Various carbon materials differing in surface area, nanostructure and grade of graphitization were used to support ruthenium catalysts for the steam reforming of glycerol. Activity measurements ...performed in a traditional reactor indicated that the reaction is structure sensitive and the initial activity for hydrogen production of the catalysts decreases with decreasing Ru particle size. Among the most active catalysts, stability in reaction was first related with the graphitization degree of the support. Ru-nanotubes was the most stable with the time on stream, whereas Ru-activated carbon underwent an abrupt deactivation due to carbon gasification. Ru-graphite, the other catalyst with highest initial activity, suffered from a slow deactivation, most likely due to coke formation derived from the olefins produced during reaction. The use of a membrane reactor prevents the formation of these undesired compounds, enhancing the hydrogen yield and rendering a very stable catalyst.
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•New catalysts for controlling the selectivity in the ethanol transformation into acetaldehyde.•Interaction of Ru metallic precursors with KL zeolite.•Molecular designing of ...catalytically active surfaces sites.•Modifications of the KL acid-base surface properties during the incorporation of Ru nanoparticles.
Four Ru/KL-zeolite catalysts containing 2wt% of Ru were prepared from Ru3(CO)12, RuNO(NO3)3, Ru(C5H7O2)3 and RuCl3 precursors. The evolution of electronic structure and local chemical environment of ruthenium in the samples named Ru(c)/KL, Ru(n)/KL, Ru(acac)/KL and Ru(Cl)/KL was studied by in-situ XANES during temperature-programmed reduction. Also by CO chemisorption and transmission electron microscopy (TEM) the sizes of the Ru nanoparticles were determined. Activity and selectivity of the catalysts were evaluated in the transformation of ethanol, under kinetic conditions, in a fixed bed flow reactor, at 523K–573K. Characterization of the samples shows that metal dispersion values follow the trend Ru(c)/KL≥Ru(n)/KL > Ru(Cl)/KL≥Ru(acac)/KL. Activity of the catalysts is in the order Ru(acac)/KL≥Ru(c)/KL>Ru(n)/KL≥Ru(Cl)/KL. The TOF values, however, are in the same order of magnitude for all the samples, nonetheless the Ru(Cl)/KL catalyst has slightly lower TOF at all the reaction temperatures. Selectivity towards the dehydrogenation product, acetaldehyde, follows the trend Ru(c)/KL>Ru(n)/KL=Ru(acac)/KL>>Ru(Cl)/KL, this being 100% for Ru(c)/KL. Selectivity towards acetaldehyde is highly diminished for Ru(Cl)/KL in favor of the dehydration products, diethyl ether and ethylene, the higher the decrease the higher the temperature. The catalytic results are related to the properties of the surface metal species and their location in the zeolite framework, as well as to their surroundings, as evidenced from the results of the characterization measurements, which are in turn influenced by the different nature of the metal precursor.
► Enthalpies of CO adsorption on Ru/carbon catalysts ex-RuCl3 are diminished. ► Oxygen groups at activated carbon surfaces do not modified the Ru surface sites. ► Homogeneity of surface site ...distributions is higher for Ru-CNTs than for Ru-HSAG. ► A possible poisoning effect of Cl species over Ru surface sites has been detected.
Chemisorption of CO combined with microcalorimetry has been applied to study the nature, number and adsorption strength distribution of surface sites exposed by carbon-supported Ru catalysts. A comparative analysis of the CO chemisorption on different Ru catalysts, prepared using two different metal precursors, RuCl3·xH2O and Ru(NO)(NO3)3, has been carried out. An activated carbon and the corresponding derivative where oxygen surface groups were incorporated, as well as carbon nanotubes and a high surface area graphite, were used as catalytic supports. Based on previous temperature programmed reduction studies, all the catalysts were reduced under hydrogen flow at 523K or at 573K. The CO adsorption differential enthalpy profiles show that Ru(NO)(NO3)3 precursor produces more homogeneous surface site distribution in the Ru nanocrystals, in comparison with those prepared from RuCl3, as well as higher values of enthalpies in the medium range of coverage. As a possible explanation for this effect, residual chloride species remaining after reduction treatment in the ex-chloride catalysts, that can be anchored to the Ru nanoparticles weakening the CO adsorption, have been considered. This behavior occurs for the three studied carbon supports. On the other hand, the oxygen surface groups incorporated on the activated carbon seem not to modify the CO adsorption properties of the catalysts, independently of the precursor employed.
•We report a new low temperature solvothermal method to synthesize MoS2 nanomaterials.•The morphology of MoS2 solids depends on the electrolyte concentration (ionic strength).•Formation of MoS2 ...nanospheres occurs at low electrolyte concentration.•At higher electrolyte amounts, MoS2 nanotubes are preferentially formed.
Different MoS2 nanostructures have been obtained following an innovative one-step solvothermal method by changing the concentration and type of the electrolyte while avoiding the use of surfactant. It was found that the chemical nature of the studied electrolyte ((NH4)2CO3 or KCl) do not significantly affect the morphology and structure of the obtained MoS2 nanomaterials. Nevertheless, increasing the electrolyte concentration yields to a remarkable modification of the morphology of the resulting MoS2 from nanospheres to worm-shaped then finally to nanotubes. All the obtained nanomaterials were characterized by X-ray diffraction, (XRD), transmission electron microscopy (TEM, HRTEM), Fourier transformation infra-red spectroscopy (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS).
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•Activated carbon, graphite, carbon nanotubes and KL-zeolite as Ru-catalyst supports for hydrogenolysis of glycerol.•Ruthenium supported on activated carbon produces mainly ethylene ...glycol.•Reduction of ruthenium generates Brönsted acid sites in the KL-zeolite which enhance the selectivity toward 1,2-PDO.•Graphite and carbon nanotubes promote formation of Ruδ− species and favor largely production of methane and 1,2-propanediol.
Supported ruthenium catalysts were prepared by incipient wetness impregnation of three different carbon materials: activated carbon (AC), high surface area graphite (HSAG) and multiwalled carbon nanotubes (CNT). Another catalyst was prepared by treating KL zeolite with RuCl3·xH2O in aqueous solution. All these samples were characterized by temperature programmed reduction (TPR), CO chemisorption coupled with microcalorimetry and transmission electron microscopy (TEM). The reduced catalysts were tested in the hydrogenolysis of glycerol in the liquid phase, under a reaction pressure of 8MPa and isothermally at the reaction temperature of 453K. The CO microcalorimetry measurements evidenced that electron donor properties of graphite and carbon nanotubes promote formation of electron-rich metal species (Ruδ−) in Ru/HSAG and Ru/CNT catalysts, which not only favors formation of 1,2-propanediol from glycerol but also enhances the successive CC cleavage, with formation of undesired products, mainly methane. For Ru/KL the occurrence of Brönsted acid sites, resulting of the reduction of the chlorinated ruthenium species bonded to the zeolite framework, was verified by temperature programmed desorption (TPD) of NH3. Furthermore, observations by TEM of the Ru/KL catalyst showed an important population of metal nanoparticles lower than 1nm, part of which exhibits electron deficient character as indicated by the CO microcalorimetry. As a consequence, the transformation of glycerol into 1,2-PDO over Ru/KL seems to be promoted through formation of the intermediate acetol on acid sites, while for the scarcely acid Ru-carbon catalyst conversion of glycerol occurs mainly on metal sites, ethylene glycol being the preferred hydrogenolysis product.
In this work we report a simple procedure for synthesis of Cu and Pd catalysts supported on high surface area graphite (HSAG) by wetness impregnation technique, and further generation of metal ...nanoparticles using NaBH4 as reducing agent. The catalysts have been tested in the reduction of 4-nitrophenol to 4-aminophenol, at room temperature, in presence of NaBH4 as hydrogen source. Both Cu and Pd catalysts exhibited exceptionally high catalytic activity with the total degradation of 4-nitrophenol in less than 45 s. Taking into account that Cu is cheaper than Pd, we focused our investigation on studying the catalytic properties of Cu nanoparticles supported over two lab prepared graphene-materials (one N-doped and other undoped) and the commercial HSAG. The maximum catalytic activity was obtained with Cu supported on undoped graphene due to the combination of two parameters: small metal particle size and the unique properties of graphene generated by its electron transference ability. However, recyclability of both Cu/graphene-materials fell after 5 consecutive runs, while Cu/HSAG displayed high stability even after 10 cycles. In order to rationalize these findings, it is postulated that copper nanoparticles in Cu/HSAG are located at the edges of the graphite layers, where a stronger metal-support interaction takes place.
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•The nitric acid treatment creates oxygen groups on the activated carbon surface.•Nitroxyl nitrate precursor generates oxygenated groups at the surface of Ru/C catalysts.•Carboxyl groups promote ...formation of 1,2-propanediol from glycerol.•Oxygenated groups enhance the CC and the CO cleavage of glycerol.•Formation of ethylene glycol is inhibited by effect of carboxyl acid sites.
Ruthenium catalysts supported on activated carbons, original (AC) and treated with nitric acid (AC-Ox) were prepared by incipient wetness impregnation from either chloride (Cl) or nitroxyl nitrate (n) precursors. These catalysts were characterized by TG, XPS, TEM, TPD-MS and CO adsorption microcalorimetry and evaluated in the hydrogenolysis of glycerol in the liquid phase, at 453K and 8MPa. Studies by TEM show that ruthenium particles supported on AC-Ox are larger than on AC, without any effect of the nature of the metal precursor. However, adsorption of CO on the ex-chloride catalysts is inhibited in comparison with that of the ex-nitroxyl nitrate catalysts. Catalysts characterization by TG, TPD-MS and XPS reveals that the nitric acid treatment and the nitroxyl nitrate precursor generate oxygenated groups on the carbon surface, which provide acid properties to the catalysts, although they are partly destroyed during the reduction treatment applied to the catalysts. The sequence of the overall TOF, Ru(Cl)/AC<Ru(n)/AC<Ru(Cl)/AC-Ox≈Ru(n)/AC-Ox, reasonably parallels the population increase of surface acid groups. Participation of the COOH groups in the transformation of glycerol into 1,2-propanediol is verified by using the admixture Ru(Cl)/AC+AC-Ox as catalyst. In this case, since AC-Ox was not thermally treated and no loss of oxygenated groups occurred, TOF and selectivity toward 1,2-propanediol improve in comparison with those of the more active catalysts.
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•Symmetric and asymmetric hollow fibre reactors have been developed: SHFR and AHFR.•SHFR and AHFR performance was compared with that in a FBR during the GSR reaction.•The catalyst ...particle size was determined by the pore dimension of the finger-like and sponge-like regions.•SHFR and AHFR allow for an enhanced H2 production.•AHFR showed lager glycerol conversion than in SHFR.
In this study, NiO/MgO/CeO2 catalysts with Ni content from 5% to 30% were synthetized by sol–gel method and tested in a fixed-bed reactor (FBR) in the glycerol steam reforming (GSR) reaction. The catalysts were characterized by N2 adsorption isotherms at −196°C (SBET), X-ray diffraction (XRD), H2 temperature programmed reduction (H2-TPR), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX). The 20% NiO/MgO/CeO2 catalyst, which showed the highest catalytic activity in GSR reaction, was selected to be deposited in the finger-like region of the asymmetric Al2O3 hollow fibre and the sponge-like region of the symmetric Al2O3 hollow fibre in the development of the asymmetric hollow fibre reactor (AHFR) and symmetric hollow fibre reactor (SHFR), respectively. The impregnated ceramic substrates were characterized by scanning electron microscopy (SEM), EDX and TEM. The performances of the AHFR and SHFR were compared with that in a conventional FBR during the GSR reaction. Both AHFR and SHFR were operating at “dead-end” configuration at a temperature range from 250°C to 550°C, atmospheric pressure and in a reactant mixture of steam and glycerol (16:1 molar ratio). At 550°C the glycerol conversion in the AHFR and SHFR was 70% and 46%, respectively, which are 5 and 2 times higher than that obtained in the FBR. The different performances of the AHFR and SHFR could be explained due to the unlike catalyst particle size deposited in the asymmetric and symmetric substrates, 8nm and 3nm, respectively.
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► Graphite, zirconia and KL as supports of Ru-catalysts for hydrogenation of citral. ► Graphite promotes formation of Run− species and favors formation of geraniol and nerol. ► ...Stabilization of Ru0Zrn+ species in Ru/zirconia activates the carbonyl group of citral. ► Geometrical effects within Ru/KL enhance the selectivity toward unsaturated alcohols. ► KL-zeolite improves formation of unsaturated alcohols more than graphite and zirconia.
Ruthenium catalysts supported on KL zeolite, ZrO2 and graphite, prepared by incipient wetness impregnation, were characterized by N2 adsorption, H2 chemisorption, TEM and CO adsorption microcalorimetry and tested in the selective hydrogenation of citral in the liquid phase, at 5MPa and 323K. Characterization studies reveal that graphite promotes formation of electron-rich metal species (Run−) that difficult hydrogenation of the conjugated CC double bond of citral and indirectly favor the production of geraniol and nerol. For Ru/ZrO2 catalyst, the Ru↔ZrO2 interaction at the interface, with formation of Ru0Zrn+ species as evidenced by the TEM and CO adsorption microcalorimetry measurements, activates the carbonyl group of citral and enhances the selectivity towards unsaturated alcohols. Characterization measurements of Ru/KL evidence that metal nanoparticles placed inside of the zeolite channels block part of ruthenium loading and hinder the free transit of citral through the channels, thus reducing the overall hydrogenation activity of the catalyst. Furthermore, this steric hindrance impedes adsorption the conjugated CC double bond of citral and forces hydrogenation of the carbonyl group in terminal position. On the other hand, the fact that active sites on occluded particles are more resistant to poisoning than those easily accessible to citral enhances the selectivity towards geraniol and nerol, as the reaction time increases. The comparative analysis of the results indicates that, in order to increase the selectivity towards unsaturated alcohols, geometrical effects derived from the size, shape and location of ruthenium particles in the KL structure are more effective than the electronic modifications induced by graphite or zirconia on the ruthenium particles.
Modifications of texture and surface properties of a commercial activated carbon (Norit GF-40) were performed by several treatments in order to study their effects on the selective adsorption of ...nitromethane from nitromethane/water vapor mixtures. Characterisation of the samples by nitrogen adsorption and thermal analysis showed that HNO
3
treatments produce important losses of porosity and surface area, accompanied of an increase of oxygenated functional groups on the surface of carbon, which are progressively removed by heating at temperatures between 573 and 1073 K. All this leads to a drastic decrease of the adsorption capacity per gram of adsorbent with respect to the raw carbon, which offers, on the other hand, the best adsorptive performance. Oxidation by H
2
O
2
does not practically affect its textural properties and introduces an important amount of oxygen functional groups at the surface, but changes in the adsorptive properties of carbon are insignificant. Sample oxidised by H
2
O
2
and subsequently treated by diethylentriamine shows a decrease in adsorption capacity, without any relevant loss of surface area. The raw carbon treated at high temperature that exhibits the highest surface area and where surface functional groups are absent, showed the greatest adsorption capacity for nitromethane, being much more selective for nitromethane than for water, in nitromethane-water mixtures. Adsorption capacity values for nitromethane on the different samples are related to the extent of the surface area, while water vapour adsorption seems to depend on the population of functional groups at the surface, which may work as adsorption sites.
