Redistribution dynamics of ultrathin vanadium oxide layers under catalytic conditions and activation of diffusion by surface acoustic waves

The reaction-induced redistribution of vanadium oxide supported on noble metal single crystal surfaces (inverse model catalysts), and the influence of surface acoustic waves (SAW) on the composition of a bimetallic Rh/Pt surface are studied. Previously, the movement and coalescence of macroscopic, two-dimensional vanadium oxide islands on Rh(111) during catalytic methanol oxidation was explained with a polymerization / depolymerization mechanism. To investigate how general this mechanism is, the reaction dynamics of vanadium oxide on Rh(111), Rh(110) and Pt(111) are investigated in a number of catalytic reactions. Island formation and coalescence are observed in ammonia, CO, and methanol oxidation on VOx/Rh(111) in the 0.0001 mbar range. Exchanging oxygen by NO as oxidizing agent results in an inverse pattern, i. e. holes in a dense vanadium oxide film instead of vanadium oxide islands surrounded by bare Rh surface. Spectroscopic LEEM reveals, that NO influences the width of the interface vanadium oxide island / bare Rh(111), thus indicating a change in the line tension. The line tension possibly explains the complementary types of pattern formation. Indications for a Rh surface oxide under reaction conditions are found. In the 0.0001 mbar range on Rh(111), oscillating vanadium oxide islands occur. A tentative mechanism is proposed, based on phase transitions inside the vanadium oxide islands, which result from gradients in the oxygen coverage. With near ambient pressure LEEM, turbulent redistribution dynamics are observed during methanol oxidation at 0.02mbar. On VOx/Rh(110) island formation occurs, but no island coalescence is seen. Instead, a wealth of chemical wave pattern is found: traveling interface modulations (TIMs), traveling wave fragments and target pattern, as well as chemical waves propagating over both, the bare Rh(110) substrate and macroscopic vanadium oxide islands. TIMs are explained by a mechanism based on the reversible creation of surface defects at the interface. The system VOx/Pt(111) is characterized by the reversible diffusion of V into the Pt bulk under reaction conditions. As a consequence, no pattern formation occurs. A strong effect of the metallic support on the behavior of VOx catalysts is demonstrated by the different types of pattern formation in VOx/Rh(111), VOx/Rh(110), and VOx/Pt(111). In addition to different types of pattern formation, also the selectivity and catalytic activity is strongly influenced by the support. Whereas formaldehyde is the main product in catalytic methanol oxidation on VOx/Rh(111), no formaldehyde production is detected on VOx/Rh(110) and VOx/Pt(111). The influence of SAWs on the diffusive intermixing of a Rh/Pt surface is investigated by laterally resolved X-ray spectroscopy. The results are compared to Auger spectroscopy measurements on the thermal diffusion of Rh into the Pt bulk on a Pt(100) single crystal and on polycrystalline Pt. At 445 K, a SAW-induced intermixing of Pt and Rh is detected. In thermal diffusion experiments, the onset of Rh diffusion into the Pt bulk is found to occur around 500 to 550 K. The experiments are a first step towards verifying the working hypothesis, that structural defects caused by SAWs are the main reason for a SAW-induced increase in catalytic activity reported in literature

Reactivity and Stability of Ultrathin VOx Films on Pt(111) in Catalytic Methanol Oxidation

The growth of ultrathin layers of VOx (< 12 monolayers) on Pt(111) and the activity of these layers in catalytic methanol oxidation at 10−4 mbar have been studied with low-energy electron diffraction, Auger electron spectroscopy, rate measurements, and with photoemission electron microscopy. Reactive deposition of V in O2 at 670 K obeys a Stranski–Krastanov growth mode with a (√3 × √3)R30° structure representing the limiting case for epitaxial growth of 3D-VOx. The activity of VOx/Pt(111) in catalytic methanol oxidation is very low and no redistribution dynamics is observed lifting the initial spatial homogeneity of the VOx layer. Under reaction conditions, part of the surface vanadium diffuses into the Pt subsurface region. Exposure to O2 causes part of the V to diffuse back to the surface, but only up to one monolayer of VOx can be stabilized in this way at 10−4 mbar

Kinetics of low pressure ammonia oxidation over Rh(111)

The kinetics of the NH3 + O2 reaction over a Rh(111) single crystal catalytic surface was explored in the 10-6 mbar pressure range at tempera-tures between 300-900 K. Selectivity towards N2 and NO products, and reactive sticking coefficients were monitored in situ using differentially pumped quad-rupole mass spectroscopy (QMS). © 2018, Latin American Applied Research

Dynamics of ultrathin V-oxide layers on Rh(111) in catalytic oxidation of ammonia and CO

Catalytic oxidation of ammonia and CO has been studied in the 10(-4) mbar range using a catalyst prepared by depositing ultra-thin vanadium oxide layers on Rh(111) (thetaV approximately 0.2 MLE). Using photoemission electron microscopy (PEEM) as a spatially resolving method, we observe that upon heating in an atmosphere of NH3 and O2 the spatial homogeneity of the VOx layer is removed at 800 K and a pattern consisting of macroscopic stripes develops; at elevated temperatures this pattern transforms into a pattern of circular VOx islands. Under reaction conditions the neighboring VOx islands become attracted by each other and coalesce. Similar processes of pattern formation and island coalescence are observed in catalytic CO oxidation. Reoxidation of the reduced VOx catalyst proceeds via surface diffusion of oxygen adsorbed onto Rh(111). A pattern consisting of macroscopic circular VOx islands can also be obtained by heating a Rh(111)/VOx catalyst in pure O2

On the promotion of catalytic reactions by surface acoustic waves

Surface acoustic waves (SAW) allow to manipulate surfaces with potential applications in catalysis,sensor and nanotechnology.SAWswere shown to cause astrong increase in catalytic activity and selectivity in many oxidation and decomposition reactions on metallic and oxidic catalysts. However,the promotion mechanism has not been unambiguously identified. Using stroboscopic X-ray photoelectron spectro-microscopy, we were able to evidence asub-nano-second work function change during propagation of 500 MHz SAWs on a9nm thick platinum film. We quantify the work function change to 455 meV.Such asmall variation rules out that electronic effects due to elastic deformation (strain) play amajor role in the SAW-induced promotion of catalysis.In asecond set of experiments,SAW-induced intermixing of afive monolayers thick Rh film on top of polycrystalline platinum was demonstrated to be due to enhanced thermal diffusion caused by an increase of the surface temperature by about 75 K when SAWs were excited. Reversible surface structural changes are suggested to be amajor cause for catalytic promotion

Optical stimulated-Raman sideband spectroscopy of a single 9Be+ ion in a Penning trap

We demonstrate optical sideband spectroscopy of a single 9Be+ ion in a cryogenic 5 tesla Penning trap using two-photon stimulated-Raman transitions between the two Zeeman sublevels of the 1s22s ground state manifold. By applying two complementary coupling schemes, we accurately measure Raman resonances with and without contributions from motional sidebands. From the latter we obtain an axial sideband spectrum with an effective mode temperature of (3.1±0.4) mK. These results are a key step for quantum logic operations in Penning traps, applicable to high-precision matter-antimatter comparison tests in the baryonic sector of the standard model

Microarray-based approach identifies microRNAs and their target functional patterns in polycystic kidney disease

Background: MicroRNAs (miRNAs) play key roles in mammalian gene expression and several cellular processes, including differentiation, development, apoptosis and cancer pathomechanisms. Recently the biological importance of primary cilia has been recognized in a number of human genetic diseases. Numerous disorders are related to cilia dysfunction, including polycystic kidney disease (PKD). Although involvement of certain genes and transcriptional networks in PKD development has been shown, not much is known how they are regulated molecularly. Results: Given the emerging role of miRNAs in gene expression, we explored the possibilities of miRNA-based regulations in PKD. Here, we analyzed the simultaneous expression changes of miRNAs and mRNAs by microarrays. 935 genes, classified into 24 functional categories, were differentially regulated between PKD and control animals. In parallel, 30 miRNAs were differentially regulated in PKD rats: our results suggest that several miRNAs might be involved in regulating genetic switches in PKD. Furthermore, we describe some newly detected miRNAs, miR-31 and miR-217, in the kidney which have not been reported previously. We determine functionally related gene sets, or pathways to reveal the functional correlation between differentially expressed mRNAs and miRNAs. Conclusion: We find that the functional patterns of predicted miRNA targets and differentially expressed mRNAs are similar. Our results suggest an important role of miRNAs in specific pathways underlying PKD

Resolved-sideband cooling of a single 9^9Be+^+ ion in a Penning trap

Manipulating individual trapped ions at the single quantum level has become standard practice in radio-frequency ion traps, enabling applications from quantum information processing to precision metrology. The key ingredient is ground-state cooling of the particle's motion through resolved-sideband laser cooling. Ultra-high-presicion experiments using Penning ion traps will greatly benefit from the reduction of systematic errors offered by full motional control, with applications to atomic masses and $g$-factor measurements, determinations of fundamental constants or related tests of fundamental physics. In addition, it will allow to implement quantum logic spectroscopy, a technique that has enabled a new class of precision measurements in radio-frequency ion traps. Here we demonstrate resolved-sideband laser cooling of the axial motion of a single $^9$Be$^+$ ion in a cryogenic 5 Tesla Penning trap system using a two-photon stimulated-Raman process, reaching a mean phonon number of $\bar{n}_z = 0.10(4)$. This is a fundamental step in the implementation of quantum logic spectroscopy for matter-antimatter comparison tests in the baryonic sector of the Standard Model and a key step towards improved precision experiments in Penning traps operating at the quantum limit.Comment: 6 pages, 5 figure

Resolved-sideband cooling of a single 9Be+ ion in a cryogenic multi-Penning-trap for discrete symmetry tests with (anti-)protons

Manipulating the motion of individual trapped ions at the single quantum level has become standard practice in radio-frequency ion traps, enabling sweeping advances in quantum information processing and precision metrology. The key step for motional-state engineering is ground-state cooling. Full motional control also bears great potential to explore another regime of sensitivities for fundamental physics tests in Penning traps. Here we demonstrate the key enabling step by implementing resolved-sideband cooling on the axial mode of a single 9Be+ ion in a 5 Tesla cryogenic Penning trap. The system has been developed for the implementation of high-precision antimatter experiments to test the fundamental symmetries of the standard model with the highest accuracy in the baryonic sector. We measure an axial phonon number of ‾=0.10⁢(4) after cooling and demonstrate that the axial heating rate in our system is compatible with the implementation of quantum logic spectroscopy of (anti-)protons