Water Desorption Peak Research Articles

Thermal desorption kinetics of H2O and H2 from rapidly solidified Al-Zn-Mg alloy powders

The thermal desorption of water and hydrogen from a rapidly solidified Al-Zn-Mg alloy powder was studied by means of temperature programmed desorption mass spectrometry (TPD-MS). By applying the Kissinger formalism, an activation energy of 137 ± 7 kJ/mol and a pre-exponential factor of 5.2 × 1011 s−1 were deduced for the primary broad water desorption peak, strongly conforming to the two pure phases of Al(OH)3, gibbsite and bayerite. Desorption of hydrogen is assigned to the oxidation of aluminum by water evolving during dehydration of the surface hydroxide layer upon heating. Overall, a complex mechanochemical mechanism is captured, with desorption profiles that are indicative of two types of oxidation processes: (1) fast, non-kinetic oxidation, initiated at ∼377 °C, followed by, (2) a slower, diffusion controlled oxidation event. The former may be viewed as a solid experimental evidence for the breakage of the surface oxide film, occurring at a relatively low temperature (<400 °C) during degassing. This breakage exposes either Al or other alloying elements to available water, which trigger in turn a fast oxidation reaction from which hydrogen is evolved.

Thermal Properties of the Stationary Current in Mesoporous Pt/TiO2 Structures in an Oxyhydrogen Atmosphere

We report on the effect of temperature on the electric current induced in the mesoporous Pt/TiO2 structure by the room temperature surface chemical reaction of hydrogen and oxygen,13,14 which helps to unveil the physical origin of this current and the related electromotive force (chemi-EMF). We found that the temperature dependence of this reaction current has a clear multipeak structure, suggesting that at least two distinct processes contribute to the current generation. We suggest that the output current represents the interplay of both chemical and electrical processes, evidenced by the metastability of the room temperature reaction and by matching one of the current peaks with a water desorption peak for TiO2.

Water Adsorption on a Co(0001) Surface

We have investigated the adsorption of water on a Co(0001) surface by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 130 K, the interaction between adsorbed water (H2O(a)) and Co(0001) is quite weak, and water adsorption forms an intact, nonwetting, and three-dimensional layer on Co(0001). The monolayer and multilayer of H2O(a) could be distinguished by XPS, and they desorb molecularly upon heating but only giving a single water desorption peak. An ordered p(2 × 2) LEED pattern was observed at low exposures of water. At room temperature, water adsorbs dissociatively on Co(0001), forming chemisorbed atomic oxygen (O(a)) and atomic hydrogen (H(a)). Upon heating, H(a) recombines into H2 desorbing from the surface, whereas O(a) remains on the surface. The interaction of water with Co(0001) is greatly influenced by the presence and nature of the oxygen species on Co(0001). Water adsorption on Co(0001) precovered with 0.45 ML O(a) at 130 K forms a mixture layer of OH(a) and H(a) via the reaction of 2H2O(a) + O(a) → 3OH(a) + H(a). However, on oxide-like Co(0001) surface, the water decomposition is completely passivated even at room temperature. These results broaden our fundamental understanding of water interaction with metal surfaces and provide insights into the water-involved catalytic reactions catalyzed by cobalt.

Water-Enhanced Low-Temperature CO Oxidation and Isotope Effects on Atomic Oxygen-Covered Au(111)

Water-oxygen interactions and CO oxidation by water on the oxygen-precovered Au(111) surface were studied by using molecular beam scattering techniques, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Water thermally desorbs from the clean Au(111) surface with a peak temperature of approximately 155 K; however, on a surface with preadsorbed atomic oxygen, a second water desorption peak appears at approximately 175 K. DFT calculations suggest that hydroxyl formation and recombination are responsible for this higher temperature desorption feature. TPD spectra support this interpretation by showing oxygen scrambling between water and adsorbed oxygen adatoms upon heating the surface. In further support of these experimental findings, DFT calculations indicate rapid diffusion of surface hydroxyl groups at temperatures as low as 75 K. Regarding the oxidation of carbon monoxide, if a C (16)O beam impinges on a Au(111) surface covered with both atomic oxygen ( (16)O) and isotopically labeled water (H 2 (18)O), both C (16)O (16)O and C (16)O (18)O are produced, even at surface temperatures as low as 77 K. Similar experiments performed by impinging a C (16)O beam on a Au(111) surface covered with isotopic oxygen ( (18)O) and deuterated water (D 2 (16)O) also produce both C (16)O (16)O and C (16)O (18)O but less than that produced by using (16)O and H 2 (18)O. These results unambiguously show the direct involvement and promoting role of water in CO oxidation on oxygen-covered Au(111) at low temperatures. On the basis of our experimental results and DFT calculations, we propose that water dissociates to form hydroxyls (OH and OD), and these hydroxyls react with CO to produce CO 2. Differences in water-oxygen interactions and oxygen scrambling were observed between (18)O/H 2 (16)O and (18)O/D 2 (16)O, the latter producing less scrambling. Similar differences were also observed in water reactivity toward CO oxidation, in which less CO 2 was produced with (16)O/D 2 (16)O than with (16)O/H 2 (16)O. These differences are likely due to primary kinetic isotope effects due to the differences in O-H and O-D bond energies.

FTIR and NMR Study of the Adsorbed Water on Nanocrystalline Anatase

A hydrated anatase sample with crystal size of 11 nm, prepared by a method involving a calcination treatment, has been studied by FTIR, 1H MAS NMR, and TPD techniques, to determine the characteristics of the hydroxyls and water adsorbed on its surface. The 1H NMR and FTIR spectra of the sample, hydrated and evacuated at increasing temperature, show that most of the hydrated anatase water, weakly adsorbed and highly mobile, is forming the multilayer of water molecular arrangements (MAs). The easy desorption of this water at room-temperature facilitates the observation of less mobile adsorbed water, whose removal originates two overlapped water desorption peaks, with maxima at 353 and 373 K, observed in the TPD profile. Most of this less weakly adsorbed water is desorbed from the MAs second layer, originating the low-temperature maximum. On the other hand, the removal of some water bound to bridging O2- anions, interacting with terminal hydroxyls, contributes to the maximum at 373 K. The TPD peak with maxim...

Using scanning tunneling microscopy to characterize adsorbates and reactive intermediates on transition metal oxide surfaces

Scanning tunneling microscopy (STM) is demonstrated to be a powerful tool to characterize adsorption and reaction on oxide surfaces by imaging molecular adsorbates and reactive intermediates. The molecules were used to probe surface structure and to study surface reactivity spatially at the atomic level. Results for three systems are presented: alcohol adsorption on WO 3(0 0 1), carboxylates on the anatase polymorph of TiO 2, and propene adsorption on a PdO monolayer on Pd(1 0 0). When the alcohols were exposed to the WO 3(0 0 1)-c(2×2) surface at room temperature the molecules could not be imaged. Heating the surface to temperatures above a water desorption peak associated with alcohol deprotonation, however, allowed 1-propoxide to be imaged. The images reveal that the alkoxide has no preference for defects, rather it binds to W 6+ ions exposed on the fully oxidized c(2×2) surface. Temperature-programmed desorption revealed that alkoxides at these sites undergo only dehydration reactions. To probe the structure of the unusual (1×4) reconstruction on anatase (0 0 1), formic and acetic acid adsorption were used. Following dissociative adsorption, both formate and acetate adsorb solely centered atop the bright rows that define the surface reconstruction, and the molecules are always at least two lattice constants apart. This result may be attributed to carboxylates bridge-bonded to Ti atoms at the center of the bright rows. This finding eliminates several suggested models of the reconstruction and suggests that a recently proposed ad-molecule model is a good representation of the surface structure. Propene was observed to initially randomly adsorb on the PdO monolayer. At higher coverages, however, the adsorbates cluster, disrupting the surface structure and causing the adsorption rate adjacent to the clusters to increase. Temperature-programmed reaction revealed that once propene adsorbs, the oxide monolayer catalyzes its oxidation at lower temperatures than metallic Pd, but that the propene sticking coefficient on the ordered oxide layer is a factor of 5 lower.

Direct reaction of adsorbed molecular oxygen with hydrazine on a clean Pt(111) surface

The oxidation of hydrazine on the clean Pt(111) surface has been investigated by temperature-programmed reaction spectroscopy (TPRS) in the temperature range 130–800 K. Direct reaction of molecular oxygen is observed on the Pt(111) surface for the first time, as indicated by the desorption of nitrogen beginning at 130 K with a maximum rate at 145 K, below the molecular oxygen dissociation temperature. Direct reaction of hydrazine with adsorbed molecular oxygen results in the formation of water and nitrogen. With excess hydrazine, all surface oxygen is reacted, forming water. When only adsorbed atomic oxygen is present, the low-temperature nitrogen yield decreases by a factor of 3 and the peak nitrogen desorption temperature increases to 170 K. No high-temperature (450–650 K) nitrogen desorption characteristic of nitrogen atom recombination is seen, indicating that during oxidation the nitrogen-nitrogen bond in hydrazine remains intact, as observed previously for hydrazine decomposition on the Pt(111) surface and hydrazine oxidation on rhodium. Two water desorption peaks are observed, characteristic of desorption-limited (175 K) and reaction-limited (200 K) water evolution from the Pt(111) surface. For low coverages of hydrazine, only the reaction-limited water desorption is observed, previously attributed to water formed from adsorbed hydroxyl groups. When excess hydrazine is adsorbed, the usual hydrazine decomposition products, H 2, N 2 and NH 3, are also observed. No nitrogen oxide species (NO, NO 2 and N 2O) were observed in these experiments, even when excess oxygen was available on the surface.

Preparation and Characterization of Fine Shirasuballoons

Hollow glass microspheres named Shirasuballoons have been prepared from nitric volcanic materials named Shirasu or Hakudo. Fine Shirasuballoons were desired to improve their physical and chemical properties. Preparation conditions of fine Shirasuballoons are examined for five kinds of vitric volcanic materials (Shinjo-Shirasu, Kakuto-Shirasu, Yoshida-Shirasu, Biei-Hakudo and Nakano-Hakudo). Fine Shirasuballoons with particle sizes under 20μm and bulk densities under 0.5g/cm3 were prepared by heating rapidly the milled powders except Shinjo-Shirasu using a fluidized bed furnace. Foaming conditions are discussed by water contents and foaming temperatures. Evolved gases from the raw materials on heating in vacuum were almost water. Two water desorption peaks are observed; a large amount of water desorption peak at temperature ranges of 200-300°C and a small amount of water desorption peak around 700°C, where the latter peak was not observed for the Shinjo-Shirasu with a low water content. Existence states of water are interpreted by infrared absorption spectrum. The shell thickness of fine Shirasuballoons produced is closely related to the water content of the raw material, which was interpreted by the phenomenon that an increase in water content of glass results in a decrease in viscosity at a foaming temperature.

Long- and short-range stabilization of water by fluorine on Ag(110)

Water and fluorine were co-adsorbed on Ag(110) in an effort to model the metal/electrolyte interface and to compare the hydration of adsorbed fluorine with previous studies of hydroxyl and chlorine hydration. Thermal desorption of coadsorbed water reveals a series of fluorine-stabilized states with peak temperatures above that for the clean surface. At low water coverage water desorbs in a broad peak (B) at 260 K, representing strong hydration of fluorine, greater than that of Cl or OH. ESDIAD (electron stimulated desorption, ion angular distribution) measurements showed emission from this adsorption state along the [11̄0] direction. With increasing water coverage, a (1 × 2) low-energy electron diffraction pattern and ESDIAD emission along [001] coincide with a new water desorption peak (A 1) at 216–230 K. Two more thermal desorption peaks, A 2 at 190–200 K and A 3 at 165–175 K, develop with additional water coverage. The solvation number N s, representing the average number of stabilized water molecules per fluorine atom, ranges from 13 in the limit of low fluorine coverage to 2 at high fluorine coverage. For solvation numbers greater than 6, the fluorine/water interactions must extend beyond nearest neighbors forming either a second hydration shell or some form of ordered water layer. As fluorine coverage increases, the form of stabilization shifts from a long-range to a more direct, short-range interaction. The surface solvation of fluorine is compared with that of OH and Cl on Ag(110), as well as with the gas phase, single-ion hydration of F −, OH − and Cl −.

酸化物超電導体粉末のガス吸脱着特性

Temperature programmed desorption (TPD) measurements in vacuum were examined for the oxide superconducting powders of synthesized high Tc Bi-Pb-Sr-Ca-Cu (Bi system) and YBa2Cu3O7-y (Y system). Oxygen desorption peak was observed around 470°C for the powder of Bi system after annealing at 400°C in oxygen atmosphere. Oxgen desorption peaks of Y system have been separated in four peaks. The desorption peak at 400-460°C is owing to the desorption of oxygen in Cu(1) plane. Two desorption peaks at 650-670°C and at 790°C seem to originate from impurity phases. The desorption peak at 900°C arises from phase separation reaction.To estimate sensitivity to water and carbon dioxide of the powders of Bi and Y system, TPD measurements were carried out after exposure to water vapor or carbon dioxide. TPD spectra of Bi system show no remarkable difference from those after annealing in oxygen atmosphere. However, two water desorption peaks were observed for the powders of Y system after exposure to water vapor at room temperature, which suggests that chemical reaction have proceeded.

Cleavage of N-H bonds by active oxygen on Ag(110): I. Ammonia

Ammonia adsorbs without dissociation on clean Ag(110) with a binding energy of 11 kcal/mol. Coadsorption of ammonia and atomic oxygen at 105 K produces adsorbed hydroxyl groups and NHx species. Coadsorption of ammonia and molecular oxygen leads to the stabilization of molecular oxygen, as is shown by the increase in the desorption peak temperature of dioxygen from 180 to 210 K. The reaction of ammonia with both forms of adsorbed oxygen produces the same products at the same temperatures. Water desorbs in a series of peaks at 310, 340, and 400 K resulting from hydroxyl recombination and hydrogen transfer from NHx species to adsorbed oxygen atoms. NO and N2 desorb together at 530 K. Oxygen recombination at 590 K only occurs following small ammonia doses such that excess oxygen persists on the surface. No hydrogen was seen to desorb under any reaction conditions. Vibrational spectroscopy shows that NH groups persist on the surface at temperatures well into the water desorption peak at 310 K and possibly to significantly higher temperatures, indicative of the difficulty of N-H bond cleavage by metallic silver.

Cleavage of N-H bonds by active oxygen on Ag(110): I. Ammonia

Ammonia adsorbs without dissociation on clean Ag(110) with a binding energy of 11 kcal/mol. Coadsorption of ammonia and atomic oxygen at 105 K produces adsorbed hydroxyl groups and NH x species. Coadsorption of ammonia and molecular oxygen leads to the stabilization of molecular oxygen, as is shown by the increase in the desorption peak temperature of dioxygen from 180 to 210 K. The reaction of ammonia with both forms of adsorbed oxygen produces the same products at the same temperatures. Water desorbs in a series of peaks at 310, 340, and 400 K resulting from hydroxyl recombination and hydrogen transfer from NH x species to adsorbed oxygen atoms. NO and N 2 desorb together at 530 K. Oxygen recombination at 590 K only occurs following small ammonia doses such that excess oxygen persists on the surface. No hydrogen was seen to desorb under any reaction conditions. Vibrational spectroscopy shows that NH groups persist on the surface at temperatures well into the water desorption peak at 310 K and possibly to significantly higher temperatures, indicative of the difficulty of N-H bond cleavage by metallic silver.

Temperature-programmed-desorption of phenol from oxide surfaces

Interactions between phenol molecules and surfaces of SiO 2−Al 2O 3 (SA), MgC and ZrO 2 were studied by using a temperature-programmed-desorption (TPD) method and an infrared spectroscopy. Desorption of phenol gave a single peak in the TP[ profiles of SA and ZrO 2 at 363 and 783 K, respectively, but complicated profiles were found on MgO. A part of the adsorbed phenol was found to decompose to benzene and water at 803 K on ZrO 2, 853 K on MgO and above 873 K on SA, Desorption of water was also found below the decomposition temperatures; SA and ZrO 2 gave a similar profile in which the water desorption peak appeared at around 473 K. No desorption of water was found on MgO at lower temperatures for MgO evacuated at 1073 K but a new peak appeared at around 538 K for MgC evacuated at 873 K. The difference in the desorption temperatures on the three samples was explained by considering the individual surface acid-base properties and OH concentration at the surface. The relative difficulty for phenol desorption and easiness for its decomposition on ZrO 2 were interpreted in terms of its weak acid and weak base properties.

Differential scanning calorimetry probes acidity strength distribution in catalytic materials

Differential scanning calorimetry (DSC) effects obtained with some catalytic solids such as amorphous silica-alumina and γ-alumina appear as an endothermic peak in the lower temperature region in a DSC thermogram, while desorption of presorbed triethylamine (TEA) from the acid sites of these solids appears as exothermic peaks in a higher temperature region. Other materials such as zeolites show a number of endothermic water-desorption peaks or a broad band covering a wide temperature range that crucially interfere with TEA-desorption peaks, and a precise evaluation of the acidity distribution in these materials is impossible. In the present work, an easy solution to this problem is found by nullifying the DSC effects caused by water desorption from the catalytic solid itself and only TEA-desorption peaks appear in the thermogram. The advantages of this DSC method over others are mentioned.

Effect of chemisorbed water on the two-dimensional condensation of water and argon on CaF2

The desorbability and regeneration of surface hydroxyls on CaF2, on which the two-dimensional condensation of water occurs, have been investigated by measuring the water adsorption isotherm, the surface water content and the i.r. spectra. Three water desorption peaks were found: A(25–150 °C), B(150–250 °C) and C(250–500 °C). Chemisorbed water exhibiting peaks A and B could be regenerated by exposing the heated surface to water vapour and are responsible for the occurrence of the two-dimensional condensation of water. Chemisorbed water corresponding to peak C could not be reproduced and it is considered to be in inner layers of the crystal. The homogeneous surface responsible for the two-dimensional condensation of water and argon was estimated for samples treated at various temperatures, and it was found that the homogeneous surface for water adsorption decays markedly on heat treatment at 250–500 °C, but recovers to the same extent as the original surface on exposing the surface to water vapour. The homogeneous surface for water adsorption is not homogeneous but heterogeneous for Ar adsorption, and vice versa.

Interaction of H 2O with basal and stepped faces of rhenium: An ESDIAD study

The adsorption and dissociation of water has been examined between 80 and 500 K on two different Re surfaces — Re(0001) and a stepped vicinal Re[16(0001) × 2(1 1 ̄ 01)] — using TDS, LEED, AES and ESDIAD (Electron Stimulated Desorption Ion Angular Distribution). Above 250 K, the adsorption is dissociative on both surfaces but the extent of dissociation is larger on a stepped than on a basal face. During adsorption, evolution of hydrogen is observed after an induction period. This phenomenon is related to the formation of OH radicals. The adsorption on Re(0001) is molecular at 80 K. Several steps could be observed: adsorption of isolated molecules, formation of bilayer clusters ordered in two ( 3 × 3 )R30° structures and finally the formation of a multilayer. Situations of non-uniform coverage of the surface have been found in which multilayer patches, bilayer clusters and uncovered areas are simultaneously present. Three molecular water desorption peaks can be observed. Peaks A (180 K) and B (150 K) are produced by desorption from the first bilayer and peak C (140–150 K) is related to the desorption from the multilayer. Furthermore, molecules in the first bilayer are partly dissociated as shown by desorption of hydrogen between 250 and 450 K. ESDIAD and LEED experiments have shown that desorption, manifested by peak A, occurs after a reconstruction of the bilayer, ordered in two (2 × 2) structures, resulting from a displacement of water molecules from the on-top sites to three fold sites. The latter molecules dissociate between 190 and 240 K giving OH ads + H ads. Dissociation of OH would occur only above 360 K. On stepped Re dissociative adsorption occurs on steps, even at 80 K, while adsorption remains molecular on terraces. This might be accounted for by the lowering of the activation barrier of the dissociation on steps.