Thermal Programmed Desorption Research Articles

Ultraviolet Irradiation Hardening of Hafnia Films Prepared by Sol-Gel Processes

Sol-gel-derived hafnia films containing formic acid as an organic ligand were hardened by ultraviolet (UV) irradiation. Transparent hafnia sols were prepared by treating hafnium hydroxide with a formic acid aqueous solution. The gel films were hardened to scratch hardness of over 9H by UV irradiation using a high-pressure mercury lamp for 2.5-10 min. A formato ligand was found to be bridged to Hf ions in a bond involving two metal ions by analyzing the FTIR spectra of the hafnia powders before and after UV irradiation. The hardening process of the hafnia gel films was investigated using the thermal programmed desorption (TPD) technique. The amount of CO and CO 2 generated from the formato ligands decreased after UV irradiation, as did the amount of H 2 O generated from Hf-OH in the gel, indicating that the formato ligands and OH in the gel were eliminated by UV irradiation, resulting in pencil hardness of over 9H.

Controlled mechano-chemical synthesis of nanostructured ternary complex hydride Mg2FeH6 under low-energy impact mode with and without pre-milling

The nanostructured ternary complex hydride Mg2FeH6 has been successfully synthesized in the present work by controlled reactive mechanical alloying (CRMA) of elemental Mg and Fe powders under low-energy impact mode in the magneto-mill Uni-Ball-Mill 5. The average nanograin (crystallite) size of Mg2FeH6 is within the range of ∼2–6nm as estimated by XRD. The highest yield of Mg2FeH6, close to ∼34wt.% as determined by XRD and DSC, is observed after 270h of direct (no pre-milling) CRMA. The formation of MgO during CRMA is identified as the primary factor that greatly hinders the initial formation of Mg2FeH6. Pre-milling of elemental Mg–Fe mixture can accelerate formation of Mg2FeH6 but the pre-milling time has not yet been optimized in the present work. The persistent presence of unreacted Fe even after CRMA for as long as 370h is observed in all powders synthesized in this work. DSC hydrogen desorption temperatures obtained in the present work are much lower than those reported by other researchers for the Mg–Fe–H system. These desorption temperatures (onset and peak) are close to or substantially lower than 300°C irrespective of whether the desorbing hydride is either β-MgH2 or Mg2FeH6 although it must be noted that the lowest DSC desorption peak temperature is associated with the hydrogen desorption from Mg2FeH6. Hydrogen desorption peak temperatures determined by thermally programmed desorption (TPD) are, on average, at least 50°C lower than those determined by DSC. Within the range of ∼1 to ∼10nm nanograin (crystallite) size of β-MgH2 and Mg2FeH6 no systematic correlation is observed between the DSC and TPD desorption peak temperatures of both hydrides and their nanograin sizes.

Étude de l'effet des additifs Fe, Co, Ni sur l'interaction Rh–toluène sur Rh/Al 2O 3

Effect of added Fe, Co, Ni on Rh–toluene interaction for alumina-supported rhodium catalysts. A series of catalysts Rh–M/Al 2O 3 (M = Fe, Co, Ni, Pb) was prepared by solution-impregnation method. The catalysts modified by Fe, Co, Ni were studied by thermal programmed desorption (TPD) of toluene, then tested in hydrogenation reaction of toluene between 25 and 250 °C. The effect of lead, which is known as a poison of catalyst, was also studied. The results of toluene TPD show a decrease of Rh–toluene interaction strengths in the presence of Fe, Co and Ni, which are represented by a significant increase of the fraction of toluene desorbing molecularly from the rhodium surface. Consequently, these catalysts, modified by Fe, Co and Ni, give a best catalytic activity and stability. However, the lead affects noticeably the activity and stability of Rh/Al 2O 3 catalyst. To cite this article: F. Benseradj et al., C. R. Chimie 7 (2004) .

Thermal desorption of Li from Si(1 1 1)

Desorption of Li from the Si(1 1 1) surface has been studied by thermal programmed desorption (TPD) spectroscopy. Apart from multilayer desorption TPD spectra show two peaks, which are both assigned to submonolayer desorption. They correspond to coverages of 0.27 and 1 monolayer, respectively. Desorption orders, activation energies, and attempt frequencies (pre-factors) of all features have been determined as function of Li coverage, where possible.

Retention of Hydrogen in Well-Ordered Pyrolytic Graphite as a Result of its Interaction with Atomic Hydrogen

Interaction of hydrogen atoms with highly oriented pyrolytic graphite (0001) surface at near room temperature was studied by means of thermal programmed desorption (TPD), atomic force (AFM) and scanning tunneling (STM) microscope. Bubbled surfaces were observed after irradiation of graphite with atomic hydrogen. The average height of bubbles was about 3–5 nm and the lateral size up to 100 nm. AFM and STM tips are not only used for surface imaging but also for destruction of bubbles. The possibility of bubble destruction with STM and AFM tips without noticeable traces of remaining substance of the bubbles on the surface, and a flat bottom of the produced craters may be regarded as a further support in favor of our previous hypothesis of hydrogen intercalation between upper graphite layers.

Molecular oxygen adsorption and desorption from single crystal diamond (1 1 1) and (1 1 0) surfaces

Abstract In this paper, we study the interaction of molecular oxygen with single crystal diamond (1 1 1) and (1 1 0) surfaces using thermal-programmed desorption (TPD), electron energy loss spectroscopy (EELS) and Auger electron spectroscopy (AES). The clean diamond surfaces were prepared by an ex-situ hydrogenation of the diamond samples in hydrogen plasma followed by an in-situ thermal annealing. Reconstructed clean C(1 1 1)-(2×1) and C(1 1 0) surfaces were obtained. It was found that molecular oxygen is easily chemisorbed on the clean diamond surfaces at room temperature. Carbon monoxide was the only product of thermal desorption from both surfaces. Desorption of CO from both surfaces was found to obey first-order kinetics. Apart from a low temperature desorption peak present in all TPD spectra, two CO desorption peaks at 790 and 1030 °C were observed for the C(1 1 1)-(2×1) surface, whereas, only one desorption peak in the 760–890 °C range was observed in case of the C(1 1 0) surface. This CO desorption was interpreted to have resulted from thermal decomposition of bridge COC and on-top CO bonded oxygenated species formed on the topmost surface layer as a result of molecular oxygen adsorption. The activation energies and pre-exponential factors for CO desorption were determined for both C(1 1 1)-(2×1) and C(1 1 0) surfaces. The thermal stability of the bridge COC and on-top CO species on the diamond surfaces is discussed.

Layer-by-Layer Structure in Ultrathin Aniline and Pyridine Films on Ag(111)

Ultrathin (1−10 layers) aniline and pyridine films deposited on a Ag(111) surface at 90 K have been examined using thermal programmed desorption (TPD). Multiple desorption peaks have been observed ...

TPD and XPS study of the CO adsorption on transition-SP metal systems: Pd and Al

CO adsorption properties of the bimetallic Pd/Al systems have been studied using thermally programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under UHV conditions. The samples were prepared by vacuum evaporation of Pd onto pure Al substrates. Measured TPD and XPS spectra of CO from bimetallic surfaces were compared to the ones of clean bulk metals. The deposited Pd ad-layers exhibited strong interaction with Al support, which determined the chemical activity of Pd towards interaction with CO. The formation of PdAl alloy of noble metal-like electronic structure with the d-band centered at 4.5eV below Fermi level was observed. The Pd 3d5/2 core level exhibited binding energy 1.8eV higher than that of the surface atoms of bulk Pd with further slight increase finishing at 2.1eV. The thermal desorption spectra of CO from the Pd/Al system consists of two peaks, both lower than those from Pd foil, indicating a distinct weakening in the strength of the CO adsorption bond. Heating induced alloying, an appearance of surface oxygen and a carbon contamination on bimetallic surfaces were observed. The correspondence of these effects with the character of electronic interaction between Pd and Al is discussed.

Adsorption of water on epitactic NiO(1 0 0)

The water adsorption and desorption on NiO(100) films on a Ag(100) substrate was studied with XPS, ultra-violet photoelectron spectra (UPS) and thermally programmed desorption (TPD). Two different adsorption states for the water in the monolayer can be distinguished. In the TPD spectra one signal with a narrow peak at 200 K and a second broad desorption peak at approximately 220 K are observed. The TPD signal at 200 K shows the typical behavior for a first order desorption kinetic. The quantity of the water adsorbed in this state strongly depends on the degree of surface order.In TPD spectra measured on defect rich surfaces the 200 K desorption peak is significantly reduced. The onset of the broad TPD signal at 220 K shifts strongly to higher temperatures with decreasing water coverage, like a second order desorption kinetic or the desorption of repulsively interacting adsorbent. The UPS shows only molecular water and no signal from OH groups, so a second order desorption can be excluded.

NO adsorption and dissociation on Co{1010}

Nitric oxide adsorption and dissociation on Co{101̄0} have been studied using reflection absorption infrared spectroscopy (RAIRS), low energy electron diffraction (LEED) and thermal programmed desorption (TPD). At 100 K, NO is found to absorb molecularly on the Co{101̄0} surface with an estimated 0.5 ML coverage at saturation. At low coverages, three adsorption sites, on-top, twofold and threefold, are occupied, while from moderate coverage to saturation, bridge-bonded NO becomes dominant. A sharp c(2×4) LEED pattern is observed at saturation, and a corresponding surface structure is proposed for the pattern. For all precoverages of NO, dissociation is complete on heating the surface, but the temperature at which dissociation occurs depends critically on NO coverage and on the presence of the dissociation products. TPD spectra indicate that no species other than N 2 desorbs below 630 K; c(2×6) and p(2×3) LEED patterns are observed after annealing to 600 K, and removal of O adatoms by H 2 shows that these are due to N adsorption. A first-order unimolecular reaction mechanism is proposed for NO dissociation on Co{101̄0}, with a strong dependence of the activation energy for surface dissociation on both NO coverage and the coverage of the dissociation products, N and O.

Temperature programmed desorption using time-of-flight mass spectrometry

A new method of thermal programmed desorption (TPD) using time-of-flight mass spectrometry (TOFMS) is developed for simultaneous detection, in wide mass ranges, of large molecules and their surface reaction products. It provides a three-dimensional (3D) TPD spectrum that contains a complete set of conventional two-dimensional (2D) TPD spectra of all the masses in the mass range of interest. As an example, we present a 3D TPD spectrum of (methylcyclopentadienyl)Ir(1,5-cyclooctadiene), molecular weight 379.53 AMU, on a rhodium surface. The 3D TPD spectrum comprises TOF mass spectra, from 1 to 385 AMU, accumulating 10 000 scans every 0.6 s. By covering the whole mass range of interest in one TPD experiment, the new TOFMS-TPD eliminates guesswork required in TPD using quadrupole mass spectrometry that limits the number of masses to follow.

Temperature-programmed desorption study of water-gas shift and methane steam-reforming reactions over Li/MgO catalyst

The water-gas shift (WGS) and methane steam-reforming (MSR) reactions over 1% Li/MgO catalyst were investigated using thermal-programmed desorption (TPD) technique with isotope labelled compounds (C 16O, H 2 18O, C 16O and CD 4). The experimental results show that WGS reaction over 1% Li/MgO catalyst takes place in the temperature range 400–800 K following a Langmuir–Hinshelwood mechanism. The highest reaction rate was observed at 617 K and the estimated activation energy, from the desorption peak of reaction product, is 158 kJ/mol. The experimental results suggest that the contribution of WGS side reaction to the total amount of CO 2 and H 2, formed in the oxidative coupling of methane (OCM), is important. The MSR reaction between CD 4 and water over Li/MgO catalyst could not be evidenced by TPD method due to the weak adsorption of CD 4. However, a room-temperature isotope exchange between CD 4 and the hydroxyl groups of 1% Li/MgO could be observed.

Thermal Chemistry of CH3 on Si/Cu(100)

Photoelectron spectroscopy (PES), thermal programmed desorption (TPD) studies, and scanning tunneling microscopy (STM) investigated the interaction and chemistry of CH{sub 3} (generated by the thermal cracking of azomethane) on Si/Cu(100). Si was deposited on Cu(100) by the thermal decomposition of SiH{sub 4} at 420 K. STM of adsorbate-free Si/Cu(100) at a less than saturation coverage of Si revealed a surface that contained large domains of a Cu{sub 2}Si structure. These Cu{sub 2}Si domains coexisted with regions that were believed to be lower in fractional Si coverage. TPD results showed that (CH{sub 3}){sub 3}SiH desorbed near 200 K from CH{sub 3}/Si/Cu(100) prepared with a low Si concentration. With increasing Si concentration a (CH{sub 3}){sub 3}SiH desorption state appeared near 420 K, in addition to the 200 K state. The two observed TPD states of (CH{sub 3}){sub 3}SiH at 200 and 420 K were believed to be due to the thermal reaction of CH{sub 3} with the low Si density and high Si density (i.e., Cu{sub 2}Si) regions, respectively. At a saturation coverage of Si, when the well ordered Cu{sub 2}Si phase covered the surface, only the 420 K peak was present during CH{sub 3}/Si/Cu(100) TPD. Results also suggested that (CH{sub 3})Simore » and possibly some (CH{sub 3}){sub 2}Si intermediates predominated on the surface below room temperature, and (CH{sub 3}){sub 3}Si species were formed on the surface only at temperatures between 250 and 390 K. Surface hydrogen needed for the final evolution of (CH{sub 3}){sub 3}SiH was generated from methyl groups at temperatures above 390 K on the Si-saturated Cu(100).« less

An examination of the initial oxidation of a uranium-base alloy (U–14.1 at.% Nb) by O 2 and D 2O using surface-sensitive techniques

The corrosion resistance of uranium is greatly enhanced by alloying with niobium. In this study the initial stages of corrosion of a specific uranium-base alloy (U–14.1 at.% Nb) by O 2 or D 2O have been examined using the surface specific techniques of X-ray photoelectron spectroscopy (XPS), thermal programmed desorption (TPD), static secondary-ion mass spectroscopy (SSIMS), and sputtered neutrals mass spectroscopy (SNMS). XPS studies of the U–14.1 at.% Nb surface following oxidation using O 2 at 300 K indicate production of a thin oxide overlayer of stoichiometric UO 2.0 intermixed with Nb 2O 5. The same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O 2; although, niobium is much less oxidized exhibiting a mixture of NbO and Nb. Contrary to previous XPS literature, SNMS depth profiling studies reveal that oxidation by O 2 is much greater (as judged by oxide layer thickness) than that exhibited by D 2O. An oxide layer thickness of less than 20 Å was created using D 2O as an oxidant at 300 K with exposures >3500 L (oxide layers created from O 2 are significantly greater at much smaller exposures). Formation of a critical density of Nb 2O 5 is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O − (O 2−) or OD −/OH − into the oxide/metal interface region. The domains of stability of hydroxyl formation have also been followed using TPD, SSIMS and XPS. Maximal surface hydroxyl concentrations ( Θ rel=0.30) are obtained at a surface temperature of 175 K for these experimental conditions.

A study of deuterium interaction with diamond (110) single crystal surface by TPD, EELS and LEED

The present paper describes, for the first time, the desorption of deuterium from a degraded and well-defined microwave (MW) treated Di(110) surface. The interaction of deuterium with the Di(110) surface was studied by thermal programmed desorption (TPD), low energy electron diffraction (LEED), and low energy electron energy loss spectroscopy (EELS). It was found that hydrogen MW plasma treatment yielded (111) faceting on the well-defined Di(110):H surface. Heating the hydrogenated surface at 1100 °C in vacuum resulted in hydrogen desorption. Activated low-coverage deuterium adsorption on the Di(110) surface was performed in situ. From the well-defined surface, deuterium desorbed, yielding a single desorption peak, γ 1 , at ∼960 °C. Due to numerous deuterium adsorption/desorption cycles, the well-defined Di(110) surface was gradually degraded. From the degraded Di(110) surface a broad desorption peak, γ 2 , centered at ∼660 °C, in addition to the γ 1 peak, was detected in the TPD spectrum. It is suggested that the γ 1 peak is associated with C–D bond breaking on the diamond structure. By contrast, it is suggested that the γ 2 peak is connected with desorption from different sites on non-diamond carbon. Desorption from the γ 1 sites was found to obey first-order kinetics. The activation energy and pre-exponential factor of deuterium desorption from γ 1 sites were determined to be 75.5 kcal/mol and 3×10 12 s −1 , respectively. A possible mechanism describing the first-order desorption kinetics is discussed. This discussion includes a comparison with other reports and with our own results on deuterium desorption from the Di(111) and Di(100) surfaces.

Preparation and properties of new acid catalysts obtained by grafting alkoxides and derivatives on the most common supports. Part III – grafting titanium alkoxides and sulphate derivatives on silica

Some catalysts have been prepared by grafting different amounts of titanium isopropoxide dissolved in toluene on a silica support and by steaming and calcinating the products obtained. A catalyst has been prepared also by first treating with pure sulphuric acid titanium isopropoxide, grafting the titanium alkoxide sulphate on silica and again by steaming and calcinating the product obtained. All the catalysts prepared have been characterized using different techniques such as: electrochemical potentiometric titrations to evaluate the PZC (point of zero charge), the hydroxyl densities and the surface dissociation equilibrium constants; TGA (thermal gravimetric analyses) to evaluate the hydroxyl densities; TPD (thermal programmed desorption) of organic bases such as pyridine and lutidine; and XPS (X-ray photoemission spectroscopy). All catalysts have also been tested by determining the performances in three different reactions: dehydration of methanol, epoxidation of cyclooctene with cumene hydroperoxide and isomerization of 1-butene to 2-butene. The behaviour of the catalysts prepared shows that grafting titanium on silica strongly modifies the original surface and the catalysts obtained with the mentioned grafting techniques are different from those obtained by impregnation and coprecipitation.

Interaction of methanol and water on MgO(100) studied by ultraviolet photoelectron and metastable impact electron spectroscopies

The coadsorption 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 by thermal programmed desorption (TPD). Methanol wets the MgO surface and adsorbs nondissociatively within the first monolayer with the hydroxyl group oriented toward the substrate. In coadsorption experiments, methanol wets a water precovered MgO surface; however, adding water to a methanol precovered MgO surface does not yield a water-only surface. Essentially, a constant fraction of the preadsorbed methanol remains within the topmost surface layer during the buildup of the water multilayer. Temperature-dependent measurements show that the adsorption and desorption dynamics of both water and methanol are governed by intermolecular interactions rather than adsorbate–substrate interactions.

Thermal-programmed desorption (TPD) of deuterium from Di(111) surface: presence of two adsorption states

Abstract Deuterium desorption from diamond (111) surface was investigated by thermal-programmed desorption (TPD), electron energy loss spectroscopy (EELS) and low energy electron diffraction (LEED). Annealing of diamond (111) deuterated surface results in (2 × 2)/(2 × 1) reconstruction. Activated deuterium adsorption on Di(111) surface, pre-treated in MW hydrogen plasma, was performed in situ for doses in the 0.14–30 L range (1 L  10 −6 Torr × s). Within this dose-range the (2 × 2)/(2 × 1) reconstruction did not revert into the (1 × 1) structure. The deuterium coverage for the highest adsorption dose (30 L) was estimated as ∼0.5 ML. It was found that deuterium desorbs out of two distinct adsorption sites, β 1 and β 2 from the reconstructed (111) diamond surface. No thermal-induced redistribution of deuterium between the β 1 and β 2 adsorption sites was observed. Based on the TPD peak shapes and their dependence on coverage, a first order kinetics was assumed for the desorption process. The β 1 and β 2 adsorption states were attributed to monohydride (C–D) formation on Di(111) planes and diamond edges, respectively. The desorption kinetic parameters were calculated with the following results: β 1 (high temperature TPD peak): K = 5 × 10 12 s −1 , E act = 80 kcal mol −1 and β 2 (low temperature TPD peak): K = 5 × 10 12 s −1 ; E act = 67 kcal mol −1 .

CO adsorption and CO oxidation on Rh(100)

The adsorption of CO on a Rh(100) surface and the interaction of CO with p(2 × 2) and (2 × 2)p4g oxygen layers on Rh(100) at different temperatures have been studied by means of Thermal Programmed Desorption (TPD), X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). As a measure of the reaction rate the changes in the partial pressure of CO 2 (mass 44) during titration of the oxygen layers with CO were used. The composition of the surface layer and the surface structure were monitored at various stages of the reaction. The CO O 1s XPS spectra showed that CO adsorption occurs in two bonding configurations characterized by O 1s binding energies of 532.4 and 531.5 eV, assigned to occupation of on-top and bridge sites, respectively. The on-top sites are occupied first yielding an ordered c(2 × 2) layer at coverage of 0.5 ML. At higher coverages a fraction of CO adsorbs in bridge sites and conversion to compressed p(4√2 × √2) and ‘split’ (2 × 1) structures occurs. Three CO desorption states were observed in the CO TPD curves which correlate well with the development of the three ordered structures with increasing CO coverage. The desorption parameters of each ordered structure and their variations with CO coverage were evaluated. The rate of CO oxidation reaction during titration of p(2 × 2) and (2 × 2)p4g layers was studied at temperatures ranging from 290 to 670 K. The variation of the CO 2 production revealed dependence on the temperature, reactant surface structure and coverages. It has been found that the following factors are of importance for the reaction rate: (i) the mobility of CO which at the same reaction temperature is affected by the actual oxygen surface structure; (ii) the ordering of the reactants and the structural changes in the course of the titration reaction.

Low temperature sticking and desorption dynamics of oxygen on Pt(111)

The adsorption/desorption dynamics of oxygen on Pt(111) have been studied by angle resolved thermal programmed desorption (TPD) over the temperature range from 20 to 1000 K. The first monolayer of physisorbed O2 formed by dosing at 20 K completely converts to the chemisorbed state at 30–40 K. From 100 to 160 K, chemisorbed O2 either desorbs with a cosine angular distribution or dissociates to form adsorbed O atoms. At low coverages all the molecules thermally dissociate upon heating. It follows that the minimum energy pathway leading to thermal dissociation of adsorbed O2 lies below the energetic barrier for thermal desorption. Nevertheless, the O2 angular distribution from associative desorption of adsorbed O atoms at 720 K is sharply peaked around the surface normal (∼0.2 cos ϑ + 0.8 cos5 ϑ) for both low and saturation O atom coverages. The emergence of an exit channel barrier i desorption at high temperatures is discussed within the context of transition state theory and the potential of mean force.