Competitive Adsorption Of Oxygen Research Articles

The effect of carbon monoxide on the oxidation of propane over a glass fiber based platinum catalyst

Deep oxidation of hydrocarbons over platinum catalysts underlies the majority of processes for purification of industrial waste gases. Since waste gases usually contain carbon monoxide, it is important to investigate its effect on the oxidation kinetics of hydrocarbons. This paper considers results of the study on the oxidation kinetics of propane over a glass fiber platinum catalyst in the presence or absence of CO in the reaction mixture. It was found that at low temperatures the presence of CO strongly hinders the oxidation of propane, whereas upon temperature elevation this detrimental effect is replaced by the beneficial effect of CO. As shown by numerical modeling, this bidirectional effect can be attributed to the competitive adsorption of oxygen, propane and CO on the catalyst active sites.

High temperature Zircaloy-4 oxidation in water vapour-containing environments examined with Raman imaging and labelled oxygen

High Temperature oxidation of Zircaloy-4 was studied in N2–O2–H2O gas mixtures using labelled 18O2, thermogravimetry, Raman imaging and mass spectrometry. Samples were either exposed to N2–18O2–H2O gas mixtures, or two-stage oxidised first in 18O2, then in H216O - N2 or 16O2 - N2 atmospheres. In the pre-transition, diffusion-controlled regime, mostly steam contributes to the zirconia formation, while in the nitrogen-catalysed post transition regime both oxygen and steam contribute to the oxidation. These results were tentatively explained by competitive adsorption of oxygen and steam at the gas/solid interfaces, and the formation of porosity in the oxide layers.

WO3 porous nanosheet arrays with enhanced low temperature NO2 gas sensing performance

WO3 porous nanosheet arrays have been prepared by a facile chemical bath deposition. The as-prepared nanosheet arrays are found to be precursors of tungstite and hydrotungstite. The nanosheets become porous upon annealing of the precursors due to the removal of crystalline water. Constructed from porous nanosheets with ultra-thin thickness of 20 nm, WO3 arrays exhibit enhanced low temperature NO2 gas sensing performance. A high response of 460 toward 10 ppm NO2 is achieved at a working temperature of 100 °C. The superior sensing performance of the WO3 porous nanosheet arrays compared to the thick WO3 layer is ascribed to the high degree of participation in surface reaction with the gas for the nanosheet arrays. The temperature-dependant gas response to NO2 is interpreted by the competitive adsorption of oxygen and NO2 at low temperature and their desorption at high temperature.

Catalytic Combustion of Methane over Pt–Ce Oxides under Scarce Oxygen Condition

Nanocrystalline Pt–Ce oxides were prepared by citric acid method and impregnation method, and then mounted on cordierites to investigate the catalytic behavior for methane oxidation under scarce oxygen environment. XRD, H2-TPR, TEM, and XPS were employed to investigate the relationship between physicochemical characteristics and catalytic performances. Pt–CeO2 showed good activity for oxygen conversion in fuel rich condition, especially during extinction step, with a huge hysteresis loop. By varying the oxygen concentration, Pt–CeO2 catalysts were poisoned by oxygen along ignition, and the competitive adsorption of oxygen and methane inhibits the activation of methane. Owing to the interaction between Pt and CeO2, and a more homogeneous dispersion of Pt in CeO2 compared with that of Pt/CeO2 prepared by impregnation, Pt–CeO2 catalysts maintained high activity during calcination and pretreatments.

Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N 2–O 2–H 2O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H 2 18O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs +-SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H 2O (g)/O 2 ratio rather than the absolute level of H 2O (g). The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H 2O (g)/O 2 ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H 2O (g)/O 2 -ratios, and the healing of any such scale is promoted.

Noble metal and sulfuric acid modified TiO2 photocatalysts: Mineralization of organophosphorous compounds

Photocatalytic oxidation by oxygen of air in water suspension of TiO2-based catalysts was carried out for dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP)—simulants of warfare agents. Active photocatalysts were prepared via surface modification of standard photocatalyst Degussa P25 with platinum and palladium. The developed catalysts were about three-fold more active than traditionally best photocatalyst Degussa P25. Kinetic curves of DMMP oxidation on TiO2 and Pt/TiO2 are well approximated by the Langmuir–Hinshelwood model with competitive adsorption of oxygen and organophosphorus compound. The increase of activity of Pt/TiO2 is linked with higher oxygen adsorption constant or reaction rate coefficient. Photocatalytic oxidation can be scaled up to a larger reactor with the same reaction rates expressed as mmoll−1min−1 in mineralization of DMMP. The batch recirculating reactor with total volume 3l utilizes photocatalyst deposited over porous support and demonstrated higher oxidation rate and catalyst stability compared to suspended photocatalyst. Main criteria for mass-transfer process in the batch recirculating reactor were calculated to understand how concentration cross gradient depends on flow rate. The concentration of oxygen in reaction mixture changed with the stirring or recirculation rate and exerts strong influence on the oxidation rate.

High Temperature Schottky Barrier on n-Type SrTiO3 and Its Sensitivity to Ambient Gases

Metal or oxide electrodes (Pt, Au, Ag, (La, Sr)CoO3) were deposited on single crystals of 0.02 mol% Nb doped SrTiO3 by pulsed laser deposition. Current-voltage and capacitance-voltage responses were measured using three-terminal electrode configuration. Under high oxygen partial pressures, clear rectification behaviors were observed. Diffusion model well explained the current vs. voltage relationship with ideality factors close to unity. The barrier height varied reversibly with oxygen partial pressure, and was almost independent of the electrode materials, which suggested that the Fermi level at the interface was pinned by the surface states. The origin of the surface states was discussed in terms of oxygen adsorption or oxidative formation of metal vacancies around the surface. Chemical interaction between the surface and oxygen and resulting cation rearrangement was concluded to play an important role from the long stabilization time on oxygen partial pressure change. The water vapor pressure dependence of the barrier height was also explained by competitive adsorption of oxygen and water.

Comparative study on photocatalytic oxidation of four organophosphorus simulants of chemical warfare agents in aqueous suspension of titanium dioxide

Photocatalytic oxidation by oxygen of air was carried out for dimethyl methyl phosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl phosphoramidate (DEPA) in different concentration. The initial rate of organophosphorus compounds consumption increases with the initial concentration at relatively low concentrations but decreases at higher initial concentrations. If the concentration is higher than the concentration in maximum, the rate decreases because of the lack of adsorbed oxygen. These summit-like dependences are well approximated by one site Langmuir–Hinshelwood equation with competitive adsorption of oxygen and organophosphorus compound. Parameters of the Langmuir–Hinshelwood equation are reported. Complete mineralization of the organophosphorus compounds at the end of reaction was evidenced by the total organic carbon concentration profiles. These profiles have sigmoidal shape. GC–MS technique was used to identify intermediates of TEP and TMP oxidation. The main intermediates are dimethyl phosphate and methyl phosphate in the case of TMP and diethyl phosphate and ethyl phosphate in the case of TEP. The set of intermediates shows that photocatalytic oxidation proceeds primarily at α carbon atoms of TEP. The distribution of intermediates corroborates that photocatalytic oxidation is initiated by reaction with hydroxyl radicals.

Adsorption of nitric oxide on nickel(755): competitive adsorption of oxygen and nitrogen atoms along a step line of nickel(755)

The adsorption of NO on Ni(755) was studied by means of temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). A peculiar behavior of TPD spectra for N/sub 2/ can be explained by a competitive adsorption of oxygen and nitrogen atoms along a step line of Ni(755). The adsorption energy of oxygen atoms along a step line is higher than that of nitrogen atoms.

Selective reduction of nitric oxide over noble metals

Laboratory studies of the partitioning of CO between NO and O 2 under net lean conditions over the noble metals Ir, Rh, Pt, and Pd showed that over Ir alone is the CO + NO reaction favored over the CO + O 2 reaction. Furthermore, Ir exhibited a particle size dependence such that small Ir crystallites were more effective for reducing NO than large particles. Space velocity experiments showed that NO and O 2 react simultaneously with CO over Ir. NO reduction is positive order in NO concentration under the conditions studied. The NO reaction is enhanced by CO (CO is not inhibiting) but inhibited by competitive adsorption of oxygen. The conversion of oxygen was determined by CO availability and was slightly retarded by NO. A reaction mechanism is proposed for the reduction of NO in the presence of excess O 2 at 550 °C. We hypothesize that a different site requirement for NO and O 2 adsorption may be important in determining the selectivity of the catalyst when the number of vacant adsorption sites is limited. Ir differs from Rh in that Ir is able to maintain high NO conversion at elevated temperatures in the presence of excess oxygen. Rh, on the other hand, promotes NO reduction at lower temperatures than does Ir. Rh is expected to make a greater contribution to both CO oxidation and NO reduction during catalyst warmup than Ir. Iridium can be lost from the catalyst under our reaction conditions, presumably via volatilization.