Radioisotope Tracer Method Research Articles

Hydrodesulfurization and Hydrogenation Reactions on Noble Metal Catalysts: I. Elucidation of the Behavior of Sulfur on Alumina-Supported Platinum and Palladium Using the 35S Radioisotope Tracer Method

Hydrodesulfurization (HDS) reactions of 35S-radioisotope-labeled dibenzothiophene (DBT) were carried out over a series of γ-Al2O3-supported noble-metal-containing catalysts at 5.00 MPa and at 260 and 280°C. The amount of sulfur (STOTAL) accommodated on the catalyst and the amount of labile sulfur (S0) participating in the reaction were determined using a direct method, the 35S radioisotope pulse tracer method, which has recently been developed by the authors. It was observed that both STOTAL and S0 increased linearly with an increase in active metal loading. At the same time, it was found that the sulfided test noble metal catalysts corresponded to a S/Pt (Pd) ratio of 0.25 and that almost all the labile sulfur on these catalysts was mobile in the HDS reaction. Further, the activities of both HDS and hydrogenation reactions over the bimetallic catalyst (Pt–Pd) were higher than those of the monometallic catalysts tested together, whereas the synergetic effects observed on the Pt–Pd catalyst were not as significant as in typical Co–Mo catalyst cases.

Hydrodesulfurization of dibenzothiophene and hydrogenation of phenanthrene on alumina-supported Pt and Pd catalysts

Hydrodesulfurization (HDS) reactions of dibenzothiophene (DBT) and hydrogenation (HYD) of phenanthrene (PHE) were carried out over several alumina-supported noble metal catalysts at temperatures between 200°C and 400°C and under 50 kg/cm 2 pressure. A dual noble metal catalyst, Pt(2)–Pd(10), showed similar HDS results and better HYD promotion compared with a conventional Co–Mo catalyst. Further, 35 S radioisotope tracer method showed that almost no labile sulfur was accumulated on Pt/Al 2O 3 catalyst in the HDS of DBT or HYD of PHE, although this catalyst showed an HDS activity comparable to that of the Co–Mo catalyst. This indicated that the mechanism of DBT HDS on noble catalysts was different from that on the conventional Mo-based catalyst.

Elucidation of hydrodesulfurization mechanism on molybdenum‐based catalysts using 35S radioisotope pulse tracer methods

Elucidation of the hydrodesulfurization (HDS) mechanism on molybdenum‐based catalysts using radioisotope tracer methods and reaction kinetics is reviewed. Firstly, to investigate the sulfidation state in Mo/Al2O3 and Co–Mo/Al2O3 catalysts, presulfiding of these catalysts has been performed using a 35S pulse tracer method. Secondly, HDS of radioactive 35S‐labeled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts and cobalt‐promoted molybdena–alumina catalysts in a pressurized flow reactor to estimate the behavior of sulfur on the working catalysts. Finally, sulfur exchange of a 35S‐labeled catalyst with hydrogen sulfide was performed to estimate the relationship between the amount of labile sulfur and catalytically active sites.

Sorption of chromium hydroxide sol on silica studied by radiotracing

This study was undertaken to investigate the sorption behavior of chromium hydroxide sol on silica type adsorbent in aqueous solution. Radioisotopic tracer method was used to assess data. This technique allowed sensitive kinetic and equilibrium measurements in both direction: sorption and desorption, which are strongly dependent on solution conditions, such as pH, ionic strength or the presence of complexing species. Total removal of sol particles could not be achieved, even under the most favorable experimental conditions (acidification).

The competitive exchange of hydrogen and cobalt ions on calcium-montmorillonite

The sorption of cobalt ion on calcium-montmorillonite was studied by the radioisotopic tracer method at different pH values. In dilute solutions the reaction has to be treated as a ternary exchange of cobalt, calcium, and hydrogen ions because of the high affinity of hydrogen ion to montmorillonite. Equilibrium constant for calcium-hydrogen exchange was determined.

Elucidation of molybdenum-based catalysts using a radioisotope tracer method Part 1.—Hydrodesulfurization activity and structure of sulfided molybdena–alumina catalysts

Hydrodesulfurization of radioactive 35 S-labelled dibenzothiophene was carried out over a series of sulfided molybdena–alumina catalysts containing 6–20 wt.% molybdena at temperatures in the range 280–380°C and at a pressure of 50 kg cm -2 . The sulfur exchange data and hydrodesulfurization activity were simultaneously obtained and the effect of molybdenum content on the structure of sulfided catalysts was estimated. The sulfur exchange rates were approximately the same for all samples at the same temperature and apparent activation energies of hydrodesulfurization reactions were 20±2 kcal mol -1 for all catalysts. These results suggest that the mechanism of hydrodesulfurization and the nature of active sites did not vary with molybdenum content. On the other hand, the amount of labile sulfur increased linearly with the molybdenum content up to 2.89 atom nm -2 but then levelled off with further loading of molybdenum. Thus, it was suggested that a monolayer dispersion of molybdenum sulfide on alumina was maintained up to 2.89 atom nm -2 but some crystallites of molybdenum sulfide would be formed when molybdenum was added beyond 2.89 atom nm -2 . Moreover, the sulfur exchange rate hardly varied with the molybdenum content and the hydrodesulfurization rates of dibenzothiophene on the catalysts paralleled the amount of labile sulfur.

Elucidation of molybdenum-based catalysts using a radioisotope tracer method Part 2 Promotion effect of cobalt on molybdena/alumina catalyst

Hydrodesulfurization (HDS) reactions of radioactive 35S-labelled dibenzothiophene were carried out over a series of cobalt-promoted molybdena/alumina catalysts at temperatures between 240 and 320°C and under 50 kg cm-2 pressure. HDS and hydrogenation (HYD) activities of Mo/Al2O3 catalysts were remarkably enhanced with the addition of cobalt, the maximum promotional effect occurring with a Co/Mo molar ratio of ca. 0.5. The amount of labile sulfur and the rate constant of sulfur exchange increased significantly with the addition of cobalt, indicating that cobalt makes the sulfur more mobile and that the active phases in the promoted catalysts are different from that in the unpromoted catalyst. The promoting effect of cobalt was thus attributed to the formation of more active sites. On the other hand, the amount of labile sulfur increased linearly with the Co/Mo ratio up to ca. 0.5 whereas the rate constant of sulfur exchange remained almost constant. Moreover, the enhancement in catalytic activity did not change significantly with the cobalt content. The increase in the catalytic activity with the cobalt content was thus ascribed to an increase in the number of the same active sites.

Study of Hydrodesulfurization of Dibenzothiophene on Ni-Mo/Al2O3, Mo/Al2O3, and Ni/Al2O3 Catalysts by the Use of Radioisotope 35S Tracer

The radioisotope tracer method has been used to quantify the behavior of sulfur on sulfided Ni-Mo/Al2O3, Mo/Al2O3, and Ni/Al2O3. The apparent activation energies of HDS reaction for DBT for the three catalysts were 20 ± 1 kcal/mol. The formation rate constants of 35S-H2S were determined and the amount of labile sulfur on the sulfided catalysts was estimated by tracing the changes in radioactivities of the unreacted 35S-DBT and the formed 35S-H2S during the HDS reaction of 35S-labeled dibenzothiophene (35S-DBT). It was deduced that ca. 75% of sulfur in the sulfided Mo/Al2O3 was related to HDS reaction at infinite rate of HDS. Compared with the amounts of labile sulfur in the sulfided Ni-Mo/Al2O3, Mo/Al2O3, and Ni/Al2O3, it was determined that the amounts of labile sulfur were 1.6, 9.8, and 18.4 mg sulfur/g catalyst at 280°C for the three catalysts, respectively. And it was suggested that the sulfur in the form of NiS on the sulfided Ni-Mo/Al2O3 was not labile and that the sulfurs attached to both Mo and Ni atom were more labile and related to HDS. The promotion of Ni for Mo-based catalysts was attributed to the sulfur bonded to both Mo and Ni in the MoS2 phase being more labile.

Fabrication and Characteristics of a High Pressure Tiny Pump for Gas Circulation in Radioactivity Measurement of Gaseous Samples.

A high pressure tiny pump operatable in a range of (0.5-5) ×105 Pa was needed for gas circulation in the newly developed direct measurement method of radioactivities of gaseous samples, yet, such a pump was not commercially available. A tiny air bubbling pump of electromagnetic vibrating-reed type for small fish raising was remodeled by putting it inside a high pressure stainless steel vessel and vibrating the reed with an electromagnet being set outside the vessel wall. Gas flow rate (Pa·m3/s) was measured at atmospheric pressure both by a method using a measuring-cylinder and by radioisotope (RI) tracer method, and at other pressures by RI tracer method. The flow rate decreased with increasing pressure and viscosity of the gas. The pump was very useful for the present purpose. Details of remodeling and characteristics of the pump are described.

The inhibition of bovine heart hexokinase by 2-deoxy- d-glucose-6-phosphate: characterization by 31P NMR and metabolic implications

The glucose analog, 2-deoxy- d-glucose (2DG), has been used widely for studying the initial steps in the metabolism of glucose by radio-isotope tracer methods and by 31P NMR. In the rat heart perfused with acetate/2DG (both 5 mM) plus insulin, trapping of phosphorus by 2-deoxy- d-glucose-6-phosphate (2DG6P) results in a steady state exhibiting high 2DG6P (55 mM) and low ATP concentrations but near-normal function, as observed in an earlier 31P NMR study. In order to understand how the 2DG6P concentration is stabilized, we studied the inhibition of a mammalian hexokinase by 2DG6P in vitro by a 31P NMR technique. Inhibition, previously unobserved, was found. It is similar to inhibition by G6P in that it is competitive with ATP and not competitive with 2DG, but the inhibition constant (1.4 mM) is much larger. The experimental protocol includes provisions for enzymatic destruction of stray inhibitors such as G6P. The results show that the high 2DG6P and low ATP concentrations found in the steady state of the perfused heart should strongly reduce the rate of phosphorylation of sugars by hexokinase.

The exchange of 54Mn and 45Ca ions on montmorillonite

The interfacial reaction between calcium-montmorillonite and the manganese ion is studied by a radioisotopic tracer method. At high manganese concentration an expression for the sorbed manganese is found, which indicates a small extent of adsorption. A relation is found for the separation of ion exchange and adsorption. The derived equation was found suitable for use in the calcium-montmorillonite-manganese ion system. A similar equation can be applied for the exchange of three ions.

Hydrogenation Mechanism of Coal Tar Pitch for Carbon Fiber(Part 3). Effects of Hydrotreatment on Pyrolysis Reactivity of Each Component in Coal Tar Pitch for High Performance Carbon Fiber.

Coal tar pitch for high performance carbon fiber (HPCF) was hydrotreated using tritium labelled tetralin as donor solvent at 450°C for 120min. The hydrogen transfer in this process was determined quantitatively according to results obtained from radioisotope tracer method. The hydrotreatment of pitches can be explained by a general mechanism such as involved in the hydrotreatment of petroleum heavy oil or coal liquefaction. The tritiated pitch was heat-treated in the range of 30 to 1, 000°C under nitrogen atmosphere. It was found that the reaction of carbon-carbon bond cleavage and the evaporation of volatile components were basically completed at about 600°C. The dehydrogenation of polynuclear aromatic rings was sustained up to 1, 000°C. The model based on the first order rate expression was able to analyse TGA data successfully. Activation energies and frequency factors, for the nonisothermal heat treatment of pitches and each component of pitches, were calculated. The pyrolysis activation energies of the pitch and major components (HIS-BS and BIS-THFS fractions) in the pitch decreased when the pitch was hydrotreated.

Effect of carbon dioxide on electrochemical stability of gas diffusion electrodes in alkaline solution

By means of radioisotope (3H and 14C) tracer methods, a study was undertaken to investigate the role of CO2 impurity [10% (m/o)] contained in O2 feed gas on the electrochemical stability of gas diffusion electrodes which performed the oxygen reduction in alkaline solution. The electrodes were prepared by supporting a perovskite oxide catalyst on three kinds of carbon carriers; hydrophilic activated carbon, hydrophobic acetylene black and furnace-type carbon black. For the electrodes with the carriers mixed by activated carbon and acetylene black, a polarization loss of the electrode and a dissolution of CO2 into electrolytic solution were observed during the supply of CO2 to O2 feed gas. Under a low gas flow rate of the feed gas, a reversible polarization responsible to the supply of CO2 was observed in the electrodes with high contents of activated carbon carrier. However, an irreversible fatal polarization loss accompanied with the progressive wetting of the electrode was observed under a high gas flow rate of the feed gas, leading to the decay of three phase equilibrium even when cutting off the supply of CO2. On the other hand, neither the dissolution of CO2 into electrolytic solution nor the wetting of the electrode induced by the introduction of CO2 were detected for the electrode with furnace-type carbon black which was able to construct a compactly packed electrode structure.

On the adsorption of iron oxide—hydroxide sol on alumina and silica

The adsorption of α-FeO (OH) colloidal particles was investigated on coarsely dispersed adsorbents by the radioisotopic tracer method. On the basis of the Langmuir representation, the sol adsorption is usually considered as a pseudo first-order process, and changes in the adsorbed amount with the equilibrium sol concentration can be described by the Langmuir adsorption isotherm. However, the adsorption of the iron oxide-hydroxide sol on aluminum oxide (pH 4.3) was not found to be first order. An expression was derived which made possible the simple treatment of experimental data even in such cases. In the case of adsorption of iron oxide-hydroxide on SiO2 (pH 4.3–5) the amount adsorbed by unit mass of adsorbent (the specific adsorbed amount) changed with the sol/adsorbent ratio at a given equilibrium sol concentration, i.e., the Langmuir adsorption isotherm proved unsuitable for the quantitative description of adsorption.Results suggest that the surface properties of the SiO2 change by changing the sol/adsorbent ratio; the SiO2 becomes positive by adsorbing ferric hydroxo complex species from solution. However, if ferric ions in sufficiently high concentration are added to the ultrafiltrate at pH 3.6 before the adsorption measurement, the SiO2 will acquire a net positive charge; as a consequence, the measured data lie along a straight line by plotting the linearized Langmuir isotherm, i.e., amounts of particles adsorbed at equilibrium will be independent of the sol/adsorbent ratio. The effect of indifferent electrolytes on the iron oxide-hydroxide sol/SiO2 (or silica) system (pH 3.6) is interpreted in terms of interactions between surfaces of like charge. The special effect of the fluoride ion is attributed to secondary processes of a chemical nature and cannot be interpreted simply by the DLVO theory taking into account only interactions between double layers.

Effects of concentration and temperature of alkaline solutions on the electrochemical stability of a gas diffusion electrode

The working life of a gas diffusion electrode performing in 1, 5 and 10 M KOH was investigated at 25, 55 and 80° C. During the performance, the wetting of the electrode could be monitored by means of a radioisotope tracer method with a tritiated aqueous solution of KOH. The polarization became less in 1 M KOH with increasing temperature due to the progress in the moderate wetting of the electrode by the electrolytic solution. Upon increasing the concentration of the electrolytic solution, the working life was prolonged at 25°C, although a large polarization was observed. The working life at 80°C was considerably shorter at all concentrations of the electrolytic solution. It is suggested that the carbon carrier of the electrode is corroded appreciably by the electrolytic solution at high temperatures.

Characterization of the Three‐Phase Equilibrium on a Porous Electrode for Fuel Cell

The stability of the three‐phase interface involving the solid catalyst on carbon carrier, liquid electrolyte solution, and fuel gas found in a porous electrode was investigated by means of a radioisotope tracer method for the electrodes prepared from carbon carriers with various fine structures. Higher electrode activity was observed for the electrode prepared with a perovskite‐type catalyst and a carrier which consisted of a secondary particle crowd due to the aggregation of fine particles. The operating life of the electrode depended remarkably on how much water vapor permeated from the aqueous electrolyte solution into the electrode. The electrode activity decreased abruptly when the permeation of the water vapor exceeded a critical amount. The reactivity of an electrode was found to improve not only by catalytic activity but also by the stabilization of the three‐phase equilibrium in the porous electrode which is promoted by many micropores and a large pore volume formed by the crowd of secondary particles of the carbon carrier.

The study of ion exchange processes in systems of zinc(II)-calcium-soils by a radioisotopic tracer method

Interfacial processes between zinc ions and different calcium soils (clayey soil, humate, bentonite, and sandy soil) were studied by a radioisotopic tracer method. Kinetic studies have shown that the process can be divided into two first-order steps, the activation energy of which is determined. The rate-determining step is diffusion. The sorptions of zinc ion on humate, clayey soil, and bentonite were found to be similar, but in the case of sandy soil, the process was slightly different. The process reaches an equilibrium state on each type of soil in 30–60 min. In equilibrium the amount sorbed is practically the same on humate, bentonite, and clayey soil, but significantly less on sandy soil. In the case of the interaction between calcium bentonite and zinc ions another interfacial process, which has to be taken into account in the evaluation of the resultant reaction, also takes place besides ion exchange.

The interfacial processes between calcium—bentonite zinc ion

The interfacial reaction between calcium-bentonite zinc ion was studied by a radioisotopic tracer method. Both the quantity of zinc ions incorporated into the solid and the quantity of calcium ions leaving the solid were determined. A comparison of the two quantities and the Langmuir representation obtained suggest that other processes can occur besides ion exchange. These processes were separated; ion exchange and adsorption isotherms were constructed, and the isotherm parameters determined. The real ion exchange process was evaluated on the basis of the law of mass action as well. The equilibrium constant of the process and the surface activity coefficients of calcium and zinc ions were determined. The variation of the surface activity coefficient as a function of surface concentration is explained by supposing exchange sites of different binding energy.

Creatine kinase-catalyzed ATP-phosphocreatine exchange:: Comparison of 31P-NMR saturation transfer technique and radioisotope tracer methods

Unidirectional fluxes from ATP to phosphocreatine, catalyzed by the MM isoenzyme of creatine kinase, were measured by both the 31P-NMR saturation transfer technique and radioisotope tracer ([γ- 32P]ATP) method. It was found that at 30–37°C and pH 7.4, over a wide range of[phosphocreatine]/[creatine] (from 0.2 to 5.0) ratios, both methods gave the same results, showing that magnetization transfer allows determination of real fluxes under ‘physiological’ conditions. However, at[PCr]/[Cr] ratios higher than 5 ([ADP] free < 30 μM) or at lower temperatures (t < 15°C,[PCr]/[Cr] ∼ 1), the fluxes assessed by saturation transfer were somewhat faster than those detected by the radioisotope tracer method. These data imply that under physiological conditions phosphoryl group transfer is actually the rate-determining step of the creatine kinase reaction. In contrast, at high[PCr]/[Cr] ratios or at lower temperatures, control may be shifted from phosphoryl group transfer or distributed among other steps of the reaction.

A radioisotopic tracer method for measurement of solids concentration in a settling bed of solids

AbstractA new and improved radiotracer technique provides information on both the particulate concentration and the movement during sedimentation of monodispersed and binary mixtures. In studies of sedimentation with 44 and 62 μm ion exchange particles, several solids concentrations, three fluid viscosities, and three mixtures with different ratios of the two size particles, the results all lie close to a correlation proposed by Richardson and Zaki.