrate species mainly on the Al2O3 support. An initial development of a microkinetic model to describe H2-assisted NO oxidation over Ag/Al2O3 was conducted using a set of transient data. The single role of H2 to remove inhibiting nitrate species on active sites was examined. In the further model development, a global kinetic model of H2-assisted C3H6-SCR, including NO oxidation, C3H6 oxidation and C3H6-SCR in the presence and absence of H2, was proposed. This model was based on dual roles of H2 to remove inhibiting nitrates from active sites and simultaneously form more active Ag sites. The model could effectively capture a wide range of feed concentrations and temperatures, including temperature-programmed and transient experiments.The influence of H2 on NO oxidation over Pt/Al2O3 as a DOC catalyst was evaluated with various feed mixtures. Formation of Pt oxide has been known to lower the NO oxidation activity over Pt/Al2O3. The role of H2 to retard the Pt oxide formation was investigated. This resulted in a temporal enhancement in NO2 yield due to H2 addition during temperature ramp experiments. In addition, the effect of C3H6 and CO to influence the NO oxidation was also investigated. Addition of H2 mainly serves to weaken the inhibition effect of C3H6 and to a much lesser degree CO. This is mainly due to an enhancement of lower temperature C3H6 oxidation. The promotional effects of H2 to increase NO2 yield was proposed as a result of effects of H2 on surface chemistry and/or reactions. These effects could be clearly distinguished from exothermal heat effects from mainly H2 but also C3H6 and CO oxidation
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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