Complete flat surfaces with two isolated singularities in hyperbolic 3-space

We construct examples of flat surfaces in H³ which are graphs over a two-punctured horosphere and classify complete embedded flat surfaces in H³ with only one end and at most two isolated singularities.     

Stress‐dependent correlations for resonant Raman bands in graphite with defects

We have studied the Raman features characteristics of defects generated in graphite under high stress conditions. Defects are generated in pristine highly oriented pyrolytic graphite by squeezing the samples in a high‐pressure anvil cell and monitored in situ by Raman spectroscopy. On the basis of our Raman measurements and existing literature correlations, we conclude that vacancies and grain boundaries are generated during compression–decompression cycles, being the defects mostly generated during the decompression stage. Our results demonstrate that the relative intensities of the D, D′, and (D + D′) bands are strongly correlated. Which is important for practical application of Raman spectroscopy in the characterization of carbon materials is that such correlations are essentially constant over the whole stress range covered in the experiments (~7 GPa). An additional interesting result concerns the relative intensities of the denoted 2D₁ and 2D₂ contributions, which are correlated with the intensity of the G band; the intensity ratio between both features is modified by stress indicating that the stress affects the stacking order of pristine graphite. Finally, we find that the decrease in intensity of the 2D₂ band with decreasing crystallite size found in existing studies on unstrained graphite remains under stress conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Characterization of Supported Gold Catalysts with 197Au Mössbauer Effect Spectroscopy

We report on a ¹⁹⁷Au Mössbauer study of several types of supported gold catalysts. Differences in particle size show up in the Mössbauer spectra by a change in the relative weight of the spectral contribution of the surface atoms. The presence of ionic gold in active gold catalysts is not observed. The spectra can be interpreted in terms of bulk-like contributions from the inner-core atoms plus contributions from the outermost atoms at the surface of the particles.     

Minimal Surfaces Obtained by Ribaucour Transformations

We consider Ribaucour transformations between minimal surfaces and we relate such transformations to generating planar embedded ends. Applying Ribaucour transformations to Enneper's surface and to the catenoid, we obtain new families of complete, minimal surfaces, of genus zero, immersed in R ³, with infinitely many embedded planar ends or with any finite number of such ends. Moreover, each surface has one or two nonplanar ends. A particular family is obtained from the catenoid, for each pair (n,m), nm, such that n m0 is an irreducible rational number. For any such pair, we get a 1-parameter family of finite total curvature, complete minimal surfaces with n+2 ends, n embedded planar ends and two nonplanar ends of geometric index m, whose total curvature is –4(n+m). The analytic interpretation of a Ribaucour transformation as a Bäcklund type transformation and a superposition formula for the nonlinear differential equation = e^-2 is included.     

Al2O3基Ni-Cu双金属催化剂上乙酰丙酸氢解（英）

Inexpensive γ-alumina-based nickel-copper bimetallic catalysts were studied for the hydrogenolysis of levulinic acid,a key platform molecule for biomass conversion to biofuels and other valued chemicals,into γ-valerolactone as a first step towards the production of 2-methyltetrahydrofurane.The activities of both monometallic and bimetallic catalysts were tested.Their textural and chemical characteristics were determined by nitrogen physisorption,elemental analysis,temperature-programmed ammonia desorption,and temperature-programmed reduction.The monometallic nickel catalyst showed high activity but the highest bγ-product production and significant amounts of carbon deposited on the catalyst surface.The copper monometallic catalyst showed the lowest activity but the lowest carbon deposition.The incorporation of the two metals generated a bimetallic catalyst that displayed a similar activity to that of the Ni monometallic catalyst and significantly low bγ-product and carbon contents,indicating the occurrence of important synergetic effects.The influence of the preparation method was also examined by studying impregnated- and sol-gel-derived bimetallic catalysts.A strong dependency on the preparation procedure and calcination temperature was observed.The highest activity per metal atom was achieved using the sol-gel-derived catalyst that was calcined at 450 ℃.High reaction rates were achieved;the total levulinic acid conversion was obtained in less than 2 h of reaction time,yielding up to 96%γ-valerolactone,at operating temperature and pressure of 250 ℃ and 6.5 MPa hydrogen,respectively.     

3D Raman mapping of uniaxially loaded 6H‐SiC crystals

Raman spectroscopy is used to investigate the three‐dimensional stress distribution in 6H‐silicon carbide (SiC) specimens subjected to stresses up to 3.7 GPa along the c‐axis. Specifically, the relative Raman shift of the longitudinal optic phonon of 6H‐SiC is used to evaluate the local stress across the bulk crystal. For this purpose, an anvil device with opposed 6H‐SiC and sapphire specimens was used. After subjecting the anvils to uniaxial load, several series of two‐dimensional Raman maps were registered at different depths in the 6H‐SiC anvil. The analysis of the Raman spectra reveals an exponential decay of the stress as a function of the depth. A novel phenomenological Grüneisen‐like model is introduced here to account for such observation. On the contrary, the in‐plane stress analysis shows a radial Gaussian‐like distribution regardless the depth, a distinct behavior that is attributed to the appearance of shear stress components. The suitability of both models and their applicability to other materials are discussed, along with some future directions. Copyright © 2013 John Wiley & Sons, Ltd.     

Raman scattering in low-dimensional HfTe5

Nine long-wavelength phonons have been observed by Raman scattering in the quasi-one-dimensional crystal HfTe₅. The assignment of these phonon modes is made on the basis of symmetry arguments using the correlation method. The similarity between the HfTe₅ and the ZrTe₃ chain-like crystal structures instigates the attribution of the most characteristic modes like the high-frequency chalcogen diatomic-mode and the low frequency rigid-chain mode. Although no new line appears even at 14 K, the irregular temperature dependence of the spectra suggests the possibility of a structural instability connected with a large peak in the electrical resistivity and with the X-ray diffraction anomalies.     

PROMOTIONAL EFFECT OF Sn ON NO OXIDATION OVER Pt/Al2O3 AS PROMISING GAS EXHAUST CATALYTIC CONVERTER. SELECTIVE Pt AND Sn DISPERSION DETERMINATION

NO oxidation was investigated over Pt-Sn/g-Al₂O₃. The addition of Sn to 1%Pt/g-Al₂O₃ enhances NO oxidation to NO₂. This effect is related to the enhancement of chemisorbed oxygen on the metallic surface produced by Sn.     

An investigation of the promotional effect of propylene or propane traces on the activity of sulfatedPt/g-Al2O3andPt-Sn/g-Al2O3inCH4combustion

Summary A strong promoting effect of the presence of C₃H₈or C₃H₆was determined for the combustion of CH₄in excess oxygen, over pre-sulfated 1%Pt/g-Al₂O₃and pre-sulfated 1%Pt-2%Sn/g-Al₂O₃catalysts.</o:p>     

Platinum-palladium catalysts in hydrogenation of methylbenzenes

Hydrogenation of benzene, toluene, o,m and p-xylene over Pt, Pd and Pt–Pd/Al₂O₃ catalysts has been studied. Experimental data show that when Pd is added to Pt catalysts, a decrease in activity and changes in selectivity occur. This is explained by the formation of electron-deficient and electrorich metallic species.

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