Núria J. Divins

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.

Hydrogen from Bioethanol

Bioethanol, ethanol produced from biomass by fermentation, is the most promising renewable source for hydrogen production. Ethanol is advantageous over other conventional substrates because it is readily available, easy to obtain from biomass and to transport, CO 2 neutral and safe to handle. Ethanol is a well-established source of hydrogen by catalytic steam reforming, oxidative reforming or partial oxidation. An efficient catalyst for hydrogen production from ethanol has to dissociate the C–C bond, maintain a low CO concentration and be stable under catalytic operation. Noble metal-based catalysts perform well; they are stable and exhibit high activity. However, they are expensive and need high temperatures to be active. Nickel- and cobalt-based catalysts are inexpensive but under reaction conditions, they suffer from sintering and deactivation by carbon deposition. Supports with redox properties, based on CeO 2 , can oxidize carbon residues and prevent extensive carbon deposition due to its oxygen storage capacity and high oxygen mobility. The use of catalytic membrane reactors, with simultaneous generation and separation of hydrogen, appears as an attractive approach to simplify on-site/on-demand ethanol reformers and to reduce downstream separation costs. Looking for mobile applications, microreactor technologies have been applied to ethanol reforming processes as well. Recently, photocatalytic generation of hydrogen at room temperature from water–ethanol mixtures has been accomplished over noble metals supported over semiconductors.

Improved thermal stability of oxide-supported naked gold nanoparticles by ligand-assisted pinning

We report a method to improve the thermal stability, up to 900 °C, of bare-metal (naked) gold nanoparticles supported on top of SiO(2) and SrTiO(3) substrates via ligand-assisted pinning. This approach leads to monodisperse naked gold nanoparticles without significant sintering after thermal annealing in air at 900 °C. The ligand-assisted pinning mechanism is described.

Macroporous silicon microreactor for the preferential oxidation of CO

A macroporous silicon micromonolith containing ca. 40,000 regular channels of 3.3 μm in diameter per square millimeter has been successfully functionalized with an Au/TiO2 catalyst for CO preferential oxidation (CO-PrOx) in the presence of hydrogen. The functionalization of the silicon microchannels has been accomplished by growing a SiO2 layer on the channel walls, followed by exchange with a titanium alkoxyde precursor and decomposition into TiO2 and, finally, by anchoring carbosilanethiol dendron protected pre-formed Au nanoparticles. Catalytically active centers at the Au-TiO2 interface have been obtained by thermal activation. With this method, an excellent homogeneity and adherence of the catalytic layer over the microchannels of the macroporous silicon micromonolith has been obtained, which has been tested for CO-PrOx at 363-433 K and λ=2 under H2/CO=0-20 (molar). The macroporous silicon micromonolith converts ca. 3 NmL of CO per minute and mL of micro reactor at 433 K under H2/CO=20, suggesting that it could be particularly effective for hydrogen purification in low-temperature microfuel cells for portable applications.

Fabrication and characterization of a fuel flexible micro-reformer fully integrated in silicon for micro-solid oxide fuel cell applications

A novel design of a fuel-flexible micro-reactor for hydrogen generation from ethanol and methane is proposed in this work. The micro-reactor is fully fabricated with mainstream MEMS technology and consists of an array of more than 20000 through-silicon vertically aligned micro-channels per cm2 of 50 μm in diameter. Due to this unique configuration, the micro-reformer presents a total surface per projected area of 16 cm2/cm2 and per volume of 320 cm2/cm3. The active surface of the micro-reformer, i.e. the walls of the micro-channels, is homogenously coated with a thin film of Rh- Pd/CeO2 catalyst. Excellent steam reforming of ethanol and dry reforming of methane are presented with hydrogen production rates above 3 mL/min·cm2 and hydrogen selectivity of ca. 50% on a dry basis at operations conditions suitable for application in micro-solid oxide fuel cells (micro-SOFCs), i.e. 700-800ºC and fuel flows of 0.02 mLL/min for ethanol and 36 mLG/min for methane (corresponding to a system able to produce one electrical watt).