H2-induced promotion of CO oxidation over unsupported gold

The kinetics and mechanism of the preferential oxidation of carbon monoxide in the presence of hydrogen (PrOx) over an unsupported gold powder (mean particle size 20 nm and free of silver) have been investigated using flow fixed bed catalytic testing and diffuse reflectance infrared Fourier transform spectroscopy coupled to mass spectrometry (operando DRIFTS or DRIFTS-MS). It is shown that the presence of H₂ has a favourable effect on the oxidation of CO, either by strongly accelerating the reaction or by preventing the catalyst deactivation, depending on the conditions used. Variation of the hydrogen partial pressure has allowed us to determine partial reaction orders for both CO oxidation and H₂ oxidation under PrOx conditions. An infrared band at 2113 cm⁻¹, corresponding to on-top CO adsorption on metallic gold, has been observed below 150 °C. In addition, adsorbed hydroxyl groups gradually develop simultaneously to gas-phase water in the course of the reaction at increasing temperatures. The promotional effect of hydrogen is ascribed to highly oxidative H_xO_y intermediates formed from the interaction between H₂ and O₂ on the gold surface.     

Concrete floor-bovine claw contact pressures related to floor roughness and deformation of the claw

The intention of this research was to study the impact of concrete floor surface roughness on a bovine claw model and to assess the deformation of the bovine claw model under load. The pressure distribution between the floor and the claw is the key method in this research. Monitoring foot-to-ground pressure distributions may provide insight into the relation between high local pressures and foot lesions. Concrete floor samples were made with 5 different finishing methods. Their roughness was determined by measuring the heights of the “peaks and the valleys” of the surface with a high-precision laser beam. The smoothest surface was the sample finished with a metal float (surface roughness R_a = 0.062 mm) and the roughest surface occurred with the heavily sandblasted sample (surface roughness R_a = 0.488 mm). The roughness of the concrete floor samples was related to the mean and peak contact pressures that can occur in a laboratory test bench between floor and bovine claw. It was found that the claw itself has approximately 2 times more effect on these contact pressures than the surface roughness. Peak pressures found were high enough (up to 111 MPa) to cause damage to the bovine claw sole horn. The strains occurring in the horn wall were measured and related to the floor-finishing method and the load. Strain gauge measurements indicated that it is difficult to predict what kind of deformation of the claw wall will occur at a certain location. Different strains will occur for different floor-finishing methods. The corresponding stresses in the horn wall did not exceed the yield stress (14 and 11 MPa for dorsal and abaxial wall horn, respectively).     

Highly Nonstoichiometric YAG Ceramics with Modified Luminescence Properties

Y₃Al₅O₁₂ (YAG) is a widely used phosphor host. Its optical properties are controlled by chemical substitution at its YO₈ or AlO₆/AlO₄ sublattices, with emission wavelengths defined by rare-earth and transition-metal dopants that have been explored extensively. Nonstoichiometric compositions Y_3+xAl_5-xO₁₂ (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over two or more sublattices simultaneously, producing new local coordination environments for the activator ions. However, YAG typically behaves as a line phase, and such compositions are therefore challenging to synthesize. Here, a series of highly nonstoichiometric Y_3+xAl_5-xO₁₂ with 0 ≤ x ≤ 0.40 is reported, corresponding to ≤20% of the AlO₆ sublattice substituted by Y³⁺, synthesized by advanced melt-quenching techniques. This impacts the up-conversion luminescence of Yb³⁺/Er³⁺-doped systems, whose yellow-green emission differs from the red-orange emission of their stoichiometric counterparts. In contrast, the YAG:Ce³⁺ system has a different structural response to nonstoichiometry and its down-conversion emission is only weakly affected. Analogous highly nonstoichiometric systems should be obtainable for a range of garnet materials, demonstrated here by the synthesis of Gd_3.2Al_4.8O₁₂ and Gd_3.2Ga_4.8O₁₂. This opens pathways to property tuning by control of host stoichiometry, and the prospect of improved performance or new applications for garnet-type materials.     

Resolved-detrimental surface crystallization in yttrium lanthanum gallate glasses for optical fiber applications

Gallium-rich heavy metal oxide glasses have become highly attractive optical materials since they exhibit a wide transparency window spanning from the ultraviolet ∼270 nm up to the mid-infrared (IR) region ∼6 μm making them promising for a future integration in optical fiber devices. Nonetheless, in most composition, surface crystallization is a key limiting factor for optical fiber drawing using the classical preform-to-fiber method. Herein, taking advantage of structural information from vibrational spectroscopies (Raman/IR) and ⁷¹Ga Solid-State Nuclear Magnetic Resonance, we describe the key role of lanthanum and yttrium rare—earth elements on the glass structure and their impact on the capability to draw those new glass compositions into optical fibers. This approach emphasizes that yttrium ions as compared with lanthanum ones favor the glass disorder, increasing significantly the fraction of GaO₅ units with respect to GaO₄. That, combined with thermal analysis and examination of the crystallization behaviors, highlights that Y₂O₃ prevents the glass devitrification during the glass shaping. The smaller yttrium radius is believed to be the key physical parameter preventing the precipitation of the Ba_yGa_5-yGe_y+1La_3-yO₁₄ (y = 0, 1, 2, 3) langasite-type crystal phase. This study remains particularly relevant and opens up the way for the development of highly robust power scaled fiber devices operating from the visible up to the challenging mid-IR domain.