Oxygen Recombination Reaction Research Articles

Hydrogen and oxygen recombination reaction on Pt–Ni and Pt–Co based alloys using density functional theory

Density functional theory (DFT) calculations were used to investigate the surface performance of Pt, Ni, Co, and PtxTM1-x (0 ≤ x ≤ 1) alloys, as well as reaction intermediates (O, H, OH, OH + H, H2O) on these surfaces for H2/O2 recombination. The activity of the PtxTM1-x alloys towards H2/O2 recombination reaction was probed using adsorption energies and reaction energies. The Pt3Co, Pt3Ni and PtNi3 alloys were found to be stable along the (111) miller index, with strong surface adsorption occurring on the PtNi3 (111) surface and weaker adsorption on the Pt3Co (111) surface. Enhanced reactivity was observed on the Pt3Ni and Pt3Co (111) surfaces for the (O* + H* → OH*) reaction step, while the Pt (111) surface was most suited for the (OH* + H* → H2O) reaction step. The OH* formation reaction step was inhibited on the PtNi3 (111) surface due to the strong surface absorption of the reaction intermediates. Overall, these results suggest that the Pt3Co (111) surface is a promising alternative catalyst for H2/O2 recombination compared to pristine Pt due to its performance in the O + H adsorption and OH* formation steps.

Kinetics of non-premixed hydrogen–oxygen catalytic recombination reaction at ambient temperature

This study proposes the use of the hydrogen–oxygen catalytic recombination reaction to safely eliminate the leaked hydrogen in a confined environment. Experiments on the hydrogen–oxygen reaction catalyzed by using Pt/C as a catalyst are conducted at ambient temperature in a small cylindrical vessel. The macroscopic kinetic process of the hydrogen–oxygen recombination reaction is investigated, and the effects of the reaction parameters, such as the initial hydrogen volume fraction and catalyst layer position, on the reaction temperature and hydrogen conversion are examined. The reaction temperature and temperature rise rate are shown to reach the maximum values when the initial hydrogen fraction is 70 vol%. When the initial hydrogen fraction is ≤ 67 vol%, the hydrogen conversion reaches 100%. After the initial hydrogen fraction is > 67 vol%, the hydrogen conversion decreases significantly, and the hydrogen conversion is only 53% for the initial hydrogen fraction is up to 80 vol%. Moreover, the position of the catalyst layer has a significant effect on the reaction rate and heat distribution inside the vessel. When the catalyst layer is near the bottom of the reaction vessel, the reaction rate is accelerated and the released heat accumulates at the bottom of the vessel. The influence law of the aforementioned factors can provide a technical reference for applications of the hydrogen–oxygen catalytic reaction.

On the Correlation between the Oxygen in Hydrogen Content and the Catalytic Activity of Cathode Catalysts in PEM Water Electrolysis

Hydrogen has a central role in future energy storage scenarios. In order to realise high market penetrations for proton exchange membrane water electrolyzers (PEMWE), the investment costs need to be reduced without compromising operational costs [1, 2, 3]. Today only the usage of precious platinum group metals (PGM) as electrocatalysts allows a good electrochemical polarisation behaviour and accordingly minimal operating costs. Platinum is known to be an outstanding catalyst for both hydrogen oxidation and evolution [1, 2, 3, 4]. However, platinum also catalyses the formation of reactive oxygen species when oxygen permeates from anode to cathode. The intermediates of the recombination reaction are sufficiently reactive to attack the polymeric backbone of the electrolyte membrane which leads to a performance loss due to material degradation and eventually to a cell failure [1, 5, 6].For this reason, not only the polarisation behaviour but also the hydrogen-oxygen recombination characteristics need to be evaluated in order to find new cathode catalysts for PEMWE. In this contribution, the polarisation behaviour of five different cathode catalysts was investigated. The ordinary electrochemical characterisation (polarisation curves, electrochemical impedance spectroscopy and linear sweep voltammetry) was extended by measuring the permeated oxygen in the cathodic product gas to draw conclusions on the recombination characteristics of the used catalysts.In general, the studied PGM-free catalysts showed lower activities for the oxygen reduction reaction and higher oxygen in hydrogen volume fractions compared to the PGM catalysts. However, all investigated electrocatalysts still show an activity towards the recombination reaction. Ex situ linear sweep voltammetry measurements have shown that the activity for the recombination reaction of oxygen and hydrogen correlates with the activity for the oxygen reduction reaction. In particular, a commercial PGM-free cathode catalyst based on a Ti suboxide showed a comparably good polarisation behaviour for the hydrogen evolution reaction similar to platinum and the lowest activity for the oxygen reduction reaction at the same time. Further, also the influence of the used PTL material on the measurable oxygen crossover was investigated. The results show that also carbon based PTL materials contain catalytically active sites for the recombination of permeated oxygen with hydrogen.Literature:[1] U. Babic, J. Elec. Soc., 2017, 164.[2] K. Ayers, Annu. Rev. Chem. Biomol. Eng., 2019, 10.[3] M. Carmo, Int. J. Hydrogen Energ., 2013, 38.[4] C. V. Pham, ChemElectroChem, 2018, 5.[5] M. Chandesris, Int. J. Hydrogen Energ., 2015, 3.[6] S.A. Grigoriev, Int. J. Hydrogen Energ., 2014, 35. Figure 1

Inhibition of H2-O2 recombination using amino functional groups to anchor oxygen for enhancing photocatalytic hydrogen generation

The recombination reaction of hydrogen and oxygen on the photocatalyst surface is a universal phenomenon, and it competes with the photocatalytic water splitting. To improve the photocatalytic hydrogen production efficiency, the rapid and effective separation of generated hydrogen and oxygen is one of the important means to inhibit this reverse reaction. Herein, theoretical simulation find that the amino-functionalized titanium dioxide (AF-TiO2) has excellent thermodynamic stability, and shows a strong oxygen adsorption capacity as well as is easy to release hydrogen compared with bare TiO2. So the amino functional groups on the catalyst surface can be used as an anchor sites for oxygen to separate well hydrogen and oxygen. Based on the theoretical results, the amide-functionalized AF-TiO2/Pt photocatalyst is synthesized by solvothermal method. A series of structural tests find that the AF-TiO2/Pt surface contains abundant and stable amide groups, which act as oxygen adsorption sites to reduce hydrogen-oxygen recombination. Therefore, AF-TiO2/Pt catalyst shows a high and stable photocatalytic hydrogen production activity under UV light triggering in pure water, which is 14.25 times than the H2 evolved over TiO2/Pt. Research ideas and design methods in this paper might provide a new way for the preparing solar-driven high active photocatalyst.

The Influence of pH on the Morphology and Electrochemical Behavior of Pd–Pt thin Films Prepared by Electro-less Deposition

Pd–Pt thin films over titanium substrates were coated through electro-less deposition method as a function of pH values. It is concluded that highly acidic medium is helpful in achieving high amount of Pd–Pt coatings. The prepared thin films of different Pd–Pt ratios were characterized using the techniques of scanning electron microscopy and Energy dispersive X-ray spectroscopy. The catalytic efficiency was evaluated under static air conditions for hydrogen and oxygen recombination reaction. The developed catalysts have been found to be highly efficient for removal of hydrogen. Comparison of the hydrogen evaluation reaction, HER by Pd/Pt bimetallic catalysts with different percentages of Pd and Pt was measured and concluded that the hydrogen removal efficiency of catalyst having 35 wt % palladium and 65 wt % platinum (Pd35Pt65) prepared at pH value 1.2 was the best. The catalytic efficiency of all catalysts was in the range of 67% whereas the best sample was Pd35Pt65 deposited at a pH value of 1.2 and its efficiency was of the order of 86.7 ± 2%. Thus, it is concluded that these films may be used as sensors to monitor HER reactions and may be used at places where removal of atomic hydrogen is desired to avoid accidents.

The activity enhancement of photocatalytic water splitting by F- pre-occupation on Pt(100) and Pt(111) co-catalyst facets

The reversible hydrogen and oxygen recombination reaction in semiconductor dispersion highly impede the yield increase of photocatalytic water splitting to hydrogen and oxygen. In order to inhibit such reverse reaction, it is necessary to reduce the degree of hydrogen and oxygen adsorption and activiation over co-catalyst on semiconductor. It is known that partial occupation of Pt sites by halogen ions with high electronegativity can decrease the adsorption and activation degree of hydrogen and oxygen molecules over Pt/TiO2, by this way, hydrogen and oxygen recombination reaction can be significantly inhibited, and the overall water splitting activity can be remarkably enhanced. Nevertheless, the detail inhibition mechanism on the reverse recombination reaction by halogen ions occupation on different Pt sites is still unclear. In the present work, by comparing the fluorine ions occupation on different Pt facets, we found that the fluorine ions occupation could decrease the numbers of adsorption sites on Pt(100) and Pt(111) surface. On Pt(100) facet, a fluorine ion occupation affect the four adjacent adsorption sites for H2 and O2 molecules adsorption, while, on Pt(111) surface, a fluorine ion occupation could affect the six adjacent adsorption sites for H2 and O2 molecules adsorption. The difference of fluorine ions occupation on the Pt(100) and Pt(111) facets led to the different adsorption strength of hydrogen and oxygen molecules, which further induced the activiation difference of hydrogen and oxygen on Pt co-catalyst. The results of density functional theory (DFT) calculation indicated that the hydrogen and oxygen adsorption energies on F/Pt(100) were higher than that on F/Pt(111) surface. The H2/O2-TPD, cyclic voltammetry and in-situ XPS experimental results also verified that the hydrogen and oxygen adsorption on F/Pt(100) surface was stronger than that of F/Pt(111) surface. The hydrogen and oxygen recombination experimental results showed that the hydrogen and oxygen recombination rates over F/Pt(100) surface was higher than that of F/Pt(111) surface. Upon light irradiation, the F/Pt(100)/TiO2 photocatalyst exhibited lower activity for overall water splitting than F/Pt(111)/TiO2 photocatalyst. Results in this paper provide a new avenue to promote seimiconductor catalyst for over-all water splitting by tuning the adsorption sites of hydrogen and oxygen on co-catalyst surface.

Effect of titanium surface roughness on oxygen catalytic recombination in a shock tube

Surface roughness and microscopic morphology are key factors influencing oxygen catalytic recombination. Titanium is extensively used as a material for metallic thermal protection systems (TPS) in the design of hypersonic and reusable launch vehicles. In this study, the effect of titanium surface roughness on oxygen catalytic recombination is experimentally investigated. The efficiency of the oxygen recombination reaction is determined by evaluating the measured heat-transfer rates while considering the existing theory of binary gas mixtures. The surface of the test models was coated with either titanium or silicon dioxide, and for each model, four different levels of surface roughness were prepared. It is shown that, with an increase in surface roughness, oxygen recombination efficiency increased on both the titanium and silicon dioxide surfaces. Surface topography was characterized in terms of the roughness factor (Ф = A/Ag), i.e., the ratio of the actual surface area (A) to the projected surface area (Ag) using an atomic force microscope. The relationship between the roughness factor and oxygen recombination efficiency was examined.

The inhibition of hydrogen and oxygen recombination reaction by halogen atoms on over-all water splitting over Pt-TiO2 photocatalyst

Semiconductor photocatalysts for overall water splitting into H2 and O2 require metal cocatalyst, such as Pt, to catalyze H2 evolution efficiently. However, these metal cocatalysts can also catalyze hydrogen and oxygen recombination to form water. In this paper, we found that the pre-adsorbed halogen atom catalyst could inhibit the reverse reaction of water formation from H2 and O2 due to the decrease of adsorption energies of H2 and O2 on Pt. The adsorption energy decrease of H2 and O2 followed the order of F/Pt < Cl/Pt < I/Pt < Br/Pt. H2-TPD results exhibited similar dependence. This inhibition was achieved via the occupation of halogen atom on the Pt surface sites, and thereby the adsorption and activation of hydrogen and oxygen molecules were decreased. The occupation difference of halogen atoms are determined by radius of halogen ions, which further leads the different activity for H2 and O2 recombination. By inhibition of water formation reverse reaction, the over-all water splitting over various TiO2 photocatalysts has been achieved. Isotope experiments with D2O and H218O confirmed the over-all water splitting to H2 and O2. This study may help scientist to develop high-efficient photocatalyst for overall water splitting.

A Potential Use γ-Al2O3 Coated Cordierite Honeycomb Reinforced Ti0.97Pd0.03O2−δ Catalyst for Selective High Rates in coupling reactions

The current work was mainly focused on a potential use of γ-Al2O3 coated cordierite honeycomb reinforced Ti0.97 Pd0.03 O2 catalyst for selective high rates in coupling reactions. The Titanium isopropoxide {Ti (OC3H7)4}, Pdcl2 and glycine are the precursors used for the synthesis of Ti0.97Pd0.03O2−δ. In typical coating of (3 % atom Pd on TiO2), 9.7 mmol of TiO(NO3)2 solution, 0.3 mmol PdCl2 and 11 mmol glycine were dissolved to make a solution. The obtained sample of the monolith before and after coating and catalytic activity were studied by using PXRD, IR, and Mass spectroscopy. TEM and SEM. The surface image of the catalyst coated monolith was characterized by FEI Quanta scanning electron microscope. In the coupling reactions have been investigated in view of their potential use in the industrial passive autocatalytic recombines (PAR). The recombination reaction of hydrogen and oxygen has been monitored using Ti0.97Pd0.03O2−δ catalyst

High rates of catalytic hydrogen combustion with air over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ Ti 0.97 Pd 0.03 O 2 - δ coated cordierite monolith

$$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta }$$ was coated on $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ -coated honeycomb structured cordierite monolith (AHCM) by solution combustion method using dip-dry-heat process. This is a modified conventional method to coat the catalysts on honeycomb structured cordierite monolith (HCM). Formation of $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ on AHCM was confirmed by XRD. The XPS spectra of Pd(3d) core level confirmed Pd ion in $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ is in the form of +2 state. The surface morphology of coated catalyst was unchanged by long time exposure to hydrogen combustion reaction; i.e., $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination reaction which indicated the stability of coating on monolith. $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ showed high rates of $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination compared to 2 atom% Pd(metal)/ $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ , $$\hbox {Ce}_{0.98}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta } $$ , $$\hbox {Ce}_{0.98}\hbox {Pt}_{0.02}\hbox {O}_{2-\updelta } $$ , $$\hbox {Ce}_{0.73}\hbox {Zr}_{0.25}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta }$$ , $$\hbox {Ti}_{0.99}\hbox {Pd}_{0.01}\hbox {O}_{2-\updelta } $$ and $$\hbox {Ti}_{0.98}\hbox {Pd}_{0.02}\hbox {O}_{2-\updelta } $$ . The activation energy of $$\hbox {H}_{2} + \hbox {O}_{2}$$ recombination reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ is 7.8 kcal/mol. The rates of reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ at 60 $$^{\circ }\hbox {C}$$ are in the range between 10 and 20 $$\upmu \hbox {mol/g/s}$$ . The rate of reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ increased with increase in the concentration of $$\hbox {H}_{2}$$ . For 50 mL of $$\hbox {H}_{2}$$ , it showed rates of the reaction around 36.45 $$\upmu \hbox {mol/g/s}$$ at room temperature and 230 $$\upmu \hbox {mol/g/s}$$ at 60 $$^{\circ }\hbox {C}$$ . It was found that the rate of reaction due was lower due to hindering effect by adsorption of other gas molecules on the catalytic site. Finally, we propose a mechanism of hydrogen and oxygen recombination reaction over $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ . $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ catalysts were coated by solution combustion method on $$\upgamma \hbox {-}\hbox {Al}_{2}\hbox {O}_{3}$$ coated honeycomb structured cordierite monolith. Among the catalysts, $$\hbox {Ti}_{0.97}\hbox {Pd}_{0.03}\hbox {O}_{2-\updelta } $$ coated monolith showed high catalytic activity for hydrogen combustion with air. The proposed mechanism of $$\hbox {H}_{2} + \hbox {O}_{2}$$ reaction clarified that $$\hbox {Pd}^{+2}$$ ion state is unaltered in $$\hbox {Ti}\hbox {O}_{2}$$ lattice.

A role of Au-content in performance of Pd-Au/SiO2 and Pd-Au/Al2O3 catalyst in the hydrogen and oxygen recombination reaction. The microcalorimetric and DFT studies

The thermal effects and activity of silica and alumina supported bimetallic Pd-Au catalysts (of various Pd/Au ratio) in the exothermic H2 and O2 recombination reaction have been investigated in view of their potential use in the industrial passive autocatalytic recombiners (PAR). The catalysts were prepared by the colloid-based reverse “water-in-oil” microemulsion method which provided metal particles of size in a very narrow range (4–7nm). In both SiO2 and Al2O3 − series catalysts the Pd-Au particles aggregated to some extent, especially strongly in alumina-series samples. The H2+O2 reaction has been monitored using Microscal gas-flow through microcalorimeter at temperature of 22°C and atmospheric pressure. The observed pattern of changes in both the heat evolution and the conversion of hydrogen seem to reflect the effect of water and/or other oxygen-containing surface species (like OH) on the activity/deactivation of catalysts. The nature of support and the composition of metal particles (Pd/Au ratio) played a role. Deactivation of alumina supported catalysts was stronger than silica supported counterparts. Among all studied catalysts, the best behavior was offered by low Au content-containing Pd-Au-0.1/SiO2 (Pd90Au10) catalyst. Its almost stable activity during the catalytic run may be attributed to relatively weak interactions with water molecules and/or other oxygen-containing species (like OH), intermediates formed in the hydrogen oxidation. It may be supposed that electronic modification of palladium sites by gold assisted by the surface composition of Pd-Au particles reflecting in “surface arrangement of Pd and Au-atoms” are decisive. This experimental observation seems to correlated with the DFT calculation indicating that besides the number of Au atoms, their location with respect to the Pd, e.g “surface arrangement of Au” is more important for the energy/strength of interaction with water molecules.

Recombination coefficient of atomic oxygen on ceramic materials in a CO2 plasma flow for the simulation of a Martian entry

In order to design heat shields for space vehicles, materials must be characterized in simulation conditions close to those encountered in space environments. The most important conditions for simulating a Martian entry phase of space vehicles (high temperatures, low pressure CO2 plasma…) are achieved through the MESOX set-up associating a solar radiation concentrator and a microwave plasma generator. After the previous determination of the thermal flux of recombination transferred to the material (thermal approach, accommodation), a chemical approach was developed for evaluating the recombination coefficient of atomic oxygen and/or carbon monoxide by Optical Emission Spectroscopy. The gas temperature above the material, close to the surface where recombination occurs, was evaluated using the rotational temperature of a CO band in the Ångström system. The recombination coefficients of atomic oxygen with O and/or CO on surface were determined in function of temperature-up to 2200K according to the material resistance-for three sintered ceramic materials: SiC, Si3N4 and Al2O3 that can be used in space. An Arrhenius fit is obtained for each material leading to the activation energy of the atomic oxygen recombination reaction in CO2 plasma and these results are compared to the ones previously obtained in air plasma (Earth entry).

Activity and deactivation of Pd/Al2O3 catalysts in hydrogen and oxygen recombination reaction; a role of alkali (Li, Cs) dopant

Activity and deactivation of alkali (Li, Cs) doped 2%Pd/Al2O3 catalyst in the exothermic H2 and O2 reaction have been studied in view of potential application in the passive autocatalytic recombiners (PAR), the safety devices applied in the nuclear plant containments to lower the explosion risk associated with hydrogen release. The catalysts have been prepared with impregnation method and characterized by BET, XPS, TEM and STEM techniques. The role of humidity in the H2/O2 recombination reaction has been studied in a flow laboratory reactor using water saturated reaction mixture (0.5 vol% H2). The time-on-stream behavior of catalyst in contact with reaction mixture of high H2 concentration (7.2 vol%) has been studied using Microscal gas-flow through microcalorimeter. The thermal effects accompanying the H2 conversion during slow deactivation of catalysts have been monitored. The pattern of changes in both the heat evolution and the H2 conversion in the Pd-catalyzed H2/O2 recombination reaction seem to reflect the inhibiting effect of humidity present in the reaction mixture and water molecules formed in the reaction. The presence of both Li and Cs enhanced deactivation of Pd/Al2O3 catalyst and Cs-dopant displayed more pronounced effect. On the other hand, the alkali dopants do not essentially affect the amount of heat evolved during the recombination process ranging from 187 to 220 kJ/mol H2, e.g. being lower than the heat of water formation (246 kJ/mol). The detrimental effect of Li,Cs dopant has been attributed to higher affinity to water, promoting retention of water molecules/films on the catalyst surface and blockage of active sites due to enhanced adsorption of surface species, like H2O/OH.

Humidity induced deactivation of Al2O3 and SiO2 supported Pd, Pt, Pd-Pt catalysts in H2 + O2 recombination reaction: The catalytic, microcalorimetric and DFT studies

The thermal effects and activity of silica and alumina supported monometallic Pd, Pt and bimetallic Pd-Pt catalysts (of various Pd/Pt ratio) in the exothermic H2 and O2 recombination reaction have been investigated in view of their potential use in the industrial passive autocatalytic recombiners (PAR). The pattern of changes in both the heat evolution and the conversion of hydrogen observed in the H2/O2 reaction seem to reflect the effect of water formation on the activity/deactivation of the studied catalysts. The catalysts have been prepared using the colloid-based reverse “water-in-oil” microemulsion method and characterized by XRD, XPS, SEM, EDS techniques. The recombination reaction of hydrogen and oxygen has been monitored using Microscal gas-flow through microcalorimeter as well as a laboratory flow microreactor. The humidity present in the reaction mixture and the water molecules formed in the recombination reaction both inhibited the activity of the tested catalysts. The nature of support and the type of metal played a role in such water-poisoning effect. Deactivation of alumina supported catalysts was stronger than silica supported counterparts and Pt was more prone to the water inhibition compared to Pd. The most promising catalysts exhibiting the lowest amount of evolved heat accompanied by high and stable activity are silica supported Pd-rich bimetallic Pd-Pt particles whereas alumina wash-coat-Pt is the catalyst of chose in the PAR reactors. The observed activity/water-poisoning relations found confirmation in the DFT calculations concerning the interactions between water molecules and mono-(Pd, Pt) as well as bi (Pd-Pt) metallic clusters as a function of cluster size and Pd/Pt compositions.

A Mechanistic Model of a Passive Autocatalytic Hydrogen Recombiner

Abstract : A passive autocatalytic hydrogen recombiner (PAR) is a self-starting device, without operator action or external power input, installed in nuclear power plants to remove hydrogen from the containment building of a nuclear reactor. A new mechanistic model of PAR has been presented and validated by experimental data and results of Computational Fluid Dynamics (CFD) simulations. The model allows to quickly and accurately predict gas temperature and composition, catalyst temperature and hydrogen recombination rate. It is assumed in the model that an exothermic recombination reaction of hydrogen and oxygen proceeds at the catalyst surface only, while processes of heat and mass transport occur by assisted natural and forced convection in non-isothermal and laminar gas flow conditions in vertical channels between catalyst plates. The model accounts for heat radiation from a hot catalyst surface and has no adjustable parameters. It can be combined with an equation of chimney draft and become a useful engineering tool for selection and optimisation of catalytic recombiner geometry.

Atomic Oxygen Concentrators for Material Exposure Acceleration Tests in Low Earth Orbit

Atomic oxygen concentration technology was investigated with two types of concentrators. One is a “horn-type” andtheotherisa“multiplering-type”concentrator.Bothofthemweredesignedwithacomputersimulationusinga hard-cube model. It was made clear that the primary factor for limiting the concentration factor is the gas buildup near the focal point. In the ground test using a laser-detonation source, which gave an intense atomic oxygen beam pulse, the concentration factor of 2–3, which is lower than the theoretical expectations, is obtained. However, it is suggested that the low concentration factor is due to the recombination reaction of atomic oxygen into molecular oxygenbythehighpeak fluxinthepulsedatomicoxygenbeam.The fluxlimitoftheconcentrationfactorisnotacase in the real low Earth orbit space environment, and the multiple ring-type concentrator can be expected to achieve a concentration factor of over 40 in a low Earth orbit exposure condition. Nomenclature A = area of quartz C = concentration factor d = thickness of quartz E = energy F = flux f0 = resonant frequency

Oxygen-driven relaxation processes in pre-irradiated Ar cryocrystals

Relaxation processes in oxygen-containing Ar cryocrystals pre-irradiated by low-energy electrons are studied with the focus on the role of the diffusion-controlled atom-atom recombination reaction of oxygen in the relaxation cascades. The results of real-time-correlated measurements of thermally stimulated phenomena are presented. The experiments have been performed using activation spectroscopy methods—thermally stimulated exoelectron emission and spectrally resolved thermally stimulated luminescence. Solid evidence for the radiative mechanism of electron detrapping triggering the relaxation cascades is obtained.

Comparative experimental study of gas evolution and gas consumption reactions in sealed Ni–Cd and Ni–MH cells

Abstract The behavior of the sealed Ni–Cd and Ni–MH systems are compared experimentally with regard to their ability to consume gaseous products generated during the overcharge stage of these systems. It was found that the Ni–Cd system could only consume oxygen, while the Ni–MH system possesses the additional ability to adsorb hydrogen and to catalyze the recombination reaction of hydrogen and oxygen. The internal pressure within both sealed Ni–Cd cells and sealed Ni–MH cells can be kept well under control during the charge/overcharge processes if the rate of overcharge is not too high and if there is sufficient surplus of charging capacity at the negative electrodes. However, the internal pressure can rise to dangerously high levels if the rate of overcharge is too high or there is a deficiency of the charging capacity at the negative electrodes. The various factors that may affect the surplus of charging capacity of the negative electrodes are also discussed.

Recombination coefficient of atomic oxygen on ceramic materials under earth re-entry conditions by optical emission spectroscopy

Abstract To develop heat shields for space vehicles, materials must be characterized in simulation conditions close to those in space environments. The most important conditions for simulating the Earth re-entry phase of space vehicles (high temperatures, low pressure air plasma,…) are achieved through the MESOX set-up associating a solar radiation concentrator and a microwave plasma generator. After determining the thermal recombination flux transferred to the material (thermal approach, accommodation), we developed a chemical approach for evaluating the recombination coefficient of atomic oxygen γO by Optical Emission Spectroscopy. We also measured the gas temperature above the material, close to the surface where recombination occurs, using the rotational temperature of nitrogen. The recombination coefficients of atomic oxygen were determined in function of the temperatures of two types of silica materials. Finally, the activation energy of the atomic oxygen recombination reaction was obtained for each material. Results showed that the microstructure of the topmost layer has an important effect and that the recombination coefficient of β-cristobalite is four times higher than that of quartz.

Ni/MH Rechargeable Battery with Zr-Based Hydrogen Storage Alloy Electrode Modified by High Surface Area of Ni Powder : Inner Cell Pressure Characteristics

Efforts to improve the surface catalytic activity of electrodes has been made to decrease the internal pressure of a sealed Ni-MH battery in which the low-cost alloys are used as the anode. To improve the surface catalytic activity of the Zr-based alloy which is closely related to the inner cell pressure behavior in a sealed cell, the electrode was fabricated by mixing the alloy with Cu powder and filamentary Ni powder. By replacing 25% of carbon black by filamentary Ni powder, the inner cell pressure has been suppressed significantly. The main reason for the reduction of inner cell pressure is the increase of overall surface catalytic activity of the Zr-based alloy electrode by addition of the filamentary Ni powder with high surface area. The filamentary Ni powder-mixed electrode has a surface area comparable to that of carbon black. It is assured that the Ni powder plays a role in enhancing a specific surface catalytic activity for the oxygen recombination reaction on the MH surface. © 2002 The Electrochemical Society. All rights reserved.