The adsorption of acetic acid on clean and oxygen-covered Au/Pd(100) alloy surfaces

The adsorption of acetic acid is studied on clean and oxygen-covered Au/Pd(100) alloys as a function of gold content by temperature-programmed desorption and reflection–absorption infrared spectroscopy. Au/Pd(100) forms ordered alloys such that, for gold coverages above ~ 0.5 monolayers, only isolated palladium atoms surrounded by gold nearest neighbors are present. Predominantly molecular acetic acid forms on Au/Pd(100) alloy surfaces for gold coverages greater than ~ 0.56 ML, and desorbs with an activation energy of ~ 59 kJ/mol. Heating this surface also forms some η¹-acetate species which decompose to form CO and hydrogen. On alloy surfaces with palladium–palladium bridge sites, η¹-acetate species initially form, but rapidly convert into η²-species. They thermally decompose to form CO and hydrogen, with a small portion rehydrogenating to form acetic acid between 280 and 321 K depending on gold coverage. The presence of oxygen on both Pd(100) and Au/Pd(100) alloys facilitates acetate dehydrogenation so that only η²-acetate species form on these surfaces. The presence of oxygen also serves to stabilize the acetate species.     

Oxygen adsorption on stepped Pd(100) surfaces

We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on density-functional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2 × 2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.     

Multi-species time-history measurements during high-temperature acetone and 2-butanone pyrolysis

High-temperature acetone and 2-butanone pyrolysis studies were conducted behind reflected shock waves using five species time-history measurements (ketone, CO, CH₃, CH₄ and C₂H₄). Experimental conditions covered temperatures of 1100–1600 K at 1.6 atm, for mixtures of 0.25–1.5% ketone in argon. During acetone pyrolysis, the CO concentration time-history was found to be strongly sensitive to the acetone dissociation rate constant k₁ (CH₃COCH₃ → CH₃ + CH₃CO), and this could be directly determined from the CO time-histories, yielding k₁(1.6 atm) = 2.46 × 10¹⁴ exp(?69.3 [kcal/mol]/RT) s^?1 with an uncertainty of ±25%. This rate constant is in good agreement with previous shock tube studies from Sato and Hidaka (2000) [3] and Saxena et al. (2009) [4] (within 30%) at temperatures above 1450 K, but is at least three times faster than the evaluation from Sato and Hidaka at temperatures below 1250 K. Using this revised k₁ value with the recent mechanism of Pichon et al. (2009) [5], the simulated profiles during acetone pyrolysis show excellent agreement with all five species time-history measurements. Similarly, the overall 2-butanone decomposition rate constant k_tot was inferred from measured 2-butanone time-histories, yielding k_tot(1.5 atm) = 6.08 × 10¹³ exp(?63.1 [kcal/mol]/RT) s^?1 with an uncertainty of ±35%. This rate constant is approximately 30% faster than that proposed by Serinyel et al. (2010) [11] at 1119 K, and approximately 100% faster at 1412 K. Using the measured 2-butanone and CO time-histories and an O-atom balance analysis, a missing removal pathway for methyl ketene was identified. The rate constant for the decomposition of methyl ketene was assumed to be the same as the value for the ketene decomposition reaction. Using the revised k_tot value and adding the methyl ketene decomposition reaction to the Serinyel et al. mechanism, the simulated profiles during 2-butanone pyrolysis show good agreement with the measurements for all five species.     

CO adsorption on clean and oxidized Pt3Ti(111)

CO adsorption on clean and oxidized Pt₃Ti(111) surfaces has been investigated by means of Auger Electron Spectroscopy (AES), Thermal Desorption Spectroscopy (TDS), Low Energy Electron Diffraction (LEED) and High Resolution Electron Energy Loss Spectroscopy (HREELS). On clean Pt₃Ti(111) the LEED patterns after CO adsorption exhibit either a diffuse or a sharp c(4 × 2) structure (stable up to 300 K) depending on the adsorption temperature. Remarkably, the adsorption/desorption behavior of CO on clean Pt₃Ti(111) is similar to that on Pt(111) except that partial CO decomposition on Ti sites and partial CO oxidation have also been evidenced. Therefore, the clean surface cannot be terminated by a pure Pt plane. Partially oxidized Pt₃Ti(111) surfaces (< 135 L O₂ exposure at 1000 K) exhibit a CO adsorption/desorption behavior rather similar to that of the clean surface, showing again a c(4 × 2) structure (stable up to 250 K). Only the oxidation of CO is not detectable any more. These results indicate that some areas of the substrate remain non-oxidized upon low oxygen exposures. Heavily oxidized Pt₃Ti(111) surfaces (> 220 L O₂ exposure at 1000 K) allow no CO adsorption indicating that the titanium oxide film prepared under these conditions is completely closed.     

Strained c(4 × 2) CoO(1 0 0)-like monolayer on Pd(1 0 0): Experiment and theory

We report on an interface-stabilized strained c(4 × 2) phase formed by cobalt oxide on Pd(1 0 0). The structural details and electronic properties of this oxide monolayer are elucidated by combination of scanning tunneling microscopy data, high resolution electron energy loss spectroscopy measurements and density functional theory. The c(4 × 2) periodicity is shown to arise from a rhombic array of Co vacancies, which form in a pseudomorphic CoO(1 0 0) monolayer to partially compensate for the compressive strain associated with the large lattice mismatch (～9.5%) between cobalt monoxide and the substrate. Deviation from the perfect 1:1 stoichiometry thus appears to offer a common and stable mechanism for strain release in Pd(1 0 0) supported monolayers of transition metal rocksalt monoxides of the first transition series, as very similar metal-deficient c(4 × 2) structures have been previously found for nickel and manganese oxides on the same substrate.     

CO and H2O adsorption and reaction on Au(310)

We have studied desorption of ¹³CO and H₂O and desorption and reaction of coadsorbed, ¹³CO and H₂O on Au(310). From the clean surface, CO desorbs mainly in, two peaks centered near 140 and 200 K. A complete analysis of desorption spectra, yields average binding energies of 21 ± 2 and 37 ± 4 kJ/mol, respectively. Additional desorption states are observed near 95 K and 110 K. Post-adsorption of H₂O displaces part of CO pre-adsorbed at step sites, but does not lead to CO oxidation or significant shifts in binding energies. However, in combination with electron irradiation, ¹³CO₂ is formed during H₂O desorption. Results suggest that electron-induced decomposition products of H₂O are sheltered by hydration from direct reaction with CO.     

Oxygen-covered tungsten crystal shape: Time effects,equilibrium, surface energy and the edge-rounding temperature

The equilibrium crystal shape (ECS) of an oxygen-covered tungsten microcrystal is studied as a function of temperature. A specially designed ultrafast crystal quenching setup with the cooling rate of 6000 K/s allows to draw conclusions about ECS at high temperatures. The edge-rounding transition is shown to occur between 1300 K and 1430 K. The ratio of surface free energies γ(111)/γ(211) is determined as a function of temperature.     

Interaction of HNCO with Au(111) surfaces

The surface chemistry of isocyanic acid, HNCO, and its dissociation product, NCO, was studied on clean, O-dosed and Ar ion bombarded Au(111) surfaces. The techniques used are high resolution energy loss spectroscopy (HREELS) and temperature-programmed desorption (TPD). The structure of Ar ion etched surface is explored by scanning tunneling microscopy (STM). HNCO adsorbs molecularly on Au(111) surface at 100 K yielding strong losses at 1390, 2270 and 3230 cm^? 1. The weakly adsorbed HNCO desorbs in two peaks characterized by T_p = 130 and 145 K. The dissociation of the chemisorbed HNCO occurs at 150 K to give NCO species characterized by a vibration at 2185 cm^? 1. The dissociation process is facilitated by the presence of preadsorbed O and by defect sites on Au(111) produced by Ar ion bombardment. In the latter case the loss feature of NCO appeared at 2130 cm^? 1. Isocyanate on Au(111) surface was found to be more stable than on the single crystal surfaces of Pt-group metals. Results are compared with those obtained on supported Au catalysts.     

Sulphur adsorption on Au{1 1 0}: DFT and LEED study

The adsorption of sulphur on clean reconstructed Au{1 1 0}-(1 × 2) surface was studied using density functional theory (DFT) and quantitative low energy electron diffraction (LEED) calculations. The results show that the sulphur atoms form a (4 × 2) ordered structure which preserves the missing row reconstruction of the clean surface. The sulphur atom is found to adsorb on threefold hollow sites, on the {1 1 1} microfacets that border the trenches of the missing rows.     

Highly sensitive hydrogen detection by medium energy Ne+ impact

Hydrogen atoms on solid surfaces were measured directly by elastic recoil detection analysis (ERDA) using medium energy (100–150 keV) Ne⁺ ions with an excellent sensitivity of (～ 1 × 10¹² H/cm²) without any absorber foils and time-of-flight techniques. An electrostatic toroidal analyzer acquired H⁺ ions with energy around 11 keV recoiled from Si(111)-1 × 1-H surfaces. The H⁺ fraction strongly depends upon emerging angle and takes a value more than 50% at the angle below 70° and a saturated value of 17% at the angle above 80° with respect to surface normal. We detected H atoms on the reduced TiO₂(110) exposed to water molecules at room temperature (2 L) and estimated the absolute amount of H to be ～ 2.0 × 10¹⁴ H/cm² corresponding to ～ 38% (～ 0.38 ML) of the bridging oxygen atoms.     

The adsorption and reaction of vinyl acetate on Au/Pd(100) alloy surfaces

The surface chemistry of vinyl acetate monomer (VAM) is studied on Au/Pd(100) alloys as a function of alloy composition using temperature-programmed desorption and reflection–adsorption infrared spectroscopy. VAM adsorbs weakly on isolated palladium sites on the alloy with a heat of adsorption of ~ 55 kJ/mol, with the plane of the VAM adsorbed close to parallel to the surface. The majority of the VAM adsorbed on isolated sites desorbs molecularly with only a small portion decomposing. At lower gold coverages (below ~ 0.5 ML of gold), where palladium–palladium bridge sites are present, VAM binds to the surface in a distorted geometry via a rehybridized vinyl group. A larger proportion of this VAM decomposes and this reaction is initiated by CO bond scission in the VAM to form adsorbed acetate and vinyl species. The implication of this surface chemistry for VAM synthesis on Au/Pd(100) alloys is discussed.     

Structural and dynamical properties of Ru(0 0 0 1) surface

I performed density functional theory (DFT) calculations combined with MD simulations to study the structural relaxation of Ru(0001) surface. The surface relaxation of the topmost layer is found to be about ?4% at absolute zero temperature. Using MD simulations in the temperatures range of 50 K and 900 K, the effect is found to be minor on the surface relaxation as compared to Pd (1 1 1) clean surface. The effect of surface vibration is also investigated using a LEED code and shows no effect of the vibrational level on the IV curves, which rules out any disagreement between proper theory and LEED results of well prepared surfaces.     

Low temperature oxidation of Fe2+ surface sites on the (2 × 1) reconstructed surface of α-Fe2O3(011­2)

Temperature programmed desorption (TPD), electron energy loss spectroscopy (ELS) and low energy electron diffraction (LEED) were used to study the interaction of molecular oxygen with the (2 × 1) reconstructed surface of hematite α-Fe₂O₃(011­2) under UHV conditions. The (2 × 1) surface is formed from vacuum annealing of the ‘ideal’ (1 × 1) surface and possesses Fe²⁺ surface sites based on ELS. While O₂ does not stick to the (1 × 1) surface at 120 K, the amount of O₂ that can be reversibly adsorbed at 120 K on the (2 × 1) surface was estimated to be ～ 0.5 ML (where 1 ML is defined as the Fe³⁺ surface coverage on the ideal (1 × 1) surface), with additional O₂ that is irreversibly adsorbed based on subsequent H₂O TPD. Molecularly and dissociatively adsorbed O₂ modifies the surface chemistry of H₂O both in terms of enhanced OH stability (relative to either the (1 × 1) or (2 × 1) surfaces) and in the blocking of H₂O adsorption sites. While O₂ adsorption at 120 to 300 K does not transform the (2 × 1) surface into the (1 × 1) surface, the influence of O₂ on the (2 × 1) surface involves both charge transfer from surface Fe²⁺ sites and formation of an ordered c(2 × 2) structure resulting from O₂ dissociation.     

N2 emission-channel change in NO reduction over stepped Pd(211) by angle-resolved desorption

A sharp change in the N₂ emission channel from N₂O(a) → N₂(g) + O(a) to N(a) + N(a) → N₂(g) has been found at around 500 K in a steady-state NO + D₂ reaction over stepped Pd(211) = [(S)3(111) × (100)] by means of angle-resolved desorption. The desorbing N₂ is highly collimated at around 30° off normal toward the step-down direction below about 500 K due to the intermediate N₂O decomposition, whereas, above 500 K, the near normally directed desorption due to the recombination of N(a) is relatively enhanced. The N₂O decomposition channel is promoted when the reaction is carried out with hydrogen (deuterium) and the channel change is accelerated by quick changes of the amounts of surface hydrogen and oxygen (or NO(a)) into the opposite directions, and enhanced nitrogen removal as ammonia on the resultant hydrogen-rich surface. In the steady-state NO + CO reaction, the N₂ emission channel gradually changes above 500 K toward recombination. A model for the off-normal N₂ emission is briefly described.     

Modeling midwave infrared muzzle flash spectra from unsuppressed and flash-suppressed large caliber munitions

Time-resolved infrared spectra of firings from a 152 mm howitzer were acquired over an 1800–6000 cm^?1 spectral range using a Fourier-transform spectrometer. The instrument collected primarily at 32 cm^?1 spectral and 100 Hz temporal resolutions. Munitions included unsuppressed and chemically flash suppressed propellants. Secondary combustion occurred with unsuppressed propellants resulting in flash emissions lasting ～100 ms and dominated by H₂O and CO₂ spectral structure. Non-combusting plume emissions were one-tenth as intense and approached background levels within 20–40 ms. A low-dimensional phenomenological model was used to reduce the data to temperatures, soot absorbances, and column densities of H₂O, CO₂, CH₄, and CO. The combusting plumes exhibit peak temperatures of ～1400 K, areas of greater than 32 m², low soot emissivity of ～0.04, with nearly all the CO converted to CO₂. The non-combusting plumes exhibit lower temperatures of ～1000 K, areas of ～5 m², soot emissivity of greater than 0.38 and CO as the primary product. Maximum fit residual relative to peak intensity are 14% and 8.9% for combusting and non-combusting plumes, respectively. The model was generalized to account for turbulence-induced variations in the muzzle plumes. Distributions of temperature and concentration in 1–2 spatial regions demonstrate a reduction in maximum residuals by 40%. A two-region model of combusting plumes provides a plausible interpretation as a ～1550 K, optically thick plume core and ～2550 K, thin, surface-layer flame-front. Temperature rate of change was used to characterize timescales and energy release for plume emissions. Heat of combustion was estimated to be ～5 MJ/kg.     

Hydrogen permeation of tungsten phosphate glass thin films

Thin films of tungsten phosphate glasses were deposited on a Pd substrate by a pulsed laser deposition method and the flux of hydrogen passed thorough the glass film was measured with a conventional gas permeation technique in the temperature range 300–500 °C. The glass film deposited at low oxygen pressure was inappropriate for hydrogen permeation because of reduction of W ions due to oxygen deficiency. The membrane used in the hydrogen permeation experiment was a 3-layered membrane and consisted of Pd film (~ 20 nm), the glass film (≤ 300 nm) and the Pd substrate (250 µm). When the pressure difference of hydrogen and thickness of the glass layer were respectively 0.2 MPa and ~ 100 nm, the permeation rate through the membrane was 2.0 × 10^? 6 mol cm^? 2 s^? 1 at 500 °C. It was confirmed that the protonic and electronic mixed conducting glass thin film show high hydrogen permeation rate.     

Mid-wavelength type II InAs/GaSb superlattice infrared focal plane arrays

We have demonstrated 384 × 288 pixels mid-wavelength infrared focal plane arrays (FPA) using type II InAs/GaSb superlattice (T2SL) photodetectors with pitch of 25 μm. Two p-i-n T2SL samples were grown by molecular beam epitaxy with both GaAs-like and InSb-like interface. The diode chips were realized by pixel isolation with both dry etching and wet etching method, and passivation with SiN_x layer. The device one with 50% cutoff wavelength of 4.1 μm shows NETD ∼ 18 mK from 77 K to 100 K. The NETD of the other device with 50% cutoff wavelength at 5.6 μm is 10 mK at 77 K. Finally, the T2SL FPA shows high quality imaging capability at the temperature ranging from 80 K to 100 K which demonstrates the devices’ good temperature performance.     

InAs/GaSb strained layer superlattice detectors with nBn design

We have investigated the electrical and optical properties of an nBn based Type-II InAs/GaSb strained layer superlattice detector as a function of absorber region background carrier concentration. Temperature-dependent dark current, responsivity and detectivity were measured. At T = 77 K and V_b = 0.1 V, with two orders of magnitude change in doping concentration, the dark current density increased from ～0.3 mA/cm² to ～0.3 A/cm². We attribute this to a depletion region that exists at the AlGaSb barrier and the SLS absorber interface. The device with non-intentionally doped absorption region demonstrated the lowest dark current density (0.3 mA/cm² at 0.1 V) with a specific detectivity D¹ at zero bias equal to 1.2 × 10¹¹ Jones at 77 K. The D¹ value decreased to 6 × 10¹⁰ cm Hz^1/2/W at 150 K. This temperature dependence is significantly different from conventional PIN diodes, in which the D¹ decreases by over two orders of magnitude from 77 K to 150 K, making nBn devices a promising alternative for higher operating temperatures.     

STM and HREELS investigation of gas phase silanization on hydroxylated Si(100)

The gas phase anhydrous reaction of glycidoxypropyldimethylethoxysilane (GPDMES) with a model hydroxylated surface has been investigated using high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). Water dissociation on the clean reconstructed (2 × 1)-Si(100) surface was used to create an atomically flat surface with ~ 0.5 ML of hydroxyl groups. Exposure of this surface to GPDMES at room temperature under vacuum was found to lead to formation of covalent Si–O–Si bonds although high exposures (6 × 10⁸ L) were required for saturation. STM images at the early stages of reaction indicate that the reaction occurs randomly on the surface with no apparent clustering. The STM images together with semi-empirical (AM1) calculations provide evidence for hydrogen bonding interactions between the oxygen atoms in the molecule and surface hydroxyl groups at low coverage.