Hydrogen Chemisorption on Gallium Oxide Polymorphs (original) (raw)
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Adsorption and Decomposition of Methanol on Gallium Oxide Polymorphs
The Journal of Physical Chemistry C, 2008
The adsorption of methanol was studied on three gallia polymorphs (R, , and γ), pretreated under oxygen or hydrogen at 723 K. Their Brunauer-Emmett-Teller surface areas were in the range 12-105 m 2 g-1. Methanol (or methanol-d 3) chemisorbs on the gallium oxides both molecularly, as CH 3 OH S (or CD 3 OH S ,) and dissociatively, as methoxy (CH 3 O or CD 3 O) species, at 373 K. The quantification of the total amount of chemisorbed methanol at this temperature allowed us to determine the number of available surface active sites per unit area (N S), which is in the range 1-2 µmol m-2 for the oxygen pretreated oxides at 723 K. The density of active sites was moderately smaller (∼25%) after pretreating the oxides under hydrogen at 723 K. The temperature-programmed surface reaction of adsorbed methanol and methoxy was followed by mass spectrometry and infrared spectroscopy under He flow, up to 723 K. It was found that, upon heating above 473 K, methoxy oxidized to methylenbisoxi (H 2 COO) and, then, to formate (HCOO) species, and traces of dimethyl ether were also detected. Surface formate species further decompose to give CO(g) and CO 2 (g) at temperatures higher than 573 K, with the concurrent generation of OH and H species over the surface, which react toward H 2 (g). It is suggested that the CO 2 production implies the removal of lattice oxygen, generating a surface oxygen vacancy, which can be restored by water molecules from the gas phase. Thus, gallia can be envisaged as a promising support for the steam reforming of methanol, as long as a (noble) metal officiates/ acts as a rapid H 2 releaser from the surface.
Thermal vaporization and deposition of gallium oxide in hydrogen
Journal of Nuclear Materials, 1999
The thermodynamics of gallium oxide vaporization and deposition in Ar-6% H 2 at elevated temperatures is described. It is shown that Ga 2 O 3 vaporizes in H 2 as Ga 2 O(g) at elevated temperatures. During thermal processing the Ga 2 O(g) moves to cooler zones of the furnace, back reacts with H 2 (g) and H 2 O(g) and condenses out as Ga(l) and Ga 2 O 3 (s). Upon removal from the furnace, the exposed Ga forms a ubiquitous surface oxide of Ga 2 O 3. X-ray photoelectron spectroscopy (XPS) was used to examine heat treated Ga 2 O 3 powders and vaporization products deposited onto SiO 2 and Cu substrates. In agreement with the thermodynamic predictions, these data demonstrate that the deposition product contained Ga 2 O 3 and metallic Ga. Analysis of the XPS spectra also revealed an intermediate oxidation state for Ga. The precise bonding of this state could not be demonstrated conclusively, but it is suggested that it may be solid Ga 2 O. For coherent product deposition on Cu the metallic Ga concentration increases and the Ga 2 O 3 concentration decreases with sputtering depth, suggesting the metallic Ga in the outermost layers of the deposit is readily oxidized during air exposure.
Real-time x-ray studies of gallium adsorption and desorption
Journal of Applied Physics, 2006
Real-time grazing-incidence small-angle x-ray scattering has been employed to study the adsorption and desorption of Ga on c-plane sapphire and Ga-polar GaN surfaces. Formation of self-organized liquid Ga nanodroplets has been observed on sapphire during Ga exposure from an effusion cell at high flux. Subsequent to the Ga deposition, the nanodroplets were nitridated in situ by a nitrogen plasma source, which converted the droplets into GaN nanodots. In addition to the droplet studies, at lower Ga flux, the adsorption and desorption of Ga have been studied in the predroplet regime. For identical processing conditions, significantly different Ga adsorption/desorption rates were observed on sapphire and GaN surfaces.
1997
Methane in air can be detected by the conductivity increase of Ga 2 O 3 films. Films (200 µm) of β-Ga 2 O 3 were prepared by depositing a suspension of β-Ga 2 O 3 powder (Johnson Matthey; 32102; 99,99%) on alumina substrates. The films were exposed to 20 kPa O 2 for 15 min at 934 K. In thermal desorption spectroscopy (TDS, β = 4,6 K/s, UHV conditions) only O 2 occured at temperatures above 934 K. On reduction in 100 Pa H 2 for 5 min at 800 K, only a suboxide, Ga 2 O (above 880 K), indicating a destabilisation of the lattice [1], a broad hydrogen peak (440-930 K) and the formation of water (700-900 K) were observed. No Ga 2 O 3 and O 2 were found in desorption. At temperatures between 260 K and 934 K the film was exposed to methane (100 Pa, 5 min). For exposure temperatures between 630 K and 934 K, CO, CO 2 , H 2 , and small amounts of CH 4 and the suboxide Ga 2 O appeared in desorption. A reaction scheme for the decomposition of methane is proposed. It includes the adsorption of CH 4 , the dissociation of CH 4 , the desorption of H 2 O and the formation of oxygen vacancies. These vacancies and the adsorbed hydrogen both acting as donors may explain the conductance increase on exposure to methane observed by other authors.
Journal of Physical Chemistry B, 2006
Methanol adsorption on -Ga 2 O 3 surface has been studied by Fourier transform infrared spectroscopy (FTIR) and by means of density functional theory (DFT) cluster model calculations. Adsorption sites of tetrahedral and octahedral gallium ions with different numbers of oxygen vacancies have been compared. The electronic properties of the adsorbed molecules have been monitored by computing adsorption energies, optimized geometry parameters, overlap populations, atomic charges, and vibrational frequencies. The gallia-methanol interaction has different behaviors according to the local surface chemical composition. The calculations show that methanol can react in three different ways with the gallia surface giving rise to a nondissociative adsorption, a dissociative adsorption, and an oxidative decomposition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The molecule is nondissociatively adsorbed by means of a hydrogen bond between the alcoholic hydrogen atom and a surface oxygen atom and a bond between the alcoholic oxygen atom and a surface gallium atom. Two neighbor oxygen vacancies on tetrahedral gallium sites produce the dissociation of the methanol molecule and the formation of a bridge bond between two surface gallium atoms and the methoxy group.
Sensor Actuator B Chem, 2000
Ž. Infrared emission spectroscopy IRES was used for studying in situ gas-surface interactions and adsorbed species on Ga O at 2 3 elevated temperatures used for gas sensors. Regarding the permanent presence of humidity in most gas sensor applications, the interaction of H O with the sensor surface was observed under working conditions in order to emphasize the benefits of IRES for the understanding 2 of gas sensing mechanisms. The coadsorption of water and H ,C H , acetone and ethanol was investigated to point out the role of 2 2 4 humidity in the sensitive reactions to organic gases. Screen printed Ga O films were studied in the temperature range from 2508C up to 2 3 6508C. These results were correlated with simultaneous conductivity measurements. The evident influence of the addition of water on the surface chemistry of Ga O during the adsorption of hydrogen-containing gases is confirmed by this work. q 2000 Elsevier Science S.A. 2 3 All rights reserved.
Study of Adsorption and Decomposition of H 2 O on Ge(100)
The Journal of Physical Chemistry B, 2005
The adsorption and decomposition of water on Ge(100) have been investigated using real-time scanning tunneling microscopy (STM) and density-functional theory (DFT) calculations. The STM results revealed two distinct adsorption features of H 2 O on Ge(100) corresponding to molecular adsorption and H-OH dissociative adsorption. In the molecular adsorption geometry, H 2 O molecules are bound to the surface via Ge-O dative bonds between the O atom of H 2 O and the electrophillic down atom of the Ge dimer. In the dissociative adsorption geometry, the H 2 O molecule dissociates into H and OH, which bind covalently to a Ge-Ge dimer on Ge(100) in an H-Ge-Ge-OH configuration. The DFT calculations showed that the dissociative adsorption geometry is more stable than the molecular adsorption geometry. This finding is consistent with the STM results, which showed that the dissociative product becomes dominant as the H 2 O coverage is increased. The simulated STM images agreed very well with the experimental images. In the real-time STM experiments, we also observed a structural transformation of the H 2 O molecule from the molecular adsorption to the dissociative adsorption geometry.
Gallium–Hydrogen Bond Formation on Gallium and Gallium–Palladium Silica-Supported Catalysts
Journal of Catalysis, 2002
catalysts has been investigated by in situ X-ray photoelectron and transmission infrared spectroscopies (XPS and FTIR). The precursors, calcined in dry air at 673 K, yield PdO and Ga 2 O 3 . Upon exposure to hydrogen reduction at 423 K, the fraction of reduced gallium ions (Ga δ+ cations, δ < 2) was 16% on Ga-promoted Pd/SiO 2 , whereas no Ga 3+ was reduced over Ga/SiO 2 materials. It is suggested that this promotional reduction effect of the noble metal over gallium(III) is accomplished through hydrogen spillover from metallic palladium, early reduced at 423 K. Higher reduction temperatures (723 K) lead to highly dispersed metallic palladium crystallites on the catalysts surface, and to the reduction of about 23-28% of the total gallium content in both the Ga/SiO 2 and the Ga-Pd/SiO 2 materials. Above 473 K and under hydrogen flow, a band at 2020 cm −1 developed over all these reduced gallium-containing catalysts, which was assigned to Ga δ+ -H bond stretching. A linear relationship was found between the intensity of this infrared signal and the total gallium or Ga δ+ loading over the catalysts, at 723 K and 760 Torr of flowing H 2 . We propose that gallium-hydrogen bond formation can be achieved on Ga/SiO 2 by heterolytic hydrogen dissociation on the Ga δ+ cations, which are stabilized on the silica surface, to yield additional GaO-H bonds. In the Ga-Pd/SiO 2 catalysts the process is further aided by the metal particles. The Ga δ+ -H species were unstable below 450 K and/or decomposed under evacuation but could be immediately regenerated after restoring H 2 pressure. The impact of the formation of this gallium-hydrogen bond over the hydrogenation of carbon dioxide to oxygenated compounds is also discussed. c 2002 Elsevier Science (USA)