1,720,958 research outputs found
CO2 hydrogenation and ethanol steam reforming over Co/SiO2 catalysts: Deactivation and selectivity switches
No full textTwo 20 wt.% Co/SiO2 catalysts have been prepared using silica gel as a support and cobalt acetate or nitrate as precursors. They were characterized before and after reaction using XRD, skeletal IR and DR-UV–vis-NIR spectroscopies and FE-SEM microscopy. Fresh catalysts are active in CO2 methanation at atmospheric pressure but rapidly deactivate at 623–673 K, due to formation of encapsulating carbon. However, they retain a more stable activity in producing CO by the reverse water gas shift (rWGS) reaction. Methanation and rWGS appear to occur on independent sites. The Co/SiO2 catalyst produced from Co-acetate precursor shows stable activity in Ethanol Steam Reforming (ESR) at 873−973 K, producing also carbon whiskers (nanotubes) that, however, do not cause catalyst deactivation at the laboratory conditions and timescale. The catalyst produced starting with Co-nitrate is even more active in ESR but it deactivates fast. At lower temperature or when deactivated with respect to ESR, both catalysts shift to high activity in ethanol dehydrogenation to acetaldehyde, showing that the ability to activate water is mostly lost. The differences observed among catalysts produced from cobalt acetate and cobalt nitrate precursors can be associated to the different pH of the impregnating solution. The higher pH of cobalt acetate solution results in surface dissolution / hydrolysis of silica and this may be at the origin of the formation of cobalt silicate surface species, which can also stabilize the support against high temperature reactivity. This might favor larger Co particles formation, less active, but also giving rise to inert cobalt species
Modeling of laboratory steam methane reforming and CO2 methanation reactors
No full textTo support the interpretation of the experimental results obtained from two laboratory-scale reactors, one working in the steam methane reforming (SMR) mode, and the other in the CO2 hydrogenation (MCO2) mode, a steady-state pseudo-homogeneous 1D non-isothermal packed-bed reactor model is developed, embedding the classical Xu and Froment local kinetics. The laboratory reactors are operated with three different catalysts, two commercial and one homemade. The simulation model makes it possible to identify and account for thermal effects occurring inside the catalytic zone of the reactor and along the exit line. The model is intended to guide the development of small size SMR and MCO2 reactors in the context of Power-to-X (P2X) studies
Support effects in metal catalysis: a study of the behavior of unsupported and silica-supported cobalt catalysts in the hydrogenation of CO2 at atmospheric pressure
No full textTwo 20 wt.% Co/SiO2 catalysts were prepared using silica gel as a support and cobalt acetate and nitrate as precursors, respectively. As a comparison unsupported amorphous cobalt nanoparticles (10–80 nm) were synthesized by decomposing cobalt carbonyl. They were characterized before and after reaction using XRD, IR spectroscopy in the range of framework vibrations, DR-UV–vis-NIR spectroscopies and FE-SEM microscopy. They are active in CO2 methanation at atmospheric pressure. However, unsupported cobalt and the silica-supported catalyst prepared using cobalt acetate deactivate fast at 623–673 K, also due to formation of encapsulating carbon. On the other hand, they retain more stable activity in the reverse water gas shift (rWGS) reaction, producing CO. Methanation and rWGS appear to occur on independent sites. The silica-supported catalyst produced from cobalt nitrate show more stable methanation activity, attributed to the presence of smaller cobalt particles (10–20 nm) that resist deactivation. For both unsupported CoNPs and Co/SiO2, the significant parameters for catalysts deactivation have been determined for CO2 methanation. The differences observed among catalysts produced starting from cobalt acetate and cobalt nitrate, can be associated to the different pH of the impregnating solution
Unsupported cobalt nanoparticles as catalysts: Effect of preparation method on catalytic activity in CO2 methanation and ethanol steam reforming
No full textA study on CO2 methanation and steam methane reforming over commercial Ni/calcium aluminate catalysts
No full textThree Ni-based natural gas steam reforming catalysts, i.e., commercial JM25-4Q and JM57-4Q, and a laboratory-made catalyst (26% Ni on a 5% SiO2-95% Al2O3), are tested in a laboratory reactor, under carbon dioxide methanation and methane steam reforming operating conditions. The laboratory catalyst is more active in both CO2 methanation (equilibrium is reached at 623 K with 100% selectivity) and methane steam reforming (92% hydrogen yield at 890 K) than the two commercial catalysts, likely due to its higher nickel loading. In any case, commercial steam reforming catalysts also show interesting activity in CO2 methanation, reduced by K-doping. The interpretation of the experimental results is supported by a one-dimensional (1D) pseudo-homogeneous packed-bed reactor model, embedding the Xu and Froment local kinetics, with appropriate kinetic parameters for each catalyst. In particular, the H2O adsorption coefficient adopted for the commercial catalysts is about two orders of magnitude higher than for the laboratory-made catalyst, and this is in line with the expectations, considering that the commercial catalysts have Ca and K added, which may promote water adsorption
Ni/SiO2-Al2O3 catalysts for CO2 methanation: Effect of La2O3 addition
No full textNi-based catalysts, 13.5 Ni wt.%, with enhanced thermal stability and catalytic performance for CO2 methanation have been synthesized with different La2O3 loadings. Catalysts have been extensively characterised by: XRD, IR, H2-TPR, IPA-TPD, UV–vis-NIR, FE-SEM and tested in CO2 methanation. SiO2 addition to Al2O3 support decreases the activity for CO2 methanation, while lanthanum acts as suitable promoter by strongly increasing catalytic performances. Silica presence successfully inhibits the formation of crystalline perovskites phases, stabilizes support morphology, and allows the introduction of high La2O3 loadings, allowing a better control of acid-base properties. 37 % wt. La2O3 addition gives rise to even higher performances than those previously observed, i.e. 83 % CH4 yield at 573 K. Reaction orders for CO2 and H2 have been determined; La- addition is confirmed to be responsible for a reduction in the CO2 reaction order, suggesting a stronger CO2 adsorption and the possible role of these species as a reactant reservoirs
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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