Search

In this study, we use zeolite Y as a support for the synthesis of Sn and Ga doped zeolites aimed at the isomerization of glucose to fructose. Though these materials are inactive in water, they are active in methanol and we could ascertain a reaction pathway involving a hydride shift for the interconversion of glucose to fructose and mannose, and a Brønsted acid pathway with the formation of a methyl fructoside intermediate and its hydrolysis to fructose if water was added afterwards. By using characterizations comprising: chemisorption, XPS, XRD, HAADF-STEM and EXAFS; it was possible to demonstrate that a straightforward impregnation protocol for the preparation of our catalysts, led to Sn/Y mainly consisting of small SnO2 clusters on the external surface of the zeolite, whereas Ga/Y consisting of highly dispersed Ga species mostly inside the zeolite pores; and a catalytic activity that appears to be dominated by Brønsted acid sites.

Some materials that are active heterogeneous catalysts for the breakdown of non-ionic aromatic solutes in water are found to act as potentiometric sensitizers for same solutes. As an example, here the aromatic water pollutant, benzyl alcohol, was sensed with a limit of detection below its potability limit of 19 μM. Our findings are rationalized on the grounds that both catalysis and sensing rely on adhesion of analyte/substrate on the sensitizer/catalyst. Specifically, a set of powdered transition metal-doped zeolites and related frameworks that catalyze the oxidation of waterborne aromatic pollutants were dispersed in phase transfer matrices. Matrices were introduced into water-gated thin film transistors that act as potentiometric transducers. Potentiometric sensing of non-ionic waterborne pollutants is limited to molecules with a ‘free’ molecular dipole, i.e., a dipole that is not locked in the molecular plane. The present work establishes an application for catalysts beyond catalysis itself. The use of catalysts as sensitizers is recommended for wider uptake and in reverse, to screen candidate catalysts.

International audience

Rhodium nanoparticles or rhodium organometallic complexes are mainly used in catalysis for reduction or hydroformylation reactions. In this work instead, we explored the capabilities of Rh nanoparticles as an oxidation catalyst, applied to the oxidation of benzyl alcohol to benzaldehyde under very mild conditions (100 °C, and atmospheric pressure) as a model reaction. Here we report the preparation of novel Rh/C catalysts by using an impregnation protocol, with particular emphasis on the pre-treatment of the carbon supports by using HNO3 and HCl, as well as the characterization of these materials by using an array of methods involving TEM, XPS and XRPD. Our preparation method led to a wide Rh particle size distribution ranging from 20 to 100 nm, and we estimate an upper limit diameter of Rh nanoparticles for their activity towards benzyl alcohol oxidation to be ca. 30 nm. Furthermore, a HNO3 pre-treatment of the activated carbon support was able to induce a smaller and narrower particle size distribution of Rh nanoparticles, whereas a HCl pre-treatment had no effect or sintered the Rh nanoparticles. We rationalise these results by HNO3 as an acid able to create new nucleation sites for Rh on the carbon surface, with the final effect of smaller nanoparticles, whereas for HCl the effect of sintering was most likely due to site blocking of the nucleation sites over the carbon surface. The roles of acid centres on the carbon surfaces for the oxidation reaction was also investigated, and the larger their amounts the larger the amounts of by-products. However, by treatment with HNO3 we were able to convert neutral or basic carbons into supports capable to enhance the catalytic activity of Rh, and yet minimised detrimental effects on the selectivity of the oxidation to benzaldehyde.

International audience

Ultra-low frequency photocurrent (PC) self-oscillation has been investigated in aIn0.15Ga0.85As/Al0.15Ga0.85As quantum-well (QW) diode in details as a function of temperature,excitation power and wavelength. The PC oscillation frequency increases with increasing temperatureand illumination power at excitation wavelengths below the leading n = 1 heavy-hole excitonresonance line under reverse bias conditions. The illumination wavelength dependence shows a clearevidence for beating due to two oscillators when photoexcitation by shorter wavelength below 1050nm is used. These results suggest that the low-frequency PC self-oscillation with a characteristicfrequency of about 0.01-0.1 Hz is caused by oscillating electric fields due to two-types ofphotogenerated charge carriers trapped at deep localized centers within the QW regions.

25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 545-548, Intermediate band solar cells (IBSCs) are expected to realize high efficiency operations. The cell structures based on multi-stacked quantum dots (QDs) are widely investigated both experimentally and theoretically. Partial filling of QDs and operation under concentrations are viable methods to increase the carrier generation via intermediate band (IB). We have investigated performance of IBSCs by using self-consistent driftdiffusion method under different doping densities in IB region, solar concentrations and relation of absorption coefficients of IB. We used GaAs material parameters except for absorption coefficients. Absorption coefficients were simplified and effectively related to QD based structure. If absorption coefficients are sufficiently large, effect of doping in IB region becomes important under low concentrations. However, high doping densities in IB region can degrade optical carrier generations. On the other, these effects are reduced by photofilling under high concentrations. If these absorption coefficients are small, large open circuit voltages are expected but IBSC operation would not be achieved. As a result, estimation of optimal dopant density in IB region is very important under low concentrations. This condition can be relaxed by photofilling effects under high concentrations.

A simple method to fabricate high-quality nanoparticles including spherical carbon onions and elongated fullerene-like nanoparticles similar to nanotubes in large quantities without the use of vacuum equipment is reported. The nanoparticles are obtained in the form of floating powder on the water surface following an arc discharge between two graphite electrodes submerged in water. High-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy images confirm the presence of spherical carbon onions with diameters ranging from 4 to 36 nm. The specific surface area of the floating powder was found to be very large, 984.3 m2/g, indicating that the material is promising for gas storage. From the surface area measurements, the mean particle diameter was calculated to be 3.7 nm. This value is close to the lower limit of the carbon onions observed in HRTEM. However, closer HRTEM observations also reveal that some carbon onions are not well crystallized. The large specific surface area c...