Silicone microreactors for the photocatalytic generation of hydrogen

A silicone microreactor with 500 mu m-width microchannels coated with a Au/TiO2 photocatalyst was manufactured and tested for the photocatalytic generation of hydrogen from gaseous water-ethanol mixtures under dynamic conditions. The manufacture of the microreactor included the fabrication of a polylactic acid (PLA) mold with a 3D printer and casting with polydimethylsiloxane (PDMS) prepolymer. After curing, the silicone microreactor was peeled off and the microchannels were coated with a Au/TiO2 photocatalyst prepared by impregnation of preformed Au nanoparticles over TiO2, and sealed with a thin silicone cover. The microreactor was tested at room temperature and atmospheric pressure under different operational conditions (photon irradiance, residence time, photocatalyst loading, and water ethanol ratio). Hydrogen production rates of 5.4 NmL W-1 h(-1) were measured at a residence time of 0.35 s using a H2O:C2H5OH molar ratio of 9:1, a photocatalyst load of 1.2 mg cm(-2) and a UV irradiance (365 nm) of 1.5 mW cm(-2) achieving an apparent quantum efficiency of 9.2%. The photogeneration of hydrogen with commercial bioethanol was also tested. A long-term photocatalytic test of two days revealed a stable hydrogen photoproduction rate. The use of silicone microreactors represents an attractive and customizable solution for conducting photochemical reactions for producing hydrogen at low cost. (C) 2016 Elsevier B.V. All rights reserved.Postprint (published version

L'edat d'or de l'or

A finals del segle passat, l'ús de l'or com a catalitzador tenia un interès escàs a causa de la seva reconeguda manca d'activitat. L'investigador japonès Masatake Haruta, però, va demostrar que, quan l'or es presenta dividit molt finament, aleshores la seva capacitat catalítica augmenta d'una manera extraordinària. Aquest article, amb una intencionalitat divulgativa clara, explica que la immobilització, tant de nanopartícules com d'àtoms de l'or aïllats sobre un suport insoluble, incrementa enormement el potencial catalític d'aquests sistemes en nombroses reaccions químiques d'interès industrial

Els dendrímers o l'estètica molecular

[cat] En aquest article s'explica què s'entén per dendrímer i se subratllen els trets estructurals més característics d'aquestes noves supramolècules, les estratègies sintètiques més habituals i les maneres de funcionalitzar-les per tal de fer-ne ús en diferents i variades aplicacions, com són l'encapsulació de petites espècies químiques, la catàlisi, la formació de nanopartícules i la síntesi de cristalls líquids, entre d'altres.[eng] This article explains the term dendrímer and emphasises the most characteristic structural features of these new supramolecules. It also reports the most usual synthetic strategies and the ways in which these molecules are funtionalised for use in a wide range of applications. These include the encapsulation of small chemical species, catalysis, the formation of nanoparticles and the synthesis of liquid crystals

Hydrogen photoproduction from ethanol-water mixtures over Au-Cu alloy nanoparticles supported on TiO2

Au/TiO2, Au0.75Cu0.25/TiO2, Au0.5Cu0.5/TiO2 and Au0.25Cu0.75/TiO2 photocatalysts prepared from pre-formed Au and Au-Cu alloy nanoparticles of controlled composition and size were loaded over ceramic honeycombs (2 mg cm(-2)) and tested in an optical fiber photoreactor illuminated with UV LEDs (2.6 mW cm(-2)) to continuously produce hydrogen from water and ethanol mixtures in gas phase at W/F = 4 g min L-1 and 298 K (where W is the weight of the catalyst and F is the flow rate). The photocatalytic honeycombs were characterized by high resolution transmission electron microscopy, high-angle annular dark-field imaging, energy dispersive X-ray, X-ray photoelectron spectroscopy, and UV-Vis spectroscopy. The yield of hydrogen generation was Au0.75Cu0.25/TiO2 > Au0.5Cu0.5/TiO2 similar to Au/TiO2 > Au0.25Cu0.75/TiO2 >> bare TiO2, thus demonstrating that the addition of small quantities of copper to conventional TiO2-supported gold photocatalysts promotes the photocatalyic activity, likely by providing effective charge transfer between Au and Cu in the alloy nanoparticles.Postprint (published version

CO oxidation and COPrOx over preformed Au nanoparticles supported over nanoshaped CeO2

Au/CeO2 (0.25% wt. Au) catalysts were prepared by anchoring preformed Au nanoparticles over ceria polycrystals, cubes and rods and tested in the oxidation of CO and COPrOx. The use of preformed Au nanoparticles assured a constant Au particle size (ca. 5 nm by HRTEM) for all samples, which allowed to a precise assessment of the effect of the morphology of nanoshaped ceria on catalytic activity. The catalytic performance of the Au/CeO2-rods was much better than that of the Au/CeO2-polycrystals and Au/CeO2-cubes both in the oxidation of CO and COPrOx reactions. The Au/CeO2-rods exhibited the highest amount of oxidized Au and Ce(III) species by XPS, whereas in the Au/CeO2-cubes gold was totally metallic and the amount of Ce(III) was minimum. An intermediate situation was encountered in the Au/CeO2-polycrystals. Considering the differences in the oxidation states ofAu and Ce and the factthat all samples were prepared with preformed metallic Au nanoparticles of the same size, the results indicate that the intrinsic nature of the different ceria surfaces exerts a prominent role in the gold-ceria interaction and in the electron density transfer from Au to Ce, which in turn has a strong effect on catalytic activity. Gold nanoparticles strongly interact with CeO2-{1 1 0} surfaces with respect to CeO2-{1 1 1} and CeO2-{1 0 0}, even when Au nanoparticles are prepared separately and simply deposited by impregnation.Postprint (author's final draft

Effect of temperature on the gas-phase photocatalytic H2 generation using microreactors under UVA and sunlight irradiation

The effect of temperature on the photocatalytic hydrogen generation from a gaseous water-ethanol mixture has been tested in a silicone microreactor containing nine microchannels of 500 µm (width) × 1 mm (depth) × 47 mm (length) coated with Au/TiO2photocatalyst under UVA irradiation. Kinetic analyses have indicated that the hydrogen production rate follows the Langmuir-Hinshelwood model. The effect of temperature from 298 to 348 K has been determined by thermodynamic parameters, such as enthalpy (¿H¿), entropy (¿S¿) and Gibbs free energy (¿G¿) of activation, using the transition state theory (TST). The apparent rate constants (kapp) are higher by increasing the temperature, and the activation energy has been determined to be 24 ± 1 kJ·mol-1. In order to evaluate if solar concentration could be used to enhance the photoproduction of hydrogen, the reaction has also been conducted under direct sunlight using a solar concentrator of about 1 m in diameter. Finally, the microreactor has been scaled out by a factor of ca. 10 to a device containing thirty-two microchannels of 500 µm (width) × 1 mm (depth) × 117.5 mm (length). The specific (i.e. per irradiated area of catalyst) hydrogen production rates of both microreactors using sunlight are very similar suggesting that this technology could lead to viable solar hydrogen production.Postprint (author's final draft

CO Total and preferential oxidation over stable Au/TiO2 catalysts derived from preformed au nanoparticles

CO preferential oxidation (PROX) is an effective method to clean reformate H2 streams to feed low-temperature fuel cells. In this work, the PROX and CO oxidation reactions were studied on preformed Au nanoparticles (NPs) supported on TiO2 anatase. Preformed Au NPs were obtained from gold cores stabilized by dodecanethiols or trimethylsilane-dendrons. A well-controlled size of ca. 2.6 nm and narrow size distributions were achieved by this procedure. The catalysts were characterized by high-resolution transmission electron microscopy and ex situ and in situ X-ray photoelectron spectroscopy (XPS). The XPS results showed that the preformed Au NPs exhibited high thermal stability. The different ligand-derived Au catalysts, as well as a conventional gold catalyst for comparison purposes, were loaded onto cordierite supports with 400 cells per square inch. The activity and selectivity of the samples were evaluated for various operation conditions. The catalyst prepared using dodecanethiol-capped Au NPs showed the best performance. In fact, CO conversions of up to 70% at 40% CO2 selectivity and 90% O2 conversion were observed operating at 363 K in H2-rich atmospheres. The performance of the best catalysts was subsequently tested on stainless steel microreactors. A 500-hour stability test was carried out under a real post-reformate stream, including 18 vol.% CO2 and 29 vol.% H2O. A mean CO conversion of ca. 24% was measured for the whole test operating at 453 K and a gas hourly space velocity (GHSV) of 1.3 × 104 h−1. These results reveal our dodecanethiol-and carbosilane-derived Au catalysts as extremely promising candidates to conduct a PROX reaction while avoiding deactivation, which is one of the major drawbacks of Au/TiO2 catalysts.Fil: Divins, Núria J.. Universidad Politécnica de Catalunya; EspañaFil: Lopez, Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Planta Piloto de Ingeniería Química. Universidad Nacional del Sur. Planta Piloto de Ingeniería Química; ArgentinaFil: Angurell, Inmaculada. Universidad de Barcelona. Facultad de Química; EspañaFil: Neuberg, Stefan. Fraunhofer Institute of Microengineering and Microsystems; AlemaniaFil: Zapf, Ralf. Fraunhofer Institute of Microengineering and Microsystems; AlemaniaFil: Kolb, Gunther. Fraunhofer Institute of Microengineering and Microsystems; AlemaniaFil: Llorca Piqué, Jordi. Universidad Politécnica de Catalunya; Españ

CO total and Preferential Oxidation over stable Au/TiO2 catalysts derived from preformed Au nanoparticles

CO preferential oxidation (PROX) is an effective method to clean reformate H2 streams to feed low-temperature fuel cells. In this work, the PROX and CO oxidation reactions were studied on preformed Au nanoparticles (NPs) supported on TiO2 anatase. Preformed Au NPs were obtained from gold cores stabilized by dodecanethiols or trimethylsilane-dendrons. A well-controlled size of ca. 2.6 nm and narrow size distributions were achieved by this procedure. The catalysts were characterized by high-resolution transmission electron microscopy and ex situ and in situ X-ray photoelectron spectroscopy (XPS). The XPS results showed that the preformed Au NPs exhibited high thermal stability. The different ligand-derived Au catalysts, as well as a conventional gold catalyst for comparison purposes, were loaded onto cordierite supports with 400 cells per square inch. The activity and selectivity of the samples were evaluated for various operation conditions. The catalyst prepared using dodecanethiol-capped Au NPs showed the best performance. In fact, CO conversions of up to 70% at 40% CO2 selectivity and 90% O2 conversion were observed operating at 363 K in H2-rich atmospheres. The performance of the best catalysts was subsequently tested on stainless steel microreactors. A 500-hour stability test was carried out under a real post-reformate stream, including 18 vol.% CO2 and 29 vol.% H2O. A mean CO conversion of ca. 24% was measured for the whole test operating at 453 K and a gas hourly space velocity (GHSV) of 1.3 × 104 h−1. These results reveal our dodecanethiol- and carbosilane-derived Au catalysts as extremely promising candidates to conduct a PROX reaction while avoiding deactivation, which is one of the major drawbacks of Au/TiO2 catalysts

Magnetite-supported palladium single-atoms do not catalyse the hydrogenation of alkenes but small clusters do

The activity of supported noble metal catalysts strongly depends on the particle size. The ultimate small-size limit is the single-atom catalyst (SAC), which maximizes the catalytic efficiency in the majority of the examples. Here, we investigate the catalytic behavior of Pd SACs supported on magnetite nanoparticles and we unambiguously demonstrate that Pd SACs are absolutely inactive in the hydrogenation of various alkene substrates. Instead, Pd clusters of low atomicity exhibit outstanding catalytic performance.Postprint (author's final draft