Jordi Llorca

The effect of gold loading and particle size on photocatalytic hydrogen production from ethanol over Au/TiO2 nanoparticles

Catalytic hydrogen production from renewables is a promising method for providing energy carriers in the near future. Photocatalysts capable of promoting this reaction are often composed of noble metal nanoparticles deposited on a semiconductor. The most promising semiconductor at present is TiO₂. The successful design of these catalysts relies on a thorough understanding of the role of the noble metal particle size and the TiO₂ polymorph. Here we demonstrate that Au particles in the size range 3-30 nm on TiO₂ are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3-12 nm range. The high hydrogen yield observed makes these catalysts promising materials for solar conversion.

Nanofaceted PdO Sites in PdCe Surface Superstructures: Enhanced Activity in Catalytic Combustion of Methane†

An open superstructure: A Pd/CeO2 catalyst prepared by solution combustion synthesis is three to five times more active for CH4 combustion than the best conventional palladium-based systems. The catalyst contains an ordered, stable Pd-O-Ce surface superstructure (see picture; cyan arrow is a square-planar Pd site, red arrow is an undercoordinated O atom) and is an example of ultra-highly dispersed, stable PdO within an oxide carrier.

The effect of doping CeO2 with zirconium in the oxidation of isobutane

The preparation of a CeO2ZrO2 mixed oxide of composition Ce0.8Zr0.2O2 with its characterization and its use as a catalyst in the oxidation of isobutane is reported, and compared with the reactivity of pure CeO2. The formation of an homogeneous, fluorite-type solid solution is observed; the material is characterized by a higher reducibility and a higher capacity of oxygen uptake compared to pure CeO2. The catalytic activity in the oxidation of isobutane is not greatly affected by the introduction of ZrO2, but the selectivity to isobutene is significantly enhanced. This enhancement has been attributed to an increased oxygen mobility and to an increased activity for the Ce4+/Ce3+ redox couple, occurring as a consequence of the creation of surface and bulk defects in the solid solution, induced by the introduction of the smaller Zr4+ cation in the fluorite lattice.

In situ magnetic characterisation of supported cobalt catalysts under steam-reforming of ethanol

Various supported cobalt catalysts (Co/MgO, Co/Al2O3, Co/SiO2, Co/TiO2, Co/V2O5, Co/ZnO, Co/La2O3, Co/CeO2 and Co/Sm2O3), were characterised by magnetic measurements under ethanol steam-reforming conditions, in situ diffuse reflectance infrared spectroscopy and UV-Vis-NIR diffuse reflectance spectroscopy. Their magnetic behaviour was analysed as a function of reaction temperature and hydrogen treatment and was related to the cobalt species in the catalysts and to their catalytic behaviour in the steam-reforming of ethanol. The catalysts were prepared from Co2(CO)8 and used without previous calcination or reduction. All samples showed a paramagnetic or diamagnetic behaviour before reaction. The catalysts that performed well in the steam-reforming of ethanol presented under reaction conditions, metallic (ferromagnetic) cobalt particles and oxidised cobalt species. An easy exchange between small metallic cobalt particles (produced under H2 or under ethanol steam-reforming conditions) and oxidised cobalt species (produced under ethanol steam-reforming conditions) was found in these catalysts.

influence of the support on surface rearrangements of bimetallic nanoparticles in real catalysts

Supported nanoparticles make the reaction faster Several techniques now allow surface structures used as catalysts to be probed during exposure to reactive gases, as opposed to under vacuum conditions. Divins et al. used near-ambient-pressure x-ray photoelectron spectroscopy to compare the effect of reaction gases on unsupported palladium-rhodium nanoparticles versus ones on a reducible cerium oxide support. For the reaction of ethanol with steam to produce hydrogen, the supported nanoparticles were more reactive and less prone to reduction and surface rearrangement. Science, this issue p. 620 A cerium oxide support rendered palladium-rhodium nanoparticles more reactive and harder to reduce under reaction conditions. Catalysts used for heterogeneous processes are usually composed of metal nanoparticles dispersed over a high–surface-area support. In recent years, near-ambient pressure techniques have allowed catalyst characterization under operating conditions, overcoming the pressure gap effect. However, the use of model systems may not truly represent the changes that occur in real catalysts (the so-called material gap effect). Supports can play an important role in the catalytic process by providing new active sites and may strongly affect both the physical and chemical properties of metal nanoparticles. We used near-ambient pressure x-ray photoelectron spectroscopy to show that the surface rearrangement of bimetallic (rhodium-palladium) nanoparticles under working conditions for ethanol steam reforming with real catalysts is strongly influenced by the presence of a reducible ceria support.

A Phenomenological Study of the Metal-Oxide Interface : The Role of Catalysis in Hydrogen Production from Renewable Resources

The truth about Cats: The metal-oxide interface of a Pd-Rh/CeO{sub 2} catalyst was studied in the context of developing active, selective and durable solid catalytic materials for the production of hydrogen from renewables. The presence of a stable contact between finely dispersed transition-metal clusters (Pd and Rh) on the nanoparticles of the CeO{sub 2} support leads to a highly active and stable catalyst for the steam reforming of ethanol.

A novel and simple route to catalysts with a high oxygen storage capacity: the direct room-temperature synthesis of CeO2–ZrO2 solid solutions

High-energy mechanical alloying of pure CeO2 and ZrO2 at room temperature results in the formation of a single phase Ce1–xZrxO2 fluorite structured solid solution in all the examined composition ranges; the compounds are characterized by a high oxygen storage capacity and an excellent reducibility.

Direct production of hydrogen from ethanolic aqueous solutions over oxide catalysts

Steam-reforming of ethanol over ZnO gives highly effective production of CO-free H2: 5.1 mol of H2 per mol of reacted ethanol is formed at 723 K under 100% ethanol conversion.

Fast and efficient hydrogen generation catalyzed by cobalt talc nanolayers dispersed in silica aerogel

Cobalt talc nanolayers dispersed in silica aerogel constitute an active nanocomposited material with outstanding catalytic properties for the generation of hydrogen by ethanol steam reforming at low temperature. Delamination of talc particles into individual nanolayers of ca. 1.8 nm thick readily occurs under the reforming conditions, which results in a strong enhancement of the exposed catalytic active area. The presence of aerogel assures the immobilization of the talc nanolayers resulting from the delamination while maintaining an excellent mass transfer of products and reactants to the surface of the talc nanolayers. A fast and reversible surface activation for the steam reforming of ethanol occurs at 580–590 K under the reforming conditions. Magnetic characterization and in situ X-ray photoelectron spectroscopy (XPS) show the non-reversible formation of metal cobalt ensembles during activation, which escape HRTEM detection. This material appears as a good candidate for on-board hydrogen generation from ethanol–water mixtures for mobile and portable fuel cell applications.

Cobalt hydrotalcite for the steam reforming of ethanol with scarce carbon production

Co/Mg/Al hydrotalcite-type catalysts have been loaded onto ceramic honeycombs and tested in the ethanol steam reforming (ESR) reaction for producing hydrogen under practical conditions. In contrast with previously reported cobalt-based systems, the formation of carbon was scarce. This has been ascribed to the unique formation of traces of metallic cobalt particles under reaction conditions, as inferred from HRTEM, magnetic measurements, and in situ X-ray photoelectron spectroscopy (XPS) experiments. The best catalytic performance has been exhibited by the catalyst derived from the hydrotalcite with a Co:Mg:Al molar ratio of 1:2:1, where CoAl spinel and CoO strongly interacting with MgO phases have been identified.