We present high-pressure x-ray photoelectron spectroscopy (HP-XPS) and first-principles kinetic Monte Carlo study addressing the nature of the active surface in CO oxidation over Pd(100). Simultaneously measuring the chemical composition at the surface and in the near-surface gas phase, we reveal both O-covered pristine Pd(100) and a surface oxide as stable, highly active phases in the near-ambient regime accessible to HP-XPS. Surprisingly, no adsorbed CO can be detected during high CO(2) production rates, which can be explained by a combination of a remarkably short residence time of the CO molecule on the surface and mass-transfer limitations in the present setup.

Two new urea-bipyridine derived bridged organosilanes (P5 and P6) have been synthesized and their hydrolysis–condensation under nucleophilic catalysis in the presence of Eu3+ salts led to luminescent bridged silsesquioxanes (M5-Eu and M6-Eu). An important loading of Eu3+ (up to 11%w) can be obtained for the material based on the 6,6′-isomer. Indeed the photoluminescence properties of these materials, that have been investigated in depth (photoluminescence (PL), quantum yield, lifetimes), show a significantly different complexation mode of the Eu3+ ions for M6-Eu, compared with M4-Eu (obtained from the already-reported 4,4′-isomer) and M5-Eu. Moreover, M6-Eu exhibits the highest absolute emission quantum yield value (0.18 ± 0.02) among these three materials. The modification of the sol composition upon the addition of a malonamide derivative led to similar luminescent features but with an increased quantum yield (0.26 ± 0.03). In addition, M6-Eu can be processed as thin films by spin-coating on glass substrates, leading to plates coated by a thin layer (∼54 nm) of Eu3+-containing hybrid silica exhibiting one of the highest emission quantum yields reported so far for films of Eu3+-containing hybrids (0.34 ± 0.03) and an interesting potential as new luminescent solar concentrators (LSCs) with an optical conversion efficiency of ∼4%. The ratio between the light guided to the film edges and the one emitted by the surface of the film was quantified through the mapping of the intensity of the red pixels (in the RGB color model) from a film image. This quantification enabled a more accurate estimation of the transport losses due to the scattering of the emitted light in the film (0.40), thereby correcting the initial optical conversion efficiency to a value of 1.7%.

CO oxidation over Pd(100) and Pd 75 Ag 25 (100) has been investigated by a combination of near-ambient pressure X-ray photoelectron spectroscopy, quadrupole mass spectrometry, density functional theory calculations and micro-kinetic modeling. For both surfaces, hysteresis is observed in the CO 2 formation during heating and cooling cycles. Whereas normal hysteresis with higher light-off temperature than extinction temperature is present for Pd(100), reversed hysteresis is observed for Pd 75 Ag 25 (100).The reversed hysteresis can be explained from dynamic changes in the surface composition. At the beginning of the heating ramp, the surface is rich in palladium which gives a CO coverage that poisons the surface until the desorption rate becomes sufficiently high. The thermodynamic preference for an Ag rich surface in the absence of adsorbates promotes diffusion of Ag from the bulk to the surface as CO desorbs.During the cooling ramp, an appreciable surface coverage is reached at temperatures too low for efficient diffusion of Ag back into the bulk. The high concentration of Ag in the surface leads to a high extinction temperature and, consequently, the reversed hysteresis.

The sol-gel fabrication of bridged silsesquioxane thin films containing trivalent lanthanide ions (Ln 3+ ) has been achieved from a 4,4 0 -diureido-2,2 0 -bipyridine bridged organosilane (P4) in the presence of Eu 3+ or Tb 3+ salts. Crack-and defect-free thin films ($50 nm) of optical quality were successfully deposited on glass substrates by spin-coating after optimizing the spinning rate and solvent system. Ellipsometry investigations revealed a significant decrease in the refractive index of the films with ageing of the corresponding precursor sols. In contrast, the photoluminescence properties of the films are independent of precursor ageing time, although significant differences are observed with respect to the corresponding bulk materials. Compared to the bulk materials, the films exhibit a significant blueshift of the excitation spectra and an increase of the excited state lifetime. The optical conversion efficiencies (h opt ) of the F4-Eu and F4-Tb-based collectors were also determined and potential applications of the coatings in such areas as luminescent solar concentrators are discussed.

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Abstract:The applicably of multi-objective optimization to ellipsometric data analysis is presented and a method to handle complex ellipsometric problems such as multi sample or multi angle analysis using multi-objective optimization is described. The performance of a multi-objective genetic algorithm (MOGA) is tested against a single objective common genetic algorithm (CGA). The procedure is applied to the characterization (refractive index and thickness) of planar waveguides intended for the production of optical components prepared sol-gel derived organicinorganic hybrids, so-called di-ureasils, modified with zirconium tetrapropoxide, Zr(OPr n ) 4 deposited on silica on silicon substrates. The results show that for the same initial conditions, MOGA performs better than the CGA, showing a higher success rate in the task of finding the best final solution. ©2010 Optical Society of America

We have studied the oxidation of CO over Rh(100) using high pressure x-ray photoelectron spectroscopy under CO and O2 pressures ranging from 0.01 to 1 mbar. The results show a very low or no conversion for the CO covered surface found at low temperatures, while the activity rises slightly when the temperature is high enough for some CO to desorb, exposing surface sites for dissociative O2 adsorption. As the temperature is increased further, more CO desorbs and oxygen replaces CO as the dominating species at the surface. At the same time we find a sudden increase in the reactivity, such that all CO that reaches the surface is instantly transformed into CO2. We find that the O coverage in the active state is highly dependent on the total pressure and, although we do not detect any presence of a surface oxide as in previous surface x-ray diffraction studies, the highest O coverage indicates that the surface is close to being oxidized.