Aerogel Catalysts Research Articles

Effect of Ni/Al atomic ratio of mesoporous Ni–Al 2O 3 aerogel catalysts on their catalytic activity for hydrogen production by steam reforming of liquefied natural gas (LNG)

Mesoporous Ni–Al 2O 3 ( XNiAE) aerogel catalysts with different Ni/Al atomic ratio ( X) were prepared by a single-step sol-gel method and a subsequent CO 2 supercritical drying method. The effect of Ni/Al atomic ratio of mesoporous XNiAE aerogel catalysts on their physicochemical properties and catalytic activity for steam reforming of liquefied natural gas (LNG) was investigated. Textural properties and chemical properties of XNiAE catalysts were strongly influenced by Ni/Al atomic ratio. Nickel species were highly dispersed on the surface of XNiAE catalysts through the formation of surface nickel aluminate phase. In the steam reforming of LNG, both LNG conversion and hydrogen yield showed volcano-shaped curves with respect to Ni/Al atomic ratio. Average nickel diameter of XNiAl catalysts was well correlated with LNG conversion and hydrogen yield over the catalysts. Among the catalysts tested, 0.35NiAE (Ni/Al = 0.35) catalyst with the smallest average nickel diameter showed the best catalytic performance. The highest surface area, the largest pore volume, the largest average pore size, and the highest reducibility of 0.35NiAE catalyst were also partly responsible for its superior catalytic performance.

Development of a Co–Ni bimetallic aerogel catalyst for hydrogen production via methane oxidative CO 2 reforming in a magnetic assisted fluidized bed

A novel nano-sized Co–Ni bimetallic aerogel catalyst was synthesized via the sol–gel process followed by the supercritical drying method. The catalyst exhibited at least 28% higher activity and 15% higher H 2 selectivity than those of a monometallic cobalt aerogel catalyst in methane oxidative CO 2 (Oxy-CO 2) reforming. Cold-model experiments revealed that channels were alleviated and the agglomerate size was reduced when the catalyst was applied in a magnetic assisted fluidized bed (MAFB). Owing to the improved fluidization quality of the catalyst in the MAFB reactor, CH 4 conversion was raised by 12% and 7% as compared with those in the fixed bed reactor and the conventional fluidized bed reactor, respectively. Furthermore, the catalytic performance was quite stable during a 50 h reaction. This stable performance can be illustrated both by the superior catalytic property of the Co–Ni bimetallic aerogel catalyst and the intensified gas–solid interaction in the MAFB reactor.

Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel–alumina aerogel catalyst

A mesoporous nickel–alumina aerogel catalyst (NiAE-SS) was prepared by a single-step sol–gel method and a subsequent CO 2 supercritical drying method for use in hydrogen production by steam reforming of liquefied natural gas (LNG). For comparison, a nickel catalyst supported on alumina aerogel (Ni/AE-IP) was also prepared by an impregnation method. The effect of preparation method of supported nickel catalysts on their physicochemical properties and catalytic performance in the steam reforming of LNG was investigated. NiAE-SS catalyst retained superior textural properties compared to Ni/AE-IP catalyst. Nickel species were finely dispersed on the surface of both Ni/AE-IP and NiAE-SS catalysts through the formation of surface nickel aluminate phase. Although both Ni/AE-IP and NiAE-SS catalysts exhibited a stable catalytic performance, NiAE-SS catalyst showed a better catalytic performance than Ni/AE-IP catalyst in terms of LNG conversion and hydrogen yield. High nickel surface area, high nickel dispersion, and well-developed mesoporosity of NiAE-SS catalyst played an important role in enhancing the catalytic performance in the steam reforming of LNG. Uniformly distributed metallic nickel particles in the NiAE-SS catalyst were also responsible for its high catalytic performance.

Iron-oxide aerogel and xerogel catalyst formulations: Characterization by 57Fe Mössbauer and XAFS spectroscopies

Iron in various iron-oxide aerogel and xerogel catalyst formulations (> or =85% Fe(2)O(3); < or =10% K, Co, Cu, or Pd) developed for possible use in Fischer-Tropsch synthesis (FTS) or the water-gas-shift (WGS) reaction has been examined by (57)Fe Mössbauer spectroscopy. The seventeen samples consisted of both as-prepared and calcined aerogels and xerogels and their products after use as catalysts for FTS or the WGS reaction. Complementary XAFS spectra were obtained on the occurrence of the secondary elements in some of the same materials. A broad, slightly asymmetric, two-peak Mössbauer spectrum was obtained from the different as-prepared and calcined catalyst formulations in the majority of cases. Such spectra could only be satisfactorily fit with three quadrupole doublet components, but no systematic trends in the isomer shift and quadrupole splitting parameters and area ratios of the individual components could be discerned that reflected variations in the composition or preparation of the aerogel or xerogel materials. However, significant reductions were noted in the Mössbauer effective thickness (recoilless absorption effect per unit mass of iron) parameter, chi(eff)/g, determined at room temperature, for aerogels and xerogels compared to bulk iron oxides, reflecting the openness and lack of rigidity of the aerogel and xerogel structures. Mössbauer measurements for two aerogels over the range from 15 to 292K confirmed the greatly diminished nature of this parameter at room temperature. Major increases in the effective thickness parameter were observed when the open structure of the aerogel or xerogel collapsed during calcination resulting in the formation of iron oxides (hematite, spinel ferrite). Similar structural changes were indicated by increases in this parameter after use of iron-oxide aerogels as catalysts for FTS or the WGS reaction, during which the iron-oxide aerogel was converted to a mixture of nonstoichiometric magnetite and the Hägg carbide, chi-Fe(5)C(2), or nonstoichiometric magnetite, respectively.

Depletion of CO oxidation activity of supported Au catalysts prepared from thiol-capped Au nanoparticles by sulfates formed at Au–titania boundaries: Effects of heat treatment conditions on catalytic activity

We have investigated the effects of heat treatment in air and H 2/Ar on Au/titania-coated silica aerogel catalysts prepared through adsorption of thiol-capped Au nanoparticles (AuNPs). Sulfur atoms remained on the support surfaces in the form of sulfates upon calcination at temperatures below 773 K. X-ray absorption fine structure and in situ infrared absorption measurements of the catalysts suggested that some of the sulfates affected the catalytic properties of the AuNPs through the interactions between AuNPs and sulfates. The turnover frequency for CO oxidation reaction of the catalysts was enhanced by a factor of 3–5 upon the removal of the sulfates through calcination at 873 K or H 2/Ar treatment at 673 K after calcination at 673 K. These results reveal the importance of the contribution of Au–support interfaces to the CO oxidation efficiency on Au/TiO 2 catalyst systems.

Biodiesel synthesis using K2CO3/Al-O-Si aerogel catalysts

In this study, catalysts for fatty acid methyl esters (FAME or biodiesel) synthesis with K2CO3 as the active component on an alumina/silica support were synthesized using the sol-gel method, which was followed by drying the ?dense? wet gels with supercritical carbon dioxide to obtain the aerogels. The prepared catalysts were characterized by XRD analysis, FTIR spectroscopy and N2 physisorption at 77 K, and tested in the methanolysis of sunflower oil. The effects of reaction variables, such as reaction time, temperature and methanol to oil molar ratio, on the yield of FAME were investigated. The aerogel catalysts with K2CO3 as the active component on an alumina/silica support exhibited good activity in the methanolysis of sunflower oil. The leaching of potassium when the catalyst was in contact with pure methanol under the working conditions of methanolysis was also tested in this study, indicating that it occurred only at higher temperatures, while at lower ones, it was negligible.

Electrochemical characterization of Pt/carbon xerogel and Pt/carbon aerogel catalysts: first insights into the influence of the carbon texture on the Pt nanoparticle morphology and catalytic activity

Platinum catalysts were prepared by impregnation/reduction of two carbon supports with different pore textures: one carbon aerogel and one carbon xerogel. Impregnation with H2PtCl6 was followed by reduction in aqueous phase with NaBH4, filtration, drying and subsequent reduction by H2. The catalysts were characterized by widely used physico-chemical methods (N2 adsorption, transmission electron microscopy, X-ray diffraction and CO chemisorption); from these techniques, no significant difference could be detected between the two samples. Actual Pt surface areas measured by coulometry of the electrochemical COads stripping are comparable for both samples. However, the peak position and charge below each electrooxidation peak points towards different fraction of small/large particles within these two samples. In addition, COads stripping shows that a fraction of the Pt particle surface is not electrochemically active. Pronounced differences observed in the specific activity towards O2 reduction reaction were then explained by structural differences in Pt particles, undetectable by physico-chemical characterization techniques.

Cobalt ferrite aerogels by epoxide sol–gel addition: Efficient catalysts for the hydrolysis of 4-nitrophenyl phosphate

Porous cobalt ferrite aerogel catalysts were obtained by 1,2-epoxide sol–gel process and investigated in the hydrolysis of 4-nitrophenyl phosphate. These materials were synthesized by reacting cobalt and iron salts with propylene oxide in methanol, dried by supercritical carbon dioxide, and calcined between 200 and 800 °C. The catalysts were characterized using Fourier Transform Infrared (FTIR) spectroscopy, nitrogen adsorption–desorption technique, and powder X-ray diffraction (XRD). The “as-prepared aerogel” surface exhibits M–OH groups that disappear after annealing, which enhances the spinel structure. This was coupled with a better crystallinity revealed by XRD peaks sharpness. The crystallite sizes were found to be between 6.3 and 28.1 nm. In addition, the catalysts revealed high porosities that decrease as the annealing temperature increases. The catalysis showed that the catalytic activity significantly rely on the synthesis procedure and mainly the calcination temperature. Samples calcined at 600 °C and above did not show any catalytic activity, however, the highest catalytic efficiency was for those calcined at 200 °C with 100% selectivity for the 4-nitrophenol. The correlation of the characterization techniques and the catalysis tests revealed that the catalytic properties of these sol–gel materials are due to the existence of residual surface OH groups.

Synthesis and Purification of Single-Walled Carbon Nanotubes Prepared Over CoO/MgO Aerogel Catalyst

Synthesis and Purification of Single-Walled Carbon Nanotubes Prepared Over CoO/MgO Aerogel Catalyst

Influence of support porosity and Pt content of Pt/carbon aerogel catalysts on metal dispersion and formation of self-assembled Pt–carbon hybrid nanostructures

Three carbon aerogels with different meso–macropore networks were used as supports for Pt catalysts using [Pt(NH 3) 4]Cl 2 as precursor salt. Results obtained showed mesopore volume and mean mesopore size to be important parameters that control Pt particle size and dispersion in catalysts containing 2 wt.% Pt. Once the most appropriate porosity to obtain the highest dispersion was determined, the metal content was increased to 20 wt.% Pt. However, the mean Pt particle size only increased from 1 to 2 nm, indicating the importance of an appropriate mesoporosity for obtaining a high dispersion at high metal loading. Mean Pt particle size was always slightly smaller by transmission electron microscopy than by H 2 chemisorption, because some Pt particles were not reduced during pre-treatment, as confirmed by X-ray photoelectron spectroscopy. Finally, transmission electron microscopy observations of catalysts with metal loading of 8–20 wt.% before pre-treatment showed the formation of self-assembled Pt–carbon hybrid nanorods and nanowires. To the best of our knowledge, this is the first observation of this phenomenon in Pt/C catalysts.

Structures and CO oxidation activities of size-selected Au nanoparticles in mesoporous titania-coated silica aerogels

We have prepared Au/Titania-coated silica aerogel catalysts from Au nanoparticles (AuNPs) having well-regulated sizes in the range 1.4–6.4 nm, through heat-treatment of thiol-capped AuNPs, size-separation through fractional precipitation, and adsorption of the AuNPs on the support in a nonpolar solvent. The Au particle size was preserved during calcination of the catalyst samples at 673 K and subsequent CO oxidation. The turnover frequency for CO oxidation of the catalysts changed drastically when their sizes varied from 4 to 5 nm, but it did not change appreciably in other size regions. X-ray absorption spectroscopy and X-ray diffraction measurements revealed that lattice contraction and structural changes become prominent at Au diameters of less than 4–5 nm. Thus, the structural changes that occurred in the AuNPs in this diameter region are consistent with the enhanced catalytic activity.

Effects of preparation methods for V2O5-TiO2 aerogel catalysts on the selective catalytic reduction of NO with NH3

A series of V2O5-TiO2 aerogel catalysts were prepared by the sol-gel method with subsequent supercritical drying with CO2. The main variables in the sol-gel method were the amounts of V2O5 and when the vanadium precursor was introduced. V2O5-TiO2 xerogel and V2O5/TiO2 (P-25) were also prepared for comparison. The V2O5-TiO2 aerogel catalysts showed much higher surface areas and total pore volumes than V2O5-TiO2 xerogel and impregnated V2O5/TiO2 (P-25) catalysts. The catalysts were characterized by N2 physisorption, X-ray diffraction (XRD), FT-Raman spectroscopy, temperature-programmed reduction with H2 (H2-TPR), and temperature-programmed desorption of ammonia (NH3-TPD). The selective catalytic reduction of NOx with ammonia in the presence of excess O2 was studied over these catalysts. Among various V2O5-TiO2 catalysts, V2O5 supported on aerogel TiO2 showed a wide temperature window exhibiting high NOx conversions. This superior catalytic activity is closely related to the large amounts of strong acidic sites as well as the surface vanadium species with characteristics such as easy reducibility and monomeric and polymeric vanadia surface species.

Nanosized CuO–Zr xCe 1 − xO y aerogel catalysts prepared by ethanol supercritical drying for catalytic deep oxidation of benzene

Nanosized CuO–Zr x Ce 1 − x O y (0 ≤ x ≤ 0.5) solid solution oxide catalysts were prepared using a co-precipitation method followed with ethanol supercritical drying and calcination at 800 °C. The structural characteristics and redox behaviors were investigated by using X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, and temperature programmed reduction (H 2-TPR). The results revealed that the as-prepared samples not only possessed large specific surface areas, small crystal sizes and high homogeneity of constituents, but also exhibited high catalytic activity for the complete oxidation of benzene. The structural properties and the catalytic performances of the aerogel catalysts strongly depended on the Ce/Zr molar ratio. The most highly active catalyst, CuO–Ce 0.5Zr 0.5O y , could oxidize benzene at 300 °C and kept robust in a 50 h long-term duration test, implying it would be a good and cost-effective alternative to noble metal catalysts for the complete oxidation of VOCs at lower temperatures.

Water assisted synthesis of clean single-walled carbon nanotubes over a Fe 2O 3/Al 2O 3 binary aerogel catalyst

Water assisted synthesis of clean single-walled carbon nanotubes over a Fe 2O 3/Al 2O 3 binary aerogel catalyst

The Stability of Platinum–Carbon Aerogel Catalysts upon Repeated Potential Cycles

A sense of stability: The stability of carbon-supported platinum catalysts at high potentials is important for the commercialization of fuel cells for homes and cars. The electrochemical active surface (EAS) area of a Pt-C aerogel catalyst was found to increase up to 500 cycles, in contrast to that for the so-called Tanaka catalyst which decreases with the number of repeated potential cycles.

Aerogel VO x/MgO catalysts for oxidative dehydrogenation of propane

VO x /MgO aerogel catalysts were synthesized using three different preparation methods: by mixing the aerogel MgO support with dry ammonium vanadate, by vanadium deposition from a precursor solution in toluene, and by hydrolysis of a mixture of vanadium and magnesium alkoxides followed by co-gelation and supercritical drying. The latter aerogel technique allowed us to synthesize mixed vanadium–magnesium hydroxides with the surface areas exceeding 1300 m 2/g. The synthesized catalysts were studied by a number of physicochemical methods (XRD, Raman spectroscopy, XANES and TEM). A common feature of all synthesized samples is the lack of V 2O 5 phase. In all cases vanadium was found to be a part of a surface mixed V–Mg oxide (magnesium vanadate), its structure depending on the synthesis method. The VO x /MgO mixed aerogel sample had the highest surface area 340 m 2/g, showed higher catalytic activity and selectivity in oxidative dehydrogenation of propane compared to the catalysts prepared by impregnation and dry mixing. The addition of iodine vapor to the feed in 0.1–0.25 vol.% concentrations was found to increase to propylene yield by 40–70%.

Alkylsilylated Gold Loaded Magnesium Oxide Aerogel Catalyst in the Oxidation of Styrene

Aerogel-prepared magnesium oxide (A-MgO) was modified by attachment of gold particles. Subsequent modification of its surface with alkylsilylation of n-octadecyltrichlorosilane and chlorotrimethylsilane exhibits a high catalytic activity for oxidation of styrene with tert-butyl hydroperoxide in liquid phase. Catalytic results show that the introduction of gold on the surface of A-MgO gives a higher catalytic activity in styrene oxidation compared to titanium dioxide. The effect of surface modification of A-MgO was studied with XRD, UV-Vis DR, FTIR and nitrogen adsorption analysis.

Removal of Elemental Mercury (Hg(0)) by Nanosized V2O5/TiO2 Catalysts

Novel reactive nanosized V2O5/TiO2 catalysts (aerogel, xerogel, and impregnated catalysts) for the removal of Hg(O) and their synthesis methods have been introduced in this research. Aerogel catalyst has the highest surface area among the catalysts synthesized in this research and contained reactive monovanadates on its surfaces resulting in higher reactivity for the Hg(O) removal than impregnated and selective catalytic reduction catalysts. XPS analyses on the surfaces of nanosized catalysts after the removal of Hg(O) suggest that adsorbed Hg(O) oxidatively transformed to HgO by surface vanadates (mono- and poly vanadates), consistent with the Mars-Maessen mechanism. Early column breakthrough has been observed at temperatures above 300 degrees C due mainly to the desorption of Hg(O) from the catalyst surfaces. The decrease in Hg(O) concentration and increase in catalyst content in a column reactor have increased the removal of Hg(O), indicating that the removal is a heterogeneous surface-limited reaction. At 400 degrees C, the catalysts under air flow have shown a higher Hg(O) removal because gas-phase oxygen from the flow could provide an oxygen-rich environment for producing more oxidized vanadate species on their surfaces. No significant difference in the Hg(O) removal between different gas types (nitrogen and air) has been observed at 100 degrees C.

Studies on the Structure and Electrochemical Performance of Pt/Carbon Aerogel Catalyst for Direct Methanol Fuel Cells

Platinum catalysts supported on carbon aerogel (Pt/CA) with different pore structures were synthesized by an ethylene glycol liquid-phase reduction method. The Pt/CA catalysts were characterized by X-ray diffraction, transmission electron microscopy, and cyclic voltammetry. The effect of the pore structures of CAs on the electrochemical performance of Pt/CA for a direct methanol fuel cell was also analyzed. Experimental results indicate that the Pt/CA catalysts synthesized have achieved an extremely uniform Pt dispersion. The size of the Pt nanoparticles is 2.3 nm. The electrochemical active area and mass activity of the Pt/CA catalyst prepared on a CA with R/C (resorcinol/cetyltrimethylammonium bromide molar ratio) = 125 reach 87.4 m2 g−1 and 395.3 A g−1, respectively, which are significantly higher than those of commercial Pt/C catalysts.

Ru/Cabon Aerogel触媒を用いるH2雰囲気下でのCO選択酸化反応

Ru/Cabon Aerogel触媒を用いるH2雰囲気下でのCO選択酸化反応