Molecular scale investigations of the reactivity of magnetite with formic acid, pyridine, and carbon tetrachloride

The (I 11) surface of magnetite, a dominant growth and fracture surface of this mineral, has been studied using Scanning Tunneling Microscopy (STM) at atomic resolution. In line with previous work, this surface shows three possible terminations which can be related to different level slices through the bulk structure. The reactivities of these different surface terminations have been explored by exposing them, under highly controlled conditions, to formic acid, pyridine, and carbon tetrachloride and undertaking further imaging at atomic resolution. These investigations have, themselves, helped to discriminate between competing models of surface structure. The so-called A' surface termination we now regard as exposing 1/4 monolayer of tetrahedrally coordinated Fe ions over a close packed oxygen layer, and the A surface termination as being these same Fe ions but each capped by a single oxygen. The so-called B surface termination, previously thought to expose 1/2 monolayer of equal numbers of octahedral and tetrahedral Fe ions over a close packed oxygen layer, we now regard as this same arrangement but again with each Fe capped with an oxygen. For all three molecules, the A' surface is most reactive but the reactions observed are markedly different. Formic acid undergoes dissociation at the magnetite surface, apparently chemisorbing at the A' surface via a bidentate non-bridging complex. On the same A' surface, pyridine is chemisorbed through a monodentate linkage via the 'basal' nitrogen of the molecule. For both formate and pyridine, a weaker interaction (a 'physisorption') was observed with the A and B surfaces, interpreted as involving attachment of the intact molecule. The exceptions to this were where the interaction involved chemisorption at defects on A and B type surfaces. The behavior of carbon tetrachloride on the magnetite surface is very different to the other molecules studied. Only the A' surface is significantly reactive, and the molecule undergoes a series of temperature-dependant dissociation and surface chemical reactions. These involve sorption of intact CCl4 molecules at the lowest temperatures, dissociation into CCl2 and Cl species at around room temperature, and removal from the magnetite of surface oxygens to form OCCl2 and then Fe to form FeCl2 at successively higher temperatures. At around room temperature, both strongly bonded Cl atoms and weakly bonded CCl2 molecules appear to co-exist on the same (A' type) surface, a situation not previously observed in iron oxide systems. (c) 2006 Elsevier Inc. All rights reserved

Spillover Reoxidation of Ceria Nanoparticles

Interest in resolving the mechanisms behind ceria's activity has been intense due to the numerous industrial applications including those in heterogeneous catalysis. In this work, we study the reduction and reoxidation of ultrathin CeO2(111) nanoislands on Rh(111) and Pt(111) substrates, so-called inverse model catalysts, with a combination of real and reciprocal space techniques based on X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy. Soft X-ray microfocused illumination was employed to reduce the ceria islands, which we are able to control by varying the oxygen partial pressure within the measurement chamber. Low energy electron diffraction measurements of the irradiated ceria films demonstrate the formation of an ordered array of oxygen vacancies leading to a (√7 × √7)R19.1° superstructure attributed to the ι-phase (Ce7O12)(111). Resonant photoelectron spectroscopy provides the required high sensitivity to detect small changes in Ce3+ concentration. The high spatial resolution of the XPEEM allows us to determine that the reduction of the ceria occurs initially at the interface of the islands with the Rh support. Reoxidation of the CeO2–x(111) to CeO2(111) proceeds via spillover of activated oxygen adsorbed on the Rh(111) surface as a (2 × 2) overlayer. Our results highlight the important role that the noble metal plays in the regeneration of the stoichiometric ceria surface, a vital step in many reactions on ceria. This differs from the commonly proposed Mars–van Krevelen model in which reoxidation involves direct reaction of the ceria with O2

Analysis of Uncertainties in Monte Carlo Simulated Organ and Effective Dose in Chest CT: Scanner- and Scan-related Factors

In Monte Carlo simulation of CT dose, many input parameters are required (e.g. bowtie filter properties and scan start/end location). Our goal was to examine the uncertainties in patient dose when input parameters were inaccurate. Using a validated Monte Carlo program, organ dose from a chest CT scan was simulated for an average-size female phantom using a reference set of input parameter values (treated as the truth). Additional simulations were performed in which errors were purposely introduced into the input parameter values. The effects on four dose quantities were analyzed: organ dose (mGy/mAs), effective dose (mSv/mAs), CTDIvol-normalized organ dose (unitless), and DLP-normalized effective dose (mSv/mGy cm). At 120 kVp, when spectral half value layer deviated from its true value by  ±1.0 mm Al, the four dose quantities had errors of 18%, 7%, 14% and 2%, respectively. None of the dose quantities were affected significantly by errors in photon path length through the graphite section of the bowtie filter; path length error as large as 5 mm produced dose errors of  ≤2%. In contrast, error of this magnitude in the aluminum section produced dose errors of  ≤14%. At a total collimation of 38.4 mm, when radiation beam width deviated from its true value by  ±  3 mm, dose errors were  ≤7%. Errors in tube starting angle had little impact on effective dose (errors  ≤  1%); however, they produced organ dose errors as high as 66%. When the assumed scan length was longer by 4 cm than the truth, organ dose errors were up to 137%. The corresponding error was 24% for effective dose, but only 3% for DLP-normalized effective dose. Lastly, when the scan isocenter deviated from the patient\u27s anatomical center by 5 cm, organ and effective dose errors were up 18% and 8%, respectively

Self-assembled metallic nanowires on a dielectric support:Pd on rutile TiO ₂(110)

International audiencePalladium nanoparticles supported on rutile TiO2(110)-1 x 1 have been studied using the complementary techniques of scanning tunneling microscopy and X-ray photoemission electron microscopy. Two distinct types of palladium nanoparticles are observed, namely long nanowires up to 1000 nm long, and smaller dotlike features with diameters ranging from 80-160 nm. X-ray photoemission electron microscopy reveals that the nanoparticles are composed of metallic palladium, separated by the bare TiO2(110) surface

Scanning tunnelling microscopy of suspended graphene

Suspended graphene has been studied by STM for the first time. Atomic resolution on mono- and bi-layer graphene samples has been obtained after ridding the graphene surface of contamination via high-temperature annealing. Static local corrugations (ripples) have been observed on both types of structures