Supercritical fluid chromatography of metoprolol and analogues on aminopropyl and ethylpyridine silica without any additives

Metoprolol and a number of related amino alcohols and similar analytes have been chromatographed on aminopropyl (APS) and ethylpyridine (EPS) silica columns. The mobile phase was carbon dioxide with methanol as modifier and no amine additive was present. Optimal isocratic conditions for the selectivity were evaluated based on experiments using design of experiments. A central composite circumscribed model for each column was used. Factors were column temperature, back-pressure and % (v/v) of modifier. The responses were retention and selectivity versus metoprolol. The % of modifier mainly controlled the retention on both columns but pressure and temperature could also be important for optimizing the selectivity between the amino alcohols. The compounds could be divided into four and five groups on both columns, with respect to the selectivity. Furthermore, on the aminopropyl silica the analytes were more spread out whereas on the ethylpyridine silica, due to its aromaticity, retention and selectivity were closer. For optimal conditions the column temperature and back-pressure should be high and the modifier concentration low. A comparison of the selectivity using optimized conditions show a few switches of retention order between the two columns. On aminopropyl silica an aldehyde failed to be eluted owing to Schiff-base formation. Peak symmetry and column efficiency were briefly studied for some structurally close analogues. This revealed some activity from the columns that affected analytes that had less protected amino groups, a methyl group instead of isopropyl. The tailing was more marked with the ethylpyridine column even with the more bulky alkyl substituents. Plate number N was a better measure than the asymmetry factor since some analyte peaks broadened without serious deterioration of symmetry compared to homologues.     

Formation, structure and magnetism of the metastable defect fluorite phases AVO3.5+x (A=In, Sc)

We report the preparation and stability of ScVO_3.5+x and the novel phase InVO_3.5+x. AVO_3.5+x (A=Sc, In) defect fluorite structures are formed as metastable intermediates during the topotactic oxidation of AVO₃ bixbyites. The oxidation pathway has been studied in detail by means of thermogravimetric/differential thermal analysis and in-situ powder X-ray diffraction. The oxidation of the bixbyite phase follows a topotactic pathway at temperatures between 300 and 400 °C in air/carbon dioxide. The range of accessible oxygen stoichiometries for the AVO_3.5+x structures following this pathway are 0.00x0.22. Rietveld refinements against powder X-ray and neutron data revealed that InVO_3.54 and ScVO_3.70 crystallize in the defect fluorite structure in space group Fm-3 m (227) with a=4.9863(5) and 4.9697(3)Å, respectively with A³⁺/V⁴⁺ disorder on the (4a) cation site. Powder neutron diffraction experiments indicate clustering of oxide defects in all samples. Bulk magnetic measurements showed the presence of V⁴⁺ and the absence of magnetic ordering at low temperatures. Powder neutron diffraction experiments confirmed the absence of a long range ordered magnetic ground state.     

Reversible Modification of the Structural and Electronic Properties of a Boron Nitride Monolayer by CO Intercalation

We demonstrate the reversible intercalation of CO between a hexagonal boron nitride (h‐BN) monolayer and a Rh(111) substrate above a threshold CO pressure of 0.01 mbar at room temperature. The intercalation of CO results in the flattening of the originally corrugated h‐BN nanomesh and an electronic decoupling of the BN layer from the Rh substrate. The intercalated CO molecules assume a coverage and adsorption site distribution comparable to that on the free Rh(111) surface at similar conditions. The pristine h‐BN nanomesh is reinstated upon heating to above 625 K. These observations may open up opportunities for a reversible tuning of the electronic and structural properties of monolayer BN films.     

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO-O Complex**

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.     

Surface oxide development on aluminum alloy 6063 during heat treatment

We report on the influence of oxygen partial pressure for the development of surface oxides covering the industrial aluminum alloy standard 6063 at temperatures ranging from room temperature to 500° C. Using an array of synchrotron-based techniques, we followed the change in oxide thickness, chemical composition, and the lateral distribution of alloying elements. The impact of the oxygen chemical potential is most visible at high temperatures where the oxide composition changes from mostly Al based to mostly Mg based. This is in stark contrast to the ultra-high vacuum (UHV) conditions where only a partial compositional transition is observed. The microscopy data demonstrate that in the UHV case, Mg segregation onto the surface occurs firstly at grain boundaries at 300° C and secondly at sites over the entire surface at 400° C. Further, the initial oxide thickness is 45 Å, as determined by XPS and XRR, decreases in all observed cases after heating to 300° C. At higher temperatures, however, the oxygen partial pressure highly influences the resulting oxide thickness as evident from our X-ray reflectivity data.     

A single h-BN layer on Pt(1 1 1)

The structure and formation of an ultrathin hexagonal boron nitride (h-BN) film on Pt(1 1 1) has been studied by a combination of scanning tunneling microscopy, low energy electron diffraction, low energy electron microscopy, X-ray absorption and high resolution core level spectroscopy. The study shows that a single boron nitride layer is formed on Pt(1 1 1), resulting in a coincidence structure. High resolution scanning tunneling microscopy (STM) images of the h-BN ultrathin film display only one of the atomic species in the unit cell. Probing the boron and nitrogen related local density of states by near edge X-ray absorption fine structure measurements we conclude that the nitrogen sublattice is visible in STM images. The growth of the single hexagonal boron nitride layer by vapourized borazine in the pressure range of 1×10^-6–1×10^-8 at 800 °C is further studied by low energy electron microscopy, and reveals that the number of nucleation sites and the perfection of the growth is strongly pressure dependent. A model for the single, hexagonal, boron nitride layer on Pt(1 1 1) is proposed.     

Coexistence of aging states in spin glasses

The influence of temperature variations on the aging process of a Cu(Mn) spin glass has been investigated. It is found that the dynamics of the aging process is not very susceptible to small temperature variations around the measurement temperature. The astonishing possibility for the spin glass system to house two coexisting aging states is demonstrated.     

Na and K on Al(100) studied by low-energy electron diffraction and high-resolution core-level spectroscopy

The temperature effects on sub-monolayers of V deposited at the TiO₂(001) surface have been studied by ultraviolet and X-ray photoelectron spectroscopies, UPS and XPS, from 300 up to 623 K.

V coverages, Θ_v, between 0.2 and 0.7 monolayers (ML) were deposited by an e-beam evaporator at 300 K. The V 2p_3/2 core line region exhibits two well-defined components whose relative intensity depends on Θ_v. These two components, assigned to different oxidation states of V, are correlated with two features, with a dominant V 3d character, detected within the TiO₂(001) band gap of the UPS valence band spectra.

UPS and XPS measurements performed after in-situ thermal treatments show unambiguous and reproducible changes of these spectral components. After annealing at 623 K only the higher binding energy component is present in the V 2p_3/2 spectra; the Ti 2_p core lines recover the typical symmetry of the clean and stoichiometric TiO₂(001) surface and the higher binding energy feature only is detected in the TiO₂ band gap. These data suggest that, within the volume probed by XPS and UPS, Ti ions have a mainly d⁰ configuration, while V has a single and stable open-shell configuration, as revealed by the significant intensity detected within the TiO₂ band gap. These annealing-induced changes are due, as suggested by the O 1s/Ti 2p core line intensity trend, to an oxygen diffusion from the TiO₂ bulk to the surface. Finally, a detailed analysis of the data indicates that different V/TiO₂(001) interfaces exhibit different behaviours after annealing treatments, depending on Θ_v. For Θ_v = 0.7 ML, V interdiffuses into the TiO₂ sub-surface layers, whereas for Θ_v = 0.2 ML it remains at the surface. This finding is consistent with a rearrangement of V atoms. which under annealing occupy first the energetically most favorable surface sites (Θ_v = 0.2 ML) before interdiffusing into the TiO₂ lattice (Θ_v = 0.7 ML).