No abstract

First-principles-based kinetic Monte Carlo simulations and kinetic experiments are used to explore CO oxidation over Pt/CeO2. The simulations compare CO oxidation over a ceria-supported ∼1 nm particle with simulations of a free-standing particle and Pt(111). The onset of the CO oxidation over ceria supported Pt is shifted to lower temperatures compared to the unsupported systems thanks to a Mars–van Krevelen mechanism at the Pt/CeO2 interface perimeter, which is not sensitive to CO poisoning. Both the Mars–van Krevelen mechanism and the conventional Langmuir–Hinshelwood mechanism over the Pt nanoparticle are contributing to the conversion after the reaction onset. The reaction orders in CO and O2 are compared experimentally for Pt/CeO2  and Pt/Al2O3. The reaction orders over Pt/CeO2  are close to zero for both CO and O2, whereas the corresponding reaction orders are −0.75 and 0.68 over Pt/Al2O3. The measured zero orders for Pt/CeO2  show the absence of CO/O2 site competition and underline the relevance of interface reactions. The measurements for Pt/Al2O3 indicate that the main reaction path for CO oxidation over Pt is a conventional Langmuir–Hinshelwood reaction. The results elucidate the interplay between condition-dependent reaction mechanisms for CO oxidation over Pt supported on reducible oxides.

Ultrasound optical tomography (UOT) is an imaging technique based on the acoustooptic effect that can perform optical imaging with ultrasound resolution inside turbid media, and is thus interesting for biomedical applications, e.g. for assessing tissue blood oxygenation. In this paper, we present near background free measurements of UOT signal strengths using slow light filter signal detection. We carefully analyze each part of our experimental setup and match measured signal strengths with calculations based on diffusion theory. This agreement between experiment and theory allows us to assert the deep tissue imaging potential of ∼ 5 cm for UOT of real human tissues predicted by previous theoretical studies [ Biomed. Opt. Express 8, 4523 (2017)] with greater confidence, and indicate that future theoretical analysis of optimized UOT systems can be expected to be reliable.

No abstract

No abstract

A two-step seeded-growth method was refined to synthesize Au@Pd core@shell nanoparticles with thin Pd shells, which were then deposited onto alumina to obtain a supported Au@Pd/Al2O3 catalyst active for prototypical CO oxidation. By the strict control of temperature and Pd/Au molar ratio and the use of l-ascorbic acid for making both Au cores and Pd shells, a 1.5 nm Pd layer is formed around the Au core, as evidenced by transmission electron microscopy and energy-dispersive spectroscopy. The core@shell structure and the Pd shell remain intact upon deposition onto alumina and after being used for CO oxidation, as revealed by additional X-ray diffraction and X-ray photoemission spectroscopy before and after the reaction. The Pd shell surface was characterized with in situ infrared (IR) spectroscopy using CO as a chemical probe during CO adsorption–desorption. The IR bands for CO ad-species on the Pd shell suggest that the shell exposes mostly low-index surfaces, likely Pd(111) as the majority facet. Generally, the IR bands are blue-shifted as compared to conventional Pd/alumina catalysts, which may be due to the different support materials for Pd, Au versus Al2O3, and/or less strain of the Pd shell. Frequencies obtained from density functional calculations suggest the latter to be significant. Further, the catalytic CO oxidation ignition-extinction processes were followed by in situ IR, which shows the common CO poisoning and kinetic behavior associated with competitive adsorption of CO and O2 that is typically observed for noble metal catalysts.

No abstract