1,721,046 research outputs found
Uniform fast growth of hydrotalcite-like compounds
No full textSmall size, monodispersed hexagonal particles of Mg,Al-CO3 hydrotalcite-like compounds have been synthesized in short periods of time by the microwave-hydrothermal method. An in-depth study of the influence of the irradiation time and the process temperature has been carried out. Well-crystalized materials with relatively high specific surface area values have been prepared in 30 min at 150 °C. © 2006 American Chemical Society
Microwave-treated layered double hydroxides containing Ni2+ and Al3+: The effect of added Zn2+
No full textNi containing layered double hydroxides (LDHs) have been prepared by precipitation and hydrothermally treated under microwave irradiation for different periods of time. The solids have been calcined at three temperatures corresponding to stable phases formed during thermal decomposition of LDHs. The properties of the irradiated samples and of the calcined products were studied in order to ascertain whether the ageing treatment under microwave irradiation modifies not only the properties of the layered materials, but also the properties of the calcined products. A structural and textural study was carried out by PXRD, FT-IR and Vis-UV spectroscopy, thermal analyses (DTA and TG), N2 adsorption/desorption at -196 °C and TEM microscopy; the reducibility of the nickel species was studied as well by TPR. The results show that the microwave treatment leads to better crystallized LDHs with modified thermal stability and reducibility. In addition, the degree of crystallinity of the layered precursors and their textural properties determine the properties of their thermal decomposition products. © 2006 Elsevier Inc. All rights reserved
Microwaves and layered double hydroxides: A smooth understanding
No full textMicrowave-hydrothermal treatment (MWHT), a modification of conventionalhydrothermal treatment, has been used during post-treatment of different layered doublehydroxides (LDHs). In some cases, microwaves (MWs) have been used simultaneously withurea hydrolysis or for reconstruction of the LDH structure. The main advantages of replacingthe conventional furnaces by MW ovens are a noticeable reduction in the time required tocomplete the process to obtain well-crystallized materials, and modification of their particlesize distribution and textural and thermal properties. MW radiation leads to an increase inthe rate of urea hydrolysis and consequently to fast precipitation of LDHs. Finally, thememory effect of Ni,Al-based LDHs is also improved. © 2009 IUPAC
Microwave-assisted reconstruction of Ni,Al hydrotalcite-like compounds
No full textThe microwave-assisted reconstruction of Ni,Al hydrotalcite-like compounds (HTlcs) with Ni/Al molar ratios 2/1 and 3/1 has been studied. Mixed oxides obtained after calcination of the HTlcs are immersed in three different solutions containing carbonate, distilled water and an aqueous NH3 solution, and then heated at different temperatures for increasing periods of time under microwave radiation. The evolution of the structure during the treatment is followed by powder X-ray diffraction, FT-IR and vis-UV spectroscopies and SEM and TEM microscopies. Full recovery of the original layered structure is achieved in short periods of time for the 2/1 samples when the calcined HTlcs are rehydrated in the Na2CO3 solution, but more drastic conditions are necessary for the 3/1 samples and the reconstruction seems not to be complete. Finally, only a partial reconstruction is observed in distilled water or NH3 aqueous solution. © 2008 Elsevier Inc. All rights reserved
Stabilization of Co2+ in layered double hydroxides (LDHs) by microwave-assisted ageing
No full textCo-containing layered double hydroxides at different pH have been prepared, and aged following different routes. The solids prepared have been characterized by element chemical analysis, powder X-ray diffraction, thermogravimetric and differential thermal analyses (both in nitrogen and in oxygen), FT-IR and Vis-UV spectroscopies, temperature-programmed reduction and surface area assessment by nitrogen adsorption at -196 °C. The best conditions found to preserve the cobalt species in the divalent oxidation state are preparing the samples at controlled pH, and then submit them to ageing under microwave irradiation. © 2006 Elsevier Inc. All rights reserved
Effect of post-synthesis microwave-hydrothermal treatment on the properties of layered double hydroxides and related materials
No full textThe effect of microwave-hydrothermal treatment (MWHT) on the properties of Mg,Al-layered double hydroxides with the hydrotalcite-like structure intercalated with carbonate and organic anions (terephthalate, adipate and dodecyl sulphate) and on their thermal decomposition products is studied. All solids have been characterised by element chemical analysis, powder X-ray diffraction, FT-IR spectroscopy, thermal analysis, transmission electron microscopy, specific surface area determination by N2 adsorption at - 196 °C and particle size distribution. The MWHT yielded a larger improvement of the crystallinity for carbonate-containing samples than for the organic-intercalated ones; however, although the crystallinity is not greatly modified for the latter, the thermal properties and particle size distribution are significantly affected by the ageing treatment. © 2009 Elsevier B.V. All rights reserved
Influence of microwave radiation on the textural properties of layered double hydroxides
No full textTwo series of layered double hydroxides (LDHs) with compositions Mg,Al-CO3 and Mg,Cr-CO3 were prepared by microwave and conventional hydrothermal treatment to study the role of microwave radiation on their physicochemical properties. Aging temperature was 125 °C in all cases. The samples were fully characterised by element chemical analysis, powder X-ray diffraction, differential thermal analysis and thermogravimetric analysis, 27Al MAS NMR, FT-IR and UV-Vis diffuse reflectance spectroscopies, and specific surface area and porosity assessment by N2 adsorption at -196 °C. In all cases, the only crystalline phase obtained was hydrotalcite which crystallinity is further enhanced by microwave hydrothermal treatment than by conventional hydrothermal treatment. The microwave-aged samples also have larger amounts of interlayer water and smaller particle size than those prepared by conventional hydrothermal ageing. The textural properties of the solids are strongly modified during the ageing treatment, and large specific surface areas values are observed at shorter times for those samples subjected to microwave radiation. © 2006 Elsevier Inc. All rights reserved
Nanosize cobalt oxide-containing catalysts obtained through microwave-assisted methods
No full textNanosized cobalt mixed oxide, with high stability of Co(II), were obtained by controlled thermal decomposition of layered double hydroxides (LDHs) with Co, Zn and Al in the brucite-type sheets. The effect of the microwave ageing treatment of the LDHs into the properties of the calcined products was studied. For this reason, the precursors were submitted to increasing periods of time to microwave-hydrothermal treatment. The use of microwave ageing yields to well-crystallised compounds without oxidation of Co2+ species to Co3+. The structure features of oxides remain during whole ageing process; the microwave-hydrothermal treatment yield, in all cases, spinel-like compounds. Studies of surface properties, N2 adsorption/desorption and the TEM micrographs, show that the textural properties are strongly modified depending on the irradiation time were submitted to, altering both the pore size distribution and the particle size of the nano-oxides. © 2007 Elsevier B.V. All rights reserved
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
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