Presence Of High Partial Pressure Research Articles

Stability of lanthanum in sulfate and phosphate systems and implications for selective rare earths extraction

The stability of Rare Earth Elements (REE), both sulfates and phosphates, was investigated using monazite ore, a residue from a synthetic rutile manufacturing process and a high-grade lanthanum sulfate sample. It was demonstrated that REE sulfates are converted into phosphates at temperatures within the range of 450–800 °C. This reaction depends on excess phosphorus present in compounds that are less stable than monazite. The presence of high partial pressures of P2O5 in the system will reconvert REE sulfates into insoluble REE polyphosphates – REE(PO3)3. Relatively high partial pressure may be attained by the decomposition of phosphorus compounds present in the sample, such as monocalcium phosphates and phosphoric acid generated by the addition of sulfuric acid. The direct contact with phosphoric acid leads, in turn, to the formation of monazite-like compounds. The gas–solid mechanism is believed to be the most suitable due to the easier access of the gas phase to the REE sulfate particles, thus making it possible to achieve the drastic decrease (conversion into phosphate) in REE extraction observed at 800 °C. This study’s results are expected to contribute to a better understanding of the factors favoring selectivity in REE extraction over iron from monazite ores.

On the Mechanism of the Disproportionation of ZrCo Hydrides

Among the various getter materials the interalloy ZrCo has been selected by the ITER team as the reference material for the storage of hydrogen isotopes at the tritium plant because of its excellent getter properties, which are comparable to those of uranium. Only certain conditions, such as the presence of high partial pressure of H2 at relatively low temperatures (350°C to 400°C), or during repeated hydrogen absorption-desorption heat cycles, have been a matter of concern, because under these conditions ZrCo can lose its gettering properties. Indeed, under repetitive loading/deloading cycling, the getter hydride (ZrCoH3) tends to disproportionate, i.e., to convert into ZrH2 and ZrCo2 and thus show a significant performance degradation of its gettering properties. Disproportionation is a major drawback as it fixes almost irreversibly part of the hydrogen (hence, tritium) into a ZrH2 form.To understand the underlying mechanism leading to the disproportionation, a detailed investigation has been undertaken. Using thermal analytical methods and based on crystallographic considerations, we came to the conclusion that the driving force for such disproportionation has to be attributed to the hydrogen occupation (taking place during the hydridation) of the various crystallographic sites available to it. During the hydridation process ~4% of hydrogen goes into the less-stable 8f2 and 8e sites, where the Zr-H distance is shorter than the ZrH2 distance. Therefore, during the dehydridation process these sites are not releasing the hydrogen, but rather they are generating the very stable ZrH2, thus leading to the partial disproportionation of the material.Therefore, we may conclude that ZrCo it is not adequate for the storage of tritium and other hydrogen isotopes within the tritium plant of ITER, and consequently, we would not recommend it for such use.

Study on Deactivation of Co/ZrO2/SiO2 Catalyst For Fischer-Tropsch Synthesis

A Co/ZrO2 /SiO2 catalyst active for synthesis of heavy hydrocarbons from syngas was subjected to a stability test of 500 h on a laboratory scale. The catalyst showed a considerable decrease in CO conversion, while the hydrocarbon distribution hardly changed with time on stream. The regeneration of deactivated catalyst with H2 at 673 K partially restored the activity. XRD, FT-IR and TGA identified a hydrated cobalt silicate species in the catalyst near the reactor outlet. Thus, the cause of deactivation was proposed mainly to be the formation of inactive hydrated silicate between Co metal and silica in presence of high partial pressure of water.

Hydrogen on unsupported ruthenium sulfide: Thermodesorption and 1H NMR studies

The interaction of transition metal sulfides with hydrogen is a complex process. Understanding this process is important due to the very large number of industrial processes dealing with these systems. The main utilization of these catalysts concerns catalytic hydrotreating which involves hydrogenation reactions and C-S, C-N, and C-O bond cleavage reactions carried out in the presence of high partial pressures of hydrogen. The complexity of the interaction of hydrogen with metal sulfides is related to its double role, as reactant and as modifier of the concentration of active sites. As a matter of fact, the interaction of hydrogen with the surface of the catalyst provokes its partial reduction, leading to coordinatively unsaturated metal ions which are directly involved in the adsorptive and catalytic properties. To tackle this fundamental problem of hydrogen adsorption on ruthenium sulfide, the authors have taken the fully sulfided state as starting point and evaluated the influence of progressive desulfurization on the amount of adsorbed hydrogen. The nature of the adsorbed species was investigated using thermodesorption and [sup 1]H NMR. 20 refs., 3 figs.

Analysis of Substrates for Methanogenesis in Anaerobic Sludges Using Specific Inhibitors

We examined what kind and how much of the specific inhibitor is effective for the estimation availabilities of substrates for methanogens in anaerobic sludges. The chloroform (0.1%, v/v) inhibited 90% of the methanogenesis in an anaerobic sludge (LD), which was cultivated with glucose, and was more effective than 2-bromoethanesulfonic acid (BES, 0-200 mM). Hydrogen, acetate and propionate were linearly accumulated when the methanogenesis was inhibited by 0.1% of chloroform. The propionate accumulation rate in the presence of chloroform was identical with that in the presence of high partial pressure of hydrogen (H2:CO2 = 20:80) which inhibits syntrophic H2- producing fatty-acids-oxidizing bacteria. This shows that the methanogens utilized hydrogen and acetate converted by syntrophic hydrogen-producing propionate-oxidizing bacteria through interspecies hydrogen transfer. Using chloroform (0.1%) as the specific inhibitor, in the LD sludge 49% of methane was estimated to be derived from propionate. The H2 + CO2 and acetate from the other pathway (NS), contributed 2 and 34% of the methanogenesis, respectively. This method was also applied to a municipal sewage digester sludge (MD). In the MD sludge, methanogenesis from propionate, H2 + CO2 and acetate via the NS pathway were 16, 2 and 78%, respectively. Major substrates for methanogenesis, therefore, could be estimated in anaerobic sludges.

Isolation and Characterization of a Thermophilic Bacterium Which Oxidizes Acetate in Syntrophic Association with a Methanogen and Which Grows Acetogenically on H 2 -CO 2

We previously described a thermophilic (60 degrees C), syntrophic, two-membered culture which converted acetate to methane via a two-step mechanism in which acetate was oxidized to H(2) and CO(2). While the hydrogenotrophic methanogen Methanobacterium sp. strain THF in the biculture was readily isolated, we were unable to find a substrate that was suitable for isolation of the acetate-oxidizing member of the biculture. In this study, we found that the biculture grew on ethylene glycol, and an acetate-oxidizing, rod-shaped bacterium (AOR) was isolated from the biculture by dilution into medium containing ethylene glycol as the growth substrate. When the axenic culture of the AOR was recombined with a pure culture of Methanobacterium sp. strain THF, the reconstituted biculture grew on acetate and converted it to CH(4). The AOR used ethylene glycol, 1,2-propanediol, formate, pyruvate, glycine-betaine, and H(2)-CO(2) as growth substrates. Acetate was the major fermentation product detected from these substrates, except for 1,2-propanediol, which was converted to 1-propanol and propionate. N,N-Dimethylglycine was also formed from glycine-betaine. Acetate was formed in stoichiometric amounts during growth on H(2)-CO(2), demonstrating that the AOR is an acetogen. This reaction, which was carried out by the pure culture of the AOR in the presence of high partial pressures of H(2), was the reverse of the acetate oxidation reaction carried out by the AOR when hydrogen partial pressures were kept low by coculturing it with Methanobacterium sp. strain THF. The DNA base composition of the AOR was 47 mol% guanine plus cytosine, and no cytochromes were detected.

Mutants of Rhizobium japonicum with Increased Hydrogenase Activity

Some strains of Rhizobium japonicum can use hydrogen as an energy source for growth under microaerophilic conditions. Mutant strains have been selected that use hydrogen in the presence of high partial pressures of oxygen. The mutants contain more hydrogenase than the parent strain, both as free-living cells and as bacteroids in nitrogen-fixing soybean root nodules.

Cloud chamber study of the gas phase photooxidation of sulfur dioxide

Nucleation and growth promises to be a powerful quantiative detector and amplifier for many chemical physical phenomena. We have utilized vapor phase nucleation in a study of the photo-oxidation (by disproportionation) of SO2 through adaptation of the upward diffusion cloud chamber. Our results are unique to the extent that the photolysis of SO2 is performed in the presence of high partial pressures of water vapor. At the moment (because of interference from the formation of a sulfuric acid mist) it does not seem possible to study this reaction under these conditions by any other means. Our detection scheme is based on the nucleation of water droplets by H2SO4, a product of photolysis. In these experiments H2SO4 molecules are detected in real time at concentration levels in the neighborhood of 108 cm−3, and reaction rates of the order of 108 cm−3 s−1 are also detected. The photolysis of SO2 to form SO3 and SO (by disproportionation) can result from uv excitation in either the singlet (2900 Å) or triplet (3800 Å) absorption bands of SO2. Quantitative experimental results obtained from excitation in these two respective bands (the singlet excitation leads to intersystem crossing to the triplet manifold) are entirely consistent with one another. Furthermore, the results are consistent with mechanisms previously proposed and verified for the disproporationation taking place in the absence of water vapor, with one exception. The crucial step in this mechanism involves SO2 in the 3B1 state as the principal intermediate which participates in disproportionation through a reaction with a ground state SO2 molecule. The rate constant for this step yielded by previous studies is k1a=4.2×107 l mole−1s−1 while our result is approximately 2×1011 l mole−1 s−1. This is nearly as large as the estimated collision frequency rate constant, namely, 4×1011 l mole−1 s−1. The increased rate constant in the presence of water vapor is rationalized by assuming that SO2 in our experiments, mixed with water vapor at 283% relative humidty, is hydrated so that the activated complex contains an incipient H2SO4 molecule which is stable enough to present only a negligible activation energy. By adopting a suitable strategy, our rate constants are determinable without appeal to the theory of nucleation. However, direct use of the existing theory of nucleation together with the observed rate of nucleation yields a value for the rate constant in satisfactory agreement with that derived by the means which avoids nucleation theory. In this sense our experiments also provide some confirmation of existing nucleation theory. The paper contains a detailed account of the many special technical problems (many of them solved) involved in the adaptation of the cloud chamber to the detection of chemical physical processes. In this sense the accomplishments are twofold: (1) the investigation of the gas phase photoinduced disproportionation of SO2 in the presence of water vapor, and (2) the demonstration of the use of the diffusion cloud chamber as a quantiative tool for the detection and amplification of chemical physical phenomena.

The Influence of Partial Pressure of Carbon Dioxide upon Carbon Metabolism in the Tomato Leaf

The incorporation of radioactive carbon into various photosynthetic products was investi gated with tomato plants in atmospheres containing between 40 and 1400 parts/106 carbon dioxide. A significantly greater proportion of 14C entered sucrose and alcohol-insoluble material at high concentrations of carbon dioxide. Incorporation into glycine and serine was significantly greater at lower carbon-dioxide concentrations. The pool size of these inter mediates was also determined and it was concluded that in the presence of high partial pressures of carbon dioxide the flow of carbon from the photosynthetic cycle through the C2 pathway is decreased.