Special Issue "Aerogel Catalyst"

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A special issue of Catalysts (ISSN 2073-4344).

Deadline for manuscript submissions: closed (30 September 2012)

Special Issue Editors

Guest Editor
Dr. Theophilos Ioannides

Institute of Chemical Engineering Sciences (FORTH/ICE-HT), Stadiou Str., Platani, P.O. Box 1414, GR-26504 Patras, Greece
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Phone: 302610965264
Fax: +30 2610 965 223
Interests: heterogeneous catalysts; catalytic Processes; new materials; membrane processes
Guest Editor
Dr. Nathalie Job

Département de Chimie appliquée, Bât. B6, Université de Liège, Allée de la Chimie 3, 4000 Liège 1, Belgium
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Special Issue Information

Dear Colleagues,

The term aerogel describes a material obtained by supercritical extraction of the liquid of a gel, itself consisting of a solid three-dimensional network that ensnares a liquid medium. Drying by transition from the liquid to the supercritical phase does away with the capillary forces, which act in evaporation and cause partial or total collapse of the pore network. Hence, supercritical drying leads to materials with low density, high specific surface area, large pore volume and very versatile pore size. The first aerogels were produced from silica gels in 1937. Since then, the sol-gel technique has expanded to other inorganic materials such as alumina or titanium oxide, for instance, then to carbon in the late 1980s.

Extending the original definition, literature includes in aerogel-like materials a large variety of nanostructured porous solids. One can cite, for instance, materials obtained by supercritical drying of precipitates, which maintain a loose structure with non-agglomerated primary particles.  Cryogels synthesized by freeze-drying of gels, or xerogels, i.e. materials prepared via subcritical drying but that nevertheless maintain a substantial fraction of the original gel pore texture, constitute an interesting alternative to the (costly) supercritical drying.

The inherent properties of aerogels and aerogel-like supports render them very attractive in catalytic applications, as shown by the constantly renewed interest of research groups for this technology through the years. Besides high dispersion of the active phase, made possible by high surface areas, the high pore volume and tunable pore size of aerogels lead to the possibility of designing catalyst supports that facilitate the diffusion of reactants and products to and from the active sites. The indubitable improvements in catalytic performance obtained through use of aerogel supports should of course outweigh their higher processing cost. To this end, the development of efficient manufacture processes is crucial and should constitute the last step towards large-scale use of catalysts supported on these fascinating engineered supports.

Dr. Theophilos Ioannides
Dr. Nathalie Job
Guest Editors

Keywords

  • sol-gel
  • aerogels
  • xerogels
  • cryogels
  • catalysts
  • catalyst supports
  • cogelation
  • nanostructured materials

Published Papers (6 papers)

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Research

Open AccessArticle Partial Oxidation of n-Butane over a Sol-Gel Prepared Vanadium Phosphorous Oxide
Catalysts 2013, 3(1), 11-26; doi:10.3390/catal3010011
Received: 2 October 2012 / Revised: 5 December 2012 / Accepted: 8 January 2013 / Published: 16 January 2013
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Abstract
Vanadium phosphorous oxide (VPO) is traditionally manufactured from solid vanadium oxides by synthesizing VOHPO4∙0.5H2O (the precursor) followed by in situ activation to produce (VO)2P2O7 (the active phase). This paper discusses an alternative synthesis method
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Vanadium phosphorous oxide (VPO) is traditionally manufactured from solid vanadium oxides by synthesizing VOHPO4∙0.5H2O (the precursor) followed by in situ activation to produce (VO)2P2O7 (the active phase). This paper discusses an alternative synthesis method based on sol-gel techniques. Vanadium (V) triisopropoxide oxide was reacted with ortho-phosphoric acid in an aprotic solvent. The products were dried at high pressure in an autoclave with a controlled excess of solvent. This procedure produced a gel of VOPO4 with interlayer entrapped molecules. The surface area of the obtained materials was between 50 and 120 m2/g. Alcohol produced by the alkoxide hydrolysis reduced the vanadium during the drying step, thus VOPO4 was converted to the precursor. This procedure yielded non-agglomerated platelets, which were dehydrated and evaluated in a butane-air mixture. Catalysts were significantly more selective than the traditionally prepared materials with similar intrinsic activity. It is suggested that the small crystallite size obtained increased their selectivity towards maleic anhydride. Full article
(This article belongs to the Special Issue Aerogel Catalyst)
Open AccessArticle Carbon Xerogel Catalyst for NO Oxidation
Catalysts 2012, 2(4), 447-465; doi:10.3390/catal2040447
Received: 20 July 2012 / Revised: 28 September 2012 / Accepted: 9 October 2012 / Published: 17 October 2012
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Abstract
Carbon xerogels were prepared by the polycondensation of resorcinol and formaldehyde using three different solution pH values and the gels were carbonized at three different temperatures. Results show that it is possible to tailor the pore texture of carbon xerogels by adjusting the
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Carbon xerogels were prepared by the polycondensation of resorcinol and formaldehyde using three different solution pH values and the gels were carbonized at three different temperatures. Results show that it is possible to tailor the pore texture of carbon xerogels by adjusting the pH of the initial solution and the carbonization temperature. Materials with different textural properties were obtained and used as catalysts for NO oxidation at room temperature. The NO conversions obtained with carbon xerogels were quite high, showing that carbon xerogels are efficient catalysts for NO oxidation. A maximum of 98% conversion for NO was obtained at initial concentration of NO of 1000 ppm and 10% of O2. The highest NO conversions were obtained with the samples presenting the highest surface areas. The temperature of reaction has a strong influence on NO oxidation: the conversion of NO decreases with the increase of reaction temperature. Full article
(This article belongs to the Special Issue Aerogel Catalyst)
Open AccessArticle Tailoring Synthesis Conditions of Carbon Xerogels towards Their Utilization as Pt-Catalyst Supports for Oxygen Reduction Reaction (ORR)
Catalysts 2012, 2(4), 466-489; doi:10.3390/catal2040466
Received: 6 August 2012 / Revised: 24 September 2012 / Accepted: 9 October 2012 / Published: 17 October 2012
Cited by 12 | PDF Full-text (881 KB) | HTML Full-text | XML Full-text
Abstract
Carbon xerogels characterized by different textural, structural and chemical properties were synthesized and used as supports for Pt catalysts for the application in polymer electrolyte fuel cells. Synthesis conditions were varied in order to synthesize carbon xerogels following the sol-gel method. These included
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Carbon xerogels characterized by different textural, structural and chemical properties were synthesized and used as supports for Pt catalysts for the application in polymer electrolyte fuel cells. Synthesis conditions were varied in order to synthesize carbon xerogels following the sol-gel method. These included the reactants ratio (precursor/formaldehyde), the catalyst concentration (precursor/catalyst ratio) and type (basic and acid), the precursor type (resorcinol and pyrogallol) and the solvent (aqueous or acetone based). Stoichiometric mixtures of resorcinol and formaldehyde yielded well polymerized gels and highly developed structures. Slow gelation, favored by the presence of acetone as solvent in the sol and low catalyst concentration, resulted in higher polymerization extent with a highly mesoporous or even macroporous texture and more ordered structure, as evidenced by XPS and Raman spectroscopy. Small Pt particles of ca. 3.5 nm were obtained by using carbon xerogels characterized by an ordered surface structure. The specific activity towards the oxygen reduction reaction, i.e., the limiting catalytic process in low temperature fuel cells, is significantly favored by highly ordered carbon xerogels due to a metal-support enhanced interaction. Nevertheless, surface defects favor the distribution of the metallic particles on the surface of carbon, which in the end influences the effectiveness of the catalyst. Accelerated degradation tests were conducted to evaluate catalyst stability under potential cycling conditions. The observed decay of performance was considerably lower for the catalysts based on ordered carbon xerogels stabilizing Pt particles in a higher extent than the other xerogels and the commercial carbon black support. Full article
(This article belongs to the Special Issue Aerogel Catalyst)
Open AccessArticle Carbon Aerogel-Supported Pt Catalysts for the Hydrogenolysis and Isomerization of n-Butane: Influence of the Carbonization Temperature of the Support and Pt Particle Size
Catalysts 2012, 2(4), 422-433; doi:10.3390/catal2040422
Received: 18 July 2012 / Revised: 3 September 2012 / Accepted: 24 September 2012 / Published: 12 October 2012
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Abstract
Carbon aerogels prepared at different carbonization temperatures and with varying mesopore volumes were used as supports for Pt catalysts to study the n-C4H10/H2 reaction. Mean Pt particle size depended on the mesopore volume of the support, showing
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Carbon aerogels prepared at different carbonization temperatures and with varying mesopore volumes were used as supports for Pt catalysts to study the n-C4H10/H2 reaction. Mean Pt particle size depended on the mesopore volume of the support, showing a linear decrease when the mesopore volume increased. The turnover frequency (TOF) for hydrogenolysis was much higher than for isomerization in catalysts supported on carbon aerogels obtained at 900–950 °C. However, both TOF values were similar in catalysts supported on the carbon aerogel obtained at 500 °C. TOF for hydrogenolysis and isomerization were related to the mean Pt particle size in catalysts supported on carbon aerogels obtained at 900–950 °C. In addition, both reactions showed a compensation effect between the activation energy and pre-exponential factor, indicating that they have the same intermediate, i.e., the chemisorbed dehydrogenated alkane. Full article
(This article belongs to the Special Issue Aerogel Catalyst)
Open AccessArticle Co-Fe-Si Aerogel Catalytic Honeycombs for Low Temperature Ethanol Steam Reforming
Catalysts 2012, 2(3), 386-399; doi:10.3390/catal2030386
Received: 1 August 2012 / Revised: 6 September 2012 / Accepted: 7 September 2012 / Published: 19 September 2012
Cited by 5 | PDF Full-text (1567 KB) | HTML Full-text | XML Full-text
Abstract
Cobalt talc doped with iron (Fe/Co~0.1) and dispersed in SiO2 aerogel was prepared from silica alcogel impregnated with metal nitrates by supercritical drying. Catalytic honeycombs were prepared following the same procedure, with the alcogel synthesized directly over cordierite honeycomb pieces. The composite
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Cobalt talc doped with iron (Fe/Co~0.1) and dispersed in SiO2 aerogel was prepared from silica alcogel impregnated with metal nitrates by supercritical drying. Catalytic honeycombs were prepared following the same procedure, with the alcogel synthesized directly over cordierite honeycomb pieces. The composite aerogel catalyst was characterized by X-ray diffraction, scanning electron microscopy, focus ion beam, specific surface area and X-ray photoelectron spectroscopy. The catalytic layer is about 8 µm thick and adheres well to the cordierite support. It is constituted of talc layers of about 1.5 µm × 300 nm × 50 nm which are well dispersed and anchored in a SiO2 aerogel matrix with excellent mass-transfer properties. The catalyst was tested in the ethanol steam reforming reaction, aimed at producing hydrogen for on-board, on-demand applications at moderate temperature (573–673 K) and pressure (1–7 bar). Compared to non-promoted cobalt talc, the catalyst doped with iron produces less methane as byproduct, which can only be reformed at high temperature, thereby resulting in higher hydrogen yields. At 673 K and 2 bar, 1.04 NLH2·mLEtOH(l)−1·min−1 are obtained at S/C = 3 and W/F = 390 g·min·molEtOH−1. Full article
(This article belongs to the Special Issue Aerogel Catalyst)
Open AccessArticle Large Mesopore Generation in an Amorphous Silica-Alumina by Controlling the Pore Size with the Gel Skeletal Reinforcement and Its Application to Catalytic Cracking
Catalysts 2012, 2(3), 368-385; doi:10.3390/catal2030368
Received: 28 June 2012 / Revised: 27 August 2012 / Accepted: 31 August 2012 / Published: 13 September 2012
Cited by 8 | PDF Full-text (1648 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Tetraethoxy orthosilicate (TEOS) was used not only as a precursor of silica, but also as an agent which reinforces the skeleton of silica-gel to prepare an aerogel and resultant silica and silica-alumina with large pore size and pore volume. In this gel skeletal
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Tetraethoxy orthosilicate (TEOS) was used not only as a precursor of silica, but also as an agent which reinforces the skeleton of silica-gel to prepare an aerogel and resultant silica and silica-alumina with large pore size and pore volume. In this gel skeletal reinforcement, the strength of silica aerogel skeleton was enhanced by aging with TEOS/2-propanol mixed solution to prevent the shrink of the pores. When silica aerogel was reinforced by TEOS solution, the pore diameter and pore volume of calcined silica could be controlled by the amount of TEOS solution and reached 30 nm and 3.1 cm3/g. The results from N2 adsorption measurement indicated that most of pores for this silica consisted of mesopores. Silica-alumina was prepared by the impregnation of an aluminum tri-sec-butoxide/2-butanol solution with obtained silica. Mixed catalysts were prepared by the combination of β-zeolite (26 wt%) and prepared silica-aluminas with large mesopore (58 wt%) and subsequently the effects of their pore sizes on the catalytic activity and the product selectivity were investigated in catalytic cracking of n-dodecane at 500 °C. The mixed catalysts exhibited not only comparable activity to that for single zeolite, but also unique selectivity where larger amounts of branched products were formed. Full article
(This article belongs to the Special Issue Aerogel Catalyst)

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