CO-H2-O2 reaction on a catalytic surface: A computer simulation study

The oxidation of carbon monoxide to form carbon dioxide and the oxidation of hydrogen to form water are the reactions of environmental and industrial importance. These two reactions have been studied independently by Monte Carlo computer simulation using Langmuir-Hinshelwood mechanism but no effort has been made to study the combined CO-H₂-O₂ reaction on these lines. Keeping in view the importance of this 3-component system, the surface coverages and production rates are studied as a function of CO partial pressure for different ratios of H₂ and O₂. The diffusion of reacting species on the surface as well as their desorption from the surface is also introduced to include temperature effects. The phase diagrams of the system are drawn to observe the behavior of these atoms/molecules on the surface and the production of CO₂ and H₂O are determined at different concentrations of H₂. The results are compared with 2-component systems.     

CO2 motion in carbon dioxide deuterohydrate determined by applying maximum entropy method to neutron powder diffraction data

Crystal structures of carbon dioxide deuterohydrate were studied by neutron powder diffraction at temperatures from 10 to 200 K. Maps of scattering length density distribution were obtained using a maximum entropy method (MEM), which clarified the motion of CO₂ molecules in the hydrate. In small cages, the carbon atom of the CO₂ molecule is at the center of the cage, and the oxygen atoms of CO₂ revolve freely around the carbon atom. In large cages, the carbon atom also is at the center of the cage, but the oxygen atoms tend to revolve around the carbon atom along the plane parallel to the hexagonal facets of the cage.     

Study on corrosion properties of pipelines in simulated produced water saturated with supercritical CO2

This work aims at systematically investigating the corrosion properties of three pipeline steels in static simulated produced water (SPW) saturated with supercritical carbon dioxide using weight-loss tests. SEM, XRD and XPS were employed to study the chemical composition and structure of the corroded surface. The results showed that the corrosion rates of the tested steels significantly decreased with increasing the exposure temperature and time in static SPW saturated with SC-CO₂. The surface film on the corroded surface, which markedly influenced the CO₂ corrosion behavior of the samples, was mainly composed of (Fe, Ca)CO₃ and α-FeOOH. Inhomogeneous element distribution of carbon, oxygen, calcium and iron in the surface film was observed. (Fe, Ca)CO₃ formed at a lower temperature was more stable than that formed at elevated temperatures.     

Residual surface stress measurements in YBa2Cu3Ox superconductors

XRD and residual surface stress (sin² ψ) measurements were carried out on YBa₂Cu₃O_x superconductors with varying oxygen stoichiometry (6.3 < x < 7.0). Slopes of the surface strain versus sin² ψ were plotted against oxygen content for certain reflections. Compressional surface stress has been found along the c-axis, while a tensile surface stress has been observed along the ab-plane. Both surface stresses were found to vary slightly with oxygen content. These findings qualitatively agree with a very small hydrostatic pressure effect on T_c for fully oxygenated YBa₂Cu₃O_x (x = 7) compared to oxygen deficient material at the surface.     

Characterization of LaMnAl11O19 by FT-IR spectroscopy of adsorbed NO and NO/O2

The nature of the NO_x species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on LaMnAl₁₁O₁₉ sample with magnetoplumbite structure obtained by a sol-gel process has been investigated by means of in situ FT-IR spectroscopy. The adsorption of NO leads to formation of anionic nitrosyls and/or cis-hyponitrite ions and reveals the presence of coordinatively unsaturated Mn³⁺ ions. Upon NO/O₂ adsorption at room temperature various nitro-nitrato structures are observed. The nitro-nitrato species produced with the participation of electrophilic oxygen species decompose at 350 °C directly to N₂ and O₂. No NO decomposition is observed in absence of molecular oxygen. The adsorbed nitro-nitrato species are inert towards the interaction with methane and block the active sites (Mn³⁺ ions) for its oxidation. Noticeable oxidation of the methane on the NOx⁻-precovered sample is observed at temperatures higher than 350 °C due to the liberation of the active sites as a result of decomposition of the surface nitro-nitrato species. Mechanism explaining the promoting effect of the molecular oxygen in the NO decomposition is proposed.     

Investigation of chemisorbed oxygen, surface segregation and effect of post-treatments on La0.8Sr0.2MnO3 powder and screen-printed layers for solid oxide fuel cell cathodes

In order to better understand the mechanism of the reaction of oxygen reduction at the surface of strontium doped lanthanum manganites (LSM) cathodes in solid electrolyte fuel cells (SOFC), the surface properties of La_0.8Sr_0.2MnO₃ powders and screen-printed layers have been characterised by various techniques.Strontium enrichment at the surface has been evidenced by X-ray photoelectron spectroscopy according to the conditions of annealing (temperature, oxygen pressure) and polarisation treatments of the samples.The interaction between oxygen and La_0.8Sr_0.2MnO₃ for SOFC cathodes has been studied by thermo-programmed desorption, in situ infrared spectrometry and calorimetry. The results indicate that various adsorbed oxygen species may exist on the surface of LSM depending on temperature.The presence of various adsorbed oxygen species and the surface Sr segregation are important factors to consider in the mechanism of oxygen reduction at LSM SOFC cathodes since they could be responsible for many discrepancies between the interpretations that can be found in the literature data.     

Further investigation on the formation mechanisms of (NH4)2SiF6 synthesized by dry etching technique

The validity of two formation mechanisms of ammonium silicofluoride (ASF), which are proposed to take place when a silicon surface is exposed to the vapor of HNO₃/HF acid mixture is investigated. Of the two proposed mechanisms regarding the synthesis of ASF on silicon surface, validity of the first predicting the release of hydrofluosilicic acid (H₂SiF₆) at the intermediate stage is examined by FTIR spectroscopy and the second mechanism suggesting O₂ release is investigated using the Winkler technique. IR absorbance bands of SiF₆²⁺ are observed on the fresh samples prepared at low (1/100) HNO₃/HF volume fractions. No significant amount of oxygen is detected during the synthesis of ASF films on silicon surface by dry etching technique. These two observations together provide firmer support for the validity of the second mechanism.     

Structure and surface characterization of ZrO2-Y2O3-Cr2O3 system

3-mol% Y₂O₃ and 0.3 to 3-mol % Cr₂O₃ co-doped ZrO₂ nanopowders were synthesized using co-precipitation technique and investigated by terms of X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Structural analysis shows no significant impact of chromium on powders structure except of presence of small amount of m-phase. Surface analysis reveals segregation of yttrium and chromium atoms to the surface along with surface enrichment by oxygen that can be attributed to residual water. Chromium surface atoms present in three oxidation states with catalytically active Cr²⁺ sites possibly controlling m-phase appearance through lattice distortion.     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

Synthesis and electronic behaviors of TiO2/carbon clusters/Cr2O3 composite materials

Nano-structured TiO₂/carbon clusters/Cr₂O₃ composite material has been successfully obtained by the microwave treatment of a TiO(acac)/Cr(acac)₃/epoxy resin complex. The compositions of the composite materials were determined using ICP, elemental analysis and surface characterization by SEM-EDX, TEM and XRD. ESR spectral examinations suggest the possibility of an electron transfer in the process of TiO₂ → carbon clusters → Cr₂O₃ with an oxidation site at TiO₂ particles and a reduction site at Cr₂O₃ particles. The preliminary experimental results show that the calcined materials could decompose methylene blue under visible-light irradiation.     

On the use of a O2:SF6 plasma treatment on GaAs processed surfaces for molecular beam epitaxial regrowth

Preparation of processed GaAs surface cleaning in view of molecular beam epitaxy regrowth by means of a O₂SF₆ microwave plasma has been investigated. Photoemission, Auger electron spectroscopy, atomic force microscopy and secondary ion mass spectrometry have been used for characterization. The O₂SF₆ plasma treatment was found to be very efficient for decontaminating the GaAs surface and leads to the formation of an oxide layer that can be taken off by a thermal or low-temperature H-plasma-assisted deoxidation. The levels of oxygen and carbon contaminants at the regrowth interface were measured to be in the range of a standard homoepitaxial layer-epiready substrate interface. Fluorine was observed to be eliminated upon deoxidation while sulphur is present, particularly in the case of low temperature grown layers. This plasma treatment was found to be efficient for preparation of processed GaAs surfaces for molecular beam epitaxial regrowth.     

Electron stimulated desorption of H3O from 316L stainless steel

Surface ions generated by electron stimulated desorption from mass spectrometer ion source grids are frequently observed, but often misidentified. For example, in the case of mass 19, the source is often assumed to be surface fluorine, but since the metal oxide on grid surfaces has been shown to form water and hydroxides, a more compelling case can be made for the formation of hydronium. Further, fluorine is strongly electronegative, so it is rarely generated as a positive ion. A commonly used metal for ion source grids is 316L stainless steel. Thermal vacuum processing by bakeout or radiation heating from the filament typically alters the surface composition to predominantly Cr₂O₃. X-ray photoelectron spectral shoulders on the O 1s and Cr 2p_3/2 peaks can be attributed to adsorbed water and hydroxides, the intensity of which can be substantially increased by hydrogen dosing. On the other hand, the sub-peak intensities are substantially reduced by heating and/or by electron bombardment. Electron bombardment diode measurements show an initial work function increase corresponding to predominant hydrogen desorption (H₂) and a subsequent work function decrease corresponding to predominant oxygen desorption (CO). The fraction of hydroxide concentration on the surface was determined from X-ray photoelectron spectroscopy and from the deconvolution of temperature desorption spectra. Electron stimulated desorption yields from the surface show unambiguous H₃O⁺ peaks that can be significantly increased by hydrogen dosing. Time of flight secondary ion mass spectrometry sputter yields show small signals of H₃O⁺, as well as its constituents (H⁺, O⁺ and OH⁺) and a small amount of fluorine as F⁻, but no F⁺ or F⁺ complexes (HF⁺, etc.). An electron stimulated desorption cross-section of σ⁺ ∼ 1.4 × 10⁻²⁰ cm² was determined for H₃O⁺ from 316L stainless steel for hydrogen residing in surface chromium hydroxide.     

K-Shell energy loss spectra of 2.5 keV electrons in CO2 and N2O

Energy loss spectra of 2.5 keV electrons, scattered by CO₂ and N₂O through small angles, have been studied in the regions around the carbon, nitrogen and oxygen K-edges. With the exception of the oxygen K-shell spectrum of nitrous oxide, the spectra are similar to those observed¹ for the diatomic molecules, N₂ and CO, in that the spectra are dominated by one very intense discrete transition while above the K-edge considerable structure is observed in addition to the normal K-continuum. This structure represents the simultaneous transitions of K-shell and valence shell electrons. The oxygen K-shell spectrum of nitrous oxide is unique in that the higher energy discrete peaks are more intense relative to the first discrete peak compared with the other spectra, while additional continuum structures are very weak. The observed spectra for the carbon K-shell of carbon dioxide and the terminal nitrogen K-shell of nitrous oxide are not in agreement with predictions based on the core analogy model which has previously been shown¹ to apply in the case of N₂ and CO.     

Synthesis and electrochemical properties of Li4Ti5O12/C composite by the PVB rheological phase method

Spinel Li₄Ti₅O₁₂/C powders were synthesized successfully by a simple rheological phase method using polyvinylbutyral (PVB) as both template and carbon source. The structure and morphology characteristics of the composite were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy and transmission electron microscopy. The XRD results showed that the composite had a good crystallinity. Its average particle size was about 2.1 μm with a narrow size distribution as a result of homogeneous mixing of the precursors. The in situ carbon coating produced by decomposition of PVB played an important role in improving electrical conductivity, thereby enhancing the rate capacity of Li₄Ti₅O₁₂ as anode material in Li-ion batteries. The Li₄Ti₅O₁₂/C composite, synthesized at 800 °C for 15 h under argon, containing 0.98 wt% of carbon, exhibited better electrochemical properties in comparison with the pristine Li₄Ti₅O₁₂, which could be attributed to the enhanced electrical conductive network of the carbon coating on the particle surface.     

Gas-sensing properties of perovskite La0.875Ba0.125FeO3 nanocrystalline powders

La_0.875Ba_0.125FeO₃ nanocrystalline powders have been prepared by a sol-gel method. The structure, conductance and gas-sensing properties were investigated. La_0.875Ba_0.125FeO₃ crystallizes as a perovskite phase with the orthorhombic structure. The La_0.875Ba_0.125FeO₃ based sensor shows good sensitivity and selectivity to alcohol gas. The highest sensitivity to 500 ppm alcohol gas reached was 58 at 170 °C. The adsorption of O₂ on the La_0.875Ba_0.125FeO₃ (0 1 0) surface was studied with the first-principles calculation based on the density functional theory. The results show that the surface states are near the Fermi energy level and that the Fe ion plays an important role in the process of oxygen adsorption, which affects the gas-sensing properties.     

Enhanced gas-sensing behaviour of Ru-doped SnO2 surface: A periodic density functional approach

A theoretical study on Ru-doped rutile SnO₂(1 1 0) surface has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA-RPBE) level with a periodic supercell approach. Electronic structure analysis was performed based on the band structure and partial density of states. The results provide evidence that the electronic structures of SnO₂(1 1 0) surface are modified by the surface Ru dopant, in which Ru 4d orbital are located at the edge of the band gap region. It is demonstrated that molecular oxygen adsorption characteristics on stoichiometric SnO₂(1 1 0) surface are changed from endothermic to exothermic due to the existence of surface Ru dopant. The dissociative adsorption of molecular oxygen on the Ru_5c/SnO₂(1 1 0) surface is exothermic, which indicates that Ru could act as an active site to increase the oxygen atom species on SnO₂(1 1 0) surface. Our present study reveals that the Ru dopant on surface is playing both electronic and chemical role in promoting the SnO₂ gas-sensing property.     

Leed and thermal desorption studies of small molecules (H2, O2, CO,CO2, NO,C2H4, C2H2 AND C) chemisorbed on the rhodium (111) and (100) surfaces

The chemisorption of H₂, O₂, CO, CO₂, NO, C₂H₄, C₂H₂ and C has been studied on the clean Rh(111) and (100) surfaces. LEED, AES and thermal desorption were used to determine the surface structures, disordering and desorption temperatures, displacement and decomposition characteristics for each species. All of the molecules studied readily chemisorbed on both surfaces. A large variety of ordered structures was observed, especially on the (111) surface. The disordering temperatures of most ordered surface structures on the (111) surface were below 100°C. It was necessary to adsorb the gases at 25° C or below in order to obtain well-ordered surface structures. Chemisorbed oxygen was readily removed from the surface by H₂ or CO gas at crystal temperatures above 50°C. CO₂ appears to dissociate to CO upon adsorption on both rhodium surfaces as indicated by the identical ordering and desorption characteristics of these two molecules. C₂H₄ and C₂H₂ also had very similar ordering and desorption characteristics and it is likely that the adsorbed species formed by both molecules is the same. Decomposition of ethylene produced a sequence of ordered carbon surface structures on the (111) face as a result of a bulk-surface carbon equilibrium. The chemisorption properties of rhodium appear to be generally similar to those of iridium, nickel and palladium.     

Methanol decomposition on the β-Ga2O3 (1 0 0) surface: A DFT approach

Density functional theory (DFT) cluster model calculations on methanol reactions on the β-Ga₂O₃ (1 0 0) surface have been realized. β-Ga₂O₃ structure has tetrahedral and octahedral ions and the results of gallia-methanol interaction are different depending on the local surface chemical composition. The surface without oxygen vacancies is very reactive and produces the methanol molecule decomposition. The unsaturated surface oxygen atoms strongly oxidize the methanol molecule. CO₂ and H₂O molecules are produced when methanol reacts with a free oxygen vacancy surface on octahedral gallium sites. On the other hand, H₂CO is found after the reaction of this molecule with a free O vacancy surface on tetrahedral gallium sites. A weak interaction between the remaining CO₂ molecule and the oxide surface was found, being this molecule easy to desorb. Otherwise, H₂CO has a stronger surface bond and it could suffer a later oxidation.     

Effect of H2 and O2 plasma etching treatment on the surface of diamond-like carbon thin film

In this study, we investigated the surface properties of diamond-like carbon (DLC) films for biomedical applications through plasma etching treatment using oxygen (O₂) and hydrogen (H₂) gas. The synthesis and post-plasma etching treatment of DLC films were carried out by 13.56 MHz RF plasma enhanced chemical vapor deposition (PECVD) system. In order to characterize the surface of DLC films, they were etched to a thickness of approximately 100 nm and were compared with an as-deposited DLC film. We obtained the optimum condition through power variation, at which the etching rate by H₂ and O₂ was 30 and 80 nm/min, respectively. The structural and chemical properties of these thin films after the plasma etching treatment were evaluated by Raman and Fourier transform infrared (FT-IR) spectroscopy. In the case of as-deposited and H₂ plasma etching-treated DLC film, the contact angle was 86.4° and 83.7°, respectively, whereas it was reduced to 35.5° in the etching-treated DLC film in O₂ plasma. The surface roughness of plasma etching-treated DLC with H₂ or O₂ was maintained smooth at 0.1 nm. These results indicated that the surface of the etching-treated DLC film in O₂ plasma was hydrophilic as well as smooth.     

Surface state of TiO2 treated with low ion energy plasma

The effect of low pressure radio frequency (rf) plasma treatment on TiO₂ surface states has been studied using X-ray photoelectron spectroscopy. Three different oxidation states of oxygen in untreated TiO₂ powder were observed, which suggests the existence of adsorbed water and carbon on the surface. The ratio of oxygen to titanium (O/Ti) was decreased for the low ion dose plasma treated samples due to desorption of water from the surface. In the case of Ti 2p about 20% of surface states were converted to Ti³⁺ 2p_3/2 state after plasma treatment with a very good stability, whereas untreated TiO₂ remained mostly as Ti⁴⁺ state. A rapid decrease in the ratio of carbon to titanium (C/Ti) at TiO₂ surface was also observed after plasma treatment and more than 90% of carbon atoms were removed from the surface. Therefore, the plasma treatment of TiO₂ has advantages to surface carbon cleaning, increasing O⁻ and Ti³⁺ surface states, hence improving the activity of TiO₂ for different environmental, energy and biological applications.