Temperature-programmed Desorption Data Research Articles

Hydrogen adsorption in nanoporous nickel(II) phosphates.

Hydrogen sorption in the nanoporous nickel phosphates VSB-1 and VSB-5 has been studied with a combination of BET, temperature programmed desorption (TPD), and inelastic neutron scattering (INS) measurements. H(2) BET isotherms for VSB-1 are similar to those seen in nanoporous zeolites, while VSB-5 adsorbs substantially more hydrogen due to a steep initial uptake at low partial pressures. TPD data show that hydrogen interacts strongly with VSB-5, with desorption peaks at 109 and 149 K in a nitrogen flow, whereas the absence of similar peaks for VSB-1 suggests a weaker interaction. INS spectra of the rotational tunnel transition of the adsorbed H(2) also reveal a strong interaction with the VSB-5 host. These data strongly suggest the existence of coordinatively unsaturated Ni(2+) sites accessible to H(2) molecules in the pores of VSB-5.

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.

The Tribological Properties of Monolayer KCl Films on Iron in Ultrahigh Vacuum: Modeling the Extreme-Pressure Lubricating Interface

The friction coefficients of thin KCl films deposited onto clean iron in ultrahigh vacuum are measured using a tungsten carbide tip. A rapid decrease is found in the friction coefficient from ∼ 2 for clean iron to 0.27 ± 0.03 after the deposition of ∼40 A of KCl. Based on previous contact resistance measurements, this was proposed to be due to the completion of the first layer of KCl. The first-layer KCl coverage was measured by adsorbing deuterium onto an iron surface partially covered by KCl, where deuterium selectively adsorbs onto the iron. This revealed that the first monolayer is complete after the deposition of ∼40 A of KCl and that the first-layer KCl film coverage ΘKCl(1) is given by ΘKCl(1) = 1 - exp(-0.39±0.02t), where t is the film thickness. XPS data suggest that heating a KCl film to ∼550 K causes it to wet the surface. This leads to decreases in the friction coefficients for thin KCl films in accord with the idea that friction is reduced by the first monolayer of KCl on iron. Temperature-programmed desorption data indicate that KCl in the first monolayer is ∼5 kJ/mol more stable than the multilayer consistent with the wetting behavior. Finally, the kinetic data are analyzed to suggest that the first-layer film is ∼2.6 A thick.

Carbon Monoxide on γ-Alumina Single Crystal Surfaces with Gold Nanoparticles

The adsorption and oxidation reaction of CO on γ-Al2O3 single-crystal thin films with Au nanoparticles were studied as a function of the particle size using reflection−adsorption infrared spectroscopy, temperature-programmed desorption (TPD), and the molecular beam technique in a temperature range of 93 K < Ts < 213 K. It was found that CO undergoes a precursor adsorption on the Au particles ending up in an on-top position. The activation energy of this process is calculated from the molecular beam data to (Ed − Ea) = 3.53 kJ/mol. The amount of CO adsorbed increases with increasing particle size. Two types of adsorption sites could be deduced from the TPD data. The oxidation of CO, however, was never observed in the present experiments.

Ammonia adsorption on Zr(0001): effect of electron bombardment on hydrogen production

AbstractWith the ultimate goal of modifying the surface chemistry of zirconium, we present a temperature‐programmed desorption (TPD) study of ammonia adsorbed on Zr(0001) followed by soft electron bombardment. The TPD data indicate that water accompanies the thermal desorption of ammonia near 730 K, which is a result of the mixing of oxygen atoms from the subsurface region with ammonia dissociation products. Nitrogen is left at the surface following TPD and can be removed only by repeated sputtering cycles. These thermal effects occur with or without electronic excitation. Electron bombardment does, however, result in the desorption of hydrogen gas in TPD near 310 K, indicating the formation of new hydrogenic species at the surface via electronic excitation. The electron‐induced cross‐sections for the production of these species are determined to be in the 1–5(×10−18) cm2 range. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Characterization of and imprint results using indium tin oxide-based step and flash imprint lithography templates

As compared to quartz relief structures currently employed, indium tin oxide-based (ITO-based) step and flash imprint lithography templates offer advantages in terms of electron-beam writing, scanning electron microscope inspection, and pattern transfer. The material properties of the ITO have been completely characterized, and data is presented for resistivity, crystal structure, transmission, stress, surface roughness, adhesion, composition, and etch characteristics. For a 600 Å annealed ITO film, the resistivity is approximately 3.5×102 Ω/sq and the optical transmission 77% at 365 nm. The atomic film composition was found to be 31.5% In, 4.3% Sn, and 64.2% O by x-ray photoelectron spectroscopy. When contrasted with SiO2 films, temperature programmed desorption data suggests that hydroxyl groups on the ITO surface bind H2O less strongly but are more abundant. Ultimate verification of compatibility was obtained by imprinting features into an etch barrier layer using an ITO-based template. The fidelity of 20 nm isolated and semi-isolated features, the smallest features present on the template, was successfully reproduced via the imprinting process.

Oxidation of Methanol to Formaldehyde on Vanadia Films Supported on CeO2(111)

The reaction of methanol to formaldehyde on both V2O3 and V2O5 films supported on CeO2(111) was studied by temperature programmed desorption (TPD). Methanol was found to react on monolayer and submonolayer vanadia films to produce formaldehyde, while multilayer films were found to be inactive for this reaction. The kinetics and mechanism of the dehydrogenation of adsorbed methoxides was found to be dependent on the oxidation state of the vanadium cations in the supported vanadia layer. On films that contained V3+ (i.e., V2O3), adsorbed methoxy species produced via dissociative adsorption of methanol underwent dehydrogenation to formaldehyde at 600 K. In contrast, on films that contained predominantly V5+ (i.e., V2O5), adsorbed methoxy species underwent dehydrogenation to produce formaldehyde at 540 K. Kinetic parameters for the methoxide dehydrogenation to formaldehyde on both V2O3 and V2O5 films were determined from the TPD data. The TPD results also provide insight into the thermal stability of supporte...

Adsorption and decomposition of triethylindium on an InP( [formula omitted])-(2×4) surface studied by STM, TPD, and HREELS

Adsorption and decomposition of triethylindium (TEI) on an InP(0 0 1)-(2×4) surface have been studied by scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and high-resolution electron energy loss spectroscopy (HREELS). Filled-state STM images indicate that TEI molecules are preferentially adsorbed at the center of the hetero(mixed-)-dimer rows along the [1 1 ̄ 0] direction. A line profile of empty-state STM images concludes that TEI is adsorbed on the P atom at the surface. TPD data and HREELS spectra show that ethyl radical and ethylene are evolved at about 380 and 435 K, respectively, as decomposition products. The HREELS spectra suggest that TEI is adsorbed molecularly at RT and that vibration modes related to hydrocarbon disappeared at about 500 K. Adsorption sites and the decomposition mechanism are discussed in terms of the (2×4) surface structure.

H 2O interaction with bare and Li-precovered TiO 2: studies with electron spectroscopies (MIES and UPS(HeI and II))

TiO 2(1 1 0) single crystals and titania thin films on Mo(1 1 0), bare and precovered by Li atoms, were exposed to water at 120 K. During the adsorption process metastable impact electron spectroscopy (MIES) and ultraviolet photoemission spectroscopy (UPS)(HeI and II) spectra were collected in situ. Spectroscopic and temperature-programmed desorption data were obtained during the annealing of water multilayers grown at 120 K. For the adsorption on the bare TiO 2 surface we conclude that an initial dissociative adsorption at defect sites is followed by molecular adsorption up to multilayer formation. From the exposure dependence of the MIES and UPS signals we conclude that, even before the completion of the first adlayer, no isolated water molecules are sampled, and strong lateral interaction between adjacent adsorbates stabilizes molecular water. On TiO 2(1 1 0), precovered by about 1 ML of Li atoms, water is adsorbed dissociatively both at 120 K and at room temperature until the first, mixed layer consisting of Li and OH species, is completed. On top of the first layer additional water is adsorbed molecularly. Annealing experiments show that the first layer remains intact when the second layer forms.

Self-diffusion in ceria

Ceria (CeO2) is an oxygen storage material vital to the proper functioning of automobile three-way catalysts and is typically viewed as an anion conductor. Prior experimental work using temperature programmed static secondary ion mass spectrometry (TPSSIMS) has indicated that for rutile TiO2, a prototypical oxide, the mobile species are Ti cations rather than O anions. To further expand on the mobile species in CeO2 we have investigated the diffusion of both cerium and oxygen ions by TPSSIMS. The CeO2(111) film was heteroepitaxially grown by molecular beam epitaxy on a yttria stabilized zirconia substrate. Although high quality low-energy electron diffraction patterns and Auger electron spectroscopy spectra free of impurity signals were obtained after just a few sputtering and annealing cycles, further cleaning was necessary to remove intense alkali and alkaline earth signals observed in SSIMS. The CeO2(111) surface was slightly enriched in O18 by first annealing the film in UHV at 830 K and then exposing the 130 K crystal to O218. TPSSIMS data in conjunction with temperature programmed desorption data demonstrate that surface oxygen atoms begin to exchange with the bulk around 550 K. Physical deposition of submonolayer amounts of isotopically enriched cerium (136Ce) in an O218 background allowed the simultaneous study of the diffusion of both cerium and oxygen ions. Surface cerium cations were found to be immobile with no diffusion into the bulk for temperatures up to 900 K, the highest temperature studied.

Lithium titanate pebbles reprocessing by wet chemistry

An original dissolution method for irradiated Li 2TiO 3 in aqueous H 2O 2 was developed. One could easily obtain fine Li 2TiO 3 powders from the solution through drying and calcination. Li 2TiO 3 pebbles (size ∼0.6 mm, above 90% TD) were obtained from the `reprocessed' powders. These solutions were also suitable for the formation of a sol emulsion in 2-ethyl-hexanol-1, from which gelled microspheres of lithium titanate could be obtained. Locally prepared Li 2TiO 3 reprocessed and supplied pebble batches were tested for tritium release by temperature programmed desorption (TPD) methods in He + 0.1%H 2 (R-gas) after their short irradiations in a thermal neutron flux. The relative TPD data were compared. A qualitative correlation was developed between peak characteristics and pebble microstructure.

Spectroscopic studies of ethylene adsorption on oxygen-modified Ag(1 1 1) at high pressures

2-Iodoethanol adsorbs on Ag(1 1 1) to form 2-hydroxyethyl species which react at ∼230 K to eliminate water and ethylene and to deposit species of stoichiometry C 2H 4O. At high coverages, this forms predominantly oxametallacyclic species which exhibit vibrational modes at 759, 972 and 1069 cm −1. The oxametallacycle thermally decomposes forming acetaldehyde. The reflection-absorption infrared spectra of both ethylene and ethylene oxide adsorbed on oxygen-activated Ag(1 1 1) at 300 K under a pressure of 1 Torr show that they react to form both acetaldehyde and ethylene oxide species. Ethylene oxide adsorbs at high pressures via the oxygen atom with the molecular plane tilted with respect to the (1 1 1) surface. All of the adsorbed ethylene oxide and the majority of the acetaldehyde desorb on heating to 390 K, with the remainder of the acetaldehyde being removed by heating to 450 K. These results are in accord with previous temperature-programmed desorption data which showed the formation of acetaldehyde, ethylene oxide and CO 2 in this temperature range.

Ammonia decomposition on Ir(100): from ultrahigh vacuum to elevated pressures

Ammonia decomposition on Ir(100) has been studied over the pressure range from ultrahigh vacuum to 1.5 Torr and at temperatures ranging from 200 to 800 K. The kinetics of the ammonia decomposition reaction was monitored by total pressure change. The apparent activation energy obtained in this study (84 kJ/mol) is in excellent agreement with our previous studies using supported Ir catalysts (Ir/Al2O3 82 kJ/mol). Partial pressure dependence studies of the reaction rate yielded a positive order (0.9±0.1) with respect to ammonia and negative order (−0.7 ±0.1) with respect to hydrogen. Temperature-programmed desorption data from clean and hydrogen co-adsorbed Ir(100) surfaces indicate that ammonia undergoes facile decomposition on both these surfaces. Recombinative desorption of N2 is the rate-determining step with a desorption activation energy of ∼63 kJ/mol. Co-adsorption data also indicate that the observed negative order with respect to hydrogen pressure is due to enhancement of the reverse reaction (NHx + H → NHx+1, x=0–2) in the presence of excess H atoms on the surface.

The adsorption and structure of carbon monoxide on ethylidyne-covered Pd(1 1 1)

A quarter of a monolayer of carbon monoxide (CO) adsorbs on ethylidyne-covered Pd(1 1 1) (ethylidyne saturation coverage, 0.25 monolayers) at 300 K, compared to the saturation coverage of 0.55 monolayers for CO on clean Pd(1 1 1). Infrared and temperature-programmed desorption data reveal that CO adsorbs on the hexagonal-close-packed, threefold hollow site on ethylidyne-covered Pd(1 1 1). This, in turn, suggests that ethylidyne species on Pd(1 1 1) occupy face-centered-cubic, threefold hollow sites. Atop sites become occupied when an ethylidyne-covered Pd(1 1 1) surface is pressurized with 10 −2 Torr of CO, and no additional CO appears to adsorb when the surface is pressurized with 5 Torr of CO.

Partially decarbonylated tetrairidium clusters on MgO: structural characterization and catalysis of toluene hydrogenation

Supported [HIr 4(CO) 11] −, prepared by adsorption of [Ir 4(CO) 12] from n-pentane solution onto MgO powder that had been partially dehydroxylated in vacuum at 400°C, was identified by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The clusters were decarbonylated to various degrees by treatment in He or H 2 and characterized by infrared and EXAFS spectroscopies. When the decarbonylation was carried out in H 2, the iridium aggregated; infrared data show that the CO ligands reacted with H 2, resulting in the formation of water and hydrocarbons. When the decarbonylation was carried out in He, the tetrahedral cluster frame remained unchanged; the fully decarbonylated clusters, modeled as Ir 4 tetrahedra, had an Ir–Ir first-shell coordination number of 3.0±0.1 at a distance of 2.67±0.01 Å. The partially decarbonylated tetrairidium clusters chemisorbed hydrogen. Temperature-programmed desorption (TPD) data show that the strength of bonding of hydrogen to the clusters increased with increasing cluster decarbonylation. The completely decarbonylated clusters were characterized by low chemisorption capacities (H/Ir and CO/Ir values, e.g. 0.27 and 0.39, respectively). Chemisorption and EXAFS data show that the amount of hydrogen or CO chemisorbed on Ir 4 increases with increasing size of the iridium clusters and aggregates, consistent with a support effect that is maximized for the smallest clusters, Ir 4. Data representing the catalytic hydrogenation of toluene show that the rate depends on the amount of hydrogen on the clusters — and not just on the degree of decarbonylation — consistent with the suggestion that iridium atoms free of CO ligands are not sufficient for catalysis to proceed. Toluene adsorbed on the support is implicated in the catalysis.

Ultra-high vacuum investigation of the surface chemistry of zirconium

A description of a new ultra-high vacuum chamber for studying zirconium surface chemistry is presented. The design includes low-energy electron diffraction (LEED) and scanning tunneling microscopy, a gas-handling system for controlled gas exposure, a mass spectrometer for temperature programmed desorption (TPD), an electron gun for electronically inducing surface chemistry, and Auger electron spectroscopy (AES). Schematic representations of the apparatus are described, followed by the first application of TPD and LEED in this system to water adsorption on Zr(0 0 0 1). Water adsorption at 180 K followed by linear heating results in water desorption in a broad TPD feature near 550 K. Data from LEED indicate that this adsorption does not result in ordered layers until 700 K annealing, and that formation of ordered structures depends on exposure and annealing conditions. These TPD and LEED data indicate a competition between the kinetics of recombination and desorption with those of diffusion involving the subsurface regions.

Water and Methanol Adsorption on MgO(100)/Mo(100) Studied by Electron Spectroscopies and Thermal Programmed Desorption

The adsorption of methanol (CH3OH) and water (D2O) on the MgO(100)/Mo(100) surface at 100 K has been studied by metastable impact electron spectroscopy (MIES), ultraviolet photoelectron spectroscopy (UPS (HeI)), and temperature programmed desorption (TPD). To acquire detailed information regarding the initial stages of adsorption, TPD data, providing information about the relative surface coverage and the molecule-surface interaction, are compared directly with MIES data, which provide information about the electronic structure of the adsorbed molecules. For water adsorption, a prominent multilayer desorption feature develops in the TPD before the entire surface is covered, indicating 3D growth on the partially covered MgO(100)/Mo(100) surface. On the other hand, the formation of a methanol multilayer desorption feature coincides with complete coverage of the surface.

Dipole–dipole interactions among CH3Cl molecules on Ru(001): Correlation between work function change and thermal desorption studies

Work function change measurements (ΔΦ) combined with temperature programmed desorption (TPD) were employed to study layer growth mechanism and the CH3Cl dipole–dipole interactions on Ru(001), over the temperature range of 97 K–230 K. The activation energy for desorption (Ea) and the molecular dipole moment (μ) both decrease from 55.9 kJ/mol and 2.44 D, at the zero coverage limit, to 38.6 kJ/mol and 1.27 D, at one monolayer. This coverage dependence originates from strong dipolar lateral repulsion among neighbor CH3Cl molecules. Using a model introduced by Maschhoff and Cowin (MC) [J. Chem. Phys. 101, 8138 (1994)], the isolated adsorbed molecule’s dipole moment, μ0 (2.35 D) and polarizability α(8.1×10−24 cm3), were extracted from TPD data. These values agree very well with μ0 (2.12 D) and α(9.2×10−24 cm3) obtained from work function change measurements by employing the same MC model. The ability to simulate both TPD and work function change data over a wide coverage range within the framework of a single electrostatic model has been demonstrated. It enabled better understanding of fine details of surface dipolar interactions.

Temperature-Programmed Desorption and Reflectance Absorption Infrared Spectroscopy of H2O:HBr Thin Films of Varying Stoichiometry from &lt;1:1 to 5:1

Temperature-programmed desorption (TPD) and reflection absorbance infrared spectroscopy (RAIRS) were used to study H2O:HBr thin films of varying stoichiometry from <1:1 to 5:1. These films were deposited on a Ag(110) crystal at T = 100 K. The TPD data showed a single desorption peak near 180 K in the H2O+ (m/e = 18) mass channel for all of the films investigated. In the HBr+ (m/e = 82) mass channel, a single desorption peak near 180 K was observed for films with a H2O:HBr stoichiometry ≥3:1, indicating sublimation of the H2O:HBr film. For films with a H2O:HBr stoichiometry <3:1, HBr desorption began at temperatures as low as 125 K. The HBr desorption peak was broad and continuous until the entire H2O:HBr film desorbed near 180 K. RAIR spectra of the H2O:HBr films were also recorded as a function of film temperature. The infrared data in conjunction with TPD allowed for changes in the spectra to be interpreted in terms of changes in film crystallinity and film composition.

Temperature-Programmed Desorption of Oxygen Surface Complexes on Acenaphthylene-Derived Chars: Comparison with Oxygen K-Edge XANES Spectroscopy

The interpretation of temperature-programmed desorption (TPD) data relating to the decomposition of oxygen surface complexes on carbonaceous materials is complex. Oxygen K-edge XANES spectroscopy was used to characterize a series of acenaphthylene-derived chars oxidized by either partial combustion or low-temperature oxygen chemisorption prior to their study by TPD. Evidence for the presence of mobile hydrogen species was provided by the observation that H2 and H2O were evolved at high temperatures during TPD. A constant CO:CO2 ratio for the desorption products evolved during the TPD of the partially combusted chars at desorption temperatures below ∼1000 K was attributed to surface reactions of mobile oxygen surface species controlling the product ratio. This indicates that desorption product ratios may be an unreliable guide to the functional groups undergoing decomposition, especially for chars with high hydrogen contents. Above ∼1000 K, the CO:CO2 ratio was observed to increase sharply. This was attributed to scavenging of C(O) by mobile hydrogen species, although the possibility that different functional groups might have different mobilities was also considered. The TPD profiles of the surface oxygen complexes formed during oxygen chemisorption at 473 K contained additional peaks not present in the profiles of the samples combusted at 773 K. These additional peaks were attributed to the formation of a wider range of acid-like surface complexes during low-temperature oxygen chemisorption than during combustion. This suggests that care must be taken when applying low-temperature chemisorption data to combustion scenarios.