Special Issue "Aerogel Catalyst"

Guest Editor
Dr. Theophilos Ioannides
Institute of Chemical Engineering Sciences (FORTH/ICE-HT), Stadiou Str., Platani, P.O. Box 1414, GR-26504 Patras, Hellas, Greece
Website: http://www.iceht.forth.gr/staff/ioannides.html
E-Mail: theo@iceht.forth.gr
Phone: +30 2610 965 264
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
Website: http://reflexions.ulg.ac.be/cms/c_22809/job-nathalie
E-Mail: nathalie.job@ulg.ac.be

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

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

Type of Paper: Article
Title: MgO Aerogels as CO₂ Adsorption Catalysts
Authors: Winny Dong, Tanya Faltens and Mingheng Li
Affiliation: Chemical and Materials Engineering California State Polytechnic University, Pomona, CA, USA; E-Mails: winnydong@csupomona.edu; tafaltens@csupomona.edu; minghengli@csupomona.edu
Abstract: CO₂ capture on solid sorbents is an important technique in the context of greenhouse gas emission reduction as well as adsorption enhanced reaction processes. Traditional CO₂ adsorbents such as hydrotalcites, alumina and zeolite have been extensively investigated. This work will focus on the study of CO₂ adsorption on sol-gel derived MgO aerogels, which could have a great potential because of their large surface area (~ 500 m² g^-1). In this study amorphous MgO aerogels were compared with Alumina, crystalline (low surface area) MgO, and amorphous MgO xerogels. A thermogravimetric analyzer (TGA) was employed where the weight gain of the sample due to CO₂ adsorption was measured. CO₂ was introduced at different partial pressures starting from 0.0204 atm and going on to 0.0417 atm, 0.0638 atm, 0.087 atm, 0.111 atm, 0.5 atm, and finally 1 atm. Adsorption isotherms and the adsorption capacity of each material were determined. Data showed that the adsorption capacity of the MgO aerogel at 1 atm partial pressure of CO₂ was 0.314 mmol CO₂/g adsorbent. This was more than twice the CO₂ capacity of the next highest adsorbent. It was also observed that MgO aerogel has faster CO₂ adsorption kinetics and is less prone to N₂ adsorption as compared to Alumina. The CO₂ capacity was correlated to surface area, porosity, and structure.

Type of Paper: Article
Title: 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
Authors: Carlos Moreno-Castilla*, Francisco Carrasco-Marín and Marta B. Dawidziuk
Affiliation: Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain
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 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 ºC. However, both TOF values were similar in catalysts supported on the carbon aerogel obtained at 500 ºC. TOF for hydrogenolysis and isomerization were related to the mean Pt particle size in catalysts supported on carbon aerogels obtained at 900–950ºC. 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.

Type of Paper: Article
Title: Tailoring Synthesis Conditions of Carbon Xerogels towards Their Utilization as Pt-Catalyst Supports for ORR in DMFC
Authors: Cinthia Alegre, David Sebastián, Estela Baquedano, María Elena Gálvez, Rafael Moliner and María Jesús Lázaro
Affiliation: Instituto de Carboquímica (CSIC-Spanish National Research Council), C/. Miguel Luesma Castán, 4; 50018 Zaragoza, Spain; E-Mails: cinthia@icb.csic.es (C.A.); dsebastian@icb.csic.es (D.S.); megalvez@icb.csic.es (M.E.G); rmoliner@icb.csic.es (R.M.). mlazaro@icb.csic.es (M.J.L.)
Abstract: Carbon xerogels with different textural and structural properties were synthesized, and used as supports for Pt catalysts for the oxygen reduction reaction (ORR). Different synthesis conditions were employed in order to synthesize 8 carbon xerogels following the sol-gel method. Two different precursors, resorcinol and pyrogallol, and two different catalysts, Na₂CO₃ and H₂SO₄, were used. The molar ratio precursor (P)/formaldehyde (F) was modified in the synthesis of the different xerogels, as well as the molar ratio precursor (P)/catalysts (C). Either distilled water or acetone, were used as solvents. Pt catalysts were synthesized by impregnation with H₂PtCl₆ and reduction with formic acid. Among all xerogels synthesized, the one prepared under P/C = 800, P/F = 0.5, Na₂CO₃ as catalyst and water as solvent, reached the highest development of porous structure, with large pore volume, what lead to the highest active catalyst towards ORR, with a catalytic activity comparable to that of Pt/Vulcan. Accelerated degradation tests were also carried out.  After these tests, polarization curves showed that Pt/Vulcan followed by Pt/CX-1 (intermediate textural properties) were the most active catalysts towards the ORR.
Keywords: carbon; xerogel; platinum; catalyst, oxygen; reduction; reaction