Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.     

The kinetics and mechanism of catalytic reactions by molecular beam relaxation spectroscopy: HCOOH decomposition

The reactive scattering of formic acid from Ni(110) was studied over the temperature range of 175–920 K with MBRS in the millisecond time region by employing a modulation frequency of 36.8 Hz. The steady-state carbon and oxygen composition of the surface varied over the range of temperatures studied. For beam fluxes of 10¹³ molecules/cm² sec the onset of decomposition on the steady-state surface occurred at 300 K. By 400 K decomposition was essentially complete, and the products CO₂, CO, H₂ and H₂O were detected. All reaction events were prceded by a common step, and the products were then produced by a series/parallel mechanism. The rate constants measured for H₂ and H₂O formation indicated stringent limitations on the efficiency of second-order collisions on the surface for producing gaseous products. This study illustrates the use of MBRS for surface reaction mechanistic studies in the millisecond time scale.     

Surface-mediated NH and N addition to styrene on Ag(110)

In this paper, we report the investigation of the reaction of adsorbed NH and N with styrene on Ag(110) using temperature-programmed reaction spectroscopy. Using O2 and NH3 as the starting reagents, NH and N species were deposited on a Ag surface. The reaction of styrene on NH- and N-covered Ag surface appears to yield 2-phenylaziridine and benzonitrile, with additional products HCN and NH3. The formation of aziridine, the nitrogenous analogue of styrene epoxide, is proposed to be due to the cycloaddition of adsorbed NH to the carbon-carbon double bond. These results suggest that Ag-based heterogeneous catalysts may be useful for the aziridination of olefins.     

Apparent spin polarization decay in Cu-dusted Co/Al2O3/Co tunnel junctions

Co/Al2O3/Co magnetic tunnel junctions with an interfacial Cu layer have been investigated with in situ growth characterization and ex situ magnetotransport measurements. Cu interlayers grown on Co give an approximately exponential decay of the tunneling magnetoresistance with xi approximately 0.26 nm while those grown on Al2O3 have a decay length of 0.70 nm. The difference in decay lengths can be explained by different growth morphologies, and in this way clarifies a present disagreement in the literature. For monolayer coverage of Cu, we show that the tunneling spin polarization is suppressed by at least a factor of 2 compared to Co and beyond approximately 5 ML it becomes vanishingly small.     

Identification of the intermediates in the dehydration of formic acid on Ni(110) by high resolution electron energy loss vibrational spectroscopy

High resolution electron energy loss vibrational spectroscopy was used to study the intermediate formed in the dehydration reaction for formic acid on Ni(110) and Ni(110)(4 × 5)C. On the carbided surface only the formate was observed. The frequencies of the asymmetric and symmetric OCO sketch indicated a monodentate configuration. On the clean surface a mixed adlayer of CO and HCOO formed. No losses expected for formic anhydride were observed. Lateral interactions between CO and HCOO appear to be responsible for the autocatalytic decomposition of the formate.     

NEXAFS study of HCOO/Ag(110): Evidence for dynamic bending

The structure of the surface formate on Ag(110) at 300 K was studied by near-edge X-ray absorption fine structure (NEXAFS) at the carbon K-edge. Interpretation of the NEXAFS spectra of the formate intermediate with a localized bond picture is inconsistent with the findings of previous vibrational studies. Rather, the resonances must be assigned to transitions to delocalized molecular orbitals, whereupon the results agree with the bidentate configuration deduced from the vibrational experiments. A bonding geometry is determined with respect to the surface normal in which the O-O direction lies parallel to the plane of the surface; no azimuthal ordering was detected. The molecular plane appears to be inclined at an angle of 30 ± 15°. This apparent tilt may be due to a dynamical motion of the species about the surface normal in which the molecular plane oscillates and rotates about the surface normal. These results indicate different bonding geometries for formate on Ag(110) and Cu(110).     

Structural studies of formate and methoxy groups on the Cu(100) surface using NEXAFS and SEXAFS

Local chemisorption geometries of formate (HCO₂) and methoxy (CH₃O) groups on Cu(100) were examined by means of surface extended (SEXAFS) and near-edge X-ray absorption fine structure (NEXAFS) measurements above the O K edge. At 300 K the oxygen of the formate group are equivalent and asymmetrically located near the four-fold hollow site yielding two CuO distances between 2.31 and 2.45 Å. These distances are at least 0.3 Å longer than typical CuO distances of surface and bulk compounds due to a Cu-C steric interaction. The CO bonds and the OCO angle of the formate are estimated to be $\text{1.27 ± 0.04}\text{A}\text{?}$ and 127 ± 7°, respectively. At 200 K the methoxy group has a CuO distance of $\text{1.97 ± 0.05}\text{A}\text{?}$ and the CO axis is tilted with respect to the surface normal. The exact chemisorption site of the methoxy goup could not be determined, but the atop site is ruled-out.     

The oxidation of methanol on a silver (110) catalyst

The oxidation of methanol was studied on a Ag(110) single-crystal by temperature programmed reaction spectroscopy. The Ag(110) surface was preoxidized with oxygen-18, and deuterated methanol, CH₃OD, was used to distinguish the hydroxyl hydrogen from the methyl hydrogens. Very little methanol chemisorbed on the oxygen-free Ag(110) surface, and the ability of the silver surface to dissociatively chemisorb methanol was greatly enhanced by surface oxygen. CH₃OD was selectively oxidized upon adsorption at 180 K to adsorbed CH₃O and D₂¹⁸O, and at high coverages the D₂¹⁸O was displaced from the Ag(110) surface. The methoxide species was the most abundant surface intermediate and decomposed via reaction channels at 250, 300 and 340 K to H₂CO and hydrogen. Adsorbed H₂CO also reacted with adsorbed CH₃O to form H₂COOCH₃which subsequently yielded HCOOCH₃ and hydrogen. The first-order rate constant for the dehydrogenation of D₂COOCH₃ to DCOOCH₃ and deuterium was found to be (2.4 ± 2.0) × 10¹¹ exp(?14.0 ± 0.5 $\text{kcal}\text{mole}$ · RT)sec^?1. This reaction is analogous to alkoxide transfer from metal alkoxides to aldehydes in the liquid phase. Excess surface oxygen atoms on the silver substrate resulted in the further oxidation of adsorbed H₂CO to carbon dioxide and water. The oxidation of methanol on Ag(110) is compared to the previous study on Cu(110).