Desorption Spectroscopy Research Articles

NO Decomposition on an Mn-Deposited Pd(100) Surface

The influence of Mn deposited on a Pd(100) surface on the adsorption, dissociation and desorption properties of NO has been studied using infrared reflection absorption spectroscopy (IRAS), temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). On the Mn/Pd(100) surface, only the NO adsorbed on the Pd was observed at 320 K. Thermal dissociation of NO did not occur on the clean Pd(100) surface; it did occur, however, on the Mn/Pd(100) surface at 320 K. A Pd-Mn alloy was formed by deposition of Mn onto the Pd(100) surface; the formation of the Pd-Mn alloy was correlated with the activity of NO dissociation, assuming that it was the active site for this dissociation. The oxygen produced from the dissociation of NO was found to destroy the Pd-Mn alloy, forming MnOx. No desorption of oxygen from MnOx on Pd(100) was observed below 1200 K.

Reactivity of Hydroxyl Species from Coadsorption of Oxygen and Water on Ni(110) Single-Crystal Surfaces

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H2O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.

Deuteron implantation into hexagonal silicon carbide: defects and deuterium behaviour

Results of the comprehensive study of deuterium-implanted hexagonal SiC (4H and 6H) using optical absorption and infrared measurements, elastic recoil detection analysis, thermal desorption and positron annihilation spectroscopies are reported. It is shown that implanted deuterium mainly forms bonds with lattice atoms. The amount of deuterium in the form of interstitial molecules and in vacancies is considerably smaller. Ion implantations with fluences exceeding 10 10 D + /cm -2 create point defects in concentrations sufficiently high for complete positron trapping. Recrystallisation of the amorphised SiC does not remove the positron traps.

Thermalized-positron induced ion-desorption from surfaces

A system of slow-positron-induced desorption spectroscopy has been constructed and applied to proton desorption from polycrystalline Ni surfaces. The cross section for the ion desorption at a positron incident energy of 1.9 keV is more than two orders of magnitude larger than that by electron-stimulated desorption with the similar incident energies. The results suggest that ion-desorption is caused by the thermalized positrons coming back from the bulk to the surface.

Europium adsorption on a tungsten surface with various degrees of oxidation

The kinetics of europium adsorption on a W(100) face with various degrees of oxidation were studied by thermal desorption and Auger electron spectroscopy. The spectrum of Eu atoms desorbed thermally from the W(100) face consists of three successively filling desorption phases whose desorption activation energy decreases from 3 to 2.1 eV with an increase in the surface coverage. The thermodesorption spectrum of Eu atoms from the W(100) face coated with a monatomic oxygen film contains five successively forming desorption phases, with the desorption activation energy increasing to 4 eV for the high-temperature phase. The oxidized W is reduced by europium, and the desorption of the W oxides is replaced by that of EuO. After a monolayer film has formed, the Eu film adsorbed on tungsten starts to grow in the form of three-dimensional crystallites. As the degree of W oxidation increases, the Eu film becomes less nonuniform, until a solid Eu film starts to grow on bulk W oxides and completely screens the tungsten Auger signal.

Decomposition of NO on Tungsten Carbide and Molybdenum Carbide Surfaces

The decomposition of 15NO on C/W(111), C/W(110), and on monolayer and bulk C/Mo/W(111) surfaces is compared based on temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) measurements. Our results indicate that the decomposition of 15NO occurs readily over all surfaces, and the only 15N-containing reaction products are 15N2 and 15N2O under our experimental conditions. Much higher surface reactivity for 15NO decomposition was observed over the more open-structured C/W(111) surface, with a value of 0.68 15NO/W, in contrast to the surface reactivity of 0.24 15NO/W over the close-packed C/W(110) surface. The selectivity of these two 15N-containing reaction products depends on the structure of the substrates as well. The more open-structured C/W(111) surface favors the production of 15N2, with a product selectivity of 15N2 being approximately 87%. In contrast, the selectivity to 15N2 is only about 52% on C/W(110). In addition, we have investigated the decomposition of 15NO on C/Mo surfaces that were epitaxially grown on W(111). The selectivity of 15N2 on C/Mo/W(111) surfaces is ∼88%, which is very similar to that observed on C/W(111). Finally, the general similarity between the DeNOx chemistry on carbides and on Pt-group metals will also be discussed.

TOTAL MERCURY DETERMINATION IN NAPHTHAS BY EITHER ATOMIC FLUORESCENCE OR ABSORPTION SPECTROSCOPY

ABSTRACT Mercury determination in light hydrocarbon fractions and natural gas condensates, has long been an important issue in the oil industry. Mercury has to be monitored, because of the many problems encountered with this metal in reforming and processing. The main objective of this study was to develop a fast reliable methodology to quantify total mercury in naphtha fractions. Since there is no certified standard analysis procedure for Hg in naphtha, two methodologies with different operating principles were implemented, namely: atomic fluorescence spectroscopy with thermal desorption (AFS-TD) and atomic absorption spectroscopy with electrothermal atomization (AAS-ETA). The statistical treatment applied was paired t-test (95% confidence level), which showed that both techniques are statistically equivalent.

Imaging of hydrogen distribution on solid surfaces by desorption spectroscopy

Two-dimensional hydrogen analysis on solid surfaces has been accomplished using an electron-stimulated desorption (ESD) microscope (the so-called scanning “protoscope”) with a finely focused electron gun. A typical pulsed electron beam has a pulse width of 220 ns at 400 eV of primary electron energy. Spatial resolution of the scanning protoscope is 1 μm for hydrogen resolution. We describe the potential of TOF-ESD and two-dimensional distribution analysis of hydrogen on the solid surface using the protoscope. Hydrogen images are presented for a copper-mesh of 1000 lines and bundled PbO fibers as a demonstration.

Development of an in vitro screening method for evaluating decontamination of sulfur mustard by reactive dermal formulations.

New barrier creams known as topical skin protectants (TSPs) recently have been formulated and demonstrated to be effective in delaying skin penetration of the blistering warfare agent sulfur mustard (HD). To further inactivate or neutralize HD, compounds that react with the toxic agent must be incorporated in the formulations, which then become reactive topical skin protectants (rTSPs). Specific and fast screening methods are necessary to assess the decontaminating activity of rTSPs. A headspace sampling technique in conjunction with thermal desorption and gas chromatography-mass spectrometry was evaluated as a potential screening method. A lower detection limit of 1 ng and a coefficient of variation of <20% were observed for the repetitive measurements of residual HD. Using this method, we evaluated a candidate rTSP. The percentage recovery of HD applied to the rTSP decreased by 35% over a 20-min time period compared with a non-rTSP. The candidate formulation showed an instant decontamination of the HD simulant dibutyl sulfide (decontamination was achieved in 2 s). The instrument set-up has the potential to accommodate multiple samples and can be automated. The method can be extended also to test reactive dermal formulations with toxic organophosphorus compounds.

Photoinduced Reaction of Digermane with Si(111)

The photoinduced reaction of digermane with the Si(111) surface using ultraviolet irradiation has been studied using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Hydrogen and germane desorption yields and relative Ge/Si AES signals are used to determine the reactivity of digermane. UV irradiation during or after dosing of the Si crystal surface at low temperatures (≤120 K) enhances the reactivity of digermane compared to similar doses without UV irradiation. Adsorption and photoexcitation of digermane at 110 K lead to significantly more reaction that at 120 K. We also find that UV irradiation after dosing the Si(111) surface with digermane enhances the reactivity more than simultaneous UV irradiation at 110 K, but the opposite is true at 120 K. Evidence is also found for precursor-mediated adsorption, and this is used to rationalize the dramatic changes with temperature. Photoexcitation of digermane at low temperatures leads initially to deposition of a-Ge:H.

Modification of formate stability by alloying: the Cu(100)-c(2 × 2)-Pt system

Room temperature deposition of Pt on Cu(100) has been studied by LEED, AES and desorption spectroscopy indicating that sub-monolayer Pt growth at 300 K leads to formation of a poorly ordered c(2 × 2) surface alloy co-adsorbed with Pt microclusters. In contrast to the Cu(100)-c(2 × 2)-Pd system, a high degree of crystalline perfection requires thermal activation which leads to surface Pt atoms switching to second layer sub-surface sites to form a c(2 × 2) underlayer below a copper monolayer. The presence of Pt in the second layer leads to electronic perturbation of the outermost copper monolayer. Formic acid adsorption on the Cu(100)-c(2 × 2)-Pt sub-surface alloy leads to formation of a formate intermediate with reduced stability relative to clean Cu(100) signalled by a ~30 K downward shift in the simultaneous CO2/H2 evolution. The formate decomposition on the Cu(100)-c(2 × 2)-CuPt underlayer follows first order decomposition kinetics and the decomposition activation energy is reduced from 119 kJ mol-1 (Cu(100)) to 110 kJ mol-1 on the CuPt underlayer alloy. Thicker Pt films with Pt coverages of 1 ML up to 2.5 ML containing significant quantities of Pt in the outermost layer in a local c(2 × 2) environment lead to an additional downward shift in the formate decomposition temperature to 406 K corresponding to a decomposition activation energy of 104 kJ mol-1. This activation energy corresponds to the formate stability on a mixed CuPt bimetallic site. Thus, the surface chemistry of Pt in Cu3Pt alloys differs significantly from that of pure Pt resulting in a much increased stability for the formate intermediate.

Chemisorption and Reaction of NO and N2O on Oxidized and Reduced Ceria Surfaces Studied by Soft X-Ray Photoemission Spectroscopy and Desorption Spectroscopy

We have examined the chemisorption and reaction of NO and N2O on ceria surfaces by soft X-ray photoelectron spectroscopy (SXPS), X-ray absorption spectroscopy, and isothermal and temperature programmed desorption (TPD) spectroscopy. Samples include highly oriented CeO2(100) and CeO2(111) thin films with surfaces of variable oxidation states. CeO2(111) thin films were grown in situ allowing control over the initial oxidation state of the ceria surface, quantitatively determined by valence band, Ce 4d, and Ce 3d photoemission. Following NO exposure, various N-containing surface species were observed by N 1s photoemission, and the distribution of these species depended upon surface oxidation state, exposure, and adsorption temperature. These species included N2O, nitrite, NO−, and three states believed to be associated with atomic or anionic forms of N. Nitrite and N2O are seen on a fully oxidized surface while NO−, N2O, and dissociation products are observed on a reduced surface. The primary reaction of NO with reduced ceria is reoxidation of the ceria by the NO, both by NO− formation and by NO dissociation, leading to immediate and thermally induced N2 desorption. Adsorption of NO at 150 K is predominantly molecular while exposure to NO at 400 K leads to a thermally activated nitride which desorbs at temperatures above 500 K. Dissociative adsorption of NO leads to the displacement of N3− at high exposures.

Determination of durability-enhancing admixtures in concrete by thermal desorption and pyrolysis gas chromatography-mass spectrometry

Two methods involving thermal desorption and pyrolysis gas chromatography-mass spectrometry are described for the determination of two durability-enhancing admixtures, a corrosion inhibitor and a shrinkage reducing agent (SRA) in hardened portland cement mortar and concrete. The corrosion inhibitor, based on calcium nitrite, is selectively measured by the in situ pyrolytic conversion of 2-aminobiphenyl to 2-phenylphenol. The SRA, which consists of a mixture of isomeric glycol and ether compounds, can be thermally desorbed directly from powdered concrete. For both durability-enhancing agents, the recovery rate generally exceeds 93% with no interference evident from several common chemical admixtures. Cement composition does not appear to significantly affect desorption efficiency for the SRA additive. Thermal desorption and pyrolysis gas chromatography are shown to be rapid, selective, and highly convenient methods for both qualitative and quantitative estimates of two important chemical admixtures.

Reductive coupling desorption of methanol on reduced SrTiO3(110)

The adsorption and reaction of methanol with SrTiO3(110) has been studied using temperature-programmed desorption and X-ray photoelectron spectroscopy. The main organic products are methane and ethane, though the exact ratio of these depends on sample history and defect levels in the surface and bulk. It is likely that the methanol dissociates to form adsorbed methyl groups associated with reduced Ti3+ and Ti2+ centres which were formed by Ar+ bombardment. We tentatively associate ethane formation with the latter sites and dialkyl formation, while methane production may be associated with the former sites and monoalkyl formation.

A New Development Of Scanning Hydrogen Imaging System And Its Application

ABSTRACTScanning type electron-stimulated desorption (ESD) spectroscopy to detect hydrogen on solid surfaces has been described. To analyze a two-dimensional distribution of hydrogen on solid surface, a pencil-type fine-focused electron gun, of which spot size is less than 300 nm at 800 eV, has been developed using a thermal field emitter. The lateral resolution of analysis is achieved less than 1 μm.Letters were drawn on a hydrogen-terminated Si(100) surface by irradiation of continuous electron beam to remove hydrogen from the surface, and by the following scanning ESD measurement, a clear letters were confirmed on this surface. Direct lithography by an electron beam on silicon surfaces has been realized.

Interaction of Hydrogen and Methane with Inp100) and GaAs(100) Surfaces

The interaction of thermally activated hydrogen atoms and methane molecules with Inp100) and GaAs(100) surfaces was studied by X-ray and UV-induced photoelectron spectroscopy (XPS, UPS), high resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and desorption spectroscopy (DS). In most cases the interaction causes a strong decomposition of the surface due to a preferential loss of the group-V and an enrichment of the group-III elements. Hydrogenation of the clean Inp100) 4×2 surface can be divided into three stages. First, there is a saturation of dangling bonds, where the 4×2 reconstruction is preserved. Then, there is a breaking of the In dimers present at the surface, resulting in a 4×1 LEED structure. Finally, a loss of phosphorus and the build-up of metallic indium droplets follows. Bombardment of Inp100) surfaces with methane ions results in the formation of In–C and P–C species. Hydrogen exposure of GaAs(100) surfaces is more effective, since it changes the surface structures already at the initial stages of interaction. This is corroborated by the HREELS and DS data, which give strong evidence for a preferential loss of arsenic. Desorption spectra were taken during the hydrogen exposure and they show for low hydrogen pressure directly the desorption of AsH3. For high hydrogen pressure (p ≥ 1×10−4 Torr) GaH3 is detected in addition. The intensity ratio of desorbing species (AsH3/GaH3) decreases with increasing hydrogen pressure. After extrapolation a value of one results at a pressure of p = 1×10−3 Torr. Models for the interaction of hydrogen and methane with Inp100) and GaAs(100) are discussed in detail.

A Study of the Brönsted Site Acidity of Crystalline and Amorphous Aluminosilicates: 1. Desorption of Ammonia, Dehydration of 2-Propanol and Cracking of Cumene

The acidity of the H-form aluminosilicates, which include ZSM-5, mordenite, Y-faujasite, Al-containing aerosils and silica gels, and the layering of aluminosilicate compounds on an aerosil surface was measured by temperature-programmed mass spectrometry desorption (TPD) of NH3. It was found that the desorption energies of NH3 decrease in the order: H-M &gt; H-ZSM-5 &gt; Al-aerosils&gt; H-Y &gt; CVD Al-silicates&gt; Al-silica gels. The catalytic activities of the studied samples in 2-propanol dehydration and cumene cracking have been shown to decrease in the same order.

Thermal‐ and electron‐stimulated chemistry of a cyclotriphosphazene lubricant on a magnetic disk with a hard carbon overcoat

The thermal and electron induced decomposition of a bis(4‐fluorophenoxy)‐tetrakis(3‐tri‐ fluoromethylphenoxy)cyclotriphosphazene(code‐named X‐1P) lubricant on a magnetic disk are studied by temperature‐programmed desorption spectroscopy, electron‐stimulated desorption, and Auger electron spectroscopy. X‐1P decomposes on the surface with a threshold temperature for dissociation at ∼570 K, ∼100 K lower than the thermal decomposition temperature of fluid molecules. Electron impact also damages X‐1P molecules. Depending on the electron energies, dissociative ionization and dissociative electron attachment are both likely mechanisms for the electron‐induced dissociation. The cross section for the dissociative electron attachment by 3 eV electrons is roughly 9×10−18 cm2. For both 8 and 25 eV electrons the cross sections are larger.

Observation of C60 cage opening on Si(111)-(7×7)

The ‘‘opening’’ of C60 on Si(111)-(7×7) has been directly observed by scanning tunneling microscopy and correlated with Auger electron spectroscopy (AES) and temperature-programmed desorption spectroscopy. Isolated C60 clusters are observed on Si(111)-(7×7) after annealing the surface to 620 K. Annealing the surface to 1020 K causes the C60 cages to open and thereby cover more of the surface. Also evident is that the opened clusters agglomerate on the surface. The opening of the C60 cage is correlated with an increase in the amount of silicon—carbon bonding and with the increase in the carbon-to-silicon AES peak height ratio with increasing annealing temperature. The ratio increases since the opened cages cover more of the substrate silicon atoms, reducing AES emission from the substrate.

Atomic Layer Growth of SiO2 on Si(100) Using the Sequential Deposition of SiCl4 and H2O

AbstractThis study explored the surface chemistry and the promise of the binary reaction scheme:(A) Si-OH+SiCl4 → Si-Cl + HCl(B) Si-Cl + H2O → Si-OH + HClfor controlled SiO2 film deposition. In this binary ABAB… sequence, each surface reaction may be self-terminating and ABAB… repetitive cycles may produce layer-by-layer controlled deposition. Using this approach, the growth of SiO2 thin films on Si(100) with atomic layer control was achieved at 600 K with pressures in the 1 to 50 Torr range. The experiments were performed in a small high pressure cell situated in a UHV chamber. This design couples CVD conditions for film growth with a UHV environment for surface analysis using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The controlled layer-by-layer deposition of SiO2 on Si(100) was demonstrated and optimized using these techniques. A stoichiometric and chlorine-free SiO2 film was also produced as revealed by TPD and AES analysis. SiO2 growth rates of approximately 1 ML of oxygen per AB cycle were obtained at 600 K. These studies demonstrate the methodology of using the combined UHV/high pressure experimental apparatus for optimizing a binary reaction CVD process.