The characterization of oxygen surface arrangements on a heated model Pt3Pb bimetallic catalyst: A case study by SIMS

SIMS in the low damage mode is applied to characterize different surface arrangements of O on a heated polycrystalline Pt₃Pb sample which is exposed to several oxygen doses. Small oxygen doses at 600 K lead to a surface characterized by O in on-top positions principally. At higher doses oxygen saturation occurs which is probably accompanied by a transition to a reconstructed state of the treated surface. At 790 K immediately place-exchange between O and the substrate metal atoms happens after all doses supplied. The SIMS results are elucidated using recent presentations of the interaction of electronegative species with metallic surfaces. The connection between work function and oxygen surface arrangement is discussed in detail.     

Reactions of azomethane on a clean,H-covered,and O-covered Pt(111) surface

The reactions of azomethane were studied on a clean, H-covered, and O-covered Pt(111) surface at different coverages of azomethane, and ratios of hydrogen or oxygen to azomethane, by temperature programmed desorption. On a clean surface, dehydrogenetion, together with breaking of the NN bond to produce HCN and H₂ were the major reactions. Small amounts of methylamine and cyanogen were also observed. On an H-covered surface at high hydrogen coverage, methane was the dominant product, in addition to H₂. This adsorbed hydrogen promoted breaking of the CN bond. In addition, the dehydrogenation product HCN was also observed, as was methylamine. On the O-covered surface, CO, CO₂, H₂O, and small amounts of NO were observed together with the products typical for a clean surface. Comparison of the results with those for acetylene, ethylene, and diazomethane is made.     

The formation and decomposition kinetics of alkylidynes on Pt(111)

The formation and decomposition kinetics of ethylidyne and propylidyne on Pt(111), were studied using static secondary ion mass spectrometry and temperature programmed desorption. For the maximum amounts of dissociatively adsorbed ethylene and propylene formed during adsorption at 200 K and subsequent temperature programmed desorption, the following activation energies (E) and pre-exponential factors (A) are determined: (a) for ethylidyne formation: E = 17±1 kcal mol⁻¹ and A = 1×10^12±1 s⁻¹; (b) ethylidyne decomposition: E = 27 ±2 kcal mole⁻¹ and A = 6×10^11±1 s⁻¹; (c) propylidyne formation: E = 17.5 ± 2 kcal mol⁻¹ and A = 7×10^12±1 s⁻¹; and (d) propylidyne decomposition: E = 22.5±2 kcal mol⁻¹ and A = 4×10^{11 ± 1} s⁻¹.     

Inelastic electron tunneling spectroscopy of a model catalyst: CO on Rh/Al2O3

Using tunneling spectroscopy we have studied the preparation and behavior of dispersed rhodium model catalysts supported on alumina. Samples were prepared by vacuum evaporation from Rh metal or Rh₂O₃ sources onto an oxidized Al film and CO was adsorbed in-situ. The tunnel junctions were formed by adding a Pb top electrode and the vibrational spectra of the adsorbed species were measured. We observed qualitatively different spectra when the preparation procedure was varied. Special care was taken to monitor and control background gases. We obtained different results from Rh of Rh₂O₃ sources and the presence of oxygen or water affects the vibrational spectra of the adsorbed CO. We also study the effect of the Rh thickness on the spectral intensity. Other experiments were measurement of the superconducting tunneling spectra of the Pb and a TEM study of Rh particle size. Previously reported data from tunneling and IR measurements are compared with the present work. Based on these results, we conclude that there are two species present, either a linear Rh-CO or doubly (geminal) adsorbed Rh(CO)₂ depending upon the degree of dispersion and oxidation of the Rh. The evidence also indicates that in both instances a dispersed form of Rh, rather than relatively large Rh metal particles, is responsible for the observed spectra.     

Photoreduction of safranine by substituted anilinomethanesulphonates

The reactions between the triplet state of safranine and various anilines and substituted anilinomethanesulphonates (RAMS) have been studied by monitoring the formation of the semireduced form of the dye using flash photolysis. The reaction with RAMS at low concentrations and anilines corresponds to the formation of an encounter complex which allows electron transfer or deactivation. The Hammett reaction constant for poor electron-donating substituents is −2.7. At high RAMS concentrations, the semi-reduced dye yield decreases for increasing quencher concentrations. This effect is ascribed to the formation of a ground state ion-pair complex (K ≈ 140) with an unfavourable configuration for electron transfer. When excited, this complex is rapidly deactivated by static quenching.     

Parametres thermodynamiques des defauts ponctuels dans le chlorure de strontium pur ou dope par des cations monovalents ou trivalents

The electrical conductivity of a great number of SrCl₂ crystals, either purified or doped with Na⁺, K⁺, Rb⁺, Gd³⁺ and Ce³⁺, has been studied. The microscopic models used for the interpretation of the results have been justified by dielectric loss measurements, ITC and EPR (for Gd³⁺). The thermodynamic parameters of point defects have been derived, over the temperature range 420–860 K, from the fitting of experimental data to the equations of the Lidiard-Debye-Hückel theory. The values of the enthalpies of Cl vacancy mobility (0.30 eV) and Cl interstitial mobility (0.76eV), association of Cl vacancy with Na⁺ (0.53 eV), K⁺ (0.42 eV), Rb⁺ (0.40 eV) and ionic Frenkel defects formation (2.20 ± 0.11 eV) have been used in the interpretation of the experimental results over a large concentration range. The solubility enthalpy of Na ⁺ is 1.2 ± 0.1 eV; the solubility of the rare earths is very small at low temperature. The thennodynamic parameters which have been found have been compared with other known experimental values and with theoretical results.     

Torsion-vibration interaction in CH3OH

The strong torsion-vibration interaction in CH₃OH has traditionally been dealt with by letting the torsional barrier height depend on vibrational excitation, and letting the vibrational energy depend on torsional excitation. By including an explicit interaction term in the Hamiltonian this is avoided, and apart from an anomaly which is presumably caused by the OH bending mode, the relative location of the vibrational ground state and the CO stretch state is well reproduced for torsional states n = 0, 1, and 2 by adjusting a single interaction constant.     

Optical properties of Nd3+ pairs in LiYF4-existence of a short range interaction

Optical studies of Bridgman grown LiYF₄ crystals weakly doped with Nd³⁺ have been made in the vicinity of the ⁴I_9/2→⁴G_5/2 transition. By selective excitation with a narrow pulsed laser or with a CW monomode laser, excitation spectra and dynamical characteristics of infrared and ultraviolet (up-converted) fluorescences produced by satellites associated with Nd³⁺ pairs have been obtained. From the results, an assignment of the satellites to the various classes of pairs has been possible. The existence of a short range interaction, probably superexchange, has been proved which confirms previous observations in another fluoride compound, LaF₃.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.     

Comparative infrared study of silicon and germanium nitrides

Silicon and germanium nitride (Si₃N₄ and Ge₃N₄) are isomorphic compounds. They have been studied in the β-phase which crystallises in the hexagonal system. The space group is P6₃/m (C_6h²).The IR transmission spectra of these two nitrides are very similar but the absorption frequencies of germanium nitride are shifted to the lower values in comparison with silicon nitride. We noted that the atomic mass effect is the only cause of this shift for the streching modes but not for the bending modes.